Peter Sheldon

A photoemission determination of the band diagram of the Te/CdTe interface

Two experiments designed to assist in understanding the physics of certain back contacts on p‐type CdTe solar‐cell devices are described. In the first experiment, x‐ray photoelectron and Auger electron spectroscopies are used to show that etching CdTe in HNO3:H3PO4 results in a Te layer on the CdTe surface. In the second experiment, photoemission spectroscopy is used to explore the electronic properties of evaporated Te deposited on thin‐film, polycrystalline p‐CdTe in an effort to develop a band diagram for the Te/p‐CdTe interface. The motivation for developing the band diagram derives from previous observations that chemically etching polycrystalline p‐CdTe solar‐cell device material before application of the back contact reduces the series resistance of the device. The key results are that the evaporated Te overlayer is p type and that the valence‐band offset between Te and p‐CdTe is favorable for low‐series‐resistance contact, ΔEv=0.26±0.1 eV.

Cathodoluminescence of Cu(In,Ga)Se2 thin films used in high-efficiency solar cells

Cathodoluminescence spectroscopy and spectrum imaging are employed to investigate Cu(In,Ga)Se2 (CIGS) thin films used in high-efficiency solar cells. We have found a nonuniform spatial distribution for the photon energy. The shift by decade of the emission spectrum is also found to depend systematically on the location of excitation. In addition, the photon energy at grain boundaries is not affected by the external excitation. A model for radiative recombination to be applied to these chalcopyrite compounds should explain these results, and some suggestions are considered.

EFFECT OF NITRIC-PHOSPHORIC ACID ETCHES ON MATERIAL PROPERTIES AND BACK-CONTACT FORMATION OF CDTE-BASED SOLAR CELLS

Forming a low-resistance contact to p-type CdTe is a critical issue for successful commercialization of CdTe-based photovoltaic devices. One solution to this problem has been to incorporate surface pretreatments to facilitate contact formation. In this article, the effects of a nitric–phosphoric (NP) acid pretreatment on material properties and device performance are investigated for polycrystalline CdTe-based devices. We demonstrate that the NP acid pretreatment, when applied to CdTe thin films, forms a thick, highly conductive Te layer on the back surface of the film and on exposed grain boundaries. When etched under optimal conditions, this results in CdS/CdTe devices with reduced series resistance and enhanced performance. On the other hand, we find that the NP etch preferentially etches grain boundaries. Overetching can result in complete device failure by forming shunt paths that extend to the heterointerface. Therefore, carefully controlling the etch concentration and duration is critical to optimi...

Development of Cu-doped ZnTe as a back-contact interface layer for thin-film CdS/CdTe solar cells

The full potential of thin‐film, CdS/CdTe photovoltaic solar cells will not be realized until issues relating to the fabrication of environmentally stable, low‐resistance, and easily manufactured contacts to the p‐CdTe layer are addressed. One alternative that provides the required contact parameters employs a Cu‐doped ZnTe(ZnTe:Cu) interface layer between the p‐CdTe and the outer metal contact. Thin films of ZnTe:Cu containing various concentrations of metallic Cu are produced by rf‐magnetron sputtering. Additionally, the effect of incorporating small amounts of excess Zn into the sputtering target is studied. We find that the electrical resistivity of ZnTe:Cu films deposited at 300u2009°C, and prepared with Cu concentrations of ∼0.45 at.u2009%, is much higher than would be expected from studies of films doped with higher Cu concentrations (∼6 at.u2009% Cu). We also find that postdeposition heat treatment significantly reduces the electrical resistivity of the films containing ∼0.45 at.u2009% Cu. However, compositional ...

