Efficiency Nonfullerene Organic Solar Cells

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Long Ye, Wenchao Zhao, Sunsun Li, Subhrangsu Mukherjee, Joshua H. Carpenter, Omar Awartani, Xuechen Jiao, Jianhui Hou,* and Harald Ade*

Organic solar cells (OSCs) made of donor/acceptor bulk-heterojunction active layers have been of widespread interest in converting sunlight to electricity. Characterizing of the complex morphology at multiple length scales of polymer:nonfullerene small molecular acceptor (SMA) systems remains largely unexplored. Through detailed characterizations (hard/soft X-ray scattering) of the record-efficiency polymer:SMA system with a close analog, quantitative morphological parameters are related to the device performance parameters and fundamental morphology–performance relationships that explain why additive use and thermal annealing are needed for optimized performance are established. A linear correlation between the average purity variations at small length scale (≈10 nm) and photovoltaic device characteristics across all processing protocols is observed in ≈12%-efficiency polymer:SMA systems. In addition, molecular interactions as reflected by the estimated Flory–Huggins interaction parameters are used to provide context of the room temperature morphology results. Comparison with results from annealed devices suggests that the two SMA systems compared show upper and lower critical solution temperature behavior, respectively. The in-depth understanding of the complex multilength scale nonfullerene OSC morphology may guide the device optimization and new materials development and indicates that thermodynamic properties of materials systems should be studied in more detail to aid in designing optimized protocols efficiently.

Dr. L. Ye, Dr. S. Mukherjee,[+] J. H. Carpenter, Dr. O. Awartani, Dr. X. Jiao, Prof. H. Ade Department of Physics and Organic and Carbon Electronics Lab (ORaCEL) North Carolina State University Raleigh, NC 27695, USA E-mail: [email protected] W. Zhao, S. Li, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, China E-mail: [email protected] [+]Present

address: Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

DOI: 10.1002/aenm.201602000

Adv. Energy Mater. 2016, 1602000

1. Introduction

Organic small molecules mixed with polymer semiconductors are being extensively investigated for electronic devices for highly efficient solar energy conversion and charge transport, enabling current field-effect transistor mobility larger than 5 cm2 (V s)−1[1–4] and single-junction solar cells device efficiencies greater than 11%,[5,6] respectively. Despite known disadvantages such as weak absorption in the visible region, relatively high cost, and limited tunability of energy levels, fullerene derivatives with large nonplanar spherical configuration are the predominant acceptor materials used in organic solar cells (OSCs) during the past two decades. Presently, nonfullerene small molecular acceptors (SMAs) garnered significant interest due to their easily designed and functionalized electronic and absorptive properties through judicious choices or modifications of the molecular skeletons and flexible side chains.[7,8] Power conversion efficiencies (PCEs) ranging from 8% to 12% are achieved by various groups[9–17] using different processing strategies and/ or introducing novel combinations of SMAs and polymeric donors, indicating the great potential of nonfullerene OSCs in replacing their fullerene-based counterparts and revolutionizing OSCs yet again. Unfortunately, a majority of the polymer:SMA systems reported in the literature[18–25] have relatively inferior performances due to the relatively high energy loss or poor blend morphology. Currently available donor polymers along with hundreds of novel SMAs create an incredibly large pool of polymer:SMA pairs that would be difficult to optimize without a complete characterization and fundamental understanding of the complex and often multilength scale morphology and the thermodynamic drivers and kinetic trapping during processing. Over recent years, it has become clear for the OSC community that the phases even in the most efficient polymer:SMA systems are not pure.[26,27] Analysis of the size scale of the nanomorphology of polymer:fullerene blends can be achieved through the study of probe-based atomic force microscopy (AFM) and the use of transmission electron microscopy (TEM),

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High-Efficiency Nonfullerene Organic Solar Cells: Critical Factors that Affect Complex Multi-Length Scale Morphology and Device Performance

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Figure 1. a) Chemical structures of the ITIC-family SMA materials (ITIC, IT-M, IT-DM) and donor polymer PBDB-T. R is 2-ethylhexyl and R1 is hexyl (C6H13). b) Schematic illustration of the fabrication process of high-efficiency nonfullerene OSCs. Notes: 1% DIO is used as additive and TA represents thermal annealing at 160 °C for 30 min.

