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

photovoltaic cells

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2013 Nanotechnology 24 484007

(http://iopscience.iop.org/0957-4484/24/48/484007)

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Nanotechnology 24 (2013) 484007 (9pp) doi:10.1088/0957-4484/24/48/484007

Chloroboron (III) subnaphthalocyanineas an electron donor in bulkheterojunction 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, Japan2 Department of Materials Science and Engineering, University of California-Los Angeles, Los Angeles,CA 90095, USA

E-mail: zrhong@ucla.edu, kid@yz.yamagata-u.ac.jp and yangy@ucla.edu

Received 3 June 2013, in final form 9 July 2013Published 6 November 2013Online at stacks.iop.org/Nano/24/484007

AbstractIn this work, chloroboron (III) subnaphthalocyanine (SubNc) was used as an electron donor,combined with a [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) or fullerene C70acceptor in bulk heterojunction photovoltaic cells. In spite of the limited solubility of SubNcin 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 thetemperature-dependence of the carrier mobilities. SubNc:C70 bulk heterojunctions were alsofabricated via thermal co-evaporation, demonstrating an efficiency of 4.4%. This result showsthat SubNc is a promising material for photovoltaic applications via various processingtechniques, 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

Organic semiconductor material based bulk heterojunction(BHJ) photovoltaic (PV) cells have been considered asa promising approach for making large-size, flexible andlight-weight solar cell devices [1–4]. In a BHJ device, ablend film of electron donor (p-type conjugated polymer orsmall molecule (SM)) and electron acceptor (n-type fullereneand their derivatives) acts as the PV active layer to collectlight and generate electricity. Such a BHJ structure is usefulto overcome the relatively short exciton diffusion length,normally on the order of 10 nm in neat organic films. Toachieve high efficiency BHJ cells, some basic requirementsshould be met for donor and electron materials, such asstrong and broad absorption in the visible and near IR regionfor efficient light harvesting; well matched energy level ofdonor and acceptor material for high open circuit voltage

(Voc) and efficient exciton dissociation; and sufficient carriermobility for charge collection [5, 6]. Recently, SM based BHJcells have underwent great development and demonstratedcompetitive power conversion efficiency (PCE) with polymerBHJs [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 beenrealized [12]. In principle, highly thermal stable materialscan be deposited by using a vacuum co-evaporation processwhile the soluble materials can be spin-coated from solution.In particular, some SM materials possessing high thermalstability and solubility can be deposited from both vacuumco-evaporation and solution processes. For example, severalmerocyanine dyes [13] and a squaraine dye DIB-SQ [6, 8, 11]have been reported as promising materials for both solution-and vacuum co-evaporation processed BHJ cells.

10957-4484/13/484007+09$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 24 (2013) 484007 G Chen et al

Figure 1. (a) Device architecture and (b) schematic energy level diagram of the SubNc:PC70BM bulk heterojunction photovoltaic cell.

