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.