Heterojunction solar cell

Heterojunction solar cells, variously known as Silicon heterojunctions or Heterojunction with Intrinsic Thin Layer, are a family of photovoltaic cell technologies based on a heterojunction formed between semiconductors with dissimilar band gaps. They are a hybrid technology, combining aspects of conventional crystalline solar cells with thin-film solar cells.
Silicon heterojunction-based solar panels are commercially mass-produced in high volumes for residential and utility markets., Silicon heterojunction architecture has the highest cell efficiency for mass-produced silicon solar cells. In 2022–2024, SHJ cells overtook Aluminium Back surface field solar cells in market share to become the second-most adopted commercial solar cell technology after conventional crystalline PERC/TOPCon, increasing to up to 10% market share by 2032.
Solar cells operate when light excites the absorber substrate. This creates electron–hole pairs that must be separated into electrons and holes by asymmetry in the solar cell, provided through chemical gradients or electric fields in semiconducting junctions. After splitting, the carriers travel to opposing terminals of the solar cell that have carrier-discriminating properties. For solar cells to operate efficiently with a low probability of mutual annihilation of the carriers, absorber substrates and contact interfaces require protection from passivation to prevent electrons and holes from being trapped at surface defects.
SHJ cells generally consist of an active crystalline silicon absorber substrate which is passivated by a thin layer of hydrogenated intrinsic amorphous silicon, and overlayers of appropriately doped amorphous or nanocrystalline silicon selective contacts. The selective contact material and the absorber have different band gaps, forming the carrier-separating heterojunctions that are analogous to the p-n junction of traditional solar cells. The high efficiency of heterojunction solar cells is owed mostly to the excellent passivation qualities of the buffer layers, particularly with respect to separating the highly recombination-active metallic contacts from the absorber. Due to their symmetrical structure, SHJ modules commonly have a bifaciality factor over 90%.
As the thin layers are usually temperature sensitive, heterojunction cells are constrained to a low-temperature manufacturing process. This presents challenges for electrode metallisation, as the typical silver paste screen printing metallisation method requires firing at up to 800 °C; well above the upper tolerance for most "buffer layer" materials. As a result, the electrodes are commonly composed of a low curing temperature silver paste, or uncommonly a silver-coated copper paste or electroplated copper.

History

The heterojunction structure, and the ability of amorphous silicon layers to effectively passivate crystalline silicon has been well documented since the 1970s. Heterojunction solar cells using amorphous and crystalline silicon were developed with a conversion efficiency of more than 12% in 1983. Sanyo Electric Co. filed several patents pertaining to heterojunction devices including a-Si and μc-Si intrinsic layers in the early 1990s, trademarked "heterojunction with intrinsic thin-layer". The inclusion of the intrinsic layer significantly increased efficiency over doped a-Si heterojunction solar cells through reduced density of trapping states, and reduced dark tunnelling leakage currents.
Research and development of SHJ solar cells was suppressed until the expiry of Sanyo-issued patents in 2011, allowing various companies to develop SHJ technology for commercialisation. In 2014, HIT cells with conversion efficiencies exceeding 25% were developed by Panasonic, which was then the highest for non-concentrated crystalline silicon cells. This record was broken more recently in 2018 by Kaneka corporation, which produced 26.7% efficient large area interdigitated back contact SHJ solar cells, and again in 2022 and 2023 by LONGi with 26.81% and 27.09% efficiency respectively. In 2023, SHJ combined with Perovskite in monolithic tandem cells also recorded the highest non-concentrated Two-junction cell efficiency at 33.9%.
In 2023, heterojunction solar panels have been fabricated with efficiencies up to 23.89%.
SHJ solar cells are now mass-produced on the gigawatt scale. In 2022, projects planned for the establishment or expansion of SHJ production lines totaled approximately 350 GW/year of additional capacity. Over 24 manufacturers are beginning or augmenting their heterojunction production capacity, such as Huasun, Risen, Jingang, LONGi, Meyer Burger and many more.

Utility scale projects

In early 2022, a 150 MW heterojunction solar farm was completed by Bulgarian EPC company Inercom near the village of Apriltsi in Pazardzhik Province, Bulgaria—the largest HJT solar farm at the time, according to a press release by module supplier Huasun. In 2023, the same supplier announced a further 1.5 GW supply deal of HJT modules to Inercom.

Advantages

Performance

Efficiency and voltage

SHJ has the highest efficiency amongst crystalline silicon solar cells in both laboratory and commercial production. In 2023, the average efficiency for commercial SHJ cells was 25.0%, compared with 24.9% for n-type TOPCon and 23.3% for p-type PERC. The high efficiency is owed mostly to very high open-circuit voltages—consistently over 700 mV—as a result of excellent surface passivation. Since 2023, SHJ bottom cells in Perovskite tandems also hold the highest non-concentrated Two-junction cell efficiency at 33.9%. Due to their superior surface passivation, heterojunction cells generally have a lower diode saturation current density than other silicon solar cells, allowing for very high fill factor and voltage; and hence record high efficiency.

