Heterojunction solar cells (HJT), variously known as Silicon heterojunctions (SHJ) or Heterojunction with Intrinsic Thin Layer (HIT), 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. As of 2023, Silicon heterojunction architecture has the highest cell efficiency for mass-produced silicon solar cells. In 2022–2024, SHJ cells overtook Aluminium back surface field (Al-BSF) solar cells in market share to become the third-most adopted commercial solar cell technology after conventional crystalline PERC (Passivated Emitter Rear Cell) and TOPCon (Tunnel Oxide Passivated Contact), with a projected increase to up to 10% market share by 2032. It is not expected to outgrow these technologies due to having a similar theoretical efficiency ceiling to TOPCon but higher production costs. Solar cells operate when light excites the absorber substrate. This creates electron–hole pairs that must be separated into electrons (negative charge carriers) and holes (positive charge carriers) 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 (known as selective contacts). For solar cells to operate efficiently with a low probability of mutual annihilation of the carriers (recombination), 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 (denoted as a-Si:H; the "buffer layer"), 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. (now a subsidiary of Panasonic Group) 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" (HIT). 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 (IBC) 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 (mostly Chinese) manufacturers are beginning or augmenting their heterojunction production capacity, such as Huasun, Risen, Jingang (Golden Glass), 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 (world record efficiency) and commercial production (average efficiency). 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 (such as TOPCon), allowing for very high fill factor and voltage; and hence record high efficiency.
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