Kent J. Price

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.

Effect of CdCl 2 Treatment on the Interior of CdTe Crystals

An essential processing step in CdTe/CdS polycrystalline solar cells is heat treatment in CdCl 2 . We present photoluminescence results from single crystals of CdTe that have been exposed to CdCl 2 treatments at 387 C similar to those used in actual cell fabrication. Using sub band gap excitation from a tunable diode laser, we probe states in the interior of the crystal. We show that high-purity (99.998 percent) CdCl 2 treatment results in the appearance of a 1.45 eV donor-acceptor transition that is likely due to a Cl-Cu center. Low purity (99.7 percent) CdCl 2 treatment results in the appearance of the 1.45 eV line and a 1.555 eV Cu-related emission. These results indicate that the CdCl 2 treatment has an effect on the interior of CdTe grains, in addition to its already well established effect on grain boundaries in polycrystalline CdS/CdTe devices. They also imply that CdCl 2 treatment may result in the incorporation of Cu into the CdTe grains. The results will be related to the effects of CdCl 2 on polycrystalline CdS/CdTe devices that have been observed by other groups. This work is supported by NREL

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.

Visible and x-ray spectroscopy studies of defects in CdTe

We have used electroluminescence (EL), photoluminesence (PL), and synchrotron x-ray fluorescence (XRF) to study some of the properties of defects related to Cu and Na in polycrystalline thin films of CdTe. Some samples were prepared by ion implantation of CdTe single crystals followed by thermal anneal to remove the damage. Others were prepared by magnetron sputtering on quartz or borosilicate glass followed by vapor CdCl/sub 2/ heat treatment. EL was studied from sputtered CdS/CdTe solar cell structures. We find that EL is very sensitive to defects induced by light soaking; ion implantation-induced damage can be annealed at 400 C to yield good PL; and synchrotron XRF shows the presence of substantial copper in films sputtered from nominally pure CdTe.

Open-Circuit Voltage Decay in CdTe/CdS Solar Cells

Open-Circuit Voltage Decay (OCVD) is a common technique used to characterize numerous semiconductor devices. However, to the knowledge of the authors, the technique has not previously been applied to CdTe-based solar cells. We use a simple setup consisting of a function generator, rectifying diode, and digital oscilloscope to measure the dark open circuit voltage decay as a function of time across a CdTe solar cell. We find the decay to be described by the equation v(t) = v 0 + A 1exp (–t/τ1) + A 2 exp (–t/τ2) where v is the voltage, t is time, τ1 and τ2 are characteristic decay times, and A1, A2 and v0 are constants. The two characteristic decay times are on the order of 10 μs and 500 μs. The relative contribution of the two decay times depends on the magnitude of the initial applied voltage pulse. We will describe preliminary results on the correlation between the OCVD and solar cell performance, including the effects of light-soaking on OCVD behavior.