Defect Passivation towards Highly Efficient and Stable Metal Halide Perovskite Solar Cells
Metal halide perovskite material had emerged as a rising star in the photovoltaic application due to its excellent optoelectronic properties. Perovskite solar cells with a power conversion efficiency of over 25% have been demonstrated within a few years. However, the stability issues of this material still prohibit the commercialization of perovskite solar cells because of the low ion-migration activation energy, phase instability at room temperature, and sensitivity to the external environments. Attacked by the external environments through the defects in the perovskite has been considered one of the most critical issues of the instability of perovskite solar cells. Since the defects are preferred formed at the surface and grain boundaries of the perovskite, developing facile and effective strategies to passivate the defects at surface and grain boundaries is essential to enhance the stability and efficiency of perovskite solar cells.
In Chapter two, 1,3,7-Trimethylxanthine, a commodity chemical with two conjugated carboxyl groups better known by its common name caffeine, improves the performance and thermal stability of perovskite solar cells based on both MAPbI3 and CsFAMAPbI3 active layers. The strong interaction between caffeine and Pb2+ ions serves as a "molecular lock" that increases the activation energy during film crystallization, delivering a perovskite film with a preferred orientation, improved electronic properties, reduced ion migration, and greatly enhanced thermal stability. A champion stabilized efficiency of 19.8% and retain over 85% of their efficiency under continuous annealing at 85�C in nitrogen. Later on, in Chapter three, CuBr2 was introduced into the all-inorganic perovskites to control the crystallization and passivate the grain boundary defects, therefore a power conversion efficiency of over 16% was realized.
Apart from grain passivation by modulating the crystallization process of perovskites, a facile surface-induced secondary grain growth by utilizing the surface anisotropic was developed to enlarge the grain size to reduce the grain boundaries. As a result, grain size as large as 4 microns was realized through the oleylammonium treatment, the power conversion efficiency of 16.58% was achieved with 4,000-hour shelf stability.
In Chapter 5, the chemical environment of a functional group that is activated for defect passivation was systematically investigated with theophylline, caffeine and theobromine. When N-H and C=O were in an optimal configuration within the molecule, hydrogen-bond formation between N-H and I assisted the primary C=O binding with the antisite Pb defect to maximize surface-defect binding. A stabilized power conversion efficiency of 22.6% of photovoltaic device was demonstrated with theophylline treatment.