Multi-junction solar cell

Multi-junction (MJ) solar cells are solar cells with multiple p–n junctions made of different semiconductor materials. Each material's p-n junction will produce electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the cell's sunlight to electrical energy conversion efficiency.

Traditional single-junction cells have a maximum theoretical efficiency of 33.16%.^[2] Theoretically, an infinite number of junctions would have a limiting efficiency of 86.8% under highly concentrated sunlight.^[3]

As of 2024 the best lab examples of traditional crystalline silicon (c-Si) solar cells had efficiencies up to 27.1%,^[4] while lab examples of multi-junction cells have demonstrated performance over 46% under concentrated sunlight.^[5][6][7] Commercial examples of tandem cells are widely available at 30% under one-sun illumination,^[8][9] and improve to around 40% under concentrated sunlight. However, this efficiency is gained at the cost of increased complexity and manufacturing price. To date, their higher price and higher price-to-performance ratio have limited their use to special roles, notably in aerospace where their high power-to-weight ratio is desirable. In terrestrial applications, these solar cells are emerging in concentrator photovoltaics (CPV), but cannot compete with single junction solar panels unless a higher power density is required.^[10]

Tandem fabrication techniques have been used to improve the performance of existing designs. In particular, the technique can be applied to lower cost thin-film solar cells using amorphous silicon, as opposed to conventional crystalline silicon, to produce a cell with about 10% efficiency that is lightweight and flexible. This approach has been used by several commercial vendors,^[11] but these products are currently limited to certain niche roles, like roofing materials.