DOI: 10.1039/C6TA00677A
Enhanced Organic Solar Cells Efficiency through Electronic and Electro-optic Effects Resulting from Charge Transfers in Polymer Hole Transport Blends ‡c
‡d
Calvyn T. Howells, Khalid Marbou, Haeri Kim, Kwang Jin Lee, Benoît Heinrich, Sang Jun Kim, Aiko Nakao, Tetsua g h dhi d j e b Aoyama, Seiichi Furukawa, Ju-Hyung Kim, Eunsun Kim, Fabrice Mathevet, Stéphane Mery, Ifor D.W. Samuel, Amal Al a a gk fl d h Ghaferi, Marcus S. Dahlem, Masanobu Uchiyama, Sang Youl Kim, Jeong Weon Wu, Jean-Charles Ribierre, Chihaya h d bdg Adachi, Dong-Wook Kim, Pascal André* ab
ab
e
f
g
a- Masdar Institute of Science and Technology, Abu Dhabi, UAE b- School of Physics and Astronomy, University of St Andrews, SUPA, St Andrews, UK c- Clean Energy Research Center, Korea Institute of Science and Technology (KIST), Seoul, Republic of Korea d- Department of Physics and CNRS-Ewha International Research Center (CERC), Ewha W. University, Seoul, Republic of Korea e- Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg (UMR 7504), Strasbourg, France fEllipso Technology (Co. Ltd.), Suwon, Republic of Korea g- RIKEN, Wako, Saitama, Japan. Email:
[email protected] h- Kyushu University, Center for Organic Photonics and Electronics Research (OPERA), Fukuoka, Japan iDepartment of Chemical Engineering, Pukyong National University, Busan Republic of Korea jInstitut Parisien de Chimie Moléculaire, Chimie des Polymères, CNRS-UMR 8232, Université Pierre and Marie Curie, Paris, France k- Graduate School of Pharmaceutical Sciences, University of Tokyo, Tokyo, Japan lDepartment of Physics, Ajou University, Suwon, Republic of Korea and ‡ These authors contributed equally to this work.
We demonstrate that blending fluorinated molecules in PEDOT:PSS hole transport layers (HTL) induces charge transfers which im pact on both charge extraction and photogeneration within organic photovoltaic (OPV) devices. OPVs fabricated with m odified HTL and two photoactive polymer blends led systematically to power conversion efficiencies (PCE) increases, with PTB7:PC70 BM blend exhibiting PCE of ~ 8.3 %, i.e. ~ 15 % increase compared to pristine HTL devices. A reduced device-to-device characteristics variations was also noticed when fluorinated additives were used to modify the PEDOT:PSS. Shading lights onto the effect of HTL fluorination, we show that the morphology of the polymer:PCBM blends remains surprisingly unaffected by the fluorinated HTL surface energy but that, instead, the OPVs are impacted not only by the HTL electronic properties (work function, dipole layer, open circuit voltage, charge transfer dynamic) but also by alteration of the complex refractive indices (photogeneration, short circuit current density, external quantum efficiencies, electro-optic modelling). Both mechanisms find their origin in fluorination induced charge transfers. This work points towards fluorination as a promising strategy toward combining both external quantum e fficiency modulation and power conversion efficiency enhancement in OPVs. Charge transfers could also be used more broadly to tune the optical constants and electric field distribution, as well as to reduce interfacial charge recombinations within OPVs.
Introduction Organic and hybrid organic-inorganic optoelectronics are the subject of intensive research partially motivated by the potential to achieve low processing cost devices, for instance via roll-to-roll and inkjet printing processes, and by the promise to deliver exciting mechanical properties such as 1-4 lightweight, flexibility and stretchability. Organic materials for solution based processes range from low melting point semiconductors, suitable for liquid electronics, to large polymers, soluble in various solvents.5-8 Ongoing molecular engineering efforts aim at combining solution properties with optimized energy levels and charge transport properties to design high performance devices including transistors, 9,10 light 11,12 4,6 2,13 emitting diodes or cavities, memories, and solar cells. Organic photovoltaic (OPV) devices in both single and multijunction configuration have now reached in research
|1
environment the 10% efficiency seen as the breaking point announcing technology transfer and commercialization. 1,14-20 Among the suitable device structures, bulk hetero-junction (BHJ) solar cells are probably the most studied, with their specific morphology being seen as close to ideal: a) nanophase structuration to increase exciton dissociation, b) bi-continuous percolation network to allow efficient charge collection, c) donor (acceptor) rich phases near the hole (electron) transport layer to reduce charge recombination while increasing the charge selectivity of the electrodes. Various approaches have been developed to tune the phase separation and control the BHJ morphology. These include thermal and solvent vapour treatments, as well as the addition of small molecules or cosolvents, and the resulting structures have been investigated by ellipsometry, electron tomography, dynamic secondary ion mass spectroscopy, X-ray photoemissi on.2 1-29
DOI: 10.1039/C6TA00677A Poly(3,4-ethylenedioxythiophene: poly(styrene sulfonate) (PEDOT:PSS, Fig. 1a1) is widely used as hole transport layers in optoelectronic devices. Sulfonate groups withdraw electrons from the PEDOT backbone and then transform the chain electronic state from neutral to polycationic. The PSS serve then the dual purpose of oxidizing PEDOT moieties and stabilizing in aqueous media the otherwise insoluble polymer. Stimulated by its unusual properties, PEDOT:PSS intensive study revealed that subtle changes in the molecule conformation could lead to dramatic alteration of its electronic properties. For this purpose a wide range of alcohol, acid, surfactant and polymer molecules have been used as cosolvents or as post-treatments of spin-coated films leading to conductivities up to 3000 S/cm and work functions ranging from 4.0 to 5.7 eV.30-37 For instance, McCarthy et al. have recently reported on methanol and formic acid spray treatments of PEDOT:PSS films resulted in a 3 to 4 orders of magnitude improvement in sheet resistance values;37 while Lipomi et al. have developed stretchable electrodes based on ultraviolet/ozone treated PDMS surfaces and the addition of Zonyl fluorosurfactant to a PEDOT:PSS solution. 34 The resulting materials have been used for light emitting diodes, 38-40 transistors,41-44 heat flux sensors,45-47 and solar cells.48-53 In the latter case, PEDOT:PSS doping has been shown to increase photovoltaic efficiencies in both standard and inverted 53-60 configurations. In some cases, solvent treatments based on alcohol or acid derivatives have made possible the fabrication of ITO free devices with compara ble PCE’s.48,50,52, 61,6 2 Fluorinated materials have solution properties orthogonal to both water and oil derivatives. This can be of interest to create barriers or to control solubilization, surface properties, or even to complete chemical reactions in original environments. 63-66 Fluor is also widely used in push-pull molecular design due to its high electro-negativity.67 A nonionic ethoxylated fluorosurfactant was used to develop PDMS based stretchable electrodes for P3HT:PC60BM OPVs.50 A similar nonionic material was blended with PEDOT:PSS to develop inverted solar cells with tunable efficiency and longer lifetime.68 (Heptadecafluoro-1,1,2,2-tetra-hydrodecyl)triethoxysilane was spin-coated on-top of PEDOT:PSS layers subsequently annealed to create a silica based fluorinated spacer above which pentacene was deposited. This interface facilitated 3D single crystalline growth, and the development of interfacial dipole moments through the accumulation of negative charges which enhanced the built-in potential across the devices and resulted in increased open-circuit voltage, hole transport and 49 device efficiency. Germack et al. studied SiO2, PEDOT:PSS and poly(thienothiophene):perfluorinated ionomer inter-faces with P3HT:PC60BM showing that whilst segregation at the buried interface near the HTL could be strongly affected by its surface energy, devices made with the latter two blends lead to OPVs 69 -7 1 with similar characteris tics. However, little has been achieved to discriminate between the relative impacts of work function, conductivity, interfacial and optical property alterations including charge transfer. Herein, we gain original insight into this complex situation relying on a combina tion of experimental investiga tion including atomic
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a1
O
a2
O
C2F4 CFCF2 OCF2CF OCF2CF2SO3H CF3
S
a3 SO3H
b1
CH2(CH2)4CH3
CF3(CF2)7SO3H
b2 O
S
c1
C60
O
S
S F
c2
OR
O
S S COOR
OR
C70
O
R=
Fig. 1. Molecular structures of PEDOT: PSS (a1), PFI (a2), FOS (a3), P3HT:PC60 BM (b1-2), PTB7: PC70 BM (c1-2).
force microscope, Kelvin probe, conductivity, device fabrication and characterisation, wetting, grazing incidence wide angle X-ray scattering, transient absorption and spectroscopic ellipsometry measurements, as well as devices electro-optical modelling. These, we show, provide a full picture of the system and are essential to gain insight into overlapping or complementary effects of fluorination agents (FAs). Two anionic fluorinated materials, i.e. perfluorinated ionomer (PFI, Fig. 1a2) and perfluorooctane sulfonic acid (FOS, Fig. 1a3) were selected to mix with PEDOT:PSS. Whilst FOS presents stronger environmental risks compared to PFI, the choice was motivated by their similar composition (fluorinated and sulfonic acid) allowing to compare the effects of fluorination with polymeric and small molecules on both PEDOT:PSS electronic properties and on device performances. We focused on PEDOT:PSS:FA solutions with weight ratio of 6:1:30, which was selected to observe noticeable effects without altering substantially the HTL conductivity while adding an excess of insulating materials. Fig. 1b/c shows the two photoactive blends, P3HT:PC60BM and PTB7:PC70BM, which were used in this work. P3HT:PC60BM is certainly the most studied OPV model system; whereas, PTB7:PC70BM is a relatively newer model system. Interestingly, PTB7 is a low band-gap material, in which charge separation along its backbone appears to be enhanced by intramolecular charge separation associated with the alternating donor-acceptor groups delocalizing the excitons, lowering their binding energy and reducing charge carrier recombination.72 PCEs up to 5.6 % have been achieved on semi-transparent substrate,73 8.7 % when it was blended with high mobility polymer, 18,74 9.2 % 75 with an inverted device structure, and up to 10.1 % when dual side nanoimprint process were implemented.19 A range of studies have been carried out on the effects of additives,29,76-80 molecular weight,81 processing,82 and substrate,83,84 on the BHJ morphology and efficiency. The combination of both photoactive blends is, however, convenient to strengthen and draw relevant comparisons .83, 85
DOI: 10.1039/C6TA00677A In the present work, a stronger effect of the PEDOT:PSS fluorination is observed with PTB7 based devices than with P3HT OPVs, but both types of material display the same trend with HTL fluorination. We then show that the optoelectronic properties (work function, dipole layer, refractive index) of fluorinated PEDOT:PSS contribute greatly to the overall enhancement of the device efficiencies and their spectral modulations. In addition, and somewhat unexpectedly when compared with other systems, the variations of the wetting property and surface energy of the HTL layer appear to have no detectable influence on the photoactive blend morphology (crystallinity, orientation and composition profile), and then does not appear to be relevant to the variations of the performance of organic solar cells herein observed. In contrast, the electro-optical experimental and modelling results point towards the effect of charge transfers on optical constants to explain the OPVs efficiency spectral variation with HTL fluorination. This is an essential set of information which needs to be understood to take advantage of unusual features and compositions in BHJ devices.
a1
b1
c1
a2
b2
c2
b3
Results & Discussion PEDOT:PSS Fluorination The first row of Fig. 2 presents typical atomic force microscopy (AFM) images. In solution, PEDOT:PSS forms micelles around a PEDOT crystalline core, which has been shown by scattering techniques to grow when PEDOT:PSS is subject to solvent treatments.35 Once spin-coated, AFM images reveal bright and dark areas, which are commonly associated with PEDOT and PSS rich regions.33,48,50-52,68 In the case of the mixed materials, PSS and the fluorination agents compete to stabilize the PEDOT polymeric chains. The morphology of the images is noticeably influenced by the presence of the fluorination agent leading to larger bright domains. This feature is more visible with the polymeric than with the surfactant fluorination (Fig. 2b1 vs c1). However, FOS based thin films reveals a finer substructure of smaller grains aggregated to form the bright regions. These variations would be consistent with conformational or aggregation changes of the polymer chains when co-solvents or fluorination agents are added. The finer substructure observed with FOS could results from its surfactant nature leading to a more effective distribution than with the polymer PFI. Also noticeable is the very small variation of the RMS and height between peaks and valley, which are only slightly larger in the fluorinated materials compared to pristine PEDOT-PSS thin films (Table 1, Fig. S1). These variations in the RMS and Peak to Peak values are not large enough to have any impact on device performances, which are presented in the following section. Surface potential maps were obtained by Kelvin probe force (KPFM) microscopy and are displayed in the 2nd row of Fig. 2. Surface potential maps show three distinct average values, i.e. -220 mV, -720 mV and -250 mV for the pristine, PFI and FOS mixed PEDOT:PSS, respectively. Local variations of the surface
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Fig. 2. PEDOT:PSS layers: mixed (a), fluorinated with PFI (b) and FOS (c) topographic images obtained by atomic force microscopy (1), surface potential maps obtained by Kelvin probe force microscopy (2), surface potential on the topography for the PFI based PEDOT:PSS sample (3).
potential are relatively small and of the order of 20 mV. As illustrated in Fig. 2b3 for the PFI based PEDOT:PSS sample, when the surface potential is overlaid onto the topography, the surface potential appears to be independent of the surface profile; Fig. S2 confirms this characteristic for all the samples. From KPFM measurements, the local work function was deduced and its average values are presented in Table 1. Both fluorinated PEDOT:PSS films present a deeper work function than the pristine films. This is consistent with fluorinated materials located at the polymer-air interface due to their higher ionization potentials compared to alkyl chains.39,86 For comparison purposes, the work functions of the thin films were also measured with a macroscopi c Kelvin probe (Table 1).