but these widely used tools can not readily determine the multiple phase and complex domain details of nonfullerene OSC systems due to the low contrasts of compositionally similar materials. The domain composition variations at various length scales have been rarely studied in nonfullerene devices and remain unexplored for representative, high-efficiency polymer:SMA systems. Correlating the quantitative domain characteristics with device function is extremely challenging and identification of the important morphological parameters that govern the nonfullerene OSC performance is thus a pressing need for a better understanding of the morphology evolution via various optimization strategies and their underlying thermodynamic drivers. Among a wide range of newly reported SMAs, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC)[25] is extensively utilized and has delivered several record efficiencies in nonfullerene OSCs. In some cases, ITIC-based devices even outperformed their analogue fullerene-based OSC devices in both efficiency and stability.[6,12,25] Aiming to achieve higher open-circuit voltage (Voc), we recently designed and prepared two novel ITIC analogues[17] named IT-M and IT-DM (see Figure 1a) with upshifted lowest unoccupied molecular orbital levels. When combined with a wide band gap donor polymer named poly[(2,6-(4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′] dithiophene-4,8-dione)][28,29] (PBDTBDD-T, abbreviated to PBDBT), both SMA devices afforded 11%–12% efficiency[17] subject to combined treatments of incorporating a trace amount of solvent 1602000 (2 of 10)


additive 1,8-diiodooctane (DIO) during casting and subsequent thermal annealing (TA). This processing method is also used in the optimization of a previous high-efficiency PBDB-T:ITIC system.[6] Inspired by these consistent observations, we therefore focus on revealing the critical factors that affect the multilength scale morphologies and device performance of this class of highefficiency nonfullerene OSCs processed at different conditions (see Figure 1b). The detailed morphological features including molecular packing and domain composition variations at different length scales are investigated using resonant soft X-ray scattering (R-SoXS)[30] and complementary characterization methods. We find that higher composition variation at smaller length scales (≈10 nm) and improved π–π coherence lengths synergistically yield and explain the superior device performance of current nearly 12% efficiency systems using optimal processing conditions. Thus, the degree of mixing in the amorphous mixed domains is shown to control fill factors (FFs) and needs to be precisely controlled for designing new polymer:SMA pairs with even higher FFs. Additionally, the morphology development of two high-efficiency systems PBDB-T:IT-M and PBDB-T:IT-DM at various processing conditions is discussed in the context of their likely thermodynamic phase diagrams.

2. Results and Discussion 2.1. Nonfullerene Device Performance The photovoltaic properties of devices based on PBDB-T coupled with IT-M or IT-DM (the chemical structures are shown in



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2.2. Molecular Packing, Texture, and Coherence Lengths In order to understand the detailed morphology that lead to the higher performance for the optimized devices, we first extract molecular packing and crystalline texture observed by grazing incidence wide angle X-ray scattering (GIWAXS) at beamline 7.3.3 of the Advanced Light Source.[32] The π–π stacking in the out-of-plane (OOP) direction of organic thin film is well-known to benefit charge transport between anode and cathode of OSCs.[33,34] We note that the (010) peak of the neat polymer film is located at around 1.73 Å−1 along the vertical (qz) axis and the neat small molecule films have a distinct (010) peak at 1.8 Å−1, characteristic of π–π stacking, and corresponding to a d-spacing of 3.5 Å (see Figure S1, Supporting Information). From the GIWAXS 2D patterns shown in Figure 3a,b and the OOP 1D line-profiles (Figure S2, Supporting Information), there exist similar lamellar and (010) diffraction features, while the peak breadth of (010) peak varies with the processing condition. For a clear comparison, the two π–π stacking peaks can be

differentiated by multiple-peak fitting of the OOP line-profiles. Thus, the π–π coherence lengths (L) can be quantified by the full width at half-maximum of the (010) diffraction peak via the Scherrer Equation. As shown in Figure 3c,d, the π–π coherence length of the SMAs (LA) is slightly increased after annealing. The π–π coherence lengths of the polymer (LD) are substantially increased by a factor of ≈2 from 2–4 to 5–6 nm after the addition of DIO. Higher π–π coherence length is known to be beneficial for the charge transport characteristics.[33] Although these observations could partially explain the increase in device efficiency with annealing and additive, other morphological parameters, the average composition variation (or purity variations), and length scales of the compositional domains across the photoactive layers may also contribute to the variations in OSC efficiency, which we will discuss below.

