Alan L. Fahrenbruch

Photovoltaic properties of ZnO/CdTe heterojunctions prepared by spray pyrolysis

An extended investigation has been made of the electrical and photovoltaic properties of heterojunctions prepared by spray‐pyrolysis deposition of thin ZnO films on single‐crystal p‐type CdTe. The principal experimental variables were the substrate temperature and the postdeposition temperature for annealing in H2. Under actual sunlight the optimum cell showed an open‐circuit voltage of 0.54 V, a short‐circuit current of 19.5 mA/cm2, and a solar efficiency (referred to the active area) of 8.8%, the highest value obtained to date for an authentic heterojunction on CdTe. The nature of the forward transport mechanism has been investigated, and a tunneling model in which bulk and interface deep traps control the forward characteristics is shown to provide good correlation with the experimental data.

Transport mechanisms in ZnO/CdS/CuInSe2 solar cells

The transport mechanisms in ZnO/CdS/CuInSe2 solar cells prepared by ARCO (now Siemens) Solar Inc. have been analyzed by measurements of current versus voltage at different temperatures in the dark, short‐circuit current versus open‐circuit voltage at different temperatures in the light, spectral response of quantum efficiency, and junction capacitance. In the dark, recombination in the depletion region and/or thermally assisted tunneling are the dominant transport mechanisms. The observation of a smaller open‐circuit voltage than would be predicted from the dark transport parameters is the result of a small change in the transport parameters under illumination, probably without a change in transport mechanism.

II‐VI photovoltaic heterojunctions for solar energy conversion

Several different II‐VI heterojunctions show possible promise for photovoltaic conversion of solar energy. Two of these are p‐CdTe/n‐CdS and p‐CdTe/n‐Zn0.35Cd0.65S, which have a maximum solar efficiency of 17 and 23%, respectively. We report here specifically on the properties of p‐CdTe/n‐CdS cells prepared (a) by close‐spaced vapor transport of CdTe onto single‐crystal CdS, and (b) by two‐source vacuum evaporation of CdS onto single‐crystal CdTe. Cells with efficiency of 4.0% have been produced without detailed attention to optimization of cell design; these cells have quantum efficiencies as high as 0.85.

Sputtered oxide/indium phosphide junctions and indium phosphide surfaces

The reason that sputtered indium‐tin oxide (ITO)/InP solar cells junctions with large lattice mismatch have the same efficiency as CdS/InP junctions with good lattice match is shown to be that sputtered ITO/InP junctions actually consist of n+‐ITO/n‐InP/p‐InP buried homojunctions. To demonstrate this and to show that the homojunction formation is caused by thermal damage to the InP surface during sputtering deposition rather than from impurity diffusion from the oxide, a series of five different metal oxide/InP junctions have been formed by sputtering of the oxide, all with high solar efficiency. Junctions have been prepared both from single‐crystal InP : Cd and from epitaxial crystal films of InP : Zn. The effects of sputter deposition of the oxide have been simulated by sputter etching of the InP surface, and the effects analyzed through measurements of the properties of Au/InP junctions, and of the Hall effect and photoluminescence of InP surfaces. Some heat treatment of the sputtered cells is required...

Photovoltaic properties of n‐CdS/p‐CdTe heterojunctions prepared by spray pyrolysis

Heterojunction solar cells of n‐CdS/p‐CdTe have been prepared by deposition of CdS films by spray pyrolysis on single‐crystal CdTe to produce solar efficiencies greater than 6% without optimization or correction for reflection loss. Cells prepared by this method therefore compare favorably with cells prepared by vacuum evaporation of CdS. In addition, the highest open‐circuit voltage (0.74 V) observed to date in an efficient n‐CdS/p‐CdTe solar cell has been produced by this fabrication process.

Low resistance contacts to p-type cadmium telluride

A survey of metal contacts to p-type CdTe has revealed that the best contact is one prepared by vacuum deposition of CuAu onto a surface that has been etched with K2Cr2O7:H2SO4 . The contact resistivity on 0.5 ohm-cm p-type CdTe is between 0.1 and 0.5 ohm-cm2 . The contact resistivity does increase if subjected to heat treatment, but is stable in air at room temperature for at least several months. Contacts of CuAu and Au on a chromate etched surface were compared in detail with similar metal contacts on a methanol-bromine etched surface through measurements of current vs. voltage as a function of temperature, photovoltaic response, and Auger analysis. CuAu contacts on a chromate etched surface are describable in terms of a thermally assisted tunneling model with a barrier height of about 0.60 eV. Au contacts on either a chromate etched or a methanol-bromine etched surface can be described equally well by a thermally assisted tunneling model or by a thermionic emission model. In either case the barrier height is about 0.1 eV larger on the methanol-bromine etched surface than on the chromate etched surface. Auger analysis indicates that chromate etching produces a tellurium rich surface that plays a key role in determining subsequent contact properties.

Ohmic contacts and doping of CdTe

Abstract Fabrication of ohmic contacts to CdTe is strongly dependent on obtaining sufficient carrier density in the material. Acceptor levels greater than 10 17 cm −3 are easily obtained in single-crystal CdTe but are very difficult to obtain for polycrystalline material. Consequently, contacts with a specific resistivity of about 0.2 ω cm 2 , which is adequate for efficient solar cells, can be obtained readily with single crystals but not with polycrystalline CdTe. This paper reviews the current art and science of contacting p-CdTe and its relation to doping the material.

II–VI compounds in solar energy conversion

Abstract The principal advantages of II–VI compounds for terrestrial solar photovoltaics are low cost, direct bandgap, and the ease of deposition of good quality films of these materials by a variety of growth methods. Existing solar cell technology shows that a solar efficiency of 10% for all thin film, II–VI compoud cells will be reached within a year or two. This paper outlines the useful II–VI compounds and their preparation in this and discussions many of the heterojunctions which show promise. The interaction between the structural parameters of these films such as growth morphology and grain size and the electrical properties of the heterojunctions fabricated from them is crucial to producing efficient cells. Still very little research has been done in this area. The effect of grain boundaries in thin films and heterojunction lattice mismatch is demonstrated by simple models in this paper in order to gain a perspective on the required film properties and cell conjunctions. Finally experimental results are presented for a number of heterojunctions involving II–VI compounds, focusing particularly on the CdS/CdTe cell.

Photovoltaic determination of optical‐absorption coefficient in CdTe

Optical‐absorption coefficients for CdTe greater than 103 cm−1, calculated from the spectral‐response curve of a CdS/CdTe heterojunction solar cell produced in our laboratory, are in agreement with recent optical measurements on thin films and bulk samples of CdTe. This technique allows high values of absorption coefficient to be measured in bulk samples, bypassing the problems associated with optical measurements in thin films. The high values of absorption coefficient determined for CdTe encourage its use as a thin‐film solar‐cell material.

Properties of ZnO films deposited onto InP by spray pyrolysis

Abstract The properties of thin films of ZnO deposited onto single-crystal p-type InP by spray pyrolysis were investigated. The films are uniform and polycrystalline, and exhibit an optical transmission above 85%. The film sheet resistance exhibits a broad peak as a function of deposition temperature, and post-deposition annealing in hydrogen yields resistivities as low as (6−8)×10 -3 ωcm and sheet resistances of 85–100 ω/□. Variations in orientation, grain size, purity and stability as functions of spray parameters and post-deposition processing are presented, and the correlation between film structure, conductivity and stability is discussed.