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Chloroboron (III) subnaphthalocyanine as an electron donor in bulk heterojunction photovoltaic cells

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 24 (2013) 484007 (9pp)

doi:10.1088/0957-4484/24/48/484007

Chloroboron (III) subnaphthalocyanine as an electron donor in bulk heterojunction photovoltaic cells Guo Chen1 , Hisahiro Sasabe1 , Takeshi Sano1 , Xiao-Feng Wang1 , Ziruo Hong1,2 , Junji Kido1 and Yang Yang2 1

Department of Organic Device Engineering, Graduate School of Science and Engineering, Research Center for Organic Electronics (ROEL), Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan 2 Department of Materials Science and Engineering, University of California-Los Angeles, Los Angeles, CA 90095, USA E-mail: [email protected], [email protected] and [email protected]

Received 3 June 2013, in final form 9 July 2013 Published 6 November 2013 Online at stacks.iop.org/Nano/24/484007 Abstract In this work, chloroboron (III) subnaphthalocyanine (SubNc) was used as an electron donor, combined with a [6,6]-phenyl-C71 -butyric acid methyl ester (PC70 BM) or fullerene C70 acceptor in bulk heterojunction photovoltaic cells. In spite of the limited solubility of SubNc in organic solvents, the solution processed device exhibited an efficiency of 4.0% under 1 sun, AM1.5G solar irradiation at room temperature, and 5.0% at 80 ◦ C due to the temperature-dependence of the carrier mobilities. SubNc:C70 bulk heterojunctions were also fabricated via thermal co-evaporation, demonstrating an efficiency of 4.4%. This result shows that SubNc is a promising material for photovoltaic applications via various processing techniques, such as vacuum deposition and wet coating. S Online supplementary data available from stacks.iop.org/Nano/24/484007/mmedia (Some figures may appear in colour only in the online journal)

1. Introduction

(Voc ) and efficient exciton dissociation; and sufficient carrier mobility for charge collection [5, 6]. Recently, SM based BHJ cells have underwent great development and demonstrated competitive power conversion efficiency (PCE) with polymer BHJs [7–12]. For the technology to make a SM BHJ device, in spite of the typical vacuum co-evaporation processing, solution processing has also been widely used and 8.0% of PCE from a solution processed SM BHJ cell has been realized [12]. In principle, highly thermal stable materials can be deposited by using a vacuum co-evaporation process while the soluble materials can be spin-coated from solution. In particular, some SM materials possessing high thermal stability and solubility can be deposited from both vacuum co-evaporation and solution processes. For example, several merocyanine dyes [13] and a squaraine dye DIB-SQ [6, 8, 11] have been reported as promising materials for both solutionand vacuum co-evaporation processed BHJ cells.

Organic semiconductor material based bulk heterojunction (BHJ) photovoltaic (PV) cells have been considered as a promising approach for making large-size, flexible and light-weight solar cell devices [1–4]. In a BHJ device, a blend film of electron donor (p-type conjugated polymer or small molecule (SM)) and electron acceptor (n-type fullerene and their derivatives) acts as the PV active layer to collect light and generate electricity. Such a BHJ structure is useful to overcome the relatively short exciton diffusion length, normally on the order of 10 nm in neat organic films. To achieve high efficiency BHJ cells, some basic requirements should be met for donor and electron materials, such as strong and broad absorption in the visible and near IR region for efficient light harvesting; well matched energy level of donor and acceptor material for high open circuit voltage 0957-4484/13/484007+09$33.00

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c 2013 IOP Publishing Ltd Printed in the UK & the USA

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Figure 1. (a) Device architecture and (b) schematic energy level diagram of the SubNc:PC70 BM bulk heterojunction photovoltaic cell.

