Final Report: Two Dimensional Computer Simulation of Bilateral Silicon Solar Cells

Bilateral solar cells can convert albedo light (sunlight reflected from the earth) incident on the back side of the cell to improve the power to weight ratio of satellite arrays operating in Low Earth Orbits. However, the high energy radiation trapped in the Van Allen Belt surrounding the earth limits the possible improvement of solar cell electrical output by degrading the minority carrier diffusion length. The purpose of this work is to design cells to be able to collect efficiently albedo-generated carriers at end-of-life(EOL). The FORTRAN program Solar Cell Analysis Program in Two Dimensions is used to model four cell geometries for base resistivities of 1.0 to 1240. Ω—cm. The EOL efficiencies and normalized output power are compared for all cells. All the thicker (250. micron) cells modeled peak in performance within the 10.-40. Ω—cm base resistivity range both with and without albedo illumination. It is found that alternative geometries to the standard solar cell can be used to better collect albedo-generated carriers at EOL. The etched multiple vertical junction cell(22%) and the 50. micron thick standard cell(45%) show the most improvement in normalized output power over the best one- sun illuminated standard cell. Albedo light is modeled as 40. milliwatts /cm2 (AMI.5 spectrum), or 30% of one sun AM0.0 incident power. Values for the damage coefficient, Kj, are found in the literature for irradiation by 1.0 MeV electrons. Radiation induced degradation is modeled by SCAP2D through degradation of the minority carrier lifetimes. Solar cell output parameters are compared for four cells, the standard cell (for varying thicknesses), the etched multiple vertical junction cell, and the tandem junction cell. The physical phenomena responsible for poor cell performance at EOL are discussed

Concentration‐dependent optical‐absorption coefficient in n‐type GaAs

The doping-dependent, near-band-edge optical-absorption coefficient CY(h v) was deduced from optical transmission measurements in n-type GaAs thin films. The selenium-doped films were grown by metalorganic chemical-vapor deposition and do ed to produce room-temperature electron concentrations from 1.3 x 10” to 3.8X 1018 cm- P . The transmission measurements covered photon energies between 1.35 and 1.7 eV and were performed on double heterostructures with the substrate removed by selective etching. The results show good qualitative agreement with previous studies and good quantitative agreement, except for the heavily doped samples. For na=3.8 X 10” cme3, a( 1.42 eV\u3e is approximately four times that reported by previous workers. Secondary-ion-mass spectrometry measurements on flms grown under differing conditions demonstrate that a(hv) is sensitive to electrically inactive dopants and supports the hypothesis that precipitates or compensation influenced previous measurements. These comprehensive results on high-quality, uncompensated material should prove useful for fundamental studies of optical transitions in n-type GaAs as well as for modeling optoelectronic devices

Microsecond Lifetimes and Low Interface Recombination Velocities in Moderately Doped n-GaAs Thin Films

We have observed lifetimes greater than 1 ps in moderately doped, thin film, n-GaAs/A1a,Gae,As double heterostructure membranes formed by etching away the substrate. We attribute these ultralong lifetimes to enhanced photon recycling caused by the removal of the substrate. Nonradiative recombination in the bulk and at the interfaces is very low; the upper limit of the interface recombination velocity is 25 cm/S.-Such long lifetimes in GaAs doped at N,= 1.3 X 10” cme3 suggest that thin-film solar cells offer a potential option for achieving very high efficiencies

A study of minority carrier lifetime versus doping concentration in n‐type GaAs grown by metalorganic chemical vapor deposition

Time‐resolved photoluminescence decay measurements are used to explore minority carrier recombination in n‐type GaAs grown by metalorganic chemical vapor deposition, and doped with selenium to produce electron concentrations from 1.3×1017 cm−3 to 3.8×1018 cm−3. For electron densities n0\u3c1018 cm−3, the lifetime is found to be controlled by radiative recombination and photon recycling with no evidence of Shockley–Read–Hall recombination. For higher electron densities, samples show evidence of Shockley–Read–Hall recombination as reflected in the intensity dependence of the photoluminescence decay. Still, we find that radiative recombination and photon recycling are important for all electron concentrations studied, and no evidence for Auger recombination was observed

Recombination and absorption in n-type gallium arsenide

This work recounts the efforts to completely characterize recombination, absorption and luminescence in n-type GaAs grown by metal-organic chemical vapor deposition (MOCVD) and doped with selenium to create electron concentrations, n\sb{\rm o}, from 1.3 $\times$ 10\sp{17} cm\sp{-3} to 3.8 $\times$ 10\sp{18} cm\sp{-3}. To investigate recombination, photoluminescence (PL) decay was observed on double heterostructures (DH\u27s) with thicknesses ranging from 0.25 to 10 $\mu$m. For n\sb{\rm o} $\u3c$ 10\sp{18} cm\sp{-3}, recombination is dominated by radiative transitions with no evidence of Shockley-Read-Hall (SRH) recombination. For higher electron concentrations, samples show evidence of SRH recombination as reflected in the intensity dependence of PL decay. Even so, radiative recombination and photon recycling are important for all n\sb{\rm o} studied. No evidence for Auger recombination is found, but an upper limit on the Auger coefficient is determined. Transmission measurements on thin-film DH\u27s were used to deduce the near band edge absorption coefficient, $\alpha$(h$\nu$), over the photon energy range of 1.35 $\le$ h nu $\le$ 1.7 eV. A strong dependence of $\alpha$(h$\nu$) on the electron concentration was found as expected, based on previous work. Differences in the absorption tail, which increase with increasing n\sb{\rm o}, were attributed to compensation in the material studied previously. PL decay on thin-film membranes (for which the substrate is removed) yielded lifetimes greater than 1 $\mu$s for n\sb{\rm o} = 1.3 $\times$ 10\sp{17} cm\sp{-3}. This lifetime was enhanced three times over that observed with the substrate intact, because of enhanced photon recycling. By suppressing bulk recombination in this manner, it is found that the AlGaAs/GaAs interface recombination velocity, S, has an absolute upper limit of 25 cm/s. A more reasonable value is estimated as S $\le$ 11 cm/s. The absorption and recombination measurements are used to compute B, the radiative recombination coefficient, with self-consistent results. It is found that B decreases significantly with increasing n\sb{\rm o}. This is qualitatively in agreement with previous work, but the results differ quantitatively. This work describes completely the near band edge absorption properties and the recombination characteristics of high-quality n-GaAs grown by MOCVD. The results fill a void of understanding of intrinsic recombination in moderately doped n-GaAs and should be of value for the characterization and modeling of GaAs devices