Organic-inorganic lead-halide perovskite for stable optoelectronics

• Main Author: Guo, Xintong
• Other Authors: Chen Xiaodong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: https://hdl.handle.net/10356/136986
• Institution: Nanyang Technological University

Despite their emergence as promising materials for low-cost and high performance optoelectronics, hybrid organic-inorganic lead-halide perovskites’ instability towards moisture and heat stress remains a serious obstacle that needs to be tackled for commercialization. In this project, two methods to improve the intrinsic stability of perovskite materials are demonstrated. The performance and stability of the optoelectronic devices are also investigated. The first method is surface passivation by cesium oleate to modify the perovskite/hole transporting material (HTM) interface. The organic species in the passivator are anchored on the perovskite surface while the Cs cations passivate the perovskite’s surface and grain boundaries. As a result, enhanced hydrophobic character of perovskite film is realized upon passivation, evidenced by high water contact angle of 107.4 degree and improved stability at ambient condition (relative humidity of ~70%, room temperature). Concomitantly, the proposed passivation strategy leads to increase amount of cesium concentration within the films, resulting in beneficial enhanced thermal stability of the film at 85oC. Improved open- circuit voltage (Voc) and unsacrificed short-circuit current density (Jsc) were obtained from the treated devices, leading to power conversion efficiencies of over 18%. When stored in a humid environment (relative humidity of ~55%), devices with cesium oleate passivation maintain 88% of its initial PCEs after 720 hours, degrading two times slower than those of the control. This method demonstrates a strategy of coating 3D perovskites with unique combination of inorganic cation and long chain organics to provide hydrophobicity and moisture stability to the solar absorber layer, while maintaining good device performances. The other method is to substitute 3D perovskite with 2D/3D mix-dimensional perovskite, PEA2MAn-1PbnI3n+1 (n=1, 2, 3, 5, where n refers to the nominal composition of the perovskite starting precursor solution). The bulky organic cations provide a natural moisture barrier, which prevents the degradation of 3D perovskite layers. The mix-dimensional perovskite- based photodetectors show much improved moisture stability compared to 3D perovskite (i.e. MAPbI3) based device, upon exposure to ambient environment. The resulted devices also exhibit inspiring performance of high on/off ratio more than four orders of magnitude under 1 V bias and 100 mw cm-2 irradiance. This attributes to their suppressed dark current and selectively enhanced photocurrent through the combination of 2D/3D layers. Therefore, mix- dimensional perovskite is a promising material to substitute 3D perovskite in photodetector due to their outstanding performance and enhanced atmospheric stability. This project also offers an idea by designing a self-protect energy harvesting system to improve the extrinsic stability of perovskites and to prolong the lifetime of their optoelectronic devices. The bio-inspired integrated system shows capability of being responsive to the environment, such as sunlight or weather. During nights or cloudy days, when humidity levels are generally much higher than sunny days, the system will close to protect the perovskite solar cells through the movement of soft actuator. This self-protect energy harvesting system can be realized through a series of devices integration and circuit design, and it shows potential to further improve the extrinsic stability of perovskite based optoelectronic devices.