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Blending of n-type semiconducting polymer and PC BM for an efficient electron-selective material to boost the performance of the planar perovskite solar cell You-Hyun Seo, Jun-Seok Yeo, NoSoung Myoung, Sang-Youp Yim, Minji Kang, Dong-Yu Kim, and Seok-In Na ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02478 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016
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Blending of n-Type Semiconducting Polymer and PC61BM for an Efficient Electron-Selective Material to Boost the Performance of the Planar Perovskite Solar Cell You-Hyun Seo,a‡ Jun-Seok Yeo,b†‡ NoSoung Myoung,c Sang-Youp Yim,c Minji Kang,b Dong-Yu Kim,b Seok-In Naa*
a
Professional Graduate School of Flexible and Printable Electronics and Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea b
School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 500-712, Republic of Korea
c
Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology, Gwangju, 500-712, Republic of Korea
KEYWORDS: perovskite solar cells, planar structures, small molecules, polymer binders, charge extractions, interfacial layers 1 ACS Paragon Plus Environment
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ABSTRACT The highly efficient CH3NH3PbI3 perovskite solar cell (PeSC) is simply achieved by employing a blended electron-transport layer (ETL) consisting of PC61BM and P(NDI2ODT2). The high molecular weight of P(NDI2OD-T2) allows for a thinned ETL with a uniform morphology that optimizes the PC61BM ETL more effectively. As a result of this enhancement, the power conversion efficiency of a PC61BM:P(NDI2OD-T2)-based PeSC is 25% greater than that of the conventional PC61BM based-PeSC; additionally, the incorporation of P(NDI2OD-T2) into PC61BM attenuates the dependence of the PeSC on the ETL-processing conditions regarding its performance. It is revealed that, in addition to the desirable n-type semiconducting characteristics of PC61BM:P(NDI2OD-T2)—including a higher electron-mobility and a more-effective electron-selectivity of a blended ETL for an efficient electron-extraction—the superior performance of a PC61BM:P(NDI2OD-T2) device is the result of a thinned and uniformly covered ETL on the perovskite layer.
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1. INTRODUCTION The major attention garnered by methylammonium lead tri-halide, a class of organicinorganic hybrid perovskites, is due to the efficient performance and competitive price regarding its function as a promising light-harvester in the field of solution-processible photovoltaics.1-5 Since this perovskite material was first introduced as a light-absorber into the dye-sensitized solar cell (DSSC) by Miyasaka and co-workers,6 the power conversion efficiencies (PCEs) of perovskite solar cells (PeSCs) have been rapidly improved over the last three years and currently exceeds 20%.7 Along with the progress in the device-efficiency, the device-structure has also evolved, changing from the complicated structure of the mesoscopic DSSC to simple planar geometry for a low-cost fabrication.8 In the case of a planar structure, adopting the organic solar cell (OSC) configuration is far more advantageous because its device structure can be simply fabricated through solution and lowtemperature processes using a flexible system.9,10 Further, a variety of well-developed printing techniques developed for OSC fabrication can be easily applied in manufacturing of planar PeSCs, and these facilitate the compatibility of the planar PeSCs with roll-to-roll processing.11 To date, however, the planar PeSCs of an OSC structure still exhibited a relatively low PCE compared with mesoscopic PeSCs of a DSSC structure. Currently employed strategies to enhance the device-efficiency of planar PeSCs involve the development of perovskite materials and the optimization of the morphology of photo-active perovskite film in the planar system.12,13 In addition to the optimization of photo-active layer, the interfacial layers that are in contact with both the perovskite film and the metal electrodes, should also be centrally considered and engineered for the production of efficient planar PeSCs.14-16 Generally, the planar PeSCs comprises the following structure: transparent electrode of indium tin oxide (ITO)/hole-transport layer (HTL)/perovskite active layer/electron3 ACS Paragon Plus Environment
