Thickness Effect of Bulk Heterojunction Layers on the Performance

Research Article pubs.acs.org/journal/ascecg

Thickness Effect of Bulk Heterojunction Layers on the Performance and Stability of Polymer:Fullerene Solar Cells with Alkylthiothiophene-Containing Polymer Sungho Nam,†,‡ Myeonghun Song,† Hwajeong Kim,†,§ Donal D. C. Bradley,*,‡,∥ and Youngkyoo Kim*,† †

Organic Nanoelectronics Laboratory and KNU Institute for Nanophotonics Applications, Department of Chemical Engineering, School of Applied Chemical Engineering, Kyungpook National University, Daegu 41566, Republic of Korea ‡ Departments of Physics, Division of Mathematical, Physical and Life Sciences, University of Oxford, Oxford OX1 3PU, United Kingdom § Priority Research Center, Research Institute of Advanced Energy Technology, Kyungpook National University, Daegu 41566, Republic of Korea ∥ Departments of Engineering Science, Division of Mathematical, Physical and Life Sciences, University of Oxford, Oxford OX1 3PJ, United Kingdom S Supporting Information *

ABSTRACT: We report a pronounced thickness effect of bulk heterojunction (BHJ) layers on the performance and stability of inverted polymer solar cells with the BHJ layers of poly[(4,8-bis(5-(octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4- b]thiophene-2-carboxylate] (PBDT-TS1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The thickness of the BHJ layers was varied from 40 to 120 nm by changing solution concentrations and spincoating speeds. The results showed that the film thickness considerably affected the performance and stability of devices. The power conversion efficiency reached ca. 9% at the thickness of 80 nm by the optimized nanoscale phase separation between donor and acceptor components. However, the devices with 120 nm-thick BHJ layers showed better device stability under continuous illumination with a simulated solar light due to the well-maintained surface morphology and nanostructure in addition to the improved morphological volume stability. KEYWORDS: Thickness, Bulk heterojunction, Device stability, Morphology, Volume stability

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INTRODUCTION

PSCs have had active layer thicknesses ranging from 80−150 nm.18,25,26 Despite significant effort being focused on PCE improvement, the device stability/degradation issues have not yet been fully resolved. The degradation stems from thermal instability of the nanostructure/morphology for BHJ layers, photooxidation of polymers, and chemical oxidation at the metal interface.4,27−30 The PSC degradation mechanism is rather complicated and multiple factors for degradation are combined. Therefore, critical issues affecting PSC device stability enhancement should be resolved for PSC commercialization. Of various electron-donating polymers for PSCs, poly[(4,8bis(5-(octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiopheneco-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PBDT-TS1) has attracted keen interest because the two-dimensional structure of benzodithiophene (BDT) and thienothiophene (TT) units in the backbone and the presence of linear alkylthio

Polymer solar cells (PSCs) with bulk heterojunction (BHJ) layers consisting of electron-donating and -accepting materials have been extensively studied for renewable energy sources because of their intriguing features including flexibility, lightweight, and solution-based fabrication.1−6 Recently, power conversion efficiencies (PCEs) of PSCs have reached up to 12% for single-stack cells.7−10 This is attributed to extensive studies on morphology control via thermal/solvent annealing, addition of additives, interface engineering of chargecollecting buffer layers, newly synthesized electron-donating/ accepting materials, and a comprehensive understanding of device operation mechanisms in PSCs.11−22 One of the key challenges for improving solar cell efficiency is to find an optimal BHJ layer thickness, which should be limited to enable the separated charges to reach their respective electrodes.23,24 There is a compromise between strong light absorption and efficient charge transport/collection because of the relatively low charge carrier mobilities and short exciton diffusion lengths (∼10 nm). Recently, most of the efficient © 2017 American Chemical Society

