Russell M. Geisthardt

Improved CdTe Solar-Cell Performance by Plasma Cleaning the TCO Layer

A hollow-cathode plasma-cleaning source, designed for uniformity, was added to the load-lock region of an existing single-vacuum CdTe-cell fabrication system. This plasma source cleans the transparent-conductive-oxide layer of the cell prior to the deposition of the CdS and CdTe layers. This plasma exposure enables both thinner CdS layers and enhanced cell voltage. The net result is a reduction in CdS thickness by approximately 20 nm, while maintaining the same cell voltage or, equivalently, an increase in voltage of as much as 80 mV for the same thickness of CdS. Maps that are generated by electroluminescence and light-beam-induced current show modest uniformity improvement with plasma-cleaning treatment.

Status and Potential of CdTe Solar-Cell Efficiency

The status of the highest efficiency CdTe solar cells is presented in the context of comparative loss analysis among the leading technologies for single- and polycrystalline photovoltaic materials. The Shockley-Queisser limit of a single-junction cell, with acknowledgement of variations from standard conditions, is used for reference. The highest CdTe currents achieved are comparable with the best single-crystal cells and superior to other thin-film cells. Voltages match those of multicrystalline Si, but lag behind those of CIGS and crystalline Si, and considerably lag behind crystalline GaAs. The potential for still higher CdTe efficiency will likely require a combination of reduced bulk recombination, smaller back-contact barriers, device structures with advantageous internal fields, and transparent emitters with minimal band offsets.

Sputtered, oxygenated CdS window layers for higher current in CdS/CdTe thin film solar cells

The efficiency of manufactured CdS/CdTe photovoltaic modules can be greatly improved through better collection of available current from the solar spectrum. Typically, only light absorbed in the CdTe absorber layer is collected by the device and light absorption in other layers is lost. A major loss occurs in the CdS window layer, which is strongly absorbing for photon energies above 2.4 ev. RF sputter deposition of CdS in the presence of oxygen has been shown to increase transmission in this spectral region, leading to higher device current. In this work films were examined for transmission and stability upon heating to process temperatures. Numerous devices were made from oxygenated CdS films which showed increased quantum efficiency at wavelengths below 500 nm.

Performance Limits and Status of Single-Junction Solar Cells With Emphasis on CIGS

Limitations in performance and the status of single-junction solar cells are reviewed. Conversion efficiency in single-junction solar cells is systematically analyzed in terms of energy conversion efficiency, the Shockley-Queisser (SQ) efficiency limit, and two remaining efficiencies, i.e., optical efficiency and electrical efficiency normalized to the SQ limit of performance parameters. There is a weak dependence of the SQ efficiency limit in single-junction solar cells on the bandgap of the absorber and much stronger dependence of the electrical efficiency. In the case of Cu(In,Ga)(Se,S)2 (CIGS) solar cells, the bandgap can be varied through In/Ga and/or Se/S content from 1.04 to 1.7 eV, which allows considerable opportunity to optimize the bandgap. Although the record CIGS cells have a bandgap below 1.2 eV, the SQ efficiency limit predicts slightly higher efficiencies for bandgaps near 1.34 eV.

Nonuniformity Characterization of CdTe Solar Cells Using LBIC

Light-beam-induced current measurements have been used for characterization of nonuniformities in cadmium telluride (CdTe) solar cells. Spectral dependence, voltage-bias dependence, and resolution dependence are used for detailed characterization of nonuniformities in junction quality, window thickness, and absorber band gap. These tools were applied to CdTe cells and used to identify thin regions in the CdS layer, regions of modified band gap, and weak diode regions. In addition, an improved procedure has allowed for shorter measurement times without discernable loss of accuracy.

Reduction of window layer optical losses in CdS/CdTe solar cells using a float-line manufacturable HRT layer

Buffer, or high-resistance transparent (HRT) layers have been shown to increase the efficiency of CdS/CdTe solar cells. CdS/CdTe cells were fabricated on numerous float-line-manufacturable TCO and TCO/HRT-coated substrates. The behavior of the best-performing buffer layer was examined over a range of CdS thickness, and the device performance data show gradual dependence of open-circuit voltage and fill factor on CdS thickness below a critical value. The effect of fractional pinhole area is modeled, and it is proposed that pinholes do not dominate device behavior with thinner CdS, and instead band alignment effects explain the observed phenomena. Modeling indicates that the ohmic behavior of the buffer has a relatively small effect on devices with pinholes; instead, improving the diode quality of areas with thin or no CdS has a dominant effect.

Molybdenum oxide and molybdenum oxide-nitride back contacts for CdTe solar cells

Molybdenum oxide (MoOx) and molybdenum oxynitride (MoON) thin film back contacts were formed by a unique ion-beam sputtering and ion-beam-assisted deposition process onto CdTe solar cells and compared to back contacts made using carbon–nickel (C/Ni) paint. Glancing-incidence x-ray diffraction and x-ray photoelectron spectroscopy measurements show that partially crystalline MoOx films are created with a mixture of Mo, MoO2, and MoO3 components. Lower crystallinity content is observed in the MoON films, with an additional component of molybdenum nitride present. Three different film thicknesses of MoOx and MoON were investigated that were capped in situ in Ni. Small area devices were delineated and characterized using current–voltage (J-V), capacitance–frequency, capacitance–voltage, electroluminescence, and light beam-induced current techniques. In addition, J-V data measured as a function of temperature (JVT) were used to estimate back barrier heights for each thickness of MoOx and MoON and for the C/Ni p...

Light-beam-induced-current characterization of CdTe solar cells

In this work, light-beam-induced-current (LBIC) measurements are discussed as a characterization tool for nonuniformities in solar cells. A modification of the measurement procedure has reduced the measurement time by at least a factor of 5, which results in a 10 minute scan time for a 10,000-pixel image. A general strategy for using LBIC and electroluminescence as a nonuniformity characterization suite is presented, which allows for thorough characterization of nonuniformities and identification of their causes. These tools are presented in the specific context of CdTe solar cells.

Compact accelerated life testing with expanded measurement suite

An accelerated-life-testing (ALT) system has been built at the Colorado State University Photovoltaics Laboratory with an emphasis on versatility and periodically performing a suite of electronic measurements on stressed devices. The setup utilizes a scientific oven with a footprint of 17 × 17 inches as a stress chamber and four commercially available 40 W broad-spectrum LED arrays. A preliminary study has been performed on Cadmium Telluride (CdTe) devices. Devices were held at elevated temperature and were exposed to nominally one-sun illumination. Measurements taken of stressed devices include J-V, QE, C-V, electroluminescence (EL) and light-beam-induced current (LBIC).

Growth and characterization of Cd 1−x Mg x Te thin films for possible application in high-efficiency solar cells

Expanded band gap ternary alloys, such as Cd1-xMgxTe, could be beneficial for formation of high-efficiency CdTe solar cells structures, such as multi-junction and electron reflector devices. Cd1-xMgxTe thin films were grown by side-by-side co-evaporation from CdTe and Mg precursors. Optical measurements reveal increased band gap with higher Mg incorporation and lateral band gap grading across the substrate. SEM imaging denotes a grain size decrease with Mg incorporation. XPS analysis indicates Mg directly replaces Cd in the film. TEC10/CdS/Cd1-xMgxTe structures with and without CdCl2 treatment demonstrate photovoltaic diode behavior similar to typical CdS/CdTe devices. LBIC and QE measurements register grading consistent with band gap grading of the film. Although successful, refinement of Cd1-xMgxTe thin film co-evaporation is needed to improve spatial uniformity for large area deposition.