Molten Salt Strategy for Reproducible Evaporation of Efficient Perovskite Solar Cells

Abstract

Vacuum evaporation is promising for the scalable fabrication of perovskite solar cells (PSCs). Nevertheless, the poor thermal conductivity of metal halide powder leads to unfavorable temperature inhomogeneity, which destabilizes the evaporation rate, posing a major challenge to the reproducible deposition of perovskite films, particularly by coevaporation. Herein, a molten salt strategy is reported for sequentially vacuum evaporation of PSCs. The molten salt increases the thermal conductivity of metal halides and greatly homogenizes the temperature, which stabilizes the evaporation rate and the composition of the resulting perovskite films. The PSCs yield power conversion efficiencies (PCEs) of ≈24% with exceptional reproducibility. The unencapsulated PSCs maintain 85% of the initial PCE after 3600 h of maximum power point tracking and maintain 85% of the initial PCE after being heated at 60 °C for 3000 h. The molten salt strategy opens a new avenue for the application of evaporation in perovskite optoelectronics.

Substoichiometric Mixing of Metal Halide Powders and Their Single-Source Evaporation for Perovskite Photovoltaics

“…This demonstrates that halide exchange between the precursors takes place close to and during evaporation and points toward the possibility of actively tuning halide exchange and premelted mixes in the future. This calls to mind the molten salt approach of Li et al previously discussed in this manuscript, where they melted the mixed salts before usage. Rapid interfacial halide diffusion has also been studied in the case of stacked metal halide perovskite thin films by Hautzinger et al They show that spin coating CsPbI 3 nanocrystals on top of a formed CsPbBr 3 nanocrystal thin film (then annealing at 50 °C to remove the solvent) results in considerable halide interdiffusion, which can be prevented with single-layer graphene between the two inorganic perovskites.…”

“…Eventually, when approaching 1 h of mixing, the only contribution came from the desired Cs 2 AgBiBr 6 perovskite phase. Li et al adjoined a salt melting step after the mechanical mixing of the powders (CsI + PbI 2 , CsI + PbI 2 + PbCl 2 , CsI + PbI 2 + PbCl 2 + PbBr 2 ) and were, according to their differential scanning calorimetry (DSC) measurements, able to obtain new phases distinct from the precursors, albeit without diffraction measurements to confirm the presence of a new crystallographic phase. All of the compositions investigated in this study feature a low cesium content (in the 0.1 Cs/1 Pb range).…”

Efficient and Thermally Stable Wide Bandgap Perovskite Solar Cells by Dual‐Source Vacuum Deposition

“…[60] Similar approaches have been previously used by us and others to reduce the number of deposition sources in the vacuum processing of multication/halide perovskites. [53,61] MAPb(I 1x Br x ) 3 perovskite films were prepared by dual source deposition starting from MAI and mixtures of increasing bromide/iodide ratios (Figure S4, Supporting Information), specifically Pb(I 0.9 Br 0.1 ) 2 , Pb(I 0.8 Br 0.2 ) 2, and Pb(I 0.7 Br 0.3 ) 2 . Previous works highlighted the difficulties associated with the sublimation control of MAI, [62,63] hence we have adopted a recently reported protocol based on a two source/two sensors system, which ensures a reliable and reproducible deposition process.…”

Applications of vacuum vapor deposition for perovskite solar cells: A progress review

“…The thermally evaporated PSCs with N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'diamine (NPB) as hole transport material (HTM) showed negligible aging after stored in air for 189 days (Figure 2f). Park et al proposed a highly oriented butylammonium Ruddlesden-Popper (RP) perovskite as a surface passivation layer via vacuum deposition (Figures 2b and 2c) [34] . The RP perovskite passivation layer can considerably reduce the trap density of bulk perovskite and enhance the charge transport.…”