In the research field of SM based PV cells, phthalocyanine (Pc) and metallophthalocyanine (MPc) have been widely used as donor materials since the first efficient CuPc based planar heterojunction (PHJ) PV cell was reported by Tang in 1986 [14–16]. Recently, a non-planar-pyramid-shaped Pc-related compound, chloroboron (III) subnaphthalocyanine (SubNc), has been considered as a photoactive material for PV cells due to its excellent physical properties, such as strong absorption, high thermal stability, moderate solubility in organic solvents etc [17–19]. One of the significant advantages of SubNc is that it can be thermally evaporated in high vacuum, and it is also soluble in organic solvents due to its non-planar molecular structure. This allows us to make SubNc thin films via both vacuum evaporation and solution processes. By using SubNc as a donor material, Verreet et al made a vacuum evaporation processed PHJ cell SubNc/C60 with an efficiency of 2.5% with a Voc of 0.79 V, a short circuit current density (Jsc ) of 6.1 mA cm−2 and a fill factor (FF) of 0.49 [17]. Ma et al fabricated a similar SubNc/C60 PHJ cell with an efficiency of 1.5% by using a spin-coating process to make the SubNc layer [18]. More recently, photovoltaic cells based on SubNc/C60 PHJ reached a PCE of over 3% [19]. Nevertheless, SubNc is a promising material, which is expected to generate a PCE of over 6%, considering its bandgap and highest occupied molecular orbital (HOMO) level. Yet only the PHJ device structure was used in previous works, limiting the exciton collection efficiency due to insufficient donor/acceptor interface. Therefore in this work, we explore the possibility to incorporate SubNc into a BHJ device structure in view of the fact that the large donor–acceptor interface in the BHJ active layer results in efficient charge carrier separation and hence high photocurrent [20]. SubNc has strong absorption ranging from 600 to 730 nm. We thus used it as an electron donor, combined with [6,6]-phenyl-C71 -butyric acid methyl ester (PC70 BM) or fullerene C70 , which cover the blue and green region, as an electron acceptor to make BHJ cells. As a result, efficient light harvesting throughout the visible and near IR range was achieved, and ∼50% enhancement of quantum yield was realized, in comparison with the published PHJ cells.

Technology Corp.; MoO3 (purity: 99.99%) and bathocuproine (BCP) (sublimated, purity: 99.99%) were purchased from Aldrich Chemical Co. and Dojindo Laboratories, respectively. SubNc and C70 were purified two times by vacuum sublimation, and the high purity MoO3 and BCP were used as received. ITO coated glass substrates were cleaned using detergent, de-ionized water, acetone and isopropanol in an ultrasonic bath successively, and then dried in an oven at 80 ◦ C for 12 h. 2.2. Material characterization UV–visible (UV–vis) absorption spectra were obtained using a SHIMADZU MPC-2200 UV–vis spectrophotometer. Photoluminescence (PL) spectra were measured using a FluoroMax-2 (Jobin-Yvon–Spex) luminescence spectrometer. The solution for UV–vis absorption and PL measurement was at a concentration of 1 × 10−5 mol l−1 in o-dichlorobenzene (ODCB). Thin films for UV–vis absorption and PL spectra were prepared by spin-coating solution on quartz substrate. The HOMO level was determined by photoelectron yield spectroscopy (PYS) under vacuum (∼10−3 Pa) [21]. X-ray diffraction (XRD) pattern for films were collected using a high-resolution XRD diffractometer (SmartLab, Rigaku Co.). The films for PYS and XRD measurements were prepared by spin-coating solution on ITO substrates. Atomic force microscopy (AFM) images were collected in air on a Veeco AFM using a tapping mode. The films for AFM measurement were prepared by spin-coating SubNc:PC70 BM solution on the MoO3 coated ITO substrates. 2.3. Device fabrication and characterization The BHJ cells adopting the device structure of ITO/MoO3 / SubNc:PC70 BM/BCP/Al (figure 1(a)) were fabricated as follows: substrates were exposed to UV ozone for 30 min and immediately transferred into a high-vacuum (1 × 10−6 Pa) chamber for deposition of 5 nm MoO3 . Photoactive layers were fabricated via spin-coating SubNc:PC70 BM solution (in ODCB) on the MoO3 coated ITO surface in a N2 -filled glove box, and the film thickness was tuned via controlling solution concentration (the total concentration of the mixed SubNc and PC70 BM from 30 to 42 mg ml−1 ) and spin-coating speed (600–2000 rpm min−1 ). Then the film was thermally treated at 120 ◦ C for 10 min to remove solvent residue. Finally, the

2. Experimental details 2.1. Materials Patterned indium–tin–oxide (ITO) coated glass substrates, SubNc, PC70 BM and C70 were purchased from Luminescence 2

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Table 1. Summary of the key parameters in 60 ± 5 nm-thick SubNc:PC70 BM BHJ cells with various blend ratios. Weight ratio

Jsc (mA cm−2 )

Calculated Jsc a (mA cm−2 )

Voc (V)

FF

PCE (%)

1:1 1:3 1:5 1:7

7.0 ± 0.1 8.7 ± 0.2 9.2 ± 0.2 9.0 ± 0.2

6.88 8.63 9.29 9.22

0.90 ± 0.01 0.90 ± 0.01 0.90 ± 0.01 0.90 ± 0.01

0.36 ± 0.01 0.44 ± 0.01 0.44 ± 0.01 0.43 ± 0.01

2.2 ± 0.1 3.3 ± 0.2 3.6 ± 0.2 3.3 ± 0.2

a

The Jsc was calculated from the integration of EQE spectra.