Table 1 PEDOT:PSS based thin film morphology as characterized by AFM in phase mode in Fig. 2: root mean square (RMS, 0.1 nm) and peak-to-valley height (h PtV), nanoscopic and macroscopic work functions (Wf-KPFM, Wf-mKP 0.02 eV). HTL Fluorination RMS (nm) hPtV (nm) W f-KPFM (eV) W f-mKP (eV)
x
PFI
FOS
1.5 11 2 4.70 0.02 5.20
2.5 17 1 5.40 0.03 5.72
2.0 14 2 4.90 0.03 5.56
DOI: 10.1039/C6TA00677A Table 2 PEDOT:PSS based thin film conductivity (, mS/cm), just after annealing and followed by spin-coating pure solvent on-top before the electrode evaporation. HTL Fluorination
x
PFI
FOS
-chlorobenzene 1,2-dichlorobenzene
0.70 0.05 0.69 0.04 0.65 0.07
0.55 0.02 0.95 0.09 0.83 0.02
0.49 0.03 0.55 0.02 0.53 0.02
The values differ slightly from those deduced from the KPFM measurements, i.e. between 5 and 12 %, however, they follow the same trend. The Wf-mKP values of the fluorinated samples are much deeper than the value obtained with the pristine PEDOT:PSS film, and PFI-Wf-mKP is also deeper than FOS-Wf-mKP. Consistent observations were made with the ionization potential deduced from UPS measurements (Fig. S3). The main mechanisms of PEDOT:PSS conductivity are associated with the acid groups protonating the PEDOT and with charge hopping among the PEDOT polymer chains. The first column of Table 2 shows the conductivity of the spincoated thin films, just after annealing as described in the experimental section. HTL is consistent with the literature values, however, it is noticeable that the PFI- and FOS- based samples present a slight decreased conductivity. The weak variation of PEDOT:PSS conductivity when fluorinated with PFI is consistent with the literature.87 Alongside of the already mentioned slight morphology alterations of the thin films observed by AFM (Fig. 2-row 1), both PFI and FOS should at least preserve the PEDOT protonation otherwise induced by PSS. The fluorinated nature of the fluorination agents is likely to favour the formation of larger and better connected PEDOT domains. However, the insulating PSS is then partially replaced by another insulating molecule, which overall increases the ratio of insulating to conductive materials in the HTL. Spin-coated PEDOT:PSS films are known to present an upper PSS rich phase, 39,40,88 hence the apparent decrease of the conductivity observed for the two fluorinated samples was assumed to result from a larger fraction of insulating material sitting at the top of the upper interface of both fluorinated PEDOT:PSS thin films. To assess this hypothesis, as well as the becoming of this insulating interfacial layer when an organic semiconductor thin film is spin-coated on the PEDOT:PSS, conductivity measurements were also completed after spin-coating a pure organic solvent on top of the annealed hole transport layer. Pure chlorobenzene and di-chlorobenzene were chosen to mimic the effect of spin-coating P3HT:PC60BM and PTB7:PC70BM, respectively. The second and third rows of Table 2 show that the conductivity of pristine PEDOT:PSS films is unaltered by the organic solvent spin-coating process. FOS based thin films display a slight increase, ~ 10 %, of the films conductivity. PFI based PEDOT:PSS thin films show a more noticeable conductivity increase, i.e. ~ 50 % and ~ 70 % for dichlorobenzene and chlorobenzene, respectively. The apparent higher values of the fluorinated films are likely due to a better connectivity between the electrodes and the underlying PEDOT.
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a1
a2
b1
b2
Fig. 3. Schematic of the devices (1) and the flat band energy diagram (2) associated with P3HT:PC60BM (a) and PTB7:PC70BM (b).
The relative effect variation induced by spin-coating pure solvents on PEDOT:PSS fluorinated by small and large molecules could result from a different balance between the shear forces and the hydrophilic-fluorophilic-lipophilic character of each compound. Whilst, the observed enhancement is relevant, no dramatic change was observed and the obtained values remained within the range of conductivi ty usually associated with pristine PEDOS:PSS layers. Photovoltaic Devices and Time Resolved Spectroscopy The fluorinated-PEDOT:PSS thin films were used as hole transport layers in OPVs. The devices were based on two types of active semiconductor materials, P3HT:PC60BM (Fig. 1b) and PTB7:PC70BM (Fig. 1c). Fig. 3 presents both the device structures and their associated energy diagrams. Aluminium was evaporated on top of the P3HT:PC60BM active layer and the devices were characterized directly (Fig. 3b). In contrast, PTB7:PC70BM electron conduction layer relies on evaporation of both calcium and aluminium (Fig. 3a).81,85 Consequently, these devices were encapsulated to address the high reactivity of the calcium layer under ambient conditions . The normalized absorbance spectra characteristic of the semiconductor materials used in this investigation are presented in Fig. 4a/b1. PTB7 covers a wider spectral range than P3HT, and the relatively large fraction of fullerene derivatives can be noticed in the high energy part of the absorbance of the blend. Fig. 4a2 and b2 present the current density as a function of the applied voltage for both P3HT:PC60BM and PTB7:PC70BM devices, respectively. The characteristics of the devices are summarized in Table 3. Unfluorinated PEDOT:PSS based devices display consistent efficiencies and characteristics as those reported in the literature. Incidentally, one could notice that chlorobenzene led to higher efficiency PTB7 solar cells when compared with OPVs prepared wih dichlorobenzene as a solvent. For both type of solar cell and both fluorination, the PCE of the device
DOI: 10.1039/C6TA00677A 5
J (mA/cm )
2
P3HT
0.6 0.4
3
10
a3
x PFI FOS
PC60BM
0.8
ODnorm
a2
1
0 -5
0.2
ODnorm
0.8
J (mA/cm2)
PC70BM PTB7
0.6 0.4 0.2
b3
x PFI FOS
400 500 600 700 Wavelength (nm)
800
-1
10
-3
10
-5
10
0
10
1
-5 -10
10
x PFI FOS
-1
10
-3
10
-5
10
-15
0.0 300
x PFI FOS
-7
-10 5
b2
PTB7:PC70BM
|J (mA/cm2)|
0.0
b1 1.0
10
2
P3HT:PC60BM
|J (mA/cm )|
a1 1.0
-20 -1.0
-7
-0.5
0.0 0.5 Voltage (V)
1.0
10
-1.0
-0.5
0.0 0.5 Voltage (V)
1.0
Fig. 4. P3HT:PC60 BM (a) and PTB7:PC70 BM (b). Normalized absorbance spectra of films (1) made of the conjugated polymer (blue line and square), electron acceptor (red line and circles) and a blend of the two materials (black line). Solar cells prepared from these blends (2), as described in the experimental section, and Dark J-V curves (3) with the PEDOT:PSS layer being pristine (■), mixed with PFI (▲) and with FOS (●).
Table 3 Best (mean and standard deviation) device performances*: power conversion efficiency (PCE), open circuit voltage (VOC), short circuit current density (J SC), and fill factor (FF), external quantum efficiencies (EQE*) at 450 nm and 0 V bias.
PTB7:PC70BM P3HT:PC60BM
HTL Fluorination PCE (%) VOC (V) FF (%) J SC (mA/cm2) EQE* (%) PCE (%) VOC (V) FF (%) J SC (mA/cm2) EQE* (%)
x
PFI
FOS
2.53 (2.49±0.07) 0.59 (0.59±0.01) 48.5 (49.7±2.6)
2.96 (2.91±0.05) 0.62 (0.64±0.01) 54.0 (52.9±0.7)
2.91 (2.85±0.05) 0.63 (0.64±0.02) 53.3 (53.0±0.9)
8.84 (8.48±0.32) 8.86 (8.60±0.17) 8.63 (8.47±0.12) 55.4 57.7 59.8 7.18 (6.77±0.26) 7.49 (7.30±0.14) 8.26 (8.12±0.15) 0.67 (0.68±0.01) 0.68 (0.68±0.01) 0.70 (0.70±0.01) 67.2 (63.5±2.3) 65.0 (64.3±1.3) 69.3 (68.8±1.7) 15.94 (15.76±0.36) 16.89 (16.71±0.23) 16.94 (16.78±0.17) 72.5 77.1 79.4
* The OPV area was 0.08 cm2 and the statistic was established with at least 7 devices.
shown to increase when compared with the performance obtained with the pristine hole conduction layer. The highest PCE is obtained with PEDOT:PSS:FOS for PTB7:PC70BM, while there is only a marginal difference between FOS and PFI based OPVs in the case of P3HT:PC60BM. For the two types of device, the fluorination of PEDOT:PSS translates into an increase of VOC, with a maximum variation of 45 mV and 33 mV for P3HT and PTB7 based devices, respectively. This is consistent with the work function alteration induced by the HTL fluorination (Table 1) and suggests a lower amount of recombination in the device. This is thought to occur by an alteration of the band bending and the internal electric field at the HTL/heterojunction interface which prevent electrons from recombining at the hole-extracting electrode.89-91 The fill factor is more sensitive and the least understood but depends on the charge accumulation at the electrodes, i.e. balanced charge extraction, and molecular charge recombination, which
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depending upon the system can be geminate or nongeminate.92-95 P3HT:PC60BM fill factors increase with the fluorination of the HTL, with a maximum difference of ~ 5.5 %, i.e. ~11.3 % relative variation. This is consistent with the VOC variation and could be associated with a decrease of the resistive losses. The average FF values of PTB7:PC70BM devices displays a similar trend, however the very small FF difference relative variation compared to pristine HTL device and the relatively large standard deviation prevents us to draw any reliable conclusion from the FF variation of the low band-gap BHJ devices. The JSC data display a relatively low variation, ~ 0.2 mA/cm2, when the fluorination agents are used to fabricate the P3HT:PC60BM devices. However, a larger increase, ~ 0.9 mA/cm2, occurs in the case of PTB7:PC70BM, which suggests that in this later case a larger amount of charges being photogenerated upon fluorination of the HTL. Incidentally, we note that the devices fabricated with either of the additive in the HTL systematically present lower standard variations of the PCE, FF and JSC. This is observed for both polymer:PCBM blends, as well as for both fluorinated additives, and as a consequence is attributed to the fluorinated nature of the additives. The dark J-V curves are presented in Fig. 4a3 and b3. The classical “diode curve” shape is observed for the unfluorinated PEDOT:PSS based devices.85 From 1.0 to 0.5 V the curves display the usual bell shape; at 0.5 V bias, a change of slope is noticeable with a much slower decay, which changes -2 2 plateau around 10 mA/cm for very small positive external bias. In reversed bias, the current density increases slowly and continuously to reach a maximum absolute value, which is slightly larger than of 10-2 mA/cm2. Whilst the fluorinated PEDOT:PSS devices present exactly the same bell shape between 1.0 and 0.5 V, the slope of the continuous decay is unaltered below 0.5 V, with the leakage current density -4 2 reaching a minimum absolute value close to 10 mA/cm for
DOI: 10.1039/C6TA00677A 100
a1
a2
60
20
(x10-3)
Norm. T/T
1.0 0.8 0.6
350
450 550 650 750 Wavelength (nm)
b1
350
450 550 650 750 Wavelength (nm)
b2
P3HT: PC60BM
Fluorination
40
0
Table 4 Transient absorption fit parameters and calculated errors for both P3HT:PC60 BM (probe = 640 nm) and PTB7:PC70BM (probe = 810 nm) deposited on top of PEDOT:PSS based thin films and photoexcited at room temperature with a pump = 400 nm pulsed beam.
