Research On Flexible Organic Optoelectronic Devices Based On Ultrathin Metal Electrodes

Taking the advantages of organic materials,such as flexible folding,light weight and portability,flexible organic optoelectronic devices have become the representatives of organic devices with superior performance and wide application,marking a major trend in the research and development of electronic devices,and especially enjoying a promising application in wearable electronic equipment.However,the performance of flexible organic optoelectronic devices still cannot meet the needs of practical applications.One of the challenges is the flexible transparent electrode with high photoelectric properties,flexibility and mechanical stability,which is also the core problem for various scientific research teams to make breakthroughs.Playing an important role in providing transmission channels for photons and electrons in organic optoelectronic devices,transparent electrodes determine the performance of organic optoelectronic devices to a certain extent.The evaluation of the transparent electrodes of flexible optoelectronic devices are mainly from the aspects of transmittance,conductivity,surface roughness,work function,stability,flexibility,mechanical strength and machinability.At present,the most commonly used transparent electrodes are ITO electrodes,with the advantages of high transmittance and conductivity,and good ohmic contact with organic functional layer.However,due to ITO's fragility,complex fabrication process,high costs,scarce materials and the incompatibility between its high temperature fabrication technique and the flexible substrate,ITO is not suitable as flexible electrode.Therefore,a new transparent electrode with high transmittance,good conductivity and flexibility,simple production process and low costs needs to be proposed to replace ITO.Metal films with simple fabrication process are highly conductive and ductile such as Au,Ag,Al,etc.,which are commonly used as electrode materials.However,the transmittance of thick metal films in the visible wavelength region is low.Even though its optical properties can be improved by reducing the film thickness to less than 10 nm,the surface morphology and conductivity are usually poor in such ultrathin case owing to the Volmer-Weber growth mode.Here,we propose several substrate modification schemes to suppress the Volmer-Weber growth mode and to fabricate the ultrathin metal transparent electrode with good photoelectric characteristics.Colligating the desirable optoelectronic properties of proposed ultrathin metal electrodes with further plasmonic pattern design,the flexible organic optoelectronic devices have demonstrated excellent photoelectric performance,flexibility and mechanical stability,providing technical support for further application in wearable devices.Our specific research includes the following aspects:1.Hybrid modification scheme of metal seed layer combined with substrate has been proposed to suppress the Volmer-Weber growth mode of Au thin films,fabricate the ultrathin Au electrode with thickness of only 4.4 nm and apply it to transparent bottom electrodes of flexible organic photovoltaic devices(FOPVs).With the non-conductive polymer S-1805 as the substrate modification layer to provide the chemical binding between Au atoms and the slip of evaporated metal atoms on the substrate surface,further combined with Ag as the seed layer,effectively inhibited the 3D island growth mode of the deposited ultrathin Au film.Based on the hybrid modification scheme,the prepared ultrathin S-1805/Ag/Au electrode had ultra-smooth surface morphology(surface roughness is 0.37 nm),high transmittance(78%@550 nm)and good conductivity(sheet resistance is about 70?/sq).S-1805/Ag/Au has been applied on the transparent bottom electrode of FOPVs to replace ITO.Compared with devices based on ITO electrode,FOPVs based on ultrathin metal electrode have shown good photoelectric performance and mechanical stability.2.Composite seed layer combined with antireflection film has been proposed to design ultrathin Ag flexible transparent conductive film based on the structure of MoO₃/Au/Ag/NPB(MAN structure),and apply it to the transparent top electrode of inverted FOPVs.With MoO₃ and Au as the composite seed layer,the Volmer-Weber growth mode has been successfully suppressed and the percolation threshold of evaporated ultrathin Ag film has been reduced.Compared with the ultrathin Ag film based on single seed layer MoO₃,the sheet resistance has decreased from 30?/sq to 7?/sq.Further introducing NPB as the antireflection layer,the transmittance of ultrathin Ag film has improved(>80%@550 nm).In the Inverted OPVs based on MAN structure ultrathin Ag electrode,TPBi has been applied as electrode modification layer.By optimizing the thicknesses of antireflection layer and electrode modification layer,the powerconversion efficiency of FOPVs has reached 6.02%,and the device performance degradation can be ignored after 3000 bending cycles.3.Tunable surface plasmon-polariton(SPP)resonance based on plasmonic microstructured Ag-Al alloy electrode has been realized by adjusting the alloy electrode composition,and the light extraction efficiency of organic light-emitting devices(OLEDs)based on ultrathin metal electrodes has been improved.The periodic plasmonic microstructure has been introduced to the ultrathin transparent metal anode by nanoimprinting technology,which was transferred layer-by-layer transfer during the fabrication of OLEDs,together with the co-deposition technique,the microstructured Ag-Al alloy electrode with a period of 290 nm has been fabricated in OLEDs to excite SPP mode.By changing the deposition rate of Ag and Al to adjust the component molar ratio of the alloy cathode,the tunable SPP resonance in microstructured OLEDs has been realized without changing the grating period.In OLEDs,the resonance wavelength of SPP mode excited by plasmonic microstructured Ag_0.909Al_0.091 alloy cathode has matched with the emission peak of OLEDs,and the photons captured by SPP mode have been effectively extracted.The luminance and current efficiency of OLEDs based on microstructured Ag_0.909Al_0.091 alloy cathode have been increased by25%and 21%,respectively.