Examination Committee

Prof Shuhuai YAO, MAE/HKUST (Chairperson)
Prof Zhiyong FAN, ECE/HKUST (Thesis Supervisor)
Prof Ni ZHAO, Department of Electronic Engineering, The Chinese University of Hong Kong (External Examiner)
Prof Johnny SIN, ECE/HKUST
Prof Hoi Sing KWOK, ECE/HKUST
Prof He YAN, CHEM/HKUST

Abstract

Organometallic trihalide perovskites are promising materials for photovoltaic applications, which have demonstrated a rapid rise in photovoltaic performance in a short period of time, surpassing 22%. In this thesis, we have developed new vacuum techniques to make perovskite film with high quality and fabricate rigid and flexible solar cell devices on planar and textured structure with high efficiency. Firstly, a facile one-step chemical vapor deposition (CVD) method was reported to fabricate planar heterojunction perovskite solar cells and after optimization a solar power conversion efficiency (PCE) of up to 11.1% was achieved. Then, based on this approach, metal precursor and alloying technique were employed to fabricate Sn-rich perovskite solar cell with over 14% PCE to make less-toxic and efficient perovskite device. In this work, a large grain size perovskite film (reaching 5 µm) has been grown directly from molten eutectic alloy at 185 °C. In addition to CVD process, two-step deposition (TSD) and layer by layer alternating (LBLA) evaporation techniques for fabrication of thin film perovskite solar cell were proposed and optimized. It was found that the film fabricated by LBLA is very smooth and uniform with good crystallinity leading to obtain an efficient perovskite device with 15.9% PCE. 
 
Additionally, in order to fabricate large-scale and flexible devices, sputtering technique of ZnO (electron transfer layer (ETL)) was used. Since, ZnO film has a reaction with methylammonium iodide (MAI) at 100 °C, the ETL layer was modified by incorporation of reduced graphene oxide (rGO) into ETL to prevent the reaction. Moreover, it was concluded that by reducing the annealing temperature for perovskite film less than 85 °C, this problem is solved. Afterward, the above techniques have been employed to fabricate flexible device due to their room temperature processing. Using these techniques and by applying anti-reflection (AR) on the device, highly efficient, flexible, and water-repellent perovskite solar cell based on willow glass was fabricated. 
 
In the last part of the thesis, the perovskite film was deposited directly on inverted nanocone plastic substrate using the above vacuum techniques. Based on this structure, flexibility and the mechanical properties of solar cell devices were studied. Beside excellent flexibility, our experiments, coupled with mechanical simulation, demonstrated that a nanostructured template can greatly help to relax stress and strain upon device bending, which suppresses crack nucleation in different layers of a perovskite solar cell.