Table 2. Summary of the key parameters in the SubNc:PC70 BM (1:5) BHJ cells with various active layer thicknesses. Thickness (nm)

Jsc (mA cm−2 )

Calculated Jsc a (mA cm−2 )

Voc (V)

FF

PCE (%)

50 ± 5 60 ± 5 70 ± 5 75 ± 5 85 ± 5

8.1 ± 0.2 9.2 ± 0.2 10.2 ± 0.2 10.3 ± 0.2 10.0 ± 0.2

8.06 9.29 10.34 10.25 10.02

0.90 ± 0.01 0.90 ± 0.01 0.90 ± 0.01 0.90 ± 0.01 0.90 ± 0.01

0.45 ± 0.01 0.44 ± 0.01 0.42 ± 0.01 0.41 ± 0.01 0.38 ± 0.01

3.2 ± 0.2 3.6 ± 0.2 3.8 ± 0.2 3.8 ± 0.2 3.3 ± 0.2

a

The Jsc was calculated from the integration of EQE spectra.

Table 3. Summary of the key parameters in the optimized SubNc:PC70 BM (1:5, 75 nm) BHJ cell evaluated at 25 and 80 ◦ C, respectively. Temperature (◦ C)

Jsc (mA cm−2 )

Calculated Jsc a (mA cm−2 )

Voc (V)

FF

PCE (%)

25 80

10.3 ± 0.2 11.6 ± 0.2

10.25 11.43

0.90 ± 0.01 0.83 ± 0.01

0.41 ± 0.01 0.50 ± 0.01

3.8 ± 0.2 4.8 ± 0.2

a

The Jsc was calculated from the integration of EQE spectra.

Table 4. Comparison of device performance of SubNc:PC70 BM (1:5, 75 nm) and SubNc:C70 (1:5, 75 nm) BHJ cells. Active layer

Jsc (mA cm−2 )

Voc (V)

FF

PCE (%)

SubNc:PC70 BM (1:5, 75 nm) SubNc:C70 (1:5, 75 nm)

10.3 ± 0.2 12.1 ± 0.2

0.90 ± 0.01 0.74 ± 0.01

0.41 ± 0.01 0.47 ± 0.01

3.8 ± 0.2 4.2 ± 0.2

a calculated Jsc within 3% experimental difference from the measured Jsc under simulated solar light.

substrates were transferred back to the high-vacuum chamber where BCP (6 nm) and Al (100 nm) were deposited as the cathode. The active area of cells is 0.09 cm2 defined by the overlap of the ITO anode and Al cathode. BHJ devices with the structure of ITO/MoO3 (5 nm)/SubNc:C70 (1:5, 75 nm)/BCP(6 nm)/Al(100 nm) were also made via thermal co-evaporation of both SubNc and C70 in high vacuum. The active layer had a thickness of 75 nm, considering the optimal thickness of the solution processed devices. To get the average data of device performance (as shown in tables 1–4, S1–S3 available at stacks.iop.org/Nano/24/ 484007/mmedia), two batches of devices (16 cells per batch) were fabricated and tested for each experimental condition of the SubNc:PC70 BM and SubNc:C70 systems. The hole-only and electron-only devices used the structures of ITO/MoO3 /SubNc:PC70 BM (or SubNc:C70 )/MoO3 /Al and ITO/Ca/SubNc:C70 (or SubNc:C70 )/BCP/Al, respectively, to characterize carrier mobility in blend films [8]. Currentdensity–voltage (J–V) and external quantum efficiency (EQE) characterizations of PV cells were carried out on a CEP-2000 integrated system made by Bunkoukeiki Co. Bright-state J–V characteristics were measured under simulated 100 mW cm−2 AM1.5G irradiation from a Xe lamp with an AM1.5 global filter. EQE spectra were collected using a Xe lamp, monochromator, chopper, and lock-in amplifier. The integration of EQE data over AM1.5G solar spectrum yields

