An Easily Accessible Cathode Buffer Layer for Achieving Multiple High

Nov 13, 2015 - High performance PSCs with an identical Ba(OH)2 buffer layer were fabricated based on the multiple light-harvesting PBDT-TS1:PC71BM, Pf...
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An Easily Accessible Cathode Buffer Layer for Achieving Multiple High Performance Polymer Photovoltaic Cells Wenchao Zhao,† Long Ye,*,‡ Shaoqing Zhang,‡ Huifeng Yao,‡ Mingliang Sun,*,† and Jianhui Hou*,‡ †

Institute of Material Science and Engineering, Ocean University of China, Qingdao 266100, P. R. China State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

‡

S Supporting Information *

ABSTRACT: Here we report a successful efficiency improvement strategy in both conventional and inverted polymer solar cells (PSCs) based on multiple polymer blends, using a feasible and commercially available cathode buffer layer (CBL), namely barium hydroxide [Ba(OH)2], to modify the photoactive blend and cathode contacts. High performance PSCs with an identical Ba(OH)2 buffer layer were fabricated based on the multiple light-harvesting PBDTTS1:PC71BM, PffBT4T-2OD:PC71BM, and PBDT-TS1:N2200 blends. The conventional PSC with Ba(OH)2 as the CBL showed a higher power conversion efficiency (PCE) of 9.65% based on the PBDT-TS1:PC71BM system under the illumination of 100 mW/cm2. For the inverted cells based on the PffBT4T2OD:PC71BM system, the PCE can be improved from 4.26% (without CBL) to 9.02% after inserting the Ba(OH)2 buffer layer. More importantly, the Ba(OH)2 buffer layer presents similar positive effects in the conventional and inverted allpolymer devices based on a new combination, i.e., the PBDT-TS1:N2200 system. The dramatic enhancement in device performance resulted from the suitable work function of Ba(OH)2, extremely high transmittance, and excellent film-forming capability. Therefore, inserting Ba(OH)2 as the CBL is a simple, low-cost, and widely applicable method to simultaneously improve the conventional and inverted photovoltaic device performance.

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INTRODUCTION Since the pioneering works in the 1990s,1,2 bulk-heterojunction (BHJ) polymer solar cells (PSCs) comprising conjugated polymers as electron donors and fullerene derivatives or polymers as electron acceptors have attracted intensive attention because the PSC technology offers flexible, low cost, and large area energy harvest products using solution processing techniques.3−12 Enormous research efforts have been made in the PSC field, such as novel materials of BHJ blends,9−14 novel processing methods,15−23 and electrode buffer layers.24−32 These research efforts have led to a dramatic advance in the power conversion efficiencies (PCEs) of PSC devices. Single-junction PSC devices consist of two main configurations, i.e., conventional PSC [ITO/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/BHJ blend/ CBL/metal electrode] and inverted PSC [ITO/CBL/BHJ blend/molybdenum oxide (MoO3)/metal electrode].3−8 In addition to the innovation of active layer materials or BHJ blends, novel cathode buffer layers (CBLs) between BHJ blends and cathode metals have played a vital role in promoting the performance of conventional and/or inverted PSCs, affording superior electron transport and hole-blocking characteristics.15−48 Solution-processable transition metal oxides19−32 and organic/polymeric polyelectrolytes33−40 have been successfully used as CBLs in PSCs. For instance, Tan © XXXX American Chemical Society

