Origin of the Improved Power Conversion Efficiency of Pentacene/C60 Heterojunction Photovoltaic Devices through the Purification of Donor Material

The authors report the impact of the crystallinity property of the electron donor on the performance of a pentacene/C60 organic photovoltaic device. After subjecting pentacene to sublimation twice, all the photovoltaic parameters showed significant improvements leading to enhancement of the power conversion efficiency from 0.9 to 2.2% under air mass 1.5G solar illumination. This is attributed to the well-packed molecular structure in the pentacene thin film, as observed by X-ray diffraction, which leads to high carrier mobility and hence high photocurrent. Moreover, the elimination of microscopic pinholes or defect sites due to the improvement in the degree of the pentacene thin film reduces the dark current and therefore increases the photovoltage. The external quantum efficiency and space-charge limited current are used to analyze the relationship between the quality of thin film electron donor and device performance.