2.3. Domain Spacings and Composition Variations at Two Length Scales The PCE improvements of both polymer:SMA devices processed at DIO/TA condition are mostly attributed to increases in Jsc and FF, parameters that are also generally affected by the film thickness and interfacial layers used. As the thickness of all these samples is very similar (100 ± 10 nm) and an identical device configuration is utilized, we can conclude that the improved Jsc and FF in both blends processed using DIO/ TA are primarily due to a more favorable nanomorphology. As the atomic compositions of PBDB-T and SMA (IT-M or IT-DM) exhibit minimal contrast, it is difficult to obtain quantitative information from TEM images, even if energy filtering is used. The R-SoXS technique[30] at beamline 11.0.1.2[35] of the Advanced Light Source utilizes the unique optical contrast between the donor polymer and SMA near the carbon 1s absorption edge to achieve relatively high sensitivity in organic thin films. The real dispersive part of the complex refractive index (n), 1-δ, and the imaginary absorptive part, β, for the polymer and SMAs are unique fingerprints and provide a scattering contrast proportional to Δn2 = Δβ2 + Δδ2. In transmission R-SoXS, the path length of the incident soft X-rays through the sample affects the scattering intensity following the Beer– Lambert Law I = I0e−αt, where α is the linear absorption coefficient and t is the film thickness. Following the assumption of a globally isotropic 3D morphology and previously established protocols,[30c] the Lorentz corrected circular averaged

Table 1. Photovoltaic properties of the fullerene-free OSCs under AM1.5G 100 mA cm−2 and OOP π–π coherence lengths of the polymer and SMA. Blends PBDB-T:IT-DM

PBDB-T:IT-M

a)The

Condition

Voc [V]

Jsc [mA cm−2]

FF [%]

PCEa) [%]

LDb) [nm]

LAc) [nm]

As-cast

0.97 ± 0.01

14.57 ± 0.24

65.39 ± 0.59

9.26 ± 0.11 (9.33)

2.01

4.71

DIO

0.97 ± 0.01

15.69 ± 0.17

68.44 ± 0.82

10.41 ± 0.14 (10.54)

1.98

6.35

DIO/TA

0.96 ± 0.01

16.29 ± 0.18

68.78 ± 1.48

10.79 ± 0.34 (11.25)

2.48

5.77

As-cast

0.95 ± 0.01

15.41 ± 0.39

66.70 ± 0.86

9.74 ± 0.24 (10.10)

1.70

2.65

DIO

0.95 ± 0.01

16.91 ± 0.18

69.15 ± 0.81

11.11 ± 0.14 (11.24)

1.89

5.66

DIO/TA

0.94 ± 0.00

17.31 ± 0.24

70.70 ± 1.50

11.48 ± 0.25 (12.05)

2.19

5.54

best values are provided in the brackets; b)LD is the coherence length of donor polymer; c)LA is the coherence length of SMA.




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Figure 1a) are investigated in inverted (glass/indium tin oxide (ITO)/ZnO/photoactive blend/MoO3/Al) devices, where the photoactive blends with a polymer:SMA blend ratio of 1:1 by weight were prepared under three different conditions in parallel, namely As-cast, DIO, and DIO/TA (see Figure 1b). The averaged performance parameters for 30 devices are summarized in Table 1 and the best-performing J–V characteristics are displayed in Figure 2. The Voc values remain at the excellent 0.95–1 V level for all six devices. The PBDB-T:IT-DM devices with DIO demonstrate an average PCE of 10.41%, while the PCEs of the As-cast control devices are much lower with an average value of 9.26%. Average PCE, short-circuit current density (Jsc), and FF were all improved by additional thermal annealing of the DIO devices. Thermal annealing and usage of solvent additives collectively lead to simultaneous improvements of both Jsc and FF. The improved Jsc values are consistent with the external quantum efficiency (EQE) spectra (Figure 2b,d). Likewise, a similar trend was observed for the PBDB-T:IT-M blends. The champion device performances of 11.25% and 12.05% are respectively achieved for the PBDB-T:IT-DM and PBDB-T:IT-M devices using DIO/TA processing condition. These SMAs-based OSCs significantly outperform the fullerene-based device[31] and all-polymeric devices[28,29] with respect to the PCEs using the identical donor PBDB-T, respectively.

