V. Parikh

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.

Hg1-xCdxTe as the Bottom Cell Material in Tandem II-VI Solar Cells

We have measured the electrical, optical and morphological properties of as-grown Hg 1-x Cd x Te films prepared by r.f. sputtering. The Hg 1-x Cd x Te films were grown at substrate temperatures ranging from 25°C to 150°C. Films grown at temperatures lower than or equal to 70°C were highly resistive (≥10 5 Φ-cm) and were not measurable by our Hall apparatus. Optical transmission data show that the band gap of these films ranged from 0.8 eV-1.5 eV, satisfying the optimum band gap criteria. Plan view and cross sectional SEM studies indicate that the films grown at 85°C and 100°C have larger grains with compact grain boundaries and these films typically yield the best cell performance. Spectroscopic ellipsometry studies are being used to estimate the band-gap as a function of substrate temperature. We have also studied the effect of various back contacts and have fabricated complete solar cells. We found that Cu-Au serves as the best ohmic back contact to CdS/HgCdTe solar cell. Our preliminary results of J-V analyses on the complete solar cell show that the efficiency is mainly limited by the short-circuit current. Electrical-bias-dependent QE measurements indicate voltage-dependent current collection mainly in the long wavelength region. Further optimization of growth parameters and CdCl2 treatment needs to be carried out to improve the cell performance.

Thin-film tandem cells with thin CdTe

Models using realistic parameters indicate that two-terminal, double-junction tandem solar cells based on CdTe or CuInGaSe2 and related alloys should be able to reach 25% efficiencies. 1 Key to achieving this is high transparency for below-band-gap photons. Cells with transparent back contacts (TBC) and CdTe thickness between 2.4 μm and 0.3 μm show over 65% average transmission for photons below bandgap. The short circuit current is not compromised as a 10.6% cell with only 0.5 μm CdTe is demonstrated with a Jsc of 23.1mA/cm2. For the bottom cell we have focused on optimizing Hg1-xCdxTe, where a single junction efficiency of 6.7% was obtained. Calculations based on the transmission of cells and the quantum efficiency of the best HgCdTe cell indicate that efficiency gains can be made by making the CdTe layer thinner. Finally, a four terminal cell with 7.6% efficiency is discussed.

Transparent back contacts and interconnect junctions for CdTe top cells

The development of a good, low resistance transparent back contact (TBC) to the superstrate type CdS/CdTe or Cd1-xMgxTe, Cd1-xZnxTe, or Cd1-xMnxTe alloy solar cell is crucial for achieving the best possible four-terminal or two-terminal polycrystalline thin-film tandem solar cell. We find that ZnTe:N/ITO serves as a good TBC to CdS/CdTe solar cell yielding 9.1% power conversion efficiency even without the use of intentional copper at the back contact. A transparent back contact permits device measurements with light incident from the contact side. Such contact-side J-V measurements show that the short-circuit current of ~2 mum thick cells with this TBC is reduced to almost half when measured from the contact side. This difference in current matches quite well with the integrated current calculated from QE done under weak DC white light bias. Similar experiments performed on 0.68 mum cells showed little difference in current between glass-side and contact-side illumination consistent with a stronger collection field in the thin cells

polycrystalline thin film tandem solar cells cascaded by ZnTe/ZnO interconnects

The present work focuses on the optimization of an interconnect junction for II-VI tandem solar cells. We have used ZnTe:N/ZnO:Al and ZnTe:Cu/ZnO:AI as interconnect junctions between a CdS/CdTe top cell and a CdS/HgCdTe bottom cell. Separate annealing studies of ZnTe:Cu show electrical and optical properties that are stable to 387/spl deg/C when the ZnTe:Cu is sputter deposited at /spl sim/320/spl deg/C. J-V measurements show that the V/sub OC/ of the tandem cell with ZnTe:Cu is much lower (/spl sim/820 mV) than the V/sub OC/ of the tandem cell (/spl sim/1031 mV) with ZnTe:N. One of the possible reasons is the interdiffusion of Cu into CdS, probably making it intrinsic and changing the junction properties of the top cell. Quantum efficiency measurements of the tandem cell with ZnTe:N show that the current is limited by the bottom cell, while the current seems to be limited by both the top cell and the bottom cell in the case of the tandem cell with ZnTe:Cu/ZnO:AI as the interconnect junction.

Spatial and Temporal Variations in Electronic Transport Through a CdTe-Based Schottky Barrier

We study the electric current through metal-semiconductor junctions of a type used in thin-film PV for back contacts. To concentrate on one type of junction we used the symmetric structures of rf-sputtered CdTe layer sandwiched between two Cr contacts. Along with the conventional measurements, the current-sensing contact mode AFM was employed to measure the current-voltage characteristics and current variations with time under fixed voltage. We found that (i) the electric current flow is laterally strongly nonuniform; (ii) it chaotically varies over time; (iii) this behavior did not correlate with surface topography. We interpret our observations in terms of defect assisted tunneling through time-dependent defect pathways.