Structural and optoelectronic properties of CdCl2 activated CdTe thin films modified by multiple thermal annealing

- Systematic study on the influence of multiple annealing on CSS CdTe properties
- Structural and optoelectronic changes in accordance with CdCl2 mobility in CdTe
- Removal of oxygen and CdCl2 impurities by H2 and subsequent vacuum annealing
- Homogeneous chlorine doping of CdTe films in result of isothermal annealing
- Substantial improvement of CdTe optoelectronic properties has been achieved

Traditional CdCl2 thermal treatment of CdTe/CdS heterostructure results in grain growth, sintering and incorporation of Cl into CdTe lattice, substantially improving performance of the cells. The process is complicated by high vapor pressure of CdCl2 and presence of oxygen. Considering volatility of CdCl2 and presence of residual oxidation products we provide systematic investigation results on the influence of subsequent multiple annealing in hydrogen (H2), vacuum and closed isothermal conditions on the properties of CdCl2 activated CdTe films (thickness 2 μm) deposited by close spaced sublimation at low (250 °C) and high (500 °C) substrate temperature onto roughened glass. Structural and optoelectronic properties of thermal annealed films were compared. High resolution X-ray analysis demonstrates the shift of the main (111) peak in accordance with mobility of CdCl2 impurity between ambient and CdTe crystal in good correlation with optoelectronic properties. CdCl2 activated CdTe films have high resistivity and high dark to light resistance ratio. Subsequent hydrogen and vacuum thermal annealing increases the resistivity and photoconductivity of the films. The dark resistivity has been sharply decreased by about four orders of magnitude in result of isothermal annealing at 600 °C. By controlling the thermal annealing conditions, substantial improvement of CdTe optoelectronic properties has been achieved.