A. Compaan

All-sputtered 14% CdS∕CdTe thin-film solar cell with ZnO:Al transparent conducting oxide

Radio-frequency (rf)-sputtered Al-doped ZnO was used as the transparent front contact in the fabrication of high efficiency superstrate configuration CdS∕CdTe thin-film solar cells. These cells had CdS and CdTe layers also deposited by rf sputtering at 250°C with the highest processing temperature of 387°C reached during a post-deposition treatment. The devices were tested at National Renewable Energy Laboratory and yielded an efficiency of 14.0%, which is excellent for a CdTe cell using ZnO and also for any sputtered CdTe solar cell. The low-temperature deposition process using sputtering for all semiconductor layers facilitates the use of ZnO and conveys significant advantages for the fabrication of more complex multiple layers needed for the fabrication of tandem polycrystalline solar cells and for cells on polymer materials.

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.

Effects of nonuniformity in thin-film photovoltaics

We discuss the physical origin and effects of micrononuniformities on thin-film photovoltaics. The key factors are the large device area and the presence of potential barriers in the grain boundaries (for polycrystalline films) and in device junctions. We model the nonuniformity effects in the terms of random microdiodes connected in parallel through a resistive electrode. The microdiodes of low open circuit voltages are shown to affect macroscopically large regions. They strongly reduce the device performance and induce its nonuniform degradation in several different modes. We support our predictions by experiments, which show that the device degradation is driven by the light-induced forward bias and is spatially nonuniform.

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.

Raman measurements of temperature during cw laser heating of silicon

We present the first Raman data from laser‐heated silicon obtained in situ during the laser irradiation. Analysis of the Stokes to anti‐Stokes ratios yields temperatures at the center of the laser spot which range from room temperature to the melting point as laser power is increased. The data agree well with the nonlinear heat diffusion calculations of Lax for Gaussian beam profiles when the temperature dependence of the silicon absorption coefficient is included. In addition, we show that analysis of the spectral profile of the Raman line can yield information on strain effects produced in the laser‐heated spot.

Plasmons, photoluminescence, and band‐gap narrowing in very heavily doped n‐GaAs

The optical properties of extremely heavily doped n‐GaAs are examined. Metastable electron concentrations up to 3.2×1019 cm−3 were produced by pulsed‐laser annealing of Si‐implanted GaAs. These very heavily doped layers give plasmon Raman shifts up to 1700 cm−1 and photoluminescence bandwidths of greater than 410 meV. The low‐energy edge of the photoluminescence indicates a band‐edge narrowing proportional to ∼n1/3 and equal to ∼200 meV at the highest electron concentration.

Optical properties of CdTe1−xSx(0⩽x⩽1): Experiment and modeling

Spectral ellipsometry at 300 K, in the range 0.75–5.4 eV, has been used to determine the optical constants e(E)[=e1(E)+ie2(E)] of a series of CdTe1−xSx (0⩽x⩽1) films fabricated by a laser-deposition process. The measured e(E) data reveal distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E1, E1+Δ1 doublet and E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs (also E0+Δ0 CP) plus excitonic and band-to-band Coulomb enhancement (BBCE) effects. In addition to evaluating the energies of these various band-to-band CPs, our analysis also makes it possible to obtain information about the binding energies of not only the three-dimensional exciton associated with E0 but also the two-dimensional exciton related to the E1, E1+Δ1 CPs. Our results will be compared to previous experiments and m...

Wiring-up carbon single wall nanotubes to polycrystalline inorganic semiconductor thin films: low-barrier, copper-free back contact to CdTe solar cells.

We have discovered that films of carbon single wall nanotubes (SWNTs) make excellent back contacts to CdTe devices without any modification to the CdTe surface. Efficiencies of SWNT-contacted devices are slightly higher than otherwise identical devices formed with standard Au/Cu back contacts. The SWNT layer is thermally stable and easily applied with a spray process, and SWNT-contacted devices show no signs of degradation during accelerated life testing.

Polycrystalline Cd1−xZnxTe thin films on glass by pulsed laser deposition

Polycrystalline thin films of Cd1-xZnxTe on glass were grown by pulsed laser deposition using an XeCl excimer laser. X-ray diffraction, optical absorption, Raman scattering and energy dispersive x-ray analysis were used to characterize these films. The grains show a predominant orientation with considerable amounts of for the alloys. The band gap energy exhibits a small amount of bowing and the phonons display the expected two-mode behavior. Substrate temperatures above 275°C resulted in optical quality films with good stochiometry for deposition rates below ∼ 1 A/ pulse.