Photophysics and Photoelectrochemistry of Perovskite Semiconductors for Optoelectronic Applications

鈣鈦礦半導體材料的光物理及光電化學研究及其在光電器件中的應用

Student thesis: Doctoral Thesis

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Award date6 Aug 2020

Abstract

With the rapid growth of the energy consumption and strict environment standard, fossil energy cannot fulfill the mankind’s needs. As an inexhaustible supply of natural resources, solar energy has undoubtedly become an important energy source due to its unlimited supply and clean characters. Recently, a new generation of hybrid organic-inorganic halide perovskite (HOIHP) based thin film solar cells (TFSC) has quickly emerged with a high solar-to-efficiencies of over 23%. Although the significant progress of HOIHP-based TFSC has been achieved in the past 10 years, several magor stumbling blocks restrict its further development, such as poor stability, toxic lead containing and unclear working principle. More importantly, perovskite materials are potentially promising alternatives in diverse fields due to their outstanding optoelectronic properties. Therfore, a series of research projectsrelated to perovskite materials has been systematically investigated to enhance the performance and reduce the cost in different areas, e.g., solar cells and electrochemiluminescence (ECL) devices.

Firstly, for the first of time, a methylammonium cadmium halides show photoelectrochemical (PEC) response with excellent humidity and chemical resistance because the oxidation state of almost all cadmium compounds is +2 are demonstrated. The photocurrent densities of two-dimensional (2D) (MA)2CdCl4 perovskites are about 0.30 mA/cm2 under 100 mW/cm2 irradiation. The interaction between these methylammonium cadmium halides and water vapor is studied by probing film morphology and characterizing single crystal structure. It is shown that H2O is able to complex with the perovskite, forming a hydrate product with a molecular formula of MACd3Cl7·3H2O upon humidity exposure. This causes a decrease in absorption and a recognizable change in the crystal structure of the material. When compared to methyl-ammonium lead iodide (MAPbI3), the PEC stability of 2D layered (MA)2CdCl4 perovskites with BQ/BQ•− redox couples (where BQ is benzoquinone) in CH2Cl2 is enhanced from 50 h to 600 h.

Next, Carbon-based conductive materials have been recognized as promising alternatives to noble metals as the electrode in optoelectronic devices. Herein, by utilizing energetic plasma ion implantation, Ni-doped TiN nanowire (NWs) modified graphitic carbon nanofibers (CNF) are designed and prepared as the candidates of the platinum (Pt) counter electrode for low cost hybrid perovskite-based liquid-junction photoelectrochemical solar cells (LPSCs). Notably, the photoelectrochemical (PEC) response of Ni/TiN/CNF counter electrodes is almost identical to that of a typical Pt counter electrode. From electrochemical investigations, i.e., cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), it is observed that the CNF-based materials show a similar redox activity compared with the Pt counter electrode, indicating low charge-transfer resistance (Rct) and large capacitance (C). The LPSCs, with a configuration of p-Rb0.05FA0.95PbI3/ BQ (2 mM), BQ •− (2 mM)/Ni/TiN/CNF-based counter electrode, exhibit an open-circuit photovoltage of 1.00 V and a short-circuit current density of 7.02 mA/cm2 under 100 mW/cm2 irradiation. The overall optical-to-electrical energy conversion efficiency is 5.06%. The PEC solar cell shows good stability for 5 h under irradiation.

Finally, a zero-dimensional cesium lead bromide (Cs4PbBr6) perovskite with outstanding photoluminescence (PL) properties has been synthesized. However, the electron transfer dynamics and electrochemical behaviors of Cs4PbBr6 emitters still remain unclear. Herein, we prepared the Cs4PbBr6 perovskite emitters with the investigation of the exciton transport and electrochemical dynamics by utilizing temperature-dependent transient photoluminescence (TRPL) and electrogenerated chemiluminescence (ECL) techniques, respectively. Stronger electronic coupling of Cs4PbBr6 emitters arises from the overlap of electronic wavefunctions, indicating that the increased possibility for the generation of electrochemiluminescence. Through the determination of diffusion coefficient and electron-transfer rate by using an electrochemical method, it is confirmed that effective heterogeneous charge transfer at the interface of electrode and electrolyte results in a red-shifted ECL emission in the presence of benzoyl peroxide (BPO) as the co-reactant.

In summary, the research projects in this dissertation present an attempt to fabricate a cost-effective and highly efficient LPSC by modifying the CNF, synthesizing a 2D layered (MA)2CdCl4 perovskite semiconductor with enhanced long-term stability to overcome the major limitations of LSPC devices, and preparing 0-D Cs4PbBr6 perovskite emitters with ECL emissions to broaden the application of the perovskite materials. These studies not only unlock the strong potential of liquid-junction perovskite solar cells, but also provide more evidences for further application of perovskite materials.