and collaborators presented a solution processed CBL based on cerium oxide (CeOx), which exhibited efficient light trapping for the BHJ blend layer, and thus greatly improved device performance can be achieved in the conventional PSC.41 Alternatively, Zhang and Li et al. reported easy accessible fullerene-based and fullerene-free small molecular CBLs, which exhibited enhanced performance in the conventional PSCs employing various BHJ blends as the active layers.42,43 Recently, Ge et al. recently reported a novel CBL, i.e., a nonconjugated small-molecule electrolyte, which can be applied in conventional and inverted dingle junction PSC devices and has achieved considerable PCEs of ∼10% based on a poly((4,8bis((2-ethylhexyl)oxy)benzo(1,2-b:4,5-b′)dithiophene-2,6diyl)(2-(((2-ethylhexyl)oxy)carbonyl)-3-fluorothieno(3,4-b)thiophenediyl)):[6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM) BHJ blend.39 Unfortunately, a wide range of these CBLs were complicated, being prepared either by hightemperature thermal annealing (>150 °C) or chemical synthesis/purification.19−23 Moreover, solution-processed CBLs with potential application in both conventional and inverted PSC devices based on various BHJ blends have seldom been investigated. Therefore, to explore a double-purpose CBL Received: September 30, 2015 Revised: November 6, 2015

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Figure 1. Device diagram and energy level diagram of conventional PSCs (a) and inverted PSCs (b) employing Ba(OH)2 film as the cathode buffer layers.

Figure 2. Molecular structures of the BHJ materials involved in this work: PBDT-TS1, PffBT4T-2OD, PC71BM, and N2200.

Figure 3. (a) UPS spectra of the bare ITO and Ba(OH)2 buffer layer on ITO/glass surface. (b) Optical transmission spectra of Ba(OH)2 film (2 nm) on the quartz plate. XPS spectra of (c) Ba3d and (d) O1s profiles of the Ba(OH)2 film on the ITO substrate, with thermal annealing or not.

transistors (OFETs),50 and other organic devices51 as a solution-processed n-type interlayer. However, Ba(OH)2 has not been simultaneously utilized in conventional and inverted PSCs based on polymer:fullerene and polymer:non-fullerene BHJ blends yet. In this study, we explored and successfully applied a low-temperature solution-processed Ba(OH)2, as a

with a simple solution process should be an important task in the field of PSC and hybrid photovoltaic technology. As a less-studied type of inorganic salt, barium oxide or barium hydroxide [Ba(OH)2] has outstanding charge injection ability and has been explored to improve device performance in polymer light-emitting diodes (PLEDs),49 organic field effect B

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The Journal of Physical Chemistry C CBL in the fabrication of conventional and inverted PSCs (Figure 1a,b) based on several high-efficiency BHJ blends 48,52−54 such as PBDT-TS1:PC 71 BM, PffBT4T2OD:PC71BM, and a novel all-polymeric composite PBDTTS1:N2200 (see Figure 2). Specifically, the Ba(OH)2 CBL is simply prepared by spin-coating from a methanol solution on ITO and then thermal annealing at a low temperature (100 °C) in the inverted PSCs. In the conventional device case, the Ba(OH)2 film is directly deposited on top of the active layer before evaporating metal electrodes, and no further treatments are needed. The PCE of the inverted PSCs with the Ba(OH)2 as the CBL reached 9.52% and 9.02% based on PBDTTS1:PC71BM and PffBT4T-2OD:PC71BM blends, respectively, under the illumination of AM 1.5G, 100 mW/cm2. Moreover, high PCEs of 9.65%, 8.78%, and 5.22% were also observed in conventional PSCs based on PBDT-TS1:PC71BM, PffBT4T2OD:PC71BM, and PBDT-TS1:N2200 blends, respectively, after the application of Ba(OH)2 as a novel CBL. Our results indicate that inserting Ba(OH)2 as a CBL is a very simple and effective method to improve the device efficiency of both conventional and inverted PSCs. The great enhancement in device performance resulted from the suitable work functions of Ba(OH)2, extremely high transmittance, and excellent filmforming capability.