In the research field of SM based PV cells, phthalo-cyanine (Pc) and metallophthalocyanine (MPc) have beenwidely used as donor materials since the first efficient CuPcbased planar heterojunction (PHJ) PV cell was reported byTang in 1986 [14–16]. Recently, a non-planar-pyramid-shapedPc-related compound, chloroboron (III) subnaphthalocyanine(SubNc), has been considered as a photoactive material forPV cells due to its excellent physical properties, such asstrong absorption, high thermal stability, moderate solubilityin organic solvents etc [17–19]. One of the significantadvantages of SubNc is that it can be thermally evaporatedin high vacuum, and it is also soluble in organic solvents dueto its non-planar molecular structure. This allows us to makeSubNc thin films via both vacuum evaporation and solutionprocesses. By using SubNc as a donor material, Verreet et almade a vacuum evaporation processed PHJ cell SubNc/C60with an efficiency of 2.5% with a Voc of 0.79 V, a short circuitcurrent density (Jsc) of 6.1 mA cm−2 and a fill factor (FF) of0.49 [17]. Ma et al fabricated a similar SubNc/C60 PHJ cellwith an efficiency of 1.5% by using a spin-coating processto make the SubNc layer [18]. More recently, photovoltaiccells based on SubNc/C60 PHJ reached a PCE of over3% [19]. Nevertheless, SubNc is a promising material, whichis expected to generate a PCE of over 6%, considering itsbandgap and highest occupied molecular orbital (HOMO)level. Yet only the PHJ device structure was used inprevious works, limiting the exciton collection efficiencydue to insufficient donor/acceptor interface. Therefore inthis work, we explore the possibility to incorporate SubNcinto a BHJ device structure in view of the fact that thelarge donor–acceptor interface in the BHJ active layerresults in efficient charge carrier separation and hence highphotocurrent [20]. SubNc has strong absorption ranging from600 to 730 nm. We thus used it as an electron donor, combinedwith [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) orfullerene C70, which cover the blue and green region, as anelectron acceptor to make BHJ cells. As a result, efficientlight harvesting throughout the visible and near IR rangewas achieved, and ∼50% enhancement of quantum yield wasrealized, in comparison with the published PHJ cells.

2. Experimental details

2.1. Materials

Patterned indium–tin–oxide (ITO) coated glass substrates,SubNc, PC70BM and C70 were purchased from Luminescence

Technology Corp.; MoO3 (purity: 99.99%) and bathocuproine(BCP) (sublimated, purity: 99.99%) were purchased fromAldrich Chemical Co. and Dojindo Laboratories, respectively.SubNc and C70 were purified two times by vacuumsublimation, and the high purity MoO3 and BCP were usedas received. ITO coated glass substrates were cleaned usingdetergent, de-ionized water, acetone and isopropanol in anultrasonic bath successively, and then dried in an oven at 80 ◦Cfor 12 h.

2.2. Material characterization

UV–visible (UV–vis) absorption spectra were obtainedusing a SHIMADZU MPC-2200 UV–vis spectrophotometer.Photoluminescence (PL) spectra were measured using aFluoroMax-2 (Jobin-Yvon–Spex) luminescence spectrometer.The solution for UV–vis absorption and PL measurement wasat a concentration of 1 × 10−5 mol l−1 in o-dichlorobenzene(ODCB). Thin films for UV–vis absorption and PL spectrawere prepared by spin-coating solution on quartz substrate.The HOMO level was determined by photoelectron yieldspectroscopy (PYS) under vacuum (∼10−3 Pa) [21]. X-raydiffraction (XRD) pattern for films were collected using ahigh-resolution XRD diffractometer (SmartLab, Rigaku Co.).The films for PYS and XRD measurements were preparedby spin-coating solution on ITO substrates. Atomic forcemicroscopy (AFM) images were collected in air on a VeecoAFM using a tapping mode. The films for AFM measurementwere prepared by spin-coating SubNc:PC70BM solution onthe MoO3 coated ITO substrates.

2.3. Device fabrication and characterization

The BHJ cells adopting the device structure of ITO/MoO3/SubNc:PC70BM/BCP/Al (figure 1(a)) were fabricated asfollows: substrates were exposed to UV ozone for 30 min andimmediately transferred into a high-vacuum (1 × 10−6 Pa)chamber for deposition of 5 nm MoO3. Photoactive layerswere fabricated via spin-coating SubNc:PC70BM solution (inODCB) on the MoO3 coated ITO surface in a N2-filled glovebox, and the film thickness was tuned via controlling solutionconcentration (the total concentration of the mixed SubNc andPC70BM from 30 to 42 mg ml−1) and spin-coating speed(600–2000 rpm min−1). Then the film was thermally treatedat 120 ◦C for 10 min to remove solvent residue. Finally, the

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Table 1. Summary of the key parameters in 60± 5 nm-thick SubNc:PC70BM BHJ cells with various blend ratios.

