Investigations On Improving Micro-structure Of Cu₂ZnSnS₄-based Thin Films And Its Photovoltaic Applications

Solar energy is the most economic and environmentally friendly renewable energy resources.Solar cells based on photo-voltaic effect can convert sunlight directly into electricity directly.Various kinds of solar cells have received widespread attention.Thin film solar cells have been intensively studied due to the advantages of simple preparation technology,low cost and easy to carry.Cu In Ga Se2（CIGS）and Cd Te based thin film solar cells have realized industrialization production,but the consumption of rare or toxic elements limit their further application.The kesterite Cu₂ Zn Sn S₄（CZTS）has a suitable bandgap and a high absorption coefficient larger than 104 cm-1.The constituent elements are non-toxic and earth-abundant.Theoretical limit conversion efficiency of CZTS-based solar cell is over 30%.Therefore,it is an ideal alternative absorption material for solar cells applications.The power conversion efficiency（PCE）of CZTS-based solar cells updated rapidly from 0.66% to 12.6% in the past two decades.However,there is still a considerable gap compared with CIGS cells.In addition to the small range of single phase and the existence of more second phases and defects,there are also two main reasons for the low efficiency,i.e.the poor crystal quality of the films and the mismatching of energy band at the heterojunction interface.In view of these,we mainly researched the technical way to improve the crystalline quality and the micro-structure of CZTS-based films in this thesis.In addition,we also studied the band alignment and the related photoelectric applications.The results are as follows:1.The CZTSSe thin films were prepared by sputtering a single quaternary target followed by a rapid thermal process,the effects of the S/Se element ratio in the annealing atmosphere on the films were studied.CZTSSe absorbed layers with smooth surfaces and densely packed columnar grains were obtained by optimizing the heating curve.The different Se supply during the annealing was used to adjust theS/（S+Se）ratios in the films.The structure,optical and electrical properties can also be controlled.A Se-rich CZTSSe thin film possesses more uniform grain size,the grain size can reach about 2 μm,almost equal to the thickness of the films.This film has an optimized bandgap about 1.13 e V and relatively high hall mobility,indicating that high efficiency solar cells can be obtained by this Se-rich CZTSSe film.A CZTSSe thin film solar cell with standard structure shows a conversion efficiency of 3.38%.Analysis of the J-V characteristic shows that severe interface recombination is the major reason for the large reverse saturation current and diode ideality factor.The recombination in the bulk can be reduced due to the improved micro-structure and crystalline quality.2.We proposed that the morphology and crystallinity of solution based CZTSSe films can be improved by introducing polymer into the precursor solution.The influence of different amount of Polyvinylpyrrolidone（PVP）additive on the morphology and crystallinity were investigated.It is found that the polymer additive has no effect on the element composition and the crystal structures of the kesterite films.However,the voids on the surface of CZTSSe films can be reduced and the grain size improved dramatically after the addition of PVP.Grain size can reach more than 5 μm when the mass fractions of PVP is 5 wt%,but such an excessive additive will bring a thick small grain layer in the film.This can be eliminated by reducing the amount of PVP,when the additive amount is 1 wt%,high quality CZTSSe film without fine-grain sub-layer were obtained.The CZTSSe thin film solar cell fabricated with 1 wt% PVP shows a PCE of 4.34%,which demonstrated a promotion compared with the control device（PCE of 2.24%）without PVP addition.The enhancement of the PCE can be attribute to the improvement of the interfacial properties which reduces the recombination.3.It is important to determine the band alignments at the interface for achieving high efficiency cells.We have fabricated a CZTS/Zn O heterojunction using Cd-free buffer layer Zn O.The band alignments at the interface of CZTS/Zn O heterojunction were determined by X-ray photoelectron spectroscopy combined with first-principles calculations.The results implying that the CZTS/ZnO heterojunction has a type-Iband alignment with a small conduction band offset（CBO）which will not block the transport of electrons.We can infer that Zn O may be a suitable nontoxic buffer layer material for high efficiency CZTS solar cells.4.Wide-bandgap semiconductor gallium nitride（Ga N）has the same structure with Zn O and similar wide direct bandgap（3.4 e V）.CZTS film was deposited on an n-Ga N/sapphire substrate using sputtering method to form a p-CZTS/n-Ga N heterojunction photodiode.The spectral response measurements indicate that the response wavelength of the photodiode can be tuned from ultraviolet to visible regions via applying zero and reverse bias.Alternative visible and ultraviolet light response spectra were realized in this device.Band alignment at the interface of the p-CZTS/n-Ga N heterojunction was proposed to interpret this phenomenon.5.Cu₂ Mg Sn S₄（CMTS）thin films were successfully grown on glass substrates by sol–gel spin coating technique.The effects of the annealing temperature on structural,optical,and electrical characteristics were investigated.It is found that cations（Cu+,Mg2+,and Sn4+）share the same position and have a random distribution,the crystal structure shows a high symmetry.Thus,only diffractions of zinc-blende structure were detected.Furthermore,a suitable bandgap around 1.5 e V and the stable p-type conductivity imply that the CMTS thin film seems to be an ideal absorption material for cheap and environmentally friendly solar cells applications.