3. Results and discussion 3.1. Photophysical properties, energy level and hole mobility of SubNc The optical properties of SubNc in solution and as a thin film were investigated by UV–vis absorption and PL spectra. As depicted in figures 2(a) and S1(a) (available at stacks.iop.org/ Nano/24/484007/mmedia), the absorption of SubNc showed a high molar extinction coefficient (ε = 9.48 × 104 M−1 cm−1 ) in ODCB solution with a strong absorption band at 663 nm. For the SubNc film spin-coated from ODCB solution, the absorption peak in the long wavelength red-shifted to 697 nm, whereas each of the absorption peaks in the short wavelength resembles very closely the absorption spectrum of SubNc in solution, and all peaks are preserved during the transition from a single molecule in dilute solution to the solid state film. This suggests that there are weak intermolecular interactions between non-planar SubNc molecules and thus a low tendency to aggregate in the film [22]. Figure 2(b) shows the absorption spectra of the neat films of SubNc and PC70 BM, and the blend film of 3

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this BHJ cell as shown later; meanwhile the energy level offsets of the LUMO and HOMO between levels of SubNc and PC70 BM (>0.4 eV) are large enough to act as a driving force for efficient exciton dissociation [25]. The hole mobility of SubNc in a solid film was characterized by using a space charge limited current (SCLC) model [26]. A hole-only device with the structure of ITO/MoO3 (5 nm)/SubNc (40 nm)/MoO3 (5 nm)/Al (100 nm) was fabricated and characterized (figure S3 available at stacks. iop.org/Nano/24/484007/mmedia). Here, the SubNc film was prepared by spin-coating SubNc solution in ODCB. The calculated hole mobility of SubNc in thin film is 1.61 × 10−4 cm2 V−1 s−1 . Note that hole mobility of SubNc would increase with increasing temperature, leading to a higher mobility as 2.87 × 10−4 cm2 V−1 s−1 at 80 ◦ C, which is consistent with the improvement in the PCE at elevated temperature as shown later. 3.2. Photovoltaic cells The BHJ device was fabricated by employing a SubNc:PC70 BM blend film as the active layer sandwiched between MoO3 coated ITO and BCP modified Al (as shown in figure 1(a)). The active layer was prepared by spin-coating the SubNc:PC70 BM solution in ODCB, followed by thermal annealing at 120 ◦ C for 10 min. The pre-annealing temperature was selected from 70 to 150 ◦ C. (Figure S4 available at stacks.iop.org/Nano/24/484007/mmedia.) Then the device performance was further optimized by tuning the weight ratio of the donor and acceptor and the thickness of the photoactive layer. As a result, an optimized device based on a SubNc:PC70 BM (1:5, 75 nm) active layer was obtained with a PCE of 4.0% at room temperature and a PCE of 5.0% evaluated at 80 ◦ C. Meanwhile, we also optimized the device by using different hole transporting layers (HTLs); the result shows that the ITO/MoO3 anode is necessary and contributes to a higher Jsc and Voc than the widely used ITO/poly(3,4ethylenedioxythiophene):poly(styrenesulfonicacid)(PEDOT: PSS) anode. The result indicates that for SM based BHJ systems, MoO3 could be a more appropriate anode buffer layer due to its deep work function compared with that of PEDOT:PSS. The effect of thermally annealing the active layer on device performance is shown in figure S4 (available at stacks. iop.org/Nano/24/484007/mmedia). Comparison of the device performance of the BHJ cells based on as-cast or annealed (at 120 ◦ C for 10 min) SubNc:PC70 BM films (1:5, 65 nm) shows that the Jsc increases from 9.14 to 10.10 mA cm−2 , and the FF increases from 0.38 to 0.43 and thus the PCE improves from 3.2% to 4.0%. The absorption spectra of as-cast and annealed SubNc:PC70 BM films are almost the same as shown in figure S1(b) (available at stacks. iop.org/Nano/24/484007/mmedia). The enhancement of the Jsc and FF could be ascribed to phase separation of the donor SubNc and acceptor PC70 BM after thermal annealing, which favors charge transport in a BHJ cell. Meanwhile, the decreased internal series resistance suggests that charge transport is improved [27]. Morphology analysis of the

Figure 2. (a) Normalized UV–vis absorption and PL spectra of SubNc in ODCB solution and as a thin film; (b) normalized UV–vis absorption spectra of SubNc, PC70 BM and blend SQ:PC70 BM (1:5) films.

