Studies On Absorption Improvement In Organic Photovoltaic Cells Employing Silver Nanostructures

Organic photovoltaic cells?OPVs?have attracted extensive attention due to their plentiful materials,solution processability,low weight,high flexibility and large area production.More studies on improving the performence of OPVs are always important and meaningful,although the silicon-based solar cells dominate the new energy market and the investigation of perovskite solar cells is a hot point now.However,one key element which limits the development of OPVs is the low photoelectric conversion efficiency?PCE?.Enhancing absorption of the thin active layer is an effective approach to improve the PCE for OPVs,which can be realized by metallic nanostructures based on surface plasmon resonance.Many studies show that the metallic back electrodes with sub-nanostructures can remarkbly capture more incident photons into the active layer,but there are few studies addressing their merit and demerit from a comparison standpoint.Further more,it is rare that the short-pitch nanostructures are introduced into the back electrode in OPVs,aiming at absorption enhancement in a thin active layer.Besides,the exciton quenching probability will be increased,when the metal nanostructures contacts with the active layer directly.This undesirable electrical phenomenon can be avoided by coating the metal nanostructures with the dielectric.However,it is needed to further explore the impact of such core shell nanostructures on light absorption.For above mentioned problems,the detailed works as follows:?1?The nanohole and nanopillar-type patterned metallic electrodes are respectively introduced into OPVs,focusing on optical absorption comparison.We find their optimized integrated absorption efficiencies are almost the same?82.4%?,increased by 10%compared to that of the planar reference device,but their absorption enhancment mechnisms are different.In addition,both devices are almost angle insensitive for either TM or TE polarization.More,we find that the nanopillar-type patterned metallic electrode performs better in pratical application,because of two advantages:i.easy fabrication due to the moderate filling ratio;ii a bigger tolerance for geometric parameters approaching optimized absorption.?2?One novel OPV structure is proposed,mainly construced the short pitch hexagonal nanopillar array at the metalic back electrode.By optimizing the geometrical parameters of such back electrode,the integrated absorption efficiency in 40 nm active layer is up to 66.8%,which is increased by 81.5%with respect to that of the equivalent reference device.This high absorbing performance weakly depends on the polarization and the angle of the incident light.In addition,the amplititude of such absorption enhancement increases with the thickness of active layer decreased?from 120 nm to 40 nm?.It is attributed to the excitation of the dipole-like localized surface plasmon resonance and mutual coupling between them.?3?One OPV structure,which has the dielectric-coated comb silver nanogratings in back electrodes,is designed.Here we will theoretically investigate the effect of coating engineering on the absorption performance.The focus is to analyze the dependence of the absorption ability on the structural and dielectrical parameters of the coating dielectric.By studing the dispersion relations supported in the three layer waveguide as well as the related electric field distributions,we find that the dielectric shell has a significant modulation on the plasmon waveguide mode excited for TM polarization.This is because the introduction of the dielectric shell affects the equivalent wavelength existed in the three layer waveguide.Moreover,the results show that the adoption of the suit shell material and a thinner dielectric shell is important for absorption improvement of the active layer.Under TE polarization,the structural and dielectrical parameters of dielectric shell mainly influence the interference maximum of the plane wave,which will act on the optical absorption in the active layer.?4?The core shell nanocubes?Ag@SiO₂ NCs?and core shell nanoshperes?Ag@Si O₂ NSs?are introduced into the active layer?adjacent to the buffer layer PEDOT:PSS?of OPV sructures,respectively.And the comparative studies are carried out on their influence acting on light absorption of the active layer.The results show that the two structures are beneficial to the absorption enhancement when the size of Ag core is relatively large while the SiO₂ dielectric shell is thin.But their mechanisms of the absorption enhancement are different:the core-shell nanocubes mainly enhance absorption in the long wavelength due to the excitation of diverse local plasmon resonances?corners resonance and edges resonance?,while the core-shell nanoshperes improve the absorption in the wider wavelength range,because of the excitation of the dipole-like plasmon resonance.In addition,there aslo has the gap plasmon resonance between the two core-shell nanostructures and the silver electrode,significantly contributing to the absorption enhancement in the active layer.