Abstract
Formamidinium-lead-iodide (FAPbI3)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Record-performance FAPbI3-based solar cells have relied on fabrication via the sequential-deposition method; however, these devices exhibit unstable output under illumination due to the difficulty of incorporating cesium cations (stabilizer) in sequentially deposited films. Here we devise a perovskite seeding method that efficiently incorporates cesium and beneficially modulates perovskite crystallization. First, perovskite seed crystals are embedded in the PbI2 film. The perovskite seeds serve as cesium sources and act as nuclei to facilitate crystallization during the formation of perovskite. Perovskite films with perovskite seeding growth exhibit a lowered trap density, and the resulting planar solar cells achieve stabilized efficiency of 21.5% with a high open-circuit voltage of 1.13 V and a fill factor that exceeds 80%. The Cs-containing FAPbI3-based devices show a striking improvement in operational stability and retain 60% of their initial efficiency after 140 h operation under one sun illumination.
Highlights
Formamidinium-lead-iodide (FAPbI3)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap
The fabrication of FAPbI3-based perovskite films via the perovskite seeding growth (PSG) method is depicted in Fig. 1a and b
In the PSG method, perovskite growth commences immediately from the perovskite seeds when the alkylammonium halide salts are deposited on the PbI2 film
Summary
Formamidinium-lead-iodide (FAPbI3)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Hybrid perovskite solar cells have been developed for efficient solar energy conversion in light of the devices' high power conversion efficiencies (PCEs) and facile processing[1,2,3,4,5,6] Processing techniques such as one-step antisolvent crystallization and two-step sequential deposition methods have been developed to produce high-quality perovskite thin films[7,8,9,10]. FAPbI3-based perovskite solar cells have demonstrated superior initial performance and dark storage stability; the devices exhibit rapid performance degradation following operation at their maximum power point (MPP) even for a few hours under illumination[10,19,22,23] Their operational stabilty is far inferior to mixed cation-halide perovskites cells with high Bromide content (≥15%) via Cs or Rb incorporation (e.g., Cs-doped 1(F7,A20P,2b4I–32)70..8T5(hMesAePsbtBudr3ie)0s.1d5 emanodnstFrAat0e.83tChes0.c1r7uPcbi(aIl0.i8m3Bpro0r.1t7a)n3)c6e,1o4–f the Cs cation additive for the long-term photostability of mixed cation-halide perovskites. In addition to the deficiency of cesium incorporation, perovskite nucleation is poorly controlled in conventional two-step sequential processing: variability in the interdiffusion reaction between PbI2 and the organic compounds produces a substantial variation in device performance among processing batches[8,28,31]
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