Study of compositional stability and related optical properties of perovskite CH3NH3PbBr3 films fabricated via two-step sol-gel process using weakly coordinating isopropanol solvent

Compositional stability of perovskites is crucial for perovskite solar cells (PSCs) application because of directly affecting the performance of PSCs. Highly crystallized perovskite MAPbBr3 (MA = CH3NH3) with micronscale crystallites are usually fabricated via the one-step sol-gel process that uses strongly coordinating organic solvents. The two-step sol-gel process using weakly coordinating organic solvent is rarely used to fabricate MAPbBr3 films with relatively small crystallites. However, the two-step sol-gel process is preferred to fabricate PSCs because the residual PbBr2 can prevent the direct contact between electron- and hole-transporting layers. Thus in this work, perovskite MAPbBr3 films are fabricated via two-step sol-gel process using weakly coordinating isopropanol solvent. The compositional stability in dry ambient air and related optical properties of the films are intensively studied by a technique of post-annealing. A compositional transition from MAPbBr3 to PbBr2 occurs with increase of post-annealing temperature (Tpa). The product PbBr2 begins to form in the MAPbBr3 films at approximately 130 °C Tpa, and MAPbBr3 is fully thermally degraded to PbBr2 at 170 °C Tpa. The compositional stability of the films in dry air is inherently determined by instable organic cations (MA)+ and small electronegativity of Br−. The perovskite film initially shows a redshifted and then a blueshifted optical absorption edge with increase of Tpa, which is due to the decrease in film tensile stress and to the enhancement in thermal degradation to PbBr2, respectively. The increase in number of PbBr2 particles and defects in the films remarkably reduces the intensity of the green emission of MAPbBr3.