Weight ratio Jsc (mA cm−2) Calculated Jsca (mA cm−2) Voc (V) FF PCE (%)

1:1 7.0 ± 0.1 6.88 0.90 ± 0.01 0.36 ± 0.01 2.2 ± 0.11:3 8.7 ± 0.2 8.63 0.90 ± 0.01 0.44 ± 0.01 3.3 ± 0.21:5 9.2 ± 0.2 9.29 0.90 ± 0.01 0.44 ± 0.01 3.6 ± 0.21:7 9.0 ± 0.2 9.22 0.90 ± 0.01 0.43 ± 0.01 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:PC70BM (1:5) BHJ cells with various active layer thicknesses.

Thickness (nm) Jsc (mA cm−2) Calculated Jsca (mA cm−2) Voc (V) FF PCE (%)

50 ± 5 8.1 ± 0.2 8.06 0.90 ± 0.01 0.45 ± 0.01 3.2 ± 0.260 ± 5 9.2 ± 0.2 9.29 0.90 ± 0.01 0.44 ± 0.01 3.6 ± 0.270 ± 5 10.2 ± 0.2 10.34 0.90 ± 0.01 0.42 ± 0.01 3.8 ± 0.275 ± 5 10.3 ± 0.2 10.25 0.90 ± 0.01 0.41 ± 0.01 3.8 ± 0.285 ± 5 10.0 ± 0.2 10.02 0.90 ± 0.01 0.38 ± 0.01 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:PC70BM (1:5, 75 nm) BHJ cell evaluated at 25 and 80 ◦C, respectively.

Temperature (◦C) Jsc (mA cm−2) Calculated Jsca (mA cm−2) Voc (V) FF PCE (%)

25 10.3 ± 0.2 10.25 0.90 ± 0.01 0.41 ± 0.01 3.8 ± 0.280 11.6 ± 0.2 11.43 0.83 ± 0.01 0.50 ± 0.01 4.8 ± 0.2

a The Jsc was calculated from the integration of EQE spectra.

Table 4. Comparison of device performance of SubNc:PC70BM (1:5, 75 nm) and SubNc:C70 (1:5, 75 nm) BHJ cells.

Active layer Jsc (mA cm−2) Voc (V) FF PCE (%)

SubNc:PC70BM (1:5, 75 nm) 10.3 ± 0.2 0.90 ± 0.01 0.41 ± 0.01 3.8 ± 0.2SubNc:C70 (1:5, 75 nm) 12.1 ± 0.2 0.74 ± 0.01 0.47 ± 0.01 4.2 ± 0.2

substrates were transferred back to the high-vacuum chamberwhere BCP (6 nm) and Al (100 nm) were deposited as thecathode. The active area of cells is 0.09 cm2 defined bythe overlap of the ITO anode and Al cathode. BHJ deviceswith the structure of ITO/MoO3(5 nm)/SubNc:C70(1:5,75 nm)/BCP(6 nm)/Al(100 nm) were also made via thermalco-evaporation of both SubNc and C70 in high vacuum.The active layer had a thickness of 75 nm, consideringthe optimal thickness of the solution processed devices.To get the average data of device performance (as shownin tables 1–4, S1–S3 available at stacks.iop.org/Nano/24/484007/mmedia), two batches of devices (16 cells perbatch) were fabricated and tested for each experimentalcondition of the SubNc:PC70BM and SubNc:C70 systems.The hole-only and electron-only devices used the structuresof ITO/MoO3/SubNc:PC70BM (or SubNc:C70)/MoO3/Al andITO/Ca/SubNc:C70 (or SubNc:C70)/BCP/Al, respectively, tocharacterize carrier mobility in blend films [8]. Current-density–voltage (J–V) and external quantum efficiency (EQE)characterizations of PV cells were carried out on a CEP-2000integrated system made by Bunkoukeiki Co. Bright-state J–Vcharacteristics were measured under simulated 100 mW cm−2

AM1.5G irradiation from a Xe lamp with an AM1.5global filter. EQE spectra were collected using a Xelamp, monochromator, chopper, and lock-in amplifier. Theintegration of EQE data over AM1.5G solar spectrum yields

