CdS/CdTe interface analysis by transmission electron microscopy

Abstract

CdTe-based polycrystalline solar cells are leading candidates for terrestrial photovoltaic applications. High efficiency devices have been obtained despite large lattice mismatch between hexagonal CdS and cubic CdTe. In the present work, CdS was deposited by chemical bath deposition on Si substrates, and CdTe was deposited by close spaced sublimation. The chemical nature of the CdS/CdTe interface, structural properties, and their dependence on the fabrication parameters, e.g., substrate temperature (475–600 °C) and postdeposition CdCl2 heat treatment, were analyzed. In addition, the effects of crystallinity of CdS on the interface structural defects were examined. Small spot energy dispersive spectroscopy of the interface revealed a considerable amount of sulfur in CdTe. The concentration of sulfur, in general, was higher in the grains with higher density of structural defects and at the grain boundaries. The planar defect density in CdTe films increased with substrate temperature, whereas the threading dislocation density decreased. Interface analysis showed that the majority of the crystalline defects in the CdTe films deposited on CdS were generated at the interface. The crystallinity of CdS did not have a major influence on the interface defect generation.

Influence of deposition parameters on the properties of CdTe films deposited by close spaced sublimation

“…It is observed that the increase of the substrate temperature led to an increase in the grain size. This increase, that has also been observed in the literature 3,9,10 , is due to the low deposition rate achieved at higher substrate temperatures, as shown in Table 1, and also to the higher mobility of atoms on substrates heated at higher temperatures.…”

“…Thus, the kinetic energy of Cd atoms is critically reduced, not contributing for further grain growth. It is important to point out that some authors 9 observed even an important decrease in grain size of these films deposited at the same range of pressures. Figure 1 shows the XRD spectra of CdTe films deposited under 10 Torr, from a source kept at 750 °C, at different substrate temperatures.…”

In-depth analysis of chloride treatments for thin-film CdTe solar cells

“…The recombination current occurs due to a flow of carriers recombining in the depletion region and may take place via either direct band-to-band recombination or indirect trapping assisted recombination routes27. Trap assisted transport mechanisms are commonly observed in CdTe solar cells due to a high degree lattice mismatch between CdS/CdTe layers, leading to the formation of interfacial defects28. Although numerous transport mechanisms have been reported for CdTe solar cells, the two most common are trap dominated multi-step tunnelling29 and Shockley-Read-Hall (SRH) recombination30.…”

INFLUÊNCIA DO TRATAMENTO TÉRMICO EM PRESENÇA DE CdCl2 NO DESEMPENHO DE CÉLULAS SOLARES DE CdS/CdTe

“…Esse tratamento tem resultado em um aumento de eficiência da célula, por promover recristalização e crescimento de grão do CdTe (Durose et al, 1999). Além disso, promove interdifusão na interface CdS/CdTe, resultando na formação da camada de CdTe x S 1-x , o que leva à redução da densidade de estado interfacial entre CdS e CdTe, gerado pela diferença entre parâmetros de rede de cerca de 10% (Dhere et al, 2000). Promove ainda a passivação dos contornos de grão.…”