Defect chemistry and photo-ionic effects in bromide and iodide perovskites

Abstract

Bromide and iodide perovskites, especially their mixtures, hold great potential for opto-electronic application due to their optical absorption properties in the visible range. While it is established that these materials are mixed ionic-electronic conductors, their ionic transport properties both in the dark and under light are poorly understood. The present work deals with the defect chemistry and photo-ionic effects in halide perovskites, including iodide and bromide perovskites with special focus on the photo induced phase separation (photo de-mixing) in mixed bromide - iodide perovskites. The first part covers the defect chemical study of bromide perovskites, including 3D MAPbBr3 and 2D Dion-Jacobson (PDMA)PbBr4. The results reveal that both 3D MAPbBr3 and 2D (PDMA)PbBr4 are mixed ionic-electronic conductors. 2D (PDMA)PbBr4 show three orders of magnitude lower ionic conductivity compared with 3D MAPbBr3. This implies that dimensionality reduction is an effective strategy for reducing ion migration in these systems. From a bromine partial pressure dependent study, it is concluded that MAPbBr3 and 2D(PDMA)PbBr4 are both P-type conductor and that the surface reaction is the limiting process for the incorporation and exporation of the Br2 gas. A non-monotonic dependence of the electronic conductivity on bromine partial pressure is detected for both 2D and 3D bromide perovskites. It can be attributed to the reversible formation and dissociation of AuBrx on the gold electrode and perovskites interface. The second part covers the investigation of the thermodynamic properties of the 2D mixed halide perovskites under light. It has been shown that light can be used as a knob for inducing photo de-mixing from single phase 2D mixed halide perovskites to to I-rich and Br-rich phases. In the dark, the photo de-mixed phases re-mix with complete reversibility of both their optical and structural properties, demonstrating the full miscibility of mixed bromide-iodide perosvkites in the dark. The temperature-dependence of absorption spectra for the photo de-mixed phases gave clear evidence for a miscibility gap under light, from which photo de-mixed phases’ compositions are extracted. The photo-miscibility-gap is mapped and confirmed by various methods. The shape of the photo-miscibility-gap shows limited variation in the 0.01 - 0.1 sun illumination intensity range. The non-encapsulation of surface, however, demonstrated a widening of the photo-miscibility gap. The third part covers the kinetic analysis and mechanistic investigation of photo de-mixing in 2D mixed halide perovskites. Simultaneous monitoring of the electrical conductivity and optical absorption allows for a local probe of electronic and ionic charge carriers, and the composition evolution. Furthermore, time dependent phase distribution is investigated with the aid of top view SEM, showing that I-rich nanodomains forming along the grain boundaries at early times after light exposure with further formation of such domains also within the grain at longer times. Local elementary distribution is probed with TEM. From the temperature dependent de-mixing half-time, an activation energy for photo de-mixing of 0.39 eV is obtained. Finally, together with DFT calculation on defect formation energy of the mixture of different defect type, a mechanistic description for photo de-mixing, both from molecular and microscopic level is proposed. The last part deal with the phase stability study of mixed halide perovskites in other dimensionalities, including 3D and nanocrystal based thin films. 3D mixed halide perovskites show that similar to 2D mixed halide perovskites, photo de-mixing occur in two stage. Different from the full reversibility of 2D, photo degradation of 3D perovskites into PbI2 in the dark over long time scales is observed. The nanocrystalline mixed halide perovskite (BA-MAPb(I0.5Br0.5)3) thin films show no de-mixing in contrast to 2D and 3D under same measurement conditions. Nanocrystals mixtures show superior phase stability under the same illumination condition, with neither degradation nor de-mixing. This thesis contributes to the understanding of the defect chemistry and ion transport properties of bromide and iodide perovskites, with specific focus on photo de-mixing in mixed halide perovskites. These findings will aid compositional engineering related to halide mixtures to enable optimization of optoelectronic devices as well as the development of other emerging systems exploiting photo-ionic effects.