Study On Structure Optimization And Photoelectric Performance Of Cu₂ZnSnS₄ Quantum Dot Cell

Cu₂ZnSnS₄（ Eg=1.5 eV） is a kind of environment-friendly materials with a high optical absorption coefficient（ >104 cm-1） in the visible region. Cu₂ZnSnS₄ colloidal quantum dots（CQD） combining the advantages of CZTS materials and quantum dots is widely used in various salor cells because of its simple preparation process, multi-exciton effect and tunable optical bandgap. However, CQD solar cells with general structures such as DHJ and Schottky still exist a compromise between the absorption of light and the extraction of current photon. Inspired by organic photovoltaic device, we combined ZnO nanorod arrays and CZTS NCs to build depleted bulk heterojunction CQD solar cells with a Au / MoO₃ / CZTS / CdS /ZnO nanorod arrays / FTO structure. One-dimensional ordered ZnO nanorod arrays provide the necessary separation interfaces for the photo-generated electron- hole pairs and also the direct channels for carriers to the collecting electrode. Significantly increased exciton separation and collection efficiency ensure the DBH CQD solar cells both having a high light absorption efficiency and a good carrier extraction capacity.The contradiction of "electric thin" and "optically thick" can be solved thoroughly. Then we mainly used surface photovoltage spectrum and I-V testing system to study the impact of ZnO nanorod arrays’ length, CdS buffer layer deposition time, the number of CZTS NCs spin-coating and ligand exchange on the performance of the photovoltaic device.This thesis includes the following main aspects:1.By controlling the reaction time and the concentration of the precursor solution to obtain the ZnO nanorod arrays with appropriate diameter and different length.Then use them to assemble depleted bulk-heterojunction CQD solar cells. Using surface photovoltage spectrum and I-V testing system to explore the effect of ZnO length on device performance,we concluded that the optimized length of ZnO nanorods is about 800-1000 nm. The photoelectric conversion efficiency（PCE） of the solar cell with 800 nm ZnO reached 1.63%.2. By changing the CdS deposition time to get FTO / ZnO / CdS electrode loaded with different amounts of CdS, and then spin-coated the same number layers of CZTS built DBH CQD solar cells.According to experimental data, we know that when CdS deposited15 min or so,the constructed depletedbulk heterojunction CQD solar cell can obtain the optimum photoelectric conversion efficiency.3. We got pure inorganic CZTS nanocrystals by ligand exchange using（NH₄）₂S, and then explored the impact of the CZTS thickness on charge transport of the electrode. Then the obtained CZTS nanocrystals were deposited onto ZnO nanorod arrays to construct depleted bulk heterojunction solar cell. The all-inorganic CZTS nanocrystal solar cells demonstrated a remarkable improvement in Jsc（from 8.14 to13.97 mA /cm2） and power conversion efficiency（from 1.83% to 3.34%） compared with surfactant capped CZTS nanocrystals. Using surface photovoltage spectrum, the influence of ligand exchange of CZTS nanocrystals on the charge transport and photovoltaic performance of the nanostructured CZTS solar cells was discussed.We optimized the architecture and the performance of CZTS CQD salor cells. These works provide a new way for researchers to improve the efficiency of CZTS CQD salor cells.