Effect of cathode interface thickness on the photovoltaic parameters of bulk heterojunction organic solar cells

We investigate the role of cathode interfacial layer thickness in tuning the barrier height and charge transport in inverted bulk heterojunction organic solar cells (BHJ OSCs). The variation of cathode layer thickness significantly changed the cell parameters like open-circuit voltage (Voc) and short-circuit current density (Jsc). For better understanding of the charge transport properties in the device, photoinduced capacitance spectroscopic studies and temperature dependent studies were carried out in polythieno[3,4-b]thio phene/benzodithiophene : [6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM) based inverted BHJ OSC with ZnO as the cathode interfacial layer. These studies gave insight into the charge accumulation and barrier height at the active layer/ZnO interface. Due to the presence of trap states in the ZnO layer, charges can accumulate at the interface creating a barrier for the smooth extraction of charge carriers. The barrier height is found to increase from 0.05 to 0.13 eV as the ZnO thickness is varied from 30 to 250 nm that can be attributed to the increase in the density of trap states in the ZnO layer. Intensity-dependent studies were also performed to analyze the recombination processes in the device.