Cadmium sulfide(Cd S) buffer layers with the scale of 10 cm×10 cm are deposited by chemical bath deposition(CBD) with different temperatures and thiourea concentrations under low ammonia condition.There are obvious hexagonal phases and cubic phases in Cd S thin films under the conditions of low temperature and high thiourea concentration.The main reason is that the heterogeneous reaction is dominant for homogeneous reaction.At low temperature,Cd S thin films with good uniformity and high transmittance are deposited by adjusting the thiourea concentration,and there is almost no precipitation in reaction solution.In addition,the low temperature is desired in assembly line.The transmittance and the band gap of Cd S thin films are above 80% and about 2.4 e V,respectively.These films are suitable for the buffer layers of large-scale Cu(In,Ga)Se2(CIGS) solar cells.
Cu(In,Ga)Se2(CIGS)thin films are prepared by a single-stage process and a three-stage process at low temperature in the co-evaporation equipment.The quite different morphologies of CIGS thin films deposited by two methods are characterized by scanning electron microscopy(SEM).The orientation of CIGS thin films is identified by X-ray diffraction(XRD)and Raman spectrum,respectively.Through analyzing the film-forming mechanisms of two preparation processes,we consider the cause of such differences is that the films deposited by three-stage process at low temperature evolve from Cu-poor to Cu-rich ones and then back to Cu-poor ones.The three-stage process at low temperature results in the CIGS thin films with the(220)/(204)preferred orientation,and the ordered vacancy compound(OVC)layer is formed on the surface of the film.This study has great significance to large-scale industrial production.
Cu_2ZnSn(S, Se)_4(CZTSSe) thin films were deposited on flexible substrates by three evaporation processes at high temperature. The chemical compositions, microstructures and crystal phases of the CZTSSe thin films were respectively characterized by inductively coupled plasma optical emission spectrometer(ICP-OES), scanning electron microscopy(SEM), X-ray diffraction(XRD) and Raman scattering spectrum. The results show that the single-step evaporation method at high temperature yields CZTSSe thin films with nearly pure phase and high Sn-related phases. The elemental ratios of Cu/(Zn+Sn)=1.00 and Zn/Sn=1.03 are close to the characteristics of stoichiometric CZTSSe. There is the smooth and uniform crystalline at the surface and large grain size at the cross section for the films, and no other phases exist in the film by XRD and Raman shift measurement. The films are no more with the Sn-related phase deficiency.
We fabricate polycrystalline Cu(In, Ga)Se2 (CIGS) film solar cells on polyimide (PI) substrate at temperature of 450 °C with single-stage process, and obtain a poor crystallization of CIGS films with several secondary phases in it. For improving it further, the two-stage process is adopted instead of the single-stage one. An extra Cu-rich CIGS layer with the thickness from 100 nm to 200 nm is grown on the substrate, and then another Cu-poor CIGS film with thickness of 1.5-2.0 μm is deposited on it. With the modification of the evaporation process, the grain size of absorber layer is increased, and the additional secondary phases almost disappear. Accordingly, the overall device performance is improved, and the conversion efficiency is enhanced by about 20%.
