The Effect Of Deposition Parameters On ZnTe And ZnTe: Cu Thin Film

Improving back contact property of battery is a key technology to preparehigh-efficiency CdTe solar cell. The cause of roll—over phenomenon onⅠ-Ⅴcurve isthat p-type CdTe and the back electrode contact form a p-n knot, which is reversed tothe CdS/CdTe knot. The doping of Cu in the back contact film will improve quality ofbattery, but Cu's further diffusion into CdTe forms defects in CdTe, which leads toinstability on the cell's property. Therefore, it is necessary to study the existing stateof Cu atom in the film to control Cu consistency effectively. On the other hand, inorder to obtain fine back contact layer, the structure of ZnTe/ZnTe:Cu polycrystallinethin film must be compact. Substrate temperature and deposition rate are the mostimportant parameters which affect the film quality. The main purpose of this paper isto study the affect, and the main conclusions are as follows:Firstly, the effect of substrate temperature and deposition rate on ZnTe film andbattery property was studied. The finest grain and the most compact structure wereobtained at 70℃, and deposition rate was 0.2 nm/s. The grain size was a littlebigger when parameters changed to 70℃,0.2 nm/s or 100℃,0.6nm/s. The filmdeposited at room temperature and 0.2nm/s had the worst quality. The film grown as islands and had obvious holes in this case, which was unfavorable to preventing Cu'sdiffusion into ZnTe film. The suitable increase of deposition rate and substratetemperature had benign influence on the battery. With the improvement of thin filmquality, the defect density reduced so that the current carrier compound probabilityreduced and the current carrier collection rate increased. The battery efficiencyreaches 10.28% when the films deposited at 100℃,0.6 nm/s.Secondly, the effect of substrate temperature and deposition rate on ZnTe:Cufilm and battery property was studied. When deposition rate was 0.2 nm/s-0.3nm/s,the grain size of the film changed with the increase of temperature. Compare with thefilm deposited at room temperature, the grain size was larger and the films werelooser when deposited at 185℃and 102℃. With the increase of substrate temperature,open-circuit voltage and filling factor changed more obvious than short-circuit current.The battery efficiency increased from 7.83%(at room temperature) to 7.99%(at 102℃)and 9.02%(at 185℃).Finally, the distribution of Cu in the films was studied by XPS in order to controlCu's concentration. The deep distribution map and chemistry valence change ofvarious elements in back contact layer were studied. The variations of Cu'sconcentration with sputtering time in four kinds of samples were compared. At thebeginning, the Cu's concentration increased along with the time, this is because theadsorption of oxygen and carbon on the surface reduced. The curves reached theirpeaks, and then descended. For the sample D4 (deposited at room temperature), thepeak appeared when sputtered about 10 minutes, then the curve descended gradually.This suggested that ZnTe can't restrain Cu's diffusion. For the other kinds of samples,the peaks appeared at about 20-30 minutes, and then curves dropped sharply. Thisindicated that ZnTe restrained the diffusion of Cu when deposition parameters were modified.