Dark and illuminated J(V) characteristics of thin layered bulk heterojunction P3HT:PCBM sandwich solar cells after thermal treatment

Organic photovoltaic solar cells can offer advantages of being mechanically flexible and durable, large area devices, lightweight, made from a diversity of materials and low-cost fabrication. Their efficiency is, however, still too low for commercial exploitation. Empirical observations reveal that polymer-fullerene (P3HT:PCBM) based solar cell performance depends on thermal annealing processes employed, especially the annealing temperatures and durations. The annealing parameters are known to influence the energetics and kinetics of the blending process or morphology, but the associated physics is not fully understood. In this work, current density-voltage characteristics of P3HT:PCBM bulk heterojunction organic solar cells, thermally annealed at different temperatures, 65-160 degrees C post fabrication, were investigated under dark and illuminated conditions, and compared to their as-cast counterparts. In certain electrical regimes, as-cast devices showed higher values of current density in comparison to the corresponding annealed devices. Such performance was attributed to air-borne chemical doping of the as-cast semiconductor layer, which creates electrically conductive percolation pathways within the as-cast devices. We propose that annealing of semiconductors must be a two-step process, which first initiates decrease in conductivity, followed by its increase. As-cast devices P3HT-PCBM bulk heterojunction solar cells prepared under atmospheric conditions were observed to have comparatively superior photovoltaic performance in comparison to thermally annealed devices. The efficiency drop in the annealed counterparts is attributed to dedoping due to thermal annealing. An annealing temperature of similar to 140 degrees C was found to be optimum for power conversion efficiency in the bulk heterojunction, 1:1 by mass, P3HT:PCBM based solar cells.