Preparation And Properties Of Cu₂ZnSnS₄Thin Films And Related Solar Cells By Sputtering

The most promising renewable energy is solar energy, as it can potentially cover the world’s energyconsumption So far, three thin film materials have been used for industrial production of solar cells:a-Si, CdTe and Cu（In,Ga）（S,Se）2（CIGS）, in which CIGS solar cell reached the highest efficienciy andcan compete with polycrystalline silicon. Admittedly CIGS will also have to face some difficulties.One problem is that indium is a rare element and could run low within the next10～20years, whilethe price of indium is already now increasing rapidly.Cu₂ZnSnS₄（CZTS）, similar to the highly successful Cu（In,Ga）（S,Se）2and CuInS2materials, is apromising candidate for thin film photovoltaic devices. It has advantages such as containing no rare orexpensive elements, and cost-reduction potential for commercial systems.Before the preparation and research of CZTS thin film, SnS film’s preparation by sulfurization wasfirstly carried out in this thesis. A two-stage process for fabrication of SnS films is presented, whichincludes sputtering of Sn layers and a succeeded heat treatment in the presence of S vapour（‘sulfurisation’）. The high property of prepared SnS film shows that the sulfurization method issuitable for the preparation of chalcogenide photovoltaic thin films. The research method and thesulfurization process of SnS are useful for the preparation and research of the CZTS thin film.Based on the successful two-stage process preparation and research on the SnS film, CZTS filmswere successful prepared by a two-stage process which consists of sequential sputtering of Cu, Sn andZn layers followed by a heat treatment in the presence of S vapour （‘sulfurisation’） too.The sulfurisation reaction and morphology, element ratio, optical, electrical and photoresponsecurrent of CZTS were studied by XRD, Raman spectroscopy, SEM, EDS, UV-VIS-IRspectrophotometer and Keithely2400source meter as a function of temperature, sulfurization timeand the sort order of precursors. It is shown that the best sulfurisation temperature, the bestsulfurization time and the best precursor are500℃,2h, and glass/Zn/Sn/Cu respectively. The surfaceof the prepared CZTS film is compact and the element ratio is close to the stoichiometric.The CZTS film shows obvious photo response current and persistent photoconductivityphenomenon. The effect of applied voltage on the current decay time and the time of current jump anddrop region last are firstly studied. It is found that the decay time of photo excited current of CZTSdecreased from378s to0.341s with the electrical field reducing from1V to1×10^-4V, respectively.Under lower electrical fields, the "instantaneous" jump and drop of current appeared respectively when the light was turned on and off and their mechanism was investigated. The relationship betweenapplied voltage and decay time could be given by exponential function.The CdS and ZnS emitter layers for the CZTS based solar cells were successfully prepared bychemical bath deposition. The properties of CdS and ZnS films were systematically researched byXRD, SEM, UV-Vis-IR. It is found that the prepared CdS and ZnS films are compact and the elementratios of these films are all close to the stiochiometric. The band gap energy of CdS and ZnS are about2.5and3.9eV, respectively. The unoptimized prepared CZTS/CdS and CZTS/ZnS heterojunction thinfilm solar cells all showed obvious PV performance.Cd free completely nontoxic novel SnS/a-Si and CZTS/a-Si thin film heterojunction solar cellswere firstly fabricated and reported in this thesis. All the PV performances of SnS/a-Si and CZTS/a-Sisolar cells are very obviously. The open circuit voltage, the short circuit current density, the filledfactor and the conversion efficiency of SnS/a-Si solar cell are289m,1.55mA/cm2,38%and0.17%,respectively. The effect of i-a-Si layer on the CZTS/a-Si thin film solar cells was researched. It isfound that i-a-Si layer can obviously improve the PV performance of CZTS/a-Si solar cells. The opencircuit voltage, short circuit current density and fill factor of this solar cell all increased obviously byinserting intrinsic a-Si film （i-a-Si）, and the conversion efficiency increased about139%by insertingi-a-Si film. The open circuit voltage, short circuit current density, fill factor and conversion efficiencyof CZTS/i-a-Si/n-a-Si solar cell reached562mV,12.3mA/cm2,43.8%and3.03%, respectively.