Abstract
This paper discusses developments in the mitigation of light-induced degradation caused by boron-oxygen defects in boron-doped Czochralski grown silicon. Particular attention is paid to the fabrication of industrial silicon solar cells with treatments for sensitive materials using illuminated annealing. It highlights the importance and desirability of using hydrogen-containing dielectric layers and a subsequent firing process to inject hydrogen throughout the bulk of the silicon solar cell and subsequent illuminated annealing processes for the formation of the boron-oxygen defects and simultaneously manipulate the charge states of hydrogen to enable defect passivation. For the photovoltaic industry with a current capacity of approximately 100 GW peak, the mitigation of boron-oxygen related light-induced degradation is a necessity to use cost-effective B-doped silicon while benefitting from the high-efficiency potential of new solar cell concepts.
Highlights
One of the most significant challenges for humanity is the need to shift away from using fossil fuel technologies for electricity generation and transportation needs with a transition towards the use of renewable resources
If inappropriate parameters are used during thermal processing, it is possible to worsen the situation by increasing the defect concentration [72,74]. It appears that the rapid cool down from the high temperature plays a critical role in determining the defect concentration [75,78], possibly through a reduction in [Oi ] [79]. This influence is supported by recent studies, which have shown that the fast-firing process used for the metallisation of industrial silicon solar cells can lead to a permanent reduction in the B-O
Lee et al demonstrated an 80% reduction in the extent of light-induced degradation (LID) compared to a 60% reduction for regeneration processes performed at the cell level [15]
Summary
One of the most significant challenges for humanity is the need to shift away from using fossil fuel technologies for electricity generation and transportation needs with a transition towards the use of renewable resources. In p-type Czochralski (Cz) grown silicon, such light-induced degradation (LID) has plagued the industry for decades and has been a driver for intense research into alternative silicon materials for PV production This degradation is believed to be due to the formation of boron-oxygen (B-O) related defects [8], the actual composition of the defect is still unclear and remains a contentious issue [9,10,11]. B-O related degradation can reduce the 1-sun standard test conditions (STC) efficiency of silicon solar cells by up to 10% relative to the performance immediately after cell production [13] In practice it is much smaller than this, typically plateauing at a value of 3–4%relative for conventional aluminium back-surface-field (Al-BSF) solar cells [14] and 4–6%relative for the passivated emitter, rear cell (PERC) structure [15]. A reduction in the extent of B-O LID can be achieved by reducing either the boron or Oi concentration in the silicon material
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