Optical behaviour of γ-black CsPbI3 phases formed by quenching from 80 °C and 325 °C

The orthorhombic γ-black phase of CsPbI3 is well-known to be unstable at room temperature and strategies are needed to counteract its transformation tendency. In this paper we compare γ-black CsPbI3 thin films (∼80 nm) formed via two different routes: a fast quenching of the cubic α-phase from 325 ◦C (HT-γ) or spontaneously cooling the layer from 80 ◦C (LT-γ). The successful application of the second procedure is allowed by the use of a mother solution containing Europium with an Eu/Pb ratio as small as 5%. This has been indeed used to form both LT-γ and HT-γ thin films. The phase transition during the heating and cooling pathways is followed in situ by spectroscopic ellipsometry and x-ray diffraction analyses. We demonstrate that both γ-black phases exhibit the same absorption features and critical points as depicted in very details by the dielectric functions. Minor differences can be found in the intensity of the absorption coefficient, assigned to an improved lattice quality in the layer that has experienced the high temperature path. On the other hand, α-black and δ-yellow phases show different critical points in the optical transitions. Besides providing benchmarking optical parameters to discriminates the different phases, we demonstrate that the LT-γ phase closely competes with the HT-γ counterpart during stress tests for stability, with the first one more suited for tandem monolithic architectures that require thermal treatments under 200 ◦C.