Record High Efficiency Achievement under LED Light in Low Bandgap Donor-Based Organic Solar Cell through Optimal Design

Recently, a low bandgap donor named poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b $\prime$ ]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4b]thiophene-)-2-carboxylate-2-6-diyl)]- (PTB7-Th-) based organic photovoltaic (OPV) devices has exhibited interesting behavior when tested under indoor light. Theoretically, a PTB7-Th : [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) active layer-based OPV can show >23% power conversion efficiency (PCE) under light-emitting diode (LED) light. However, to date, the experimentally achieved PCE (~11.63%) is significantly lower than the theoretical one. Therefore, we design an indoor OPV having PTB7-Th : PC70BM active layer and low-acidic and cheaper polypyrrole : polystyrene sulfonate (PPY : PSS) as the hole transport layer (HTL), by optimizing active layer thickness and processing conditions (spin coating speed and doping concentration) of the HTL via optical simulations and experiments. The results show that the device having 100 nm thick active layer and a PPY : PSS-based HTL (PPY : PSS; weight ratio between PPY and PSS 1 : 2) coated at 5000 rpm can exhibit a record high PCE value (16.35%) during its operation under 1000 lx LED lamp. In comparison, a commercially available PEDOT : PSS-based OPV can achieve maximum 14.21% PCE under the same conditions.