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.

Recent research in organic photovoltaic (OPV) is largely focused on developing low cost OPV materials such as graphene. However, graphene sheets (GSs) blended conjugated polymers are known to show inferior OPV characteristics as compared to fullerene adduct blended with conjugated polymer. Here, we demonstrate that graphene quantum dots blended with regioregular poly(3-hexylthiophene-2,5-diyl) or poly(2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene) polymer results in a significant improvement in the OPV characteristics as compared to GSs blended conjugated polymers. This work has implications for inexpensive and efficient solar cells as well as organic light emitting diodes.

We successfully demonstrate inverted structure small-molecule (SM) solar cells with an efficiency of 7.88% using ZnO and PEIE as an interfacial layer. Modification of ZnO with a cost-effective PEIE thin layer increases the efficiency of the inverted cell as a result of reducing the work function of the cathode and suppressing the trap-assisted recombination. In addition to the high efficiency, the inverted SM solar cells are relatively stable in air compared to conventional cells.