Bulk-Defect Dominated Minority Carrier Degradation: Mechanisms and Suppression Strategies for High-Temperature Behavior of Photovoltaic n-Type Czochralski Silicon Wafers

doi:10.16553/j.cnki.issn1000-985x.2025.0259

Abstract

Abstract: The minority carrier lifetime of silicon wafers generally decays sharply after high-temperature processes in solar cell fabrication， significantly limiting further improvements in cell efficiency. To elucidate the underlying mechanism， this study systematically investigated the high-temperature behavior of photovoltaic-grade n-type Czochralski （Cz） silicon wafers through controlled heat treatment experiments， combined with multi-scale characterization techniques including minority carrier lifetime， photoluminescence（PL）， and Fourier-transform infrared spectroscopy. The results demonstrate that the significant degradation in both minority carrier lifetime and PL intensity after high-temperature treatment primarily stems from a substantial increase in bulk recombination defects， with surface conditions and process atmosphere playing a minor role. Furthermore， during heat treatment， interstitial oxygen within the silicon wafer converts into precipitated oxygen， accompanied by net inward diffusion of ambient oxygen in an oxygen-rich atmosphere. Comprehensive analysis indicates that oxygen precipitate nuclei pre-existing in the as-grown silicon wafers become activated and grow during high-temperature process， forming oxygen precipitates with high recombination activity， which is identified as the main cause of minority carrier lifetime degradation. Based on the mechanism of ‘selective activation and growth of oxygen precipitate nuclei’， this study proposes two optimization strategies： suppressing nucleus formation by optimizing the crystal cooling thermal history and eliminating existing defects through rapid thermal annealing.

Key words: n-type Czochralski silicon; minority carrier lifetime; high-temperature degradation; oxygen precipitate; rapid thermal annealing

CLC Number:

O786

WANG Pengfei, ZHANG Yuanfang, OU Ziyang, WANG Zhao, CHEN Zhancang, LIN Yao. Bulk-Defect Dominated Minority Carrier Degradation: Mechanisms and Suppression Strategies for High-Temperature Behavior of Photovoltaic n-Type Czochralski Silicon Wafers[J]. Journal of Synthetic Crystals, 2026, 55(4): 619-626.

Figures/Tables 7

Fig.1 Relationship between minority carrier lifetime and sample thickness （measured by BCT-400 and WCT-120）

Table 1 Minority carrier lifetime variation under different heat treatment conditions

Group	Temperature/℃	Gas	Oxygenation duration/s	τ_eff/μs			Instrument for τ_eff
Group	Temperature/℃	Gas	Oxygenation duration/s	Before heat treatment	After heat treatment	After surface removal	Instrument for τ_eff
1A	850	O₂	9 500	3 853	132	—	BCT-400
1B	1 000	O₂	9 500	3 787	133	119	BCT-400
2A	1 000	O₂	9 500	2 775	169	119	BCT-400
2B	1 000	N₂	0	2 840	131	—	BCT-400
3A	1 000	O₂	1 000	9 583	26	—	WCT-120
3B	1 000	O₂	2 000	10 642	11	—	WCT-120

Fig.2 Changes in PL intensity before and after heat treatment

Fig.3 Changes in oxygen content and chemical state before and after heat treatment

Fig.4 Schematic diagram of vacancy concentration evolution and defect formation during post-crystallization cooling of silicon

Fig.5 Microdefect distribution of etched silicon wafer

Fig. 6 PL test results of silicon wafers after different treatment processes. （a） Average PL intensity of A （texturing+passivation）， B （texturing+boron diffusion+passivation）， C （texturing+boron diffusion+phosphorus diffusion+passivation）， D （texturing+defects-ablation in N2+boron diffusion+phosphorus diffusion+passivation）， E （texturing+defects-ablation in O2+boron diffusion+phosphorus diffusion+passivation）；（b） PL images of first wafer in each group

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