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

3. Results and discussion

3.1. Photophysical properties, energy level and hole mobilityof SubNc

The optical properties of SubNc in solution and as a thin filmwere investigated by UV–vis absorption and PL spectra. Asdepicted in figures 2(a) and S1(a) (available at stacks.iop.org/Nano/24/484007/mmedia), the absorption of SubNc showed ahigh 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, theabsorption peak in the long wavelength red-shifted to 697 nm,whereas each of the absorption peaks in the short wavelengthresembles very closely the absorption spectrum of SubNc insolution, and all peaks are preserved during the transition froma single molecule in dilute solution to the solid state film.This suggests that there are weak intermolecular interactionsbetween non-planar SubNc molecules and thus a low tendencyto aggregate in the film [22].

Figure 2(b) shows the absorption spectra of the neatfilms of SubNc and PC70BM, and the blend film of

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Figure 2. (a) Normalized UV–vis absorption and PL spectra ofSubNc in ODCB solution and as a thin film; (b) normalized UV–visabsorption spectra of SubNc, PC70BM and blend SQ:PC70BM (1:5)films.

SubNc:PC70BM. SubNc exhibits broad absorption both inthe near-ultraviolet (NUV, 300–400 nm) and near IR (from500 nm to 750 nm) regions; PC70BM displays strongand broad absorption in the NUV and green regions(300–600 nm). Thus, the blend SubNc:PC70BM film revealsbroad absorption covering all of the visible region from 300 to750 nm, which matches the solar spectrum well for resultingin high photocurrent in the PV cell.

The PL spectra of SubNc (figure 2(a)) show emissionat 685 nm in solution and a red-shifted emission at 720 nmin a thin film with the excitation of 663 nm and 697 nm,respectively. For a blend film of SubNc:PC70BM (1:5), weclearly observed strong PL quenching in the solid films.Upon addition of 83 wt% PC70BM, the PL intensity of theSubNc in the blend film was quenched by ∼90%, as depictedin figure S2 (available at stacks.iop.org/Nano/24/484007/mmedia). Such dramatic emission quenching indicates highlyefficient exciton dissociation occurred at the SubNc/PC70BMinterface [23, 24], which shall benefit charge generation in thephotoactive layer.

The energy levels of SubNc in the solid state wereconfirmed as follows: the HOMO level was determinedby PYS as −5.3 eV; the optical bandgap Egopt wascalculated from Egopt

= 1240/λonset as 1.7 eV; and the lowestunoccupied molecular orbital (LUMO) level of SubNc wasobtained from LUMO = Egopt

+HOMO as−3.6 eV. From theenergy diagram of the SubNc:PC70BM BHJ cell (figure 1(b)),we can observe that the energy levels of the SubNc donormatch well with that of the PC70BM acceptor. The largeenergy difference between the HOMO level of SubNc andthe LUMO of PC70BM (1.3 eV) results in a high Voc in

this BHJ cell as shown later; meanwhile the energy leveloffsets of the LUMO and HOMO between levels of SubNcand PC70BM (>0.4 eV) are large enough to act as a drivingforce for efficient exciton dissociation [25].

The hole mobility of SubNc in a solid film wascharacterized by using a space charge limited current (SCLC)model [26]. A hole-only device with the structure ofITO/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 wasprepared by spin-coating SubNc solution in ODCB. Thecalculated hole mobility of SubNc in thin film is 1.61 ×10−4 cm2 V−1 s−1. Note that hole mobility of SubNc wouldincrease with increasing temperature, leading to a highermobility as 2.87 × 10−4 cm2 V−1 s−1 at 80 ◦C, whichis consistent with the improvement in the PCE at elevatedtemperature as shown later.