Received: July 5, 2017 Published: August 11, 2017 9263

DOI: 10.1021/acssuschemeng.7b02238 ACS Sustainable Chem. Eng. 2017, 5, 9263−9270

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) Chemical structure of materials and device structure for inverted polymer solar cells with the PBDT-TS1:PC71BM BHJ layer. (b) Flat energy band diagram and (c) cross-sectional FIB-prepared TEM image for the inverted polymer solar cells with the PBDT-TS1:PC71BM layers: note that p-interfaces between BHJ and MoO3 layers and n-interfaces between BHJ and ZnO layers can be made in the present devices. precursor solutions (0.45 M) were prepared by dissolving zinc acetate dihydrate and ethanolamine in 2-methoxyethanol with stirring at 60 °C for 3 h and then at room temperature before spin- coating. PBDTTS1:PC71BM (1:1.5 by weight) solutions were prepared at solid concentrations of 20, 25, and 30 mg/mL with a mixture of chlorobenzene and DIO (97:3 v/v). The resulting mixture was subjected to vigorous stirring on a magnetic stirring plate at room temperature prior to spin-coating. Thin Film and Device Fabrication. Patterned ITO-coated glass substrates were cleaned using acetone and isopropanol in an ultrasonic bath, followed by drying with a nitrogen flow. The ZnO layers (∼40 nm) were spin-coated onto the ITO-coated glass substrates and baked at 200 °C for 1 h. Next, the PBDT-TS1:PC71BM solutions were spincoated on top of the ZnO layers and dried at room temperature for 1 h inside a nitrogen-filled glovebox. The active layer thickness was varied by changing both solution concentration and spin-coating speed (see Table S1). All samples were transferred into a vacuum chamber inside an argon-filled glovebox. MoO3 (∼10 nm) and Ag (∼80 nm) were sequentially deposited on top of the BHJ layers through a shadow mask at ∼3 × 10−6 Torr, defining the active area of 0.055 cm2. The hole-only and electron-only devices were fabricated with ITO/ PEDOT:PSS/PBDT-TS1:PC 71 BM/MoO 3 /Ag and ITO/ZnO/ PBDT-TS1:PC71BM/Al, respectively. Measurements. Film thicknesses were measured using a surface profiler (Alpha Step 200, Tencor Instruments). Optical absorption and PL spectra of BHJ layers with different thicknesses were measured using a UV−vis spectrophotometer (Optizen 2120, MECASYS) and a PL spectrometer (FluoroMate FS-2, SCINCO), respectively. The solar cells were characterized under calibrated AM 1.5 G illumination (at 100 mW cm−2) using a solar cell measurement system equipped with a sourcemeter (Model 2400, Keithley) and a solar simulator (92250A1000, Newport-Oriel). The EQE spectra of the solar cells were measured using an EQE measurement system equipped with a light source (tungsten-halogen lamp (150 W), ASBN-W, Spectral Products) and a monochromator (CM110, Spectral Products) controlled by the Labview program. The nanostructures of the film samples were

chains in the BDT unit demonstrated the excellent photovoltaic properties such as broad absorption band, strong intermolecular interaction to facilitate the charge transport, and lower highest occupied molecular orbital (HOMO) energy level resulting in outstanding PCE near 10%.25,26,31,32 To date, the previous works focused on the normal-type PBDT-TS1:fullerene solar cells.25,26,33−36 However, less attention has been paid to the performance and stability of the inverted-type PBDTTS1:fullerene solar cells according to the varied thickness of the PBDT-TS1:fullerene layers even though the device performance and stability could be significantly influenced by the thickness of active layers.23,29,37−40 In this work, we fabricated the inverted-type PBDT-TS1: [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) PSCs by varying the thickness of active layers from 40 to 120 nm and investigated the correlation between active layer thickness and device performance/stability. Optimal solar cell performance was obtained with a film thickness of 80 nm because of nanoscale phase-separated percolating pathways between the PBDT-TS1 and PC71BM materials. This led to favorable charge transport and collection. Finally, device stability was tested under continuous illumination with simulated solar light (100 mW cm−2). Devices with film thickness of 120 nm exhibited relatively longer lifetimes due to the subtle change in surface morphology and nanostructure and a more stable in the bulk, compared to 40 and 80 nm-thick devices.

