Fundamentals of tin iodide perovskites: a promising route to highly efficient, lead-free solar cells

Hybrid tin-iodide perovskites are investigated as potential lead-free replacement of the lead-iodide perovskites; however, the intrinsic operational limit of these systems has not been described in detail, so far. In this work we combine advanced ab initio calculations with XRD and absorption measurements to lay out the fundamentals of formamidinium (FASnI3) and methylammonium (MASnI3) tin iodide perovskites, in comparison with the lead-halide MAPbI3 prototype. Our theoretical analysis reveals that the tin-based materials display an intrinsic photoconversion efficiency on a par with the lead perovskites, and even superior in the thick-layer limit, where the theoretical PCE reaches 30.5% for lead-halides, and 32.3% for tin-halides under AM1.5G illumination; this is the result of two competing factors: a smaller absorption cross section at the onset for stannates, and their smaller band gap of 1.36 eV, thus very close to the ideal Shockley–Queisser limit. We found the rate of photoluminescence emission extremely sensitive to the absorption spectral weight at the band extrema, resulting in B-factor as different as 7.6 × 10−9 s−1 cm3 for MASnI3 and 0.4 × 10−10 s−1 cm3 for FASnI3. The additional impact of Urbach energy and hole doping, giving rise to large Burstein–Moss effect, is described in detail.