Efficiency limits and design principles for multi-junction coloured photovoltaics

Building-integrated photovoltaics are an emerging technology with the potential to become more widely adopted as the installed capacity of solar photovoltaics (PV) continues to increase. For building and product integration, the ability to produce cells in colours other than the standard black or dark blue appearance of crystalline silicon panels plays an important role in improving the visual appearance and promoting the adoption of PV. However, to maximize power output from limited areas and use resources in the most sustainable way possible, keeping efficiency high is critical; coloured multi-junction solar cells, which can reach higher power conversion efficiencies than single-junction devices, thus become a technology platform of interest. We report the theoretical maximum possible efficiencies for coloured two-terminal solar cells with up to six junctions in the detailed balance limit, with colour produced through reflection of incident Sunlight. A wide range of colours with a relative luminance up to Y = 0.6 can be produced with <20% maximum power loss compared to a black cells. In most cases, except for colours with very high relative luminance Y ≳ 0.85 (i.e. colours which are closer to white), a two-junction coloured cell has a higher limiting efficiency than a black single-junction cell, showing the potential of coloured multi-junction cells for applications where the aesthetics of solar panels play an important role. We find that crystalline silicon provides an excellent platform as the bottom cell in developing two and three-junction coloured cells, for example in combination with perovskites or III-V semiconductor alloys with tuneable bandgaps. In addition to reporting limiting efficiencies, we report the optimal reflectance spectra and bandgap placement for achieving a range of colours, and the trends observed in these variables for colours of varying hue and relative luminance. A numerical optimization method using multi-objective differential evolution, a type of evolutionary algorithm, was used to calculate the limiting efficiencies; the method and code used to produce the results presented are made freely available.