Effect of the composition on physical properties of CdTe absorber layer fabricated by chemical molecular beam deposition for use in thin film solar cellsWe explore Cu electronic states in CdTe using photoluminescence as the main investigative method. Our results are consistent with some Cu atoms occupying substitutional positions on the Cd sublattice and with others forming Frenkel pairs of the type Cu i ϩ -V Cd Ϫ involving an interstitial Cu and a Cd vacancy. In addition, we find that Cu-doped CdTe samples exhibit a significant ''aging'' behavior, attributable to the instability of Cu acceptor states as verified by our Hall measurements. The aging appears to be reversible by a 150-200°C anneal. Our results are used to explain efficiency degradation of some CdTe solar-cell devices which use Cu for the formation of a backcontact.

We report changes in the photoluminescence (PL) spectra associated with the diffusion of Cu in CdTe thin films used in CdTe/CdS solar cells. We studied films grown by vapor transport deposition and radio-frequency sputtering as well as single-crystal CdTe. The main effects of Cu diffusion appear to be the quenching of a donor-acceptor transition associated with Cd vacancies and the increase in intensity of a lower energy band due to deep acceptor states. The changes in junction PL are consistent with the movement of Cu+ ions in the electric fields near the CdS/CdTe junction.

We report the fabrication of an 11.6% efficient, polycrystalline thin-film CdS/CdTe solar cell in which both semiconductor layers were deposited by planar-magnetron-radio-frequency sputtering at 380 °C on commercially available soda-lime float-glass substrates coated with SnO2:F. We show that the magnetron magnetic field is critical to obtaining high cell efficiency. Much stronger photoluminescence and higher electrical conductivity are found in films and cells grown with unbalanced-field magnetrons. The magnetic field dependence is interpreted as arising from the enhanced electron and ion bombardment of the film growth interface when unbalanced magnetrons are used.

We used electron beam induced current (EBIC) to measure degradation of CdTe photovoltaic cells. We have observed that: (i) the EBIC signal shows a considerable, continuous degradation depending on the electron-beam current, scan area, energy, and sample treatment; (ii) the characteristic degradation time fluctuates between different spots on the same sample; and (iii) grain boundary regions are the most effective collectors of the electron-beam generated charge carriers. Our phenomenological model relates the observed degradation to defects caused by the electron-beam generated electrons and holes.

Rutherford backscattering (RBS) and x-ray photoelectron spectroscopy (XPS) were used to investigate interdiffusion and surface properties of CdS/CdTe bilayers. The films were grown by radio-frequency sputtering and received postdeposition heat treatments similar to the ones employed in CdTe solar cell fabrication. It is found that a CdCl2 anneal strongly enhances both the diffusion of S into the CdTe layer and the surface oxidation. The diffusion of S in CdTe in this process can be described by a constant surface source model with the diffusivity given by D=3.2×10−5 exp(−1.2 eV/kT) cm2 s−1 in the temperature range studied. The change in the chemical composition of the surface following the CdCl2 anneal was analyzed by XPS showing that Te oxides and residual compounds containing Cl were present which could account for the straggling in the RBS spectrum. An HCl etch completely removes oxides and residues leaving a smooth surface.