T.A. Gessert

Fabrication procedures and process sensitivities for CdS/CdTe solar cells

This paper details the laboratory processes used to fabricate CdS/CdTe solar cells at the National Renewable Energy Laboratory. The basic fabrication technique includes low-pressure chemical vapor deposited SnO2 , chemical-bath deposited CdS, close-spaced sublimated CdTe, solution-CdCl2 treatment, and an acid-contact etch, followed by application of a doped-graphite paste. This paper also describes the results of a reproducibility study in which cells were produced by multiple operators with an average AM1·5 efficiency of 12·6%. And finally, this paper discusses process sensitivities and alternative cell fabrication procedures and reports the fabrication of a cell with an AM1·5 efficiency of 15·4%. Copyright

High‐efficiency indium tin oxide/indium phosphide solar cells

Solar cells have been fabricated by rf sputter depositing indium tin oxide onto single crystal p‐type indium phosphide. Four different substrate doping densities have been used but in all cases the dopant was zinc and the wafers were 〈100〉 oriented. The optimum doping density from the range studied was 3×1016 cm−3 and devices based on such substrates have yielded total area efficiencies up to 16.2% using the air mass 1.5 spectrum normalized to 100 mW cm−2, which correspond to active area efficiencies of 19.1%. A doping density less than the optimum yielded devices with excessive series resistance. Higher doping densities led to a marked loss of red response.

Chemical vapor deposition-formed p-type ZnO thin films

We have fabricated nitrogen-doped zinc oxide (ZnO) films that demonstrate p-type behavior by using metalorganic chemical vapor deposition. In our experiment, diethylzinc is used as a Zn precursor, and NO gas is used to supply both O and N to form a N-doped ZnO (ZnO:N) film. With these precursors, we have routinely reached an N concentration in the ZnO films of about 1–3 at. %. When the N concentration level is higher than 2 at. %, the films demonstrate p-type characteristics. The carrier concentration of the films varies from 1.0×1015 to 1.0×1018 cm−3, and mobilities are mainly in the 10−1 cm2 V−1 s−1 range. The lowest film resistivity achieved is ∼20 Ω cm.

Interdiffusion of CdS and Zn2SnO4 layers and its application in CdS/CdTe polycrystalline thin-film solar cells

In this work, we found that the interdiffusion of the CdS and Zn2SnO4 (ZTO) layers can occur either at high temperature (550–650 °C) in Ar or at lower temperature (400–420 °C) in a CdCl2 atmosphere. By integrating a Zn2SnO4 film into a CdS/CdTe solar cell as a buffer layer, this interdiffusion feature can solve several critical issues and improve device performance and reproducibility of both SnO2-based and Cd2SnO4-based CdTe cells. Interdiffusion consumes the CdS film from both the ZTO and CdTe sides during the device fabrication process and improves quantum efficiency at short wavelengths. The ZTO film acts as a Zn source to alloy with the CdS film, which results in increases in the band gap of the window layer and in short-circuit current density Jsc. Interdiffusion can also significantly improve device adhesion after CdCl2 treatment, thus providing much greater process latitude when optimizing the CdCl2 process step. The optimum CdCl2-treated CdTe device has high quantum efficiency at long wavelength,...

Advanced high-efficiency concentrator tandem solar cells

Computer modeling studies of two-junction concentrator tandem solar cells show that infrared (IR)-responsive bottom cells are essential to achieve the highest performance levels in both terrestrial and space applications. These studies also show that medium-bandgap/low-bandgap tandem pairs hold a clear performance advantage under concentration when compared to high-bandgap/medium-bandgap pairs, even at high operating temperatures (up to 100 degrees C). Consequently, two novel concentrator tandem designs that utilize low-bandgap bottom cells have been investigated. These include mechanically stacked, four-terminal GaAs-0.95-eV-GaInAsP tandem, and monolithic, lattice-matched. three-terminal InP-0.75-eV-GaInAs tandem. In preliminary experiments, terrestrial concentrator efficiencies exceeding 30% have been achieved with each of these designs. Methods for improving the efficiency of each tandem are discussed.<<ETX>>

Development of radio-frequency magnetron sputtered indium molybdenum oxide

Molybdenum-doped indium oxide (IMO), an n-type transparent conducting oxide, was deposited using radio-frequency magnetron sputtering. The effects of oxygen concentration in an argon ambient and substrate temperature on film properties were studied. Compared to undoped indium oxide (In2O3) films, IMO films demonstrated higher electron mobility and more than an order-of-magnitude higher carrier concentration. The highest conductivity IMO film demonstrated a mobility of 44 cm2 V−1 s−1 and a carrier concentration of 1.3×1020 cm−3. The properties of both In2O3 and IMO films were very sensitive to the oxygen concentration, but not to the substrate temperature. Average visible transmittance of In2O3 and IMO films were 86% and 80%, respectively. Both optical and x-ray photoelectron spectroscopy analyses indicate a possible second phase in IMO films deposited at lower (⩽1%) oxygen concentrations.