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PBDB-T:IT-DM

0

-10

0.0

0.2 0.4 0.6 Voltage (V)

0.8

As-cast DIO DIO/TA

-10

800

PBDB-T:IT-M

40

As-cast DIO DIO/TA

20

-15 0.0

500 600 700 Wavelength (nm)

60

As-cast DIO DIO/TA

-5

400

(d) 80

PBDB-T:IT-M

0

0 300

1.0

5

-20 -0.2

PBDB-T:IT-DM

40 20

-15 -20 -0.2

80 60

As-Cast DIO DIO/TA

-5

EQE (%)

Current Density (mA/cm2)

(c)

(b)

5

EQE (%)

Current Density (mA/cm2)

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(a)

0.2 0.4 0.6 Voltage (V)

0.8

1.0

0 300

400

500 600 700 Wavelength (nm)

800

Figure 2. a,c) Photovoltaic characteristics and b,d) EQE curves of the PBDB-T:IT-DM and PBDB-T:IT-M blend films under various conditions (As-cast, DIO and DIO/TA).

R-SoXS scattering profiles were subsequently normalized for the absorption and thickness (see Figure 4). An X-ray energy of 283.8 eV was selected to enhance the material contrast between polymer-rich domains and SMA-rich domains and avoid damage and fluorescence background (see Figure S3, Supporting Information). The scattering profiles for all the samples are plotted in Figure 4a–f. Similar length-scale features and a

long period (domain spacing) or the modal value of 75 ± 5 nm of the spacing distribution was obtained for all the samples. The scattering contrasts of these polymer:SMA are significantly lower compared with polymer:PCBM systems. Nonetheless, the material contrast owing to small differences in the respective near edge X-ray absorption fine structure (NEXAFS) spectra and consequently the optical constants near the edge can still result

Figure 3. 2D GIWAXS patterns of a) PBDB-T:IT-DM and b) PBDT-T:IT-M blend films as a function of processing conditions; OOP (qxy = 0) π–π coherence lengths of the polymer and SMA in the c) PBDB-T:IT-DM and d) PBDB-T:IT-M blend films with conditions as indicated.

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Full paper Figure 4. Multiple-peak fits to the circularly-averaged R-SoXS profiles obtained at 283.8 eV with two log-normal components for: a) As-cast PBDB-T:ITDM blend film, b) PBDB-T:IT-DM blend film with DIO, c) PBDB-T:IT-DM blend film with DIO/TA, d) As-cast PBDB-T:IT-M blend film, e) PBDB-T:IT-M blend film with DIO, f) PBDB-T:IT-M blend film with DIO/TA. All the R-SoXS profiles are Lorentz corrected.

in scattering from such structurally similar materials with sufficiently high signal-to-noise ratio to enable quantification. This again exemplifies the great benefits of R-SoXS as a very useful tool to characterize the morphology of organic materials. The average composition variation over the length scales probed is proportional to the square-root of the normalized integrated scattering intensity (ISI). The ISI values (over the entire q range of the measurement) were calculated from the scattering profiles to evaluate the average relative domain composition variations over all length scales for the blends.[36] The composition variation values thus obtained were normalized to that of the As-cast PBDB-T:IT-M film. Practically negligible mass density variations allow the composition variations to represent average domain purities. We further assume that the mixed domains are close to the metastable thermodynamic binary composition. The relative overall average domain purities of the PBDB-T:IT-M and PBDB-T:IT-DM films under the optimal condition (DIO/TA) were found to be 0.97 and 0.83 respectively, that agrees well with the relative average domain purities presented earlier.[17] However, the device FFs of these six polymer:SMA systems does not necessarily track the increase of overall average domain purity, which might be due to the multilength scales observed in all of the films. Similar and even more complex multilength scale morphologies have been observed and studied in some fullerene-based OSCs.[36–38] However, the precise characterization of such complex morphology has been very rarely explored for nonfullerene SMA systems. Following our previous arguments and analysis,[30] the scattering intensity integrated over the q-range qa → qb for a 2-phase system having phases labeled 1 and 2 is qb