Figure 4. AFM height images of ITO (a) and ITO/Ba(OH)2 (b).

the cathodic interface, and (iii) a smooth surface for BHJ filmforming. Recently, several novel conjugated polymers achieved outstanding progress in the PSC field, supported by the wellbalanced excellent Voc, Jsc, and FF parameters.9−14 For example, a newly developed 2D-conjugated polymer PBDT-TS1 (Figure 2), reported by Ye, Zhang, and Hou et al., is one of very few photovoltaic materials achieving ∼10% efficiency and green solvent processing in the polymer photovoltaic field.5,13,52 Soon after, Yan, Ade, and co-workers also reported a novel conjugated polymer named PffBT4T-2OD, which achieved a record PCE of 10.8% in a single-junction PSC through a substrate-annealing method.48 In this work, both the PBDTTS1:PC71BM and PffBT4T-2OD:PC71BM BHJ blends were used to verify the suitability of the Ba(OH)2 CBL in highly efficient PSCs. Figures 5a and 5c show the current density versus voltage (J−V) characteristics of the inverted and conventional polymer solar cell with Ba(OH)2 as the CBL and without a CBL under a 100 mW/cm2 standard AM 1.5G spectrum condition, respectively. The inverted device photovoltaic parameters are summarized in Tables 1 and 2. The corresponding EQE spectra of the inverted and conventional PSCs were characterized and are collected in Figures 5b and 5d. The inverted control device (ITO/polymer:PC71BM/MoO3/ Al) based on the PBDT-TS1:PC71BM BHJ blend gives a poor PCE of 4.56% with an open-circuit voltage (Voc) of 0.549 V, a short-circuit current density (Jsc) of 16.56 mA/cm2, and a fill factor (FF) of 54.81% in the absence of a CBL (see Figure 5a). For the PffBT4T-2OD:PC71BM system, a low PCE of 4.26% was also achieved in an inverted control device (Figure 6b). The impact of the annealing temperature of the Ba(OH)2 on the inverted device performances was investigated based on the PBDT-TS1:PC71BM system (see Figure S1 and Table S1). The best-performing inverted PSCs were achieved when incorporating 100 °C annealed Ba(OH)2 as the CBL. The optimization device parameters of Voc, Jsc, FF, and PCE are respectively promoted to 0.787 V, 17.94 mA/cm2, 67.42%, and 9.52% based on the PBDT-TS1:PC71BM system, respectively. Similarly, a high PCE over 9% was also recorded for the PffBT4T2OD:PC71BM-based inverted PSC. Furthermore, the Ba(OH)2 CBL was employed in conventional PSC devices based on the PBDT-TS1:PC71BM and PffBT4T-2OD:PC71BM BHJ blends. We also characterized the photovoltaic performance of different concentrations of Ba(OH)2 based on the PBDT-TS1:PC71BM system. The J−V characteristics of the PBDT-TS1:PC71BM-based device with different concentration of Ba(OH)2 as CBLs are shown in Figure S2, and the photovoltaic performances are shown in Table S2. The Voc values of the devices remained the same at ∼0.79 V, while the Jsc and FF values varied with the

