C. Kim, J-Y. Lee, P. Peumans, J. Kim
A fundamental limitation of organic photovoltaic (OPV) cells is due to the trade-off between light absorption and exciton diffusion. We show that light absorption in an OPV cell can be enhanced by replacing a conventional transparent electrode with a metal grating. In the proposed device, incident light resonantly excites surface plasmon modes in the structure, increasing the field intensity in the organic layers. As a result, high absorption efficiency can be achieved even for layers thinner than typical exciton diffusion length. We study a model device structure comprising: glass substrate / 20-nm-thick grating made of Ag / organic multilayer / 100-nm-thick Ag; the organic multilayer consists of a 10-nm-thick absorption layer of copper phthalocyanine sandwiched by optically inactive layers. Guided by the complex photonic band structure of the unperturbed device, i.e. an OPV cell with planar electrodes, we find a grating geometry that improves absorption efficiency. Our calculations, based on FDTD and FEM, show that when the organic multilayer is 80 nm thick and the grating period is 320 nm, the absorption efficiency exceeds 75% for p-polarized incident light with a wavelength of 680 nm. We discuss design guidelines for tuning the resonance wavelength and reducing the polarization dependence.