Crystalline silicon heterojunction back contact solar cells represent a leading technology in photovoltaics. However, achieving high efficiencies faces significant challenges in managing charge carrier recombination and transport. Previous research has demonstrated promising results, with reported efficiencies exceeding 26%, but these still fall short of the theoretical limits. The challenges include balancing the need for effective charge carrier collection with the need for surface passivation to minimize recombination losses. The interdigitated back contact (IBC) structure, while offering advantages such as zero-electrode shading, introduces complexities due to the numerous polarity boundaries created by the alternating rows of hole-selective contacts (HSC) and electron-selective contacts (ESC). These boundaries are susceptible to recombination, which negatively impacts overall efficiency. Furthermore, IBC cells can suffer from electrical shading effects, where not all generated carriers are effectively collected. This research addresses these challenges through the use of amorphous silicon as passivating contact layers and laser ablation for precise patterning, aiming to overcome the limitations of previous designs and fabrication methods. The study focuses on understanding and mitigating recombination losses, resistive losses, and current density losses to further boost efficiency.

Recent advancements in wafer-based crystalline-silicon (c-Si) solar cells have yielded impressive results, with normalized electrical performance (Voc × FF, normalized by the Shockley-Queisser limit) approaching 0.85. Studies by Richter et al. demonstrated n-type and p-type TOPCon solar cells with efficiencies of 25.8% and 26.0%, respectively, showing the potential of these technologies. While Front Back Contact (FBC) solar cells can excel in either electrical or optical characteristics individually, simultaneously achieving both remains a challenge. Achieving low current density loss, optimal contact resistivity, and high passivation quality simultaneously is critical for further efficiency improvement. IBC structures inherently offer zero-electrode shading, improving light utilization and lessening constraints on contact design. However, even with advanced technologies like those used by Kaneka Corporation (up to 26.7% efficiency) and LONGI Corporation (27.30% efficiency), challenges remain in balancing carrier recombination and transport within the complex IBC structure. The use of laser patterning techniques, as exemplified by the ISFH's work on POLO-IBC cells (26.1% efficiency), has shown potential but highlights the need for optimized structural design to manage recombination at the numerous polarity boundaries.

The high-efficiency HBC solar cells were fabricated on large-area Czochralski wafers using a proprietary laser patterning technique. The cell structure features a textured front surface to reduce reflection loss and a polished rear surface to enhance light absorption. Interdigitated HSC and ESC regions are precisely patterned on the rear side using laser ablation. The detailed fabrication process involves several steps: wet chemical cleaning, chemical vapor deposition (CVD) of the passivating contact layers (i-a-Si:H, n-a-Si:H, p-a-Si:H, SiNx), laser ablation to define the HSC and ESC regions, further wet etching, and finally, physical vapor deposition (PVD) of a transparent conductive oxide (TCO) layer followed by silver metallization. A simplified recombination model is employed to extract key electrical parameters such as recombination current density (J01, J02) and series resistance (Rs). Zone-resolved minority carrier lifetime testing was used to quantify J02 recombination from various regions of the solar cell (HSC, ESC, gap, HSC+gap). To further investigate polarity boundary recombination, a specialized wafer with separate HSC, gap, and ESC regions was fabricated and tested. Dark I-V measurements were conducted to quantify recombination loss using a four-probe method. Contact resistivity was determined using the transfer length method (TLM). Optical analyses, including external quantum efficiency (EQE) and reflection measurements, were performed to characterize current density losses from the front and rear surfaces, as well as electrical shading effects. Light beam induced current (LBIC) measurements were used to examine recombination at the wafer edge. Finally, Quokka 3D simulations were used to model carrier transport and recombination within the solar cell structure.

The fabricated HBC solar cell achieved a certified efficiency of 27.09% (total area) and 26.74% (designated area). Analysis revealed that recombination losses primarily originate from the HSC region and the polarity boundaries between the HSC and gap regions. The J02 recombination parameter was successfully reduced to 0.6 nA·cm⁻², significantly lower than that observed in other HBC cells. Contact resistivity analysis demonstrated that the series resistance (Rs) is heavily influenced by the contact resistivity of the HSC and ESC layers and their respective coverage areas. An equation was developed to optimize the coverage area ratio for minimizing Rs. Current density (Jsc) loss analysis showed that electrical shading and wafer edge recombination are significant factors. A simplified equation was derived to correlate the recombination current density (Jrec) with carrier transport length and effective lifetime, allowing for quick estimation of electrical shading. LBIC measurements confirmed substantial recombination at the wafer edge, accounting for a considerable portion of Jsc loss.

The findings directly address the challenges of creating high-efficiency HBC solar cells. The reduction in J02 recombination highlights the effectiveness of the laser patterning and material optimization strategies employed. The successful modeling of Rs provides a clear design guideline for minimizing resistive losses. The identification of wafer edge recombination as a dominant loss mechanism points towards the importance of improved edge passivation technologies. The simplified equation linking Jrec, transport length, and effective lifetime enables a more efficient evaluation of various structural designs and material properties for future improvements. The results demonstrate that by reducing recombination losses, resistive losses and current density losses a significant increase in efficiency is possible.

This research achieved a certified efficiency of 27.09% for an HBC solar cell, significantly advancing the technology. The key to these improvements lies in understanding and addressing recombination at the HSC/gap polarity boundary, controlling contact resistivity, and mitigating wafer edge recombination. Future work should focus on fully integrating nanocrystalline silicon technologies for improved contact properties, implementing robust wafer edge passivation strategies, and meticulously optimizing anti-reflection coatings and rear reflectors. This combined approach is projected to achieve efficiencies exceeding 27.7%.

The study primarily focuses on a specific type of HBC solar cell fabricated using a proprietary laser patterning technique. The findings may not be directly generalizable to other HBC designs or fabrication methods. Furthermore, the simplified models used in the analysis introduce inherent approximations, which may limit the accuracy of the conclusions. Future research should validate these findings across a broader range of designs and conditions.