Properties and solar cell applications of Pb-free perovskite films formed by vapor deposition

Abstract

We deposited lead-free CH₃NH₃SnBr₃ – organometal halide perovskite films using two vapor deposition-based methods (i.e., co-evaporation and sequential evaporation) using SnBr₂ and CH₃NH₃Br. We obtained comprehensive information about the structural and electronic properties of these Pb-free perovskite films. X-ray diffraction results confirmed the crystalline structure of MASnBr₃. Using UV-Vis measurements, we determined the optical bandgap of these films to be ∼2.2 eV. On the basis of ultraviolet photoemission spectroscopy results, the work function and ionization energy were measured to be 4.3 and 6.1 eV, respectively. Solar cells employing such a perovskite film in a planar structure were fabricated with various hole transport layers (HTLs) (spiro-OMeTAD, C60, and P3HT). For perovskite films prepared by co-deposition, we obtained solar cell efficiencies ranging from 0.03 to 0.35%. On the other hand, when we used sequential deposition, a higher efficiency up to 1.12% was obtained using P3HT as the HTL. We confirmed that the low efficiency of MASnBr₃ based perovskite solar cells is due to their relatively high resistance and the fast formation of Sn–Br oxide on the top surface by air exposure during fabrication. The sequential deposition method helped avoid such oxidation resulting in higher efficiencies.