Heterojunction (HJT) solar cells represent a high-efficiency photovoltaic technology that combines the advantages of crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H). The architecture of HJT cells leverages the superior electronic properties of c-Si alongside the excellent passivation and junction-forming capabilities of a-Si:H. This design results in high open-circuit voltages (Voc) and overall conversion efficiencies, making HJT cells a competitive option in the solar industry.

The core structure of an HJT solar cell consists of a thin c-Si wafer, typically n-type due to its higher carrier lifetime and reduced sensitivity to impurities. The wafer is sandwiched between intrinsic and doped a-Si:H layers, which serve multiple purposes. The intrinsic a-Si:H layer provides outstanding surface passivation, reducing recombination losses at the c-Si surface. This is critical for achieving high Voc, as surface recombination can significantly degrade cell performance. The doped a-Si:H layers, either p-type or n-type, form the heterojunction contacts, enabling efficient carrier extraction while maintaining low interface defect densities.

A key advantage of HJT cells is their low-temperature fabrication process. Unlike conventional c-Si solar cells, which require high-temperature steps for dopant diffusion and oxide growth, HJT cells are fabricated at temperatures below 250°C. This reduces thermal stress on the materials, minimizes energy consumption during production, and allows for the use of thinner wafers without warping or degradation. The low-temperature deposition of a-Si:H layers is typically achieved through plasma-enhanced chemical vapor deposition (PECVD), ensuring uniform coverage and precise control over layer thickness.

The high Voc of HJT cells is a direct result of the excellent passivation provided by the intrinsic a-Si:H layer and the high-quality heterojunction interface. Typical HJT cells achieve Voc values exceeding 730 mV, with some laboratory demonstrations surpassing 750 mV. This is significantly higher than the Voc of traditional diffused-junction c-Si cells, which usually range between 650 mV and 700 mV. The improved Voc translates to higher overall efficiencies, with commercial HJT modules reaching efficiencies above 24%, and record laboratory cells exceeding 26%.

Transparent conductive oxide (TCO) coatings play a crucial role in HJT cell performance. The TCO layer, usually made of indium tin oxide (ITO) or a similar material, serves as both a conductive electrode and an anti-reflective coating. It allows light to pass through to the active layers while facilitating lateral carrier transport to the metal grid contacts. The optical and electrical properties of the TCO are carefully optimized to minimize absorption losses and resistive losses, further enhancing cell efficiency.

Bifacial HJT solar cells are an important variant of this technology, capable of generating electricity from both the front and rear sides of the module. The symmetrical structure of HJT cells, with intrinsic a-Si:H passivation layers on both sides of the c-Si wafer, makes them naturally suited for bifacial applications. When deployed over reflective surfaces, bifacial HJT modules can achieve energy yields 10-20% higher than monofacial modules, depending on installation conditions. The rear side of the cell typically features a similar TCO and grid design as the front, though the doping profile may be adjusted to optimize bifacial performance.

The intrinsic a-Si:H layer in HJT cells is typically only 5-10 nm thick, just enough to provide effective passivation without introducing excessive series resistance. The doped a-Si:H layers are similarly thin, usually less than 20 nm, to minimize parasitic absorption while still forming an effective junction. The precise thickness and hydrogen content of these layers are critical parameters that influence passivation quality and carrier transport.

Doping in the a-Si:H layers is achieved through the incorporation of boron for p-type layers and phosphorus for n-type layers during PECVD deposition. The doping concentration is carefully controlled to balance junction quality with conductivity. Excessive doping can lead to increased defect densities and reduced passivation, while insufficient doping results in high series resistance. The optimization of these layers is a key factor in achieving high fill factors, often exceeding 80% in high-performance HJT cells.

The metal contacts in HJT cells are typically screen-printed or deposited through physical vapor deposition (PVD) techniques. To minimize shading losses, the front grid is designed with fine lines and low resistivity. Some advanced HJT cells employ copper electroplating for the front contacts, further reducing resistive losses and improving efficiency. The rear contact is often a full-area metallization, though some designs use a grid pattern similar to the front for bifacial applications.

HJT cells exhibit excellent temperature coefficients, typically around -0.25% per °C, which is superior to conventional c-Si cells. This means they retain a higher percentage of their rated power output at elevated temperatures, making them particularly suitable for hot climates. The combination of high Voc, low-temperature coefficient, and bifacial capability makes HJT technology well-suited for utility-scale solar farms as well as rooftop installations.

The manufacturing process for HJT cells involves fewer steps compared to conventional PERC or TOPCon cells, as it eliminates high-temperature processes and some wet-chemical steps. However, the requirement for precise control of thin-film deposition and the use of TCO materials can increase production costs. Ongoing advancements in deposition techniques and material utilization are expected to reduce these costs over time.

HJT technology also benefits from compatibility with thinner silicon wafers, as the low-temperature process minimizes stress-induced breakage. This is particularly relevant as the industry moves toward thinner wafers to reduce material costs. Some manufacturers are experimenting with wafers as thin as 100 µm for HJT cells, compared to the 160-180 µm thickness commonly used in conventional cells.

The stability and reliability of HJT solar cells have been extensively validated, with field data showing minimal degradation over time. The absence of high-temperature processing steps reduces the likelihood of latent defects that can cause long-term degradation. Additionally, the a-Si:H layers are less susceptible to light-induced degradation compared to bulk c-Si, contributing to stable performance over the lifetime of the module.

In terms of sustainability, HJT cells offer several advantages. The low-temperature manufacturing process reduces energy consumption during production, and the potential for thinner wafers decreases silicon material usage. Furthermore, the high efficiency of HJT modules means that fewer raw materials are required per watt of installed capacity compared to lower-efficiency technologies.

The future development of HJT technology is focused on further improving efficiency while reducing costs. Areas of research include the development of novel TCO materials with higher conductivity and transparency, optimization of the a-Si:H deposition process to increase throughput, and integration of advanced light-trapping schemes. Another promising direction is the combination of HJT technology with advanced interconnection schemes to create higher-voltage modules with reduced resistive losses.

In summary, HJT solar cells represent a sophisticated integration of crystalline and amorphous silicon technologies, delivering high efficiency through excellent passivation and junction quality. Their low-temperature fabrication, compatibility with bifacial designs, and superior temperature performance make them a compelling choice for modern photovoltaic applications. As manufacturing processes mature and scale up, HJT technology is poised to play an increasingly significant role in the global transition to renewable energy.