Laser-Induced Defects in Silicon Solar Cells and Laser Annealing

A major (~95%) part of the world solar photovoltaic power is generated using silicon-based solar cells. Further reduction in the manufacturing cost of silicon solar cell fabrication is required to compete with alternative sources of energy generation. To achieve high-efficiency and low-cost silicon solar cell devices, a low-temperature, non-vacuum, high-throughput fabrication process is required. A high-power laser process can overcome some of these challenges by significantly reducing the fabrication complexity, avoiding multiple patterning steps, and potentially replacing high-temperature, high-vacuum processes. Currently, laser processing of silicon solar cells has been investigated for laser doping, laser direct writing, laser microtexturing, and laser ablation.

However, the main challenge with the application of high-power lasers for Si solar cell fabrication lies in the ability to eliminate the generation of induced crystal defects and formation of amorphous phases due to fast thermal processes. The laser-induced damage could increase carrier recombination and will result in the deterioration of photovoltaic device performance. For a wider acceptance of laser-based silicon solar cell fabrication, three research goals are set: (1) gain a fundamental understanding of defect generation mechanisms and identify the damage-limiting laser-processing conditions; (2) demonstrate the mitigation of laser-induced defects using post laser surface annealing; and (3) demonstrate the defect-controlled and annealing processes in laser-based silicon solar cell devices.

Various types of laser-induced defects were extensively investigated, including grain boundaries, microtwins, dislocations, point defects, and oxygen incorporation. The formation of amorphous phases and internal strain were also examined. We find that the single-crystalline phase can be retained through a laser-ablation process. Laser-induced dislocation density and strain are found to increase exponentially with laser fluence, while a maximum in point-defect concentration is observed with increasing laser fluence. These experimental results are in good agreement with the simulation work done by Miao He in Prof. Leonid V. Zhigilei's research group. It is concluded that laser-induced defects can be minimized by tailoring laser-processing conditions.

Through the measurement of carrier lifetime, leakage current, drift mobility, and electrical conductivity at various laser fluences and defect densities, we find that the laser-defect induced degradation of surface electrical properties are governed by an exponential relationship. This suggests that laser-processing fluences near the silicon melting threshold should be carefully chosen for minimizing the induced defects and electrical property degradation.

Moreover, a post laser annealing technique was investigated to remove the laser-processing-induced defects, and this technique was integrated with the laser-based solar cell fabrication process. We find that a low-power long-pulse-width laser annealing process can eliminate dislocations and point defects induced by high-power laser processing.

Furthermore, we developed a laser-based method for passivating silicon surface defects by laser processing of a sol-gel TiOx thin film. We find that laser processing can produce chemical bonding at the TiOx/Si interface and lead to excellent surface passivation with a low surface recombination velocity of 6.25 cm/s.

Lastly, we demonstrated the minimization of laser-induced defects in laser-based solar cell fabrication, including laser transfer doping, laser-transferred metal contacts, and laser ablation of metals for contact isolation. Through incorporating these laser-based processes with laser annealing, we fabricated all-laser-based interdigitated back contact silicon solar cells and demonstrated that post laser annealing can considerably improve the conversion efficiency.

In summary, a fundamental understanding of laser-induced defects in Si has been provided, and a novel and viable concept of high-power laser processing with low-power laser annealing has been demonstrated for high-efficiency and low-cost silicon solar cell fabrication.