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 power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open-circuit condition in the optimized device with a Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley–Read–Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) lead to the high fill factor.