PTB7: PC70BM
EQE (%)
80
x PFI FOS
A1 CR1 (ps) A2
x
PFI
FOS
0.59 0.01 0.63 0.05 0.41 0.01
0.64 0.01 0.75 0.05 0.36 0.01
0.64 0.01 0.84 0.05 0.36 0.01
CR2 (ps)
74 3
33 3
29 2
A1 CR1 (ps) A2 CR2 (ps)
0.47 0.02 16.9 1.4 0.53 0.02 211 9
0.33 0.03 36.6 4.2 0.67 0.05 352 22
0.42 0.02 41.3 2.5 0.58 0.03 333.4 17
0.4
charge photogeneration. The relative contribution of these mechanisms differs whether P3HT:PC60BM or PTB7:PC70BM 0.0 devices are considered. The first mechanism could be 100 200 300 associated with a faster sweep of the charges out of the blend, 0 20 40 60 80 100 0 Time Delay (ps) Time Delay (ps) which consequently would not allow their recombination. Fig. 5. Device external quantum efficiencies (a) and transient absorption Using deeper work function can lead to a more favourable measurements (b) for P3HT:PC60 BM (1) and PTB7: PC70BM (2) thin films onband bending at the contact interface. This alters the energy top of PEDOT:PSS HTL pristine (■), mixed with PFI (▲) and with FOS (●). alignment within the devices and, as described by the Integer Charge Transfer model, creates a strong surface dipole, which small and positive external bias. In reversed bias, the current assists the charge extraction from the interface before they density increases slowly and continuously to reach maximum can recombine.89-91 This interpretation is supported by the absolute values under -1 V bias of the order of 10-3 mA/cm2. transient absorption measurements presented in Fig. 5b. Upon The P3HT:PC60BM dark J-V curves of both PFI and FOS mixed photoexcitation, ultrafast charge separations occur and form a with PEDOT:PSS overlap almost perfectly. PTB7:PC70BM charge transfer (CT) state, which leads to either geminate devices present the same trend, even though for reversed bias recombination or the separation of charges into the donorthose with PFI based HTL display slightly larger current density acceptor bi-continuous network. These free carriers can -2 absolute values than with FOS, which is slightly larger than 10 eventually recombine as non-geminate pairs. For the two 2 mA/cm . In other systems, such leakage current decreases types of blends herein investigated, we note that a) charge were associated with higher PCBM segregation at the anodes separation occurs over too short of a time scale to be accessed and consequently enhanced contact selectivity. Fig. 5a1 and a2 with our setup, however, b) the charge recombination (CR) present the external quantum efficiencies (EQE) of the devices. kinetics appear to be clearly altered by the fluorination of the The EQEs of pristine PEDOT:PSS devices are consistent with the HTL. The P3HT:PC60BM (Fig. 5b1) and PTB7: PC70BM (Fig. 5b2) literature for each type of semiconductors, 21,96 with samples were probed at 640 and 810 nm, respectively. The use PTB7:PC70BM covering a much broader spectral range than of these wavelengths implies that in the first blend, it is the P3HT:PC60BM, as already mentioned when discussing their P3HT cation which are probed,97,98 whereas the kinetics of the respective absorption spectra presented in Fig. 4. The EQE of PC70BM anions are monitored in the second blend.99,100 The CR the latter blend peaks around 500 nm and displays a kink at kinetics were fitted with at least two characteristic times, high energy. PTB7 devices fabricated from dichlorobenzene which values and weights are summarised in Table 4. have a rather flat EQE response across 350-750 nm spectral P3HT:PC60BM thin films present characteristic times of range. Both P3HT:PC60BM and PTB7:PC70BM show slight disappearance of the P3HT cationic species in the picosecond, increase of the EQE with PEDOT:PSS fluorination. For CR1, and tens of picosecond, CR2, ranges. CR1 could contain a illustration purposes, Table 3 presents the 450 nm EQE values contribution from geminate recombination, which will likely be for each device, these EQE measurements are independent of less affected by PEDOT:PSS fluorination. In any case, the PCE, Voc and FF but do relate to Jsc. In the case of experimental and fit uncertainties prevent discussing the CR1 P3HT:PC60BM, the same small variation and trend is observed 18 and 33 % variations with PEDOT:PSS fluorination. More for both the integral of EQE and Jsc data, with up to 8 % interestingly, by fluorination the PEDOT:PSS layer, the longer increase with PEDOT:PSS fluorination. For the PTB7 solar cells, characteristic time CR2 is drastically decreased by more 55 and fluorination induces a consistent 6 to 9 % relative variation of 60 % for PFI and FOS agents, respectively. CR2 is understood as both integrated EQE and Jsc, leading also to higher values than containing both non-geminate recombination and hole in P3HT based OPVs. transfer from the P3HT cations to the PEDOT:PSS layer. An Consequently, two enhancement mechanisms appear to be accelerated sweep of the holes from the blends is consistent involved upon HTL fluorination, i) an increased charge with the larger work functions reported in Table 1 and with extraction preventing their accumula tion, and ii) an enhanced dipole formation associated to charge transfers at the 0.2
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DOI: 10.1039/C6TA00677A interface of P3HT and fluorinated PEDOT:PSS. The negatively charged fluorinated PEDOT:PSS then accelerates the extraction of the photo-induced holes, leading to shorter CR2 values, and this is consistent with both the VOC increase and the enhanced PCE of the devices fabricated with fluorinated HTL (Table 3). In a similar manner, PTB7:PC70BM systems display a biexponential kinetic, with characteristic times in the 10s and 100s of picosecond range, i.e. slower than the P3HT:PC60BM blends. A strong effect of the HTL fluorination on the charge transfer state kinetics is clear from Fig. 5 and Table 4. The PC70BM anion disappearance characteristic times are increased by more than 117 and 145 % for CR1, as well as 67 and 59 % for CR2, upon PFI and FOS agents, respectively. This substantial effect is again consistent with observations made by monitoring the P3HT cation dynamics. Probing the PC70BM acceptors reveals that when more holes are swept out from the blends to the HTL, the recombination probability of the PC70BM anions is decreased due to a lower density of holes in the blends. This results in fewer charge recombinations at the interface of the hole-extracting electrode, and is consistent with the increased VOC when the HTL is fluorinated as presented in Table 3. This more efficient charge extraction can be associated with the lower deprotonation energy of the sulfonate resulting from a higher fluorination of the PEDOT and with a higher built-in potential in the device. This is equivalently described through the higher dipole moments of the deprotonated fluorinated sulfonic acid materials compared with alkyl chains derivatives. 86 -88, 10 1,1 02 However, the spectral features of the EQE spectra require further careful considerations as the HTL fluorination does not result in a homogeneous increase of the EQE reference spectra. On the contrary, the EQE ratios of the fluorinated and unfluorinated devices vary with the wavelength (Fig. 5a). P3HT:PC60BM OPVs display a maximum increase of 4 % of the EQE narrowly located between 400 and 500 nm. The PTB7:PC70BM devices present an EQE increase with fluorination of up to 12 % spreading between 300 and 600 nm, with stronger variations near 400 and 500 nm. This spectral variation cannot be explained by a change of transmission of the fluorinated PEDOT:PSS layer, which shows a negligible, i.e. < 1 %, variation around its maximum (Fig. S7). In addition, the two blends do not show the same EQE spectral variation with PEDOT:PSS fluorination. Several hypotheses can then be made including a) the fluorination altering the HTL surface energy, which in return impact on a1) the material packing and distribution within the blends, and a2) the thickness of the device layers, b) HTL:blend interface charge transfers altering the optical constants of the materials involved. Any of these hypotheses could be responsible of the EQE spectral variations observed in Fig. 5a as a1) would result in a change of absorption coefficients, a2) and b) would impact the electric field distribution within the devices. Interfacial Effects and Modelling The variation of the surface energy with the PEDOT:PSS fluori-
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nation was assessed by contact angle measurements, which are presented in Table S3 and Fig. S8. The fluorinated agents lead to a lower surface energy when compared with unfluorinated PEDOT:PSS. This could then provide a PCBM 69depleted region at the interface of the hole extraction layer, 71 which could contribute at preventing charge recombination at the surface of the hole-extracting electrode, and would be consistent with the increased VOC. To explain the EQE wavelength dependence, a variation of surface energy and a local change of P3HT and PTB7 concentrations relative to the PCBM electron acceptor moieties would need to be associated with an alteration of the crystalline order of the P3HT or PTB7 at least near the HTL interface. The impact of surface energy on organic semiconductor crystallinity has been documented in the context of field effect transistors, 10,49,103-105 even though most studies have focused on pentacene. In the present OPV context, an alteration of the HTL surface energy could result from a change of phase separation and blend morphology analogous to the effect obtained when the solvent is slowly evaporated from the photoactive layer or when an additive is used to control the morphology of bulk heterojunction. 15,2129,106,107 These have been suggested in P3HT:PCBM BHJ deposited on PEDOT:PSS with surface energy ranging from 50 to 70 mN/m, even though we note that ellipsometry measurements were associated with device properties and not 71 with any in-depth structural studies. P3HT:PC60BM presenting the weakest EQE spectral variation, we first focused on PTB7:PC70BM to compare the devices absorbance with and without fluorination of the HTL. Compared to the PEDOT:PSS based devices, the fluorinated OPVs do display an increased absorbance, which maximum is of the order of 6 % and which within the experimental precision would be consistent with the EQE spectral variation (Fig. S5b and S6). In addition, correlations between EQE and crystalline structure have been pointed at in systems including P3HT, 21,49,108 and PBTTPD.25 However, we note that P3HT:PC60BM does not present any obvious change of absorption, not even around 620 nm, which is associated to this polymer crystallization. Consequently and despite the contact angle data, the absorption results do not unambiguously support the hypothesis that the morphology, i.e. the crystallization, of the active layer is altered by the fluorination of the PEDOT:PSS layer. To resolve this issue, we completed a careful GIWAXS investigation, which is reported in section 4.4 of the SI. The insertion of the PCBM acceptor moieties in the conjugated polymers was shown to induce slight structural variations such as thickness, correlation length and alignment of lamellae; however, these features were shown for both P3HT:PC60BM and PTB7:PC70BM not to be affected by the fluorination of the HTL. This consequently excludes hypothesis a1). The thickness of each layer was measured both by dektak and spectroscopic ellipsometry. The techniques gave comparable thicknesses for the blends, while the deviation was smaller than 10 % for the ellipsometry and within the experimental uncertainty for the dektak. Then, there is no strong thickness alteration, which could explain the EQE spectral variation, and this rules out the hypothesis a2). It is also noticed that the abs-
DOI: 10.1039/C6TA00677A 1.48
a
nPEDOT:PSS:x
0.05
1.44
0.04 0.03
1.40
X PFI FOS
1.36 400 500 600 700 800 Wavelength (nm)
0.02
kPEDOT:PSS:x
0.06
0.01 0.00
b
Fig. 6. PEDOT:PSS refractive indices and extinction coefficients: pristine (■), mixed with PFI (▲) and with FOS (●).