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RESULTS AND DISCUSSION To understand the basic properties of Ba(OH)2, the Ba(OH)2 thin layer was spin-coated on precleaned ITO substrate from a Ba(OH)2 methanol solution with a concentration of 2 mg/mL, affording a thickness of 2 nm. Initially, the work function (WF) of the Ba(OH)2 thin film was characterized by ultraviolet photoelectron spectroscopy (UPS). Figure 3a presents the UPS profile of the pristine ITO and ITO covered with the thin Ba(OH)2 film. The WF value (∼4.7 eV) of ITO measured by UPS was consistent with the reported result,55 verifying the accuracy of the test. The WF of the Ba(OH)2 film was determined to be 3.8 eV, which distinctly decreases the WF of ITO and is suitable for electron transport. Shown in Figure 3b is the optical transmittance spectra of the Ba(OH)2 film on quartz glass in the UV−vis−NIR region. It can be observed that the transmittance of the 2 nm thick Ba(OH)2 film is above 98% in the range of 300−900 nm, which is a significant characteristic for a CBL. Shown in Figures 3c and 3d are the XPS profiles of the Ba(OH)2 without annealing and Ba(OH)2 film with annealing at 100 °C on the ITO substrates. According to published results,50 the specific peaks corresponding to Ba 3d5/2, Ba 3d3/2, and O1s are located at 779.8, 795, and 531.1 eV, respectively. Specific peaks of barium and oxygen atoms are found to be the same for the Ba(OH)2 films with and without annealing. These results indicate that no decomposition occurs after thermal annealing process of the Ba(OH)2 thin film. The surface morphologies of the 2 nm thick Ba(OH)2 film on the ITO substrate were probed by tapping-mode atomic force microscopy (AFM). Height topography images and the corresponding root-mean-square roughness (Rq) values were taken for each film and are depicted in Figure 4. After modification with the 2.0 nm Ba(OH)2 film, the Rq value of ITO surface is significantly reduced from 3.4 nm (Figure 4a) to 2.6 nm (Figure 4b). Clearly, the ultrathin Ba(OH)2 film meets the several key characteristics for acting as an efficient CBL material in PSCs: (i) extremely high transmittance for minimizing the photon loss, (ii) suitable WF for modifying C

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Figure 5. J−V and EQE curves of the PBDT-TS1:PC71BM-based inverted (a, b) and conventional (c, d) PSCs with a Ba(OH)2 CBL and without a CBL.

conventional PSC devices after the insertion of Ba(OH)2 CBLs. As illustrated in Figure S3, the Rq values and topography features are not obviously tuned, which ensures a large amount of charge collection pathways in the cathodic interfaces. A PSC consisting of a p-type semiconducting polymer donor and a n-type semiconducting polymer acceptor as the BHJ blend is emerging as a potential type of organic photovoltaic device, in which polymers enable tailoring of the chemical structures and offer superior properties, such as broad spectral coverage and highly tuned energy levels.56−71 A high mobility polymer acceptor, known as N2200, has been explored as the polymer acceptor in polymer:polymer solar cells,72−76 and considerable PCEs over 5% were achieved in some of the recent devices. Beyond the successful application of the Ba(OH)2 CBL in polymer:PCBM blends-based PSCs, Ba(OH)2 was employed as the CBL in polymer:polymer blendbased PSCs, in which the PBDT-TS1:N2200 system was introduced as the BHJ blend for the first time. In the conventional devices, the PCE of the PBDT-TS1:N2200 system can improve from 2.67% (without a CBL) to 5.22% after using Ba(OH)2 as the CBL (Figure 6c). As illustrated in Figure 6d, an improved PCE of 4.79% was also achieved in the inverted PSC device with the insertion of Ba(OH)2 as the CBL, which obviously outperformed the inverted control device

Table 1. Photovoltaic Properties of the PBDT-TS1:PC71BMBased PSC with a Ba(OH)2 CBL and without a CBL under AM1.5G 100 mW/cm2 devicea

CBL

Voc [mV]

Jsc [mA/cm2]

FF [%]

PCEb [%]

PCEc [%]

C-PSC

w/o Ba(OH)2 w/o Ba(OH)2

765 796 549 787

16.42 17.73 16.56 17.94

59.59 68.39 54.81 67.42

7.49 9.65 4.98 9.52

7.21 9.51 4.56 9.40

I-PSC a

C-PSC and I-PSC represent conventional and inverted PSC devices, respectively. bThe maximum value of the best-performing device. cThe average value of ten devices.

concentration of Ba(OH)2. When the concentration of Ba(OH)2 was 2.5 mg/mL, the conventional PSC achieved a maximum PCE of 9.65%, which is much higher than that (7.49%) of the conventional control PSC (ITO/PEDOT:PSS/ PBDT-TS1:PC71BM/Al). Likewise, a conventional PSC employing the PffBT4T-2OD:PC71BM system as the BHJ blend delivered a desirable PCE of 8.78%, outperforming that (6.42%) of the control device without a CBL (see Figure 6a and Table 2). The surface morphologies of the active layers were probed by AFM to reveal the interfacial contacts in the