3.2. Photovoltaic cells

The BHJ device was fabricated by employing a SubNc:PC70BM blend film as the active layer sandwiched betweenMoO3 coated ITO and BCP modified Al (as shown infigure 1(a)). The active layer was prepared by spin-coatingthe SubNc:PC70BM solution in ODCB, followed by thermalannealing at 120 ◦C for 10 min. The pre-annealing temper-ature was selected from 70 to 150 ◦C. (Figure S4 availableat stacks.iop.org/Nano/24/484007/mmedia.) Then the deviceperformance was further optimized by tuning the weightratio of the donor and acceptor and the thickness of thephotoactive layer. As a result, an optimized device based ona SubNc:PC70BM (1:5, 75 nm) active layer was obtainedwith a PCE of 4.0% at room temperature and a PCE of 5.0%evaluated at 80 ◦C. Meanwhile, we also optimized the deviceby using different hole transporting layers (HTLs); the resultshows that the ITO/MoO3 anode is necessary and contributesto a higher Jsc and Voc than the widely used ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonicacid)(PEDOT:PSS) anode. The result indicates that for SM based BHJsystems, MoO3 could be a more appropriate anode bufferlayer due to its deep work function compared with that ofPEDOT:PSS.

The effect of thermally annealing the active layer ondevice performance is shown in figure S4 (available at stacks.iop.org/Nano/24/484007/mmedia). Comparison of the deviceperformance of the BHJ cells based on as-cast or annealed(at 120 ◦C for 10 min) SubNc:PC70BM 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 thePCE improves from 3.2% to 4.0%. The absorption spectraof as-cast and annealed SubNc:PC70BM films are almostthe same as shown in figure S1(b) (available at stacks.iop.org/Nano/24/484007/mmedia). The enhancement of theJsc and FF could be ascribed to phase separation of thedonor SubNc and acceptor PC70BM after thermal annealing,which favors charge transport in a BHJ cell. Meanwhile,the decreased internal series resistance suggests that chargetransport is improved [27]. Morphology analysis of the

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Figure 3. AFM topographic and 3D images of a SubNc:PC70BM (1:5, 65 nm) film: (a) before and (b) after thermal annealing at 120 ◦C for10 min.

SubNc:PC70BM film (1:5, 65 nm) before and after thermalannealing is performed by AFM and XRD measurements. Theroot-mean-square (RMS) roughness increases from 0.76 nmfor 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 chargecollection, which has been observed in several SM basedPV systems [11, 28–30]. XRD spectra (figure S5 availableat stacks.iop.org/Nano/24/484007/mmedia) indicate that thereis no significant crystallinity formation in both of the as-castand annealed SubNc:PC70BM films [6, 18]. On the one hand,the amorphous nature of the films is not favorable for chargetransport. On the other hand, the small roughness and highuniformity of the films are good to reduce leakage current.Therefore the photovoltaic cells showed high shunt resistancein the dark.

In the BHJ cell, it is well known that the blend ratio ofthe donor and acceptor strongly affects the carrier mobilityin the active layer and hence the device performance. Thus,we tuned the blend ratio of SubNc and PC70BM basedon a 60 ± 5 nm-thick active layer and made devices.The J–V characteristics of BHJ cells under illumination ofAM1.5G 100 mW cm−2 are shown in figure 4(a) andtable 1, EQE spectra are displayed in figure 4(b), andthe absorption spectra of 60 ± 5 nm-thick SubNc:PC70BMfilms with various blend ratios are shown in figure S6(available at stacks.iop.org/Nano/24/484007/mmedia). The1:1 ratio device shows a Jsc of 7.0 ± 0.1 mA cm−2, aVocof 0.90 ± 0.01 V, a FF of 0.36 ± 0.01 and a PCE of2.2 ± 0.1%. The low Jsc is due to the low photoresponseof PC70BM in the shorter wavelength; meanwhile the lowFF may be ascribed to both high serial resistance and strongbias dependent charge recombination [8]. With increasingloading of PC70BM, the absorption of the blend film inthe 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 basedon a 60 nm-thick SubNc:PC70BM blend film with various blendratios.

