High-performance organic solar cells with spray-coated hole-transport and active layers

In this study, we report high performance organic solar cells with spray coated hole-transport and active layers. With optimized ink formulations we are able to deposit films with controlled thickness and very low surface roughness (<10 nm). Specifically we deposit smooth and uniform 40 nm thick films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as well as films composed of a mixture of poly(3-hexyl thiophene) (P3HT) and the C ₆₀-derivative (6,6)-phenyl C61-butyric acid methyl ester (PCBM) with thicknesses in the range 200-250 nm. To control film morphology, formation and thickness, the optimized inks incorporate two solvent systems in order to take advantage of surface tension gradients to create Marangoni flows that enhance the coverage of the substrate and reduce the roughness of the film. Notably, we achieve fill factors above 70% and attribute the improvement to an enhanced P3HT crystallization, which upon optimized post-drying thermal annealing results in a favorable morphology. As a result, we could extend the thickness of the layer to several hundreds of nanometers without noticing a substantial decrease of the transport properties of the layer. By proper understanding of the spreading and drying dynamics of the inks we achieve spray coated devices with power conversion efficiency of 3.75%, with fill factor, short circuit current and open circuit voltage of 70%, 9.8 mA cm^-2 and 550 mV, respectively. We show that spray coating can be used to deposit the organic layers in high-performance solution-processed organic solar cells. By proper engineering of the inks for the hole-transport layer and the polymer:fullerene mixture, we achieve highly uniform films with optimized and controllable thickness and low surface roughness. We demonstrate that the technique yields cells with nearly identical performance to the best spin-coated control devices.