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transport layer (ETL)/Ag or Al electrode. Despite a variety of interfacial materials that constitute the planar PeSCs, the most representative interlayers are not greatly different from the materials used in OSCs, i.e. poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) for the HTL and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) for the ETL.9,17 In particular, regarding the ETL, the outstanding performances of fullerene derivatives (PC61BM) have been observed and discussed. Previous works attributed the excellent compatibility between PC61BM and perovskite to its adequate energy levels for contact with methylammonium lead tri-halide,9 an ultrafast and efficient charge-transfer through the perovskite/PC61BM interface,18 and the capability of trap passivation at the grain boundaries in perovskite crystals.17,19 For the desirable functioning of the PC61BM interlayer without any variation of the device-performance, it is necessary to achieve the homogeneous morphology of the PC61BM film on the perovskite layer so that a leakage pathway and irregular contact with the metal electrodes are both avoided.15 In this regard, various efforts have been dedicated to the improvement of the morphology of the PC61BM layer whereby processing techniques such as spin-rates, processing solvents, and additional capping layers have been optimized;15,20,21 however, PC61BM, one of the small molecules, inevitably requires a sufficient film-thickness (> 100 nm) for film-continuity. The thick PC61BM film in the PeSC increases the series resistance of devices, because the photo-generated charges in the perovskite active layer must vertically travel along the PC61BM layer to be collected by an electrode. In consideration of above-mentioned advantages of the PC61BM interlayer, a key challenge in the further enhancement of the performance of PeSCs is a reduction of the thickness of PC61BM film without degradation in the ability of PC61BM as ETL, caused by pinholes and morphological defects in the ETLs on the perovskite layer. In organic electronic devices, the blending of small molecular semiconductors and polymer binders has been reported as an effective strategy to maximize the corresponding electrical performance of the 4 ACS Paragon Plus Environment
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devices based on the small molecules.22,23 The high-molecular-weight polymers can increase the solution-viscosity and prevent the dewetting phenomenon on the substrates, leading to high quality of organic semiconductor films.22 Very recently, Yan et al. demonstrated the better morphology and coverage of the PC61BM layer on a perovskite film by using an insulating polymer (polystyrene) as a polymeric binder.24 The high-molecular-weight polystyrene enabled the fabrication of uniform and smooth PC61BM film, thereby preventing the formation of an undesirable leakage path; however, the charge-transport through the polystyrene layer could be detrimentally affected by the electrically insulating feature of polystyrene, this could induce the precise control of the processing condition, especially with respect to its concentration. Here, we demonstrated the performance of a highly efficient PeSC based on CH3NH3PbI3, for the maximization of the ETL function of PC61BM, whereby a novel blend system involving PC61BM and a semiconducting polymer were used for the ETL. For a semiconducting polymer binder, we employed the outstanding n-type semiconductor of poly((N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5’(2,2’-bithiophene)) (P(NDI2OD-T2)). P(NDI2OD-T2) has been frequently used as an n-type semiconducting material in field-effect transistors due to its high electron-mobility,25 and the P(NDI2OD-T2) film shows a high bulk mobility in the out-of-plane direction.26 With P(NDI2OD-T2), the PeSC based on the blended ETL showed a relatively enhanced PCE of 10.57% compared to that of conventional device for which a pure PC61BM ETL was used. Furthermore, the blended-ETL-based PeSC was not sensitive to varied PC61BM:P(NDI2ODT2)-processing conditions with respect to factors such as its thickness and the concentration of P(NDI2OD-T2). An investigation on the device-analyses, ETL morphology, and chargetransfer dynamics revealed the possibility that the improved photovoltaic performance of PC61BM:P(NDI2OD-T2)-based devices originated from the thinner film-formability, the 5 ACS Paragon Plus Environment
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more homogeneous morphology of the blended ETLs from the use of a high-molecularweight P(NDI2OD-T2), and the high electron-mobility and the high charge-selectivity of the PC61BM:P(NDI2OD-T2). This result clearly highlights the great potential of a combination of fullerene derivatives and semiconducting polymers to form the ETL in perovskite solar cells for practical applications.
2. EXPERIMENTAL DETAILS Device fabrication and measurement: The patterned ITOs on the glass substrates were sequentially cleaned with deionized water, acetone, and isopropanol using ultrasonication. After drying, the cleaned substrates were treated with UV/O3 for 30 min. The 30 nm-thick PEDOT: PSS (CleviosTM P VP AI 4083, Heraeus) layers were deposited by spincoating at 5000 rpm for 40 s, followed by drying at 150 °C for 10 min in air. The prepared substrates were then transferred to an N2-filled glove box (
