Comprehensive Analysis of Energy Loss Mechanisms in Perovskite Solar Cells under Accelerated Photo-thermal Ageing

The development of perovskite solar cells has progressed rapidly over the past decade, with laboratory-achieved efficiency levels now matching those of silicon cells, and continuing to rise, while the efficiency of silicon cells has improved at much lower speed.. The ionic bonding structure of perovskite solar cells enables simple fabrication and low manufacturing costs, but this weak bonding also results in relative instability. This instability is the biggest drawback of perovskite solar cells at present. The ageing process of perovskite solar cells involves complex and diverse degradation pathways, which often overlap spatially and temporally. This thesis aims to elucidate the mechanism of energy losses in perovskite solar cells during accelerated photothermal ageing and to decouple the spatial and temporal pathways of degradation. The first part of the thesis focuses on the study of open-circuit voltage (VOC) loss. An all-inorganic wide-bandgap perovskite (CsPbI2Br) was used as the active layer, as these Br-rich perovskites suffer from a VOC deficit. The study found that the VOC deficit in CsPbI2Br stems from severe non-radiative recombination losses at the interface. Two approaches were employed to enhance VOC: 1) selecting appropriate electron and hole transport layers to reduce interfacial non-radiative recombination losses, and 2) using interfacial modification to alter the interfacial dipole and de-dope the perovskite surface, thereby achieving better energy level matching. This work provides insights into reducing energy losses from the perspectives of device structure and energy level structure regulation. The second part of the thesis focuses on the device degradation mechanism induced by two-dimensional/three-dimensional (2D/3D) unstable interfaces. Trap states caused by the discontinuity of the 3D perovskite surface are passivated using 2D perovskite, but this 2D/3D interface is relatively fragile. This work has found that the intrinsic instability of mono-ammonium-based 2D perovskite under photothermal conditions results in the structural collapse of the 2D/3D interface. This promotes the diffusion of mobile iodide ions into the transport layer and electrodes, where redox reactions occur, leading to the de-doping of the transport layer and a reduction in mobility and conductivity, ultimately causing rapid device failure. The use of stable di-ammonium-based 2D perovskite effectively maintains the stability of the passivated interface, thereby achieving long-term operational stability. This work reveals how these unstable pathways affect the long-term performance of the device.