Table 2. Photovoltaic Properties of Various BHJ Blend-Based Conventional and Inverted PSCs with a Ba(OH)2 CBL and without a CBL under AM 1.5G 100 mW/cm2 BHJ blend

device

CBL

Voc [mV]

Jsc [mA/cm2]

FF [%]

PCE [%]

PffBT4T-2OD:PC71BM

C-PSC C-PSC I-PSC I-PSC C-PSC C-PSC I-PSC I-PSC

w/o Ba(OH)2 w/o Ba(OH)2 w/o Ba(OH)2 w/o Ba(OH)2

689 774 723 772 673 796 410 802

14.74 16.11 14.33 17.14 9.43 12.57 10.78 12.50

63.20 70.43 41.16 68.18 42.05 52.14 28.46 47.76

6.42 8.78 4.26 9.02 2.67 5.22 1.26 4.79

PBDT-TS1:N2200

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Figure 6. J−V curves of conventional and inverted PSCs with a Ba(OH)2 CBL and without a CBL based on the PffBT4T-2OD:PC71BM (a, b) and PBDT-TS1:N2200 (c, d) blend systems.

Fabrication of Inverted PSC Devices. The inverted PSCs with the device configuration of ITO/Ba(OH)2/BHJ blend/ MoO3/Al were prepared. ITO substrates with a nominal sheet resistance of 15 Ω sq−1 were cleaned with detergent water, deionized water, acetone, and isopropyl alcohol in an ultrasonic bath sequentially for 20 min and then dried in an oven at 150 °C for 30 min. The CBL was spin-coated on precleaned ITO substrates from the Ba(OH)2 methanol solution (2 mg/mL) at 3000 rpm for 60 s and annealed at 100 °C for 10 min. The Ba(OH)2 solution should be filtered by a nylon filter with a diameter of 0.45 μm prior to the spin-cast. The concentration of PBDT-TS1 and PC71BM in chlorobenzene (CB) solution is 10 and 15 mg/mL, respectively.13 The concentration of PffBT4T-2OD and PC71BM in o-dichlorobenzene:chlorobenzene (DCB:CB; 1:1, v/v) solution is 9 and 10.8 mg/mL, respectively. 3% 1,8-diiodooctane (DIO) was used as solvent additives according to a previous report by Yan et al.48 The concentration of PBDT-TS1 and N2200 in chlorobenzene (CB) solution is 6 and 6 mg/mL, respectively. The PBDTTS1:PC71BM blend film thickness was ∼100 nm. The PffBT4T-2OD:PC71BM blend film thickness was ∼350 nm. The PBDT-TS1:N2200 blend film thickness was ∼110 nm. The thicknesses of the BHJ blends and Ba(OH)2 film were measured by a Bruker Dektak XT profilometer. The inverted device fabrication was completed by thermal evaporation MoO3 (10 nm) and Al (100 nm) through a mask with a 4.15 mm2 active area under vacuum at a base pressure of 1 × 10−4 Pa. Fabrication of Conventional PSC Devices. The conventional PSCs with the device configuration of ITO/ PEDOT:PSS/BHJ blend/Ba(OH)2/Al were prepared. The precleaned ITO substrates were coated with 30 nm thick PEDOT:PSS, BHJ blend layer with the same thickness of inverted PSCs, 5 nm Ba(OH)2 and 100 nm Al, sequentially. The thicknesses of the PEDOT:PSS, BHJ blend and Ba(OH)2 layers were controlled by a surface profilometer.

without a CBL. On the basis of the above results, we can conclude that Ba(OH)2 is a simple CBL with wide applicability in versatile PSCs employing different types of BHJ blends.

