Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat Manipulating the molecular structure of PEDOT chains through controlling the viscosity of PEDOT:PSS solutions to improve the photovoltaic performance of CH 3 NH 3 PbI 3 solar cells Sheng Hsiung Chang a,b, ⁎ , Wei-Nien Chen a,b , Cheng-Chiang Chen a , Shih-Chieh Yeh c , Hsin-Ming Cheng d , Zong-Liang Tseng e , Lung-Chien Chen e , Kuo Yuan Chiu f , Wen-Ti Wu c , Chin-Ti Chen c , Sheng-Hui Chen b , Chun-Guey Wu a,g a Research Center for New Generation Photovoltaics, National Central University, Taoyuan 32001, Taiwan, ROC b Deaprtment of Optics and Photonics, National Central University, Taoyuan 32001, Taiwan, ROC c Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC d Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan, ROC e Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC f Department of Applied Chemistry, National Chi Nan University, Nantou 545, Taiwan, ROC g Department of Chemistry, National Central University, Taoyuan 32001, Taiwan, ROC ARTICLE INFO Keywords: PEDOT:PSS Surface properties CH 3 NH 3 PbI 3 Energy transfer Photovoltaics ABSTRACT A series of alcohols (methanol, ethanol and isopropyl alcohol) were added to poly(3,4-ethylenedioxythiophene): (polystyrene sulfonate) (PEDOT: PSS) aqueous solutions in order to manipulate the molecular structure of the PEDOT chains in PEDOT: PSS hole transporting materials (HTMs) and thus to improve the power conversion efficiency of CH 3 NH 3 PbI 3 based solar cells. The structural and electronic characteristics of the resultant PEDOT: PSS HTMs were analyzed using atomic-force microscopy, contact angle measurement, Raman scattering spectrometry and photoelectron spectrometry. The properties of the PEDOT: PSS thin films could be controlled by tuning the viscosity of the PEDOT: PSS solutions. High viscosity PEDOT: PSS solutions resulted in linear structured PEDOT chains, which increased the work function of the PEDOT: PSS HTMs thereby improving the open-circuit voltage of the CH 3 NH 3 PbI 3 solar cells. The surface roughness and surface free energy of the PEDOT: PSS HTMs influence the structural properties of CH 3 NH 3 PbI 3 thin films, which determines the exciton dissociation at the CH 3 NH 3 PbI 3 /PEDOT: PSS interface (short-circuit current density) and the carrier recombination at the CH 3 NH 3 PbI 3 /[6,6]-phenyl-C61-butyric acid methyl ester (PC 61 BM) interface (fill factor). In addition, it is predicted that the power conversion efficiency can be further improved by increasing the crystallinity of the CH 3 NH 3 PbI 3 thin film. 1. Introduction The properties of transparent conductive electrodes (TCEs), elec- tron transporting materials (ETLs) and hole transporting materials (HTMs) have a huge influence on the efficiency of optoelectronic devices, such as liquid-crystal displays, laser diodes, light-emitting diodes, photo-detectors and solar cells. Poly(3,4-ethylenedioxythio- phene): (polystyrene sulfonate) (PEDOT:PSS) thin films have been widely used as TCEs [1–4] and HTMs [5–8] in organic optoelectronic devices because of the simplicity of the thin-film fabrication processes. PEDOT is a conjugated conductive polymer which is electrically attracted to an insulator like PSS, which can be dissolved in aqueous solutions. Thus, the PEDOT:PSS thin films can be formed using a solution process. In general, it is the PEDOT:PSS (1:2.5 wt%) and PEDOT:PSS (1:6 wt%) aqueous solutions that are used to fabricate TCEs and HTMs, respectively. The electrical conductivity of PEDOT:PSS (1:2.5 wt%) thin films can be increased a 1000 times through utilizing a solution process which involves the addition of an organic solvent additive [9] or by conducting a post immersion treatment [4]. In a past study [4], water/alcohol (methanol, ethanol or isopropyl alcohol (IPA)) co-solvent mixtures were used in the post treatment processes to enhance the electrical conductivity of PEDOT:PSS (1:2.5 wt%) thin films. The enhancement in the electrical conductivity of PEDOT:PSS (1:2.5 wt%) thin films originates from the http://dx.doi.org/10.1016/j.solmat.2016.11.020 Received 12 July 2016; Received in revised form 17 November 2016; Accepted 18 November 2016 ⁎ Corresponding author at: Research Center for New Generation Photovoltaics, National Central University, Taoyuan 32001, Taiwan, ROC. E-mail addresses: shchang@ncu.edu.tw (S.H. Chang), ericchen@dop.ncu.edu.tw (S.-H. Chen), t610002@cc.ncu.edu.tw (C.-G. Wu). Solar Energy Materials & Solar Cells 161 (2017) 7–13 0927-0248/ © 2016 Elsevier B.V. All rights reserved. crossmark