Analysis of Radiation-Induced Defects in InGaP Materials and Solar Cells

摘要

This paper reviews recent detailed experimental studies on radiation-induced defects in InGaP material and solar cells to clarify the superior radiation resistance as well as the potentialities of n~+ -p-InGaP solar cells for space applications. One of the most important results obtained was the discovery of first direct minority carrier injection enhanced annealing of the radiation-induced defect H2 in p-InGaP by deep level transient spectroscopy (DLTS) at room temperature and consequently recovery of radiation damage in InGaP n~+ -p junction solar cells. A close agreement between activation of recovery of radiation-induced defects, determined by solar cell properties (ΔE = 0.54 ± 0.10 eV) and by DLTS (ΔE = 0.51 ± 0.09 eV) for hole level H2 (E_V+0.50-0.55 eV) leads to the dominant role of this level in controlling the minority carrier life time in InGaP solar cells. An experiment based on minority carrier capture on a majority trap by the double carrier pulse DLTS technique further supports the evidence that H2 has a large minority carrier capture cross section and is an efficient non-radiative recombination center. We have investigated the mechanism involved in minority carrier injection-enhanced annealing of the H2 defect induced by 1 MeV electron irradiation in p-InGaP and shown that electron capture induces the enhancement of the annealing as a result of energy release mechanism. The present study also included the first detailed isothermal and isochronal annealing recovery of photovoltaic parameters in InGaP solar cells after 1 MeV electron irradiation and correlation with changes in the deep level transient spectroscopy (DLTS) spectra observed in p-InGaP. An apparent correlation between the recovery of short circuit current, maximum power and quantum efficiency, and the annealing of H2 (0.50-0.55 eV) and H3 (0.75 eV) defect is observed. The correlations between the recovery in the photovoltaic properties and changes in the concentration of majority levels H2 and H3 following isochronal annealing have shown that significant insight into the mechanism responsible for the recovery of radiation damage in InGaP cells. It has been concluded that H2 and H3 defects have a dominant role in governing the lifetime as well as carrier removal in InGaP solar cells and have almost same annealing behavior. The possible origin of native and radiation-induced defects has been discussed. The H2 defect is tentatively identified as phosphorus Frenkel pair. One of the native defect (E_C-0.20-0.29 eV) is identified as DX center.