Luminescent Solar Concentrators: Nanorods and Raytrace Modeling R.Bose, D.J.Farrell, A.J.Chatten, A.Büchtemann, J.Quilitz, A.Fiore, L.Manna and K.W.J.Barnham

33 rd PVSC San Diego Imperial College London Overview Introduction Making PV energy more cost-effective How the Thin Film LSC works… Why Thin Films? Nanorod LSCs Nanorod Characteristics Spectral Measurement Short-Circuit Current Measurement Modeling The Raytrace Model Fitting Nanorod Parameters Investigating the Self-Absorption

33 rd PVSC San Diego Imperial College London Making PV energy more cost-effective The LSC is inexpensive compared to PV cells Flat concentrator Static (requires no solar tracking) Collects direct and diffuse irradiation Emission can be matched to the PV cells attached Well suited for building integrated PV

33 rd PVSC San Diego Imperial College London How the Thin Film LSC works… Thin Film Substrate Luminescent center Light collection over large surface and emission out of small edges

33 rd PVSC San Diego Imperial College London Why Thin Films? More convenient fabrication Thin films allow for a flexible choice of substrates Thin film LSCs perform just as well as homogeneously doped LSCs Increased re-absorption losses in the optically dense film are balanced by reduced losses in the clear substrate Supported by experimental and computational results [1] Small separation between luminescent centers in the film can be utilized for Förster/fluorescence resonance energy transfer (FRET), which can lead to significant reduction of loss mechanisms [1]R. Bose, K.W.J. Barnham et al, 22 nd EUPVSEC, Milan (2007)

33 rd PVSC San Diego Imperial College London Nanorod LSCs Fabricated by A. Büchtemann and J. Quilitz at the Fraunhofer IAP Imperial College London

33 rd PVSC San Diego Imperial College London Nanorod Characteristics Core-shell nanorods grown at the National Nanotechnology Laboratory of CNR-INFM [2]L. Carbone, L. Manna et al, Nano Lett. 7, 2007, pp. 2942

33 rd PVSC San Diego Imperial College London Nanorod Characteristics Nanorod aspect ratio: 4 (5nm x 20nm) Luminescence quantum efficiency (QE): ~ 70% Little self-absorptions expected Anisotropic emission expected Maximal in plane perpendicular to long axis [2]L. Carbone, L. Manna et al, Nano Lett. 7, 2007, pp. 2942

33 rd PVSC San Diego Imperial College London Spectral Measurement Homogeneous 40x13x4 mm 3 Thin Film 1 / 2 50x50x3 mm 3 +9 µm / 15 µm

33 rd PVSC San Diego Imperial College London Short-Circuit Current Measurement Experimental method LSC under fairly uniform illumination from a lamp Photodetector used to measure incident light over a grid of points Same detector used to scan the emission from one edge Photon count deduced using the known detector response [3] A.J.Chatten, K.W.J.Barnham et al., Semi­conductors 38, 2004, pp. 909 LSC Incident light Photodetector

33 rd PVSC San Diego Imperial College London Incident Light Short-circuit current [µA]

33 rd PVSC San Diego Imperial College London Incident Light Short-circuit current [µA]

33 rd PVSC San Diego Imperial College London Spectra

33 rd PVSC San Diego Imperial College London Results HomogeneousThin Film 1Thin Film 2 I SC 1.39 µA 2.15 µA Photon fraction1.28 %0.26 %0.40 % Relative error± 3.8 %± 21.0 %± 2.2 %

33 rd PVSC San Diego Imperial College London The Raytrace Model Versatile model using geometrical optics Absorption/EmissionWavelength DirectionReflection/Refraction Monte Carlo Method

33 rd PVSC San Diego Imperial College London Fitting Nanorod Parameters Nanorod parameters to fit Fundamental emission spectrum Luminescence quantum efficiency (QE) Procedure Modeled the LSCs and the experimental setup Modeled photodetector using measured angular response and QE Used short-circuit current data and measured PL spectra Adjusted fundamental emission and QE until match with experimental data from PL and short-circuit current measurements

33 rd PVSC San Diego Imperial College London Fitting the Fundamental Emission Example: Thin Film 2

33 rd PVSC San Diego Imperial College London Fitting the Quantum Efficiency Homogeneous LSC QE of 67±4% In good agreement with value of ~70% for nanorods with this aspect ratio in solution Thin film LSCs Worse performance (effective QEs ~40% and ~30%) Expect homogeneous and thin film LSCs to be similar in general Lower output could be due to agglomeration of nanorods and macroscopic defects in the film

33 rd PVSC San Diego Imperial College London Investigating the Self-Absorption Do nanorods have less self-absorption than quantum dots? Absorption coefficients fitted to give equal absorption of incident light

33 rd PVSC San Diego Imperial College London Investigating the Self-Absorption Modelled a 1m x 1m x 3mm LSC under AM1.5 irradiation with Quantum Dots (commercially available green QDs) and with the nanorods (homogeneous) Both with the same amount of absorption of incident (relative difference < 0.5% ) Results QD LSC has ~7% more re-absorptions than the NR LSC The effect on the edge emission is significant – NR: 1.12% of incident light ( nm) – QD: 0.65% of incident light ( nm)  Nanorods show reduced re-absorption losses.

33 rd PVSC San Diego Imperial College London Conclusion Nanorods were successfully incorporated in LSCs The spectra in the LSC matrix were comparable to those in solution A short-circuit current measurement was carried out The LSCs were simulated with the raytrace model The fundamental emission spectra and QEs were fitted The QE of 67% for the homogeneous LSC was in good agreement with the quoted QE for rods with the given aspect ratio in solution The performance of the thin film samples was worse than predicted, possibly due to defective films Nanorods are promising as they show reduced re-absorptions

33 rd PVSC San Diego Imperial College London Thank you.

33 rd PVSC San Diego Imperial College London Rahul Bose