J. Drayton

Cadmium Telluride Solar Cells on Molybdenum Substrates

We report on the development of Mo/CdTe/CdS/indium-tin-oxide, thin-film solar cells grown by radio-frequency magnetron sputtering. This is an inverted configuration compared to the conventional glass/tin-oxide/CdS/CdTe/metal cells. Molybdenum was chosen as a substrate because its thermal expansion coefficient and the work function are close to those of CdTe. We have achieved AM1.5 conversion efficiencies of 7.8 percent on 0.05 cm 2 area devices. Our best cells had a nitrogen-doped ZnTe layer between the molybdenum and the CdTe for a somewhat improved back contact. However, we observe a significant rollover in the IV curve in forward current that indicates a back-diode effect. This implies the need for improvement of the electronic properties of the molybdenum - CdTe and possibly CdS - ITO interfaces.

Back contact and reach-through diode effects in thin-film photovoltaics

The physics of back contact effects in photovoltaic devices is revisited. We show that the back contact Schottky barrier can act in either back-diode or reach-through diode regimes. This understanding predicts that rare local spots with low back barrier hole transparency and/or weak main junctions can shunt the photocurrent thus decreasing the measured open-circuit voltage and device efficiency. We derive several more specific predictions of our model and verify them experimentally for the case of thin-film CdTe photovoltaics. Our concept has practical implications: a simple recipe leading to an efficient (13%) copper-free CdTe solar cell.

Piezo-effect and physics of CdS-based thin-film photovoltaics

We report a strong reversible piezo-effect in CdTe∕CdS photovoltaics consistent with the piezo parameters of CdS. Our finding suggests a different understanding of CdS-based solar cells including CdTe- and CuIn(Ga)Se-based devices. Because the CdS film is put into compression in the device, the piezo coupling generates surface charges and the electric field opposing that of the absorber layer. The corresponding potential barrier makes CdS insulating and the device operate in a metal-insulator-semiconductor mode. Our understanding introduces the concept of piezo-photovoltaics and suggests specific practical implications.

Piezo-photovoltaic coupling in CdS-based thin-film photovoltaics

We report a summary of our recent study of piezo- and pyroelectricity in CdS-based photovoltaics including CdTe and CuIn(Ga)Se2 absorber devices. The strong pyro- and piezo-effects in CdS significantly affect the electric field distribution and the physics of device operations. We introduce the concept of pyro-photovoltaic coupling where the photovoltaic effect and the CdS polarization strongly depend on each other. Our quantitative modeling shows that the CdS polarization turns out to be beneficial for photovoltaic technology, making it more forgiving. Three experimental setups utilized in our work were aimed at exerting mechanical stresses corresponding to the device squeezing, bending, and flexing; we used the substrate, superstrate, and flexible substrate structures. Our understanding points at previously unexplored venues in thin-film photovoltaic technology.

Magnetron sputtered CdTe solar cells on flexible substrates

We describe the application of sputter deposition processes to the fabrication of CdS/CdTe solar cells on light weight, flexible substrates in both the substrate and superstrate configurations. We have fabricated CdS/CdTe cells on molybdenum foil and Kapton polyimide. The Mo-based devices are among the best reported for substrate configuration CdTe solar cells, with efficiencies of /spl sim/7% at AM1.5.

Effect of CdTe thickness reduction in high efficiency CdS/CdTe solar cells

High efficiency CdTe solar cells are typically grown with CdTe thicknesses from 3 to 15 μm, although the thickness required for 90% absorption of the incident irradiation at 800 nm is only ∼1 μm. In this paper, we present the effect of CdTe thickness reduction on the performance of CdS/CdTe solar cells in which both the CdS and CdTe films were grown by sputtering. We produced a series of cells with different CdTe thickness (from 0.5 to 3.0 μm), and held the CdS thickness and back-contact-processing constant. The effect of CdTe thickness reduction on the diffusion of CdS into CdTe was studied using optical absorption and x-ray diffraction techniques. Only slight decreases occur in open-circuit voltage, short-circuit current, and fill factor with decrease in CdTe film thickness to 1.0 μm. Almost 10% efficient cells were obtained with 1 μm CdTe. Below 1 μm, all cell parameters decrease more rapidly, including the red quantum efficiency.

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.

Real time analysis of magnetron-sputtered thin-film CdTe by multichannel spectroscopic ellipsometry

Real time spectroscopic ellipsometry (RTSE) based on rotating-compensator modulation and multichannel detection has been implemented to characterize polycrystalline thin film CdTe deposition for photovoltaic applications. RTSE is capable of providing routine deposition information on substrate temperature T and deposition rate. It is also capable of providing detailed information on the thickness evolution of microstructure and optical properties. In this study, we highlight the differences in nucleation that occur under different CdTe deposition conditions on smooth crystalline Si wafer substrates. Differing behavior in the initial stages of deposition has been observed, ranging from layer-by-layer growth to nucleation and coalescence of 45 A thick clusters. We also consider the thickness and substrate dependence of the microstructure, comparing depositions on smooth Si wafer and rough thin film Mo substrates.

Sputtered Cd1-xMgxTe Films for Top Cells in Tandem Devices

Cd1-xMgxTe is a potential candidate for the top cell in two-terminal tandem solar cells. The close match of the lattice constant of MgTe with CdTe and the apparent complete miscibility of MgTe in CdTe and the rapid increase in band gap with Mg content gives flexibility to prepare material with the appropriate band gap for current-matching in a tandem solar cell. We have chosen to deposit Cd 1-xMgxTe thin films by RF sputtering to maintain access to low growth temperatures. Substrate temperatures greater than 250degC produced highly adhering polycrystalline films with grain size in the range of 100-150 nm and a strong preferential (111) orientation. All the films were p-type and resistive. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed that the Mg concentration in the film is significantly lower than that of the source target. Films were chloride-treated at temperatures in the range 350 to 387degC in a variety of ambients and the band gap showed narrowing at higher annealing temperature and time duration, particularly in oxygen containing ambients. XRD and AFM studies of the Cd1-xMgx Te films showed evidence of re-crystallization including grain growth and a more random crystallographic orientation of the grains. Prototype CdS/Cd1-xMgxTe solar cells were fabricated

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.