The development of interface engineering for improving stability and efficiency of perovskite solar cells and understanding meta-stability of perovskite solar cells

Abstract

The power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has increased from 3.8 to 25.2% in the last decade, making perovskite the most promising material for future solar cells. However, further PCE and stability improvement are important for successful commercialization. Therefore, the aim of this thesis is to investigate ways of increasing PCE and addressing instability of PSCs. Initial PCE increase has been observed during ambient storage for many PSCs. Through a series of experiments, the origin of the storage effect was attributed to a combination of i) defect reduction in perovskite, ii) conductivity increase, and iii) evolution of the highest occupied molecular orbital (HOMO) in spiro-OMeTAD. In particular, the HOMO level change was revealed to play a significant role in PCE improvement. In terms of strategy for improving PCE, a novel passivation technique was developed by forming 2D/3D perovskite thin layer using a mixture of formamidinium iodide and iso-butylammonium iodide on the perovskite layer. This technique achieved a maximum PCE of 21.7%, while simultaneously enhancing device light and moisture stability. The defect density reduction, the uniform surface coverage of the passivation material and the suppressed ion migration by bulky organic cation were found to be the key parameters for PCE and stability improvement. Storage effect was also studied for these passivated PSCs. It is found that the changed conduction band of passivated perovskite influenced the initial temporal change of PCE, suggesting the importance of interface band alignment by passivation and conductive materials. Also, despite significantly suppressed non-ideal recombination at the surface/interface by passivation, analysis of the dominant recombination revealed the need for defect reduction in bulk perovskite. Consequently, by engineering the composition of bulk perovskite layers to decrease defects, PCE of 22.2% was achieved. Finally, the effects of removing one of hole transport material (HTM) additives, 4-tert-butylpyridine (tBP) (via HTM solvent engineering) on device performance and thermal stability were investigated. The suppressed morphological change at high temperature for tBP-free HTM was the reason for thermal stability improvement. This work shows that comparable efficiencies can still be achieved without the use of the thermally unstable HTM dopant.