SubNc:PC70 BM. SubNc exhibits broad absorption both in the near-ultraviolet (NUV, 300–400 nm) and near IR (from 500 nm to 750 nm) regions; PC70 BM displays strong and broad absorption in the NUV and green regions (300–600 nm). Thus, the blend SubNc:PC70 BM film reveals broad absorption covering all of the visible region from 300 to 750 nm, which matches the solar spectrum well for resulting in high photocurrent in the PV cell. The PL spectra of SubNc (figure 2(a)) show emission at 685 nm in solution and a red-shifted emission at 720 nm in a thin film with the excitation of 663 nm and 697 nm, respectively. For a blend film of SubNc:PC70 BM (1:5), we clearly observed strong PL quenching in the solid films. Upon addition of 83 wt% PC70 BM, the PL intensity of the SubNc in the blend film was quenched by ∼90%, as depicted in figure S2 (available at stacks.iop.org/Nano/24/484007/ mmedia). Such dramatic emission quenching indicates highly efficient exciton dissociation occurred at the SubNc/PC70 BM interface [23, 24], which shall benefit charge generation in the photoactive layer. The energy levels of SubNc in the solid state were confirmed as follows: the HOMO level was determined by PYS as −5.3 eV; the optical bandgap Egopt was calculated from Egopt = 1240/λonset as 1.7 eV; and the lowest unoccupied molecular orbital (LUMO) level of SubNc was obtained from LUMO = Egopt +HOMO as −3.6 eV. From the energy diagram of the SubNc:PC70 BM BHJ cell (figure 1(b)), we can observe that the energy levels of the SubNc donor match well with that of the PC70 BM acceptor. The large energy difference between the HOMO level of SubNc and the LUMO of PC70 BM (1.3 eV) results in a high Voc in 4

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Figure 3. AFM topographic and 3D images of a SubNc:PC70 BM (1:5, 65 nm) film: (a) before and (b) after thermal annealing at 120 ◦ C for 10 min.

SubNc:PC70 BM film (1:5, 65 nm) before and after thermal annealing is performed by AFM and XRD measurements. The root-mean-square (RMS) roughness increases from 0.76 nm for the as-cast film to 1.83 nm for the annealed film (figure 3), which strongly suggests phase separation in the annealed film. The rough surface is of benefit to light harvesting and charge collection, which has been observed in several SM based PV systems [11, 28–30]. XRD spectra (figure S5 available at stacks.iop.org/Nano/24/484007/mmedia) indicate that there is no significant crystallinity formation in both of the as-cast and annealed SubNc:PC70 BM films [6, 18]. On the one hand, the amorphous nature of the films is not favorable for charge transport. On the other hand, the small roughness and high uniformity of the films are good to reduce leakage current. Therefore the photovoltaic cells showed high shunt resistance in the dark. In the BHJ cell, it is well known that the blend ratio of the donor and acceptor strongly affects the carrier mobility in the active layer and hence the device performance. Thus, we tuned the blend ratio of SubNc and PC70 BM based on a 60 ± 5 nm-thick active layer and made devices. The J–V characteristics of BHJ cells under illumination of AM1.5G 100 mW cm−2 are shown in figure 4(a) and table 1, EQE spectra are displayed in figure 4(b), and the absorption spectra of 60 ± 5 nm-thick SubNc:PC70 BM films with various blend ratios are shown in figure S6 (available at stacks.iop.org/Nano/24/484007/mmedia). The 1:1 ratio device shows a Jsc of 7.0 ± 0.1 mA cm−2 , aVoc of 0.90 ± 0.01 V, a FF of 0.36 ± 0.01 and a PCE of 2.2 ± 0.1%. The low Jsc is due to the low photoresponse of PC70 BM in the shorter wavelength; meanwhile the low FF may be ascribed to both high serial resistance and strong bias dependent charge recombination [8]. With increasing loading of PC70 BM, the absorption of the blend film in the shorter wavelength increases, which contributes to the

Figure 4. (a) J–V characteristics illuminated at 100 mW cm−2 (AM 1.5G solar spectrum) and (b) EQE spectra of solar cells based on a 60 nm-thick SubNc:PC70 BM blend film with various blend ratios.

photoresponse enhancement in the shorter wavelength of 1:3, 1:5 and 1:7 ratios devices from the EQE spectra. However, the photoresponse in the long wavelength decreases from the 1:3 ratio device to the 1:5 and 1:7 ratio devices due to the decreased absorption of SubNc. The competition of the 5

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Figure 5. (a) J–V characteristics illuminated under 100 mW cm−2 (AM 1.5G solar spectrum) and (b) EQE spectra of SubNc:PC70 BM (1:5) photovoltaic cells with various active layer thicknesses.