ISI =

∫ I (q ) q

2

2 dq = 2π 2V∆ρ12 φ1φ2

qa


(1)

where V is the scattering volume, Δρ12 is the optical contrast between phases 1 and 2, φ1 and φ2 are the volume fractions of phases 1 and 2. In case of a three-phase system with pure donor (d), mixed amorphous (m), and acceptor (a) phases with volume fractions φd, φm, and φa respectively and assuming volume conservation i.e., φd + φm + φa = 1, the ISI can be written as[39] 2 2 2 ISI ∝ 2π 2V ( ∆ρda φdφa + ∆ρma φmφa + ∆ρdm φ dφ m )

(2)

where Δρ2da, Δρ2ma, and Δρ2dm are contrast variations between the three phases with Δρ2da > Δρ2ma > Δρ2dm as the mixed domains are donor-rich. This is slightly different in nomenclature to some of our prior work,[37] as we make the reasonable assumption that the fraction of aggregated polymer φd is unchanged (see UV–vis absorption spectra in Figure S4, Supporting Information). Thus the volume fractions φm and φa are determined by the overall composition and the composition in the mixed domain. The scattering intensity is primarily modulated by the purity of the mixed domains. The higher the purity of the mixed domains, the larger the average purity, the larger the volume fraction φa, the larger the contrasts Δρ2dm and Δρ2ma, and thus scattering intensity. The domain spacing distribution and average composition/ purity variations at different length scales of the polymer:SMA blend films were derived from scattering profiles via multiplepeak fitting with two log-normal peaks. The bimodal features in reciprocal space can be verified from the power spectral density profiles calculated by Fourier transform of the AFM phase images (see Figure S5, Supporting Information). The peak-fitting results of the low-q and high-q peaks are listed in Table 2. The slight differences in the evolution of the low-q and high-q component peak intensities as a function of energy (Figure S6, Supporting Information) indicate the presence of two scattering contrast mechanisms. The low-q peaks centered around



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Table 2. Relative ISI (overall length scales probed), long period from the low-q and high-q peaks and relative average purity (Δρ) over the length scales corresponding to the high-q peak for the six blend films as obtained from the R-SoXS multipeak fits. Blends PBDB-T:IT-M

PBDB-T:IT-DM

Processing condition

Relative overall ISI [± 0.01]

Long period (Low-q peak) [nm]

Long period (high-q peak) [nm]

Δρ (high-q peak) [± 0.01]

As-cast

1

64.7

12.5

0.37

DIO

0.88

80.0

13.2

0.42

DIO/TA

0.97

80.1

13.4

0.46

As-cast

0.92

64.9

10.7

0.28

DIO

0.87

71.7

11.7

0.39

DIO/TA

0.83

72.7

11.5

0.33

0.1 nm−1 corresponding to a spacing (long period) of ≈70 nm is largest near the absorption edge (285.2 eV) and likely have a contribution from contrast due to orientation-correlations of acceptor-rich and/or donor-rich domains. Similar features at the largest observed length scale and originating from molecule– molecule orientation correlations with a similar energy dependence have been observed before in the case of other small molecule systems[37] and all-polymer systems.[28] Importantly, the intensity of the high-q peaks track the material contrast (Figure S6, Supporting Information) and therefore arise from donor–acceptor phase separation at the ≈10 nm (long period) nano-length scale. It may be noted that such a center-to-center spacing matches well with the typical exciton diffusion length and is one of the smallest length scales clearly observed with R-SoXS in any OSC systems. The TEM image of PBDB-T:ITM processed at the optimal condition (DIO/TA) is shown in Figure S7 (Supporting Information), and the domain spacing shows a general consistency with the R-SoXS results. However, we note that the small domains are likely to significantly overlap in 3D and can not be observed with a 2D projection method such as TEM without interferences from overlapping domains.

to a recent case study of PBDTTPD:PC71BM,[36] higher average composition variation of the highest-q peak is known to be favorable in reducing the bimolecular recombination and maximizing the charge generation in the photoactive layers. This study together with our previous observations on various materials systems (see Figure S8 in the Supporting Information) including the benchmark PBDTTPD:PC71BM,[36] and the all-polymeric PBDB-T:PBDTNDI-T,[29] show a general finding

