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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 dissimilar semiconductors. They are a hybrid technology, combining aspects of conventional crystalline solar cells with thin-film solar cells. Silicon heterojunction architecture has the highest cell collection efficiency for commercial-sized silicon solar cells.

SHJ cells generally consist of an active crystalline silicon absorber substrate passivated by a thin layer of intrinsic amorphous or nanocrystalline silicon (the "buffer layer") and appropriately doped amorphous selective contacts. The buffer layer material and the substrate have different band gaps, forming the heterojunction that is analogous to the p-n junction of traditional solar cells. The high efficiency of heterojunction solar cells is owed to the excellent passivation qualities of such intrinsic buffer layers. Although intrinsic buffer layers are effectively non-conductive, charge carriers can tunnel through as the thickness is typically less than 10 nm. It is advantageous for the passivating layer to have a higher band gap in order to minimise parasitic absorption of photons, as absorption coefficient is partially dependent on band gap.

Heterojunction cells are commercially mass produced and are commonly bifacial. 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 method requires firing at up to 800°C; well above the upper tolerance for most buffer layer materials. As a result, the electrodes are composed of a low-temperature silver 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 supressed 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, which was then the highest for single junction 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 by LONGi with 26.8% efficiency. As of 2023, this is the highest recorded efficiency for monojunction silicon solar cells.