The transformation of In2S3 film from flower-like to net-like structure by tunable microwave reaction for buffer layer in CuInS2/TiO2 solar cells

Two morphologies of In2S3 film were synthesized directly on FTO surface with microwave-assistant solution method. With reaction time/temperature/microwave power increasing, In2S3 film assembled by hierarchical flower-like structure (In2S3 flower) transformed into In2S3 film assembled by net-like structure (In2S3 net) with increased size and crystallinity. Compared to In2S3 flower, In2S3 net displayed stronger light-harvesting ability with higher charge separation, contributing to stronger photocurrent and lower charge transfer resistance. Based on the excellent photoelectrical properties, TiO2/In2S3/CuInS2/spiro-oMeTAD solar cells were fabricated with two In2S3 films as the buffer layers as compared to the device without In2S3, which displayed obviously higher photovoltaic parameters due to the decreased surface defects on TiO2 and the increased complementary absorption from In2S3. Furthermore, the device with In2S3 net presented higher short current density (Jsc) than that with In2S3 flower because the thicker In2S3 layer strengthened the contribution of complementary light-harvesting. However, it displayed lower open circuit voltage (Voc), which was attributed to the presence of defects on the surface of overgrown In2S3 for increased interfacial recombination.