PV-Seminar: Temperature dependent diffusion of residual solvent molecules during CH3NH3PbI3 layer formation and impact on solar cells

PV-Seminar: Temperature dependent diffusion of residual solvent molecules during CH3NH3PbI3 layer formation and impact on solar cells

Thursday, 16. May 2019 // 10.00

Helmholtz-Zentrum Berlin, Institute for Silicon Photovoltaics

Kekuléstraße 5, 12489 Berlin

Speaker: MSc Qin Tan, HZB, EE-IS

Abstract

Hybrid organic-inorganic metal halide perovskites are usually prepared from solutions. A solution contains precursor salts and solvents such as DMSO and DMF. During the formation of a crystalline perovskite layer, solvent molecules diffuse out. Residual solvent molecules can remain in the crystal lattice and can influence the performance of solar cells. For the analysis of the influence of residual solvent molecules on the performance of solar cells, the amount of residual molecules in perovskite layers shall be measured. In our approach, we focus on the CH3NH3PbI3 system which has the lowest complexity. For formation of the intermediate phase and for defining practically ideal boundary conditions for out-diffusion, a vacuum-assisted treatment was applied. The temperature dependent out-diffusion of DMSO from CH3NH3PbI3 precursor layers was investigated by measuring the S/Pb molar ratio in by high-resolution continuum source absorption spectroscopy (HR-CSAS) and the evolution of the S=O vibrational mode with infrared spectroscopic ellipsometry (IRSE). The diffusion coefficients were extracted by applying a diffusion model in a homogeneous layer. The diffusion coefficient of DMSO in CH3NH3PbI3 amounted to about 10-11 cm²/s at 100°C. The diffusion coefficient was activated by two processes with activation energies of 0.6 and 1.8 eV which can be explained by decomposition of DMSO complexes and by activation of DMSO trapped in the perovskite lattice. The S/Pb molar ratio had, for example, strong influence on the fill factor of solar cells. Furthermore, it seems that some residual DMSO is useful for the preparation of homogeneous CH3NH3PbI3 layers and to passivate defect states in the material. In future, it will be very interesting to apply our approach to complex systems with more cations, different preparation conditions, various layer architectures etc.