Basic Research on Tin Sulfide(SnS) Thin Film Solar Cells

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Tin sulfide (SnS) is one of the promising materials for low-cost thin film solar cell, because of a bandgap of 1.3-1.4eV and the absorption coefficient larger than 10^4cm^<-1>. In this study, we have investigated the physical properties of vacuum-evaporated SnS films and the photovoltaic properties n-CdS/p-SnS and n-SnS_2/p-SnS heterojunctions.Vacuum-deposited SnS films are always p-type conduction with a resistivity of 13-20OMEGAcm, carrier density of 6.3*10^<14>-1.2*10^<15>cm^<-3>, and a Hall mobility of 400-500cm^2/Vs. The activation energy for conduction of the SnS films was about 0.28-0.34eV.The resistivity of SnS films became about two order of magnitude larger than that of as-grown films by the doping of antimony metal. We cannot get an n-type SnS film in the present stage.The n-CdS/p-SnS heterojunctions were made by depositing n-CdS,p-SnS and Ag ohmic electrode on the transparent electrode ITO films by turns. The barrier height phi_B was estimated to be 0.56eV from the saturation current, assuming Richardson constant of a free electron. The 1/C^2-V curve is roughly straight line, the voltage intercept is 0.49V.The heterojunction behaves as a metal/semiconductor Schottky barrier, in which the CdS layr plays the role of a metal. From the analysis of the slope of 1/C^2-V plots measured at 1MHz, the width of the depletion layr at zero bias was 3.01*10^<-5>cm. The photovoltaic properties of a short-circuit current of 7mA/cm^2, an open-circuit voltage of 0.1V,and a fill factor of 0.42 for n-CdS/p-SnS heterojunction were obtained under the illumination of 100 mW/cm^2.From the analysis of Ag/n-SnS_2 and Al/p-SnS Schottky diodes, the electron affinities of n-SnS_2 and p-SnS films were 4.67eV and 3.72eV,respectively. Accordingly, SnS_2 film is not suitable for the window material of p-SnS film.