Performance improvement of organic solar cells incorporated with metallic nanoparticles

Organic solar cells (OSCs) have shown great promise in becoming the next generation of renewable energy due to its low cost, simple manufacturing process and flexibility. A method of efficiency improvement in OSCs is by incorporating metallic nanoparticles (NPs). While various reports have reported that incorporation of NPs improve OSC efficiencies due to the Localized Surface Plasmon Resonance (LSPR) effect, the investigations have lacked depth and a detailed investigation is necessary to fully understand the device mechanisms of these OSCs. In this thesis, we first investigate OSCs incorporating Au NPs into the hole collection Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer. Our theoretical and experimental results show that the very strong near field around Au NPs due to LSPR mainly distributes laterally along the PEDOT:PSS rather than vertically into the adjacent active layer, leading to minimal enhancement of light absorption in the active layer. With optical effects proven to be minor contributors to device performance improvements, we investigate the electrical properties of the OSCs and obtain insights into the detailed device mechanisms. Improvements in power conversion efficiency (PCE) of solar cells are found to originate from the enlarged active layer/PEDOT:PSS interfacial area and improved PEDOT:PSS conductivity. At high NP concentrations, reduced exciton quenching at donor/acceptor junctions is found to cause PCE deterioration. Next, the effects of Au NPs incorporated into the active layer of OSCs with a newly synthesized donor polymer are investigated in detail. Our experimental and theoretical results both show that LSPR introduced by the NPs can enhance the light absorption in the active layer of OSCs because the strong LSPR near field mainly distributes laterally along the active layer. Combined with our previous study, our results strongly suggest that NPs have to be incorporated in the active layer in order to harvest light by the LSPR effect. Meanwhile, our results show that the electrical properties of NPs improve at low concentration of NPs. When NP concentration is increased, the electrical properties deteriorates and counter-diminish the optical enhancement from LSPR and reduces the overall performance improvement. Finally, we demonstrate efficiency improvement in OSCs by ~22% through incorporating Au NPs into all polymer layers. Au NPs are found to have distinct mechanisms in improving device performance when incorporated in different polymer layers. Our results indicate that the efficiency improvement is the accumulated effects of incorporating NPs in the individual layers and that coupling is not observed in this device configuration. On the whole, our findings highlight the importance that both optical and electrical properties need to be studied and optimized simultaneously for achieving enhancement in PCE of OSCs. We have carried out a detailed study on incorporating NP in various layers and our results are highly useful for the design of high efficiency OSCs incorporating metallic NPs.published_or_final_versionElectrical and Electronic EngineeringMasterMaster of Philosoph