Reductant Engineering in Stable and High‐Quality Tin Perovskite Single Crystal Growth for Heterojunction X‐Ray Detectors

Tin perovskites have emerged as a promising alternative material to address the toxicity of lead perovskites and the low bandgap of around 1.1 eV is also compatible with tandem solar cell applications. Nevertheless, the optoelectronic performance of solution-processed tin perovskite single-crystal counterparts still lags behind because of the tin instability under ambient conditions during crystal growth and limited reductants to protect the Sn2+ ions from oxidation. Here, the reductant engineering to grow high-quality tin perovskite single crystals under ambient conditions is studied. Oxalic acid (H2 C2 O4 ) serves as an excellent reductant and sacrificial agent to protect Sn2+ ions in methanol due to its suitable redox potential of -0.49 V, and the CO2 as the oxidation product in the gas state can be easily separated from the solution. The FPEA2 SnI4 single crystal grown by this strategy exhibits low trap density perovskite surface by constructing an FPEA2 PbI4 -FPEA2 SnI4 (FPI-FSI) single crystal heterojunction for X-ray detection. An improved X-ray sensitivity of 1.7 × 105 µC Gy-1 cm-2 is realized in the heterojunction device, outperforming the control FPEA2 PbI4 counterpart.