Broadband absorption enhancement via light trapping in periodically patterned polymeric solar cells

Improved absorption is needed for thin-film organic solar cells to increase charge generation and/or reduce the thickness of the organic active film. Absorption enhancement by light trapping in a photonic crystal (PC) can be used to enhance the absorption in a patterned photo-active polymer blend. We used the finite difference time domain (FDTD) method to study the effect of combining the PC with different vertical layer configurations, including a hole transporting flash layer, used for charge separation and for increasing the photo-voltage, and a reflective metallic back electrode. The model was applied to P3HT:PCBM and pBBTDPP2:PCBM blends as the active materials, to examine the effect of different absorption spectra on the results. We found that the combination of PC and a reflector electrode is synergistic for enhancing the absorption in such devices, while the effect of the flash layer is minor. The combination of lateral and vertical reflections by the PC and the reflector, respectively, results in significant light scattering and enhancement of the absorption in a wide range of wavelengths. An increase of 30% to 60% (depending on the active material) in the number of absorbed photons of the solar spectrum was calculated compared to un-patterned blends. Broadband absorption enhancement in a realistic cell configuration is expressed as absorption enhancement of solar photons in a wide range of PC periods, i.e., the presence of periodic structuring is more important for absorption enhancement of the solar irradiation than the dimension of its period. (C) 2013 AIP Publishing LLC.