Alex Freundlich

Material Challenges for Terawatt Level Deployment of Photovoltaics

In this work several photovoltaic technologies, ranging from silicon to thin films, multi-junction and solar concentrator systems are analyzed. The estimates of the energy production limits are established for each technology, based on available global material reserves. It is shown that many existing technologies, albeit playing an important in the present sub-GW energy production levels, are affected by severe material shortages, preventing their scale-up to the terawatt range. This is the case for thin film solar cells technologies based on CdTe and CIGS where the showstopper is the scarcity of tellurium and indium respectively. Despite the abundance of silicon, crystalline Si solar cells will hardly surpass the few terawatt range as further scale-up will be impeded by the global reserves of silver, commonly used as electrode material. For amorphous silicon and dye sensitized thin film technologies, avoiding the use of ITO transparent conductive oxides (ZnO for a-Si and SnO2 for dye sensitized cells) should favor access to terawatt levels. For existing III-V concentrator cells, operating under moderate concentration (<200X), one to two terawatt year level can be afforded by simply substituting Ge substrates by more abundant GaAs wafer. The development of reliable epitaxial solar cell lift-off techniques should enable the attainment of multi-Terawatt levels

MBE growth of sharp interfaces in dilute-nitride quantum wells with improved nitrogen-plasma design

Analysis of structural and luminescence properties of GaAsN epilayers grown by molecular beam epitaxy (MBE) and chemical beam epitaxy on GaAs (001) substrates indicates the possibility of fabricating high nitrogen content (x > 0.03) alloys. The conventional plasma source design where nitrogen flux is controlled using a manual shutter was first implemented. Investigation of structural and optical properties by photoluminescence, high-resolution x-ray diffraction, secondary-ion mass spectrometry, and electron microscopy indicated the presence of thin parasitic layers formed during nitrogen plasma ignition, as well as significant N contamination of GaAs barrier layers, which could severely affect carrier extraction and transport properties in targeted devices. In order to overcome these limitations, a gate-valve-activated run-vent design was implemented that allowed the plasma to operate continuously during MBE growth, while N plasma flux changes during growth were monitored. The potential of this design for...

Dilute nitride multi-quantum well multi-junction design: a route to ultra-efficient photovoltaic devices

The current high-efficiency triple junction (Al)InGaP (1.9eV)/GaAs(1.42eV)/ Ge(0.66eV) design for a solar cell can be improved upon by the use dilute nitrides to include a sub-cell in the 1eV range. Addition of a small percentage of nitrogen to III-V semiconductor alloys (such as GaAsN) enables us to achieve the required bandgap, however these bulk dilute nitride structures suffer from a reduced minority carrier lifetime, decreasing the overall current output. The route suggested herein is to include dilute nitride multi-quantum wells (with thicknesses much lesser than the minority carrier diffusion length) within the intrinsic region of a GaAs subcell. Modeling has been done for this structure to obtain the confined energies of the electrons and holes, as well as the absorption coefficient and thereby the spectral response of the 4-junction cell. The results show that it is possible to achieve with the appropriate current matching, a conversion efficiency of ~40% under AM0 (1 sun) with up to ~18 mAcm-2 short circuit current.

Silicon PV cell production on the Moon as the basis for a new architecture for space exploration

A method is described by which silicon photovoltaic (PV) devices can be directly deposited onto the lunar regolith using primarily lunar materials. In sequence, a robotic “crawler” moving at slow speed sequentially melts the top layer of regolith and deposits a conducting layer, a doped silicon, a top conducting grid, and an antireflective coating by vacuum evaporation techniques. Concentrated solar energy is utilized as the energy source. Development of this capability would significantly lower the cost of electrical energy on the Moon and would enable a range of other activities, including lower cost propellant production, human outposts with complete food-growth capabilities, and advanced materials production. Low cost energy could affect the economics of propellants in space by allowing the extraction of solar wind hydrogen from the lunar regolith. This would allow the economical export of propellants and other materials to space, first to an Earth-Moon Lagrangian Point and potentially to low Earth orbit.

Investigation of dilute-nitride alloys of GaAsNx (0.01 < x < 0.04) grown by MBE on GaAs (001) substrates for photovoltaic solar cell devices

Dilute-nitride GaAsNx epilayers were grown on GaAs (001) substrates at temperatures of ∼450 °C using a radio-frequency plasma-assisted molecular/chemical beam exitaxy system. The concentration of nitrogen incorporated into the films was varied in the range between 0.01 and 0.04. High-resolution electron microscopy was used to determine the cross-sectional morphology of the epilayers, and Z-contrast imaging showed that the incorporated nitrogen was primarily interstitial. {110}-oriented microcracks, which resulted in strain relaxation, were observed in the sample with the highest N concentration ([N] ∼ 3.7%). Additionally, Z-contrast imaging indicated the formation of a thin, high-N quantum-well-like layer associated with initial ignition of the N-plasma. Significant N contamination of the GaAs barrier layers was observed in all samples, and could severely affect the carrier extraction and transport properties in future targeted devices. Dilute-nitride quantum-well-based photovoltaic solar cells were fabri...

