Degradation mechanism(s) of GaAs solar cells with Cu contacts

Abstract

Substrate-based GaAs solar cells having a dense Au/Cu front contact grid with 45% surface coverage were exposed to accelerated life testing at temperatures between 200 and 300 • C. TEM analysis of the front contacts was used to gain a better understanding of the degradation process. During accelerated life testing at 200 • C only intermixing of the Au and Cu in the front contact occurs, without any significant influence on the J-V curve of the cells, even after 1320h (55 days) of accelerated life testing. At temperatures ≥ 250 • C a recrystallization process occurs in which the metals of the contact and the GaAs front contact layer interact. Once the grainy recrystallized layer starts to approach the window, diffusion via grain boundaries to the window and into the active region of the solar cells occurs, causing a decrease in V oc due to enhanced non-radiative recombination via Cu trap levels introduced in the active region of the solar cell. To be a valid simulation of space conditions the accelerated life testing temperature should be < 250 • C in future experiments, in order to avoid recrystallization of the metals with the GaAs contact layer.

Temperature-Induced Degradation of Thin-Film III–V Solar Cells for Space Applications

“…After 55 days at 200 ○ C the cell on an Au carrier shows a smooth Au/GaAs interface at the back (see figure 4a), whereas the cell on a Cu carrier shows an intermixed Au/Cu layer and a GaAs contact layer that appears to have been almost fully consumed by the Au/Cu matrix already after 3.7 days at 200 ○ C (see figure 4b). This undulation of the metal/GaAs interface and intermixing of the Au and Cu is similar to the undulation and intermixing previously observed for substrate-based cells after 1320 h (55 days) at 200 ○ C (see figure 6b in reference [35]). The intermixing of the Au and Cu is in agreement with the observed colour change of the carrier foil (see figure 3).…”

“…The significant decrease in J sc observed for the thin-film cells on Cu carriers is in sharp contrast with the small changes (typically 1-2%) observed for cells on an Au carrier and substrate-based cells with and without Cu (both in this study and in previous work [34], [35], [40]). This indicates that the degradation process causing the decrease in J sc is specific for thin-film cells on a Cu carrier and therefore of interest for further investigation.…”

“…Unfortunately experimental determination is difficult and hence there are very few activation energies reported for solar cell degradation [29], [32], [33] and none of these concern (Cu) diffusion. The European Cooperation for Space Standardization (ECSS) standard for In previous studies [34], [35] a substrate-based model system utilizing a 45% coverage front contact grid was used to investigate the effects of Cu diffusion on GaAs solar cells. In these studies it was shown that at temperatures < 250 ○ C Cu diffusion has no significant effect on the J-V characteristics of (substrate-based) solar cells.…”

Metal diffusion barriers for GaAs solar cells

“…non temperature-induced) lightand/or electrically-induced degradation mechanisms these operating conditions were excluded in the test procedure used in this study. In a previous study 20 we used the abbreviation ALT for this test procedure, but as this might be confusing we will refer to the adapted test procedure as accelerated ageing testing (AAT). The main difficulty with this accelerated testing approach is that it requires a known activation energy.…”

Enhancement of photocurrent in GaInNAs solar cells using Ag/Cu double-layer back reflector

“…Furthermore, Cu has also been used as a part of the back side contact in GaAs solar cells, but Cu is also known to diffuse easily into III-V semiconductors and thus to reduce the V oc . [20][21][22] In our approach, we have studied GaInNAs solar cells with double-layer Ag/Cu reflectors exhibiting high reflectance, good adhesion, and low contact resistance. Moreover, a thin Ag layer acts as a diffusion barrier, preventing Cu diffusion into the semiconductor structure even when the cell is subjected to thermal annealing.…”