Abstract
Silicon solar cells containing boron and oxygen are one of the most rapidly growing forms of electricity generation. However, they suffer from significant degradation during the initial stages of use. This problem has been studied for 40 years resulting in over 250 research publications. Despite this, there is no consensus regarding the microscopic nature of the defect reactions responsible. In this paper, we present compelling evidence of the mechanism of degradation. We observe, using deep level transient spectroscopy and photoluminescence, under the action of light or injected carriers, the conversion of a deep boron-di-oxygen-related donor state into a shallow acceptor which correlates with the change in the lifetime of minority carriers in the silicon. Using ab initio modeling, we propose structures of the BsO2 defect which match the experimental findings. We put forward the hypothesis that the dominant recombination process associated with the degradation is trap-assisted Auger recombination. This assignment is supported by the observation of above bandgap luminescence due to hot carriers resulting from the Auger process.
Highlights
Silicon photovoltaics dominate the solar cell market and will soon provide one of the lowest cost options for future electricity supply
In the deep level transient spectroscopy (DLTS) spectra recorded in the temperature range 300–440 K for most of our as-manufactured diodes from boron-doped Cz-Si, a previously unreported broad peak has been detected, with maximum at about 390 K [Fig. 2(a)]
We suggest that state A is responsible for the fast-formed recombination defect (FRD), while A0 seems a more plausible contender for slow-formed recombination defect (SRD)
Summary
Silicon photovoltaics dominate the solar cell market and will soon provide one of the lowest cost options for future electricity supply. Mchedlidze and Weber have proposed that these states at EC−0.41 eV and EV+0.37 eV are the SRH centers responsible for BO degradation.16 They used diodes fabricated from PERC solar cells and put forward the hypothesis that previous attempts to observe the BO-related states were unsuccessful because of hydrogen contamination passivating the electrically active defects when Schottky diodes were fabricated for characterization. The model is consistent with the DLTS and PL measurements, and besides showing transformation barriers compatible with the degradation/recovery kinetics of the cells, it accounts for the resilience of Ga-doped material to LID It is suggested the BO degradation of minority carrier lifetime in solar cells containing boron and oxygen is associated with trap-assisted Auger recombination of charge carriers through the BsO2 complex, which, in its recombination active state, is a shallow acceptor
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