Investigation of abnormal behavior of light induced degradation (LID) in cost-effective industrial aluminum back surface field (Al-BSF) silicon solar cells. (Under the direction of DR. ABASIFREKE EBONG) In this research, the p-type boron-doped (B-doped) Czochralski (Cz) mono crystalline silicon (c-Si) Al-BSF solar cell was studied because of its large market share, high efficiency potential (~20%) and cost-effectiveness. The main problem of the p-type B-doped Cz silicon is the unstable efficiency due to the light induced degradation (LID).
In order to resolve this unstable-efficiency problem, this thesis work investigated the causes and mitigation of LID. The major cause of LID is the formation of boron-oxygen defect in the silicon bandgap under irradiation. The reduced lifetime due to recombination in the bandgap can be recovered by two methods including (i) ~200°C low temperature anneal or (ii) rapid thermal anneal at 630-850°C. LID can be prevented with alternative base-doping such as Ga, P, or the use of high base resistivity in boron doped Si. However, the latter is still in the research stage while the majority of the commercial solar cell is boron doped with resistivity ranging from 0.5-2 Ω-cm. Therefore, there is a greater need to fully understand how to get rid of the LID in the finished cells.
This thesis work focuses on regeneration of cells after the contact co-firing step or/and degraded state. Regeneration is a stable recovery, which encompasses degradation, recovery and stability. Two experimental set ups were conducted by controlling (i) the carrier injection (~1 sun), (ii) the temperatures (70-100°C), and (iii) time (> 24 hours). The cells went through degradation, recovery and stabilization under irradiation.
The recombination centers constituted of the boron-oxygen related defect are passivated by the effusion of hydrogen existing in the anti-reflection coating (ARC) of a solar cell. During the contact co-firing process after the screen printing, hydrogen enable to diffuse from ARC into the bulk. If the cooling rate is fast in the co-firing, then the future LID degradation is less because some of the hydrogen is retained at the recombination sites.
Regeneration is based on hydrogenation. First, the hydrogen from ARC diffuses into the bulk during the contact co-firing. Subsequently, through the regeneration process, the hydrogen in the bulk transfers to atomic and mobile hydrogen at excited states, and finally passivates defect into equilibrium. The regeneration process requires simultaneous (i) the carrier injection (> 0.1 suns), (ii) elevated temperatures (65-400°C) and (iii) the sufficient time.
Hydrogenation through regeneration process results in stable solar cells. In comparison between the results in this thesis and previous studies in literature, it can be concluded that the faster regeneration rate primarily depends on the temperatures. The higher the temperature in a proper range, the shorter the time required for regeneration.
|Commitee:||Raja, Yasin Akhtar, Zhang, Yong|
|School:||The University of North Carolina at Charlotte|
|School Location:||United States -- North Carolina|
|Source:||MAI 56/04M(E), Masters Abstracts International|
|Keywords:||Al-BSF Si, BO complex defect, Hydrogenation, Light induced degradation (LID), Regeneration, p-type B-doped solar cells|
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