D. Grecu

Radio-frequency-magnetron-sputtered CdS/CdTe solar cells on soda-lime glass

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.

Photoluminescence of Cu-doped CdTe and related stability issues in CdS/CdTe solar cells

We 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 Cui+–VCd− 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.

Photoluminescence study of Cu diffusion and electromigration in CdTe

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.

Improvement of CdTe solar cell performance with discharge control during film deposition by magnetron sputtering

We have used a combination of rf and dc excitation to control the plasma density during planar magnetron sputter deposition of CdTe films and CdTe-based solar cells. While adding dc current we have adjusted the rf power to maintain a constant deposition rate. We find that the lower plasma density obtained with negative dc currents yields films with a more highly faceted surface, more columnar growth structure, and stronger photoluminescence. The solar cells prepared with rf sputtered CdS and with CdTe deposited at reduced rf power and −70 mA of dc current produce substantially higher performance. The results indicate advantages to reducing the ion bombardment energy and flux to the growing film by the addition of dc control during rf sputtering from highly insulating semiconductor targets.

Rutherford backscattering study of sputtered CdTe/CdS bilayers

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.

Effects of heat treatment on diffusion of Cu atoms into CdTe single crystals

Angular dependence of x-ray fluorescence and x-ray absorption fine structure techniques have been used to study the diffusion of Cu atoms into the photovoltaic material CdTe. Depth profile, effective valency, and local structure of Cu atoms in a Cu-doped single crystal of CdTe were investigated before and after a second heat treatment. Enhanced Cu diffusion into the CdTe single crystal was observed as a result of heating at a moderate temperature around 200 °C, resulting in a redistribution of the Cu impurities through a broader depth profile. Some of the Cu atoms are believed either to form small complexes with Te or occupy interstitial sites in the host but accompanied by a large local lattice distortion while others substitute for Cd on the cation sites. The results thus demonstrate that these nondestructive x-ray characterization methods are useful for probing microstructural changes in CdTe photovoltaic materials/devices in which some Cu-containing compounds are used as back contacts.

Photoluminescence study of Cu diffusion in CdTe

We report changes in the photoluminescence spectra associated with the diffusion of Cu in CdTe thin films used in CdTe/CdS solar cells. Films grown by vapor transport deposition and radio-frequency sputtering as well as single crystal CdTe were included in the study. 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 broad-band transition. The PL is subsequently used to explore the effects of electric fields on Cu diffusion. The role of Te as a diffusion barrier for Cu is investigated.

Raman and RBS studies of interdiffusion in RF-sputtered CdS/CdTe solar cells

The performance of CdS/CdTe photovoltaic devices is strongly determined by the properties of the CdS/CdTe interface region which forms during the heat treatment of the solar cell. Due to interdiffusion of sulfur and tellurium across the original CdS/CdTe junction and the formation of CdSxTe1−x at the interface, material properties such as the bandgap and the absorption coefficient of the newly formed material will be changed. In order to improve our understanding of the interface and to be able to control it, near resonant Raman scattering on a series of single-phase CdSxTe1−x alloys was performed and the Stokes shifts of the longitudinal optical (LO) phonons were measured over the entire composition x. The data have been fitted according to the modified random element isodisplacement (MREI) model. The results gained from the investigation of the alloys have then been applied to study the CdS/CdTe interface region of sputter-deposited solar cells. The formation of a two-phase CdSxTe1−x alloy region at the...

Effects of Magnetic Field Configuration on Rf Sputtering for CdS/CdTe Solar Cells

We report studies of solar cells prepared by rf planar magnetron sputtering in which the films were deposited using magnetic field structures ranging from approximately balanced to strongly unbalanced in the “type II” configuration. For films grown with the unbalanced configurations, we find much stronger photoluminescence and much better cell performance than for the balanced configuration. The CdTe films show differences in electrical performance depending on magnetic field as well. These effects are interpreted as arising from the enhanced electron and ion bombardment of the film growth interface for the unbalanced magnetrons. Using two unbalanced magnetrons we have fabricated an all-rf-sputtered cell with NREL-verified efficiency of 11.66% at air mass 1.5 illumination.