Investigation of conventional ITO/MoO3/P3HT:PC61BM/Ca/Al bulk heterojunction polymer solar cells through impedance spectroscopy in view of power conversion efficiency improvements

Herein, a bulk heterojunction (BHJ) organic solar cell of configuration ITO/MoO3 (∼10 nm)/P3HT:PC61BM (∼100 nm)/Ca (∼20 nm)/Al (∼100 nm) having power conversion efficiency (PCE) ∼1.89, fill factor of ∼55, Jsc ∼5.53 mA/cm2 and Voc ∼0.62 V is characterized through impedance spectroscopy in dark as well as in illuminated conditions with varied bias voltages between 0.0 V to ±0.7 V. The impedance data shows a depressed semicircle which may be modeled as usual by an equivalent circuit consisting of resistors, capacitors and a constant phase element. The model may be based on the geometrical capacitance, chemical capacitance as well as on the recombination resistance related to charge carrier accumulation/bimolecular recombination in the active layer. A typical impedance data, both in the form of cole-cole or in Bode mode is represented in view to understand its dependence on device of desired structure. There is a correlation between the frequency at which reactance reaches a peak value and a particular device structure and, consequently, the solar cell efficiency.