CdTe is a very attractive material for high efficiency, low cost thin film solar cells. It has a direct band gap(1.4eV) with high absorption coefficient. Because CdS has little lattice mismatch and similar electron affinity, it is used as a window layer for CdTe film. For high optical transmittance, ITO film was used as a TCO layer. Therefore thin film solar cells with a ITO/CdS/CdTe structure has been in the spot light for high efficient low cost solar cells.
In this study CdS films were made by a Chemical Bath Deposition(CBD) method. Cd$(CH_3COO)_2$ and CS$(NH_2)_2$ were used to supply Cd and S source solution. This solution was heated to 90℃ for 40min. CdTe films were deposited on glass/ITO/CdS substrates by Close Space Sublimation(CSS) method. And carbon paste was used to make a back contact with CdTe at 340℃ in $N_2$ atmosphere.
But this process has some problems. CdTe films have a low carrier concentration and high resistivity. So it is difficult to make an ohmic contact with other materials having a low series resistance. And to make a low defect containing CdS/CdTe junction is not easy process. CdS films grown by CBD method has a very poor quality.
To resolve this problems, ($CdCl_2$+CdS) powder was used to improve CdS film quality during CdS heat treatment at 480℃ and 520℃ in Ar atmosphere. This heat treatment shows an improvement on the transmittance of CdS from 400 nm to 550 nm. Low Cd/Te ratio sources were employed to increase the electrical conductivity of CdTe films. If Cd/Te ratio is too low, it is difficult to make enough thickness for CdTe films. So the lowest Cd/Te ratio was limited to 0.43. According to Cd/Te ratio of source, Cd/Te ratio of CdTe films deposited by CSS process changed from 0.985 to 0.965. The carrier density also increased to $4*10^{15}㎤$. The resistivity of CdTe films also improved from $2*10^{15}Ωcm$ to $8*10^{14}Ωcm$. At Cd/Te=0.43 source ratio, the highest efficiency of 10.7% was achieved.
CdTe films were thermally treated with $CdCl_2$ powder at 440℃ in Ar atmosphere after CSS process. This heat treatment process improved CdS/CdTe junction properties by reducing junction defect. Capacitance voltage measurement showed that $CdCl_2$ heat treatment on CdS/CdTe film reduces junction defect. Defect energy level could be calculated by capacitance voltage measurement with monochromator. This measurement showed that energy level of junction defect was about 0.8eV.
