Inorganic cesium-lead mixed halide perovskite p-i-n solar cells deposited using layer-by-layer vacuum deposition technique

We report on the fabrication, electronic and device properties of inorganic cesium-lead mixed halide perovskite solar cells. The p-i-n devices were deposited using a layer-by-layer technique, where alternate layers of cesium bromide and lead iodide were deposited using vacuum deposition. The p-heterojunction layer was PTAA and the n-heterojunction layer was PCBM. It was discovered that in order to achieve the correct stoichiometry and get the best device performance, the individual thicknesses of each of the layers had to be small (4 nm and 6 nm respectively). Multiple stacks of alternating layers made up the device, with the total thickness of the perovskite to be about 400 nm.After deposition, the films were annealed at different temperatures to allow for diffusion and formation of the stoichiometric perovskite layer. It was found that when the individual layers were thin(e.g. 4 and 6 nm), one could obtain the stoichiometric perovskite by annealing at 250°C for 3 minutes, whereas thicker layers (e.g. 20 nm CsBr and 30 nm PbI2) needed annealing at higher temperatures (~350°C). The best devices were obtained by using thin alternating layers. The devices showed a voltage of 1.2 V, a current density of 12 mA/cm
²
and a fill factor of 70% for a device efficiency of ~10%. Quantum efficiency spectroscopy showed a bandgap of 1.9 eV and sub-gap quantum efficiency revealed an Urbach energy of valence band tails to be ~20 meV.