Richard L. Statler

Review of Radiation Damage to Silicon Solar Cells

This paper reviews a large number of silicon solar cell irradiation experiments performed over the last 10 years, including 1-MeV and energy spectrum electron studies, and low-(100-keV) and high-energy (up to 155-MeV) proton studies on bare and covered silicon solar cells of several types. The results of satellite flight experiments on individual solar cells are also presented, as well as data from complete solar arrays and data on the new high-efficiency solar cells. Experimental evidence indicates that the percentage of degradation is smaller in thin solar cells than in thick ones, and that cells with high resistivity (10 ¿·cm) degrade less than cells with lower resistivity (1 ¿·cm). It is shown that high-efficiency silicon solar cells produced at COMSAT Laboratories and pilot production groups of these cells retain most of their increased power output under irradiation. It is emphasized that all surfaces and edges of the solar cells must be completely shielded from the large flux protons in the space environment. Insufficiencies in the published data are noted in certain areas, and recommendations for additional research are presented. Finally, an extensive bibliography is included.

Radiation damage in silicon solar cells from low-energy protons

Recent evidence has indicated that kilovolt-energy protons are a probable cause for solar-cell array degradation in synchronous-orbit satellites. This damage can occur in the small areas of the cell that have been unprotected by the coverslip. This paper reports on solar-cell current degradation at fixed voltages in nominal 10-Ω.cm silicon solar cells with coverslips which are irradiated by 150- and 270-keV protons. The damage, which is shown to be dependent on exposed area and proton energy, can be minimized by putting adhesive on uncovered cell areas or by reducing the open area with close-fitting coverslips.

A satellite experiment to study the effects of space radiation on solar cell power generation

Abstract To study the effects of the space radiation environment on state-of-the-art solar cells, a space solar cell experiment was conducted on board a Navy satellite. There were 15 solar cell experiments on board the NTS-2 satellite launched on June 23, 1977 into a circular orbit of 12 h, 20 192 km high at an inclination of 63°. Each experiment was a separate array of five 2 cm × 2 cm solar cells. They included GaAlAs/GaAs heteroface structure cells, COMSAT high-efficiency black cells, textured cells, vertical junction cells, adhesiveless coverslides and coverslides without antireflective coatings and/or UV reflection filters. The current-voltage characteristics of the arrays were measured sequentially and telemetered as the satellite passed over the ground station at Blossom Point, MD. The average value of measurements in space on the first day of exposure were in good agreement with pre-flight solar simulator values.

Defect Clusters in Electron-Irradiated Silicon

Calculations of the formation of disordered regions in silicon due to irradiation by high energy (15-45 MeV) electrons indicate that a sufficient concentration of defect clusters is produced to affect the electrical properties of the material. Isochronal annealing of room temperature radiation-induced degradation in the short-circuit current of silicon solar cells and in the minority carrier lifetime of the p-type base region is studied up to 500°C. The existence of a low temperature (50-200°C) annealing stage is shown to be independent of dopant and oxygen impurity concentration. It is inferred that this stage, which is similar to those observed in fast neutron- and in proton-irradiated silicon, is characteristic of cluster formation.

Radiation effects in silicon solar cells

Abstract Silicon solar cells have been utilized as the principal source of electrical energy for space satellites during the past decade. Despite the reliability of these photovoltaic devices, degradation of their power output by charged particle radiation in the earths geomagnetic field has continued to be the primary problem for their use on flights of long-duration. A study of radiation damage induced by 1 MeV electrons in a variety of current silicon solar cell types has been conducted as a function of dopant impurity and resistivity of the base region. A companion study of radiation damage induced by nominal 0.2 MeV protons was performed in solar cells with coverslips having small cell areas exposed alongside the coverslip. The photovoltaic current-voltage characteristics were measured under a solar simulator emitting 140mW/cm2 at air mass zero. Irradiations were performed at room temperature to fluences of 1 × 1015 e/cm2 and 1 × 1015 p/cm2. The efficiency of 10 ohm-cm cells after large fluences was...