Bifaciality

Bifaciality refers to the ability of a solar cell to accept light from the front or rear surface. The collection of light from the rear surface can significantly improve energy yields in deployed solar arrays. SHJ cells can be manufactured with a conductive ARC on both sides, allowing a bifaciality factor above 90%, compared to ~70% for PERC cells with rear grid. Bifacial solar modules are expected to significantly increase their market share over monofacial modules to 85% by 2032.

Lifespan

By virtue of their high bifaciality, silicon heterojunction modules can exploit more advantages of glass–glass module designs compared to other cell technologies. Glass–glass modules using EPE encapsulant are particularly effective in preventing water ingress, which is a significant cause of performance degradation in PV modules. When used with the appropriate module encapsulant, a glass–glass SHJ module is generally expected to have an operational lifespan of over 30 years; significantly longer than a glass–polymer foil backsheet. Glass–glass modules are heavier than glass–polymer foil backsheet modules, however due to improvement in tempered glass technology resistance to impact damage and module designs, the glass thickness is expected to reduce, with the mainstream tending from 3.2 mm towards 2 mm or less in the 2030s. As a result, glass–glass modules are expected to become the dominant PV technology in the mid 2020s according to ITRPV.
For example, utility scale 680 W heterojunction modules with a 30-year performance derating of 93% were announced by Enel in 2022.

Temperature coefficient

The temperature coefficient refers to how the output power of a solar module changes with temperature. Typically, solar modules see a reduction in output power and efficiency at elevated temperatures. From lab testing and supplier datasheet surveys, modules fabricated with SHJ cells consistently measure an equal or lower temperature coefficient compared with Al-BSF, PERC, PERT and hybrid PERT/rear-heterojunction solar cells. This applies to a range of parameters, including open-circuit voltage, maximum power point power, short circuit current and fill factor. The temperature sensitivity of solar cells has been inversely correlated to high open-circuit voltages compared to the absorber band gap potential, as noted by Martin Green in 1982; "As the open-circuit voltage of silicon solar cells continues to improve, one resulting advantage, not widely appreciated, is reduced temperature sensitivity of device performance". Thus the low temperature sensitivity of SHJ cells has been attributed to high from well passivated contacts.

Manufacturing

Energy consumption

SHJ production lines fundamentally do not use high temperature equipment such as diffusion or metal paste curing furnaces, and on average have a lower power consumption per watt of fabricated cells. According to China PV Industry Development Roadmap, in 2022, the average electricity consumption of n-type Heterojunction cell lines was 47,000 kWh/MW, whereas p-type PERC production lines consumed about 53,000 kWh/MW and for n-type TOPCon, about 56,000 kWh/MW. It is estimated that by 2030, the power consumption of n-type Heterojunction, p-type PERC and n-type TOPCon cell production lines will drop to 34,000 kWh/MW, 35,000 kWh/MW and 42,000 kWh/MW respectively. A 2014 study estimated the energy payback time of a SHJ module to be 1.5 years, compared to 1.8 years for a regular monocrystalline module; this figure was estimated to drop to 0.94 years vs. 1.2 years respectively for a prospective module in 2020 assuming 25% efficiency. Similarly, the life-cycle CO2-equivalent emissions per kWh for 2020 SHJ modules is estimated to be 20 grams vs 25 grams for a regular monocrystalline module.

Silicon consumption

Crystalline silicon wafers used in solar cells typically have a thickness between 130 and 180 μm. The mass of consumed silicon wafer comprises a significant proportion of the cost of the solar module, and as such reducing the wafer thickness has potential to achieve significant cost reduction. Fewer photons are absorbed in thinner silicon. However, as long as surface recombination is effectively suppressed, thinner wafers can maintain—or even increase open-circuit voltages. That is, the increase in open-circuit voltage may compensate for losses in short-circuit current due to thinner wafer usage. Thinner wafers fundamentally increase open-circuit voltage, as a greater proportion of recombination occurs in the bulk of the substrate if surfaces are well passivated, therefore reducing the thickness reduces the quantity of bulk defects. As SHJ cells have excellent surface passivation, reduction in their wafer thickness is more feasible than with other crystalline silicon solar cell technologies. As such, high efficiencies have been reported over a large range of wafer thicknesses, with the minimum on the order of 50 μm. On commercial-grade n-type substrates, the optimum thickness is estimated to be 40–60 μm. This advantage is not seen in technologies with non-passivated contacts or poor surface recombination such as PERC, in which the optimum thickness is greater than 100 μm.