Finite-difference time-domain optimization of organic thin-film solar cells using photonic crystal gratings

Abstract

Organic photovoltaics (OPV) has taken off with the promise of low materials costs and fast, scalable manufacturing. However, an important factor limiting the efficiency of current OPV cells is the lengthscale mismatch between the electronic carrier extraction, and optical absorption of the organic semiconductors used. While organic semiconductors are typically strong optical absorbers, it is difficult to efficiently extract photogenerated charge carriers from them. For example, in solar cells using recently developed organic bulk heterojunctions, the active layer needs to have a thickness of less than 100 nm for efficient carrier extraction. These thin layers leave many photons unharvested, and have thus motivated much recent interest in optical design and light trapping for organic solar cells. In contrast to most inorganic cells, the active layers in organic cells have thicknesses that are far smaller than the wavelength, thereby placing them in the nanophotonic regime, where the ’conventional’ 4n 2 limit on light trapping could be easily surpassed. In this work we present detailed finite-difference time-domain calculations for light trapping nanostructures (photonic crystal gratings) introduced in organic solar cells using P3HT, MoO3, Ag and ZnO/ITO on glass as substrate. We have simulated the effect of the minimum and maximum thicknesses of each layer and founded the optimal values. Next, we have simulated the light trapping effect by using a grating in an ITO/glass substrate with different thicknesses of ITO, founding the best values for the grating geometry. The dependence with the angle of incidence and polarization of the light has been also simulated for different grating periods and thicknesses of ITO, founding enhancements in the absorption (Jsc) from 1.3% to 1.7%. Finally, contour maps of Jsc versus the height of the grating and for several filling factors have been calculated, which show total enhancement values for Jsc around 3% compared to the structures without grating.