Diffusion in CdS of Cd and S vacancies and Cu, Cd, Cl, S and Te interstitials studied with first-principles computations

Abstract

We present an ab initio study of the diffusion profiles in CdS of native, Cd and S vacancies, and interstitial adatoms Cd, S, Te, Cu, and Cl. The global minimum and saddle point positions in the bulk unit cell vary for different diffusing species. This results in a significant variation, in the bonding configurations and associated strain energies of different extrema positions along the diffusion paths for various defects. The rate-limiting diffusion barriers range from a low of 0.42 eV for an S interstitial to a high of 2.18 eV for a S vacancy. The ratelimiting barrier is 0.66 eV for Cu and Te interstitials, 0.76 eV for Cl interstitial, 0.87 eV for Cd interstitial and 1.09 eV for the Cd vacancy. The 0.66 eV barrier for a Cu interstitial is in good agreement with experimental values in the range of 0.58-0.96 eV reported in the literature. We report an electronic signature in the projected density of states for the s-and d-states of the Cu interstitial at the saddle point and global minimum energy position. In addition, we have examined the relative charge transfer experienced by the interstitials at the extrema positions through Bader analysis.techniques have allowed CdTe/CdS solar cells to emerge as a leader in the growing market of thin film module production. A number of difficulties have slowed the further improvement of CdTe/CdS thin film technologies. These include the accumulation of Cu, from the back contacts, at the CdTe/CdS interface as well as intrinsic and exotic interstitials originating in CdS and diffusing interstitials from CdTe that cross the interface into the CdS layer affecting the cell performance. The diffusion of Cu into, and its accumulation at, the CdS layer has been the most suspected cause inhibiting long term device stability [3]. All CdS/CdTe cells are exposed to processing temperatures of at least 350°C during CdCl 2 treatment and a chemical reaction between CdTe and CdS can occur, which is the driving force for bulk and grain-boundary interdiffusion of CdTe and CdS [4]. Diffusion of Cd and Te atoms from the CdTe absorption region into CdS can reduce the light transmission capability of the window in the wavelength region of 500-650 nm. The faster process of diffusion of Cd and S atoms into CdTe, in the opposite direction, is more difficult to control, especially for cell structures with ultrathin CdS films [4]. The effect of Cl in CdS is also well known. Secondary ion mass spectrometry measurements suggest that the high Cl concentration in CdS films yields better solar cell efficiency [5]. Thus, it is well known that semiconductor properties, and hence overall cell efficiencies, are affected by the presence of defects in these layers [6,7]. To be able to control the defect concentration and mobility of defects in CdS requires understanding of their migration pathways by diffusion in CdS and the structural and electronic properties of these defects. Revealing these bulk diffusion pathways directly is challenging by current experimental techniques alone. These results can be of benef...

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