Electrical performance of PTB7-Th:PC71BM solar cell when in contact with the environment

• Published in: Solar energy Vol. 208; pp. 583 - 590
• Main Authors: Araújo, F.L., Amorim, D.R.B., Torres, B.B.M, Coutinho, D.J., Faria, R.M.
• Format: Journal Article
• Language: English
• Published: New York Elsevier Ltd 15.09.2020; Pergamon Press Inc
• Subjects: Acceptor materials; Atmosphere; Butyric acid; Carrier mobility; Charge carrier mobility; Current carriers; Degradation; Electric contacts; Exposure; Heterojunctions; Inert atmospheres; Langevin coefficient reduction factor; Mobility; Organic solar cell; Photoelectric effect; Photoelectric emission; Photovoltaic cells; Recombination; Recombination coefficient; Solar cells; Solar energy; Degradation; Organic solar cell; Charge carrier mobility; Langevin coefficient reduction factor
• ISSN: 0038-092X; 1471-1257
• DOI: 10.1016/j.solener.2020.08.005

•Degradation of a PTB7-Th:PC71BM-based OSC investigated by electrical measurements.•The charge carrier mobility showed only a minor change when exposed to ambient air.•The PCE loss was correlated to the bimolecular recombination coefficient increasement.•The variation of the reduction factor with the air exposure time was also obtained. 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-ethylhexyl)-3-fluorothieno [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 current–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 coefficient vary with time of exposure to air. We conclude that the charge carrier mobility (μ) for cells made with the PTB7-Th:PC71BM blend varies very little when in contact with air, and similar influence is observed on the recombination coefficient (k), since the Langevin coefficient 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.