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EXPERIMENTAL SECTION

Materials and Solutions. PBDT-TS1 polymer (weight-average molecular weight = 28.9 kDa, polydispersity index, PDI = 2.07) and PC71BM (purity >99%) were purchased from Orgatec Materials Inc. and Nano-C, respectively. Zinc acetate dihydrate (purity >99%) and 1,8-diiodooctane (DIO) were supplied from Sigma-Aldrich. Zinc oxide 9264


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ACS Sustainable Chemistry & Engineering measured using a field-emission/scanning transmission electron microscope (FE-TEM/STEM, Titan G2 ChemiSTEM Cs Probe, FEI Company). Surface morphologies of the thin films were measured using AFM (Nanoscope V Multimode 8, Bruker), while the nanostructure of the film samples was measured using a synchrotron radiation grazing incidence X-ray diffraction (GIXD) system (X-ray wavelength = 1.2807 Å, incidence angle = 0.12°, 3C SAXS I beamline, Pohang Accelerator Laboratory). The depth profiles of atomic distribution were measured using TOF-SIMS (TOF-SIMS 5-100, ION-TOF) with a 30 keV ion beam energy (Bi+ primary ions), a 1.07 pA ion beam current, a 100 μm × 100 μm sampling area for the analysis, an energy of 0.5 keV Cs+, and a 36 nA current over a 300 μm × 300 μm area.

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RESULTS AND DISCUSSION Inverted-type PBDT-TS1:PC71BM solar cells with different film thicknesses (40, 60, 80, 100, and 120 nm) achieved by varying the solution concentration and spin-coating speed were fabricated (Figure 1a). Based on the flat energy band diagram in Figure 1b, we can expect that charge separation occurs at the interfaces between PBDT-TS1 and PC71BM domains. This is supported by the photoluminescence (PL) spectra of PBDTTS1:PC71BM blend films with different thicknesses (Figure S1). Discrete p-interfaces (BHJ−MoO3 layers) and n-interfaces (BHJ−ZnO layers) were observed from the cross-sectional focused ion beam (FIB)-prepared transmission electron microscope (TEM) image for the device in Figure 1c. It is expected that separated holes and electrons should be transported to the anode and cathode through MoO3 holecollecting and ZnO electron-collecting buffer layers, respectively. The current density−voltage (J−V) curves under illumination with simulated solar light (air mass (AM) 1.5 G, 100 mW cm−2) are shown in Figure 2a. Open circuit voltage (VOC) values are less relevant to the active layer thickness, while shortcircuit current density (JSC) values are significantly dependent on film thickness. Devices with an active layer thickness of ∼80 nm exhibit the highest JSC of 17.53 mA/cm2, whereas significantly lower values are observed for 40 and 120 nmthick devices (14.24 and 16.39 mA/cm2, respectively). For 80 nm-thick devices, the highest JSC is attributed to the relatively fast charge transport and suppressed charge recombination based on not only the lowest leakage J at reverse bias (see from −1 to 0 V for clarity), but also the relatively high forward J at 1 V from the dark J−V curves (Figure S2). In order to gain more insight into the charge transport properties in the BHJ layers with different thicknesses, the hole and electron mobilities were extracted using the Mott−Gurney law from the space-chargelimited current (SCLC) for corresponding hole-only and electron-only devices with the different film thicknesses (Figure 2b). The hole and electron mobilities (μh and μe) for the 80 nm-thick devices were 1.32 × 10−4 and 1.28 × 10−4 cm2 V−1 s−1, respectively. These results indicate that the well-balanced hole and electron transport in the 80 nm-thick devices (μh/μe = 1.03) can contribute to the higher JSC because of the minimized charge recombination. However, the devices with the 40 nmthick layers exhibited relatively slow charge transport (μh = 1.90 × 10−5 and μe = 1.79 × 10−5 cm2 V−1 s−1), while the unbalanced charge transport was measured for the devices with the 120 nm-thick BHJ layers (μh/μe = 0.67). Furthermore, the external quantum efficiency (EQE) spectra (Figures 2c and S3a) of 80 nm-thick devices exhibit higher EQE values (>65% at wavelengths 450−750 nm), while the EQE values for thinner films (80 nm) are relatively low