2.4. Morphology–Performance Relationships at Small Length Scale In order to correlate the morphology with the device performance parameters, we plot the Jsc and FF as a function of the average purity variations (Δρ) at the smaller length scales. Interestingly, the device FF has a strikingly monotonic correlation with Δρ at a characteristic length scale of ≈10 nm (Figure 5a), and the highest Δρ is obtained in the highest FF and efficiency PBDB-T:IT-M device processed at DIO/TA condition. Likewise, Jsc increases monotonically with Δρ (see Figure 5b). Although the Δρ of the PBDB-T:IT-DM film processed at DIO/TA condition is somewhat of an outlier, it is nominally within errors for FF, but not for Jsc. Excluding it, both of the FF-Δρ and Jsc-Δρ plots of the other five samples show strong linear relationships with a (R-squared) correlation coefficient above 0.9. Overall, the data scatter is likely dominated by the fact that only one device from each processing condition was analyzed and obtaining extensive statistical analysis will be extremely time consuming. The error in Δρ reflects only intrinsic measurement errors, but not variations between samples. Given that the difference in FF is only about ≈10%, being able to establish this relation showcases the advantageous capabilities of R-SoXS. According 1602000 (6 of 10)


Figure 5. a) Plot of FF of the nonfullerene OSCs versus the respective composition variation of the high-q peak (Δρ) obtained from R-SoXS. b) Plot of Jsc of the nonfullerene OSCs versus the respective Δρ.



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2.5. Molecular Interactions of Nonfullerene Systems Due to the asymmetry of polymer and SMA in molecular size, the low molecular weight SMA likely forms almost pure domains and the mixed polymer:SMA amorphous domains have some extent of residual SMA (see left upward slope of binodal and spinodal curves in Figure 6a) even if the films reach local equilibrium. As the appended side chains of the

SMA are expected to impact its solubility and molecular miscibility with the polymer donor, they could in turn affect the film morphology during the film-forming process. In our effort to reveal the fundamental factor dominating the morphology/performance of the two blend systems, we hypothesized that the methyl groups in the SMAs likely alter the molecular interactions of the PBDB-T:SMA blends. The Flory–Huggins interaction parameter (χ) is known as a fundamental metric of molecular interaction as well as the degree of mixing.[41] The experimental determination of χ parameters of polymer:SMA blends has not yet been achieved for any fullerene-free OSCs at any temperature. As illustrated in Figure 1a, the two SMAs have the same molecular skeleton and just a slight difference exists in their substituted alkyls. This allows us to make a relatively reliable comparison of the room temperature (RT)χ parameters of structurally similar materials using the empirical Hansen solubility parameters (HSP) estimated from the group contribution method[41] (details are shown in the Supporting Information). We find the estimated room temperature χ parameter of the PBDBT:IT-M blend to be significantly higher (≈2.7) than that of the PBDB-T:IT-DM blend (≈2.0). According to the Flory–Huggins polymer solution theory, amorphous mixed domains only phase separate when χ is greater than the critical point (χc). The achievable composition/purity in the amorphous mixed domains is determined by the binodal composition in the binodal curve (or coexistence curve) of the phase diagram (Figure 6a) adjust for the constant chemical potential of the SMA crystals. Thus a larger χ well above χc (≈0.8) will yield higher domain purity. We note that χ parameters this large correspond to a low miscibility and a system deep inside the spinodal curve. The relatively higher χ parameter of the PBDB-T:IT-M system is in agreement with its lower degree of mixing and thus higher average purity of the high-q peak (Δρ) at various processing conditions, as shown in Figure 6b. It is likely that higher χ parameter enables a higher purity in the mixed domains, that leads to larger average purity and higher scattering intensity. Higher purity in the mixed domains reduces bimolecular recombination, that helps to explain the higher device FF of the PBDB-T:IT-M film processed at the same processing condition.

Figure 6. a) The calculated χ parameters at RT from HSPs and inferred χ at the annealing tempeature (160 °C) in relation to spinodal and binodal curves of the polymer:SMA systems for a given weight ratio of 1:1 (the dashed lines indicate the composition in the amorphous mixed domain in the limit of complete phase separation at the respective χ and the grey horizontal line represents the critical χc ≈0.8). b) Plot of composition variation of the high-q peak (Δρ) under various processing conditions in relation to the RT, HSPs derived χ parameter of the nonfullerene OSCs.




