Gold Nanorods Coated Metallic Photonic Crystal for Enhanced Hot Electron Transfer in Electrochemical Cells
We recently demonstrated a sub-bandgap photo response with our wafer-scale Au/TiO2 metallic-semiconductor photonic crystals (MSPhC). The sub-bandgap energy with 590 nm peak could be absorbed in the form of hot electron and injected to TiO2, which provides 5.28 times more energy for photolysis than that of energy absorbed to flat TiO2. If the solar energy already absorbed above 700 nm could be injected to the catalyst, higher than 10 times improvement will be achieved, and above 20% solar to fuel efficiency will be feasible with the robust but inefficient TiO2 catalyst. In order to achieve photocurrent near and above 700 nm spectrum, we deposited gold nanorods on the surface of MSPhC to incur localized surface Plasmon (LSP) modes absorption and subsequent injection to the TiO2 catalyst.
We used electrophoretic deposition (EPD) method to deposit nanorods on the top, sidewall and bottom well surface of the photonic nanocavities. The deposition of nanorods was achieved reasonably uniform and sparse not to block the optical cavities of MSPhC. Flat gold surfaces were tested at 4 different suspension densities to get the optimum gold nanorods density. Under 10V applied electric field, positively charged gold nanorods at the concentration of 6.52×1013 #/mL could deposit MSPhC surface with the density of 230 #/µm2, which was reasonably uniform and sparse. Preliminary tests show an absorbance increase near 700 nm on flat device coated with gold nanorods. Photocurrent measurement is under way to demonstrate the enhanced hot electron transfer over full visible light and near-infrared solar spectrum.
Photonic Crystal OPV with Al2O3 Nano-cylinders
Prof Sang-Gook Kim’s group has shown that Al2O3 nano-cylinder photonic crystal structures can improve broad-band absorption due to enhancement in the cavity, waveguide and surface plasma polariton modes of the photonic crystal. Here we would like to make use of the photonic crystal to improve the absorption of organic materials in organic photovoltaic (OPV) devices. Typically, materials used in high efficiency OPV have numerous stringent requirements. Not only the absorption needs to be high across the solar spectrum, there are other factors like charge mobility, energy levels, film quality, solvent compatibility etc. that needs to be considered. This ultimately, restricts the number of possible candidates for high efficiency OPV. In our proposed device, we hope that the requirements on charge mobility and absorption can be relaxed due to the enhancement in absorption. This is a proof of concept and the idea can be extended to other semiconducting materials like quantum dots, perovskites.
A second device structure that we would like to explore is to use the nano-structured pdms patterned from the photonic crystal mold as the substrate. As the pdms is flexible and stretchable, the optical property of the device can be explored under various strains on the photonic crystal.
High Efficiency Photovoltaic Devices using Nanocavity Array
We have demonstrated in our earlier results that metal-dielectric nano-cavity arrays can improve broad-band light absorption due to the enhancement in the cavity, waveguide and surface plasma polariton resonance modes around the nanocavity. This project is to utilize this absorber structure for applications in photovoltaic devices with soft absorber materials like organic and perovskite materials, which have limited carrier lifetime and thereby requires ultra thin film thickness for efficiency charge extraction. However light absorption is insufficient if the absorber layer is too thin, and a compromise between complete light absorption and charge extraction is often needed, resulting in mediocre energy harvesting efficiencies.
The nanocavity array we fabricated can alleviate the need for thicker absorber film by enhancing broad-band absorption in the absorber material. Finite Difference Time Domain (FDTD) simulation shows that the absorption in a Poly(3-hexylthiophene-2,5-diyl):Phenyl-C61-butyric acid methyl ester (P3HT:PCBM) film is enhanced by the nanocavity structure. Most of the visible light is absorbed, leaving a black appearance on the surface of the substrate. A prototype device is under fabrication to demonstrate much higher solar energy harvesting efficiency than most reported yet.
Publications from MIT research
1. A. Elfaer, X. Li, Y. Wang, J.B. Chou, and S.G. Kim, “Gold Nanorods Coated Metallic Photonic Crystal for Enhanced Hot Electron Transfer in Electrochemical Cells,” MRS Advances, in press.
2. Chou, J.B., D. P. Fenning, Y. Wang, M. A.M. Polanco, J. Hwang, F. Sammoura, J. Viegas, M. Rasras, A. ElFaer, A. Kolpak, Y. Shao-Horn, S.G. Kim “Broadband Photoelectric Hot Carrier Collection with Wafer-Scale Metallic-Semiconductor Photonic Crystals”, 42th IEEE Photovoltaic Specialist Conference, New Orleans, 2015.
3. X. Li, Y. Wang, A. Elfaer, J. Hwang, J. B. Chou, S.G. Kim, "Surface Plasmon Assisted Hot Electron Collection in Wafer-scale Metallic-Semiconductor Photonic Crystals," to be submitted.
1. X. Li, A. Elfaer, Y. Wang, J.B. Chou, and S.G. Kim, “"Metallic-Semiconductor Photonic Crystals for Solar Water Splitting",” MTL Annual Research Conference, Jan. 2016.
2. Elfaer, Y. Wang, J.B. Chou, and S.G. Kim, “Gold Nanorods Coated Metallic Photonic Crystal for Enhanced Hot Electron Transfer in Electrochemical Cells,” MRS Fall Meeting, Boston, 2015.
3. Y. Wang, X. Li, A. Elfaer and S.G. Kim, “Full Spectrum Solar Energy Water Splitting for Storable Fuel Generation,” Masdar Institute and MIT Collaborative Research Workshop, Cambridge, December, 2015.
1. M.I.T. Serial Number: 62/181275, “Nanostructure Coated Photonic Crystals for Hot Carrier Generation and Transfer”, by Asmaa El-Faer*, Sang-Gook Kim, David Fenning, Yu Wang and Jeffrey Chou, June 18, 2015.