R. G. Bohn

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.

ZnTe:N back contacts to CdS/CdTe solar cells

We report the development of nitrogen-doped ZnTe back contacts for CdS/CdTe solar cells. Reproducible p-type doping of the ZnTe was achieved by reactive RF magnetron sputtering with Ar/N/sub 2/ gas mixtures. The conductivity of the doped films was about five orders of magnitude higher than that of intrinsic ZnTe sputtered films. These films were used as contacts for glass/SnO/sub 2//CdS/CdTe solar cells. The contact structure of ZnTe/ZnTe:N/Ni showed slightly lower initial performance but improved stability compared to our evaporated Cu/Au contacts for a 3000 hr test cycle at 100/spl deg/C.

Fabrication of CdTe solar cells by laser-driven physical vapor deposition

Abstract Polycrystalline cadmium sulfide-cadmium telluride heterojunction solar cells were fabricated for the first time using a laser-driven physical vapor deposition method. An XeCl excimer laser was used to deposit both of the II–VI semiconductor layers in a single vacuum chamber from pressed powder targets. Results are presented from optical absorption, Raman scattering, X-ray diffraction, and electrical characterization of the films. Solar cells were fabricated by deposition onto SnO 2 -coated glass with top contacts produced by gold evaporation. Device performance was evaluated from the spectral quantum efficiency and current-voltage measurements in the dark and with air mass 1.5 solar illumination.

Cadmium telluride thin‐film solar cells by pulsed‐laser deposition

We have used pulsed laser (ablation) deposition (PLD) for the fabrication of polycrystalline CdS/CdTe heterojunction solar cells on glass substrates. PLD was used for both semiconductor layers as well as a thin layer of CdCl2 deposited for a post‐deposition anneal treatment. Results are presented of optical time‐of‐flight studies of the deposition plume, characterization of the as‐deposited films, and performance of the photovoltaic devices. Results to date have yielded a 10.5% efficient cell as tested at NREL.

RF sputtered films of Cu-doped and N-doped ZnTe

Polycrystalline thin film solar cells using the CdS/CdTe structure have good efficiencies but the ideal low resistance ohmic contact to the p-type CdTe which is stable has yet to be developed. A good candidate for this contact would be p-type ZnTe produced by sputtering, which is a process that is scalable to large areas. We have successfully doped ZnTe with copper and nitrogen using RF planar magnetron sputtering. For the copper doping, resistivities as low as 0.01 ohm-cm have been achieved. Raman spectra have been used as an indicator of film quality. Nitrogen doping was achieved by introducing small amounts of molecular nitrogen into the argon sputter gas during ZnTe deposition. A minimum film resistivity of about 20 ohm-cm was obtained for films grown using approximately a 5% N/sub 2//Ar ratio at a pressure of 18 mTorr and a substrate temperature in the vicinity of 400 C. Here, again, Raman spectra were used to check on film quality by comparing to that of pure ZnTe.

Rf sputtered CdS/CdTe solar cells: effects of magnetic field, RF power, target morphology, and substrate temperature

In this paper we present a study of the influence of substrate temperature, RF power, target erosion, and magnetic field configuration on RF sputtering of CdTe and CdS. These sputtering parameters are shown to affect deposition rate, film morphology, photoluminescence efficiency, and cell performance. The magnetic field shape and strength affects the charged particle flux on the growing film and appears to have a strong influence on the final cell performance.

