Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat Unraveling the multifunctional capabilities of PCBM thin films in inverted- type CH 3 NH 3 PbI 3 based photovoltaics Sheng Hsiung Chang a,b, ⁎ , Cheng-Chiang Chen a , Lung-Chien Chen c , Chuen-Lin Tien d, ⁎⁎ , Hsin-Ming Cheng e , Wei-Chen Huang a,b , Hong-Yi Lin d , Sheng-Hui Chen b , Chun-Guey Wu a,f, ⁎ a Research Center for New Generation Photovoltaics, National Central University, Taoyuan 32001, Taiwan, ROC b Department of Optics and Photonics, National Central University, Taoyuan 32001, Taiwan, ROC c Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC d Department of Electrical Engineering, Feng Chia University, Taichung 40724, Taiwan, ROC e Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan, ROC f Department of Chemistry, National Central University, Taoyuan 32001, Taiwan, ROC ARTICLE INFO Keywords: CH 3 NH 3 PbI 3 PCBM Residual stress X-ray diffraction Raman scattering Photovoltaics ABSTRACT Comprehensive studies were carried out to explore the roles of phenyl-C 61 -butyric acid methyl ester (PCBM) thin films in inverted-type CH 3 NH 3 PbI 3 (MAPbI 3 ) based photovoltaics, including the surface morphologies, transmittance spectra, photoluminescence spectra, X-ray diffraction (XRD) patterns, Raman scattering spectra and thin-film residual stress analysis. The reduction in the photoluminescence line width indicates that the crystallinity of the MAPbI 3 thin film can be increased by covering it with the PCBM thin film. The XRD patterns and Raman scattering spectra show a reduction in the compressive stress of MAPbI 3 thin film when covered by the PCBM thin film. In addition, it is noted that the residual stress at the contact interface between the hydrophilic MAPbI 3 and the hydrophobic PCBM can be ignored as confirmed by the results obtained with a home-made Twyman-Green interferometer. Consequently, the superior optoelectronic properties of the perovskite materials, as well as the use of a multifunctional fullerene-based thin film as the capping layer allow for the high-efficiency perovskite photovoltaics. 1. Introduction Organo-lead halide perovkite (CH 3 NH 3 PbI 3 (MAPbI 3 ) and HC(NH 2 ) 2 PbI 3 (FAPbI 3 )) based photovoltaics have recently received a great deal of attention due to their high power conversion efficiency (PCE) of over 20% [1–3]. It is well known that the high PCE originates from the small absorption bandgap [4,5], high absorption coefficient [5,6], low exciton binding energy [7–10], long exciton lifetime [11–13], excellent carrier diffusion length [14,15], fast carrier response [16,17] and low Urbach energy [5,18] of the perovskite light absorbers. The device configurations of perovskite based photovoltaics can be classified as either regular-type or inverted-type structures [19,20]. In regular-type perovskite based photovoltaics, the n-type semiconductors (TiO 2 , ZnO, AZO, C 60 ) [21–24] are deposited on top of a transparent conductive oxide (TCO) to act as the electron transport layer (ETL) in the photovoltaic cells. In inverted-type perovskite based photovoltaics, p-type materials (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfo- nate) (PEDOT:PSS), NiO x , CuS, CuO x , MoO x , WO x and MoSe) [25–29] are prepared on top of a TCO to act as the hole transport layer (HTL) in the photovoltaic cells. The perovskite thin films can be easily deposited on either n-type materials or p-type materials using spin-coating methods due to the hydrophilic nature of these substrates. In general, the grain sizes in close-packed perovskite thin films are smaller than 500 nm which results in a lot of surface defects. The contact between the perovskite thin film and the capping layer is crucial for good photovoltaic performance. Thus, the valleys and surface defects in perovskite thin films have to be filled by a capping layer in order to achieve the high PCE. It is obvious that small molecule materials would be more suitable as the capping layer in perovskite based photovoltaics. The use of 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spir- obifluorene (Spiro-OMeTAD) thin films [21] and PCBM thin films [20] have been widely used as the HTL in regular-type perovskite based photovoltaics and the ETL in inverted-type perovskite photovoltaics, respectively. In general, the intrinsic discrepancy between the hydro- philic nature of perovskite thin films and the hydrophobic nature of small molecules impedes the formation of a smooth contact between the http://dx.doi.org/10.1016/j.solmat.2017.05.007 Received 8 March 2017; Received in revised form 5 May 2017; Accepted 6 May 2017 ⁎ Corresponding authors at: Research Center for New Generation Photovoltaics, National Central University, Taoyuan 32001, Taiwan, ROC. ⁎⁎ Corresponding author. E-mail addresses: shchang@ncu.edu.tw (S.H. Chang), cltien@fcu.edu.tw (C.-L. Tien), t610002@cc.ncu.edu.tw (C.-G. Wu). Solar Energy Materials and Solar Cells 169 (2017) 40–46 0927-0248/ © 2017 Elsevier B.V. All rights reserved. MARK