𝜆
Fig. 7 presents the distribution inside the devices of the modulus squared of the optical electric field, |E|2, which was calculated as a function of the incident wavelength. Fig. 7a and b are associated with P3HT:PC60BM and PTB7:PC70BM, respectively. The dimensionless parameter z/L is used for convenience purposes to materialise the different material layers forming the devices. As expected, |E|2 is not monotonous with both incident wavelength and position within the devices. Noticea bl y, the |E|2 maximum value is higher in the
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a1
a2
2.0 1.5 1.0
~
Q (x10-3)
x PFI FOS
0.8
0.5 shifted z/L
0.0
0.6
(2)
0.4
b1
b2
2.0 1.5
0.8
1.0
~
𝜆
with j, the layer under consideration, the speed of light, c, the permittivity of the free space, 0, the complex refractive index, 𝑛̃ = 𝑛 + 𝑖𝑘, the real part of refractive index, n, the extinction coefficient, k, the attenuation or absorption coefficient, , the electric field, E, and the wavelength, , of the incident light. ̃ as While preserving, for sake of simplicity, we define 𝑄 𝑄𝑗 (𝑧) 𝑘𝑗 𝑛𝑗 2 ̃ 𝑄 𝑗(𝑧) = = |𝐸𝑗 (𝑧) | (3) 2𝜋𝑐𝜀0
1.0
Q (x10-3)
4𝜋𝑘𝑗
case of PTB7:PC70BM than with P3HT:PC60BM. Similar patterns were obtained with fluorinated PEDOT:PSS as shown in Fig. S19. The vertical dashed line in Fig. 7 is a cross-section of |E|2 at 500 nm illumination. 2 A similar oscillating behaviour of |E| is observed in Fig. 8a1 and b1 for the three HTLs. Systematically, |E|2 tails off in the metal anode and its maximum occurs in the PEDOT:PSS and
z/L
𝛼𝑗 =
Fig. 7. Calculated distribution of the modulus squared of the optical electric field, |E|2 , as a function of the incident wavelength inside a photovoltaic device made of P3HT:PC60BM (a) and PTB7:PC70BM (b) spincoated on top of pristine PEDOT:PSS. The horizontal dotted lines stand for the material distribution as labelled on the right hand-side.
z/L
ence of significant thickness variation excludes any mobility change, which otherwise could have altered the PCE.109,110 To explore the last hypothesis, it is essential to consider the role of the optical constants and electric field within the devices. As described with more details in the section SI-4.5.2, fluorinated PEDOT:PSS refractive index and extinction coefficient were shown to present a red shift and an amplitude decrease of their main features when compared with pristine PEDOT:PSS thin films (Fig. 6). They are related to the excess of sulfonic acid groups brought in by the PFI and FOS agents along with the electro-negativity of the fluorinated moieties. This is equivalent to negative Burstein shifts induced on the apparent band gap when extra charges are added, for instance through doping,111-114 and consistent with the variation of n and k observed for various conjugated molecules when in 115-117 oxidized or reduced states. Noticeably, this trend was preserved when the PEDOT:PSS based thin films were covered by the conjugated polymer:PCBM blends (Fig. S17). Considering that the number of excited state at the position, z, within a device layer depends on the energy which is locally absorbed, the device efficiency is then directly proportional to the time average of the energy dissipated per second, Q, which it-self is associated with the interferences between incident and reflected light. Under normal incidence, it can be 118,119 expressed as: 2 1 𝑄𝑗 (𝑧) = 2 𝑐𝜀0 𝛼𝑗 𝑛𝑗 |𝐸𝑗 (𝑧) | (1)
0.0
0.6
0.4 0.0
0.5
1.0
2.0
IEI
2
3.0 0.0
0.5
shifted z/L
1.0 1.5 ~ -3 -1 Q (x10 nm )
2.0
Fig. 8. Cross-section at 500 nm illumination of |E|2 (1) across the photovoltaic devices made of P3HT:PC60BM (a) and PTB7:PC70 BM (b) spincoated on top of pristine PEDOT:PSS. The horizontal dashed lines correspond to the interfaces between each layer. Calculated 𝑄̃ values (2). Insert: 𝑄̃ values in the polymer:blend layers with z/L shifted to overlap the blend:anode interface. PEDOT:PSS HTL layer: pristine (■), mixed with PFI (▲) and with FOS (●).
DOI: 10.1039/C6TA00677A Table 5 Components of the complex refractive index of the polymer:PCBM blend at 500 nm, with n, the real part, and k, the extinction coefficient, used to calculate 𝑄̃ 𝑖𝑛𝑡. , the integral of 𝑄̃ across the device photoactive layer.
PTB7: P3HT: PC70BM PC60BM
HTL Fluorination n k 𝑄̃ 𝑖𝑛𝑡. (10-4) n k 𝑄̃ 𝑖𝑛𝑡. (10-4)
x
PFI
FOS
1.796 0.258 1.54 1.573 0.196 0.99
1.819 0.248 1.65 1.665 0.232 1.17
1.817 0.255 1.65 1.596 0.248 1.44
̃ was calculated by blend based layers. As described in eq. 3, 𝑄 taking into account n and k values of each layer, and is shown ̃ is equal in Fig. 8-2. Regardless of the fluorination and blend, 𝑄 to zero and very small in the metal anode and HTL, respectively. In Fig. 8a2, we notice that with P3HT:PC60BM, the ̃ is marginally larger with the fluorinated HTLs maximum of 𝑄 than with the pristine PEDOT:PSS. However, in the case of ̃ PTB7:PC70BM this trend is much more pronounced as the 𝑄 values of the fluorinated HTLs remain larger than those of the pristine HTL all across the photoactive layers. The inserts of Fig. 8-2 focus on these photoactive blend layers and matches the z/L value corresponding to the blend to HTL interfaces. The ̃ between the devices was quantified by overall variation of 𝑄 ̃ integrating the 𝑄 values as a function of z/L. The areas under ̃ curves are summarized in Table 5. When compared with the 𝑄 ̃ is of the order of 7 % in the case pristine HTL, the increase of 𝑄 of P3HT:PC60BM. PTB7:PC70BM based devices show an increase ̃ of about 18 and 35 % for PFI and FOS based HTL, of 𝑄 respectively. We confirmed with PTB7:PC70BM that such an increase could not be induced by a change of the PEDOT:PSS layer thickness (Fig. S20). We also note that comparing Table 3 ̃ and JSC are consistent with one and 5, the variations of 𝑄 another, weak for P3HT:PC60BM but pronounced for PTB7:PC70 B M. Whilst the electro-optical analysis remains partially subject to the models and fits used to analyse the spectroscopic ellipsometry data, it does highlights the potential of fluorination agent to alter the optical constants of OPV layers. It is important to keep in mind that these charge transfers are independent of any photo-excitation, and instead induced by the sulfonic acids groups pending at the apex of the fluorination agent in the HTL. The variations of the HTL work functions and refractive index differ by the fact that the former applies to photo-induced charges by sweeping them out of the photoactive blend, while the latter is spectrally resolved, as illustrated by the transient absorption, the EQE curves and the electro-optical modelling. The latter contribution has usually been neither investigated, nor taken advantage off.
probe measurements have revealed a large impact on the substrate work function, which remained very homogeneous even at the nanometer scale. The conductivity of the hole transport layers were shown to decrease slightly when the fluorinated sulfonic acid surfactant and the sulfonic ionomer were used as fluorination agent. The film conductivity was altered further when fresh solvent was spin-coated on top of the HTL. Two different photoactive polymer:PCBM blends were used to fabricate solar cells on top of un-fluorinated and fluorinated PEDOT:PSS thin films. As for both small surfactants and large polymeric macromolecules, the device efficiencies were increased, any specific influence of the fluorination agent molecular structure and conformation could be excluded. GIWAXS data showed that the interfacial surface energy and wetting properties had no effect on the morphology, crystallinity and donor-acceptor distribution of the photoactive polymer:PCBM blends. Regardless of the conjugated polymer: PCBM blend, when using the fluorinated additives, the device characteristics were shown to be systematically more reproducible from one device to another. The increased power conversion efficiency of the devices based on fluorinated PEDOT:PSS layer was shown to be solely related to the electronic and optical properties of the fluorinated hole transport layer, through an alteration of not only its work function but also its refractive index. Two distinct mechanisms lead to i) an increased charge extraction (VOC, CR) and ii) an ̃), which relative enhanced charge photogene-ration (JSC, n-k, 𝑄 contributions vary with the conjugated polymer:PCBM blend. These cannot be separated as they occur simultaneously and share the same origin, i.e. charge transfers induced by the fluorination agents. Overall, the present results shed lights onto the optoelectronic effects on BHJ OPVs of using fluorinated agent in HTL preparations and are likely to be applicable to electrode interlayers. They also suggest that whilst it can be of interest to alter the electrode with a dipole layer to aim at increasing the interfacial charge extraction, fluorination can also be used more broadly to tune the optical constants and electric field distribution within the devices. Herein, such a strategy is shown to lead to a power conversion efficiency increase of up to 15 % along with a noticeable change of the device external quantum efficiencies in the UV-visible range. Impacts will likely vary in strength and spectral range from one fluorination agent, polymeric electrode, or photoactive blend, to another, so that this work paves the way toward a broad range of materials to be systematically explored as we demonstrated that this fluorination strategy is an important and widely applicable parameter to control increase OPV characteristics.
Supplementary Information: Conclusion The fluorination of the hole transport layer by fluorinated molecules have been characterized by near-field microscopy showing that the morphology of the thin films was not drastically altered by the fluorination agent. In contrast, Kelvin
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Experimental Section and further details about phase and surface potential near-field measurements, transmittance, wetting, GIWAXS, ellipsometry, transient absorption and calculations of internal electric field and absorption .
DOI: 10.1039/C6TA00677A
Acknowledgements PA thanks the Canon Foundation in Europe for supporting his visits to the RIKEN through a personal Fellowship, and the OSC to access the facilities. KJL, HK, JHK, ESK, JWW, DWK and PA were supported by funding of the Ministry of Science, ICT & Future Planning, Korea (201000453, 2015001948, 2014M3A6B3063706). Part of this work has been carried out in the framework of CNRS International Associated Laboratory "Functional nanostructures: morphology, nanoelectronics and ultrafast optics" (LIA NANOFUNC), France. TA would like to acknowledge funding from the Japanese Society for the Promotion of Science via a JSPS KAKENHI grant (no. 22350084). JCR and CA would like to acknowledge financial support from the Regional Innovation Strategy Support Program -Kumamoto Area on Organic electronics collaboration - Ministry of Education, Culture, Sports, Science and Technology (MEXT). The authors thank Pohang Accelerator Laboratory (PAL) in South Korea for giving us the opportunity to perform the GIWAXS measurements in the frame of the proposal number "2014-1st-9A-015". The authors are grateful to MEST and POSTECH for supporting these experiments, to Drs. Tae Joo Shin and Hyungju Ahn for adjustments and assistance, as well as to other staff members from 9A U-SAXS beamline for further assistance.
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DOI: 10.1039/ C6TA00677A
Supporting Information Enhanced Organic Solar Cells Efficiency through Electronic and Electro-optic Effects Resulting from Charge Transfers in Polymer Hole Transport Blends 1.Experimental Section...................................................... 1 1.1. M aterials ................................................................... 1 1.2. Device Fabrication ................................................... 1 1.3. Characterisation Techniques .................................... 2 1.3.1.Topography and Kelvin Probe M easurements ......... 2 1.3.2.M acroscopic Kelvin Probe M easurements ............... 2 1.3.3.Conductivity M easurements ..................................... 2 1.3.4.Transient Absorption Spectroscopy.......................... 2 1.3.5.Optical Transmission ................................................ 2 1.3.6.Wetting Characterizations ........................................ 2 1.3.7.GIWAXS .................................................................. 2 1.3.8.Spectroscopic Ellipsometry ...................................... 3 1.3.9.Electric Field and Absorption Calculations .............. 3 2.Interfacial and Electronic Characterizations................... 3 2.1. Surface M orphology................................................. 3 2.2. M orphology-Wf Correlations.................................... 3 2.3. UV Photoemission Spectroscopy ............................. 4 3.Solar Cell Analysis ......................................................... 4 3.1. Jsc and EQE Consistency .......................................... 4 3.2. Dark J-V Characterizations ...................................... 4 3.3. Series and Shunt Resistances ................................... 4 4.EQE Spectral Variations with HTL Fluorination ........... 5 4.1. Active Layer Optical Density Variations ................. 5 4.2. HTL Optical Transmission ....................................... 5 4.3. Wetting Properties .................................................... 6 Polymer:PCBM Blend.............................................. 6 4.4. Optical Absorbance .................................................. 6 4.5. GIWAXS .................................................................. 6 4.5.1.Neat and fluorinated PEDOT:PSS............................ 6 4.5.2.P3HT based Bilayers ................................................ 8 4.5.3.PTB7 based Bilayers................................................. 9 4.5.4.Overall Effect of the HTL Fluorination on P3HT and PTB7 based Blend M orphology.......................... 10 4.6. Dektak and Spectroscopic Ellipsometry................. 11 4.6.1.Thicknesses............................................................. 11 4.6.2.Optical Constants.................................................... 11 4.7. Electro-Optical M odeling....................................... 12 5.Contributions ................................................................ 13 6.References..................................................................... 13
1. Experimental S ection 1.1. Materials Hydrochloric acid (HCl, 37 %), acetone, isopropanol, dichlorobenzene, chlorobenzene were all HPLC grade and purchased from Sigma-Adrich. Perfluorinated ionomer resin solution (PFI, Nafion® DE 520, 5 wt. % in an alcohols and aqueous solution including 45 % of water, with a density of 0.924 g/mL at 25 °C) and perfluorooctane sulfonic acid solution (FOS, Mw = 500.13, D, ~ 40 % in water with 1.25 g/mL density) were also from Sigma-Adrich. 1, 8-diiodooctane (DIO) was from Fluka. Poly(3, 4-ethylenedioxythiophene: poly (styrenesulfonate) (PEDOT:PSS, 1:6 weight ratio, 1 g/L ; AI4083) was from Clevios, poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5 b’]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl}) (PTB7, Mw = 92 kDa, pdi = 2.6) was from 1-M aterial; regioregular poly(3hexylthiophene) (P3HT, Mw = 57 kDa, pdi = 2.9) was from Rieken M etals. The soluble fullerenes [6,6]-phenylC60/C70 butyric acid methyl ester (PC60BM / PC70BM ) were both purchased from Solenne (99 % purity).
