Gregory M. Kimball

820 mV open-circuit voltages from Cu2O/CH3CN junctions

P-Type cuprous oxide (Cu2O) photoelectrodes prepared by the thermal oxidation of Cu foils exhibited open-circuit voltages in excess of 800 mV in nonaqueous regenerative photoelectrochemical cells. In contact with the decamethylcobaltocene+/0 (Me10CoCp2+/0) redox couple, cuprous oxide yielded open-circuit voltage, Voc, values of 820 mV and short-circuit current density, Jsc, values of 3.1 mA cm−2 under simulated air mass 1.5 illumination. The energy-conversion efficiency of 1.5% was limited by solution absorption and optical reflection losses that reduced the short-circuit photocurrent density. Spectral response measurements demonstrated that the internal quantum yield approached unity in the 400–500 nm spectral range, but poor red response, attributable to bulk recombination, lowered the overall efficiency of the cell. X-Ray photoelectron spectroscopy and Auger electron spectroscopy indicated that the photoelectrodes had a high-quality cuprous oxide surface, and revealed no observable photocorrosion during operation in the nonaqueous electrolyte. The semiconductor/liquid junctions thus provide a noninvasive method to investigate the energy-conversion properties of cuprous oxide without the confounding factors of deleterious surface reactions.

Photoluminescence-based measurements of the energy gap and diffusion length of Zn3P2

The steady-state photoluminescence spectra of zinc phosphide (Zn_3P_2) wafers have revealed a fundamental indirect band gap at 1.38 eV, in close proximity to the direct band gap at 1.50 eV. These values are consistent with the values for the indirect and direct band gaps obtained from analysis of the complex dielectric function deduced from spectroscopic ellipsometric measurements. Bulk minority carrier lifetimes of 20 ns were observed by time-resolved photoluminescence decay measurements, implying minority-carrier diffusion lengths of ≥ 7 µm.

Band alignment of epitaxial ZnS/Zn3P2 heterojunctions

The energy-band alignment of epitaxial zb-ZnS(001)/α-Zn_(3)P_(2)(001) heterojunctions has been determined by measurement of shifts in the phosphorus 2p and sulfur 2p core-level binding energies for various thicknesses (0.6–2.2 nm) of ZnS grown by molecular beam epitaxy on Zn_(3)P_(2). In addition, the position of the valence-band maximum for bulk ZnS and Zn3P2 films was estimated using density functional theory calculations of the valence-band density-of-states. The heterojunction was observed to be type I, with a valence-band offset, ΔE_V, of −1.19 ± 0.07 eV, which is significantly different from the type II alignment based on electron affinities that is predicted by Anderson theory. n^(+)-ZnS/p-Zn_(3)P_(2) heterojunctions demonstrated open-circuit voltages of >750 mV, indicating passivation of the Zn_(3)P_(2) surface due to the introduction of the ZnS overlayer. Carrier transport across the heterojunction devices was inhibited by the large conduction-band offset, which resulted in short-circuit current densities of <0.1 mA cm^(−2) under 1 Sun simulated illumination. Hence, constraints on the current density will likely limit the direct application of the ZnS/Zn_(3)P_(2) heterojunction to photovoltaics, whereas metal-insulator-semiconductor structures that utilize an intrinsic ZnS insulating layer appear promising.

Wafer-Scale Strain Engineering of Ultrathin Semiconductor Crystalline Layers

The fabrication of a wafer-scale dislocation-free, fully relaxed single crystalline template for epitaxial growth is demonstrated. Transferring biaxially-strained Inx Ga1-x As ultrathin films from InP substrates to a handle support results in full strain relaxation and the Inx Ga1-x As unit cell assumes its bulk value. Our realization demonstrates the ability to control the lattice parameter and energy band structure of single layer crystalline compound semiconductors in an unprecedented way.

Conformal GaP layers on Si wire arrays for solar energy applications

We report conformal, epitaxial growth of GaP layers on arrays of Si microwires. Silicon wires grown using chlorosilane chemical vapor deposition were coated with GaP grown by metal-organic chemical vapor deposition. The crystalline quality of conformal, epitaxial GaP/Si wire arrays was assessed by transmission electron microscopy and x-ray diffraction. Hall measurements and photoluminescence show p- and n-type doping with high electron mobility and bright optical emission. GaP pn homojunction diodes on planar reference samples show photovoltaic response with an open circuit voltage of 660 mV.

