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

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.

溶液加工型有机小分子太阳能电池（Solution-processed organic small molecule solar cells, SMSCs）的光电转换效率已突破11%。然而，目前鲜有研究关注其光照稳定性，以及所谓的“初始衰减期（burn-in period）”内的衰减根源。本文以苯并二噻吩并三噻吩若丹明:[6,6]-苯基C71丁酸甲酯（BTR:PC71BM）为活性层体系，通过调控活性层的溶剂蒸气退火时长来调控BTR相的结晶度，进而研究该溶液加工型SMSCs的初始衰减期行为。研究发现，初始衰减行为与BTR的结晶度密切相关。为探究潜在的衰减机制，我们采用掠入射X射线衍射（Grazing Incidence X-ray Diffraction, GIXD）、紫外-可见吸收光谱、拉曼光谱以及光致发光（PL）光谱，对新鲜样品与光老化共混薄膜进行了系统表征。结果表明，尽管BTR的结晶度会影响初始衰减期内的性能衰减，但该性能衰减并非源于BTR相的结晶度变化，而是与分子构象的改变密切相关——具体表现为噻吩侧链的旋转，该现象可通过拉曼光谱解析得到，且与轻微的光漂白及PL光谱变化存在关联。