Indium tin oxide (ITO) coated glass substrates (15 /square) were purchased from Xin Yan Technology Ltd. UV optical adhesive (Norland Optical Adhesive 68) and glass coverslips used for the encapsulation were purchased from Thorlabs and Fisher Scientific, respectively. 1.2. Device Fabrication An adhesive tape was used as a mask of the ITO-coated glass substrates, which were etched in HCl (37 %) for 20 min. The mask was removed and further cleaning of the substrates was completed by sonication in deionized water, acetone and isopropanol. The substrates were then dried with a nitrogen flow before being treated in an oxygen plasma asher for 5 min. Solutions with 6:1:30 weight ratio of PEDOT:PSS:fluorinated agent (FA) were prepared based on the weight concentration of the material indicated by the manufacturers (1 wt %, 5 wt% and 40 wt% for PEDOT: PSS, PFI and FOS respectively). The PEDOT:PSS, the PFI and FOS solutions were used as received. The volumes of the fluorinated agent solution (~ 278 µL and ~ 25 µL for PFI and FOS respectively) were added to 3 mL of PEDOT:PSS solution and left overnight under vigorous stirring. Before use, the solutions were sonicated, filtered (pore size 0.45 µm) and spin-coated at ~ 4 k.rpm to form 40 nm thick films on quartz substrates. With a a hotplate placed in a nitrogen-filled glovebox, the PEDOT:PSS:FA-coated ITO substrates were subsequently annealed at 120 °C for 15 min. The liphophilic organic semiconductors were dissolved by gentle stirring in a N 2-filled glove-box: PTB7: PC70BM , 5:7.5 mg in 0.5 mL, were dissolved in orthodichlorobenzene at 50 °C for a few hours. After the solution was cooled down to room-temperature, 3 % v/v DIO was added to the solution, which was left to stir for further 5 min before being spin-coated at 1 k.rpm to obtain a ~100 nm thick film; P3HT:PC60BM 5:5 mg were dissolved in 0.5 mL chlorobenzene and stirred overnight at 70 °C. The solution was spin-coated at 1 k.rpm on top of the annealed PEDOT:PSS based layer. The ~80nm thick P3HT:PC60BM based devices were then annealed under inert and dry atmosphere at 130 °C for 15 min. The top electrode was evaporated under a 2.10-6 mBar vacuum: P3HT:PC60BM was coated with a 200 nm thick aluminum layer, while PTB7:PC70BM was covered first by a 20 nm calcium layer on-top of which a 200 nm aluminum layer was evaporated. PTB7:PC70BM devices were immediately removed from the evaporator and encapsulated with a UV optical adhesive and a glass coverslip, whilst P3HT:PC60BM devices were not encapsulated. Devices were then removed from the glovebox, masked and characterized in air with a Keithley 2400 source-measure unit and a K.H. Steuernagel AM 1.5G solar simulator providing an illumination intensity of 100 mWcm2 verified with an NREL-calibrated monosilicon detector and a KG-5 filter. The solar cell area was 0.08 cm2 and their characteristics were extracted from the J-V curves with the power conversion efficiency (PCE or η) determined with the standard following equation V I FF (1) OC SC Pin where VOC is the open-circuit voltage, ISC the short-circuit
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current, FF the fill factor, and Pin the incident light power. The external quantum efficiency (EQE) was measured with an incident photon to charge carrier efficiency setup, made of an NPL-calibrated photodiode, a Keithley 6517A picoammeter and a TM c300 monochromator. 1.3. Characterisation Techniques 1.3.1. Topography and Kelvin Probe Measurements The surface work function of the sample was measured using a scanning probe microscopy (SPM ) system (XE100, Park Systems Co.) with a controlled glove box environment. All the measurements were performed under N2 atmosphere and ambient temperature. The samples were heated at 120 °C for 20 min to remove water adsorbates just prior to the SPM experiments. Conductive Pt-coated Si cantilevers (NT-M DT) were used for the measurements. The topography was obtained using AC mode with a resonance frequency of 280 kHz. KPFM images were simultaneously acquired by applying to the tip an AC modulation voltage of 2 V amplitude and 20 kHz frequency. For the estimation of the sample work function, the Pt coated tip (4.7 eV) was calibrated with the highly ordered pyrolytic graphite (HOPG) reference sample. To extract the work function (Wf-KPFM = Wsample ) from the data, first the contact potential difference (VCPD) was measured, and then the work function of the SPM tip (Wtip) was calibrated. Finally, Wf-KPFM of the sample was deduced from the equation (2), with q the electronic charge of an electron.1-3 ( Wtip Wsample ) VCPD (2) q 1.3.2. Macroscopic Kelvin Probe Measurements As described in section 1.2, the PEDOT:PSS-based films were prepared on ITO substrates by spin-coating. The samples were then annealed at 120 °C for 15 min on a hot plate in a N 2-filled glove-box. The work function of the film was then measured using a Kelvin probe (FAC-1, RIKEN KEIKI) with a measured area of 1 cm in diameter. The calibration was carried out by gold thin plates giving a 5.10 eV value. 1.3.3. Conductivity Measurements The pristine and fluorinated PEDOS:PSS films were prepared by spin-coating and thermal annealing, as described above but these steps were repeated three consecutive times to form thicker films required for reliable measurements. Once the thick films obtained, thermal annealing was completed for 20 min. The prepared PEDOT:PSS films were ~100 nm thick for the pristine film, ~250 nm for the PFI-based film, and ~85 nm for the FOS-based film, respectively. Gold was then deposited onto each sample through a shadow mask with a pattern of two contact pads. The contact length and width of the two contact pads were 8.5 mm and 3.0 mm, respectively. After cutting the part which is not to be measured in each sample (i.e., outside of the two contact pads), the sheet resistance and conductivity between the two contact pads were determined using an Agilent B1500A semiconductor device analyzer in a N 2-filled glove-box. For the post spin-coating treatments with pure organic solvents, chlorobenzene and di-chlorobenzene solvents were individually spin-coated at 1 k.rpm onto the pristine and fluorinated films prepared with the same procedure. The post spin-coating treatments were performed before the deposition of the gold contact pads, and the samples
were left at room temperature in a N 2-filled glove-box and overnight for the solvent to evaporate. 1.3.4. Transient Absorption S pectroscopy Femtosecond transient absorption measurements were carried out using pump and probe pulses generated by a Ti:Sapphire regenerative amplifier (Spitfire Pro XP, Spectra-Physics) working with an 800 nm output. A M ai Tai laser composed of a mode-locked Titanium-doped sapphire (Ti3+ :Al2O 3) laser (Tsunami) and of a diodepumped continuous wave Nd:YVO 4 laser (M illennia). The former was used as the seeding laser for the regenerative amplifier. The latter was used to pump the Tsunami. The regenerative amplifier was based on a Q-switched intracavity frequency doubled Nd:YLF laser operating at a repetition rate of 5 kHz and delivered 40 fs long pulse centered at 800 nm. The beam was split into two. The first led to the 400 nm pumping beam obtained by second harmonic generation using a beta barium borate (BBO) crystal after the Ti:Sapphire regenerative amplifier. The pump beam was attenuated to 1.0 mW using neutral density filters located in front of the sample. The second beam was directed to a 2 mm thickness sapphire crystal to generate a white light continuum in the visible range from which the 640 nm probe beam was selected. The time delay between the pump and probe beam was varied up to 500 ps using a delay line. The time intervals for the on-set and decay measurements were 100 fs and 5 ps in stepping motor, respectively. Pump light was modulated using a mechanical chopper at 220 Hz and the differential transmission T/T of the probe beam was determined as a function of the delay time with a detection setup including photodiode and lock-in. A filter cutting the light below 635 nm was used to reduce the potential impact of scattered light from the pump beam. The pump ( = 400 nm, P ~ 1 mW, ~ 0.5 mm) and probe ( = 640 nm, P ~ 0.01 mW, ~ 10 μm) beams hit the substrates on the polymer side and the differential transmission was calculated as T T ( ,t ) [Ton ( ,t ) Toff ( ,t )] Toff ( ,t ) (3) where Ton and Toff correspond to the sample transmission with the pump beam on and off, respectively. The charge separation (i.e. onset, rise) and recombination (i.e. decay, recovery) times were measured with 100 fs and 10 ps time interval, respectively. The values were extracted from the T/T curves plotted as a function of the delay time and fitted with eqs. (9) and (10), respectively. T T (t ) .[1 exp ( t CS)] (4)
T T (t ) .exp ( t CR )
(5)
1.3.5. Optical Transmission Transmission measurements were completed with a Perkin-Elmer Lambda 950 UV/Vis/NIR spectrometer with ITO coated substrates as references. 1.3.6. Wetting Characterizations The contact angle measurements were measured with a positioning stage, dosing needle and a Nikon D5200 with a macroscopic lens and analysed with Image J software as described by Stalder et al.4 1.3.7. GIWAXS Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) measurements were conducted at PLS-II 9A U-SAXS beamline of Pohang Accelerator Laboratory (PAL) in Korea. The X-rays from the vacuum undulator (IVU) were monochromated using Si(111) double crystals
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and focused on the detector using K-B type mirrors. Patterns were recorded with a 2D CCD detector (Rayonix SX165). The sample-to-detector distance was about 225 mm for energy of 11.08 keV (1.119 Å). 1.3.8. S pectroscopic Ellipsometry Spectroscopic Ellipsometer (SE from Ellipso technology Co., Inc.) was used in the measurement of the rotating polarizer type. The spectroscopic ellipsometry measurements were performed in the spectral range of 1.25.2 eV (~ 240-1000 nm) for three angles of incidence ranging from 60° to 70° with a 5° step. This approach improves the accuracy of the calculations allowing the determination of film thicknesses and optical refractive index values. The measured ellipsometric angles Ψ and Δ are defined from the ratio of the reflection coefficients rp and rs for the p- and s- polarizations, respectively, (i.e., polarization of the electric field parallel and perpendicular to the plane of incidence) according to rp (6) tanei rs From the analysis of the SE measurement data, the dielectric function of a certain material is determined. For this, the optical response of the measured samples was modelled in the Tauc-Lorentz dispersion formula,5 which included multiple oscillators.
i,TL E
r E r
2
p
Eg
. i 2 E2
c
for E > Eg for E ≤ Eg
d
(8)
1.3.9. Electric Field and Absorption Calculations The invariant imbedding method is a powerful tool for handling the electromagnetic wave propagation in onedimensional inhomogeneous media. The M axwell’s equations were applied to the amplitude of the electric field. The exact differential equations satisfied by the reflection coefficient and the electric field amplitude were obtained with respect to medium size. These were
Figure S 2.
b
2
And the experimental ellipsometric data were fitted using the Levenberg-M arquardt algorithm for minimizing the mean-squared error (MSE). N 1 mod MSE iexp ) 2 (imod iexp ) 2 ] [( i 2 N M i 1 (9) where N is the number of (α, β) pairs, M is the number of fitted parameters in the model. The superscripts mod and exp indicate model-generated and experimental data, respectively.
a1
a
(7)
1 AEoC E Eg E E Eo 2 C 2 E 2
i,TL E 0
2. Interfacial and Electronic Characterizations 2.1. S urface Morphology The topography images were Fourier filtered to remove from the data the frequency components associated with very weak signal, and the frequency regime affected by electrical and mechanical noise. The images were then Fourier Transformed to obtain the 2D Power Spectrum Density (PSD) presented in Figure S1a-c. The 2D-PSD was then radially integrated to reflect the root mean squared (RMS) roughness of the sample surface. The resulting spectra confirmed that the fluorination had little impact on the PEDOT:PSS layer roughness, which as a consequence cannot be used to explain the variations of the device characteristics when fluorinating their HTLs.
a2
-7
d
10
x PFI FOS
-8
10
PSD (µm4)
i,TL E i,L i,T
supplemented with the initial conditions from Fresnel formulas.6,7 Using this method, we obtained the exact solutions for the reflection and transmission coefficients of incident waves and electric field amplitudes inside the organic photovoltaic (OPV) media.
