H.L. Cotal

Radiation response of heteroepitaxial n+p InP/Si solar cells

The effect of 1 MeV electron and 3 MeV proton irradiation on the performance of n+p InP solar cells grown heteroepitaxially on Si (InP/Si) substrates is presented. The radiation response of the cells was characterized by a comprehensive series of measurements of current versus voltage (I–V), capacitance versus voltage (C–V), quantum efficiency (QE), and deep level transient spectroscopy (DLTS). The degradation of the photovoltaic response of the cells, measured under simulated 1 sun, AM0 solar illumination, is analyzed in terms of displacement damage dose (Dd) which enables a characteristic degradation curve to be determined. This curve is used to accurately predict measured cell degradation under proton irradiation with energies from 4.5 down to 1 MeV. From the QE measurements, the base minority carrier diffusion length is determined as a function of particle fluence, and a diffusion length damage coefficient is calculated. From the C–V measurements, the radiation-induced carrier removal rate in the base...

Electron and proton irradiation-induced degradation of epitaxial InP solar cells

Abstract The degradation of epitaxial, shallow homojunction n+p InP solar cells under 1 MeV electron and 3 MeV proton irradiation is presented. The data measured under 3 MeV proton irradiation are analyzed in terms of displacement damage dose which is the product of the particle fluence and the calculated non-ionizing energy loss (NIEL)[1]. A characteristic proton degradation curve is derived from which the cell degradation under any energy proton irradiation can be calculated. The data measured under 1 MeV electron irradiation is also analyzed in terms of displacement damage dose. The electron irradiation-induced degradation is correlated with the proton degradation curve by determining electron to proton dose ratios for each of the photovoltaic (PV) parameters. A comparison of the characteristic degradation curves for InP and GaAs/Ge solar cells, which was determined previously, shows InP to be intrinsically more resistant to displacement energy deposition. The base carrier concentration was measured during the irradiations, and significant carrier removal was observed. When analyzed as a function of displacement damage dose, the reduction in carrier concentration under both the 1 MeV electron and the 3 MeV proton irradiation is shown to follow the same degradation curve. From this common degradation curve, a characteristic carrier removal rate is calculated for InP under any irradiation. The junction dark current was also measured during both irradiations, and the data were fit to a three-term diode dark current equation. From the fits, the diffusion current is determined as a function of particle fluence. Changes in the diffusion current under electron and proton irradiation are shown to correlate in terms of displacement damage dose in the same way as the cell maximum power. The junction recombination current is also determined from the dark current data, and the results show the energy level of the dominant radiation-induced recombination center to be approximately the same in both the electron and proton irradiated samples. In addition, the dark current analysis indicates that the relative changes in the hole and electron lifetimes are essentially the same under both the electron and the proton irradiations. Based on these results and the overall correlation between the electron and proton damage, a detailed description of the mechanism of the radiation response of InP is developed which describes the cell degradation under any particle irradiation.

Annealing of irradiated epitaxial InP solar cells

The annealing behavior of electron, proton, and alpha particle irradiated, epitaxial n+p InP solar cells has been characterized using several techniques. Current–voltage measurements were made under simulated 1 sun, AM0 solar illumination and in the dark. The radiation‐induced defect spectra were monitored using deep level transient spectroscopy and the base carrier concentration profiles were determined through capacitance–voltage measurements. The irradiated cells were annealed at temperatures ranging from 300 up to 500 K. Some cells were annealed while under illumination at short circuit while others were annealed in the dark. These experiments produced essentially the same results independent of illumination and independent of the irradiating particle. An annealing stage was observed between 400 and 500 K, in which the radiation‐induced defects labeled H3 and H4 were removed and the carrier concentration recovered slightly. Concurrently there was a small reduction in the junction recombination current...