Figure 2. (a) Current density−voltage (J−V) curves for the inverted PBDT-TS1:PC71BM solar cells according to different film thicknesses (40, 60, 80, 100, and 120 nm) under one-sun condition (AM 1.5 G, 100 mW cm−2). (b) Hole and electron mobilities for the BHJ layers with different film thicknesses of 40, 80, and 120 nm, which were extracted from the space-charge-limited currents of hole-only and electron-only devices. (c) EQE spectra of the inverted PBDTTS1:PC71BM solar cells. (d) Optical absorption spectra of the BHJ layers with different film thicknesses of 40, 80, and 120 nm. (inset) Corresponding photographs for the PBDT-TS1:PC71BM films.

(Figure S3b) (here we note that the obtained JSC values were almost equivalent to the integration of the EQE spectra). It is noteworthy that the well-balanced charge transport for 80 nmthick BHJ layers are beneficial to higher EQE values. However, the relatively low EQE values for the thinner films (80 nm) could significantly impact the relatively low EQE values despite the higher optical absorption. The JSC value trend is explained by the SCLC mobilities and EQE spectra for devices with different film thicknesses. The detailed solar cell parameters are summarized in Figure 3 and Table 1. The JSC values gradually increase with increasing PBDT-TS1:PC71BM film thickness up to 80 nm, followed by a drop with film thicknesses >80 nm. This observation is in accordance with the inverse trend seen in the series resistance (RS). The VOC values increase with increasing film thickness up to 100 nm. Interestingly, the fill factor (FF) values marginally decrease with increasing film thicknesses up to 80 nm. This is followed by a rapid decline for devices with thicknesses >80 nm, corresponding to the shunt resistance (RSH) trend. It is noteworthy that efficient charge transport and suppressed charge recombination can be explained by the relatively low RS and high RSH for devices with 80 nm-thick BHJ layers. Thus, the remarkable PCE improvement for devices with 80 nm-thick active layers is a result of the relatively higher JSC, VOC, and FF values (JSC = 17.53 mA/cm2, VOC = 0.768 V, FF = 66.45%, and PCE = 8.88%), which is consistent with the previous report.26,33,36,41 These results show that film thickness is critical in affecting charge transport and collection, thus determining the solar cell performance. 9265


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PBDT-TS1:PC71BM solar cells with three different thicknesses (40, 80, and 120 nm) were subjected to continuous illumination with simulated solar light (100 mW cm−2) in the presence of a UV-cut filter for 6 h (as reported previously, a UV-cut filter attached to the illuminating side improves device lifetime).31 The light J−V curves gradually degrade with illumination time, irrespective of film thickness (Figure 5a). This degradation is mainly attributable to the photodegradation of the PBDT-TS1 polymer (as previously reported for p o ly ( b e nzo d it h io p he ne -co-thieno[3,4-b] th i op h en e ) (PBDTTT)-based materials in PSCs).42−46 All devices exhibited a gradual decrease in both JSC and VOC with increasing illumination time (Figures 5b and S4). The JSC and VOC values were reduced by 10% and 5%, respectively, after 6 h of illumination. However, it is interesting that the FF is more stable for 120 nm-thick devices than for 40 and 80 nmthick devices. Here we note that after 6 h of illumination, the FF values decreased by 15, 12, and 1.5% for 40, 80, and 120 nm-thick devices, respectively. As a result, the overall PCE stability of the 120 nm-thick device is better than the 40 and 80 nm-thick devices. The different device lifetimes are presumably due to the lateral and vertical distributions of the polymer donor and fullerene acceptor components within BHJ layers because degradation occurs at the surface of the thin films and/ or in the bulk layers as discussed below. We first investigated the surface morphology before and after solar light illumination by employing atomic force microscopy (AFM) in order to understand the device stability trend (Figure 6). In the case of the surface morphology of blend films before solar light illumination, the relatively large domains for the 40 nm-thick BHJ layers are shown in the phase mode AFM image, while laterally phase-separated surface morphology of the 80 and 120 nm-thick BHJ layers was observed (here we note that the lateral phase separation can be significantly influenced by the solvent drying rate and high solution concentration and low spin-coating speed can provide the sufficient time for the phase separation). Interestingly, the thin film surface for the 40 and 80 nm thick BHJ layers became smoother after 6 h illumination, based on the different root-mean-square roughness (RRMS) change (from 1.00 to 0.86 nm and from 0.91 to 0.74 nm for 40 and 80 nm-thick BHJ layers, respectively). This is evident from the relatively larger domains after illumination in the phase mode AFM images for the 40 and 80 nm-thick BHJ layers, compared with those before illumination. However, interestingly, the 120 nm-thick BHJ layers was almost marginally changed after illumination when it comes to the well-kept uniform film surface (RRMS = 0.79−0.74 nm) and the relatively finer domains in the phase mode AFM image. Here it is noteworthy that the surface of the 120 nm-thick BHJ layers became already much smoother than that of the 40 and 80 nmthick BHJ layers before solar light illumination.

