Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Novel dopant-free hole-transporting materials for efficient perovskite solar cells Islam M. Abdellah (Ph.D.) a , Towhid H. Chowdhury (Ph.D.) b,c , Jae-Joon Lee (Ph.D.) (Professor) c , Ashraful Islam (Ph.D.) (Professor) b, ⁎ , Ahmed El-Shafei (Ph.D.) (Professor) d, ⁎ a Faculty of Science, Department of Chemistry, Aswan University, Aswan 81528, Egypt b Photovoltaic Materials Group, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan c Department of Energy & Materials Engineering & Research Center for Photoenergy Harvesting and Conversion Technology(phct), Dongguk University, Seoul 04620, Republic of Korea d Polymer and Color Chemistry Program & Fiber and Polymer Science Program, North Carolina State University, Raleigh 27606, USA ARTICLE INFO Keywords: Molecular engineering Photovoltaics Perovskite solar cells Solar energy Hole transport materials ABSTRACT Two novel highly conjugated small organic molecules as hole transporting materials (HTMs) coded T(EDOT- TPA) 2 and DBT(QT-TPA) 2 were designed and developed by utilizing facile synthetic procedures with high yields. The fabricated perovskite solar cells (PSCs) utilizing these HTMs without any dopants under 1 sun il- lumination (100 mW cm -2 , AM 1.5G) and surface area of 1.02 cm 2 achieved a short circuit current (J SC = 19.23), open circuit voltage (V OC = 1.042), fill factor (FF = 0.679) and overall power conversion efficiency (PCE = 13.61%) for DBT(QT-TPA) 2 . While, T(EDOT-TPA) 2 exhibited (J SC = 20.25, V OC = 1.04, FF = 0.583, and PCE = 12.27%). These dopant free HTM based PSCs achieved superior PCEs compared to that of undoped Spiro-OMeTAD (PCE = 9.34%) based PSCs and a comparable photovoltaic performance to the PSCs using doped Spiro-OMeTAD (J SC = 20.37, V OC = 1.057, FF = 0.74, and PCE = 15.93) as the HTM under same fabrication conditions. Noticeably, the absence of additives is of significant importance, as DBT(QT-TPA) 2 and T(EDOT-TPA) 2 based PSCs still produces a J sc up to 20.25 mA cm -2 and a comparable PCE of 13.61%, which reduces the fabrication cost of cm sized PSCs. 1. Introduction A steady increase in energy consumption was one of the challenges facing humanity, and energy consumption was expected to double worldwide due to a growing population and improved living standards, and therefore more energy would be needed. New energy sources have become essential, especially solar energy sources as a clean energy source. In this regard, photovoltaics, which can be used to convert sun- based energy into electrical energy, are promising options that are unlike oil derivatives (Lund et al., 2018). The current photovoltaic (PV) market is governed by silicon-based monocrystalline silicon solar cells with power conversion efficiency (PCE) of 26% in module scale (Yoshikawa et al., 2017). Perovskite solar cells (PSCs) have attracted a great deal of interest from the solar cell research community due to the rapid improved PCE from 3.8% (Kojima et al., 2009) to 25.2% (Park, 2019) in just over a decade with advantageous easy fabrication process over silicon-based solar cells which require expensive high-vacuum deposition techniques (Kumar et al., 2013; Liu and Kelly, 2014; Shin et al., 2015). The rapid progress of PSCs performances has been at- tributed to the wide spectral absorption range, the exceptionally high absorption coefficient, high mobility, longer propagation length, and long transport lifetime of the perovskite compounds (Bhandari and Ellingson, 2018). High efficiency PSCs are fabricated with mesoporous configuration which are configured with a compact electron transport layer (ETL) followed by a mesoporous ETL, perovskite compound as the absorber with a hole transporting material (HTM) and silver or gold as the electrode (Yun et al., 2018). Numerous factors influence the performance of PSCs such as the composition of the perovskite layer, hole transport material, electron transport layer, electrode structure (Boyd et al., 2019; Cao et al., 2018; Christians et al., 2018) and environmental conditions such as humidity, light, temperature and thermal changes etc. (Bryant et al., 2016; Kong https://doi.org/10.1016/j.solener.2020.06.016 Received 25 February 2020; Received in revised form 25 May 2020; Accepted 4 June 2020 ⁎ Corresponding authors. E-mail addresses: CHOWDHURY.Towhid@nims.go.jp (T.H. Chowdhury), jjlee@dongguk.edu (J.-J. Lee), islam.ashraful@nims.go.jp (A. Islam), Ahmed_El-Shafei@ncsu.edu (A. El-Shafei). Solar Energy 206 (2020) 279–286 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T