Synergistic effect of processing additives and thermal annealing in organic solar cells: the “Morphology of Magic”†
Organic solar cells (OSCs) usually need to be optimized with the strategies of thermal annealing (TA), solvent vapor annealing (SVA), or processing additives (PA) to obtain the best performance. Here, PA and TA were used simultaneously for OSCs based on the novel organic molecules TBDT-T6ffBT and OBDT-T6ffBT. The synergistic effect of PA and TA on the active-layer morphologies was investigated by measurements of optical microscopy, atomic force microscopy, transmission electron microscopy, and grazing incident X-ray diffraction. Comprehensive results suggest that a PA can enhance the crystallinity of the active layer, whereas subsequent TA treatment can gradually undermine a well-ordered morphology with increasing TA times. However, upon using a PA with reasonable TA treatment (temperature and annealing time), the blends can develop an interpenetrating network with appropriate phase separation to facilitate charge transfer and transportation. Therefore, an OSCs device based on TBDT-T6ffBT:PC61BM showed the best power conversion efficiency (6.1%) after treatment with 0.5% 1-chloronaphthalene additive and further annealing at 130 °C for 40 s. These results demonstrate that highly efficient OSCs can be achieved through optimization of active-layer morphology via appropriate PA and TA treatment.