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EXPERIMENTAL SECTION Materials and Instruments. PBDT-TS1 (Mn = 29 kDa) was synthesized in our laboratory according to the previous works.52−54 N2200 (Mn = 138 kDa), PffBT4T-2OD (Mn = 35 kDa), and PC71BM were commercially available from Solarmer Material Inc. Ba(OH)2 power and the processing solvents used in the device fabrication process were purchased from Alfa Aesar. The PEDOT:PSS (AI 4083) and electrode materials were commercially available products without further purification. The UV−vis spectra were measured by a double-source spectrophotometer (Shanghai Lab-spectrum 1900C). A set of samples was analyzed on the Thermo Scientific ESCALab 250Xi using UPS. The gas discharge lamp was used for UPS, with helium gas admitted and the He I (21.22 eV) emission line employed. The helium pressure in the analysis chamber during analysis was approximately 2 × 10−8 mbar. The data were acquired with −10 V bias. XPS was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was approximately 3 × 10−10 mbar. Typically the hydrocarbon C1s line at 284.8 eV from adventitious carbon is used for energy referencing. AFM images were obtained on an Agilent 5400 scanning probe microscope using ac mode. The surface topography and phase images of thin film were obtained using the AFM (Multimode 8) in topping mode. Current density−voltage (J−V) characteristics were measured under the 100 mW/cm2 standard AM 1.5G spectrum using a solar simulator (AAA grade). For reliable characterizations, the simulator was calibrated by a NIM certificated silicon solar cell (KG3 color filter).77 The EQE data were collected by an integrated IPCE measurement system, namely QE-R3011 (Enli Technology Co. Ltd., Taiwan). E

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(5) Zhao, W. C.; Ye, L.; Zhang, S. Q.; Sun, M. L.; Hou, J. H. A Universal Halogen-Free Solvent System for Highly Efficient Polymer Solar Cells. J. Mater. Chem. A 2015, 3, 12723−12729. (6) Ye, L.; Zhang, S. Q.; Ma, W.; Fan, B. H.; Guo, X.; Huang, Y.; Ade, H.; Hou, J. H. From Binary to Ternary Solvent: Morphology FineTuning of D/A Blends in PDPP3T-Based Polymer Solar Cells. Adv. Mater. 2012, 24, 6335−6341. (7) Ye, L.; Jing, Y.; Guo, X.; Sun, H.; Zhang, S.; Zhang, M.; Huo, L.; Hou, J. Remove the Residual Additives toward Enhanced Efficiency with Higher Reproducibility in Polymer Solar Cells. J. Phys. Chem. C 2013, 117, 14920−14928. (8) Li, C. Z.; Chang, C. Y.; Zang, Y.; Ju, H. X.; Chueh, C. C.; Liang, P. W.; Cho, N.; Ginger, D. S.; Jen, A. K. Y. Suppressed Charge Recombination in Inverted Organic Photovoltaics via Enhanced Charge Extraction by Using a Conductive Fullerene Electron Transport Layer. Adv. Mater. 2014, 26, 6262−6267. (9) Li, Y. F. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723−733. (10) Ye, L.; Zhang, S. Q.; Huo, L. J.; Zhang, M. J.; Hou, J. H. Molecular Design toward Highly Efficient Photovoltaic Polymers Based on Two-Dimensional Conjugated Benzodithiophene. Acc. Chem. Res. 2014, 47, 1595−1603. (11) Zhang, H.; Ye, L.; Hou, J. H. Molecular Design Strategies for Voltage Modulation in Highly Efficient Polymer Solar Cells. Polym. Int. 2015, 64, 957−962. (12) Duan, C. H.; Huang, F.; Cao, Y. Recent Development of PushPull Conjugated Polymers for Bulk-Heterojunction Photovoltaics: Rational Design and Fine Tailoring of Molecular Structures. J. Mater. Chem. 2012, 22, 10416−10434. (13) Zhang, S. Q.; Ye, L.; Zhao, W. C.; Yang, B.; Wang, Q.; Hou, J. H. Realizing Over 10% Efficiency in Polymer Solar Cell by Device Optimization. Sci. China: Chem. 2015, 58, 248−256. (14) Liu, C.; Yi, C.; Wang, K.; Yang, Y. L.; Bhatta, R. S.; Tsige, M.; Xiao, S. Y.; Gong, X. Single-Junction Polymer Solar Cells with Over 10% Efficiency by a Novel Two-Dimensional Donor-Acceptor Conjugated Copolymer. ACS Appl. Mater. Interfaces 2015, 7, 4928− 4935. (15) Hu, Z. C.; Zhang, K.; Huang, F.; Cao, Y. Water/Alcohol Soluble Conjugated Polymers for the Interface Engineering of Highly Efficient Polymer Light-Emitting Diodes and Polymer Solar Cells. Chem. Commun. 2015, 51, 5572−5585. (16) Wang, F. Z.; Tan, Z. A.; Li, Y. F. Solution-Processable Metal Oxides/Chelates as Electrode Buffer Layers for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 1059−1091. (17) He, Z. C.; Wu, H. B.; Cao, Y. Recent Advances in Polymer Solar Cells: Realization of High Device Performance by Incorporating Water/Alcohol Soluble Conjugated Polymers as Electrode Buffer Layer. Adv. Mater. 2014, 26, 1006−1024. (18) Cai, W. Z.; Gong, X.; Cao, Y. Polymer Solar Cells: Recent Development and Possible Routes for Improvement in the Performance. Sol. Energy Mater. Sol. Cells 2010, 94, 114−127. (19) Chen, L. M.; Xu, Z.; Hong, Z. R.; Yang, Y. Interface Investigation and Engineering - Achieving High Performance Polymer Photovoltaic Devices. J. Mater. Chem. 2010, 20, 2575−2598. (20) Yip, H. L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994−6011. (21) Espinosa, N.; Hosel, M.; Jorgensen, M.; Krebs, F. C. Large Scale Deployment of Polymer Solar Cells on Land, on Sea and in the Air. Energy Environ. Sci. 2014, 7, 855−866. (22) Steim, R.; Kogler, F. R.; Brabec, C. J. Interface Materials for Organic Solar Cells. J. Mater. Chem. 2010, 20, 2499−2512. (23) Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. The Role of Buffer Layers in Polymer Solar Cells. Energy Environ. Sci. 2011, 4, 285−310. (24) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction

