Modeling radiation degradation in solar cells extends satellite lifetime

The near-Earth space environment includes a radiation field that can be extremely harmful to spacecraft electronics. The solar cells on the power arrays of satellites are especially susceptible to radiation damage, and the solar-cell radiation response strongly affects the full capability of the spacecraft and, ultimately, the mission's lifetime. By understanding the mechanisms that control the radiation response in solar cells, we can develop methods to mitigate the radiation effects and devise more radiation-resistant devices. As a result, mission capability and duration can be enhanced.

For the past two decades, the US Naval Research Laboratory (NRL) has maintained a basic and applied research program to develop a physics-based model to describe space solar-cell radiation response. The result, NRL's Displacement Damage Dose (DDD) methodology for predicting solar-cell degradation, is now becoming the accepted on-orbit performance prediction method. With this method, it is possible to obtain accurate on-orbit performance predictions from a minimum of ground-test data. In addition, solar cells with superior radiation resistance can be realized.

Figure 1.Schematic representation of the radiation environment surrounding the Earth. This environment is dominated by ‘belts’ consisting of a spectrum of electrons and protons that impinge upon the solar cells and degrade their performance.

To understand the performance of a solar cell in the space radiation environment, it is necessary to know how cell degradation depends on the energy of the irradiating particle (since the space environment consists of a spectrum of protons and electrons: see Figure 1). Other degradation modeling methods, in particular the Equivalent Fluence method of the Jet Propulsion Laboratory (JPL),^1,2 are based on a large data set from which the energy dependence is empirically determined (see Figure 2 for an example of such a data set). While the Equivalent Fluence method is an accurate prediction tool, it is data intensive and, thus, time consuming and costly. The DDD method, on the other hand, focuses on the primary basic mechanism for solar-cell radiation damage, displacement damage—the displacement of a cell's atom from its regular position upon interaction with incident electrons or protons. Therefore, the method can describe the solar-cell response from only one proton and two electron curves, that is, from the solar-cell data measured as a function of the number of incident protons or electrons per surface area (particle fluence). By quantifying the radiation environment and the solar-cell degradation in terms of DDD, the measured data collapse to a single curve, from which the cell performance in any radiation environment can be predicted (see Figure 3).

Figure 2.Measured degradation of a single junction gallium arsenide (GaAs) solar cell under proton, electron,²and neutron irradiation.³ These data can be used to empirically determine the energy dependence of the solar-cell degradation thereby enabling on-orbit performance prediction. P_max: Maximum power.

Figure 3.Solar-cell degradation data from Figure 2 plotted as a function of displacement damage dose. When plotted as a function of this dose, the data corresponding to particles of various energies collapse to a single curve.

The fundamental advancement from the NRL research is the introduction of the concept of non-ionizing energy loss (NIEL), which gives the rate at which energy from the irradiating particle is transferred to displacement damage.⁴ The NIEL is an analytic quantity that is calculated from first principles for a given material and irradiating particle. The DDD is then given by the product of this quantity with the particle fluence. An example of the calculated NIEL values for particles incident upon gallium arsenide (GaAs) is shown in Figure 4. An advantage of the DDD method is that the energy dependence of the solar-cell degradation is determined analytically, so much fewer empirical data than in the Equivalent Fluence method is required. Furthermore, the DDD method reveals the physics controlling the solar-cell response, and it is the first step in understanding radiation response in terms of basic material properties.

Figure 4.Nonionizing energy loss (NIEL) calculated for protons, electrons, and 1MeV neutrons incident on GaAs.⁴

Having been validated for the case of solar cells consisting of one semiconductor junction, i.e. a single-junction solar cell, the DDD method has been extended to describe the case of triple-junction solar cells,⁵ which are currently the state of the art for high-efficiency solar cells. The DDD method has now been implemented in a computer program called SCREAM (Solar Cell Radiation Effects Analysis Models)⁶ that can be used to predict the performance of a given solar-cell technology in a specific Earth orbit. The SCREAM model is now in the last stage of testing and should be generally available in early 2012. The next step in the model development consists of extending it to other optoelectronic devices, such as optocouplers and focal-plane arrays. Since the radiation response of such devices is controlled by displacement damage effects similar to solar cells, the DDD method is expected to apply to them as well.