Composition and Interface Engineering of Organic-Inorganic Hybrid Perovskites to Improve Photovoltaic Performance and Stability

Faced with the growing demands for energy in modern society, organic-inorganic metal halide perovskite materials have recently fascinated the photovoltaics (PV) research community due to a combination of their high quality optoelectronic properties and their ease in the fabrication proceed. As a result, solar cells employing the perovskite materials have been researched exponentially on their way and now the power conversion efficiency (PCE) of perovskite solar cells have been improved over 23% since the first report showing the PCE of 3% in 2009. In this thesis, I investigate compositional modification of perovskite materials and optimization of the charge transporting materials to produce high efficiency, stable and reproducible perovskite solar cells. First of all, after the reasonable performance was achieved, we have innovated a new approach of interface engineering in a perovskite layer to boost efficiency of devices. Engineering a compositional gradient with formamidinium bromide (FABr) at the rear interface between a pristine mixed perovskite ((FAPbI3)0.85(MAPbBr3)0.15) film and a hole transporting material (spiro-OMeTAD) demonstrated that charge collection is improved and charge recombination is reduced at the interface, which leads to a striking enhancement in open-circuit voltage (VOC). This result shed light on the importance of the passivation engineering on the rear surface of perovskite layers. However, beyond the improvement of efficiency, a long-term stability under moisture and continuous illumination is still remained as another challenge for market deployment of the perovskite solar cells. To develop the stability, we have developed the engineering by the surface growth of a two-dimensional (2D) perovskite, the crystal structure of A2BX4, on top of a bulk three-dimensional (3D) perovskite ABX3 film. It is well-known that the 2D perovskite has the superior stability, but suffered because of their low efficiency in the application of photovoltaics. The formation of a distinct 2D perovskite on top of the 3D perovskite (Cs0.1FA0.74MA0.13PbI2.48Br0.39) layer was proved by investigation of structural and optical properties of the stack. This embodying two different type of perovskite layer in one film had never been shown. Finally, this innovative approach led to the PCE of 21% and enhanced stability sustaining 85% of the initial value after 800 hours under full illumination. Thus, my approach of coating 2D perovskite layer make the perovskite solar cells more effective and stable for commercialization. Besides, to avoid the toxic chemical the lead free perovskite (Cs2AgBiBr6) was explored as photovoltaics and the further improvement can be expected. The last results presented in this thesis are related to optimization of the electron transporting layer (ETL) and hole-transporting materials (HTM) for efficient perovskite solar cells. It is shown that the ETL and the HTMs are playing a significant role in realizing efficient and stable perovskite solar cells.