Abstract
Lead-based organic hybrid halide perovskites (APbX3, A= organic cation group and X= Cl, Br, I) have attracted tremendous attention due to their recent success as high efficiency solar cell materials and their fascinating material properties uniquely suitable for many optoelectronic devices including, photodetectors, light emitting diodes, lasers, field-effect transistors. However, the poor ambient and operational stability of the organic cation group as well as the concern of Pb toxicity greatly hamper the practical utilization and commercialization of these devices. Consequently, Pb-free, all-inorganic halide perovskites (using Cs as the inorganic cation, IHPs) are now the subject of intense research interest. It has been suggested that antimony (Sb) can be a possible replacement of Pb (Cs3Sb2X9) in these IHPs. Until now Sb-based IHPs have been synthesized by solution processes and their device performances are far from acceptable. However, due to the very large disparity between the melting points of Sb halide and Cs halide precursors, synthesis of high quality Cs3Sb2X9 materials using chemical vapor process is still a challenge.This thesis covers the development of a two-step chemical vapor deposition (CVD) method to synthesize Cs3Sb2X9 (X= I and Br) IHPs, including microplates and thin films; detailed characterization to reveal the structural and optoelectronic properties of microplates and thin films; and the optoelectronic applications of these synthesized materials particularly in photodetection and photovoltaics.
First, in order to overcome the difficulties encountered in conventional CVD, we developed a two-step CVD method and successfully synthesized Cs3Sb2I9 microplates. The microplates show hexagonal morphology with lateral dimensions ~10-40 µm, thickness ~50-100 nm, and high crystallinity. As compared with other typical Pb-free perovskite materials such as Bi-, Sn-, Cu-based halide perovskites, the Cs3Sb2I9 microplates demonstrate excellent optoelectronic properties, including substantial enhancements in the Stokes shift, exciton binding energy (Eb), and electron-phonon coupling. To demonstrate the applications of these microplates in optoelectronics, simple photoconductive devices were fabricated. The photodetectors exhibit a respectable responsivity up to 40 mA/W and detectivity reaching 1011 Jones at 532 nm irradiation wavelength. Notably, benefiting from the excellent crystalline quality of microplates, the photodetectors achieve an ultra-fast photoresponse with fast rise and decay times of 96 μs and 58 μs, respectively. This respectable photoconductor performance can be regarded as a record among all the lead-free perovskite materials. Furthermore, due to the enhanced Eb, these photodetectors also show superior thermal stability with a wide temperature range, capable of functioning reversibly between 80 K and 380 K, indicating their robustness to operate under both low and high temperatures.
Secondly, Cs3Sb2Br9 microplates ~10-50 µm in lateral size with a smooth hexagonal morphology were obtained using the similar two-step CVD approach. The exciton absorption peak at ~2.8 eV and a bandgap of ~2.85 eV were obtained for these microplates. Similar to Cs3Sb2I9 microplates, these Cs3Sb2Br9 microplates also show a large Stokes shift of ~450 meV and Eb of ~200 meV. Photodetectors fabricated using these microplates exhibit a current on/off ratio of 2.36×102, a responsivity of 36.9 mA/W, and detectivity of 1.0×1010 Jones with a fast response of rising and decay time of 61.5 ms and 24 ms, respectively, upon 450 nm photon irradiation. The Cs3Sb2Br9 microplates and photodetectors also show good stability in ambient air without encapsulation. As a comparison with Cs3Sb2I9 microplate photodetectors, photodetectors fabricated with Cs3Sb2Br9 microplates have a lower wavelength detection spectral range with less desirable device performance in terms of responsivity, detectability, and response time.
Finally, in addition to microplate structures, synthesis of Cs3Sb2X9 (X= I and Br) thin films utilizing the two-step CVD approach was also explored. Both Cs3Sb2I9 and Cs3Sb2Br9 thin films show good crystallinity. However, the crystallinity of films is lower than their microplates counterparts. Similar to Cs3Sb2I9 and Cs3Sb2Br9 microplates, these thin films also exhibit a large stokes shift of ~570 and ~630 meV, respectively and large exciton binding energy. The bandgap was confirmed to be 2.2 eV for Cs3Sb2I9 and 2.85 eV for Cs3Sb2Br9 thin films. The films exhibit a p-type semiconducting characteristic. Photoconductive devices fabricated using these thin films exhibit a respectable performance with responsivity up to 54.5 and 3.6 mA/W and high detectivity reaching 4.3×1010 and 1.6×1010 Jones for Cs3Sb2I9 and Cs3Sb2Br9 film photodetectors, respectively. Moreover, a stable photoswitching characteristic with fast rise and decay times of 50 ms and 30 ms for Cs3Sb2I9 and 108 ms and 56.2 ms for Cs3Sb2Br9 film photodetectors were observed. In general, the performance of photodetectors fabricated using microplates is better as compared with their films. This may be due to the high crystallinity and lower defect density in the microplates compared to their films. Furthermore, solar cell structures were also fabricated using the Cs3Sb2I9 film as the absorber layer. Our preliminary results show a current density of 11.5 mA/cm2, an open-circuit voltage 0.52 V, and power conversion efficiency of 1.85% under 1.5 G solar illumination. Although the device conversion efficiency is still low, it exhibits excellent long-term stability in the ambient air without encapsulation.
All these results evidently demonstrate that the two-step CVD process is an effective approach to synthesize high quality Pb-free Cs3Sb2X9 IHP materials which have the technological potentials for next-generation high-performance optoelectronic device applications.
| Date of Award | 14 Mar 2022 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Kin Man YU (Supervisor) |