Potential Analysis of a Rear-Side Passivation for Multi-Junction Space Solar Cells based on Germanium Substrates

Abstract

For future 4-junction Ge based multi-junction solar cells, the current generated in the Ge sub-cell gets very important. We developed an efficient rear-side passivation stack, which results in minority carrier lifetimes (τ eff ) ≈ 200 µs and investigated its performance in an accelerated aging experiment (1 MeV electron irradiation). The aging caused a strong lifetime decrease down to τ eff = 4 µs, whereas the carrier mobility stayed constant. These experimental values provide the basis for Beginning-of-Life and End-of-Life solar cell simulations, which show that the potential of the rear-side passivation for 3-junction solar-cell performance is limited, but very promising for 4junction solar cells.

Application of Quasi-Steady-State Photoconductance Technique to Lifetime Measurements on Crystalline Germanium Substrates

“…> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 8 120 which shows a plateau of 140 s for n av = 10 14 -10 15 cm -3 which is the estimated n av range used in the lifetime mapping [30]. This good agreement demonstrates the validity of  eff (n av ) measurements using QSS-PC technique on c-Ge samples.…”

Assessing the operating temperature of multi-junction solar cells with novel rear side layer stack and local electrical contacts

“…In principle, a lower DB is also expected for lower doped Ge substrate and therefore for the LFC sample. However, simulations have shown that an optimized passivation can compensate the loss in DB due to a lower doping level [9]. Therefore, the difference in DB and "B between the standard and the LFC sample in Figure 3 are attributed to a good, but sub-optimal back side passivation.…”

“…In a recent work, a new design of MJ solar cell with a back mirror and point electrical contacts has shown superior Ge bottom cell performance [9]. This design was also predicted to reduce the operating temperature of the cell by reflecting the unused near infrared solar photons [9], [10].…”

Passivated, Highly Reflecting, Laser Contacted Ge Rear Side for III-V Multi-Junction Solar Cells

“…On test samples we showed good passivated Ge surfaces leading to effective minority carrier lifetimes of τ eff > 300 μs on Ge with a doping level of 2 × 10 16 at.cm −3 [9]- [11] as well as ohmic, low resistance contacts. For this article, the developed rear side structure was implemented into a III-V multijunction (MJ) solar cell process.…”

“…The development and the performance of the passivation layer and the simulated cell potential were described earlier [9]- [11]. Furthermore, the improved rear side was recently integrated successfully into a four junction solar cell device [6].…”