Interface engineering for efficient and stable perovskite photovoltaics

• Main Author: Lin, Marcus Yiquan
• Other Authors: Sam Zhang Shanyong
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Materials::Microelectronics and semiconductor materials
• Online Access: http://hdl.handle.net/10356/74513
• Institution: Nanyang Technological University

With the intensive research effort across the world, solution processed perovskite based solar cells have achieved higher power conversion efficiency than that of most of the Silicon based solar cells. However, perovskites are very sensitive to moisture because it contains -NH2 group, which is the cause of perovskite degradation under humid environments. Hence, before it can be commercialized, the problem of long-term moisture stability needs to be addressed. Therefore, to further enhance the moisture stability by maintaining its performance, two moisture passivating compounds namely (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-Heneicosafluorododecyl) phosphonic acid (Type A) and 2,3,4,5,6-Pentafluorobenzylphosphonic acid (Type B) which contain bifunctional group were applied to engineer the different interfaces of the perovskite solar. The Type A compound contains the straight chain carbon with phosphonic and fluorine functionality. On the other hand, Type B compound is cyclic carbon structure with bifunctional groups. The phosphonic acid functional groups –PO(OH)2 forms hydrogen bond with the -NH3+ in the perovskite, passivating the surface dangling bond defects. Meanwhile, the fluorine-contained hydrophobic carbon chain of the passivation materials prevents the penetration of moisture into the perovskite layer. Here, the commonly used perovskite precursor MAPbI3 was used to fabricate the device. Three different approaches of interfaces passivation were employed using above mentioned compounds as given below: i) Surface passivation of perovskite absorber against moisture i.e. Perovskite/HTM Interface (Approach 1), ii) Surface passivation of hole transporting materials i.e. HTM/Au interface (Approach 2) and iii) Surface passivation of complete device i.e. passivation on the Au surface (Approach 3). The perovskite devices passivated (approach 1) using Type B compound i.e. carbon containing ring structure achieved an enhanced device performance of PCE of 15% as compared to bare device (PCE of 14.24%) and device passivated with Type A compound (PCE of 14.51%). In addition, device passivated using Type A and Type B compound found to stable over 32 days at room temperature and 50% humidity. However, perovskite device stored without any passivation, noted the efficiency drop off from 14.24% to 12.10%. It clearly reveals that the passivating layer showing good moisture resistance and good stability. For the device passivation carried out using the second approach i.e. HTM/Au interface passivation, there was not any efficiency enhancement noted but the passivated devices were quite stable over 70 days of aging, stored at room temperature at 50% humidity. Interestingly, using the third approach that is device passivation on the surface of completed device also noted the stable device over 70 days of aging, stored at room temperature and 50% humidity, as compared to devices without any passivation. Overall, the Type A compound has produced a better moisture stability for the perovskite devices. Although there was initial performance enhancement for the Type B compound, the long-term moisture stability could not be achieved. Type A compound has proved to be a good material that improves the moisture stability without affecting the device performance. Comparing the three approaches, the surface passivation on complete device (Approach 3) produced the best stability results, achieving an enhanced PCE of 112% of initial values after 70 days of aging in room temperature and 50% humidity. Unlike Approach 1 and 2, Approach 3 did not affect the morphologies of the perovskite and HTM layer, and thus had no effect on the charge collection and transfer of the perovskite devices.