CONCLUSIONS To summarize, we fabricated multiple high-efficiency conventional and inverted PSC devices using an identical barium hydroxide [Ba(OH)2] CBL and found that the PSCs buffered with a Ba(OH)2 layer exhibited significant enhancement in Voc, Jsc, and FF, which leads to optimal PCEs of 9.65%, 9.02%, and 5.22% based on the PBDT-TS1:PC 71 BM, PffBT4T2OD:PC71BM, and PBDT-TS1:N2200 blend systems, respectively. This is one of the few works that multiple cases of highly efficient polymer solar cells can be fabricated with an identical cathode buffer layer by simple solution processing. The improved device performance due to the insertion of Ba(OH)2 as a cathode buffer layer in the versatile devices can be attributed to the well-matched interface contact, extremely high transmittance, and excellent film-forming properties. The present study offers a simple and cost-effective strategy toward versatile PSCs with high efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09575. Additional device performances of inverted and conventional PSCs with the CBLs at various conditions and surface morphologies of BHJ blends modified with and without a CBL (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Y.). *E-mail: [email protected] (M.S.). *E-mail: [email protected] (J.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program 973 (2014CB643501), the NSFC (Nos. 91333204 and 51261160496), and the Chinese Academy of Sciences (No. XDB12030200). M. Sun gratefully acknowledges financial support from the NSFC (21274134) and Qingdao Municipal Science and Technology Program (13-1-4-200-jch). J. Hou thanks the CAS-Croucher Funding Scheme for Joint Laboratories for the support.

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REFERENCES

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