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that the purity variations at the smallest length scale (10–20 nm) dominates the charge generation,[40] charge transport, and thus device performance of the OSCs with multiple-length scale morphologies, irrespective of fullerene- or nonfullerene-based systems. The information on the organization and ordering at the nano- and meso-scales, as obtained from the GIWAXS and R-SoXS data, therefore reveal a complex morphological picture in these record nonfullerene material systems. The impact of additives and annealing on the morphological parameters of these high-efficiency nonfullerene OSCs is differentiated phenomenologically — additive and annealing impact different aspects. The π–π coherence lengths of the SMAs are much higher after adding DIO, and the π–π coherence length of the polymer is slightly increased after annealing. Without DIO, the polymer and SMA tend to form small and impure or small number of domains and more random packing as the As-cast film dries very fast. In contrast, thermal annealing provides an external driving force for the self-organization of polymer chains and small molecules, enabling a higher degree of order to be reached in both blends. Subsequent thermal annealing does not change the π–π coherence lengths of PBDB-T:IT-M much while greatly improving the composition variations, i.e., the average purity, at smaller length scales. The increased FF and Jsc values in both of the high-efficiency polymer:SMA blends after a two-step optimization (DIO/TA) are mainly contributed by the increased π–π coherence lengths of polymers and higher average purity at the smaller length scales (≈10 nm). This highlights the need to achieve a high average purity as well as highly ordered packing at the smallest length scales to diminish charge recombination in the fullerene-free OSCs.

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The crossover observed in Δρ values for the DIO and DIO/TA devices (Figure 6b) imply different phase behaviors for the two PBDB-T:SMA systems. The determined solubility of two SMAs at room temperature is starkly different, and the solubility of IT-M (>60 mg mL−1) is two times higher than that of IT-DM in the processing solvent. We therefore speculate that the films are not very quenched locally as these Δρ measurements are related to very small length scales with ≈10 nm spacings and local equilibrium should be achieved readily as the DIO evaporates slowly and the thermal annealing is relatively extensive. The lower Δρ observed for PBDB-T:IT-DM post annealing is particularly revealing, as ordinarily additional processing should not lower the purity or decrease the volume fraction of the small domains, but lead to further phase separation and purification. However, this can be reconciled by taking into account the change in temperature from RT for DIO-addition to 160 °C for annealing as explained below. The PBDB-T:IT-DM system is likely an upper critical solution temperature system (χ scales with 1/T), because the DIO device achieves near binary composition at room temperature and a lower purity was achieved for DIO/TA device after thermal annealing. In contrast, the higher χ PBDB-T:IT-M system may be a lower critical solution temperature system, where χ is proportional to −1/T. DIO processing yields mixed domain purities with a binodal composition that corresponds to RT. During the higher temperature annealing step, the DIO/TA device has enough time and mobility to reach the binodal composition at the elevated temperature, with all four compositions determined by χ(T). This implies tuning the molecular interaction via molecular design or thermal annealing is a viable strategy for improving the PCE of the nonfullerene OSCs. Importantly, the impact of molecular changes on morphology and particularly the mixed amorphous regions need to be considered alongside energy level tuning objectives and understood much better if they are to be deployed in a controlled and predictable way.

3. Conclusions In this work, we have presented the first quantitative study of the multilength scale morphology of the recent recordefficiency nonfullerene OSC systems, and established the direct correlations between morphological attributes at small length scales and OSC device characteristics of two state-ofthe-art polymer:SMA blend films (PBDB-T:IT-M and PBDBT:IT-DM) processed at various processing conditions. A strong correlation between the average purity at smaller length scale (≈10nm) and the device FF/Jsc is observed. Together with the previous observations in polymer:fullerene and polymer:polymer systems, our study highlights the significance of achieving a high purity variation and highly ordered packing at the smallest length scales in both of the fullerene and nonfullerene OSCs. These observations also delineate why the use of both solvent additive and thermal annealing is needed to achieve best performance. More importantly, we find here that molecular interactions as reflected by the χ parameter and its temperature dependence could be the key factor to control the average purity at smaller length scale and thus device performance. This aspect needs to be studied more extensively 1602000 (8 of 10)


and is generally likely underappreciated in the OSC community, as not a single χ(T) measurement exists in the literature. We believe that improved nonfullerene OSCs with efficiency surpassing 15% are achievable in single junction OSCs with further manipulation of the domain composition/purity variations and molecular interactions of the polymer:SMA pairs via molecular engineering.