-9
10
-10
10
-11
10
-12
10
1
10 1/Length (µm-1)
100
Figure S 1. Fourier Transforms (FT) of AFM images. PEDOT:PSS based thin films: pristine (a), mixed with PFI (b) and with FOS (c). Radially integrated FTs (d) for PEDOT:PSS pristine (■), mixed with PFI (▲) or FOS (●). 2.2. Morphology-Wf Correlations Figure S2 shows large scale representations of the surface potential map overlaid on top of the topography for each hole transport layer (HTL). Both set of data were recorded simultaneously. The largest average surface potential variation is observed when PFI is used as a fluorination agent.
a3
Surface potential on the topography of PEDOT:PSS thin films: pristine (a), PFI- (b) and FOS-mixed (c).
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As discussed in the main manuscript, none of the sample presents any correlation between surface potential fluctuations and surface topography. This could be consistent with probing different orientations of the PEDOT crystalline grains distributed within the hole transport layer below the upper interface of the blend layer rich in PSS, PFI and FOS. AFM measurements were also carried out in non-contact mode. The literature suggests the top of the PEDOT:PSS to be rich of insulating PSS, PFI and FOS.8,9 In contrast, the present samples showed no noticeable potential variation contrast. This could be associated with a loss of resolution due to the Pt-coated Si tip required for KPFM acquisition.
Intensity (a.u.)
1.0
2
J (mA/cm )
0.4 0.2 10 5 0 Binding Energy (eV)
-5
Figure S 3. Normalised UPS spectra of PEDOT:PSS thin films spin-coated on quartz substrates: pristine (■), mixed with PFI (▲) and with FOS (●). experiments, respectively.10-14 In the case of the UPS the measurements, a further contribution could arise from UV exposure.15 These explain the value differences between the results provided by each technique. Table S 1. Ionization potential as deduced from UPS measurements (IP ) along with nanoscopic and macroscopic work functions (Wf-KPFM, Wf-m KP in eV) from PEDOT:PSS based thin films. Fluorination IP (eV)
PFI
x
FOS
5.19 0.02 7.32 0.02 7.09 0.02
W f-KPFM (eV) 4.70 0.02 5.40 0.03 4.90 0.03 W f-mKP (eV) 5.20 0.02 5.72 0.02 5.56 0.02
Dark Light
a2
0.6
Dark Light
a3
0 -5 -10 10 5
b1
b2
b3
2
J (mA/cm )
Dark Light
a1
0.8
0.0 15
2.3. UV Photoemission S pectroscopy A discharge lamp provided an He I (h = 21.22 eV) radiation with a resolution of 0.15 eV. The samples were transferred into the chamber one-by-one, and thin films were used to minimize the risk of charging. Charging and evolution of the samples when exposed the UV light source of the UPS were not observed. The UPS measurements of the photoemission onset were completed with a negative 5 V bias applied to the sample (Figure S3). Similar observations as those reported for the Kelvin probe measurements can be made with the ionization potential values deduced from the UPS measurements (Table S1). The discrepancies between the values obtained with these three different techniques are expected and ex plained by the different atmospheres and experimental conditions associated with each type of measurements, i.e ambient conditions, inert atmosphere and ultra-high vacuum for the Kelvin probe, the KPFM and UPS 5
x PFI FOS
0 -5 -10 -15 -1.0
-0.5
0.0 0.5 Voltage (V)
-1.0
-0.5
0.0 0.5 Voltage (V)
-1.0
-0.5
0.0 0.5 Voltage (v)
1.0
Figure S 4. P3HT:PC60BM (a) and PTB7:PC70BM (b) solar cells using PEDOT:PSS (1, ■), PEDOT:PSS:PFI (2, ▲), and PEDOT:PSS:FOS (3, ●) as the hole transport layer. Empty and filled symbols (square, triangle, disk) stand for dark and illuminated conditions, respectively. 3. S olar Cell Analysis 3.1. Jsc and EQE Consistency Using an AM 1.5G spectrum from NREL and the EQE data, JSC-EQE could be calculated and compared with JSC direct measurements. The values are presented in Table S2 and illustrate the good agreement between EQE and J-V. 3.2. Dark J-V Characterizations For comparison purposes, Figure S4 presents the J-V curves in the dark and under 1.0 Sun illumination.
3.3. S eries and S hunt Resistances In first approximation,16 the single diode model solar cell was used to extract series resistance, rs, and shunt resistance, Rsh, values which are presented in Table S2. rs results from the charge displacement across the materials and the contacts between the active material and the electrodes, which tend to reduce the fill factor of the device. The value of rs was estimated by calculating the slope of the V-J curve at VOC. Within the precision of the approach the assessed values of rs remains relatively
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2
P3HT:PC60BM
J SC (mA/cm ) SEQE. (k%.nm)
PFI
FOS
8.842
8.855
8.628
14.7
15.2
14.7
J SC-EQE (mA/cm )
7.92
8.21
7.85
r s (.cm2)
15
17
15
Rsh (k.cm )
0.5
0.5
0.4
J SC (mA/cm2)
15.944
16.889
16.941
SEQE. (k%.nm)
28.3
29.6
30.2
J SC-EQE (mA/cm )
16.15
16.62
16.96
r s (.cm2)
7
7
5
Rsh (k.cm2)
1.3
1.1
0.9
2
2
PTB7:PC70BM
x
2
Rsh is thought to be mostly due to defects associated with alternative current pathways within the active media. Its effect is considered most important at low illumination and at low voltages. The value of Rsh was estimated by calculating dV/dJ at 0 V, leading to values which also remains relatively unchanged with the fluorination in the case of P3HT:PC60BM devices, or would appear to decrease slightly in the case of PTB7:PC70BM devices. The apparent discrepancy between PCE-VOC-FF and rs-Rsh values suggests that more accurate device modeling would be needed to extract more reliable rs and Rsh values. Considering the dark J-V curves (Figure S4) led to Rsh values,18 which were also deemed unreliable within the experimental precision. M ultiple illumination intensity measurements and more complex modelling might overcome this situation,16,19 however, due to the complexity of the systems herein presented, these approaches fall beyond the scope of this manuscript. 4. EQE S pectral Variations with HTL Fluorination To understand the origin of the EQE spectral variations presented in Fig. 5a of the main manuscript, several hypotheses were explored. These include that the HTL fluorination could a) change the HTL transmission spectra, b) alter the surface energy at the HTL polymer:PCBM blend interface. By doing so, the blend morphology could be impacted as discussed in the main manuscript based on the literature.20-38 This should be associated with changes in the absorption spectra of the blend as well as crystallinity, orientation, domain size and/or
a Relative EQE (%)
Fluorination
4.1. Active Layer Optical Density Variations The relative variations of the EQE data are presented in Figure S5 using pristine PEDOT:PSS based devices as a reference. The EQE values obtained with P3HT:PC60BM OPVs display weaker variations than those obtained with PTB7:PC70BM devices, Figure S5a and b respectively, but both show a noticeable dependence with the excitation wavelength.
b
Relative EQE (%)
Table S 2. Short circuit current density (JSC), external quantum efficiencies integrated over the 300-800 nm spectral range (SEQE.) and JSC calculated from the EQE data (JSC-EQE); series (rs) and shunt (Rsh) resistances extracted from the the J-V curves.
polymer:PCBM profile. c) alter the thickness or change the absorption coefficient of the device layers, which either of them would impact on the electro-optical properties of the devices. In this section, we then start to analyze the differential EQE variations and address each of the above hypotheses.
12
PFI FOS
8 4 0 -4 -8 12 8 4 0 -4 -8 300
400 500 600 700 Wavelength (nm)
800
Figure S 5. Relative EQE spectral variation for P3HT:PC60BM (a) and PTB7:PC70BM (b) with pristine PEDOT:PSS based devices used as a reference for PFI (▲) and FOS (●) mixed hole transport layer. 4.2. HTL Optical Transmission Figure S6 presents the transmittance spectra of fluorinated and pristine PEDOT:PSS thin films. It is obvious from the amplitude of the variation, i.e. less than 3 %, as well as the absence of spectral deviation that the fluorination does not alter enough the photophysical properties of the hole transport layer to explain the EQE spectral variation reported in Fig. 5 of the main manuscript for both P3HT and PTB7 based OPVs. In addition, we note that the EQE of both P3HT and PTB7 based devices are not affected in the same manner as illustrated in Figure S5. As a conse100
x PFI FOS
98
T (%)
unchanged for both PTB7:PC70BM and P3HT:PC60BM devices, despite the slight increase of VOC and FF reported in Table 3 of the main manuscript. This comes in contrast with the decreased rs values observed for instance in organic-inorganic Schottky solar cells,17 and suggests that in the present more complex systems a 1 st approximation model is not sufficient to describe the current flow across the device and its interfaces when using fluorinated PEDOT:PSS.
96
94 300
400
500
600
700
800
Wavelength (nm)
Figure S 6. Transmittance spectra of PEDOT:PSS thin films spin-coated on quartz substrates: pristine (■), mixed with PFI (▲) and with FOS (●).
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quence, the absorption of the fluorinated PEDOT:PSS can be definitely ruled out (hypothesis a) and other origins have to be investigated. In this context, the variation of the surface energy when fluorinating the HTL was quantified by contact angle measurements on ITO and on the PEDOT:PSS based thin films.
a1
b1
a2
b2
Relative OD (%)
4.3. Wetting Properties Figure S7 illustrates the variation of the contact angle associated with the fluorination of PEDOT:PSS. Variations of the contact angles are visible for both PEDOT:PSS on quartz substrates, Figure S7a, and dichlorobenzene on the HTL thin films, Figure S7b. The contact angle values are listed in Table S3.
b3
a3
4.4. Polymer:PCBM Blend Optical Absorbance To focus on the photo-active blend displaying the largest EQE amplitude variations, the relative absorbance of each PTB7:PC70BM device is reported in Figure S8. Its variation turned out to be measureable but nonetheless relatively moderate, i.e. < 6 %. Compared to the PEDOT:PSS based devices, the fluorinated HTL devices do display an increased absorbance which, within the experimental precision, could be consistent with the EQE spectral variation (Figure S5b vs Figure S8). The variation of the PTB7:PC70BM absorption coefficient herein reported could be seen as consistent with the surface energy of the fluorinated PEDOT:PSS layer which could influence the PTB7 structure at the interface of the hole transport layer.
Figure S 7. Contact angle measurements of PEDOT:PSS on quartz substrates (a) and of dichlorobenzene on PEDOT:PSS based thin films (b): pristine (1), mixed with PFI (2) and with FOS (3). From the Young and Dupré equations, eq. 10 and 11 respectively,4 we obtain eq. 12 from which the adhesion energy of the dichlorobenzene on the PEDOT:PSS based surfaces can be assessed. (10) SV SL LV cosc Eadh SV LV SL
(11)
(12) Eadh LV 1 cosc with SV, SL, LV being the solid-vapor, solid-liquid and liquid-vapor interfacial tensions, respectively. θc is the equilibrium contact angle that the drop makes with the surface and Eadh is the adhesion energy defined as the amount of energy involved to separate the liquid from the surface. LV is taken as the dichlorobenzene surface tension, which value is 26.84 mN/m at 25 °C. Table S 3. Contact angle (θc ) variation with PEDOT:PSS fluorination in two distinct configurations and adhesion energy, Eadh, calculated with eq. 12. θc (°) / Eadh Picture (mN/m)
Interface
Solution
glass glass glass
PEDOT:PSS PEDOT:PSS:PFI PEDOT:PSS:FOS
11.6 / -21.2 / -37.7 / --
a1 b1 c1
PEDOT:PSS
Dichlorobenzene
2.6 / 53.7
a2
PEDOT:PSS:PFI
Dichlorobenzene
38.1 / 48.0
b2
PEDOT:PSS:FOS
Dichlorobenzene
47.8 / 44.9
c2
The calculated values the adhesion energy are listed in Table S3. There is a moderate but clear decrease of the surface energy with the PEDOT:PSS layer fluorination. Noticeably, the variation of the surface energy herein reported is nonetheless weaker than reported in the literature, as for instance when PEDOT:PSS, SiO 2 and poly(thienothiophene):Nafion were used.39
12
PFI FOS
8 4 0 -4 -8 300
400 500 600 700 Wavelength (nm)
800
Figure S 8. Relative OD variation for PTB7:PC 70BM with pristine PEDOT:PSS based devices used as a reference for PFI (▲) and FOS (●) based HTL. On the one hand, we note that PTB7 has been found to display an ordered fraction as small as fraction 20 %. 40 As a consequence, this rather small ordered fraction could leave room for a relatively strong effect of the substrate surface energy on PTB7 based photoactive blends. In contrast, annealed P3HT is known to be rather crystalline, and the PCE increase upon PEDOT:PSS fluorination was more modest than for PTB7 devices. This could be consistent with the fact that we could not observe any variation of the absorbance larger than the experimental uncertainty. On the other hand, we note a potential inconsistency in the fact that P3HT:PC60BM does not present any obvious change of absorption, not even around 620 nm, which is associated to this polymer crystallization. Along similar lines of thoughts, P3HT:PC60BM displays its strongest EQE relative variations in the UV range and not around 620 nm, Figure S5a. Consequently and despite the contact angle data, the absorption results do not unambiguously support the hypothesis that the morphology, i.e. the crystallization, of the active layer would be altered by the fluorination of the PEDOT:PSS layer. To address this ambiguity, GIWAXS measurements were undertaken. 4.5. GIWAXS 4.5.1. Neat and Fluorinated PEDOT:PS S As illustrated in Figure S9, the GIWAXS patterns of PEDOT:PSS films spincoated on top of a silicon wafer contain two broad continuous rings at roughly 1.2 and 1.8 Å-1, whose location and width are in good agreement with the previously described nanocrystalline structure. 41 The fluorination expectedly preserves the rings and the nanocrystals, as the fluorinated alkyl (FA) chains are not miscible with the PEDOT:PSS segments. Patterns should therefore contain a specific signal from domains of closepacked FA chains, thus a scattering maximum around 1.15
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a
a
b
b
c
c
d
d
Figure S 9. GIWAXS patterns with an incident angle i = 0.13° for pristine (a), PFI- (b) and FOS- (c) mixed PEDOT-PSS layers deposited on silicon substrate. Profiles extracted from the GIWAXS patterns (d). Å-1 in the molten state or a sharp first order reflection at about 1.20 Å -1 for a long-range correlated hexagonal inplane arrangement.42 The ramified architecture of PFI favors the short-range correlated organization, which gives rise to a diffuse signal overlapping the PEDOT:PSS ring. Both contributions were thus not separable for PFI, contrarily to FOS, for which a sharp elongated spot at 1.20 Å -1 is clearly observed on the equator and evidences crystallized FA chains layers lying parallel to the film surface.
