Regulation Of Interfacial Micro-structure And Photoelectric Performance Of BiOX Based Heterojunction Films

Bismuth oxyhalides is typical V-VI-VII semiconductor materials with unique electronic structure, good optical and catalytic performance. Bismuth oxyhalides have attracted increasing attentions in photovoltaic field because of its low toxicity and low cost characteristics. In this thesis, we synthesize bismuth oxyhalides and bismuth oxyhalides-based heterojunction films through an environmental friendly and green method at low temperature. The tunability of product shape, crystal structure and crystal growth directions were realized by changing the experimental conditions. The bandgaps and interfacial properties of BiOX could be adjusted by surface modification technique and self-doping with halogen element. The Mott-Schottky curve was employed to measure the semiconductor type and energy band structure of BiOXfilms. The separation, transfer behavior and lifetime of photoinduced electron-hole pairs at interface/surface of BiOX based heterojuntions were systematically investigated by electrochemical impedance spectrum (EIS), surface photovoltage technique (SPV) and transient surface photovoltage (TPV). In addition, we have elucidated the working mechanisms underlying the BiOX heterojuntions based solar cells from the interfacial microstructure of bismuth oxyhalides, and fabricated the BiOX heterojunction thin film solar cells for photoelectric applications. The main innovations and conclusions are as follows:1. Modulation of conductive type of BiOX (X = Cl, Br, I) thin films. The surface states of the semiconductors determine the type of semiconductors. Although BiOCI,BiOBr and BiOI belong to the bismuth oxyhalide semiconductors and have similar crystal structures and electronic structures, they exhibit different conductivity types due to their respective surface states. The Mott-Schottky curves and surface photovoltage results demonstrate that the BiOCl and BiOI nanosheet arrays prepared by successive ion-layer deposition (SILAR) at room temperature show the properties of n-type semiconductors, while the BiOBr films is p-type semiconductor. However, bismuth oxychloride is often studied as a p-type semiconductor material. In addition, the properties of p-type of- BiOBr have been changed by doping an appropriate amount of iodine in a BiOBr film to form n-type BiOBrl-xIx thin film. As result, the carrier density in the bismuth oxyhalide thin film was remarkably improved, and the photoelectric performance was enhanced.2. Fine regulation of the band gap of bismuth oxyhalide film through doping. The I-doped BiOBr,-xIx (x = 0.139, 0.212, 0.488) nanosheet arrays with the perfect single crystal structure were prepared by SILAR method at room temperature for the first time.The dominant growth plane was (110), the doping of iodine does not change the crystal structure of BiOBr itself, iodine can enter the BiOBr lattice replacing Br. The higher doping amount of I leads to darker color of BiOBr1-xIx film and more redshift of the absorption wavelength, consequently the bandgap value changes from 2.84 eV to 1.97 eV. The band gap of the bismuth oxyhalide films can be finely tuned by doping of I atoms.3. Fabrication of BiOX based heterojunction thin films and interfacial micro-structure regulation. BiOX based heterojunction thin films, including BiOBr/CdS,BiOBr/Bi2S3 and BiOI/Bi2S3 have been fabricated through a facile method based on SILAR and CBD techniques at low temperature. The growth of metal sulphides on BiOX nanosheet exhibited evidently crystal orientation, for example,Bi2S3 nanoparticles preferentially grow on the (110) crystal face of BiOBr.The experimental results show that the formation of bismuth oxychloride/sulfide heterojunction can effectively improve the separation efficiency of photogenerated carriers and prolong the carrier lifetime. In the case of BiOBr/CdS, the generation,separation and transfer of photogenerated charge transfer were determined by the interfacial EF and surface EF of BiOBr and CdS NPs, and were strongly dependent on the CdS NPs layer thickness and incident light wavelength. Both the interfacial electric field and surface electric field dominated the separation and transport of photoinduced charge in the heterojunction at X>360 nm, while the effect of surface EF become dominant for the heterojunction at ?<360 nm. In the case of BiOI and n-Bi2S3 junction,however, N-type BiOI and n-Bi2S3 form an isomeric junction (n-n junction), the photogenerated charge transfer process between BiOI and Bi2S3 was similar to the Z-scheme mechanism of the n-n heterojunction.4. Photovoltaic performance and mechanism of heterogeneous thin film solar cell devices based on BiOX/sulfides heterojunctions. The photoelectric conversion efficiency of the BiOX-based heterojunction thin film solar cell device is 1.26 %.Although the current efficiency is not yet high to be further optimized, it is known that the efficiency is the highest one among reported BiOX based photovoltaic devices. In addition, the deposition amount of sulfide plays an important role in the photoelectric properties of heterojunction thin films. The photovoltaic performance of the heterojunction thin films was found increase with the increase of the thickness of the sulfide nanoparticles. However, when excess sulfide nanoparticles were deposited on the surface of BiOX nanosheets, the separation and migration of photogenerated electrons are hindered, resulting in a significant decrease in photoelectric properties.The study on the photovoltaic performance and mechanism show that the efficient separation, fast transport and prolong lifetime of photogenerated carriers will improve the photovoltaic performance of heterogeneous junction thin film device.