Interface and composition engineering towards stable and efficient organic-inorganic perovskite solar cells

In the past few years, organic-inorganic perovskite solar cells have drawn considerable attention because of their excellent optoelectronic properties, easy-processability and low cost. A high power-conversion efficiency of over 22% has been achieved for organic-inorganic perovskite solar cells, which is a promising candidate as a low-cost photovoltaic technology. However, the intrinsic instability of perovskite solar cells owing to moisture or water hampers their large-scale practical application in ambient environment. This thesis targets on the development and illustrating related mechanisms the high-efficiency and stable perovskite solar cells. In the first part of this thesis, a novel hole transporting material (solution-processable sulfated graphene oxide, SGO) is exploited. It is employed as the hole transporting materials instead of hydrophilic PEDOT:PSS for organic-inorganic perovskite solar cells. An impressive power conversion efficiency (PCE) of 15.2% was achieved for the resulting perovskite devices with the planar inverted architecture. In addition, the resulting device shows a higher open-circuit voltage of close to 1.1 V than its counterparts based on PEDOT:PSS. Moreover, approximately 92% of its original PCE of the unpackaged perovskite solar cell is kept under ambient atmosphere and in the dark after around 1900 h. Moreover, without encapsulation, approximately 80% of the initial PCE for perovskite device in combination with SGO hole transporting layer is retained under 0.5 sun illumination and nitrogen atmosphere after around 500 h. Besides, ~60% of initial PCE for organic-inorganic perovskite solar cells without encapsulation is maintained under 0.5 sun light soaking and ambient atmosphere with the temperature lower than 30 °C after ~1000 h. It suggests SGO layer play a role of effectively blocking the diffusion of moisture into the perovskite film, which leads to dramatically improved photo-stability and environmental stability of unsealed perovskite devices. This research illustrates the importance of exploiting hydrophobic interfacialmaterials and inhibiting the diffusion of moisture into the perovskite solar cells, which is vital for efficient perovskite devices with impressive lifetime. The second part of this thesis develops aqueous processed [6,6]-phenylC61butyric acid methyl ester (PCBM) nanoparticles (PCBM NP). Organic-inorganic perovskite solar cells were fabricated via employing an ultrathin aqueous-processed PCBM NP layer as the electron transporting layer. PCBM in chlorobenzene (PCBM CB) have been used as electron transport layer for high-efficiency perovskite solar cells. However, its solvent is toxic and PCBM CB layer can be washed off the solvent of perovskite precursor such as DMF and DMSO. To increase its resistance to solvents of perovskite precursor, aqueous processed PCBM NP were developed. With the green and environmentally safe processing, the perovskite solar cells are fabricated and optimized as a function of the processing conditions. Then, the lifetime of devices based on PCBM CB and PCBM NP is compared (under 1 sun light soaking in nitrogen). Aggressive long-term stability measurements of perovskite devices based on PCBM NP and PCBM CB layers are carried out. The long-term stability of the unencapsulated devices based on PCBM NP and PCBM CB is compared under constant light illumination (1 sun). The PCE of the PCBM NP-based perovskite device maintains 86.2% of its original value within the 920 h. By contrast, the PCBM CB-based device exhibits worse stability, retaining only 62.7% of its original PCE after 920 h. Interface engineering plays a key role in improving the PCE and lifetime of organic-inorganic perovskite solar cells. In the third part of this thesis, composition engineering is employed to exploit an efficient and stable perovskite solar cell. The optimized perovskite solar cell based on FA0.85Cs0.15PbI2.4Br0.6 exhibits the best power-conversion efficiency and superior photo-stability. There is only slight decrease for the FA0.85Cs0.15PbI2.4Br0.6-based perovskite solar cells after ~500 h under constant 1 sun illumination in nitrogen, indicating composition engineering is vital for enhancing the photostability of perovskite solar cells