Solar concentrators use mirrors and lenses to capture light and direct it towards smaller areas of photovoltaic (PV) material where the solar energy is converted into electricity1. In this way the cost of the overall system is reduced by decreasing the area of photovoltaic material required which is typically the most expensive part of a PV solar panel1,2. However, the introduction of these optical devices to focus light onto these solar cell(s) can result in very bulky systems. Although solar concentrators can reduce solar energy costs and improve efficiencies, their weight and size therefore often limits their deployment3,4. Current solar concentrators vary widely in design and even the simple polishing of metal can result in a reflective mirror finish but such polished surfaces are very heavy and specific curved shapes are difficult and therefore expensive to manufacture5,6. Reflective film adhered to plastic mirrors is a second option but this setup often has low reflectivity when applied to complex surfaces6. Polymer mirror films are a more recent third method to gain reflectance values of >90% but require specially designed structures to gain the appropriate shapes for a given application7,8. Vacuum metalizing is therefore the current best option but this process is highly dependent on the material and surface quality it is bonded with in order to ensure a high quality mirror finish5,9. Given the limitations of all existing systems, further study into possible lightweight reflective materials and structures is important. The benefits of a lightweight, easily applied reflective material or coating would not only improve the development of solar concentrator technologies but may also be beneficial to many other disciplines where lightweight highly reflective coatings are desirable.

The white butterflies of the genus Pieris take flight before other butterflies on cloudy days when solar inputs to flight muscle warming are limited. This ability to heat up quickly on cloudy days has been anecdotally suggested to relate to the V-shaped posture they adopt whilst basking in cloudy conditions, a process we here term ‘reflectance basking’. These white butterflies do indeed show high wing reflectance based upon a unique display of pterin containing nano-beads within their individual wing scales as extensively reported by Stavenga et al.10,11,12, Giraldo et al.13,14 and Morehosue et al.15. Luke et al.16 expand on this descriptive work by removing the pterin beads and showing that overall reflectance is decreased by a third in the absence of the beads themselves. The precise arrangement of the pterin beads within the scale cell appears critical as it shows a quasi-random pattern that has recently been proposed to be optimum for efficient light manipulation17.

Here we therefore investigate if the wings, or some derivation thereof, of the white Pieris butterflies can be used to develop a novel, lightweight reflective material directly applicable to solar concentrators. To investigate if a consideration of the photonics of butterfly wings is indeed useful in solar concentrator design we chose to first answer five specific questions. First, can we prove practically that the butterflies concentrate light and indeed heat, onto their thorax? Second, is there an optimum angle with which they accomplish this and which we would therefore have to adhere to in solar concentrator design? Third, does the light reflected by the butterfly wings themselves actually match the input requirements of any given photovoltaic solar cell? Fourth, can whole butterfly wings thus be used directly to increase the output from a given solar cell? Finally, can specific sub-structures from the wing (e.g. a mono-layer of removed scale cells) or bead-like coatings (e.g. a coating of nano-beads with the same orientation and properties of the pterin beads) be used to achieve similarly improved solar cell outputs?

Butterfly wings are in fact surprisingly complex as butterflies not only have pairs of wings that are effectively linked in flight (and overlap at rest) but the scale cells on their wings also show dramatically different morphologies and orientations. Further, these scale cells can exist as complex overlapping layers therefore potentially conferring complex overall optical properties on the whole wing, as detailed extensively by the work of Vukusic et al.18,19,20 and also by Kolle et al.21. Such complex naturally occurring structures can be used for various modern applications in a process known as ‘biomicry’22,23,24, however no studies have yet examined the Pieris wing structures as a basis for reflective materials in solar photovoltaic concentrators. Johnsen and Widder25 showed that the pterin bead size is optimized for light scattering and that the two types of wing scales (‘cover’ and ‘ground’ scales) together produce wide-angle scattered light. Stavenga and co-workers also argue that to gain the full reflectance from the pierid wing a complete model including all components of the wing structure would be required. This would initially suggest that a single layer of scale cells or a thin coating of nano-beads correctly orientated would have insufficient optical performance to enhance inputs to a solar cell. One of the central aims of the research described here was therefore to see if a mono-layer of scale cells could recapitulate the reflective properties of the whole wing. Surprisingly, here we show that wings from the large white butterfly do indeed increase the efficiency of photovoltaic cells when the wings are held at a critical optimal angle for the concentration of both heat and light. Further, this whole wing configuration not only dramatically increases the power to weight ratio of the butterfly-solar cell structure but critically similar reflective properties can be achieved from a single mono-layer of removed scale cells. This work suggests that scale cell-like structures or indeed just coatings of correctly oriented nano-beads may be useful in even more lightweight coatings.