Theoretical Study On The Diffusion Mechanism Of Zn In GaSb And GaSb Thin Film Cells

Theromophotovoltaic(TPV)can directly convert Beat to electricity.Compared to the conventional photovoltaic systems,TPV systems have many advantages,such as high theoretical efficiency,no moving parts,portability,high reliability and so on.The temperature of the radiator of the TPV system is 1000?2000 K,thus the peak energy of the photon emitted by the radiator is concentrated in 1.4?2.9 ?m.With a bandgap of 0.72 cV at room temperature,a corresponding spectral response to 1.8 ?m,GaSb cell is the most commonly used TPV cell.This work addresses the study of GaSb cells.The key structure of a GaSb cell is the p-n junction,which is commonly fabricated by diffusing Zn into an n-GaSb wafer.As Zn diffusion profile has a great effect on the peroformance of the GaSb cell,understanding the diffusion mechanism of Zn in GaSb is very important to accurately control the diffusion profile and then fabricate high efficiency GaSb cells.The experimental results indicated that when the diffusion temperature and time are kept unchanged,the Zn diffusion profiles with pure Zn,Zn-Sb and Zn-Ga sources differ in the shape,surface concentration and effective surface diffusivity.Firstly,the relationship between the two surface diffusion parameters and the vapor pressure surrounding the GaSb wafer in the diffusion process was deduced based on a surface-equilibrium assumption and the substitutional-interstitial mechanism.The Ga/Sb/Zn ternary phase diagram was calculated and discussed for determining the vapor pressures in different source cases.The ratio of surface concentrations and that of effective surface diffusivities between different sources were obtained to quantitatively explain the effects of the source composition on the surface diffusion parameters.The theoretical results agree with the experimental ones.Secondly,five models with different concentrations of Ga vacancy were constructed to simulate the GaSb at different distances to the wafer surface under different sources.First principles method was used to calculate the formation energies of interstitial zinc to determine the stable site and valence.The activation energies of the dissociative mechanism and kick out mechanism were calculated to discuss the effect of the diffusion sources on the diffusion mechanisms.For the GaSb thin film cells,an experimental Zn diffusion profile with a box shape was used to predict the internal quantum efficiency(IQE)of the GaSb thin film cells,and the effects of the base region thickness,bottom surface recombination velocity and the hole Shockley-Read-Hall(SRH)lifetime were discussed.An experimental Te diffusion profile from the literature was also used to predict the IQE of the GaSb thin film cells.The theoretical results indicate that when the diffusion temperature and time are kept unchanged,the GaSb thin film cell fabricated with the Te diffusion method has a higher IQE than that fabricated with the Zn diffusion method.Moreover,a GaSb thin film cell with an epitaxial p-n junction was also discussed and compared to the GaSb thin fim cell with a diffused p-n junction.It was found that the former has a higher IQE in the main spectral response range.To further improve the efficiency,a GaSb thin film cell with one dimensional film-coupled metamaterial structure was proposed.The rigorous coupled-wave algorithm(RCWA)was used to calculate the spectral normal absorptance of the GaSb thin film cell with the proposed structure under transverse magnetic(TM)and transverse electrie(TE)waves.The results indicate that the proposed structure can greatly enhance the absorption under TM waves,which was elucidated to be attributed to the excitations of magnetic polariton(MP),surface plasmon polariton(SPP),and Fabry-Perot resonance(FP).Effects of grating period,width,spacer thickness,as well as incidence angle were discussed.Moreover,short-circuit current density,open-circuit voltage,output electric power,and conversion efficiency were evaluated for the GaSb thin film cell with a selective emitter.The effects of the temperature as well as the emittance band width of the emitter on the output electric power and the conversion efficiency were discussed.On the other hand,a GaSb thin film cell with a two dimensional film-coupled metamaterial structure was briefly discussed.Due to the symmetry,the absorption spectrum does not depend on TM or TE waves.The theoretical results indicate that the highest conversion efficiency of the GaSb thin film cell with two dimensional grating may reach 41.1%,increased approximately 74.9%than that with one dimensional grating.