Passivation of Zn3P2 substrates by aqueous chemical etching and air oxidation

Surface recombination velocities measured by time-resolved photoluminescence and compositions of Zn_(3)P_2 surfaces measured by x-ray photoelectron spectroscopy (XPS) have been correlated for a series of wet chemical etches of Zn_(3)P_2 substrates. Zn_(3)P_2 substrates that were etched with Br_2 in methanol exhibited surface recombination velocity values of 2.8 × 10^4 cm s^(−1), whereas substrates that were further treated by aqueous HF–H_(2)O_2 exhibited surface recombination velocity values of 1.0 × 10^4 cm s^(−1). Zn_(3)P_2 substrates that were etched with Br_2 in methanol and exposed to air for 1 week exhibited surface recombination velocity values of 1.8 × 10^3 cm s^(−1), as well as improved ideality in metal/insulator/semiconductor devices.

Mg doping and alloying in Zn 3 P 2 heterojunction solar cells

Zinc phosphide (Zn<inf>3</inf>P<inf>2</inf>) is a promising and earth-abundant alternative to traditional materials (e.g. CdTe, CIGS, a-Si) for thin film photovoltaics. We report the fabrication of Mg/Zn<inf>3</inf>P<inf>2</inf> Schottky diodes with VOC values reaching 550 mV, JSC values up to 21.8 mA/cm², and photovoltaic efficiency reaching 4.5%. Previous authors have suggested that Mg impurities behave as n-type dopants in Zn<inf>3</inf>P<inf>2</inf>, but combined Hall effect measurements and Secondary Ion Mass Spectrometry (SIMS) show that 10¹⁷ to 10¹⁹ cm⁻³ Mg impurities compensate p-type doping to form highly resistive Zn<inf>3</inf>P<inf>2</inf>. Further device work with modified ITO/Mg/Zn<inf>3</inf>P<inf>2</inf> heterojunctions suggests that the ITO capping layer improves a passivation reaction between Mg and Zn<inf>3</inf>P<inf>2</inf> to yield high voltages > 500 mV without degradation in the blue response of the solar cell. These results indicate that at least 8–10% efficiency cell is realizable by the optimization of Mg treatment in Zn<inf>3</inf>P<inf>2</inf> solar cells.

Direct evidence of Mg-Zn-P alloy formation in Mg/Zn 3 P 2 solar cells

Zinc phosphide (Zn<inf>3</inf>P<inf>2</inf>) is a promising and earth-abundant alternative to traditional materials (e.g. CdTe, CIGS, a-Si) for thin film photovoltaics. The record solar energy conversion efficiency for Zn<inf>3</inf>P<inf>2</inf> cells of 6% (M. Bhushan et al., Appl. Phys. Lett., 1980) used a Mg/Zn<inf>3</inf>P<inf>2</inf> device geometry that required annealing to reach peak performance. Here we report photovoltaic device results and junction composition profiles as a function of annealing treatment for ITO/Mg/Zn<inf>3</inf>P<inf>2</inf> devices. Mild annealing at 100 °C in air dramatically increases V<inf>oc</inf> values from ∼150 mV to 550 mV, exceeding those of the record cell (V<inf>oc, record</inf> = 490 mV). In devices with thinner Mg films we achieved J<inf>sc</inf> values reaching 26 mA cm⁻², significantly greater than those of the record cell (J<inf>sc, record</inf> = 14.9 mA cm⁻²). Junction profiling by secondary ion mass spectrometry (SIMS) and x-ray photoelectron spectroscopy (XPS) both show evidence of MgO and Mg-Zn-P alloy formation at the active photovoltaic junction in annealed ITO/Mg/Zn<inf>3</inf>P<inf>2</inf> devices. These results indicate that high efficiency should be realizable by optimization of Mg treatment in Mg/Zn<inf>3</inf>P<inf>2</inf> solar cells.

An in-situ method of monitoring flexible CIGS modules moisture induced degradation and lifetime prediction

Moisture induced degradation is a challenge facing flexible CIGS modules designs as the polymeric barriers usually have higher water vapor transmittance rate (WVTR) than rigid glass barriers. In the process of module design, the expected lifetime is a key point in selecting the most appropriate barriers. However, the correlation between common acceleration tests and module lifetime is still not well known. This work introduces a method of monitoring flexible CIGS module degradation in-situ by recording sheet resistance when a module is exposed to humidity and temperature. An empirical Hallberg-Peck model was employed to estimate the module lifetime.

Methodology for improved cell integration based on distributed-element circuit analysis

Improved cell integration is a critical part of narrowing the gap between cell and module efficiencies. In this paper, we present a methodology based distributed-element circuit analysis and electroluminescence imaging for optimizing the design of solar cell metallization and interconnection. The distributed-element circuit model is based on an array of equivalent circuits, each representing a 100×100 μm area of a thin film solar cell. A range of cell metallization designs are then compared within the simulation framework to deliver predictions of the effect of design changes on module efficiency. The optimized cell metallization design was tested experimentally on the manufacturing line, confirming the anticipated improvement in module efficiency. Distributed-element circuit analysis delivers reliable predictions of module efficiency and accelerates cell integration development projects.