Device design of GaSb/CdS thin film thermal photovoltaic solar cells

Abstract

Enthusiasm in the reflearch of thermo-photovoltaic (TPV) cells has been aroused because the low bandwidth semiconductors of III-V family are coming into use. GaSb, as a member of III-V family, has many merits such as high absorption coefficient, and low band gap of 0.725 eV at 300 K etc.. At preflent thermo-photovoltaic cells are usually based on GaSb wafer, and it can be manufactured by the vertical Bridgeman method. Thermo-photovoltaic cell based on GaSb films is one of the effective ways to reduce the cost of the thermo-photovoltaic system. GaSb polycrystalline films can be grown by physical vapor deposition (PVD) which has advantages in using fewer materials and energy, and also in doing little harm to the environment. Because of residual acceptor defects VGaGaSb, GaSb thin film is usually of p-type semiconductor. So we should find n-type semiconductor material to form pn junction. We choose CdS as the emission layer of a cell structure. CdS belongs to n-type semiconductor with a narrow band gap of 2.4 eV and high light transmissivity. CdS thin film grown by chemical bath deposition (CBD) has passivation properties for GaSb. CdS layers can remove native oxides from GaSb surface and reduce the surface recombination velocity of GaSb. This paper focuses on theoretical analysis of GaSb/CdS thin film photovoltaic structure. By way of AFORS-HET simulation, we analyze the defect state density and interface density in GaSb and CdS, and their effects on cell performance. According to the simulation, the defect density in GaSb absorption layer is the very important factors that affect cell performance. When GaSb defect increases, the major factor to affect the cell is the fill factor that leads to low efficiency. On condition that thereflexists high GaSb defect density, the thickness of GaSb should be kept at 1000 nm. GaSb with a thickness above 1000 nm can bring about a high recombination rate, which reduces the efficiency of the cell. As an emission layer, the defect density in CdS should not affect the cell performance obviously. When the increase of CdS defect density is of four orders of magnitude, the cell efficiency is only decreased by 0.11%. In order to demonstrate the interface between GaSb and CdS, we use an inversion layer n-GaSb according to the passivation properties of CdS thin film grown on GaSb. When the defect density of inversion layer increases, the efficiency of the cell will decrease rapidly. And the GaSb/CdS structure will act as a resistance when the defect density in the inversion layer reaches 1020 cm-3. So the defect density in GaSb layer and the interface is the very factor to affect thermo-photovoltaic performance.