Solar Energy 208 (2020) 583–590 Available online 18 August 2020 0038-092X/© 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. Electrical performance of PTB7-Th:PC 71 BM solar cell when in contact with the environment F.L. Araújo a , D.R.B. Amorim a , B.B.M Torres a , D.J. Coutinho b , R.M. Faria a, * a S˜ ao Carlos Institute of Physics, University of S˜ ao Paulo, PO Box 369, 13560-970 S˜ ao Carlos, SP, Brazil b Federal Techological University of Paran´ a (UTFPR), Toledo, PR, Brazil A R T I C L E INFO Keywords: Organic solar cell Degradation Charge carrier mobility Langevin coeffcient reduction factor ABSTRACT The goal of this work is to make a contribution to the complex subject of organic solar cells degradation when operating in contact with ambient air. For this we built solar cells of the bulk heterojunction type composed of PTB7-Th (poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethyl- hexyl)-3-fuorothieno [3,4-b]thiophene-) -2- carboxylate- 2–6-diyl)]) and PC71BM ([6,6]-Phenyl-C71-butyric acid methyl ester), respectively, as donor and acceptor materials of electrons. We present a series of cur- rent–voltage curves (J-V), both in the dark and under illumination, initially with the device immersed in an inert atmosphere and then exposing it to the ambient environment. J-V curves is then analyzed in terms of the Mott- Gurney equation, for measurements in dark, and by an analytical expression for the photocurrent, which allow us to check how the charge carrier mobility and the non-geminate recombination coeffcient vary with time of exposure to air. We conclude that the charge carrier mobility (μ) for cells made with the PTB7-Th:PC 71 BM blend varies very little when in contact with air, and similar infuence is observed on the recombination coeffcient (k), since the Langevin coeffcient reduction factor (ζ) is about 0.2 when the device is in inert atmosphere, reaching a value of almost 0.9 when exposed to air for 43 h. 1. Introduction One of the most pressing problems to be solved by humanity along the frst half of the 21st century is related to the development of new sources of clean and renewable energy (Dincer, 2000; Panwar et al., 2011). Solar energy presents itself as a virtually inexhaustible source of energy with the great advantage of being environmentally friendly. Among the alternative ways of converting solar energy into electrical energy stand out that of the photovoltaic devices, whose development based on new semiconductor materials and on devices of thin flms has shown more and more promising in the recent years (Polman et al., 2016; Kabir et al., 2018). In the scope of this new generation of photo- voltaic devices, that of organic bulk heterojunction solar cells types (BHJ-OSC) have been gaining credibility by exhibiting some processing advantages, such as lightness and fexibility, printing in large areas, and low-cost manufacturing (Ou et al., 2016; Zhang et al., 2018). These characteristics give these photovoltaic devices the versatility of being applied to any type of surface, even on curved and large area surfaces (Graham Morse et al., 2017). The active layer of BHJ-OSCs is a nano- structured system consisting of an intermixed biphasic layer, in which one is a conjugated polymer, as electron donor, and the other a highly electronegative molecule as electron acceptor. Despite the growing ef- fciency gains achieved by the BHJ-OSCs in recent years, breaking the 15% barrier (Che et al., 2018; Meng et al., 2018; Yuan et al., 2019), the stability of these cells is still far from being solved, especially due to deleterious consequences coming from the atmospheric environment (Cheng and Zhan, 2016; Cao et al., 2014; Sun et al., 2019). This adversity is worsened because photovoltaic devices usually operate in contact with air ambient, therefore, being exposed to many degradation pathways. That is, the power conversion effciency is reduced due to the attack of light, water, and mainly from the action of oxygen molecules (Kawano et al., 2006; Grossiord et al., 2012; Havlicek et al., 2018; Arshad and Maarouf, 2018). Thereby, several are the causes and effects of BHJ-OSCs degradation. The most usual is the photo-oxidation that occurs in all conjugated polymers and comes from the concomitant ac- tion of light and oxygen (Manceau et al., 2011). However, a more complete description of degradation effects in BHJ-OSCs is hampered by the diversity and complexity of the involved phenomena. For solar cells based on P3HT:PCBM, which has been the most studied BHJ-OSC so far, the observed decrease in mobility has been attributed to the oxygen- * Corresponding author. E-mail address: faria@ifsc.usp.br (R.M. Faria). Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener https://doi.org/10.1016/j.solener.2020.08.005 Received 13 June 2019; Accepted 2 August 2020