Full paperAll-Silicone-based Distributed Bragg Reflectors for Efficient Flexible Luminescent Solar Concentrators
Graphical abstract
Introduction
Flexible Luminescent Solar Concentrators (LSCs) have attracted attention as an opportunity to bring integrated photovoltaics to a range of industries from architecture to consumer electronics and leisure [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Flexibility also brings the opportunity of roll-to-roll manufacturing, leading to decreased production costs and retrofitting of existing windows and facades [10,11].
In general, LSCs are composed of transparent polymer sheets doped with randomly oriented luminescent particles, known as fluorophores. The fluorophores absorb incident sunlight, and re-emit it isotropically at longer wavelengths. A portion of the reemitted sunlight is trapped by means of total internal reflection and guided to the edges, where it can be coupled into solar cells and converted into electricity, as shown in Fig. 1a. Due to the energy shift between absorbed and emitted light, LSCs can collect light from a larger acceptance angle than conventional solar cells, and can in theory concentrate light to small areas extremely efficiently [12,13]. The broad freedom of design in colour, transparency, shape and rigidity, makes the devices an ideal PV-module candidate for integration into our everyday lives.
Unfortunately, LSCs are subject to two main sources of loss that have hindered their commercial deployment so far; quantum yield and escape cone losses. Quantum yield losses are caused due to a fraction of the absorbed light not being reemitted by the fluorophores and being lost as heat. Recent advances in new luminescent materials have led to fluorophores with near-unity quantum yields and low re-absorption, including core-shell nanocrystals [[14], [15], [16], [17]], silicon quantum dots [9,18], perovskite quantum-dots [[19], [20], [21]], and quantum-cutting technologies [22].
Whilst significant progress in reducing quantum yield losses has been made, escape cone losses still remain a major barrier particularly for larger area and flexible devices. In this case, light is either re-emitted or scattered in a direction such that it falls upon the interfaces at an angle smaller than the critical angle of the host matrix, resulting in the light escaping the larger faces of the device. The critical angle, , is given by Snell's law and can be calculated by where n is the host matrix refractive index [23]. For typical waveguides made of polymethyl methacrylate (PMMA), the escape cone losses of flat LSCs, with no re-absorption losses or additional sources of scattering, are 25 %. This can be significantly increased in the presence of fluorophore reabsorption, host matrix absorption and scattering [12].
When LSCs are bent, the introduced curvature can contribute additional losses of over 10 % [6,24]. These comprise of both aggravated quantum yield losses, due to longer path lengths, and escape cone losses due to a decrease in waveguiding efficiency. The curvature induced losses are particularly pronounced in the case of materials with overlap in absorption and emission spectra and can pose a significant barrier to viable flexible devices [24].
While in the case of flat LSCs several photonics based solutions have emerged in an attempt to curb escape cone losses, never before has there been an effort to mitigate the extra losses in flexible devices. In the rigid case, one of the most prominent solutions is to use wavelength selective mirrors to increase the waveguiding efficiency (known also as internal optical efficiency and more rigorously defined in the methods section). In the context of LSCs, wavelength selective mirrors are designed to be transmissive in the absorption range of the chosen fluorophore, whilst selectively reflecting light in the wavelength range of its emission. The result is that even light that falls within the escape cone of the host matrix is kept within the LSC, boosting the internal optical efficiency. There are currently several designs for rigid configurations. In one example, authors suggested the use of cholesteric liquid crystals [25,26]. Another promising wavelength selective mirror technology is the application of Distributed Bragg Reflectors (DBRs), which leverage thin-film interference effects to engineer suitable passbands and stopbands in their optical response [27]. Their potential has been explored theoretically [[28], [29], [30]], and demonstrated experimentally with spin-coated SiO2/SnO2 DBRs on dye-doped LSCs [31], and with DBRs on CdSe/CdS quantum dot doped LSCs [32]. There has also been a demonstration of a curved DBR, but this was applied to a flat, rigid, PMMA LSC which could not be deformed or reshaped [33].
In this paper we demonstrate a unique, all-flexible LSC-DBR design that allows for virtually any freeform LSC shape to be achieved. Both LSC and DBR systems consist entirely of silicone-based materials, resulting in mechanical consistency and compatibility throughout the device. Because of the elastomeric properties and compatibility of the chosen materials, we show that the device can be repeatedly deformed and reshaped without its performance being affected. We exhibit excellent control of the optical properties of our DBRs by engineering both the thin-film thickness and refractive index contrast, allowing us to tailor their properties precisely to partner our LSCs. With an eye to future scaling up, the DBRs, pictured in Fig. 1c, are fabricated using industrially available, single-pot solution-based processes. Upon integration with an LSC, as shown in Fig. 1b, we demonstrate a reduction of device escape cone losses by a quarter, as well as a significant reduction of efficiency dependence on curvature. Furthermore we analyse the performance of large scale LSCs by using Monte-Carlo statistical models demonstrating a clear pathway to real consumer products.
Section snippets
Thin film materials
To obtain flexible DBRs with high peak reflectance we sought to use two materials with a series of requirements. We aimed for the optical properties of the materials to provide high refractive index contrast, low absorption and low scattering. The materials should also have good adhesion and mechanical compatibility, alongside flexibility.
Our flexible DBRs are composed of alternate layers of two polydimethylsiloxane (PDMS) based materials with refractive index contrast across the entire
Conclusions
We have demonstrated for the first time an all flexible LSC-DBR combination formed entirely of silicone based components. By creating DBRs comprised of hPDMS and a titania-PDMS composite we achieve devices with strong adhesion and flexibility throughout. The excellent control we exhibit in the optical properties and thickness of the layers in the thin films allows us to design flexible DBRs with high and precisely engineered reflectances. Our DBRs display high uniformity, low scattering and
Materials and methods
A graphic summarising the fabrication steps of the LSC-DBR combination can be found in the Supplementary Materials, Section 5.
Author contributions statement
M.P. and I.P. conceived the experiments. M.P. developed and fabricated the samples and performed characterisation experiments. M.P. developed the modelling system. T.J.M. performed surface chemistry experiments. C.S. and M.P. performed ellipsometric measurements and fitting. M.P. T.S.R. and J.S. contributed to the development of the PDMS - TiO2 composite. T.L. developed and performed silver mirror deposition. S.G., I.P.P and I.P. facilitated the work and engaged with experiment design. All
Declaration of competing interest
The authors declare no competing interests.
Acknowledgements
We would like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for a Doctoral training award grant no: 1632762. The work was supported by a H2020 European Research Council (ERC) starting grant “IntelGlazing” grant no: 679891. T.J.M would like to thank the Ramsay Memorial Trust and the Royal Commission for the Exhibition of 1851 for their financial support.
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