An evaluation by solar simulation of radiation damage in silicon solar cells

From the viewpoint of the space power-systems designer, the most useful data for radiation-damaged solar cells is that of output power as a function of cell voltage, temperature, and radiation. This paper reviews the available results from laboratory radiation experiments where solar simulators were used. The solar cells studied were 1 and 10 ohm-cm n-on-p boron-doped cells, 5 and 10 ohm--cm aluminum-doped cells, and dendritic drift-field cells. Most of the experiments use 1 MeV electrons with some data for 0.5 to 2 MeV electrons and 0.5 to 2.7 MeV protons. Comparisons are made between types of cells on the basis of maximum power output and power at a fixed voltage. A fixed voltage is determined for each cell type using the value of cell voltage at maximum power after a 1 MeV electron fluence of 1016e/cm2. There is an apparent lack of agreement among experimental results in the order of 3 or 4 percent, due to spectral variations between simulators. Another reason for the spread in data is attributed to differences that may occur from one group of cells to another, even from the same manufacturer. However, taking this into account, the average power at fixed voltage for the 1 ohm-cm cells is greater than the average for 10 ohm-cm up to a fluence of 5 × 1015e/cm2, where a crossover occurs, and the 10 ohm-cm cells became superior.

Low Energy Proton Damage to Solar Cells

Various types of silicon solar cells have been irradiated with 4.6 - 4.8 Mev protons in two separate experiments. In the first experiment, variations included the bulk material, impurity concentration, and oxygen concentration; the second experiment involved the cells of various manufacturers. Changes in diffusion length, spectral response, and efficiency under sun-like illumination are presented. Annealing effects in terms of the aforermentioned parameters are given. Comparison of the effects of this damage to that of 1 Mev electrons is made. Some preliminary results on the effects of proton damage to GaAs photovoltaic cells are also mentioned.

The effects of space environment on silicon vertical junction solar cells on the LIPS III satellite

The preliminary analysis and results of a space experiment to evaluate the performance of a new generation of silicon vertical junction solar cells and three adhesives for attaching coverglass to the solar cells are presented. Two of the adhesives are used for the first time in coverglass applications where they are subjected to the space environment. SOLAREX vertical junction solar cells which are of 10 Omega -cm silicon with back surface fields (BSF) and back surface reflectors (BSR) are compared to SOLAREX planar junction cells of the same type. The results for up to 566 days in space indicate that the two types of solar cell show about the same rate of degradation in power output. There are no significant difference at this point in the performance of the three adhesives.<<ETX>>

Testing of materials for solar power space applications

Abstract This paper summarizes the results of a program initiated at the Naval Research Laboratory to test conventional and state-of-the-art solar power space systems by flying them aboard satellites. The program (approximately nine years in duration) confirmed the practicality of improvements in advanced silicon solar cells such as textured surfaces, shallow junctions, back surface field and back surface reflector techniques, as well as novel methods of bonding coverslips to conventional cells. In addition, the performance of gallium aluminum arsenide solar cells first tested in a space environment and demonstrated to be satisfactory. Finally, advanced silicon cells such as lithium-diffused and vertical junction cells, which were reported to be radiation resistant on the basis of measurements in the laboratory, were found unsuitable for extended space application.

Electron radiation damage in gallium arsenide solar cells

High efficiency liquid phase epitaxy GaAs solar cells and space quality single-crystal silicon solar cells were irradiated with high energy electrons at 0.7, 1.0, 2.0 and 10 MeV to a total fluence of 4 × 1015 electrons cm−2. Photovoltaic characteristics were measured at incremental fluence levels, and finally the power output and the critical fluence were calculated as a function of electron energy. The ratio of the critical fluence in GaAs cells to the critical fluence in silicon back-surface reflecting cells varied from 2 to 30, depending on the electron energy. A calculation was made of the comparative power degradation of solar cells in a 20 370 km circular orbit for 5 years, indicating that the power loss in GaAs cells is only 4%, while silicon back-surface field cells degrade 12% in maximum power.