Indacenodithiophene-based wide bandgap copolymers for high performance single-junction and tandem polymer solar cells

Two wide bandgap copolymers based on bulky indacenodithiophene (IDT) and alkoxylated benzothiadiazole units (PIDTBTO-T and PIDTBTO-TT) with the thiophene or thieno[3,2-b]thiophene (TT) π-bridge are designed and synthesized. The effect of π-bridge on the π-π packing, optical, carrier transport, nano-sized phase separation and photovoltaic properties of the copolymers are investigated in depth. In comparison with the PIDTBTO-T-based counterpart, the best performance solar cell based on PIDTBTO-TT exhibits a higher power conversion efficiency (PCE) of 8.15% which is mainly attributed to the formation of a fibrous network for the active layer based on PIDTBTO-TT. Furthermore, when a novel hybrid electron transport layer (PDIN:PFN) is introduced into a tandem solar cell using the PIDTBTO-TT-based device and a PTB7-Th-based device as the bottom and top cell components, respectively, the resulting solar cell exhibits an outstanding PCE of 11.15% with a large open circuit voltage of 1.70   V. To the best of our knowledge, the PCEs of 8.15% and 11.15% are the highest values reported to date for the single-junction and tandem solar cells using IDT-based copolymers, respectively. Our results demonstrate that the π-bridge modulation is effective in adjusting the charge carrier mobility and photovoltaic performance of IDT-based wide bandgap copolymers for single-junction and tandem devices.