Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation.

Abstract

Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C71 butyric acid methyl ester (BTR:PC71BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV-vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation - rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.

• Authors: Michael J Newman, Emily M Speller, Jérémy Barbé, Joel Luke, Meng Li, Zhe Li, Zhao-Kui Wang, Sagar M Jain, Ji-Seon Kim, Harrison Ka Hin Lee, Wing Chung Tsoi
• Journal: Science and technology of advanced materials (Sci Technol Adv Mater) Vol. 19 Issue 1 Pg. 194-202 ( 2018) ISSN: 1468-6996 [Print] United States
• PMID: 29511397 (Publication Type: Journal Article)