Performance optimization of low-temperature-annealed solution-processable ZnO buffer layers for inverted polymer solar cells

High-performance solution-processable ZnO thin films for use as electron-transporting layers (ETLs) of inverted-structured polymer solar cells (I-PSCs) are developed via a low-temperature annealing (<200 degrees C) sol-gel process. The properties of the low-temperature-annealed ZnO (L-ZnO) thin films (used as ETLs) are optimized based on the evaluation of the roles of the internal nanocrystal (NC) orientation and film-surface morphology in charge transport/transfer in I-PSCs. The low-temperature annealing conditions (dynamic annealing or static annealing) could be successfully manipulated to alter the NC orientation of the L-ZnO films, whereas tactical control of the precursor-coating conditions enabled the embedding of nanoripples on the film surfaces. Suppression of the preferential (002) plane NC orientation of the L-ZnO layers is beneficial for charge transport in I-PSCs; these devices should be evaluated in a manner different from field-effect transistors (FETs). The performance of ETLs is further enhanced by the development of nanoripple-embedded L-ZnO film surfaces, which furnish an increased area for contact with the active layers. The I-PSCs fabricated using the optimized L-ZnO films display a >20% higher power-conversion efficiency (PCE) than those employing the conventional L-ZnO films for a range of active materials including poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and poly(thienothiophene-co-benzodithiophenes)7-F20 (PTB7-F20)/phenyl-C71-butyric acid methyl ester (PC71BM) blends. A PCE of 6.42% is achieved for the I-PSCs using the optimized L-ZnO films and PTB7-F20/PC71BM blends as the ETL and active materials, respectively. This study presents a universal method for optimizing sol-gel-driven ZnO-based ETLs, whilst the low-temperature processability and long-term stability of the developed ETLs are beneficial for the commercialization of I-PSCs.