On-Ground Irradiation Experiments of Solar Cells

The use of a solar cell in space radiation environment requires knowledge of the degradation of current-voltage (1-У) characteristics under light illumination. Illuminated current-voltage (7-У) measurements of each sample were usually performed both before and after irradiation using an solar simulator under AM0 condition (air mass zero, 1 sun, 25 °C conditions). Air mass zero represents the solar irradiance observed in space. This degradation is typically characterized through the decrease of the maximum power (Pm), short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (FF) induced by the interaction with energetic particles present in space.

To predict the degradation of a particular electrical parameter of a solar cell in a space radiation environment, it is necessary to know how the parameter responds to different electron and proton energies: i. e., the energy dependence of the damage coefficients (DCs). A damage coefficient can be generated for any measurable physical parameter of a device, such as carrier lifetime, carrier diffusion length, shifts in threshold current, LED light output, and many other parameters. The DCs can be obtained through irradiating solar cells from ground-based radiation facilities (e. g. electron and proton accelerators) using unidirectional, mono­energetic electrons or protons. The appropriate particle test energies and fluencies are determined through radiation environmental predictions of the flight orbit. Once the energy dependence of the DCs is known, predictions of the cell performance in space can be determined for a given radiation environment.

In this section, the irradiation experimental results were presented about silicon, single-junction and triple-junction GaAs solar cells, and thin film solar cells to compare radiation effects of electrons and protons on these solar cells, and also to provide experimental data for predictions of the cell performances.

Figure 20.3 shows the normalized values of short circuit current (7sc), open circuit voltage (Voc) and maximum power data, the ratio of the value after irradia­tion to the one before irradiation, measured on GaInP2/GaAs/Ge triple-junction solar cells as a function of proton and electron fluences for different energies indicated by the open symbols and by the solid symbols, respectively [8].

For a given degradation level, the fluence level increases for decreasing electron energy indicating that the higher energy electrons do relatively more damage,

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Fig. 20.3 Normalized values of open circuit voltage (a), short circuit current (b), and maximum output power (c) of TJ GaAs solar cells as a function of proton and electron fluence for various particle energies

whereas the opposite is true for proton irradiation. This is correlated with nonion­izing energy loss (NIEL) of electron and proton in semiconductor materials. NIEL represents displacement damage energy transferred to the target lattice by an irradiating particle. The proton NIEL increases with decreasing energy, whereas electron NIEL shows a contrary trend. Thus, lower energy protons produce more displacement damage resulting in a more serious power degradations. Furthermore, as the proton slows down, its energy decreases further and the NIEL increases accordingly. This process continues until the proton eventually comes to rest. As a result, lower energy protons create more damage culminating in a peak in the defect concentration at the end of the proton track. However, as the injecting proton energy becomes too low to reach the active layer of solar cell, it may do smaller displacement damage than higher energy proton does [8, 9]. It can also be found that the fluence level was demanded higher for electron to give rise to the same degradation level than for proton, which can be attributed to the fact that NIEL of proton is far higher than that of electron at same energy.

A InGaP2/GaAs/Ge solar cell consists of three p-n junctions stacked on top of one another, where the thickness and bandgap of each subcell is specifically chosen to maximize absorption of the illumination source spectrum. The total device photovoltage is the sum of photovoltages from each subcell. The photocurrent of the TJ GaAs cell, however, is limited to the least value of the three subcells, which

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Fig. 20.4 Electron radiation-induced degradation of spectral response of a triple-junction GaInP2/ GaAs/Ge solar cell is referred to as the “current limiter.” As a result, the radiation response of the TJ GaAs cell is primarily controlled by the most radiation sensitive subcell. To show this explicitly, a measurement of the quantum efficiency (a measure of how efficiently a solar cell converts individual wavelengths of light into electricity) of a InGaP2/GaAs/Ge cell will show the effect of irradiation on the spectral response of each subcell overlapping the AM0 spectrum, which is shown in Fig. 20.4 for both before and after 1 MeV electron irradiation [10]. The integral of each of these curves with the incident spectrum yields the photocurrent. Given the wide absorp­tion range of the Ge subcell, it produces significantly more photocurrent than the top two junctions, even after irradiation, so it does not limit the current. The photocurrents of the top two subcells, on the other hand, are quite closely matched in the as-grown device. However, under irradiation, the GaAs subcell degrades much faster than the InGaP2 subcell so that it limits the current. The majority of the degradation occurs due to diffusion length degradation in the GaAs subcell, and in all cases, this subcell becomes the current-limiting junction after irradiation.

The structure of the TJ cell can be engineered to control the radiation resistance to some extent. The as-grown condition is referred to as beginning-of-life (BOL). The after irradiation condition is referred to as end-of-life (EOL). Indeed, current matching is the condition for maximum power output, and a TJ cell can be designed to achieve current matching at BOL or at EOL. Since the GaAs cell degrades more rapidly, a current matched cell at EOL will be top cell limited at BOL. This will sacrifice some of the BOL power but result in optimum EOL performance.

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Fig. 20.5 Normalized maximum output power of SJ GaAs solar cells (a) and silicon solar cells (b) as a function of proton and electron fluence for various particle energies

Some ways of attaining a top-cell limited device include thinning the top cell, decreasing absorption in the interconnecting tunnel junction, and extending the GaAs subcell absorption range. When top-cell limited at BOL, the degradation of a MJ cell will be controlled by the more radiation resistant InGaP2 top-cell until a specific irradiation level is reached where the photocurrent of the GaAs subcell is degraded to the level of the top-cell leaving the device current matched. The challenge, then, is to engineer the cell structure so that the radiation level corresponding to current matching coincides with the predicted radiation level of a specific space mission. Instead, it is the cell structure that more significantly controls the radiation-response, and it has been shown how the cell structure may be optimized for maximum BOL and EOL performance.

