Thermodynamically stabilized beta-CsPbI3-based perovskite solar cells with efficiencies > 18%

Although beta-CsPbI3 has a bandgap favorable for application in tandem solar cells, depositing and stabilizing beta-CsPbI3 experimentally has remained a challenge. We obtained highly crystalline beta-CsPbI3 films with an extended spectral response and enhanced phase stability. Synchrotron-based x-ray scattering revealed the presence of highly oriented beta-CsPbI3 grains, and sensitive elemental analyses-including inductively coupled plasma mass spectrometry and time-of-flight secondary ion mass spectrometry-confirmed their all-inorganic composition. We further mitigated the effects of cracks and pinholes in the perovskite layer by surface treating with choline iodide, which increased the charge-carrier lifetime and improved the energy-level alignment between the beta-CsPbI3 absorber layer and carrier-selective contacts. The perovskite solar cells made from the treated material have highly reproducible and stable efficiencies reaching 18.4% under 45 +/- 5 degrees C ambient conditions.