Abstract
In this contribution the influence of different types of recombination-active defects on the integral electrical properties of multicrystalline Si solar cells is investigated. Based on a previous classification scheme related to the luminescence behavior of crystal defects, Type-A and Type-B defects are locally distinguished. It is shown that Type-A defects, correlated to iron contaminations, are dominating the efficiency by more than 20% relative through their impact on the short circuit current ISC and open circuit voltage VOC in standard Si material (only limited by recombination active crystal defects). Contrarily, Type-B defects show low influence on the efficiency of 3% relative. The impact of the detrimental Type-A defects on the electrical parameters is studied as a function of the block height. A clear correlation between the area fraction of Type-A defects and both the global Isc and the prebreakdown behavior (reverse current) in voltage regime-2 (−11 V) is observed. An outlier having an increased full-area recombination activity is traced back to dense inter- and intragrain nucleation of Fe precipitates. Based on these results it is concluded that Type-A defects are the most detrimental defects in Si solar cells (having efficiencies > 15%) and have to be prevented by optimized Si material quality and solar cell process conditions.
Highlights
The main challenges of the photovoltaic industry are to reduce the costs per watt peak and to increase the durability of the solar modules
In previous publications [9, 17] we have introduced such a classification of recombination active defect structures in multicrystalline silicon (mc-Si) solar cells using the band-to-band and subband luminescence as well as the prebreakdown behavior at crystal defects based on results published by Bothe et al [18] and Schubert et al [19]
To estimate the influences of Type-A and Type-B defects, respectively, on the electrical properties of the solar cell, three mini solar cells (1 × 1 cm2) out of a 6 solar cell were prepared having a large number of Type-A defects and Type-B defects and a sample having a low number of recombination active defects according to an EL measurement
Summary
The main challenges of the photovoltaic industry are to reduce the costs per watt peak and to increase the durability of the solar modules. A major reason for this is the presence of a large number of different recombination active defect structures in mc-Si materials. These defect structures have an influence on (i) the solar cell efficiency [1,2,3] and (ii) the electrical breakdown behavior and the reliability of the solar module [4]. A promising connection was not performed so far since mc-Si solar cells suffer from a large number of crystal defects and not all crystal defects behave in the same way with respect to recombination and prebreakdown behavior. Afterwards, the knowledge obtained can be applied for each defect class on solar cell level even on industrial scale
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