Correlating the Active Layer Structure and Composition with the Device Performance and Lifetime of Amino-Acid-Modified Perovskite Solar Cells

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 AAs with varying carbon chain lengths as zwitterionic additives to enhance the PSC device stability, in air and nitrogen, under illumination. We, however, discovered that the device stability is insensitive to the chain length as the anticipated photocurrent drops as the chain length increases. Using glycine as an additive results in an improvement in the 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 the device lifetime in ambient air at 1 sun illumination is recorded. Short-circuit photoluminescence quenching of complete devices is reported, which reveals that the loss in photocurrent density observed with longer carbon chain AAs results from the inefficient charge extraction from the perovskite absorber layer. These combined results demonstrate new fundamental understandings about the photophysical processes of additive engineering using AAs and provide a significant step forward in improving the stability of high-performance PSCs.