Material and Device Stability of Lead-Tin Perovskite Solar Cells

Narrow bandgap perovskite solar cells based on mixed lead-tin perovskites tend to suffer from poor stability under operating conditions. This impedes the successful development of all-perovskite tandems. We explore the causes of this instability under extended periods of combined 65°C thermal and 1 sun illumination stressing, using a range of structural, optical, and electronic characterization techniques on lead-tin perovskite films, half-stacks and devices.<br/><br/>We show that the phase, absorbance, morphology and mobility of lead-tin perovskite films are stable on timescales that exceed those of device degradation, although we reveal an interesting pattern of phase segregation after stressing for much longer amounts of time. Additionally, we observe only a slight increase in background carrier density and a moderate decrease in charge carrier lifetime during the first few hundred hours of stressing. We simulate the impact of these properties on device performance using SCAPS, and argue that these changes can only partially account for the observed device degradation.<br/><br/>A close investigation of the EQE and J-V characteristics of devices reveals the formation of a charge extraction barrier in aged devices. We find that the impact of this barrier is hugely decreased in very fast J-V scans, suggesting that mobile ions contribute significantly to device degradation. We quantify the increasing impact of mobile ions on device performance during aging, and furthermore reveal that the extent of this impact is strongly related to the hole transport layer used in devices.<br/><br/>Ultimately, we identify a rapidly worsening impact of mobile ions during aging as the major cause of the observed device performance degradation. The segregation of a non-perovskite CsSnI₃ degradation phase and an increasing defect density are also expected to limit the stability of lead-tin perovskite solar cells over longer timescales. Finally, we propose solutions related to both bulk perovskite composition and device architecture to overcome these challenges.