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 fromthe 1:3 ratio device to the 1:5 and 1:7 ratio devices due tothe decreased absorption of SubNc. The competition of the

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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:PC70BM(1:5) photovoltaic cells with various active layer thicknesses.

photoresponse from the contribution of SubNc and PC70BMleads to a similar Jsc in the 1:3, 1:5 and 1:7 ratio devices. The1:5 ratio device shows the highest PCE of 3.8% with Jsc 9.4mA cm−2, Voc 0.90 and FF 0.44, which should be explainedby the balanced hole and electron mobilities in the 1:5 ratiodevice (table S1 available at stacks.iop.org/Nano/24/484007/mmedia).

To further optimize device performance, we keep theoptimized 1:5 ratio and tune the active layer thickness from50 to 85 nm. The device performance is shown in figure 5and table 2. From the J–V curves (figure 5(a)) of the devicesbased on various film thicknesses, we can clearly observe thatthe Jsc increases whereas FF decreases with the increase ofthe active layer thickness. The increase of Jsc is attributableto the increased absorption and thus increased photoresponsefrom both of the SubNc and PC70BM exhibited in the EQEspectra (figure 5(b)). Meanwhile, the decrease of the FF isascribed to the larger series resistance in the thicker activelayer. However, if the active layer is too thick, such as85 nm, the Jsc decreases because of the limitation of carriermobilities in donor and acceptor blend films and hence theserious charge recombination. From the systematic deviceoptimization process, we achieved the optimized PCE of4.0% with a Jsc of 10.5 mA cm−2, aVoc of 0.90 V anda FF of 0.42 from a SubNc:PC70BM (1:5, 75 nm) filmbased BHJ cell (table 2), which yielded ∼50% enhancementof quantum yield and 60% improvement in PCE comparedwith the published PHJ PV cells [17]. Considering the

Figure 6. Comparison of (a) J–V characteristics and (b) EQEspectra of the optimized SubNc:PC70BM (1:5, 75 nm) BHJ cell atroom temperature (25 ◦C) and 80 ◦C.

outdoor working conditions of solar cells [31], we tested thisoptimized device at 80 ◦C. The PCE of the device increases to5.0% with a Jsc of 11.8 mA cm−2, a Voc of 0.84 V and a FFof 0.50 (figure 6, table 3). By using the Jsc values calculatedfrom the integration of EQE spectra (tables 2 and 3), thecorrected PCEs of the optimized device at 25 and 80 ◦C were3.92% and 4.90%, respectively. These values are consistentwith the PV performance characterized under illuminationof 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 decreasesat 80 ◦C. This results in a 25% increase of PCE in theSubNc:PC70BM (1:5, 75 nm) device. Note that after coolingdown to room temperature, the device shows negligiblechange for all of the photovoltaic parameters in comparisonwith the pristine device. The absorption spectrum of the blendfilm remains almost unchanged even at a temperature as highas 120 ◦C (figure S1(b) available at stacks.iop.org/Nano/24/484007/mmedia). The improvement of the efficiency shouldbe explained by the enhancement of the carrier mobilities andthus the improvement of photocurrent extraction at highertemperature (table S1 available at stacks.iop.org/Nano/24/484007/mmedia). Although temperature dependent deviceperformance (especially Voc) has been studied [32, 33],normally the efficiency changes little due to the compromisebetween Voc and Jsc. Here, the 25% enhancement of efficiencyindicates that the SubNc:PC70BM device has promisingpotential to make solar panels for practical applications.

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Figure 7. Comparison of (a) J–V curves and (b) EQE spectra ofSubNc:PC70BM (1:5, 75 nm) BHJ cells using MoO3 andPEDOT:PSS as the hole transporting layer.

