Enhanced Light Trapping and Power Conversion Efficiency in Ultrathin Plasmonic Organic Solar Cells: A Coupled Optical-Electrical Multiphysics Study on the Effect of Nanoparticle Geometry

Abstract

Plasmonic effects associated with localized surface plasmon (LSP) resonances such as strong light trapping, large scattering cross-section, and giant electric field enhancement have received much attention for the more efficient harvesting of solar energy. Notably, even as the thickness of the active layer is significantly reduced, the optical absorption capability of a solar cell could be maintained with the incorporation of plasmonic effects. This is especially important for the development of bulk heterojunction (BHJ) organic solar cells (OSCs), where the short exciton diffusion length, low carrier mobility, and strong charge recombination in organic materials strongly favors the use of optically thin active layers (<100 nm). However, the disappointing performance improvements obtained with plasmonic effects in the majority of BHJ OSCs realized to date suggests that plasmonic effects are yet to be fully taken advantage of; for example, in thick active layer OSCs (>100 nm), the optical absorption is already high, even in the absence of plasmonic effects, while in thin active layer OSCs (<100 nm), insufficient attention has been given to the analysis of plasmonic effects, such as the impact of plasmonic nanoparticle (NP) geometrical factors on the directional scattering efficiency. In this paper, we propose and demonstrate that the geometrical tuning of spheroidal plasmonic nanoparticles (NPs) could enable the full exploitation of plasmonic effects, providing dramatic improvements to the light absorption and energy harvesting capability of ultrathin film BHJ OSCs. Our theoretical analysis demonstrates a dramatic enhancement in optical absorption of ∼60% with spheroidal NPs embedded in a BHJ OSC device with ultrathin, <100 nm active layer, as compared to an NP absent reference device. These improvements are explained according to enhanced scattering of light into the active layer plane, spectral broadening of absorption resonances, in addition to an increased plasmonic modal volume, exhibited near LSP resonances of spheroidal NPs with optimal eccentricity. The result of our coupled optical-electrical device simulations also proves that the outstanding optical absorption enhancement obtained from the proposed device indeed translates into significant electrical performance gains; such as a ∼30% increase in the short-circuit current and ∼20% improvement in the power conversion efficiency (PCE).

Optical Response of Hybrid Plasmon–Exciton Nanomaterials in the Presence of Overlapping Resonances

“…Understanding plasmon-exciton or so-called plexitonic coupling [1] in hybrid plasmonic nanostructures is important for tuning the optical response, e.g. for applications in nonlinear optics [2], organic solar cells [3], or organic lightemitting diodes [4]. In developing such nanostructures, it is important to consider strong coupling phenomena.…”

Inverted Ultrathin Organic Solar Cells with a Quasi-Grating Structure for Efficient Carrier Collection and Dip-less Visible Optical Absorption

“…Plasmonic effects offer powerful platforms for strong light scattering, extreme subwavelength confinement and extraordinary optical responses. By utilizing localized surface plasmons (LSP), plasmonic scattering, surface plasmon polaritons (SPP) and their combinations, the advantages of plasmonic effects have been employed in various applications such as antennas 1 2 3 , photodetectors 4 5 6 and light harvesting devices 7 8 9 . Plasmonic effects are particularly effective and physically relevant for efficient light harvesting in organic solar cells, where the reduction of the active layer thickness is important.…”

“…Plasmonic effects are particularly effective and physically relevant for efficient light harvesting in organic solar cells, where the reduction of the active layer thickness is important. Because of the extremely short carrier (especially hole) diffusion length of organic materials, the electrical-thickness of the active layer needs to be strictly limited, to achieve efficient charge collection 7 10 . However, at the current stage of development, the active layer thickness in conventional OSCs is usually larger than the typical electrical scale length of carrier diffusion (<100 nm) 11 12 13 14 15 , to avoid the penalty of low optical absorption in organic photoactive materials.…”

Emerging Role of the Band‐Structure Approach in Biohybrid Photovoltaics: A Path Beyond Bioelectrochemistry

“…In resonance frequency range, polarization enabled interaction of light with matter occurs at a larger area of cross‐section. Losses in the effect are attributed to surface plasmon resonance absorption whereas plasmon assisted scattering enhances the efficiency of the solar cells . Encouraged by such optoelectronic surprises given by metal nanostructures, SPR effects have been extended to a wide range of photovoltaics and of late to biophotovoltaics …”