Thermal Stability of Bulk Heterojunction Photovoltaics Revealed by Electrical Scanning Probe Microscopy

Thermal stability is very important for real application of bulk heterojunction (BHJ) solar cells. In this work, we report the thermal stability and degradation behavior of the BHJ solar cells. The evolution of the local structures and in situ charge separation and transport were revealed by electrical scanning probe microscopy (ESPM). The BHJ solar cells are composed of a 2D conjugated thienyl-substituted BDT with substituted TT (PBDTTT-C-T) or conventional semicrystalline poly(3-hexylthiophene) (P3HT) as the electron donors and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) or indene-C60 bisadduct (ICBA) as the acceptors. Among these four material pair-wise combinations, the PBDTTT-C-T/ICBA combination show highest thermal stability (the PCE declined by 10.77%) while the P3HT/PCBM combination shown worst (declined by 42.82%). Morphological and charge information with nanoscale resolution were provided. The hole transport and mobility of the P3HT-based BHJ blend films declined significantly after the aging treatment. However, for the PBDTTT-C-T-based BHJ system, the hole current was even enhanced. Physical evolution processes were proposed to depict the thermal aging effects within the active layer. This work provides new insights into the rational design of thermally stable photovoltaic systems by considering mesoscopic heterogeneity.