a. MIM withe suface plasmon excitation. b. Without.
c. (Black) Photocurrent with surface plasmons. (Red) Without.
(Via Melosh ad Wang with the American Chamical Society.)
Solar cells the only game in town? Stanford University's Fuming Wang and Nicholas Melosh may be bringing something different to the game of light to electricity.
Converting light into electricity is nothing new. Photovoltaic (PV) solar cells, solar-thermal collectors and rectannas (Microwave energy to DC) are tried and true technology being used worldwide. The pair are challenging the status quo with a new way to convert infrared (IR) and visible-light wavelengths into direct current by using the surface of the plasmon excitations in an easy to fabricate noncomplex metal-insulator-metal (MIM).
The MIM device sits back and absorbs photons from both the top and bottom electrodes. The photons get highly excited when they are absorbed, excitement they share with those in the MIM metal which then increase the energy level. After that madness, the electron is now called a “hot electron.” The new state of electrons in the metal forces hot-electron travel to the metal-insulator interface and opposite electrode. Electrode absorption of the electrodes creates opposing currents, only if there is an imbalance in the charges will a net DC gain be experienced.
To get the most out of the excited surface plasmons (SP) the team thickened the top electrode and used a prism to create a higher concentration of hot-electrons. The benefits of the addition come with more light absorption and a higher differential between the two electrodes due to electron scattering in the thicker layer.
Wang explained, "SPs are excited by incident light when the photon and SP wave vectors match with each other. For actual applications, it's more realistic to use nano-grating patterns on one electrode to excite SPs. By simply controlling the pitches of these gratings, SPs can be excited at any specific wavelength. As a result, energy conversion efficiency could be enhanced in the optical band from infrared to visible.”
With Ag laden electrodes a top efficiency of 4.3% is achieved for 640nm light. Gold produces a 3.5% capture in 780nm light. Full spectrum conversion is 2.7%. These figures places MIM tech at the lower end of the solar efficiency list. However, the team claims that with tweaking, the composition of the electrode material an increase in efficiency.
Melosh and Wang have "seen the light" of tapping into the unseen light available in the spectrum. With price and substrate choice on their side, I expect to see more from their budding technology.
Eavesdropper
