Rapid Healing: How Hydrogenation Supercharges Recovery of
Electron-Irradiation Defects in Ga-doped PERC Solar Cells
Abstract
Due to their significantly lower costs than their compound semiconductor
counterparts, there is increasing interest in using silicon solar cells
for specific cost-sensitive applications in space, particularly in low
Earth orbit (LEO). A major concern is, however, that the minority
carrier lifetime (referred to henceforth as lifetime) of silicon solar
cells experiences severe degradation in space due to the impact of
irradiation by high-energy electrons and protons. Fortunately, thermal
and hydrogenation processes can recover the lifetime losses caused by
some (potentially all) defects. In this work, we study these
radiation-induced defects and their recovery in detail using contactless
lifetime measurement and deep-level transient spectroscopy (DLTS). Both
fired and unfired industrial Ga-doped passivated emitter and rear
contact (PERC) solar cell precursors are used in this work. The
precursors were irradiated with 1 MeV electrons and annealed at 300 °C
and 380 °C, respectively. All the irradiated samples exhibited lifetime
recovery at both annealing temperatures, and the fired samples recovered
significantly quicker and reached higher saturated lifetime values.
After only ~360 s of annealing at 380 °C, the irradiated
fired samples recovered to their pre-irradiation lifetime. In contrast,
the irradiated non-fired samples required 71.5 times longer (25,740 s)
at 380 °C to reach saturation. Remarkably, longer annealing times result
in a reduction of the lifetime, which could be due to surface-related
degradation. The DLTS measurements revealed a clear reduction of
recombination active defects after annealing, including V-V
+ and C i-C s in
irradiated fired samples and V-V + in irradiated
unfired samples. This study demonstrates that the firing process is
critical for optimizing the recovery of irradiation damage in silicon
solar cells. Hydrogenation of the silicon bulk results in quicker
recovery and superior End-of-life performance compared to thermal
annealing without bulk hydrogen. Therefore, Ga PERC solar cells with
bulk hydrogenation can recover radiation-induced damage, rendering it
more suitable for missions in LEO.