Studies Of Copper-Znic-Tin-Sulfur Solar Energy Materials And Solar Cells

The hunt for inexpensive thin films for solar cells is also a hot topic of current research in recent years. Solar cells based on Cu2ZnSnS4(CZTS) thin film offer such a celebrated alternative to the current crystalline silicon, Cu(In, Ga)Se2(CIGS) and CdTe thin film solar cells due to the abundance, environmental benignity and industrial compatibility of its constituent elements. This work is focused on the fabrication and properties of Cu2ZnSnS4and Cu2ZnSn(S,Se)4thin film absorbers and Cu2ZnSnS4solar cells. The absorber layers were prepared by magnetron co-sputtering and electrochemical co-deposition, and their crystal structural, morphological, optical and electrical properties were studied, and the optimal sulfurization and selenization strategies were explored. Bifacial Cu2ZnSnS4solar cells based on ITO back contact and mono-facial Cu2ZnSnS4solar cells based on Mo back contact have been successfully achieved.First, the effect of sputtering power of CuSn target, ramping rate, sulfurization temperature, and duration on the properties of Cu2ZnSnS4films was studied. The Cu-Zn-Sn-S precursors with heavily Sn rich content were made either by co-sputtering from CuSn and ZnS targets or by electrochemical co-deposition. Additionally, we compared two sulfurization methods based on H2S gas and sulphur vapour respectively. For the co-sputtered samples, Cu2ZnSnS4absorbers sulfurized at2K/min have exhibited a better crystallographic quality as opposed to21K/min. The film obtained form45W of CuSn target and70W of ZnS target shows a desired stoichiometric composition and a1.51eV band gap, nonetheless concurrent with a small amout of SnS phases. The co-electroplated Sn rich precursors were used to help find the best sulfurization temperature and duration. It was revealed that the film grown at540℃for30min shows the best crystallographic quality, which was valid for both sulphur vapour and H2S gas. The film grown by H2S showed a uniform and compact surface, on the contrary, the film grown by sulphur vapour shows a loose surface and the crystal size was non-uniform varying from500nm to3μm. The degraded crystallinity of CZTS film grown using sulphur vapour possibly arised from the insufficient sulphur supply during the sulfurization. Besides, we can draw another conclusion from both the co-sputtered and co-electroplated Cu2ZnSnS4films that the presence of SnS2melt is beneficial to the growth of the expected CZTS. The presence of excessive Sn in the precursor also merits reducing the decomposition of CZTS and compensating the losses of Sn.Second, the interfacial reactions triggered by H2S gas between Cu2ZnSnS4absorber and ITO back contact were studied; the bifacial Cu2ZnSnS4solar cells based on ITO back contact were made and studied; the effect of sulfurization temperature (interfacial reaction) on the device performance was studied. H2S gas is so reductive that it could not noly inmpare the conductivity of ITO layer but also caused the diffusion/doping of indium into CZTS films even the loss of conductivity of ITO layer. The indium doped CZTS layer leaded to the expanded lattice parameters, the appearance of two additional vibrational frequency of lattice (227and299cm-1), and congruent optical band gap with pure CZTS layer. The best efficiency of3.4%has been achieved through bifacial illumination. The effect of sulfurization temperature on the properties of bifacial solar cells was discussed. The absorber sulfurized at520℃showed a better device performance and a composition of Zn/(In+Sn)=1.25, Cu/(In+Sn+Zn)=1.13in the upper layer, however, the bottom layer was badly-crystallized and showed a non-uniform elementary distribution.Third, monofacial Cu2ZnSnS4solar cells based on Mo back contact were made and studied; the effect of the Sn content in the precursors on the properties of absorber and mono-facial solar cells was also discussed. A5.5%efficient mono-facial Cu2ZnSnS4thin-film solar cell has been achieved. The optical and electrical properties of the cells have been characterized, and the problems for the low efficiency of the obtained cells have been discussed. The cell grown by insufficient tin content of precursors showed a bi-layered structure in the absorber. The upper layer was well crystallized and showed a tin poor content; however the bottom layer was full with numerous planar defects and evolution of ZnS secondary phases, which possibly resulted in the strong bulk recombination and great loss of open voltage. On the other hand, the cell made by tin rich precursor showed a better fill factor and shunt conductance in light performance, and showed a better crystallized absorber. The generation of Zn-rich localized features contributing to the breakdown may be responsible for part of the parasitic loses at the PV working voltage.Last, the effect of S2O32-concentration, selenization temperature and duration and selenium supply on the properties of Cu2ZnSn(S, Se)4thin films was studied. The electrochemical co-deposition is used to make Cu-Zn-Sn-S precursors for the development of large-area manufacturing. First, the effect of Na2S2O3on the selenized Cu2ZnSn(S,Se)4thin films were discussed. The film grown by5mM of Na2S2O3showed a compact and uniform surface, a single kesterite structure, and a good crystalline quality, however the composition is Zn poor and Se rich. Thus, we increased the usage amount of ZnSO4to improve the Zn content in the precursors, and we consequently conducted the following optimal experiments including the optimization of Se supply, selenization duration and temperature. The refined growth condition is0.3g of Se powder for usage, the external pressure of nitrogen in the tubular furnace resting necessarily at0.5-2Torr, the selenization temperature at550℃and duration for40min. Under these refined conditions, the obtained films were well developed Cu2ZnSn(S,Se)4films with a1.15eV band gap and a photovoltaic composition. The film deposited on ITO substrates showed faceted grains with a size of2-3micrometers, in stark contrast with the size of500nm-1μm of the films grown on Mo substrates.