Heterojunction solar cell

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 second-most adopted commercial solar cell technology after conventional crystalline PERC/TOPCon (Passivated Emitter Rear Cell/Tunnel Oxide Passivated Contact), 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 (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.