Bifacial stamping for high efficiency perovskite solar cells

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 δ-phase FAPbI ₃ films were prepared by spin-coating DMSO and a polar aprotic solvent admixed precursor solution at 65 °C and 100 °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 °C for 9 min, which resulted in complete δ → α 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 (~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.