Spectral response of electron-irradiated homoepitaxial InP solar cells

The Naval Research Laboratory has presented numerous reports on the radiation hardening of the two-terminal InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells for space use. To date, most work has been done on n/sup +/p devices. However, in order to facilitate the heteroepitaxial growth of these tandems on substrates such as Si or Ge, the polarity of the subcells may need to be reversed. With this in mind, spectral response measurements on irradiated homoepitaxial p/sup +/n InP solar cells were carried out for several fluences of 1 MeV electrons. The quantum efficiency data degraded at high wavelengths but no significant degradation was observed at low wavelengths. An important material parameter in solar cell devices is the minority carrier diffusion length (L) for which L/sub P/ was measured in the present study. This parameter was measured as a function of electron irradiation using the steady-state short-circuit current method. The degradation of L/sub P/ was plotted as a function of electron fluence and the data were fit to the standard semi-empirical model which relates /spl Delta/L to the damage constant, K/sub L/. The fit yielded values for K/sub L/ and the unirradiated minority carrier diffusion length, L/sub 0/.

Electron irradiation of two‐terminal, monolithic InP/Ga0.47In0.53As tandem solar cells

Results are presented for 1 MeV electron‐irradiated, two terminal, monolithic InP/Ga0.47In0.53As tandem solar cells. These highly efficient prototype cells show radiation resistance that is comparable to single junction InP cells. A current mismatch between the subcells does not occur until high fluence levels, that is, near 3×1015 e−/cm2. This value for the onset of current mismatch and the measured remaining absolute efficiency of 9.4% at 1×1016 e−/cm2 are excellent results reported for a tandem cell designed for space applications.

Performance of proton- and electron-irradiated, two-terminal, monolithic InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells

Radiation damage results are presented for two-terminal InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells irradiated with 1 MeV electrons and 3 MeV protons. The performance of the irradiated tandem cells are compared with each other which are, subsequently, described in terms of the component cells. The current mismatch appearing in the tandems are almost nonexistent compared to previous electron-irradiated cells. This is explained in terms of the thickness of the InP top subcell for a top cell-limited tandem. At high fluences, the proton-irradiated cell showed very good radiation resistance compared to a shallow homojunction InP cell. The increase in the recombination current appears to be the major degradation mechanism as revealed by the degradation of the open circuit voltage for the proton-irradiated cells. Annealing of the tandems showed partial recovery for the PV parameters in general but the short circuit current and the fill factor decrease at elevated temperatures for the cell irradiated with protons.

Development of p/n and n/p thick emitter InP solar cells

Both n/p and p/n InP thick emitter (0.3 /spl mu/m) space solar cells with and without Ga/sub 0.5/In/sub 0.5/P windows are studied. Both polarity cells are considered for possible growth on Ge. While not achieving the high efficiencies of thin emitter InP cells, the thicker emitter cells may provide an advantage in radiation hardness. The Ga/sub 0.5/In/sub 0.5/P window has little effect on cell performance, for either polarity.

Radiation effects of two-terminal, monolithic InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells

Two-terminal, monolithic InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells grown by the atmospheric-pressure metalorganic vapor phase epitaxy (APMOVPE) technique have been studied under successive fluences of 1 MeV electrons. Illuminated I-V results obtained at 1 sun, AM0 (25/spl deg/C) are described. Besides the high performance of these novel devices under particle irradiation, the current mismatch between the top and bottom subcells began to be noticeable at a fluence of 3/spl times/10/sup 15/ cm compared to 3/spl times/10/sup 14/ cm/sup -2/ as demonstrated in previous cells. This is a dramatic improvement in the radiation resistance. The cells exhibited beginning-of-life (BOL) efficiencies as high as 22%, and it is suggested that additional slight improvement in the radiation tolerance of the tandem cells can be achieved by further adjusting the device structure.<<ETX>>

Radiation degradation in In/sub 0.53/Ga/sub 0.47/As solar cells

This paper presents the radiation results on p/n In/sub 0.53/Ga0/sub 0.47/As bottom cells. The main conclusion is that In/sub 0.53/Ga0/sub 0.47/As solar cells display the same radiation tolerance, regardless of the cell polarity. This result is, in general, not expected. Previous results involving damage coefficients for both Si, GaAs, and InP show that the polarity of the device does indeed matter. This paper therefore gives device fabricators the liberty of choosing either type of polarity depending on their needs.<<ETX>>