Preparation Of A Molybdenum Electrode And CdS Buffer Layer For CIGS Solar Cells

Cu(In Ga)Se2-based solar cells have been one of the most promising solar cell technologies for cost-effective power generation. It is reported that the highest efficiency of CIGS solar cell could reach up to 21.7%. The back electrode Mo films and buffer layer CdS films of CIGS solar cells play important roles in improving the efficiency of the cells. Compared with other materials, Mo metal is an ideal back contact material for CIGS solar cells because of its inertness and high conductivity. CdS is one of the semiconductor materials and the forbidden band width is 2.42 eV which is close to the ideal value of the sunlight distribution. CdS film also has good transmittance of visible light which makes it as the window materials of the photovoltaic battery.It is for that preparation and optimization of the molybdenum electrode and CdS buffer layer for CIGS solar cell. Molybdenum(Mo) films were deposited by radio frequency(RF), direct current(DC) and mixed RF/DC magnetron, respectively. With changing the deposition parameters including deposition pressure and power, the films show different surface morphology and crystallinity. Lower resistivity is obtained in the DC mode and better reflectivity is obtained in the RF mode. It is shown that the crystallinity increases when the deposition pressure decreases. The crystallinity and the grain size both increase as the deposition power increasing. The lowest resistivity of the single Mo film is 34×10-6 Ω·cm when the deposition pressure is 0.1Pa and the deposition power is 300 W in DC mode.In order to obtain lower resistivity, better adhesion and better reflectivity, bilayer films and three layers films were deposited in DC and DC/RF mixed mode. They both show good adhesion and low resistivity. The Mo films deposited in DC/RF and RF/DC/RF mixed mode show better reflectivity. It is demonstrated that the resistivity of about 67×10-6 Ω·cm is achieved in DC/DC/DC mode and the resistivity of about 61×10-6 Ω·cm is achieved in RF/DC/RF mode, which is appropriate for CIGS solar cell.The chemical bath method is used for the preparation of CdS thin films. The various performance of CdS thin films are explored through orthogonal test and contrast test method under different conditions, such as the concentration of the ammonium chloride solution, concentration of thiourea, concentration of cadmium chloride solution, the amount of ammonia, water bath temperature, bath time and the change of annealing conditions. CdS thin film has high density and good crystallinity when the concentration of CdCl2 is 0.004 mol/L; CdS thin film has suitable grain size and good crystallinity when water bath temperature is 90 ℃; when reaction time is 9~13 mins, CdS thin film was smooth and dense by ion-by-ion mode; CdS thin film has a high transmittance of 95% and the forbidden band width is 2.5eV which is close to the theoretical value when the amount of ammonia is 24 ml; when the annealing is conducted at 350 ℃ keeping an hour, CdS thin film has high density and good crystallinity.