Thesis Examination Committee
Prof Hoi Sing KWOK, ECE/HKUST (Chairperson)
Prof Zhiyong FAN, ECE/HKUST (Thesis Supervisor)
Prof Levent YOBAS, ECE/HKUST
Organometallic halide perovskites have attracted enormous attention in both academic research fields and photovoltaic industries because they possess unique features for next-generation low-cost high-efficiency solar cell. Since the first perovskite solar cell (PSC) achieved an encouraging power conversion efficiency (PCE) of 3.8% in 2009, a considerable amount of efforts have been invested on the research and development of PSCs, leading to a remarkable PCE improvement to 22.7% within a decade. To date, solution methods and vacuum deposition are the two main techniques to fabricate perovskite thin films and devices. Given the fact that vacuum deposition, as a less studied method, has the potential to fabricate high-quality large-scale devices with excellent uniformity and reproducibility. This thesis focuses on vacuum deposition to develop vacuum-processable material for all-vacuum-deposited PSCs.
First, we have pioneered the optimization and application of room-temperature radio frequency (RF) sputtered SnO2 film as robust electron transport layer (ETL) into planar PSCs. A maximum PCE of 12.82% and 5.88% has been achieved on rigid and flexible devices respectively. The former device retained 93% of its initial PCE after 192-hour exposure in dry air while the latter device maintained over 90% of its initial PCE after 100 consecutive bending cycles. This result suggested sputtered SnO2 capability to replace high-temperature solution-processed ETLs, particularly TiO2 and ZnO. Second, we have developed a simple, novel, and cost-saving sequential vapor deposition method to fabricate high-quality and uniform mixed-cation mixed-halide perovskite films with microscale grain sizes. These perovskite films and the optimized RF sputtered SnO2 films were implemented for the first time into all-vacuum-deposited PSCs. A maximum PCE of 15.14% was achieved with promising stability and negligible hysteresis. This approach not only simplifies the conventional vapor deposition process of mixed-perovskite films, but also convinces the effectiveness and compatibility of vacuum-processed metal oxides with all-vacuum-deposited PSCs.