Influence of ZnO sol–gel electron transport layer processing on BHJ active layer morphology and OPV performance

• Annealing of organic photovoltaic devices on sol–gel ZnO led to PCBM clusters.
• Spectroscopy showed that residual organics after ZnO pyrolysis induced PCBM cluster formation.
• Higher PCBM cluster area fraction correlated to lower device performance.
• Higher bimolecular recombination with cluster area fraction explained lower performance.
• Removal of organics minimized PCBM clusters and restored good device performance.

We report that thermal annealing of P3HT:PCBM bulk heterojunctons (BHJs) on top of a commonly used sol–gel ZnO electron transport layer can lead to the formation of PCBM clusters, which significantly decreased short circuit current density (up to 45%), open circuit voltage (up to 35%), and fill factor (up to 15%). To understand the cause of this phenomenon, we deposited ZnO films using different sol–gel recipes and systematically studied the correlation between sol–gel ZnO processing and the morphology of the P3HT:PCBM BHJ. We showed that higher amounts of monoethanolamine (MEA), a stabilizer in the ZnO sol–gel precursor, lead to higher densities of PCBM clusters in the BHJ. In addition, we correlated the cluster area fraction with bimolecular recombination at 100 mW/cm2, as quantified by white-light biased external quantum efficiency measurements. Finally, we show that rinsing off residual organics from the ZnO surface after pyrolysis eliminated PCBM cluster formation, avoiding poor organic photovoltaic device performance.