Ploymer Solar Cells Based On Metal Nanoparticles Localized Surface Plasmon

Polymer solar cells（PSCs）are competing with traditional inorganic thin film solar cells owing to their unique properties such as low weight,low cost,and mechanical flexibility.In the past few years,research on PSCs has deeply examined almost all aspects,including materials synthesis of electron donors and acceptors,interface modification,nanoparticles doping,light trapping method,tandem structure,and so on,which led to a significant power conversion efficiency（PCE）enhancement to more than 10%.However,the still relatively low PCE,compared with inorganic solar cells,limits its further developing and application.Seen from the fundamental working mechanism of PSCs,the external quantum efficiency（EQE）is determined by multiplication by internal quantum efficiency and absorption efficiency.Despite the high absorption coefficients（>105cm-1）of many polymer materials,it is still difficult to absorb sufficient light for utilization in PSCs due to the relatively large bandgap compared to the one of silicon materials.Therefore,achieving light absorption enhancement without increasing the thickness of photoactive layer triggers the emergence of new research that highlights light trapping methods（or light management）.Recently,a variety of light-trapping techniques have been widely investigated,definitely including microcavity structure,photonic crystals,and plasmonic nanostructures.Among the proposed light trapping methods,in particular,plasmonic nanostructures based on advantageous optical properties have attracted considerable attention due to their unique tunable optical resonance features and for enhancing light absorption by means of increasing the photocurrents of PSCs.Herein,a thermally evaporation method was used to realize Au nanoparticles（Au NPs）doping into WO₃ anode buffer layer in inverted PSCs.The surface energy differences between Au and WO₃ inevitably lead to Au growing up through theprocess from nucleation,isolated island,aggregation of metal islands to continuous films along with the process of evaporation.The atom force microscopy（AFM）images indicate that critical thickness of Au film formation is 8 nm,which is in accordance with current density-voltage（J-V）and incident photon-to-electron conversion efficiency（IPCE）measurement results of optimal device performance.The power conversion efficiency with 8 nm Au is dramatically improved from 4.67±0.13% to 6.63 ± 0.17% compared to the one without Au.Moreover,the optical absorption enhancement is demonstrated by steady state photoluminescence（PL）,which agrees well with transmission spectrum.The optical and electrical improvement all suggest that thermal evaporation is the appropriate method to further enhance device performance.The surface plasmon resonance（SPR）effect of metal nanoparticles（MNPs）is effectively applied on PSCs to improve PCE.However,universality of the reported results mainly focused on utilizing single type of MNPs to enhance light absorption only in specific narrow wavelength range.The differences of surface energy between Ag,Au,and WO₃ compared by contact angle images enable Ag and Au prefer to respectively aggregate into isolated islands rather than films at the initial stage of the evaporation process,which was clearly demonstrated in the AFM measurement.The sum of plasmon-enhanced wavelength range induced by both Ag NPs（350-450 nm）and Au NPs（450-600 nm）almost cover the whole absorption spectra of active layers,which compatibly contribute a significant efficiency improvement from 4.57±0.16 to6.55 ± 0.12% compared to the one without MNPs.Besides,steady state PL measurements provide strong evidence that the SPR induced by the Ag,Au NPs increase the intensity of light absorption.Finally,ultraviolet photoelectron spectroscopy（UPS）reveals that doping Au and Ag causes upper shift of both the work function and valence band of WO₃,which is directly related to hole collection ability.We believe the surface-energy-induced dual plasmon resonance enhancement by simple thermally evaporating technique might pave the way toward higher-efficiency PSCs.