Influence of active layer thickness on the performance of P3HT: PC$_{61}$BM organic photovoltaic devices analyzed via J–V characteristics

Abstract

Optimizing the thickness of the active layer is essential to improve the performance of organic solar cells. In this study, we examined how the thickness of the active layer, made of a P3HT:PC₆₁BM blend, influences the key electrical parameters of inverted organic solar cells. Four prepared photovoltaic cells were prepared using a typical device structure of  ITO /ZnO /P3HT:PC₆₁BM /MoO₃ /Ag . The active layer of  P3HT : PC₆₁BM was deposited by spin coating with different thicknesses of 80, 150, 200, and 300 nm.  We used the different J-V characteristics to extract parameters such as the open-circuit voltage, short-circuit current density, series resistance, and shunt resistance. Based on these measurements, the results indicate that V_oc remains nearly stable between 80 nm and 200 nm, before dropping sharply at 300 nm due to recombination losses. Furthermore, J_sc increases with thickness and reaches its maximum at 300 nm, thanks to improved light absorption. The fill factor peaks (~50%) at a thickness of 150 nm, then decreases, reflecting an imbalance between charge collection and internal losses. To provide a deep understanding of this behavior, we also analyzed the resistive parameters. The series resistance R_s increases from 27.11 Ω·m² to 27.52 Ω·m² when the thickness increases from 80 to 150 nm due by the increased defect density in the layer structure with additional layer stacking, then rises to 28.87 –34.55 Ω·m² for 200–300 nm respectively because of the longer charge transport path and increased recombination. Finally, the shunt resistance R_sh increases up to 150 nm suggests improved film uniformity and better surface coverage, thus minimizing shunt defects, before decreasing at 200 and 300 nm. This suggests that increasing the material volume statistically leads to a higher density of volumetric defects.