Figure 20.5 shows the normalized maximum power data, measured on single­junction GaAs/Ge solar cells and on single-crystal 50 ^m thick n + p type BSR silicon solar cells as a function of proton and electron fluences for different energies indicated by the open symbols and by the solid symbols, respectively [11, 12]. The data of short circuit current (Zsc) and open circuit voltage (Voc) are not shown here. It can be seen that irradiation with protons and electrons at different fluences and energies degrades the properties of single-junction GaAs and Si solar cells in the same manner as for the cases of TJ GaAs solar cells.

Thin film solar cells are a rapidly emerging technology for space applications. The two primary technologies under development are a-Si and CIGS solar cells. Figure 20.6 shows the response of the maximum power Pm parameter of a-Si solar cells to the irradiations [13]. In the case of proton irradiation, relatively little variation is seen among the three energies. In the electron irradiation case, the lower energy irradiation is seen to cause the most degradation. These results are not consistent with the values of electron NIEL. This can be attributed to the absence of thermally controlled condition during irradiations, which cannot remove accumulated heat in Si materials produced by ionizing energy loss of irradiating electrons, thereby resulting in a significant annealing of radiation damage.

To predict the performance degradation of solar cells in space, knowledge of the relaxation of radiation-induced defects might be equally important. Indeed, the a-Si

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solar cells can undergo significant annealing of radiation damage at temperatures as low as 60 °C, a typical solar array temperature in space. Both a-Si and CIGS TF-PV devices have demonstrated the ability to self repair (anneal) at a substantial rate when subjected to operating temperatures of 60-80 °C, whereas conventional crystalline Si and III-V compound cells do exhibit some annealing, but at much higher temperatures which is not typical for operational spacecraft.

To demonstrate this, one set of a-Si solar cells was subjected to a 24-h, 60 °C thermal anneal after each irradiation fluence. The results measured after the 1 MeV electron irradiations are shown in Fig. 20.7 [13]. When the annealing is included, these cells display extreme radiation resistance.

Figure 20.8 shows the degradation behavior of hydrogenated a-Si (a-Si:H) solar cell as a function of 10 MeV proton fluence, and the sample temperatures were kept at 25 °C (LT) and at 58 °C (HT) during irradiations [14]. The remaining factor of Pmax at HT were normalized by the values at LT using the temperature coefficients. The results clearly showed that performance degradation was substantially affected by the sample temperature during irradiation. Figure 20.8 also displays the typical degradation behavior of crystalline Si (c-Si) space solar cells for comparison. The c-Si solar cell structure was optimized for space use and the radiation tolerance was enhanced. The comparison results clearly showed that the radiation tolerance of a-Si:H solar cells were much superior to that of c-Si solar cells. In particular, the a-Si:H cell kept 55 % of Pmax up to a fluence of 1.0 x 1014/cm2, whereas the c-Si cell completely lost its electric output by this fluence. Considering the thermal recovery occurring above room temperature, better EOL performance (better

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Fig. 20.8 Degradation curves of Pmax of the a-Si thin film solar cells irradiated with 10 MeV protons. Results of crystalline silicon space solar cell are denoted by triangle symbols in red for comparison

 

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Fig. 20.9 (a) Normalized open circuit voltage and (b) normalized short circuit current of CIGS solar cells after irradiation with electron energies of 1.0 and 3.0 MeV, and proton energies of 0.29, 1, and 10 MeV

radiation tolerance) of a-Si:H solar cells is expected for the higher temperature condition, especially at slightly higher temperature than RT environments.

Figure 20.9 summarizes the results of irradiation experiments of CIGS solar cells with different electron and proton energies [15]. The consequences of 3 MeV electron irradiation on CIGS solar cells are different from that of the 1 MeV electron irradiations, especially /sc heavily degraded. Reference [16] correlates this finding with the generation of a deeper defect defected by admittance spec­troscopy. For 10 MeV protons the situation is similar to the 1 MeV electrons: with only little losses of /sc, the loss of Voc dominates the efficiency degradation. After 0.29 MeV proton irradiation, losses of Voc and /sc equally contribute to the efficiency degradation. Protons in MeV energy range and high-energy electrons basically cause displacement damage that is uniform through the depth of a 2 ^m thick CIGS absorber. In contrast, upon irradiation with protons of energies in the range 100 keV < E < 500 keV, a highly localized displacement damage is expected [17, 18].

It should also be noted here that the CIGS cells also do display a relatively low annealing temperature, like the a-Si cells. Figure 20.10a depicts the thermal annealing of Voc and /sc of CIGS solar cells after 3 MeV electron irradiation [16]. The relaxation of /sc starts just above room temperature and result in full recovery at 360 K. In contrast, the Voc relaxation begins at 360 K, and the initial value of Voc is no reestablished at a maximum temperature of 440 K. A result for 290 keV-proton-irradiated CIGS solar cells is shown in Fig. 20.10b [17]. With annealing time of 300 s, the relaxation of both Voc and /sc begins at annealing temperature of 360 K and peaks at 400-420 K. Similarly to the data in Fig. 20.10a, isochronal annealing after proton irradiation leads to full recovery of /sc and to partial recovery of Voc. The open circle symbols at 440 K represent an additional annealing step with annealing time of 30 min. Other study on thermal annealing of CIGS solar cells after proton irradiations yields results that are consistent with the data in Fig. 20.10b [19].

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Fig. 20.10 annealing of CIGS solar cells (a) after 3 MeV electron irradiation with a

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fluenceof8 x 10 ‘ cm and (b) after 290 keV proton irradiation with a fluence of 5 x All measurements are performed at room temperature

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