Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH­(NH2)2PbI3 (FAPbI3) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δH-phase (yellow phase), with δH-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (δO-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI3 with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1–x Cs x PbI3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA0.85Cs0.15PbI3 perovskite alloy demonstrate much improved stability in a high-humidity environment; this contrasts significantly with the pure FAPbI3 film for which the α-to-δH phase transition (associated with yellowing appearance) is accelerated by humidity environment. Due to phase stabilization, the FA0.85Cs0.15PbI3 solid-state alloy showed better solar cell performance and device stability than its FAPbI3 counterparts. Our studies suggest that tuning the tolerance factor through solid-state alloying can be a general strategy to stabilize the desired perovskite structure for solar cell applications.

No abstract

No abstract

Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky 2D cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic 2D spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluoro-phenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained power conversion efficiency >13% for (F-PEA)2MA4Pb5I16 based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.