Studies On The Effects Of Au Nanoparticles On The Performance Of Organic Solar Cells

Organic solar cells (OSCs) are promising candidates in the field ofphotovoltaic technology owing to their material advantage as well ascompatibility with roll-to-roll fabrication and industry. Recent progress of OSCsdemonstrated that the power conversion efficiency (PCE) has been improved toaround12%, and industry investment on high throughput manufacturing ofOSCs at low cost has already emerged. The potential applications of OSCsinclude solar plants,energy supply for portable electric devices, semi-transparentsolar cell windows, building or public facility applications, etc.Due to the relatively short exciton diffusion length and low carrier mobilityof organic semiconductors, the featured thickness of the active layer in OSCs islimited (~100nm). This reduces the absorption efficiency, thereby a significantloss in photon-to-exciton generation, the limitation all types of thin-film solarcells face. In OSCs, a viable scheme is to boost light absorption in the activelayer to the best extent possible. To date, different light trapping methods havebeen reported for improving absorption in solar cells, including the use ofanti-reflection coatings, photonic crystals, and plasmonic nanostructures.Among them, plasmonic nanostructures are a superior approach in improvinglight absorption in solar cells due to the near-field enhancement caused by thelocalized surface plasmon (SP) resonances. Plasmonic elements used for lightmanagement can be categorized into two groups: nanogratings and nanoparticles(NPs). Plasmonic nanogratings are generally in well-organized patterns, whichrequire complicated nanofabrication techniques, bringing challenges to costmanagement. Plasmonic NPs, which can be synthesized by low-cost solution processes, are compatible with the fabrication of OSCs and offer a better choiceof light trapping components in OSCs.The performance of organic solar cells (OSCs) can be greatly improved byincorporating silica-coated gold nanorods (Au@SiO2NRs) at the interfacebetween the hole transporting layer and the active layer due to the plasmoniceffect. The silica shell impedes the aggregation effect of the Au NRs in ethanolsolution as well as the server charge recombination on the surface of the Au NRsotherwise they would bring forward serious reduction in open circuit voltagewhen incorporating the Au NRs at the positions in contact with the activematerials. As a result, while the high open circuit voltage being maintained, theoptimized plasmonic OSCs possess an increased short circuit current, andcorrespondingly an elevated power conversion efficiency with the enhancementfactor of~11%. The origin of performance improvement in OSCs with theAu@SiO2NRs was analyzed systematically using morphological, electrical,optical characterizations along with theoretical simulation. It is found that thebroadband enhancement in absorption, which yields the broadband enhancementin exciton generation in the active layer, is the major factor contributing to theincrease in the short circuit current density. Simulation results suggest that theexcitation of the transverse and longitudinal surface plasmon resonances ofindividual NRs as well as their mutual coupling can generate strong electric fieldnear the vicinity of the NRs, thereby an improved exciton generation profile inthe active layer. The incorporation of Au@SiO2NRs at the interface betweenthe hole transporting layer and the active layer also improves hole extraction inthe OSCs.