Radiation Effects in Thinned GaAs Photovoltaics Incorporating DBRs for Improved Radiation Tolerance of Multijunctions

Radiation tolerance of a triple junction solar cell can be improved by thinning the GaAs middle junction and maintaining power conversion efficiency through the use of a distributed Bragg reflector. Simulations using Sentaurus RSoft show that current density lost through reduction of the GaAs base thickness can be recovered by increasing the optical path length through the device using photonic structures such as a distributed Bragg reflector (DBR). In this paper, the simulation is further compared to experiment focusing on the GaAs middle cell, where prior to growing the diode a conductive DBR is grown epitaxially using MOCVD. Internal and external quantum efficiency, as well as performance under 1-sun illumination, is presented showing nearly complete recovery of performance to optically-thick conditions, while using half the thickness of GaAs absorber. These devices were then exposed to 1 MeV electrons with total doses of 2×10
¹⁴
, 5×10
¹⁴
, and 1×10
¹⁵
cm
^-2
. Post-radiation experimental results show improved J
_SC
remaining factor, and absolute J
_SC
, for the optically thin device incorporating a DBR. This allows an improvement of 0.36 mA/cm
²
at a fluence of 1×10
¹⁵
cm
^-2
over the optically thick control. This result was incorporated into a Ge-based triple junction simulation and predicts a 0.24% absolute efficiency improvement at 1×10
¹⁵
cm
^-2
1 MeV e
^-
fluence.