Silicon solar cells benefit from an internal Lambertian light distribution achieved through texturing, while the performance of direct-bandgap materials can be lower with an internal Lambertian light distribution than the light distribution of a planar cell. A novel analytic expression is derived for the emittance of cells with a Lambertian light distribution and partial rear reflectance. This expression enables comparison of Si, GaAs, CdTe, and CIS cells under planar and Lambertian light distributions with varying rear reflectance in the Auger limit. A Lambertian light distribution is shown to be particularly beneficial to thinner material with higher rear reflectance due to absorptance enhancement. It is found that a Lambertian light distribution increases radiative recombination in most absorbers but can reduce radiative recombination in some CIS material.
Improvements to solar cell efficiency and radiation hardness that are compatible with low cost, high volume manufacturing processes are critical for power generation applications in future long-term NASA and DOD space missions. In this paper, we provide the results of numerical simulation of the radiation effects in a novel, ultra-thin (UT), Si photovoltaic cell technology that combines enhanced light trapping (LT) and absorption due to nanostructured surfaces, separation of photogenerated carriers by carrier selective contacts (CSC), and increased carrier density due to multiple exciton generation (MEG). Such solar cells have a potential to achieve high conversion efficiencies while shown to be rad-hard, lightweight, flexible, and low–cost, due to the use of Si high volume techniques.
The GaSb-based family of materials and heterostructures provides rich bandgap engineering possibilities for a variety of infrared (IR) applications. Mid-wave and long-wave IR photodetectors are progressing toward commercial manufacturing applications, but to succeed they must move from research laboratory settings to general semiconductor production and they require larger diameter substrates than the current standard 2-inch and 3-inch GaSb. Substrate vendors are beginning production of 4-inch GaSb, but another alternative is growth on 6-inch GaAs substrates with appropriate metamorphic buffer layers. We have grown generic MWIR nBn photodetectors on large diameter, 6-inch GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures, including bulk GaSb nucleation, AlAsSb superlattices, and graded GaAsSb and InAlSb ternary alloys, were employed to bridge the 7.8% mismatch gap from the GaAs substrates to the GaSb-based epilayers at 6.1 Å lattice-constant and beyond. Reaching ~6.2 Å extends the nBn cutoff wavelength from 4.2 to <5 µm, thus broadening the application space. The metamorphic nBn epiwafers demonstrated unique surface morphologies and crystal properties, as revealed by AFM, high-resolution XRD, and cross-section TEM. GaSb nucleation resulted in island-like surface morphology while graded ternary buffers resulted in cross-hatched surface morphology, with low root-mean-square roughness values of ~10 Å obtained. XRD determined dislocation densities as low as 2 × 107 cm-2. Device mesas were fabricated and dark currents of 1 × 10-6 A/cm2 at 150K were measured. This work demonstrates a promising path to satisfy the increasing demand for even larger area focal plane array detectors in a commercial production environment.
A novel device concept utilizing the approach of selectively extracting carriers at the respective contacts is outlined in the work. The dominant silicon solar cell technology is based on a diffused, top-contacted p-n junction on a relatively thick silicon wafer for both commercial and laboratory solar cells. The VOC and hence the efficiency of a diffused p-n junction solar cell is limited by the emitter recombination current and a value of 720 mV is considered to be the upper limit. The value is more than 100 mV smaller than the thermodynamic limit of VOC as applicable for silicon based solar cells. Also, in diffused junction the use of thin wafers (< 50 um) are problematic because of the requirement of high temperature processing steps. But a number of roadmaps have identified solar cells manufactured on thinner silicon wafers to achieve lower cost and higher efficiency. The carrier selective contact device provides a novel alternative to diffused p-n junction solar cells by eliminating the need for complementary doping to form the emitter and hence it allows the solar cells to achieve a VOC of greater than 720 mV. Also, the complete device structure can be fabricated with low temperature thin film deposition or organic coating on silicon substrates and thus epitaxially grown silicon or kerfless silicon, in addition to standard silicon wafers can be utilized.
We present a hybrid thermodynamic model for multijunction solar cells with intermediate bands that demonstrates
possible improvements to conventional multijunction photovoltaic systems. Applying this model to selected tandem cell
structures shows that the performance of such hybrid solar cells is enhanced and that multiple transitions from
intermediate bands can reduce the number of material stacks and boost overall efficiency. We demonstrate the results of
detailed simulations for multiple numbers of stacks of hybrid multijunction solar cells. And, we can choose proper
materials to compose intermediate band for each junction. Furthermore, we suggest other alternative hybrid solar cell
systems to absorb moderate photon energy range and find appropriate materials for hybrid solar cells.
QD size, uniformity and density in InAs/GaAsSb material system for increasing Sb content are studied using Atomic
Force Microscopy (AFM). AFM results show that QD density and uniformity improve with Sb content increase. The
improvement of QD uniformity is ensured by the narrowing of the analysis of AFM scans. To obtain minimum VBO,
InAs/GaAsSb with various Sb compositions is investigated by PL and TRPL measurements. PL data shows a blue-shift
as excitation power increases as evidence of a type II band structure. Since the PL peak of 8 and 13 % Sb samples did
not shift while that of 15 % Sb sample is blue-shifted with increasing the excitation power it is concluded that InAs
QDs/GaAs0.86Sb0.14 would have minimum valence band offset. This tendency is supported by the change of a carrier
lifetime estimated from TRPL data
Third generation concepts in photovoltaic devices depend critically on the dynamics of ultrafast carrier relaxation and
electron-phonon interactions on very short times scales in nanostructures such as quantum wells, wires and dots. Hot
carrier solar cells in particular depend on the reduction in the energy relaxation rate in an absorber material, where hot
carriers are extracted through energy selective contacts. Here we investigate the short time carrier relaxation in quantum
well, hot electron solar cells under varying photoexcitation conditions using ensemble Monte Carlo (EMC) simulation
coupled with rate equation models, to understand the limiting factors affecting cell performance. In particular, we focus
on the potential role of hot phonons in reducing the energy loss rate in order to achieve sufficient carrier temperature for
In this work InGa0.85N p-n homojunction solar cells were grown by MOCVD on GaN/sapphire substrates and fabricated
using standard techniques. When illuminated from the backside, these devices showed 65.9% improvement in JSC and
4.4% improvement in VOC as compared to identical illumination from the front. These improvements arise from removal
of the losses from electrical contact shading on the front of the devices (11.7% of active area), as well as significant
optical absorption by the top current spreading layer. These improvements can likely be further enhanced by utilizing
double-side polished wafers, which would eliminate scattering losses on the back surface. In addition to improving
electrical characteristics of single cells, backside illumination is necessary for the realization of monolithic tandem
InGaN solar cells.
The Defense Advanced Research Projects Agency has initiated the Very High Efficiency Solar Cell (VHESC) program to address the critical need of the soldier for power in the field. Very High Efficiency Solar Cells for portable applications that operate at greater than 55 percent efficiency in the laboratory and 50 percent in production are being developed. We are integrating the optical design with the solar cell design, and have entered previously unoccupied design space that leads to a new architecture paradigm. An integrated team effort is now underway that requires us to invent, develop and transfer to production these new solar cells. Our approach is driven by proven quantitative models for the solar cell design, the optical design and the integration of these designs. We start with a very high performance crystalline silicon solar cell platform. Examples will be presented. Initial solar cell device results are shown for devices fabricated in geometries designed for this VHESC Program.