Figure S 10. GIWAXS patterns with i = 0.13° of ITO substrate, bare (a) and covered with neat (b) or FOS-based (c) PEDOT-PSS, ( i = 0.13) and profiles (d) within = 30° to -60° sector (origin on the meridian). The wideangle region is dominated by the scattering of the glass substrate. Only the contribution of molecular FOS layers (scattering maximum: DFOS = 15 Å; 30 Å) needs to be considered in the small-angle region. As the organic semiconductor morphology often depends on the preparation conditions, films investigated by GIWAXS were also deposited under experimental conditions as close as possible to those used to fabricate the devices. This includes the use of ITO glass substrates, which however renders the wide-angle region unusable,
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due to its own intense scattering. This is illustrated in Figure S10 for bare ITO substrate (a), as well as pristine (b) and FOS (c) -based PEDOT:PSS thin films. 4.5.2. P3HT based Bilayers Regarding P3HT samples, the organic semiconductor molecules self-organize in crystalline lamellae formed by alternating rows of π-stacked backbones and aliphatic layers. The GIWAXS pattern of P3HT deposited on top of PEDOT:PSS itself covering a silicon wafer is presented in Figure S11a.
This orientation of the P3HT domains is maintained in the PC60BM blends (Figure S12b) with nearly the same parameters and an even further improved flat-on alignment of the lamellae, as discussed below.
a
a
b
b
c
Figure S 11. P3HT on top of neat PEDOT-PSS deposited on a silicon wafer: GIWAXS pattern with an incident angle i = 0.12° (a), schematics of the polymer domains with short-range correlated structure and orientation (b). Only signals from P3HT remain therefore visible with the main signals being composed by a series of sharp harmonics from a 17 Å periodicity (first order at ca. 0.37 Å-1) and a somewhat broadened signal at 3.8 Å (ca. 1.65 Å-1). In agreement with structural studies on bulk P3HT powder pattern, the latter signal corresponds to the stacking distance, h, between -stacked backbones, while the set of (00l) reflections comes from lamellae formed by alternating polythiophene sublayers and alkyl chains. The (00l) reflections group lies on the meridian while h lies on the equator, which proves that P3HT is a mosaic texture of flat lying lamellae. This lamellar crystalline structure and the preferential flat-on alignment of the lamellae is illustrated schematically in Figure S11b. This orientation and the spacing values are in agreement with alignments and lattice parameters previously reported for P3HT thin films (a = 7.8 Å, b = 2hp = 7.8 Å, c = 16.0 Å, = 93.5°).43 The same morphology is found for the P3HT films deposited on top of PEDOT:PSS spincoated on ITO-glass and silicon wafer substrates. In particular, P3HT shows the same flat-on alignment of crystalline lamellae (Figure S12a and Figure S11a). This also demonstrate that this morphology is not altered by the presence of the PEDOT:PSS bottom layer, which is slightly different from the observations made with PTB7 in the following section.
Figure S 12. GIWAXS patterns of neat P3HT (a) and P3HT:PC60BM blend (b) films on top of neat PEDOT:PSS spincoated on ITO substrates ( i = 0.13°). I(q) profiles within = -3° to +3° sector (origin on the meridian) of the GIWAXS patterns (a), (b) and Figure S11a (c). Both P3HT and P3HT:PC60BM were also deposited on top of neat, and fluorinated PEDOT:PSS. No influence of the HTL fluorination could be evidenced, even if several incident angle and thus weighting of the successive P3HT film strata were investigated as illustrated in Figure S13a. Comparing Figure S13b1 and b2 reveals the further improved flat-on alignment of the lamellae when PC60BM is blended with P3HT. This is demonstrated by the decrease of the full widths at half maximum of the (001) spot, from FWHM 14-20° to ca. 7°, with the addition of PC60BM . Finally and in a similar manner as what will be seen in the following section with the PTB7 system, P3HT domains are slightly but significantly modified by the blending with PC60BM . In this case the thickness and the correlation length of lamellae are d = 16.3 ± 0.1 Å and 150 Å in the blends, instead of 16.1 ± 0.1 Å and 120 Å
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a1
a2
a
b b1
b2
Figure S 13. I0(q) profiles patterns (a) and normalised Iq() ring profiles (b) of GIWAXS of neat P3HT (1) and P3HT:PC60BM blend (2) thin films on top of pristine and fluorinated PEDOT:PSS layers. Sector profiles (a) are plotted with an offset to facilitate the comparison of shapes and maximums [sector from = -3° to +3° and incident angles i = 0.08° (top) and i = 0.13° (bottom)], while azimuthal profile intensities (b) are normalized to the maximum Iq(0) on the meridian [Iq(), q = 0.392 Å-1; width: Dq = 0.040 Å-1]. in pristine P3HT films. Noticeably, these features, i.e. and FWHM, variation with the insertion of PC60BM , were observed for both fluorinated and pristine PEDOT:PSS bottom layers. 4.5.3. PTB7 based Bilayers When silicon substrates are used for PTB7 film deposition, all the characteristic signals of the structure are visible and shown in Figure S14a. They correspond to two broad and intense structure peaks around 0.35 Å -1 and 1.31.5 Å -1: - the wide-angle ring arises from the lateral distances between irregularly piled backbones, h, and between aliphatic chains, hch, - the two semi-diffuse small-angle rings labeled as (1) and (2) in Figure S14a result from alternativing backbone rows and aliphatic layers, d1 and second order periodicity d2. These features are schematically represented in Figure S14b. Noticeably, these rings are nearly isotropic and exclude any significant alignment of the short-range correlated lamellae. As discussed below, this comes in contrast with the preferential orientations reported in literature.40,44,45 PTB7 is known to self-organize in a short range correlated structure formed by alternative rows of irregularly face-to-
Figure S 14. PTB7 on top of neat PEDOT-PSS deposited on a silicon wafer: GIWAXS pattern with an incident angle i = 0.12° (a), schematics of the polymer domains with short-range correlated structure and variable orientations (b). face piled backbones and of aliphatic layers. The direction of alternation therefore designs a lamellar-like periodicity, d1 17 Å, with in-plane periodicities overlapping contributions of lateral distances between molten aliphatic chains hch 4.5 Å and piling distances of backbones hp 3.9 Å.40 Roughly, the same structural parameters are found here for PTB7 deposited on top of PEDOT-PSS covered Si wafer. However no significant orientation can be recognized in the present study. The literature reported a slight flat-on preferential orientation of lamellae for PTB7 films directly deposited on top of silicon.40 Other references even reported more developed flat-on preferential alignment and slightly different structural parameters (d 20 Å; hp 4.2 Å).44 However, the apparent inconsistency probably comes from the absence of bottom layer in these references, since the PEDOT:PSS deposited herein expectedly modifies the anchoring of the PTB7 top layer. The comparison of Figure S14a with Figure S15a illustrates that the use of ITO glass substrates did preserve the small-angle lamellar ring of PTB7 related to the alternation of alkyl-chains and conjugated moieties, while rendering the wide-angle region unusable due to the ITO glass intense scattering hiding the contributions from lateral spacing of alkyl chains and backbones. M ore specifically, the position of the maximum and the width of the lamellar first order rings appear to be the same regardless of the substrate while the intensity is quite homogenously distributed along the ring in both cases. The substitution of substrates did not change the organization but confirmed the absence of preferential orientations of lamellae, whether the films are constituted by neat PTB7 or PTB7:PC70BM blend (Figure S15a and
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a
a1
a2
b1
b 2
b
c
Figure S 15. GIWAXS patterns of neat PTB7 (a) and PTB7:PC70BM blend (b) films on top of neat PEDOT-PSS spincoated on ITO substrates ( i = 0.13°). I(q) profiles within = -30° to -60° sector (c) of the GIWAXS patterns (a), (b) and Figure S14a. b). Figure S15b and c display the additional rings from PC70BM domains, which have no strong effect on the PTB7 fraction, as its scattering rings remain similar to those observed in neat PTB7 films. As the films were spincoated on top of pristine and fluorinated HTLs, accurate information was extracted from sector profiles, excluding the distorted meridian and equatorial zones, and from ring profiles, which are presented in Figure S16a and b, respectively. The morphology of PTB7 (Figure S16a1b1) and PTB7:PC70BM (Figure S16a2-b2) films is independent of the fluorination of the PEDOT:PSS bottom layer. In particular the same lamellar periodicity and correlation length were identified among almost randomly oriented domains (Figure S14b). M oreover, the variation of incident angle, i, used to probe the films revealed that the lamellar ring were unchanged (Figure S16a). This shows that there are no substantial morphology changes across the thickness of the films. The only relatively significant difference between profiles is the somewhat smaller lamellar periodicity found for PTB7:PC 70BM blends (d = 16.2 ±0.2 Å; 30 Å), when compared to neat PTB7 (d = 17.1 ± 0.2 Å; 30 Å), presumably in
Figure S 16. I(q) profiles (a) and normalized Iq() ring profiles (b) of GIWAXS patterns of neat PTB7 (1) and PTB7:PC70BM blend (2) thin films on top of fluorinated and pristine PEDOT:PSS layers. Sector profiles are plotted with an offset to facilitate the comparison of shapes and maximums [sector from = -30° to -60° and incident angles i = 0.08° (top) and i = 0.13° (bottom)], while azimuthal profile intensities are normalized to the average scattered intensity (dotted line) [Iq(), q = 0.349 Å -1; width: Dq = 0.060 Å -1]. relation with the incorporation of a small fraction of PC70BM in the PTB7 domains. Noticeably, this slightly reduced spacing in the blend with respect to neat PTB7 was observed for both fluorinated and pristine PEDOT:PSS bottom layers. 4.5.4. Overall Effect of the HTL Fluorination on P3HT and PTB7 based Blend Morphology Overall, with Figure S13 and Figure S16, this GIWAXS investigation shows that the fluorination of the HTL did not induce any detectable morphology variation of the blends. Even though local structural changes at the very interfaces remain possible, the absence of detectable influence of the HTL fluorination was observed for both P3HT:PC60BM and PTB7:PC70BM thin films, while the angle dependent GIWAXS measurements did not evidence any change of conjugated polymers and PCBM s concentration. The present findings are in contrast with what had been suggested in other reports considering the effect of PEDOT:PSS surface energy not complemented with morphology study.37 We note however that the present HTL fluorination study differs from earlier studies having for instance studied P3HT and PCBM profile distribution on top of pristine PEDOT:PSS, plain SiO 2 and pristine PTT:Nafion, which present a much larger surface energy variation, from 23 to 72 mN/m2, and which were probed by Near-Edge X-ray Absorption Fine Structure