Figure 6. Comparison of (a) J–V characteristics and (b) EQE spectra of the optimized SubNc:PC70 BM (1:5, 75 nm) BHJ cell at room temperature (25 ◦ C) and 80 ◦ C.

photoresponse from the contribution of SubNc and PC70 BM leads to a similar Jsc in the 1:3, 1:5 and 1:7 ratio devices. The 1:5 ratio device shows the highest PCE of 3.8% with Jsc 9.4 mA cm−2 , Voc 0.90 and FF 0.44, which should be explained by the balanced hole and electron mobilities in the 1:5 ratio device (table S1 available at stacks.iop.org/Nano/24/484007/ mmedia). To further optimize device performance, we keep the optimized 1:5 ratio and tune the active layer thickness from 50 to 85 nm. The device performance is shown in figure 5 and table 2. From the J–V curves (figure 5(a)) of the devices based on various film thicknesses, we can clearly observe that the Jsc increases whereas FF decreases with the increase of the active layer thickness. The increase of Jsc is attributable to the increased absorption and thus increased photoresponse from both of the SubNc and PC70 BM exhibited in the EQE spectra (figure 5(b)). Meanwhile, the decrease of the FF is ascribed to the larger series resistance in the thicker active layer. However, if the active layer is too thick, such as 85 nm, the Jsc decreases because of the limitation of carrier mobilities in donor and acceptor blend films and hence the serious charge recombination. From the systematic device optimization process, we achieved the optimized PCE of 4.0% with a Jsc of 10.5 mA cm−2 , aVoc of 0.90 V and a FF of 0.42 from a SubNc:PC70 BM (1:5, 75 nm) film based BHJ cell (table 2), which yielded ∼50% enhancement of quantum yield and 60% improvement in PCE compared with the published PHJ PV cells [17]. Considering the

outdoor working conditions of solar cells [31], we tested this optimized device at 80 ◦ C. The PCE of the device increases to 5.0% with a Jsc of 11.8 mA cm−2 , a Voc of 0.84 V and a FF of 0.50 (figure 6, table 3). By using the Jsc values calculated from the integration of EQE spectra (tables 2 and 3), the corrected PCEs of the optimized device at 25 and 80 ◦ C were 3.92% and 4.90%, respectively. These values are consistent with the PV performance characterized under illumination of AM1.5G spectrum, 1 sun as shown in tables 2 and 3. Compared with the device parameters at room temperature (25 ◦ C), both Jsc and FF increase, while Voc slightly decreases at 80 ◦ C. This results in a 25% increase of PCE in the SubNc:PC70 BM (1:5, 75 nm) device. Note that after cooling down to room temperature, the device shows negligible change for all of the photovoltaic parameters in comparison with the pristine device. The absorption spectrum of the blend film remains almost unchanged even at a temperature as high as 120 ◦ C (figure S1(b) available at stacks.iop.org/Nano/24/ 484007/mmedia). The improvement of the efficiency should be explained by the enhancement of the carrier mobilities and thus the improvement of photocurrent extraction at higher temperature (table S1 available at stacks.iop.org/Nano/24/ 484007/mmedia). Although temperature dependent device performance (especially Voc ) has been studied [32, 33], normally the efficiency changes little due to the compromise between Voc and Jsc . Here, the 25% enhancement of efficiency indicates that the SubNc:PC70 BM device has promising potential to make solar panels for practical applications. 6

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Figure 7. Comparison of (a) J–V curves and (b) EQE spectra of SubNc:PC70 BM (1:5, 75 nm) BHJ cells using MoO3 and PEDOT:PSS as the hole transporting layer.

Figure 8. Comparison of (a) J–V curves and (b) EQE spectra of SubNc:PC70 BM (1:5, 75 nm) and SubNc:C70 (1:5, 75 nm) BHJ cells.