Figure 3. Solar cell parameters (JSC, VOC, FF, PCE, RS, and RSH) as a function of the BHJ (PBDT-TS1:PC71BM) film thickness (all data were taken from the light J−V curves with error bars for >12 devices).

To understand the relatively high PCEs for 80 nm-thick devices, the PBDT-TS1:PC71BM nanostructure was examined by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM). The cross-sectional TEM/STEM images show a well-defined p-interface (BHJ− MoO3 layers) without interface damage during MoO3 thermal deposition as well as the conformal coating of PBDTTS1:PC71BM BHJ layers at the n-interface (BHJ−ZnO layers) (Figure 4a,b). These discrete p- and n-interfaces assist charge extraction from the BHJ layers to the anode and cathode, respectively. We observed bicontinuous interpenetrating networks of BHJ layers with nanoscale phase segregation, leading to efficient charge separation and transport (Figure 4c). This observation is further confirmed by the S- and F- mapping of the STEM images (Figure 4d) (here we note that S and F atoms refer to the PBDT-TS1 polymer as seen from the chemical structure (Figure 1a)).

Table 1. Summary of Solar Cell Performances According to the Thickness (t) of BHJ Layersa t (nm) 40 60 80 100 120 a

JSC (mA/cm2) 14.24 15.80 17.53 17.33 16.39

± ± ± ± ±

0.44 0.36 0.23 0.31 0.17

VOC (V) 0.755 0.760 0.763 0.768 0.758

± ± ± ± ±

0.006 0.005 0.005 0.005 0.005

FF (%) 67.08 66.48 66.45 60.85 57.98

± ± ± ± ±

0.49 0.79 1.07 0.68 0.34

PCE (%) 7.21 7.98 8.88 8.09 7.20

± ± ± ± ±

0.16 0.13 0.09 0.18 0.08

RS (kΩ·cm2) 0.095 0.090 0.080 0.123 0.145

± ± ± ± ±

0.006 0.005 0.005 0.005 0.006

RSH (kΩ·cm2) 14.48 13.20 12.71 10.15 8.11

± ± ± ± ±

3.26 1.83 2.99 3.10 1.97

All data were extracted from the light J−V curves for >12 devices. 9266


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Figure 4. (a) HRTEM and (b) STEM images for the device cross-section parts prepared by the FIB process. (c) HRTEM and (d, e) STEM images for the 80 nm-thick PBDT-TS1:PC71BM BHJ layers coated onto glass substrates (S and F atoms refer to the PBDT-TS1 polymer as seen from the chemical structure in Figure 1a).