Polycrystalline CdTe solar cells by pulsed laser deposition

Polycrystalline CdS/CdTe heterojunctions have been grown by pulsed laser-driven physical vapor deposition (LDPVD) using an XeCl excimer laser at 308 nm. The LDPVD process accommodates thin-film growth at low substrate temperatures (<or=300 degrees C), while permitting considerable control over the physical conditions of the deposition plume. The congruent evaporation provides good film stoichiometry and the use of multiple targets facilitates heterojunction fabrication in a single vacuum chamber. Polycrystalline films were grown on alkali-free glass and on SnO/sub 2/-coated glass from pressed powder targets in a vacuum of approximately 5*10/sup -6/ torr. Optimum growth occurred near 280 degrees C with growth rates of approximately 0.5 AA/pulse. Devices were fabricated on the films after post-growth annealing and characterized by I-V and spectral quantum efficiency measurements.<<ETX>>

Thin‐film CdTe photovoltaic cells by laser deposition and rf sputtering

Laser‐driven physical vapor deposition (LDPVD) and radio‐frequency (rf) sputtering have been used to fabricate thin‐film solar cells on SnO2‐coated glass substrates. The laser‐ablation process readily permits the use of several target materials in the same vacuum chamber and complete solar cell structures have been fabricated on SnO2‐coated glass using LDPVD for the CdS, CdTe, and CdCl2. To date the best devices (∼9% AM1.5) have been obtained after a post‐deposition anneal at 400 °C. In addition, cells have been fabricated with the combination of LDPVD CdS, rf‐sputtered CdTe, and LDPVD CdCl2. The performance of these cells indicates considerable promise for the potential of rf sputtering for CdTe photovoltaic devices. The physical mechanisms of LDPVD have been studied by transient optical spectroscopy on the laser ablation plume. These measurements have shown that, e.g., Cd is predominantly in the neutral atomic state in the plume but with a large fraction which is highly excited internally (≥6 eV) and th...

Sputtered ZnTe:N and ZnO:Al for Solar Cell Electrodes and Recombination Junctions

We have used sputtered ZnTe:N and ZnO:Al as transparent electrodes for CdTe based solar cells. ZnTe:N is reactively sputtered with 3% N 2 in the sputter gas. The ZnTe:N films typically have transmission near 85% above 750 nm and resistivity as low as 10 ohm-cm with fine grains of approximately 30 nm diameter. ZnO:Al is sputtered from a ZnO:Al 2 O 3 (2%) target. The ZnO:Al films have resistivity as low as 4 × 10 -4 ohm-cm with 80-95% transmission over the visible spectrum. To test the stability of the films, ZnTe:N films were annealed in argon, dry air and nitrogen at a range of temperatures from 200°C to 500°C. Annealing decreased the resistivity for temperatures up to 350°C and increased for temperatures greater than 350°C. ZnO:Al films were annealed in dry air at temperatures from 300°C to 550°C. Though the resistivity increased at higher annealing temperatures, there was no change in the transmission. Important to the function of a tandem solar cell is a transparent contact, like ZnTe:N/ZnO:Al, to replace the traditional metal contact to the CdS/CdTe solar cell. We investigate the ZnTe:N/ ZnO:Al bilayer as a possible recombination junction. The rf sputtered ZnTe:N and ZnO:Al films were characterized by AFM, STM, XRD, transmission, 4-point probe, and Hall measurements.

Thin film cadmium telluride photovoltaic cells

This report describes research to develop to vacuum-based growth techniques for CdTe thin-film solar cells: (1) laser-driven physical vapor deposition (LDPVD) and (2) radio-frequency (rf) sputtering. The LDPVD process was successfully used to deposit thin films of CdS, CdTe, and CdCl{sub 2}, as well as related alloys and doped semiconductor materials. The laser-driven deposition process readily permits the use of several target materials in the same vacuum chamber and, thus, complete solar cell structures were fabricated on SnO{sub 2}-coated glass using LDPVD. The rf sputtering process for film growth became operational, and progress was made in implementing it. Time was also devoted to enhancing or implementing a variety of film characterization systems and device testing facilities. A new system for transient spectroscopy on the ablation plume provided important new information on the physical mechanisms of LDPVD. The measurements show that, e.g., Cd is predominantly in the neutral atomic state in the plume but with a fraction that is highly excited internally ({ge} 6 eV), and that the typical neutral Cd translational kinetic energies perpendicular to the target are 20 eV and greater. 19 refs.