COMMUNICATION 1705382 (1 of 7) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Polymer Encapsulants Incorporating Light-Guiding Architectures to Increase Optical Energy Conversion in Solar Cells Saeid Biria, Fu Hao Chen, Shreyas Pathreeker, and Ian D. Hosein* S. Biria, F. H. Chen, S. Pathreeker, Prof. I. D. Hosein Department of Biomedical and Chemical Engineering Syracuse University 130 Smith Drive, 329 Link Hall, Syracuse, NY 13116, USA E-mail: idhosein@syr.edu The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201705382. DOI: 10.1002/adma.201705382 functionalities of the waveguide ensemble whereby significant modulations to light transmission may be attained [5] to poten- tially increase and concentrate the delivery of optical energy, which would increase conversion in solar cells. Such arrays are also effective under nonhomogeneous illumination conditions, as evidenced by previous studies on their imaging func- tionalities (i.e., spatially varying optical intensity profiles). [6] The waveguides are produced by irradiating a binary mixture of photoreactive monomers with a peri- odic array of microscale optical beams. Each beam initiates light-induced self- writing (LISW) of a microscale optical waveguide. [7] Concurrent photopolymerization-induced phase separation, whose dynamics are coaxial to the beam path, estab- lishes the high-index core, low-index cladding profile necessary to guide light. [7a,8] The performance of a silicon solar cell encap- sulated with this structure shows an increase in the external quantum efficiency (EQE) for both normal and nonnormal incidence irradiation, the latter of which indicates enhanced wide-angle collection that is critical to capturing sunlight over the course of a day and across seasons. To the best of our knowledge, this is the first proof-of-concept demonstration of processing broadband optical waveguide arrays for solar energy collection, and constitutes a new approach for achieving wide- angle solar energy conversion, and opens opportunities for significant improvements in cell performance. This approach is different than employing nanopillar arrays to excite specific optical modes to transfer optical energy to the cell. [9] Rather, in our approach, optical energy is collected and transmitted in multimodal form within the cylindrical cores of waveguides toward the cell. Figure 1a shows a schematic of the architecture, as our strategy to enhance optical energy collection. A commercial sil- icon solar cell is first “primed” with a thin layer of transparent silicone to submerge the contacts and provides a flat surface over which to lay an encapsulation structure comprising the waveguide array (hereon referred to simply as “structure”). The general synthetic approach for the structure is to irradiate a mixture of two photoreactive polymers with a collimated blue light-emitting diode (LED) (Figure 1b). We selected a blend of trimethylolpropane triacrylate (TMPTA) and an epoxide- terminated polytetramethylene oxide (PTMO), as both provide good transparency, chemical and environment resilience, and thermal stability. This system is also generalizable to other The fabrication of a new type of solar cell encapsulation architecture com- prising a periodic array of step-index waveguides is reported. The materials are fabricated through patterning with light in a photoreactive binary blend of crosslinking acrylate and urethane, wherein phase separation induces the spontaneous, directed formation of broadband, cylindrical waveguides. This microstructured material efficiently collects and transmits optical energy over a wide range of entry angles. Silicon solar cells comprising this encapsulation architecture show greater total external quantum efficiencies and enhanced wide-angle light capture and conversion. This is a rapid, straightforward, and scalable approach to process light-collecting structures, whereby significant increases in cell performance may be achieved. Waveguide Arrays In the streamlined, low-cost solar cell industry, proposed approaches to increase solar energy conversion must be fea- sibly integrated into construction of modules. Toward this end, effective means of managing light transmission and delivery to the active regions (surfaces) of solar cells is quite attractive, and can be achieved through sophisticatedly structured coatings, particularly as encapsulation materials. [1] Encapsulants overlay the surface of a cell and do not affect its inherent operation, but rather modulate the transmission characteristics of incident light to facilitate maximum conversion in the underlying cell. The encapsulation procedure is one of the final steps in module processing, thereby placing successfully developed coatings closest to market realization, as opposed to methods that fun- damentally change composition and structure of the actual cell itself. There have been numerous approaches to engineer encapsulant structures to achieve greater light collection and resultant conversion efficiencies. Examples include coatings, [2] diffraction and diffuse layers, [3] geometric optical structures, [3a] and cloaking schemes. [1,4] Herein, we report on a significant increase in optical energy conversion in solar cells by employing a thin-film encapsulation comprising a periodic array of broadband optical waveguides. The thin film inherits the light-collecting and light-guiding Adv. Mater. 2018, 1705382