Figure 5. (a) Light J−V curves for the PBDT-TS1:PC71BM solar cells with different film thicknesses of 40, 80, and 120 nm under continuous onesun condition (AM 1.5 G, 100 mW cm−2) for 6 h with a UV-cut filter. (b) Normalized JSC, VOC, FF, and PCE as a function of the light exposure time under one-sun condition for the PBDT-TS1:PC71BM solar cells with different film thicknesses of 40, 80, and 120 nm (all data were taken from the light J−V curves with error bars for >4 devices).

40 and 80 nm thick BHJ layers became less intense after illumination. However, the intensity of the (010) and (100) peaks after illumination for the 120 nm thick BHJ layers became comparable to that before illumination. These results indicate that the nanostructure of the relatively thicker film (∼120 nm) could be well-maintained even after illumination, which is in good agreement with the relatively less change in the surface morphology for the thicker film (∼120 nm) as disclosed from the AFM images. Comparing the surface morphology/nanostructure change of the BHJ layers with the device stability, the relatively thicker film (∼120 nm) was more

However, the surface morphology itself cannot account for the device stability, so we performed the synchrotron radiation grazing incidence X-ray diffraction (GIXD) measurements for the pristine PBDT-TS1 and PTBT-TS1:PC71BM BHJ layers with different film thicknesses. The (010) peak in the out-ofplane (OOP) direction and (100) peak in the in- plane (IP) direction for the PBDT-TS1 chains in the pristine PBDT-TS1 and BHJ layers were clearly observed from the 2D GIXD images (Figure 7a) before and after illumination. Taking a closer look at the 1D profiles (Figure 7b), the (010) peak in the OOP direction and the (100) peak in the IP direction for the 9267


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morphologically stable than the relatively thinner film (≤80 nm). Hence, on the basis of the present results, it is shortly concluded that the active layer thickness can play a critical role in improving the device stability. We further investigated in-depth compositional distributions of the PBDT-TS1:PC71BM films using a time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiler. As shown in Figure 8 and Figure S5, sharp S− and F− peaks at the p-interface (BHJ−MoO3 layers) indicate enrichment of the PBDT-TS1 polymer formed at the film surface (p-interface) irrespective of film thickness (irrespective of solution concentration/spin-coating speed) due to the lower surface energy of the PBDT-TS1 polymer compared to PC71BM, whereas it seems that the PC71BM molecules are evenly distributed without any aggregation (note that the S− and F− signals as a function of depth refer to the PBDT-TS1 polymer). This result implies that the accumulated PBDT-TS1 polymer near the p-interface rather than in the bulk film could be inevitably photooxidised upon illumination, leading to a decrease in JSC and VOC values irrespective of film thickness. However, the relatively stable FF values for 120 nm-thick devices might be attributed to the homogeneous vertical distribution of PBDT-TS1 polymer and PC71BM for 120 nmthick BHJ layers, compared to 40 nm-thick and 80 nm-thick BHJ layers. Hence, based on above characterization results, the device stability can be considerably governed by the homogeneous vertical distribution of two components in the BHJ layers.

Figure 6. (a) 2D GIXD images and (b) 1D GIXD profiles for the pristine PBDT-TS1 and BHJ layers coated on the ZnO layers with different film thicknesses of 40, 80, and 120 nm before and after continuous solar light illumination with one sun (100 mW/cm2) for 6 h.

Figure 7. 3D height-mode AFM images (2D phase mode: upper right) for the BHJ layers coated on the ZnO layers with different film thicknesses before and after continuous solar light illumination with one sun (100 mW/cm2) for 6 h: (a) 40, (b) 80, and (c) 120 nm. 9268


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absorption spectra, changes in solar cell parameters under continuous illumination, and TOF-SIMS depth profiles according to atom concentration (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +82-53-950-5616. Fax: +8253-950-6615 (Y.K.). *E-mail: [email protected] (D.D.C.B.). ORCID

Youngkyoo Kim: 0000-0002-9391-2636 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by grants from the Korean Government (NRF_2015R1A2A2A01003743, NRF_2016H1D5A1910319, NRF_2014R1A1A3051165, Human Resource Training Project for Regional Innovation, MOE and NRF- 2014H1C1A1066748, and Basic Science Research Program_2009-0093819).

