Improved Thermal Stability and Film Uniformity of Halide Perovskite by Confinement Effect brought by Polymer Chains of Polyvinyl Pyrrolidone

Abstract

Polyvinyl pyrrolidone (PVP) is doped to PbI2 and organic salt during two‐step growth of halideperovskite. It is observed that PVP molecules can interact with both PbI2 and organic salt, reduce the aggregation and crystallization of the two, and then slow down the coarsening rate of perovskite. As doping concentration increases from 0 to 1 mM in organic salt, average crystallite size of perovskite decreases monotonously from 90 to 34 nm; Surface fluctuation reduces from 259.9 to 179.8 nm at first, and then increases; Similarly, surface roughness decreases from 45.55 to 26.64 nm at first, and then rises. Accordingly, a kind of “confinement effect” is resolved to crystallite growth and surface fluctuation/roughness, which helps to build compact and uniform perovskite film. Density of trap states (t‐DOS) is cut down by ≈60% at moderate doping (0.2 mM). Due to the “confinement effect”, power conversion efficiency of perovskite solar cells is improved from 19.46 (±2.80) % to 21.50 (±0.99) %, and further improved to 24.11% after surface modification. Meanwhile, “confinement effect” strengthens crystallite/grain boundaries and improves thermal stability of both film and device. T80 of device increases to 120 h, compared to 50 h for reference ones.

Polyvinyl Pyrrolidone Induced “Confinement Effect” on PbI2 and the Improvement on Crystallization and Thermal Stability of Perovskite

“…Clearly, PVP has accelerated the reaction between PbI 2 and organic salt. Such behavior is different from that observed before, where rising PVP [25] or PAA [24] concentration in organic salt was observed to slow down the reaction rate between PbI 2 and organic salt. The accelerated reaction rate is due to the interaction between PVP and PbI 2 , as will be reflected from following studies on morphological and crystallographic properties.…”

“…Finally, the role of PVP polymer chains on thermal stability of and ion migration behavior in PVSK is examined. Interestingly, such an effect differs from that observed before, [25] but it could enlarge the scope of the "confinement effect" induced by polymer chains.…”

“…The shift is due to the interaction between PVP molecules with PbI 2 . [25,26] The interaction could be also reflected from FTIR study. As is shown in Figure 1c, three samples are tested, PVP power, PbI 2 •DMF (dried from PbI 2 -DMF precursor), and PbI 2 •DMF being blended by PVP (noted by PVP@PbI 2 •DMF).…”

“…Above observations indicate that adding certain amount of PVP in PbI 2 is beneficial to obtain flat and uniform PVSK films, such behavior is same to that observed in the previous study. [25] The improved grain size and flattened film surface are due to the interaction (or crosslinking behavior) between PVP molecule chains. Since PVP could react with both PbI 2 and organic salt, [25] PVP molecule chains could wrap on the particle surfer and lead to the "confinement effect" on PVSK film, which help to grow smooth film.…”

“…Another work of our group showed that varying the blending concentration of polyvinyl pyrrolidone (PVP) in organic salt could trigger a kind of "confinement effect" on PVSK crystallites, by which the average crystallite size of PVSK decreased, while the film surface was flattened. [25] In addition, the thermal stability was improved due to the coverage of PVP polymer chains on PVSK grains / crystallites. However, in these studies, only the effect of the blending concentration of polymer in organic salt was considered, while it was not clear for the PbI 2 side.…”