3:30 - 4:10 p.m.
"Spectral Aspects of Cavity Tuned Absorption in Organic Photovoltaic Films"
Concentration of light and infrared capture are two favored approaches for increasing the power conversion efficiency (PCE) of photovoltaic devices. Recent progress using both of these approaches has resulted in the rapid increase in efficiency in organic photovoltaic devices. Methods to concentrate light have focused on arranging for optically resonant structures such as plasmonic metal particles, periodic structures, and optical spacer layers. Using optical transfer matrix formalism, we demonstrate a high degree of coupling between the cavity photons and the semiconductor excitons despite the fact that the optical half-cavity formed by the polymer bulk heterojunction active layer (AL) between the aluminum cathode and indium tin oxide (ITO) anode is quite lossy. This robust coupling allows for a high degree of control over the effective absorption spectrum by merely adjusting the intrinsic device design. For example, we find that optical absorption can be finely tuned by adjusting the ITO thickness within a relatively narrow range, thus eliminating the need for a separate optical spacer. We have recently applied these methods to the design of inverted PTB7:PC71BM and shown a strategy for the spectral tuning of absorption validated by experimental results. Optical transfer matrix theory is used to guide OPV device design by optimizing the thickness of both the bulk heterojunction and the electron transport layers. By tuning device layer thicknesses, we demonstrate that a range of absorption spectra are achievable and exploit cavity effects to spectrally sculpt absorption enhancement. This technique allows for the minimization of the device thickness, important to reduce recombination, and increase efficiency. We show that the empirically found optimum device thickness is consistent with our models. Furthermore, we compare external quantum efficiency (EQE) measurements and transfer matrix simulations to demonstrate the good agreement between theory and the experimental response of fabricated devices.
Kenneth Singer is Ambrose Swasey Professor of Physics and Associate Director of the Center for Layered Polymer Systems at Case Western Reserve University. He received his B.S. summa cum laude in Physics from the Ohio State University in 1975 and Ph.D. in Physics from the University of Pennsylvania in 1981. He was a Member of Technical Staff at Bell Laboratories from 1982-1989, and Distinguished Member of Technical Staff from 1989-1990. From 1990-1993 he was the Warren E. Rupp Associate Professor of Physics at Case. Singer is a Fellow of both the American Physical Society and the Optical Society of America.
Singer's research interests are in organic optoelectronic materials, including nonlinear optical phenomena and electronic properties and transport in liquid crystals and polymers. His current projects include photonic crystals, photonics in multilayer polymer films, and materials and design for photovoltaics. He is the founder of Folio Photonics LLC, aimed at commercializing a high capacity optical data storage medium.