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DOI: 10.1039/ C6TA00677A (NEXAFS) spectroscopy.39,46 In the latter study,39 a single angle was used for Grazing incidence X-ray Diffraction characterization and no significant difference in the degree of crystalline order or crystal orientation was observed with different surface energy. Different P3HT:PCBM profile nonetheless suggested based on the models used by the authors to analyze ellipsometry and NEXAFS data. Our GIWAXS investigation did not reveal such a profile variation, which could be due to the relatively moderate surface energy variation induced herein by the fluorination of the HTL. It certainly underlines the importance of such GIWAXS measurements to directly probe the films morphology of bulk-heterojunction in the same conditions as those used to fabricate OPV devices. To summarize, in both P3HT and PTB7 cases, the fluorination of the HTL layer improved the device performances, while the structure and the alignment of the polymers were shown to remain unaffected. These results exclude the hypothesis b) formulated at the beginning of the section 4. It is then safe to conclude that, in the present study, the effect of the HTL fluorination does not directly relate to the photoactive blend film morphology. 4.6. Dektak and S pectroscopic Ellipsometry 4.6.1. Thicknesses The thicknesses of the multilayered structures have been measured by both dektak and spectroscopic ellipsometry, Table S 4. Thickness of the ITO, HTL and active layers involved in the devices and as measured by dektak and spectroscopic ellipsometry. Dimensionless parameter, z/L, indicating an interface between ITO, HTL, active layer and anode. Fluorination
PTB7:PC70BM
P3HT:PC60BM
DekT (nm) d HTL
‡
x 34 2
PFI ‡
41 3
FOS ‡
39 2 ‡
DekT (nm) d Blend
70 7
58 8
66 9
Ellip. d ITO (nm)
131.4
132.8
134.2
Ellip. (nm) d HTL
61.0
65.6
64.5
Ellip. d Blend (nm)
60.1
62.2
59.7
z / LEllip. ITO - HTL
0.70
0.71
0.71
z / LEllip. HTL - Blend
0.57
0.57
0.57
z / LEllip. Blend- Anode
0.44
0.43
0.44
DekT (nm) d Blend
63 6
637
688
Ellip. d ITO (nm)
134.3
134.5
133.6
Ellip. d HTL (nm)
60.0
63.4
64.2
Ellip. d Blend (nm)
54.1
55.7
59.2
z / LEllip. ITO - HTL
0.70
0.71
0.70
z / LEllip. HTL - Blend
0.57
0.57
0.56
z / LEllip. Blend- Anode
0.45
0.44
0.44
measured as a single layer
and are summarized in Table S4. Within the experimental and fitting precision, the values obtained with the fluorinated HTL are marginally different for each technique. The average thickness of PEDOT:PSS given by dektak measurements increases by about 15 to 20 % with the fluorination which remain close to the total standard deviation of our measurements. The dektak thickness of the blends is obtained by subtracting the HTL layer from the total thickness of the bilayer. This incidentally increases the experimental uncertainty on the blend thickness, without leading to clear trend, as for instance PFI blends would be associated with a 7 % increase and a 5 % decrease of the P3HT:PC60BM and PTB7:PC70BM film thicknesses, respectively. Within the experimental and fit uncertainty, these thicknesses appear to be independent of the HTL fluorination. Ellipsometry data provided thicker HTL films by ~ 40 % when compared with dektak measurements, an apparent discrepancy which could be explained by both the pressure of the dektak tip on the soft organic HTL thin film and the model used to fit of the ellipsometry data.47 However, when comparing the thicknesses obtained by ellipsometry for the same materials, the fluorination appears to induce a maximum of ~ 7 %, ~ 9 % and ~ 4 % variation on the HTL, PTB7:PC70BM and P3HT:PC60BM layer thicknesses, respectively. These variations are within the experimental and fit uncertainty, but also relatively small compared to controlled variation thickness variation reported in the literature which studied the effect of film thickness in OPVs.48-54 Overall and for each technique the variation of the blend thicknesses appear to be small compared with the spectral variation of the EQE, and consequently not to be the main parameter associated with the observed spectral changes of OPV efficiency. It is noticed that the lack of significant thickness variation excludes any change of mobility, which otherwise could have also altered the PCE.55,56 4.6.2. Optical Constants The optical constants of each materials in all the device configurations were deduced from the spectroscopic ellispometry data and are presented in Figure S17 for PEDOT:PSS and in Figure S18 for the photoactive layers of the devices. Based on our GIWAXS data, the ellipsometry model considers homogeneous material distribution across each layer. PEDOT:PSS refractive index and extinction coefficient, (n, k), are consistent with the literature. 57-59 n and k also preserve the same general characteristics whether pristine or fluorinated HTL were prepared (Figure S17a). However, they also present a slight red shift and variation of amplitude of their main features, pristine is “bluer” and with larger amplitudes than FOS-mixed, while PFI-mixed PEDOT:PSS presents the most red shifted and smallest amplitude. We note that a progressive blue shift of the PEDOT:PSS features, i.e. band gap, could be observed when doped with DM F or de-doped with hydrazine vapors.57,60 As a consequence, we associate the spectral red shift observed in the present study with the effect of the sulfonic acid groups of the fluorination agents on the PEDOT moieties and the electro-negative effect of the same fluorination agents. These are equivalent to negative Burstein shifts induced on the apparent band gap of semiconductor when extra charges are added, for instance, through doping.61-64 We note that the trend on the amplitude of the extinction coefficient is consistent both with the known tendency of fluorinated materials to have a reduced refractive index compared to hydrogenated
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nPEDOT:PSS:x
1.48
a1
a2
a3
b1
b2
b3
1.44
1.40
X PFI FOS
kPEDOT:PSS:x
0.05 0.04 0.03 0.02 0.01 0.00
400 500 600 700 800 Wavelength (nm)
400 500 600 700 800 Wavelength (nm)
400 500 600 700 800 900 Wavelength (nm)
Figure S 17. Refractive index, n (a), and extinction coefficient, k (b), of PEDOT:PSS pristine (■), mixed with PFI (▲) or FOS (●). HTL alone on top of ITO (1), HTL sandwiched between ITO and either P3HT:PC60BM (2) or PTB7:PC70BM (3). counter parts,65-72 and with the variation of n and k observed for various conjugated molecules in oxidized or reduced states.73-75 This situation prevents any precise quantification of the relative contribution of these effects. As illustrated in Figure S17-2 and 3, the features evidenced with the fluorinated HTL single layers is qualitatively preserved once the PEDOT:PSS is covered by either P3HT:PC60BM or PTB7: PC70BM . However, we also note that the amplitude of the extinction coefficient is systematically increased once the HTL is coated by the blends. We associate this evolution with the dipoles formed due to the charge transfer between the blends and the PEDOT:PSS. Driven by the relative energy levels of of the compounds, as shown in Table 1 and Fig. 3 of the main manuscript, electrons are transfered from the blends to the PEDOT:PSS. Figure S18 presents the optical constants of both P3HT:PC60BM and PTB7:PC70BM , which general features are in good agreement with the literature. 40,55,59,76 The extinction coefficients are also consistent with the
2.0
X PFI FOS
X PFI FOS
a2
2.0
1.8
1.8
1.6
1.6
1.4
1.4
b1 kP3HT:C60
2.2
0.4
nPTB7:C70
a1
X PFI FOS
0.2
X PFI FOS
b2 0.4
0.2
0.0 450 600 750 Wavelength (nm)
0.0 450 600 750 900 Wavelength (nm)
Figure S 18. Refractive index, n (a), and extinction coefficient, k (b), of P3HT:PC60BM (1) and PTB7:PC70BM (2) on top of pristine (■), mixed with PFI (▲) or FOS (●) PEDOT:PSS spincoated on ITO susbstrates.
kPTB7:C70
nP3HT:C60
2.2
absorbance data presented in Figure S8 as well as Figure 3a1 and b1 in the main manuscript. This is obviously expected from the relation between , the attenuation or absorption coefficient, k, the extinction coefficient, and , the wavelength. 4k (S13) Slight variations of the optical constants of the blends with the HTL fluorination can be noticed. These variations, especially for P3HT:PC60BM , appear weaker than those observed with PEDOT:PSS. In a similar manner as for this material, we associate the n and k variations to charge transfer across the HTL:blend interface. 4.7. Electro-Optical Modeling The optical constants were used to model the modulus of the squared electric field, |E|2, spectral distribution within the devices,77 an approach which has been successfully used to optimize and explain the performances of OPV devices in both normal and reverse configurations. 48,78-80 |E|2 for each device is presented in Figure S19. As commented in the main part of the manuscript, the HTL fluorination optical effect is revealed when |E|2 colored map cross-sections, Figure S20a and Figure 8-1, are used to calculate the time average of the energy dissipated per second, Q. For sake of simplicity we used 𝑄̃ = 𝑛. 𝑘 |𝐸 |2⁄ 𝜆 to compare the devices prepared with different HTL. In the case of P3HT:PC60BM , we evidenced a marginal increase of 𝑄̃ with PFI and FOS mixed PEDOT:PSS (Figure 8a2). This effect went up to 34 % increase of 𝑄̃ in the case of PTB7:PC70BM (Figure 8b2). With this stronger system, we then verified that a 20 % variation of the HTL thickness would not explain the EQE spectral variation. We considered the thickness of the pristine PEDOT:PSS layer as measured by ellipsometry and varied it by both 20 % ( 12 nm) to calculate |E|2 and 𝑄̃ as presented in Figure S20a and S20b, respectively. The |E|2 curves present similar profiles, shifted with z/L, which is consistent with the thickness variation of the HTL layer. The maxima of |E|2 located in the ITO is slightly increased for thinner HTL, while the maxima in the photoactive blends reach a similar amplitude. Figure S20b makes is easier to focus on what happens in the photoactive blends, where the 𝑄̃ curves appear to present a similar amplitude
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a1
a2
a3
b1
b2
b3
Figure S 19. Calculated distribution of the modulus squared of the optical electric field, |E|2, as a function of the incident wavelength inside the photovoltaics device made of P3HT:PC 60BM (a) and PTB7:PC70BM (b) spincoated on top of pristine (1), PFI (2) and FOS (3) fluorinated-PEDOT:PSS. The horizontal dotted lines stand for the material distribution as labelled on the right hand-side. regardless of the HTL thickness. To visualize any potential effect, we matched the position of the blend:anode interfaces. It is obvious in the insert of Figure S20b that the three 𝑄̃ curves overlap. The integral of 𝑄̃ over the z/L value corresponding to the PTB7:PC70BM layer leads to a marginal variation of -5.4 % and +2.7 % compared to the device of the nominal HTL thickness. This is much lower than the effect of the fluorination on the optical constants.
ellipsometry measurements, which were discussed with SYK, JWW and PA. SJK developed the ellipsometry models ap plied to the multilayer structure and KJL adjusted them to each system. EK completed some the dektak measurements. KJL developed the electrooptic modelling of the OPVs with PA’s feedback. PA discussed each set of the data with their contributors and wrote the manuscript. The authors could comment the sections related to their contributions and provide general feedback.
1.0
a z/L
0.8
0.4 0.0
1.5
^
1.0 0.5 0.0
0.6
1.0
2.0
2
|E|
3.0 0.0
b
2.0
Q (x10-3)
x- x x+
0.5
shifted z/L
1.0 1.5 ~ (x103 nm-1) Q
2.0
̃ values (b) Figure S 20. Distribution of |E|2 (a) and calculated 𝑄 across photovoltaic devices illuminated with a 500 nm monochromatic light, and made of PTB7:PC70BM spincoated on top of pristine PEDOT:PSS films of nominal (■), -20 % (●) and ̃ values in the polymer:blend +20 % (▲) thicknesses Insert: 𝑄 layers with z/L shifted to match the blend:anode interface.
5. Contributions PA, KM and CTH discussed and initiated the project. PA organized, supervised and coordinated it. KM reviewed the literature, prepared most of the PEDOT:PSS based samples and prepared most figures of the manuscript, except those related to transient absorption, GIWAXS and electro-optical modelling. CTH prepared and characterized the devices in IDWS labs. CTH, KM and PA discussed and analyzed the data. HK and DWK were in charge of the near-field characterization, which was discussed with PA. TA was in charge of the macroscopic Kelvin probe measurements and organized the UPS measurements with AN completing and analyzing them. SF, JHK and JCR completed the conductivity measurements in CA lab. BH, SM and FM completed and analyzed the GIWAXS measurements, they also wrote the related section. KJL completed the transient absorption measurements in JWW lab and analyzed them with PA. KJL and SJK completed the
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