Typically, most low bandgap materials have low absorption with wavelength at around 500 nm. Besides, the restrictions of active layer thickness of thin film polymer solar cells (PSCs) make the devices reduce to absorb light in long wavelength region (around 700 nm). As absorption would be a joint effect of material band properties and optical structures, well-designed light-trapping strategies for these low-bandgap PSCs will be more useful to further enhance efficiencies. We investigate the change of optical properties and device performances of organic solar cells based on our newly synthesized low-bandgap material with embedded poly-(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) PEDOT:PSS grating in the photoactive bulk heterojunction layer. Our results show that the PEDOT:PSS grating with a period of 320 nm and depth of 40 nm makes the light absorption improved in specific regions of the solar spectrum, especially the weak absorption region of our bulk heterojunction material near 500 nm and the red/near-infrared region at around 700 nm. The incident photon-to-electron conversion efficiency (IPCE) also improves with corresponding enhancement peaks. The physical understanding of the absorption enhancement will be investigated and described through our theoretical study. The power conversion efficiency is improved due to the enhancement of short circuit current. The work demonstrates the absorption enhancement of low bandgap solar cells using appropriate grating structures and provides the physical understanding of the absorption enhancement for improving the performances of organic solar cells. (C) 2012 Elsevier B.V. All rights reserved.