Modeling of defect tolerance of IMM multijunction photovoltaics for space application

Reduction of defects by use of thick sophisticated graded metamorphic buffers in inverted metamorphic solar cells has been a requirement to obtain high efficiency devices. With increase in number of metamorphic junctions to obtain higher efficiencies, these graded buffers constitute a significant part of growth time and cost for manufacturer of the solar cells. Its been shown that ultrathin 3 and 4 junction IMM devices perform better in presence of dislocations or/and radiation harsh environment compared to conventional thick IMM devices. Thickness optimization of the device would result in better defect and radiation tolerant behavior of 0.7ev and 1.0ev InGaAs sub-cells which would in turn require thinner buffers with higher efficiencies, hence reducing the total device thickness. It is also shown that for 3 and 4 junc. IMM, with an equivalent 1015 cm-2 1 MeV electron fluence radiation, very high EOL efficiencies can be afforded with substantially higher dislocation densities (<2×107 cm-2) than those commonly perceived as acceptable for IMM devices with remaining power factor as high as 0.85. The irregular radiation degradation behavior in 4-junc IMM is also explained by back photon reflection from gold contacts and reduced by using thickness optimization of 0.7ev and 1.0ev InGaAs sub-cells.

Superlattice intermediate band solar cell with resonant upper-conduction-band assisted photo-absorption and carrier extraction

In this work we propose and theoretically evaluate a superlattice intermediate band solar cell design, wherein a superlattice comprising lattice matched layers of electronically mismatched alloys and thin barriers are inserted within the intrinsic region of a wide bandgap p-i-n diode. The shallow valence band offsets in the design favor the minority hole extraction, and the intermediate levels is build through superlattice minibands formed by coupling lower band gap wells (lower E-conduction branch of mismatched alloys like GaAsN) and higher bandgap Kane-like semiconductors. In the proposed design the upper conduction band E+ of the mismatched alloys is maintained in resonance with the barrier and bandgap of host material to promote an efficient extraction of electrons and preserve the 3D nature of the upper band, thus favoring a strong intermediate to band second photon absorption. In this design carriers can be promoted either directly to the conduction band or via the intermediate band, permitting the absorption of low energy photons whilst maintaining a high cell voltage. Also the characteristic lengths of the wells are substantially smaller than typical diffusion lengths of the electronically mismatched alloy which should overcome the minority carrier losses observed in bulk like devices fabricated with these alloys. To attain the necessary combination of high and low bandgaps and low dislocation density, we use materials that are lightly strained or lattice-matched to an GaAs (or Ge) substrate. GaAsN (Sb) quantum well layers are incorporated into direct bandgap low Al content (x<;30%)) AlGaAs host material, to attain high bandgaps of 1.7-1.9 eV, and low-energy bandgaps of 1.1-1.3 eV. A preliminary detailed balance evaluation of the proposed device that incorporates calculation of the absorption properties of the SL region and the host AlGaAs crystal suggest potential for exceeding 1sun and 1000 sun efficiencies of 39% and 55% respectively.

Thin film growth in the ultra‐vacuum of space: The second flight of the Wake Shield Facility

The Wake Shield Facility (WSF) created to characterize and utilize the ultra‐vacuum of space has flown its second mission on Endeavor, STS‐69 in September, 1995. In its second flight the WSF flew free behind the Orbiter at a nominal distance of 25 nmi, grew four semiconductor thin film samples by molecular beam epitaxy, and one oxide thin film sample through the use of LEO atomic oxygen. The semiconductor thin films were in the III‐V semiconductor area with focus on AlGaAs/GaAs system. The WSF also generated an ultra‐vacuum consistent with past predictions of space wake vacuum levels. In addition, eleven cooperative experiments were flow on the WSF yielding additional information on the low earth orbit space environment.

III‐V compound semiconductor film growth in low earth orbit on the wake shield facility

Gallium Arsenide (GaAs) films, both silicon doped and undoped, have been deposited by Molecular Beam Epitaxy (MBE) in Low Earth Orbit (LEO) in an ultra vacuum environment created by the Wake Shield Facility (WSF). The WSF is a 12 foot diameter stainless steel disk that sweeps out a volume of space thus creating an ultra vacuum in its wake. It was developed specifically to take advantage of the ultra vacuum for the deposition of thin film materials. The WSF was flown for the first time on STS‐60 in February, 1993. The mission objectives were to measure the unique wake vacuum environment and to epitaxially deposit GaAs thin films. In this paper, we describe the films deposited and report on the characterization performed to date. Films were deposited in two basic structures. The first structure consisted of undoped GaAs films of thicknesses ranging from 2 to 4 μm with a thin (≊200 nm) highly silicon doped layer (n≊5×1017/cc) on top. This is basically a metal‐semiconductor field effect transistor (MESFET) st...

Dilute Nitride III-V Superlattices Lattice-Matched to Silicon

Dilute nitrides quaternary alloys of GaP1-x-yAsy Nx have attracted much attention since they exhibit a direct bandgap and their lattice constant matches the one of silicon for y=4.7x-0.1. These alloys offer interesting perspectives for the monolithic integration of III-V photovoltaics with the silicon technology. However, for practically achievable nitrogen composition (xles0.04), the band gap of GaP1-x-yAsyNx alloys lattice matched to Si is limited to about 1.75 to 1.8 eV. In an attempt to expand the operation wavelength of these dilute nitrides further toward the infrared a short period GaP1-xN x/GaAs1-yNy superlattice strain-balanced and lattice matched to silicon is devised. An eight-band Kane Hamiltonian modified to account for the strain effect and the band anti-crossing model is used to describe the electronic states of the highly strained GaP1-xNx and GaAs1-yN y ternaries. A transfer matrix method is applied to determine the electron and hole minibands of the superlattice structure, and the evolution of the band edge transition energies for different nitrogen compositions and alloying/thickness combinations. It is shown that lattice matched superlattice design, proposed here, allows for an additional bandgap shrinkage of 50-150 meV compared to quaternary alloys of similar average N concentration. The approach thus expands the direct bandgap photovoltaic material palette and offers new opportunities for two and three-bandgap lattice matched tandems operating in conjunction with Si bottom cells