4. Experimental Section Device Fabrication and Characterizations: The ITO-coated glass substrates were thoroughly cleaned by deionized water, acetone, and isopropanol twice successively at first. After being dried at 150 °C for 15 min, a thin layer of sol–gel ZnO (30 nm) was spin coated on top of ITO and then annealed at 200 °C for 1 h in ambient air. D/A blends (1:1 weight ratio) were dissolved in chlorobenzene (CB) or CB/DIO mixture (99/1, vol/vol) with a total concentration of 20 mg mL−1 and stirred at 100 °C overnight. Subsequently, the mixture was spin coated on ZnO modified ITO substrates at 3000 rpm to the optimal film thickness of ≈100 nm. For the best conditions, the active layers were also annealed at 160 °C for 30 min. Finally, 10 nm thick MoO3 film and 100 nm thick Al layer were deposited sequentially to complete the inverted device. The current–voltage curves of the PBDB-T:IT-M and PBDB-T:IT-DM based devices were measured under an AAA solar simulator (XES-70S1, SAN-EI Electric Co., Ltd) with an irradiance intensity of 100 mW cm−2 of the standard AM 1.5G spectrum. The spectral mismatch factor was calibrated to be unity via a National Institute of Metrology (NIM) certificated silicon reference cell with a KG5 filter.[42] All EQE curves were measured through the solar cell spectral response measurement system QE-R3011 (Enli Technology Ltd., Taiwan), which was calibrated by monocrystalline silicon solar cell in advance. The materials used here are the same batches with those used in our prior work.[17] Morphology Characterizations: NEXAFS reference spectra, GIWAXS, and R-SoXS measurements were respectively performed at the beamline 5.3.2.2,[43] beamline 7.3.3,[32] and beamline 11.0.1.2,[35] advanced light source (ALS), Lawrence Berkeley National Laboratory, following the previously established protocols. GIWAXS data were acquired just above the critical angle (0.13°) of the films with a hard X-ray energy of 10 keV, and Silver Behenate (AgB) was used for geometry calibration. OOP π–π coherence length (L) is estimated by the Scherrer Equation (Lc = 2 πK/Δq), where K is the shape factor (≈0.9), and Δq is the full width at half maximum of the (010) diffraction peak. R-SoXS was performed in a transmission geometry with linearly polarized photons under high vacuum (1 × 10−7 Torr) and a Peltier cooled (−45 °C) chargecoupled device (CCD) (Princeton PI-MTE, 2048 pixels × 2048 pixels) was used to capture the soft X-ray scattering 2D patterns. The raw 2D X-ray data were processed with a modified version of NIKA into 1D scattering profiles I(q).[44] Thicknesses for the samples were measured with a KLATencor P-15 profilometer. Simulation of the Phase Diagram: The binodal and spinodal curves were simulated with the following parameters of PBDB-T (Mn = 16 KDa; density: 1.15) and IT-M/IT-DM (density: 1.5), using a program developed by Prof. Enrique Gomez and Josh Litofsky.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements L.Y. and W.Z. contributed equally to this work. The authors gratefully acknowledge the support by the ONR grant N00141512322



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Received: September 7, 2016 Revised: October 11, 2016 Published online:

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and UNC-GA Research Opportunity Grant. X-ray data were acquired at beamlines 11.0.1.2, 7.3.3, and 5.3.2.2 at Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. J. Hou thanks the financial support from the National Basic Research Program 973 (2014CB643501), the NSFC (Grants Nos. 91333204, 21325419), and the Chinese Academy of Sciences (Grant No. XDB12030200). A. L. D. Kilcoyne, E. Schaible, C. Zhu, A. Hexemer, C. Wang, and A. Young of the ALS (LBNL) provided instrument maintenance. Y. Xiong is acknowledged for conducting the AFM characterizations.

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