Development of rf sputtered, Cu-doped ZnTe for use as a contact interface layer to p-CdTe

Cu-doped ZnTe films deposited by rf-magnetron sputtering have been analyzed with the intention to use this material as a contact interface in CdS/CdTe thin-film photovoltaic solar-cell devices. It is observed that unless careful attention is made to the pre-deposition conditioning of the ZnTe target, the electrical resistivity of thin films (∼70 nm) will be significantly higher than that measured on thicker films (∼1.0 μm). It is determined that N contamination of the target during substrate loading is likely responsible for the increased film resistivity. The effect of film composition on the electrical properties is further studied by analyzing films sputtered from targets containing various Cu concentrations. It is determined that, for targets fabricated from stoichiometric ZnTe and metallic Cu, the extent of Zn deficiency in the film is dependent on both sputtering conditions and the amount of metallic Cu in the target. It is observed that the carrier concentration of the film reaches a maximum value of ∼3 × 1020 cm−3 when the concentrations of Te and (Zn+Cu) are nearly equal. For the conditions used, this optimum film stoichiometry results when the concentration of metallic Cu in the target is ≈6 at.%.

Re‐evaporation effects and optical properties of molecular‐beam‐epitaxial AlGaAs/GaAs/AlGaAs wells

Elemental materials that condense on surfaces near effusion cells can be reevaporated toward substrates when heated by radiation from effusion furnaces. There they accumulate as unwanted impurities at interfaces and distribute throughout epitaxial films during growth. This effect is greatly increased when shutters are closed. Re‐evaporated aluminum is shown to degrade minority‐carrier properties of Al0.3Ga0.7As/GaAs double heterostructures. Modified temperature schedules and hardware to reduce re‐evaporation effects are suggested.

Process integration issues in thin-film photovoltaics and their impact on future research directions

Thin-film device processing relies heavily on the integration of a variety of materials and a wide array of process steps. These include in some instances, the integration of ‘wet’ (chemical etching) and ‘dry’ (deposition and plasma etching) process steps. As substrates are stepped through the process sequence, there are many surfaces that ultimately become interfaces that can have a dramatic impact on ultimate device performance. However, in thin-film R&D, these issues are not always carefully considered. This can have severe consequences when adapting an R&D-developed process into pilot production, and finally, into manufacturing. In this paper, we explore the impact of process integration issues on the microstructure and device performance for three prominent photovoltaic thin-film technologies (cadmium telluride, copper indium diselenide, and amorphous silicon). We also consider the impact of these concepts on future research directions in thin-film photovoltaics. Published in 2000 by John Wiley & Sons, Ltd.

Evaporated Te on CdTe: A vacuum-compatible approach to making back contact to CdTe solar cell devices

A commonly used process for forming low-resistance contacts to thin-film p-type CdTe involves the formation of a Te layer by etching the CdTe film in a concentrated mixture of nitric and phosphoric acids. The authors compare evaporated Te back contacts with ‘control’ back contacts formed by the usual etching process, and demonstrate that evaporating Te onto a CdTe thin film is a viable process for forming a low-resistance contact. The best efficiency achieved for a CdTe solar cell made with an evaporated Te back contact is 12.1%, whereas the efficiency of the device made with the control back contact was 11.9%. The evaporation process offers numerous advantages over acid etching, most notably vacuum compatibility amenable to large-scale production of CdTe solar cell modules.

CdS/CdTe interface analysis by transmission electron microscopy

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 (475–600u200a°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 d...

The role of oxygen in CdS/CdTe solar cells deposited by close-spaced sublimation

The presence of oxygen during close-spaced sublimation (CSS) of CdTe has been previously reported to be essential for high-efficiency CdS/CdTe solar cells because it increases the acceptor density in the absorber. The authors find that the presence of oxygen during CSS increases the nucleation site density of CdTe, thus decreasing pinhole density and grain size. Photoluminescence showed that oxygen decreases material quality in the bulk of the CdTe film, but positively impacts the critical CdS/CdTe interface. Through device characterization, they were unable to verify an increase in acceptor density with increased oxygen. These results, along with the achievement of high-efficiency solar cells (13% AM1.5) without the use of oxygen, led them to conclude that the use of oxygen during CSS deposition of CdTe can be useful but is not essential.