Bifacial stamping for high efficiency perovskite solar cells

Abstract

We report a novel approach for a fast phase transition of FAPbI(3) (FA = formamidinium) at low-temperature and the effective removal of interfacial recombination in MAPbI(3) (MA = methylammonium). This method also allows for printing (patterning) of the perovskite on a desired area. The pre-annealed MAPbI(3) and -phase FAPbI(3) films were prepared by spin-coating DMSO and a polar aprotic solvent admixed precursor solution at 65 degrees C and 100 degrees C for about 1 min, respectively, to form adduct films containing DMSO. Two films were sandwiched without pressure by a method called bifacial stamping, and annealed at 100 degrees C for 9 min, which resulted in complete phase transition of FAPbI(3) and led to a power conversion efficiency (PCE) of 18.34%. The stamped MAPbI(3) demonstrated a PCE of 20.18% that was much higher than the conventionally annealed MAPbI(3) (approximate to 17.4%) mainly due to a much higher fill factor and open-circuit voltage. Optical and structural studies revealed that DMSO-mediated ion exchange plays a vital role in the phase transition of FAPbI(3) and the surface modification of MAPbI(3). Theoretical calculation results further support the role of DMSO in the phase transition at low temperature. Stamping was applied to EAPbI(3) (EA = ethylammonium), where photoinactive yellow EAPbI(3) changed to photoactive EAPbI(3) with a PCE of 13.02% after stamping with MAPbI(3). The DMSO-mediated EA/MA ion exchange reaction during the stamping process created a new layer having a gradient solid solution of EAPbI(3) and MAPbI(3), which was responsible for the abnormally high PCE of the EAPbI(3) based perovskite solar cell. Facilitated ion transport by a Lewis base (such as DMSO) reservoir in the perovskite adduct film is suggested to be involved in the bifacial stamping procedure.