Research Article External Quantum Efficiency Improvement with Luminescent Downshifting Layers: Experimental and Modelling H. Ahmed, 1 S. J. McCormack, 1 and J. Doran 2 1 School of Engineering, Trinity College Dublin, Dublin, Ireland 2 Dublin Energy Lab, Dublin Institute of Technology, Dublin, Ireland Correspondence should be addressed to H. Ahmed; hahmed@tcd.ie Received 7 October 2015; Accepted 15 December 2015 Academic Editor: Akihide Wada Copyright © 2016 H. Ahmed et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Core-shell quantum dots CdSe/ZnS and lumogen yellow organic dye are characterized by their inclusion in luminescent downshifing (LDS) layers. Layers were deposited on top of crystalline silicon cell (c-Si), dye synthesized solar cell (DSSC), and cadmium telluride (CdTe) minimodules. External quantum efciency measurements for the solar cell/LDS devices are discussed. Experimental results were compared with an optical model developed by Rothemund, 2014. 1. Introduction Luminescent downshifing (LDS) is an optical approach to increase a solar cell’s spectral response by using luminescent materials to convert high energy photons to lower energy before the interaction with the solar cells occurs [1–7]. Te downshifed photons have wavelength which may better match the photosensitivity spectral response of the solar cell as illustrated in Figure 1. A typical LDS consists of lumines- cent species such as organic dyes and quantum dots doped in a transparent polymer sheet applied on top of PV cells [8–13]. 2. Experimental 2.1. LDS Layers and Devices Fabrication. Te luminescent materials used in this investigation were core-shell quantum dots CdSe/ZnS purchased from Cytodiagnostics, Canada, and lumogen yellow dye developed and manufactured by BASF, Germany. A clear polymer poly(methyl methacrylate) (PMMA) (Carl Roth GmbH + Co.KG) was used for the encapsulation of LDS layers. Te LDS layers were prepared as follows. Te QDs/dye were mixed in toluene and sonicated in an ultrasonic bath for 10 minutes. Te mixture was then added to PMMA solution (50 wt%), magnetically stirred for 20 minutes, and then placed in an ultrasonic bath for another 15 minutes. Prepared solutions were drop cast on glass sub- strates and cured for 72 hours at 25 ∘ C under a vacuum of 800 mbar. Uniform layers were obtained when removed from the glass substrate. Te average thickness of the layers was measured by white light interferometer technique and found to be 0.95 ± 0.05 m. Te luminescent quantum yield (LQY) measurements of LDS flms were measured using the integrating sphere method [14, 15] found to be 0.58 ± 0.08 for QD-LDS layer and 0.87 ± 0.08 for dye-LDS layer. UV/Vis/NIR absorption spectroscopy was used to mea- sure the absorption characteristics of the LDS layers investi- gated in this study. Te UV/Vis/NIR absorption spectrom- eter used was a Perkin Elmer Lambda 900. Te emission spectra were measured by optically pumping samples using a monochromated light source using a luminescence spec- trometer (Perkin Elmer LS55B). QD and dye absorption and emission spectra in a PMMA flm (0.09 wt% and 0.9 wt%) are shown in Figures 2 and 3, respectively. Monocrystalline silicon cells (2 × 2 cm, Sunrydz, Ger- many), dye-sensitized solar cells (1 × 1 cm, Solar Print, Ire- land, the details fabrication of which are confdential), and CdTe minimodules (15.24 × 15.24 cm, Advanced Solar Power INC, China) were used for assessing the downshifing efect of the LDS layers. Devices were fabricated by directly Hindawi Publishing Corporation International Journal of Spectroscopy Volume 2016, Article ID 8543475, 7 pages http://dx.doi.org/10.1155/2016/8543475