Light trapping approaches for high-performance polymer solar cells

Summary form only given. Organic photovoltaic devices (OPVs) have received much attention because they are promising alternative tools for harnessing renewable energy. Recently the power conversion efficiencies (PCEs) of these OPV devices have reached as high as 8%, opening up the possibility for their practical use as flexible, light-weight, low-cost, renewable energy systems. While the internal quantum efficiency can approach 100%, efficient light harvesting in OPVs remains one of the major limitations toward realizing high PCEs. Typically, the optimum thickness of the active layer for an OPV device is around 100-200 nm, or possibly less; such a thin layer can lead to low absorption of light. A thicker layer, however, inevitably increases the device resistance due to the low carrier mobilities of organic materials. This situation imposes a trade-off between light absorption and charge transport efficiencies in OPVs, motivating the development of a variety of light trapping techniques. In this paper, we will presents light trapping techniques for high-performance OPVs. First, we have employed indium tin oxide (ITO) as an optical spacer in inverted structures. We have found that the optical interference effect led to spatial redistribution of the optical field in the devices, resulting in favorable distribution of photogenerated excitons. The exiton quenching at the electrodes could be inhibited. Therefore, the introduction of the ITO optical spacer at an appropriate thickness increased the short-circuit current density and the overall PCE. Further, we blended gold nanoparticles (Au NPs) into the anodic buffer layer to trigger localized surface plasmon resonance (LSPR) for enhancing the performance of the OPVs. The power conversion efficiency of the OPV device incorporating the Au NPs improved to 4.24% from a value of 3.57% for the device fabricated without Au NPs. The primary origin of this improved performance was local enhancement of the electromagnetic field sur- - rounding the Au NPs. The mechanism of the plasmonic-enhanced OPVs will be discussed.