Practical considerations in tandem cell modeling

Abstract Computer modeling of two-junction, tandem solar cells is performed with an emphasis on exploring the sensitivity of cell design and performance to important, practical parameters such as subcell connectivity, incident spectrum, junction temperature, concentration ratio and top cell quantum efficiency. The accuracy of the model is verified by comparing calculated bandgap-dependent, normalized conversion efficiency temperature coefficients with those measured experimentally for state-of-the-art, single-junction cells. Examples of the effects of operational parameter variations are presented. Tandem designs based on independently connected subcells are shown to have several advantages. Based on the modeling work, novel, low-bandgap, InP-based devices have been developed which appear promising for bottom cell applications in two-junction tandems. In particular, epitaxially grown, high-performance p/n homojunctions in Ga0.47In0.53As layers lattice matched to InP substrates have been fabricated. The results of performance testing the Ga0.47In0.53As cells under mild concentration ratios suggest that a practical efficiency of at least 35% is possible for a GaAs/Ga0.47In0.53As mechanically stacked, two-junction tandem cell which is independently connected and operated under a concentration ratio of 500 suns (ASTM E891-87 direct spectrum, 25°C).

High-performance concentrator tandem solar cells based on IR-sensitive bottom cells

Abstract Computer simulations of two-junction, concentrator tandem solar cell performance show that IR-sensitive bottom cells are required to achieve high efficiencies. Based on this conclusion, two novel concentrator tandem designs are under investigation: (1) a mechanically stacked, four-terminal GaAs/GaInAsP (0.95 eV) tandem, and (2) a monolithic, lattice-matched, three-terminal InP/GaInAs tandem. In preliminary experiments, terrestrial concentrator efficiencies exceeding 30% have been achieved with each of the above tandem designs. Methods for improving the efficiency of each tandem type are discussed.

Electrical and optical properties of ion beam sputtered ZnO:Al films as a function of film thickness

Electrical and optical properties of as‐deposited, ion beam sputtered, Al‐doped ZnO films have been studied as a function of film thickness and carrier concentration. Hall effect measurements reveal that the bulk electrical resistivity of the film generally decreases with increasing film thickness. Additionally, it is observed that the rate of decreasing resistivity depends on the particular film thickness regime. For thinner films (100–200 nm), the resistivity decreases rapidly with increasing film thickness and is due to increases in both carrier concentration and Hall mobility. However, for thicker films, the resistivity decreases more slowly with increasing film thickness and approaches a nearly constant value at a thickness of 1100 nm. In this thickness regime, the slight decrease in resistivity with increasing film thickness is found to be due to an increase in carrier concentration alone. The above observations suggest the presence of at least two scattering mechanisms. It is speculated that grain ...

Device quality thin films of CuInSe2 by a one-step electrodeposition process

Abstract Fabrication of device quality polycrystalline thin films of CuInSe2 with appreciable photovoltaic activity has been achieved by a simple one-step electrodeposition process from a single solution. Controlled by a personal computer, the thin film deposition process has been upgraded to be semi-automatic with good accuracy, flexibility and reliability. The thin films are electrodeposited at constant potentials in unstirred solution. An innovative method employing a multi-potential procedure is used to produce the bilayer (indium-rich) thin films. The composition and quality of the thin films were found sensitive to the concentration of buffering agent, ethylenediamine. Using a lower concentration of ethylenediamine (less than or equal to 0.125 M) in the solution formulation, all of the deposited films, after being made into devices of CdS/CuInSe2, were found to be photovoltaically active. The thin films showed columnar structure in the growth direction. Devices made from these films showed improved spectral response in the long wavelength region. The electron-beam-induced-current measurement indicates a heterojunction device. Best parameters obtained so far for the devices made from the electro-deposited CuInSe2 thin films are Voc = 320 mV, Jsc = 13.2 mA cm−2 and a fill factor of 0.45.