Preparation And Characterization Of Thermally Evaporated Sb₂Se₃ Thin Film Solar Cells

Solar cell is a device that can transform solar energy to electricity. Among different types of solar cells, thin film solar cells have received wide attention due to their high efficiency, good flexibility and compatibility for building integration. As representatives of thin films solar cells, both copper indium gallium selenide ?CIGS? solar cells and cadmium telluride ?CdTe? solar cells have been successfully commercialized. However, In and Ga are expensive, and Cd is toxic, thus much attention was attracted to seek an alternative photovoltaic material with aboundant and low toxic elements. Recent studies have found Sb₂Se₃, as a typical V2-VI3 binary chalcogenide, has shown to be a promising alternative absorber material due to its earth abundancy, non-toxicity, suitable bandgap, and high absorption coefficient. Furthermore, Sb₂Se₃ possesses characteristics feasible for thermal evaporation, such as low melting point, high saturated vapor pressure, single phase and fixed composition. For this consideration, in this thesis, we mainly investigated thermally evaporated Sb₂Se₃ thin film solar cells as follows:?1? Exploring Sb₂Se₃ thin film solar cells fabricated by thermal evaporation. The rationale to choose thermal evaporation for Sb₂Se₃ film deposition was first discussed, followed by detailed characterization of Sb₂Se₃ film deposited onto FTO with different substrate temperatures. We then studied the optical absorption, photosensitivity and band position of Sb₂Se₃ film, and finally a prototype photovoltaic device was constructed with an encouraging power conversion efficiency of 2.1%.?2? A significant efficiency improvement to 4.8% of Sb₂Se₃ solar cells is obtained by inducting oxygen during thermal evaporation of Sb₂Se₃ films. Systematic materials and device physics characterization revealed that proper oxygen content in Sb₂Se₃ film deposition environment significantly improves the CdS/Sb₂Se₃ heterojunction quality through effective passivation of interfacial defect states, resulting in a substantial enhancement in device open circuit voltage and short circuit current density.?3? We first studied the defects in Sb₂Se₃ film by density functional theory and the results showed that the defect physics in this quasi-one-dimensional semiconductor are dramatically different from that in conventional 3-dimensional covalent semiconductors such as CdTe or Cu2ZnSnSe4. Specificially, both the defects of substituted antimony in selenium site ?Sbse? and selenium vacancy ?Vse? can be effective recombination centers. In addition, their concentration and detrimental effects can only be suppressed under a Se rich environment. For this consideration, we designed an in-situ selenium compensating strategy to passivate the defects of Sbse and VSe. In this way, a dramatic improvement of power conversion efficiency from 2.92% to 5.46% is observed. For further optimization, we systematically studied the thickness effects of Sb₂Se₃ layer on the solar cell performance and a competitive efficiency of 5.8% is received with the optimized thickness 420 nm, approximately equivalent to the length of depletion region together with carrier diffusion length.?4? Finally, we investigate the Cd-free buffer layer such as TiO2, ZnSe and ZnO prepared by vacuum-based methods. TiO2 and ZnSe thin films were deposited by thermal evaporation, and ZnO thin films were deposited by radio frequencymagnetron sputtering. Finally, all of them were used as buffer layers to fabricate Sb₂Se₃ superstrate thin film solar cells. Encouraging device performances were obtained based on these non-toxic buffer layers, thus offering the foundation for following study about Cd-free Sb₂Se₃ thin film solar cells with high efficiency.