Raymond Hoheisel

Double quantum-well tunnel junctions with high peak tunnel currents and low absorption for InP multi-junction solar cells

Lattice matched InAlGaAs tunnel junctions with a 1.18 eV bandgap have been grown for a triple-junction solar cell on InP. By including two InGaAs quantum wells in the structure, a peak tunnel current density of 113 A/cm2 was observed, 45 times greater than the baseline bulk InAlGaAs tunnel junction. The differential resistance of the quantum well device is 7.52 × 10−4 Ω cm2, a 15-fold improvement over the baseline device. The transmission loss to the bottom cell is estimated to be approximately 1.7% and a network simulation demonstrates that quantum well tunnel junctions play a key role in improving performance at high sun-concentrations.

Simulation of novel InAlAsSb solar cells

This work uses simulations to predict the performance of InAlAsSb solar cells for use as the top cell of triple junction cells lattice matched to InP. The InP-based material system has the potential to achieve extremely high efficiencies due the availability of lattice matched materials close to the ideal bandgaps for solar energy conversion. The band-parameters, optical properties and minority carrier transport properties are modeled based on literature data for the InAlAsSb quaternary, and an analytical drift-diffusion model is used to realistically predict the solar cell performance.

Mobile Solar Power

The militarys need to reduce both fuel and battery resupply is a real-time requirement for increasing combat effectiveness and decreasing vulnerability. Mobile photovoltaics (PV) is a technology that can address these needs by leveraging emerging, flexible space PV technology. In this project, the development and production of a semirigid, lightweight, efficient solar blanket with the ability to mount on, or stow in, a backpack and recharge a high-capacity rechargeable lithium-ion battery was undertaken. The 19% efficient blanket consists of a 10 × 3 solar array of 20 cm2 and single-junction epitaxial lift-off solar cells, which have an efficiency of ∼22% under AM1.5G illumination. A power-conditioning module was also developed to interface the solar panel to the battery. Thirteen systems were outfitted during a Limited Objective Experiment-1 in February 2012, and based on the results, a second version of the system is in development.

Drift-diffusion modeling of InP-based triple junction solar cells.

In this work, we use an analytical drift-diffusion model, coupled with detailed carrier transport and minority carrier lifetime estimates, to make realistic predictions of the conversion efficiency of InP-based triple junction cells. We evaluate the possible strategies for overcoming the problematic top cell for the triple junction, and make comparisons of the more realistic charge transport model with incumbent technologies grown on Ge or GaAs substrates.

Quantum-Well Solar Cells for Space: The Impact of Carrier Removal on End-of-Life Device Performance

In this paper, a detailed analysis on the radiation response of solar cells with multi quantum wells (MQW) included in the quasi-intrinsic region between the emitter and the base layer is presented. While the primary source of radiation damage of photovoltaic devices is minority carrier lifetime reduction, we found that in the case of MQW devices, carrier removal (CR) effects are also observed. Experimental measurements and numerical simulations reveal that with increasing radiation dose, CR can cause the initially quasi-intrinsic background doping of the MQW region to become specifically n- or p-type. This can result in a significant narrowing and even the collapse of the electric field between the emitter and the base where the MQWs are located. The implications of the CR-induced modification of the electric field on the current-voltage characteristics and on the collection efficiency of carriers generated within the emitter, the MQW region, and the base are discussed for different radiation dose conditions. This paper concludes with a discussion of improved radiation hard MQW device designs.

Characterization of high fluence irradiations on advanced triple junction solar cells

Reported is the characterization of irradiated InGaP2/GaAs/Ge multijunction (MJ) solar cells using the cathodoluminescence (CL) imaging/spectroscopy and electron beam induced current (EBIC) modes of scanning electron microscopy (SEM). These techniques were applied to verify the influence of radiation damage on the optoelectronic properties of each subcell in the monolithic triple junction structure and correlate them with the illuminated (AM0, 1 sun, 25°C) current-voltage (IV) and quantum efficiency (QE) characteristics.

Towards high efficiency multi-junction solar cells grown on InP Substrates

Progress toward the development of multi-junction solar cells grown on InP substrates is presented. In this material system, the optimal bandgaps for solar energy conversion are attained while the multi-junction structure is realized under lattice matched conditions. In this work, results for the characterization of material and devices of the individual sub cells are shown. For the top cell, InAlAsSb quaternary material is being developed. For the middle, InGaAsP and InGaAlAs are studied, and for the bottom, InGaAs will provide the possibility of adding multiple quantum wells for fine bandgap tunability. In addition, we will discuss electrical characterization of the tunnel diodes.

Analysis of radiation hardness and subcell I-V characteristics of GaInP/GaAs/Ge solar cells using electroluminescence measurements

The voltage degradation of GaInP/GaAs/Ge triple-junction solar cells after exposure to proton irradiation is analyzed using electroluminescence (EL) measurements. It is shown that EL measurements in combination with the reciprocity relationship allow accurate determination of the degradation of the open-circuit voltage (Voc) of each individual subcell. The impact of different proton energies on the voltage degradation of each subcell is analyzed. For solar cells exposed to extremely high radiation levels, a correlation between the degradation of the quantum efficiency of the Ge subcell and its EL properties is presented.

Development of InGaAs solar cells for >44% efficient transfer-printed multi-junctions

Transfer-printing is a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked II-IV solar cells with low cost. In this work, we describe the development of InGaAs solar cells, designed to harvest long wavelength photons when stacked in tandem with a high efficiency InGaP/GaAs/InGaAsNSb triple junction solar cell. High performance InGaAs solar cells, grown on InP by MOCVD, were achieved through a combination of detailed modeling, material development and device characterization. The transfer printing apparatus of Semprius Inc. was used to create a four-terminal device with an uncertified conversion efficiency of 44.1% at 690 suns.

Detailed Characterization of the Radiation Response of Multijunction Solar Cells Using Electroluminescence Measurements

The response of triple-junction solar cells to proton and electron irradiation is analyzed using electroluminescence (EL) measurements. This analysis allows the dark current of each individual subjunction to be determined providing insight into the radiation response mechanisms.