We found that the ITO/MoO3 anode also contributed tothe efficient performance of the SubNc:PC70BM BHJ cells. Ifwe employed the widely used PEDOT:PSS as the HTL, thedevice showed a much lower performance than MoO3 basedBHJ cells (figure 7(a), table S2 available at stacks.iop.org/Nano/24/484007/mmedia). Lower Jsc and lower Voc lead tolower PCE of the PEDOT:PSS based BHJ cell. The higherJsc in the MoO3 based BHJ cell is ascribed to the higherphotoresponse of both SubNc and PC70BM (figure 7(b)).In our previous work, we have found the Schottky junctionbetween ITO/HTL anode and fullerene to be a minorfactor affecting Voc of a BHJ cell [6, 8]. The differencein the work function of ITO/MoO3 anode (−5.7 eV) andITO/PEDOT:PSS anode (−5.0 eV) may explain the differenceof Voc in different HTL based SubNc:PC70BM BHJ cells [34].

Considering the fact that the SubNc compound can bethermally evaporated in vacuum, we also tested the BHJdevice structure based on SubNc and fullerene C70 viathermal co-evaporation. SubNc:C70 cells with a structureof ITO/MoO3(5 nm)/SubNc:C70(1:5, 75 nm)/BCP(6 nm)/Al(100 nm) were fabricated, from which we obtained a PCEof 4.2 ± 0.2% with a Jsc of 12.1 ± 0.2 mA cm−2,Voc of0.74 ± 0.01 V and a FF of 0.47 ± 0.01 (figure 8(a), table 4).Comparing device performance between SubNc:PC70BM(1:5, 75 nm) and SubNc:C70 (1:5, 75 nm) BHJ cells, boththe Jsc and FF of the thermal co-evaporation processedSubNc:C70 cell were higher than that of the solutionprocessed SubNc:PC70BM cell. It should be ascribed to more

Figure 8. Comparison of (a) J–V curves and (b) EQE spectra ofSubNc:PC70BM (1:5, 75 nm) and SubNc:C70 (1:5, 75 nm) BHJcells.

efficient charge separation and lower series resistance inthe SubNc:C70 system. By using SCLC methods [8], thehole and electron mobilities in the thermally co-evaporatedSubNc:C70 (1:5) film were characterized as 9.55 × 10−5

and 8.28 × 10−5 cm2 V−1 s−1, respectively. Note that theelectron mobility in the SubNc:C70 (1:5) film was almostfour times that in the SubNc:PC70BM(1:5) film (table S1available at stacks.iop.org/Nano/24/484007/mmedia), whichalso 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 lowerthan that of the SubNc:PC70BM case, which is consistentwith the LUMO level difference of C70 and PC70BM. TheEQE spectra of vacuum evaporated SubNc:C70 and solutionprocessed SubNc:PC70BM BHJs are shown in figure 8(b).The photoresponse of C70 is slightly stronger than that ofPC70BM in the visible range, resulting in a higher Jsc forthe SubNc:C70 cell. We also tested the SubNc:C70 deviceat 80 ◦C. As shown in figure S7 and table S3 (available atstacks.iop.org/Nano/24/484007/mmedia), the SubNc:C70 cellat 80 ◦C shows a slightly improved Jsc and FF, ∼0.07 Vdecreased Voc and thus a similar PCE compared with thedevice performance at 25 ◦C. It is different from the solutionprocessed SubNc:PC70BM (1:5, 75 nm) device, where thesignificant enhancement of Jsc and FF contributed to the 25%increase of PCE at 80 ◦C. Further improvement in efficiencyof the SubNc:C70 BHJ system can be expected via finelytuning the blend ratio and film thickness of the active layer.

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4. Conclusion

In summary, SubNc was introduced into the BHJ systemvia both solution and vacuum thermal deposition processes.The SubNc/fullerene donor–acceptor pair combination hasintensive 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 thePHJ based photovoltaic cells. Therefore, 4.0% of PCE atroom temperature, and 5.0% at 80 ◦C, have been obtainedfrom SubNc:PC70BM BHJs, which indicates that SubNcis a promising material for photovoltaic applications. Ourfindings also show that the Voc of SubNc:fullerene bulkheterojunctions is sensitive to the selection of anode buffer.The MoO3 buffer contributes to the large Voc due to its deepwork function.

Acknowledgments

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

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