The emergence of organic-inorganic hybrid halides perovskite solar cells with cation of CH3NH3+ or HC(NH2)2+ has generated enormous interests in the photovoltaic research community. There remains scope for the studies of all-inorganic metal halide perovskites. Inorganic cations, such as Cs and Rb can be further explored as substitutes for MA or FA possibly from a device stability point of view. In particular, Cs-containing inorganic perovskites have demonstrated high electron and hole mobilities: up to 2300 cm2/V·s and 320 cm2/V·s respectively. Here, we demonstrate a hole-transport-material-free planar solar cell of cesium lead bromide iodide perovskite (CsPbIBr2) deposited by dual source thermal evaporation for the first time. The addition of iodine into the bromide lowers the bandgap resulting in wider solar spectrum absorption. Compared to the hybrid MA halide perovskites, CsPbIBr2 demonstrates better thermal stability and shows promising applications to the tandem structure.

In this study, Inorganic CsI and PbBr2 precursors are simultaneously evaporated onto a compact TiO2 layer (c-TiO2) on FTO glass substrates. Post-annealing is carried out on a hot plate in a glove box to enable the full crystallization of the CsPbIBr2 perovskite. A series of experiments investigating the effect of post-anneal conditions on the crystalline structure is conducted in this work.

Preliminary thermal stability tests were performed on CsPbIBr2 films which involves heat treatment at 200℃ on a hot plate in a glove box. The XRD patterns show no detectable phase change or impurity peaks in the films after the heat treatments, and the film photos shows no obvious degradation. In addition, the top view SEM shows no obvious morphology changes. All these provide evidences that CsPbIBr2 has good thermal stability. We also tested the thermal stability of CsPbIBr2 films in ambient conditions. The XRD and film photo results show that CsPbIBr2 films do not show obvious degradation after heated at 150℃ on a hot plate for 120 min in air, indicating its better thermal stability than MAPbI3.

The PCE of this cell measured under reverse scan at 1.2V/s is 4.7% with a VOC of 959 mV, JSC of 8.7 mA/cm2, and fill factor of 56%. Under forward scan, PCE= 3.7%; JSC= 8.7 mA/cm2; VOC = 818 mV and FF=52%. This gives an average PCE of 4.2% which is 89% of the PCE measured under reverse scan. Stabilized PCE’s measured at maximum power points have also been performed. Normalised results show that they are within 87% to 95% of the highest PCE’s measured (usually under reverse scan).