Growth Mechanism of Thermally Evaporated gamma-CsPbI3 Film

Cesium lead triiodide (CsPbI3) inorganic perovskite possesses excellent thermal stability and matched bandgap for silicon-based tandem photovoltaics. The solution method with high-temperature annealing process for CsPbI3 film preparation creates challenges to scalable application and conformal growth on the textured silicon. Although additives can decrease the annealing temperature, it will introduce undesired organic components and increase material cost. Thermal co-evaporation for CsPbI3 has intrinsic advantages to overcome these issues, but the vague growth mechanism impedes the photovoltaic device development. In this study, gamma-CsPbI3 films are directly obtained through co-evaporation at 50 degrees C without any additives or high-temperature post-annealing. Focusing on the molecular thermodynamic calculations, it is proposed that the unique kinetic energy of evaporated molecules and the in-situ substrate thermal energy synergistically provide the energy prerequisite for gamma-CsPbI3 formation. Furthermore, the gamma phase stabilization is clarified by the crystal grain size effect with regard to the Gibbs free energy difference between the gamma and delta phases, which is adjusted through substrate temperature and evaporation rate. The obtained p-i-n device realizes an efficiency of 12.75%, which is the highest value for the thermally evaporated gamma-CsPbI3 photovoltaics at low temperature without additives. This study deepens the understanding of thermal evaporation process, benefiting to high-performance CsPbI3-textured silicon tandem photovoltaics.