Revolutionizing Solar Efficiency with Light-Induced Annealing in N-Type TOPCon Silicon Solar Cells
This paper provides a top-level summary of research aimed at improving TOPCon solar cell structure efficiency through a method known as light-induced annealing. See Figure 1 for a basic diagram of the cell structure. This cutting-edge process leads to a significant increase in both open-circuit voltage (VOC) and fill factor (FF). By manipulating the hydrogen's valence state through light and temperature adjustments, a marked enhancement in passivation performance is achieved. Notably, the quality of the passivation film, silicon substrate doping concentration, and the temperature settings during light-induced annealing are pivotal in driving these process enhancements.
Principles and Processes:
For optimal solar cell performance, achieving top-tier interface passivation is key to maximizing both Voc and FF while enabling efficient one-dimensional vertical transport. Passivation contacts serve as a vital strategy to meet this objective. The role of silicon oxide at the interface between poly-Si and the Si substrate is pivotal; it diminishes interface state density through chemical passivation.
In this context, most charge carriers navigate through tunneling (Figure 2), encountering hurdles due to poly-Si field effects, especially when traversing the oxide layer. Notably, heavily doped poly-Si experiences a concentration disparity between majority and minority charge carriers. This disparity significantly diminishes electron-hole recombination, leading to the creation of selective contacts primarily dominated by majority carriers.
Within the selective contact zone, multiple carrier transport prompts resistance losses, while a minority of carriers gravitate towards the metal contact area, causing recombination losses. The former contributes to contact resistance (ρc), while the latter influences interface recombination current (J0).
Light-Induced Annealing Process:
The light-induced annealing furnace follows a specific process:
The light-induced annealing process consists of two crucial steps. First, heating activates hydrogen (H) atoms in the nitride silicon passivation film. Then, light exposure is employed to regulate the valence states of atoms, enabling their bonding to defect centers in the P+ emitter and N-type base. This formation of non-recombinant centers ensures effective passivation, culminating in enhanced Voc and FF.
Results and Discussion:
During testing 15 solar cells, all within the same efficiency range underwent light-induced annealing. The results showed a notable increase in Voc and FF (Figure 3; Figure 4), while other electrical parameters remained unchanged. This outcome suggests that hydrogen passivation (Figure 5) post-light injection enhances the quality of the PN junction by effectively passivating dangling bonds on the crystalline facets, ultimately raising Voc.
Further analysis revealed that the emitter region contained boron doping, and the deposition of nitride silicon film on the surface led to the presence of both boron and hydrogen. Hydrogen atoms, located approximately 0.125 nm away from boron, displayed minimal energy on a spherical surface. These hydrogen atoms were confined to this spherical surface, allowing free rotation and forming dynamic boron-hydrogen complexes. This interaction altered energy levels within the bandgap, leading to improved passivation.
During testing, 300 solar cells were arranged into three sets, each exposed to different peak temperatures (200°C, 260°C, and 320°C) during LED light exposure (refer to Figure 6). The results demonstrated the most substantial efficiency enhancement when the peak temperature during illumination was set at 260°C.
This improvement is linked to annealing at 200°C, which notably aids in enhancing amorphous silicon passivation by decreasing the interface state density, particularly at silicon dangling bonds. It’s noteworthy that changes in the microstructure of the amorphous silicon film didn’t impact the photo-induced gain.
During testing on 600 solar cells exposed to LED light at a constant temperature of 260°C, light intensity varied from 10% to 60%. Surprisingly, the results showed that efficiency improvement remained consistent across the spectrum of light intensities under identical heating conditions. This outcome indicates that even lower levels of light can yield efficiency gains, highlighting the reduction in efficiency losses attributed to interface recombination between a-Si: H and c-Si.
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Testing involved symmetrically structured solar cells with varying poly-Si thicknesses (90 nm, 120 nm, 150 nm, and 200 nm). The findings indicated that when the poly-Si thickness in the back passivation layer was under 150 nm, light-induced annealing resulted in reduced passivation performance (Figure 7). However, when the thickness was equal to or greater than 150 nm, passivation performance remained consistent.?
Interestingly, an increase in poly-Si thickness resulted in parasitic light absorption by the polycrystalline silicon layer, reducing light utilization efficiency. Therefore, careful consideration of poly-Si layer thickness is crucial, balancing the trade-off between parasitic absorption and passivation performance.
In the study of 90 nm-thick poly-Si double-sided symmetrically structured solar cells, four different light-induced annealing processes were evaluated. However, the results (Figure 8) indicated no improvement in the passivation performance of thin poly-Si layers, regardless of adjustments in annealing temperature and light intensity.
Interestingly, the study revealed that light-induced annealing enhancements remained consistent at around 0.09% regardless of the coverage area of metal grid lines on the front surface (Table 1). This suggests that the hydrogen passivation gain remained independent of the coverage area of grid lines. Furthermore, an increase in the metal grid line contact area resulted in a broader metal contact recombination region, affecting J0 metal values and causing a reduction in Voc while increasing FF.
Solar cells with high resistivity did not show any efficiency improvement following light-induced annealing; in contrast, cells with low resistivity demonstrated a considerable enhancement (Figure 9). This discovery underlines a significant connection between the photo-induced gain and substrate doping concentration. In cases of high resistivity, low conductivity resulted from lower substrate doping. Exceeding the solid solubility properties of silicon with impurities causes crystal deposits to form, resulting in batch defects.
Following light-induced annealing, the faint concentric rings resulting from the high oxygen content in the silicon wafer were eliminated. These concentric rings are formed due to excessive oxygen content in the silicon wafer, leading to radial or helical oxygen precipitation during the high-temperature processes, observed as dark oxygen rings under electroluminescence (Figure 10). Theoretically, these oxygen rings indicate body defects in N-type silicon. After light-induced annealing, hydrogen atoms, initially distributed near the silicon surface, diffuse deeper into the silicon, further passivating batch defects and impurities, resulting in the complete elimination of the concentric rings.
Conclusion:
In conclusion, light-induced annealing of N-TOPCon solar cells leads to a significant improvement in both Voc and FF. The intensity of light exposure does not affect the magnitude of efficiency improvement in N-TOPCon cells within the same efficiency range. However, efficiency gains are more notably pronounced when the temperature during LED light exposure reaches 260°C.?
For N-TOPCon cells with thinner poly-Si layers, light-induced annealing may reduce passivation performance. The coverage area of front metal grid lines does not impact the efficiency enhancement of the light-induced annealing process.?
In N-TOPCon cells, high-resistivity silicon wafers do not exhibit significant efficiency gains, whereas low-resistivity wafers display marked improvement, indicating the influence of substrate doping concentration on the efficacy of light-induced enhancement. Lastly, light-induced annealing effectively eliminates oxygen rings in N-type silicon wafers.
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Source: @Pocket photovoltaic
Note: This article is translated, sourced, and/or adapted from the China Photovoltaic social media public account, with proper authorization obtained.?