CdS is an n-type semiconductor because of excess Cd and S vacancy and has the bandgap of about 2.4eV. CdS has been largely used as the window layer of CdS/CdTe heterojunction solar cells for its high optical transmittance and small lattice mismach with CdTe.
In this study CdS films were deposited through CBD(Chemical bath deposition) method. The solution used in CBD consisted of $Cd(CH_3COO)_2$ as Cd source, $CS(NH_2)_2$ as S source, $NH_4(CH_3COO)$ as buffer and $NH_3$ for adjusting pH. And the solution was heated in an oil bath. Reactions in the solution were affected by the composition, temperature, pH of the solution. It was able to deposit CdS films with various thickness by controlling the content of $NH_4(CH_3COO)$, concentration of the solution and temperature.
But CdS films fabricated through CBD has the disadvantage of small grain size and rough surface, etc.. And these properties badly affect the CdTe film deposited on the CdS film and the CdS/CdTe solar cell in the result. So it is necessary to improve these disadvantages through various heat treatments. Among heat treatments, the one using $CdCl_2$ has been known to increase the grain size of CdS and to decrease the interdiffusion between CdS and CdTe.
But in case of the heat treatment with only $CdCl_2$, pinholes were generated at grain boundary for the evaporation of CdS. In this study $CdCl_2+CdS$ atmosphere was used to heat treat the CdS films by evaporating $CdCl_2$ and CdS mixed powder. CdS powder was added to protect the generation of pinholes by compensating the evaporation of CdS from the CdS film.
When the content of $CdCl_2+CdS$ powder was too much CdS film was damaged and when too small the grain size of CdS film was not increased. In the case of this study 0.01g ~ 0.02g of $CdCl_2+CdS$ powder was adequate. The grain size was not increased through the heat treatment at 470℃, but was increased at 530℃ and 560℃. The heat treatment at 530℃ was considered to be adequate for the surface of CdS film was not dense in case of 560℃.
The crystal structure of CdS films was changed from cubic to hexagonal after the heat treatment, enhanced by $CdCl_2+CdS$ atmosphere. There was no infiltration of Cl into the CdS film during the heat treatment and the atomic ratio of Cd to S in the CdS film decreased to 1:1. Optical transmittance of CdS/ITO/glass was decreased after the heat treatment.
CdS/CdTe solar cells were fabricated using as-deposited and heat treated CdS films. As for solar cells using CdS films heat treated with $CdCl_2+CdS$ the efficiency was improved mainly because of the increase of $J_sc$ and $V_oc$. This improvement was considered to be the result of the increase of spectral response especially in the wavelength of 500 ~ 600nm of incident light because of the decrease of the interdiffusion of CdS and CdTe and the decrease of interface state resulted from the increase of the grain size of the CdS film.