Study On The Preparation And Properties Of Sulfide Films By Solution Methods For Solar Cells

The energy and the environment problems are two emergent problems that man encounters. To maintain the development of the society and the economy, man needs to develop the clean renewable energy. Photovoltaic technology is a very promising renewable energy technology that may resolve the energy and the environment problems. However, the electrical cost by photovoltaics now is relatively high, which blocks its large area application. One possible way to overcome this problem is to decrease the cost of solar cells which may be achieved by using low cost solar cell materials and low cost fabrication process.In this paper, we prepared several solar cell materials (SnS, Bi2S3, CdS, CuxS, ZnS and Cu2ZnSnS4) by low cost solution methods (the chemical bath deposition and the successive ion layers adsorption and reaction). The influence of the preparation on the properties of the films and the deposition mechanisms of the films were studied. Based on the films we prepared, several structures of thin-film solar cells were prepared. This paves the way for the preparation of low cost solar cells by solution methods.SnS films with novel zinc blende structure were prepared by successive ion layers adsorption and reaction. When chlorine ions are added into the cation precursor solution, the growth rate and the crystallization of the prepared films increase and the surface morphology of the films gets coarser. A mechanism was proposed to explain the above phenomena:the cations may hydrolyze and polymerize to form some inter-media product in which the coordination structure of the Sn atoms is the same with the coordination structure of the Sn atoms in zinc blende SnS. During the reaction process, the coordination structures of the Sn atoms remain, so zinc blende SnS films were obtained. When chlorine anions are added into the cation solution, they may promote the hydrolyzation and the polymerization process, so the growth rate and the crystallization of the films increase.Orthorhombic and zinc blende structured SnS films were prepared by chemical bath deposition with a novel deposition system. It is found that the pH value and the temperature are crucial factors that determine the structure of the films. The deposition mechanisms of the SnS films were discussed. It is believed that the structure of the SnS films is determined by the coordination structure of the Sn ions in the inter-media product that emerge during the deposition process (i.e., the different structure of the SnS films result from the different reaction path between the cations and the anions). The electrical resistivities of the SnS films are lower than the electrical resistivities of the SnS films that previous reported. Furthermore, the influence of the preparation parameters on the properties of the orthorhombic SnS films was studied and how the preparation influences the properties of the films is discussed. Using orthorhombic SnS films as the light absorbing layers, FTO/SnS/Bi2S3/Ag and FTO/CdS/SnS/Ag structured solar cell were prepared. The solar cells show obvious diode property, but their performances need further optimization.The chemical bath depositions of the Bi2S3films were investigated throughout. The influences of the deposition system, deposition parameters, buffer layer and additive on the structure, morphology, electrical and optical properties were studied. It is found that a SnS buffer layer can greatly promote the uniformity and the adhesion of the deposited B12S3films. When the Bi2S3films are deposited by the deposition system containing Bi(NO3)3, EDTA and TA, the films have a loose nano-rod shaped morphology. When the Bi(NO3)3and NaiS2O3are used as the bismuth source and sulfur source, the deposited B12S3films prone to show a dendritic morphology, and adhesion of the films on the substrate is poor. However, when the deposition system containing Bi(NO3)3, AC and TA are used, the deposited Bi2S3films are compact and adhered. Using this deposition system, the smoothness of the films can get better by increasing the concentration of TA, decreasing he concentration of Bi(NO3)3or adding NH4Cl into the deposition solution. Furthermore, nano-wall shaped B12S3films were prepared by the deposition system containing Bi(NO3)3, AC and TA, and the pH value and the temperature were found to be the crucial parameters to obtain the nano-wall B12S3films. Finally, solar cells with the structure of FTO/SnS/Bi2S3/Ag were prepared.Two deposition systems (the CdCl2system and the Cd(AC)2system) were used to prepare CdS films. The CdS films all show hexagonal structured and obvious (002) preferential orientation. It is found that the surface of the CdS films by Cd(AC)2system is more smooth. The electronic conductivity and the optical bandgap of the CdS films by the Cd(AC)2system are all larger than those of the CdS films by the CdCl2system.CuxS films were prepared by chemical bath deposition using CuCl2, thiourea, TEA (and AC). The films all show chalcocite structures after the anneal at200℃. It is found that the CuxS films prepared by different coordinate reagent have similar morphologies, optical and electrical properties. The annealed CuxS films have the electrical resistivity of around10-3Ω·cm, and the average optical transmittance in the visible range are around50%. ZnS films were prepared by chemical bath deposition. The optical bandgap of the ZnS films are around3.9eV, and the average optical transmittance in the band between350nm and1950nm are around85.6。The chemical bath deposited ZnS films can be used as a type of low cost anti reflectance coating. When a ZnS layer was deposited on the etched silicon wafer, the reflectance of the silicon wafer can be decreased from14%to7%. Cu2ZnSnS4films were prepared by annealing sulfide precursors which are deposited by solution methods. The influence of the structure of the precursor films on the formation of the Cu2ZnSnS4was investigated. The results show that precursor composed of a mixed CuxS and SnS film (by SILAR) and a successive ZnS film (by CBD) is more beneficial to the formation of the Cu2ZnSnS4films. The CZTS films we prepared are uniform and compact, and the optical bandgap of the film is around1.53eV.