Intensity Dependence of Current–Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells

Solution-processed small-molecule p-DTS(FBTTh2)2:PC71BM bulk heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a calcium layer between the BHJ layer and the Al cathode; for 20 nm Ca thickness, the FF is 73%, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100% in the spectral range of 600–650 nm and well over 80% across the entire spectral range from 400 to 700 nm. Analysis of the current–voltage (J–V) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the J–V curves are dominated by first-order recombination from the short-circuit condition to the maximum pow...