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The effect of post annealing in CdCl2 was investigated to elucidate the correlation between the evolution of the microstructure of CdTe thin films with the annealing time and the performance of CdS/CdTe thin-film solar cells made by in situ pulsed laser deposition (PLD). It has been found that the crystallinity of the annealed film is improved considerably as the CdTe polycrystalline grains evolved into grain clusters with a dendrite structure. While longer annealing time allows larger grain size, the increased film surface roughness outweighs the benefit of the improved crystallinity, resulting in increased structural disorders, such as pinholes. In fact, the concentration of the pinholes was found to increases monotonically with annealing time when it exceeds the optimal value, resulting in degradation of device performance and yield especially when pinholes appeared near the CdS/CdTe interface. This result illustrates the importance of controlling the microstructure of CdTe film and its interface with CdS to high-performance CdTe/CdS thin-film solar cells. [ABSTRACT FROM AUTHOR]

CdTe films have been prepared by close-spaced sublimation under different conditions. When CdTe is deposited at a low temperature, grains with a single predominant growth plane (111) adhere to the substrate and gather into scattered particles. With increasing deposition temperature, the number of CdTe grains on the substrate increases very quickly, the grains begin to form a quasi-continuous film, and (220), (311), (400), and (331) growth planes of CdTe begin to appear. When the deposition temperature and time are increased further, the CdTe grains begin to accumulate, the pin-holes formed initially begin to disappear, and grain boundaries with traces of layer growth can be observed. Different transmittance of samples of different thickness is clearly apparent but changes of the band gap, E, of the CdTe films is negligible. XPS results suggest CdTeO is generated on the CdTe film surface as the deposition time is increased. [ABSTRACT FROM AUTHOR]

The short-wavelength response for traditional CdS/CdTe thin film solar cells was dramatically restricted by the CdS window layer. In order to increase short-wavelength light collection, we tried to replace CdS with Mg x Zn 1−x O (MZO). The short-wavelength quantum efficiency ( QE ) response was obviously increased to more than 80% at 400 nm, while fill factor (FF) and open circuit voltage ( V oc ) for CdTe solar cells were decreased. This decrease was mainly caused by the weak build-in potential formed by the MZO/CdTe heterojunction, which resulted in insufficient driving force to repel the photo-generated carriers. Hence, the thin CdS buffer layer was introduced between MZO and CdTe to improve the built-in potential. It efficiently increased the build-in potential from 0.369 to 0.579 V with an obvious improvement of FF and V oc . But this interlayer caused QE losses to some degree in the short-wavelength region. To avoid these losses, a composite CdS/CdSe buffer layer was incorporated in the CdTe solar cells. CdSe acted as a protective layer to avoid the excessive CdS consumption during the high temperature deposition processing of the CdTe absorption layer. Meanwhile the formation of CdSe x Te 1−x by inter-diffusion extended the long-wavelength response. Thus, cell performance was enhanced with a relatively high build-in potential 0.517 V and a substantial improvement of the QE response. [ABSTRACT FROM AUTHOR]

Abstract: Bi2Te3 thin films were fabricated by vacuum co-evaporation and the properties of the films were investigated using X-ray diffraction, Hall measurements, and synchrotron radiation photoemission spectroscopy. The results indicate that p-type Bi2Te3 films, which have a work function of about 5.4eV and carrier concentration of the order of 1021 cm−3, are a rhombohedral phase with a strong (105) preferred orientation. A co-evaporated Bi2Te3 buffer layer was used for a back contact to CdTe thin film solar cells. C–V measurements, light and dark J–V measurements, and spectral response analysis also show that the p back contact has greatly enhanced cell performance. [Copyright &y& Elsevier]