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(1) Angmo, D.; Larsen-Olsen, T. T.; Jørgensen, M.; Søndergaard, R. R.; Krebs, F. C. Roll- to-roll inkjet printing and photonic sintering of electrodes for ITO free polymer solar cell modules and facile product integration. Adv. Energy Mater. 2013, 3, 172−175. (2) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent advances in bulk heterojunction polymer solar cells. Chem. Rev. 2015, 115, 12666−12731. (3) Zhou, H.; Yang, L.; You, W. Rational design of high performance conjugated polymers for organic solar cells. Macromolecules 2012, 45, 607−632. (4) Gevorgyan, S. A.; Madsen, M. V.; Roth, B.; Corazza, M.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M.; Krebs, F. C. Lifetime of organic photovoltaics: Status and predictions. Adv. Energy Mater. 2016, 6, 1501208. (5) Facchetti, A. Polymer donor−polymer acceptor (all-polymer) solar cells. Mater. Today 2013, 16, 123−132. (6) Hong, S.; Kang, H.; Kim, G.; Lee, S.; Kim, S.; Lee, J. H.; Lee, J.; Yi, M.; Kim, J.; Back, H.; Kim, J. R.; Lee, K. A series connection architecture for large-area organic photovoltaic modules with a 7.5% module efficiency. Nat. Commun. 2016, 7, 10279. (7) Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J. Energy-level modulation of small-molecule electron acceptors to achieve over 12% efficiency in polymer solar cells. Adv. Mater. 2016, 28, 9423−9429. (8) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient organic solar cells processed from hydrocarbon solvents. Nat. Energy 2016, 1, 15027. (9) Wan, Q.; Guo, X.; Wang, Z.; Li, W.; Guo, B.; Ma, W.; Zhang, M.; Li, Y. 10.8% efficiency polymer solar cells based on PTB7-Th and PC71BM via binary solvent additives treatment. Adv. Funct. Mater. 2016, 26, 6635−6640. (10) Baran, D.; Ashraf, R. S.; Hanifi, D. A.; Abdelsamie, M.; Gasparini, N.; Rohr, J. A.; Holliday, S.; Wadsworth, A.; Lockett, S.; Neophytou, M.; Emmott, C. J.; Nelson, J.; Brabec, C. J.; Amassian, A.; Salleo, A.; Kirchartz, T.; Durrant, J. R.; McCulloch, I. Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells. Nat. Mater. 2016, 16, 363−369. (11) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells. Nat. Mater. 2006, 5, 197−203.

Figure 8. TOF-SIMS depth profiles for the PBDT-TS1:PC71BM BHJ layers with different film thicknesses of 40, 80, and 120 nm (S and F atoms exist solely in the PBDT-TS1 polymer as seen from the chemical structure in Figure 1).

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CONCLUSIONS We fabricated the inverted PBDT-TS1:PC71BM solar cells with different active layer thicknesses from 40−120 nm to investigate the effect of film thickness on the device performance and stability. The highest PCE was achieved at 80 nm due to the well-balanced hole/electron transport and reduced charge recombination, as supported by the higher rectification ratio (low leakage and high forward current) and EQE values over the visible region, which are attributable to the formation of well-connected percolated pathways for each (PBDT-TS1 and PC71BM) component as evidenced by the TEM/STEM analysis. The devices with thinner BHJ layers (80 nm). However, better device stability under continuous solar light illumination was measured for the 120 nm-thick devices with higher FF values, which can be attributed to the relatively well-maintained surface morphology/nanostructure and better morphological volume stability. Therefore, the present study highlights the importance of active layer thickness control to optimize both device performance and stability, which is of crucial for the commercialization of polymer solar cells.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02238. Summary of solution concentration and spin-coating speed, PL spectra, dark J−V curves, EQE spectra, optical 9269


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Thickness Effect of Bulk Heterojunction Layers on the Performance

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