Theoretical studies of perovskite solar cells

Perovskite solar cells (PSCs) are solar cells implemented with perovskite absorber. Within 5 years of development, the power conversion efficiency (PCE) of PSCs has reached 22.1%. It has been proposed that the ferroelectric polarization of perovskites may affect the electronic performance of PSCs. CH3NH3PbI3 (MAPbI3) is one of the most widely used perovskites in PSCs. Methylammonium ion (MA) is believed to be involved in inducing ferroelectric polarization as single MA in vacuum shows a large dipole moment. On the other hand, MA are found to be disordered at room temperature. However, the behavior of MA alignment, and the connection between MA alignment and polarization are not fully understood. A better understanding of these phenomena are crucial in improving the PCE of PSCs. In this thesis, studies of the relationship between the performance and MA alignment, and polarisation have been undertaken. Band structures and effective masses of MAPbI3 in the α phase and the β phase were calculated with MA orientated differently. It is found that these structures have comparable energies. This is in agreement with the observed disordered MA in experiments. The influence of the orientation of MAs on band gap is smaller than 0.1 eV. There is a significant effect of orientation on the effective mass due to the large dipole moment of MA. However, overall reduced effective masses of differently orientated MA unit cells are comparable. It is also found that the electronic properties of the α phase are similar to that of the β phase. This is the reason why MAPbI3 PSCs are able to work stably near their phase transition point. These results explained that the orientation of MA and phase transition near room temperature have little influence on solar cell performance. The energy landscapes for MA reorientations in the α phase and the β phase unit cells and super cells are investigated in detail. The MA reorientation energy barrier depends on the initial and final orientations, it also depends on the orientations of its neighboring MA. The energy barrier is smaller for MA rotating from antiparallel to parallel. This suggests MA prefers parallel alignment. It is also found that the rotational energy barrier in the α phase is lower than that in the β phase. The polarization induced by MA rotation is about 6-8 μCcm−2, which is about three times higher than that produced by lead ion relaxation. Our work suggests that polarization in MAPbI3 is mainly from MA + orientations, which gives us a new understanding of the polarization of perovskite materials. To study the influence of the specific microscopic characteristics of PSC’s on solar cell power conversion performance, setting up of an appropriate transport model for numerical simulations is required. Existing models to simulate solar cell efficiency are unsuitable to study PSCs, as they are developed for silicon solar cells. Also, they tend to be overparameterised, which compromises their predictive capacity. In this thesis numerical models have been developed and implemented in efficient homemade codes. Using these models, it is found that PCEs increase with charge carrier lifetimes, mobilities and diffusion lengths. The open-circuit voltage (Voc) depends on the intensity of the exciting radiation and charge carrier lifetimes. Diffusion length and light intensity determine the saturated circuit current (Jsc). Additionally, three theoretical guidelines are proposed for PSC fabrication and optimization. It is theoretically shown that concentrator PSCs may offer advantages. We thus argue that the model developed here provides a framework for numerical modeling of perovskite– based cells and the optimization of their performance. Hysteresis is the performance difference between the forward and backward measurements. There is considerable debate about the reasons for the observed hysteresis. Polarization and ion migration are two possible reasons causing hysteresis in J-V curves for PSCs. However, there are no quantitative hysteresis simulations, also no theory to show the possibility of hysteresis induced by polarization. By considering two screening relaxation fields in numerical models, I quantitatively reproduced experimental hysteresis in J-V curves. It is theoretically shown that both polarization and ion migration can induce screening fields and then produce hysteresis. Lastly, two possible methods to reduce hysteresis in PSCs are proposed. One consists of reducing defects in thin films and at interfaces; The other consists of using polarisable materials as a charge transfer layer to compensate for the field induced by polarization or (and) ion migration. This work shows for the first time the role of polarization and suggests two ways to eliminate hysteresis. Our numerical work shows that polarization and ion migration are both possible mechanisms inducing hysteresis in the J-V curves. In order to distinguish between the role of polarization and ion migration in perovskite materials, two-dimensional periodic thin film models for relaxations of ion migration and polarization of MA are investigated. Dynamic simulations show both ion migration and MA polarization are able to build screening fields and promote charge transfer in PSCs. Lifetimes of