We found that the ITO/MoO3 anode also contributed to the efficient performance of the SubNc:PC70 BM BHJ cells. If we employed the widely used PEDOT:PSS as the HTL, the device showed a much lower performance than MoO3 based BHJ cells (figure 7(a), table S2 available at stacks.iop.org/ Nano/24/484007/mmedia). Lower Jsc and lower Voc lead to lower PCE of the PEDOT:PSS based BHJ cell. The higher Jsc in the MoO3 based BHJ cell is ascribed to the higher photoresponse of both SubNc and PC70 BM (figure 7(b)). In our previous work, we have found the Schottky junction between ITO/HTL anode and fullerene to be a minor factor affecting Voc of a BHJ cell [6, 8]. The difference in the work function of ITO/MoO3 anode (−5.7 eV) and ITO/PEDOT:PSS anode (−5.0 eV) may explain the difference of Voc in different HTL based SubNc:PC70 BM BHJ cells [34]. Considering the fact that the SubNc compound can be thermally evaporated in vacuum, we also tested the BHJ device structure based on SubNc and fullerene C70 via thermal co-evaporation. SubNc:C70 cells with a structure of ITO/MoO3 (5 nm)/SubNc:C70 (1:5, 75 nm)/BCP(6 nm)/Al (100 nm) were fabricated, from which we obtained a PCE of 4.2 ± 0.2% with a Jsc of 12.1 ± 0.2 mA cm−2 , Voc of 0.74 ± 0.01 V and a FF of 0.47 ± 0.01 (figure 8(a), table 4). Comparing device performance between SubNc:PC70 BM (1:5, 75 nm) and SubNc:C70 (1:5, 75 nm) BHJ cells, both the Jsc and FF of the thermal co-evaporation processed SubNc:C70 cell were higher than that of the solution processed SubNc:PC70 BM cell. It should be ascribed to more

efficient charge separation and lower series resistance in the SubNc:C70 system. By using SCLC methods [8], the hole and electron mobilities in the thermally co-evaporated SubNc:C70 (1:5) film were characterized as 9.55 × 10−5 and 8.28 × 10−5 cm2 V−1 s−1 , respectively. Note that the electron mobility in the SubNc:C70 (1:5) film was almost four times that in the SubNc:PC70 BM(1:5) film (table S1 available at stacks.iop.org/Nano/24/484007/mmedia), which also contributes to higher Jsc and FF in the SubNc:C70 (1:5) BHJ cell. Yet the Voc of the SubNc:C70 cell is 0.15 V lower than that of the SubNc:PC70 BM case, which is consistent with the LUMO level difference of C70 and PC70 BM. The EQE spectra of vacuum evaporated SubNc:C70 and solution processed SubNc:PC70 BM BHJs are shown in figure 8(b). The photoresponse of C70 is slightly stronger than that of PC70 BM in the visible range, resulting in a higher Jsc for the SubNc:C70 cell. We also tested the SubNc:C70 device at 80 ◦ C. As shown in figure S7 and table S3 (available at stacks.iop.org/Nano/24/484007/mmedia), the SubNc:C70 cell at 80 ◦ C shows a slightly improved Jsc and FF, ∼0.07 V decreased Voc and thus a similar PCE compared with the device performance at 25 ◦ C. It is different from the solution processed SubNc:PC70 BM (1:5, 75 nm) device, where the significant enhancement of Jsc and FF contributed to the 25% increase of PCE at 80 ◦ C. Further improvement in efficiency of the SubNc:C70 BHJ system can be expected via finely tuning the blend ratio and film thickness of the active layer. 7

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4. Conclusion In summary, SubNc was introduced into the BHJ system via both solution and vacuum thermal deposition processes. The SubNc/fullerene donor–acceptor pair combination has intensive absorption covering the near UV and visible range. Due to sufficient donor/acceptor contact in the blend films, the photocurrent was almost doubled in comparison to the PHJ based photovoltaic cells. Therefore, 4.0% of PCE at room temperature, and 5.0% at 80 ◦ C, have been obtained from SubNc:PC70 BM BHJs, which indicates that SubNc is a promising material for photovoltaic applications. Our findings also show that the Voc of SubNc:fullerene bulk heterojunctions is sensitive to the selection of anode buffer. The MoO3 buffer contributes to the large Voc due to its deep work function.

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Acknowledgments This work is financially supported by the Japan Science and Technology Agency (JST) via the Japan Regional Innovation Strategy Program by the Excellence (J-RISE), and Adaptable and Seamless Technology transfer Program (A-STEP, No. AS232Z00929D).

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