A comparison of molecular iodine evolution on the chemistry of lead and tin perovskites

Abstract

The development of perovskite solar cells (PSCs) has gone from strength to strength over the last decade, enabling low-cost, flexible and high-efficiency photovoltaic devices. However, the significance of molecular iodine (I₂) evolution within the perovskite layer on device longevity is only recently becoming realised. In lead-based perovskites, the formation of I₂ is determinantal to both the photovoltaic performance of PSCs and the long-term stability. Likewise, I₂ formation within tin perovskite is highly destructive; rapidly breaking down the composition of the perovskite layer, and severely limiting the shelf-life of photovoltaic devices. In both cases, the formation of I₂ has a significant additional impact on the stability and function of all other elements of the PSC structure including the conductive metal oxide, metal electrode and charge transport layers (CTLs). In this perspective, we highlight the key role of iodine in dictating the performance and stability of lead and tin perovskite materials. In doing so we compare the similarities and differences between the formation mechanisms of molecular I₂ in the lead and tin analogues while also considering its effect on the performance of PSCs through consideration of the various elements of the PSC structure. In discussing this challenge, we look to identify new emerging ways in which volatile iodine has been captured within other scientific fields and discuss the applicability, modification and utilisation of these strategies within PSCs. Finally, through consideration of the fundamental chemistry in these systems, we summarise the all-important role of iodine in PSCs, discuss efforts being made to mitigate the damage of I₂ evolution, manage the redox chemistry, and provide design criteria for developing iodine-resilient PSCs.