Correlating active layer structure and composition with device performance and lifetime in amino acid modified perovskite solar cells

Abstract

Additive engineering is emerging as a powerful strategy to further enhance the performance of perovskite solar cells (PSCs), with the incorporation of bulky cations and amino acid (AA) derivatives being shown as a promising strategy for enhanced device stability. However, the incorporation of such additives typically results in photocurrent losses owing to their saturated carbon backbones hindering charge transport and collection. Here we investigate the use of amino acids with varying carbon chain lengths as zwitterionic additives that enhance PSC device stability, in air and nitrogen, under illumination. We discover thatstability is insensitive to chain length however, as anticipated photocurrent drops as chain length increases. Using glycine as an additive results in an improvement in open circuit voltage from 1.10 to 1.14 V and a resulting power conversion efficiency of 20.2% (20.1% stabilized). Using time-of-flight secondary ion mass spectrometry we confirm that the AAs reside at the surfaces and interfaces of our perovskite films and propose the mechanisms by which stability is enhanced. We highlight this with glycine as an additive, whereby an 8-fold increase in device lifetime in ambient air at 1-sun illumination is recorded. Short circuit photoluminescence quenching of complete devices are reported and reveal that the loss in photocurrent density observed with longer carbon chain AAs results from inefficient charge extraction from the perovskite absorber layer. These combined results demonstrate new fundamental understandings in the photophysical processes of additive engineering using amino acids and provide a significant step forward in improving the stability of high-performance PSCs.