The Study On The Optical And Electrical Effects Of Gold Nanoparticles And The Application In Organic Light-Emitting Diodes

Organic light-emitting diodes(OLEDs) have been attracted a lot of attention due to its merits of being low-cost, high-resolution, and environment-friendly, what makes this technology even more attractive is that it can be fabricated on thin, large, and flexible substrates making it ideal for “wall paper-style” light sources. Significant efforts over the past decade have been made to improve the efficiency of OLEDs devices using a variety of processing strategies, such as synthesis high performance molecular, optimize fabricating process, fabricate the better devices structure and so on. Recently, metal nanoparticles(NPs) have shown unique optical and electrical properties, because of their small size effect, quantum effect, surface effect and macro quantum tunnel effect and some special physical effects. Utilizing the metal NPs to be extensive applied in organic optoelectronic devices for further enhancing devices performances, has becoming the research focus area. We made full use of the optical and electrical properties of metal NPs to improve the performance of OLEDs devices, and we also proposed some innovative concepts and achieved some valuable results! In optical effect aspect of gold(Au) NPs, on one hand, we utilize the Au NPs/PEDOT:PSS anode interface to enhance red, green, blue full color devices performance and white color devices performance, and “far-field” effect has been demonstrated as the enhancement mechanism firstly. On the other hand, we utilize the Au NPs/ZnO cathode interface substantially to improve the performance of iPLEDs devices, attributing to the local surface plasmon resonance(LSPR) effect of Au NPs. In electrical effect aspect of Au NPs, we utilize the Au NPs/PFN cathode interface to improve the performance of iPLEDs/cPLEDs devices. The enhancement would come from the improved conductivity and the enhanced trapping holes ability of cathode interface, the recombination of electron/hole could be controlled more balanced. The detailed discussion has been shown below:Firstly, we study the Au NPs/PEDOT:PSS anode interface in enhancing the performance of the cPLEDs devices systematically. Aiming at the limited action range(1 nm-10 nm) and the enhanced wavelength region should match the plasma resonance peak, we applied Au NPs/PEDOT:PSS anode interface to enhance the performance of the full visible(red, green, blue) light and white cPLEDs. For different emissive layer(MEH-PPV, P-PPV, PFO), the maximum brightness and luminous efficiency have been improved 30%-40%. And the maximum brightness, luminous efficiency and power efficiency have been improved 50%-60% for white color devices. The experiment and simulation results show that the enhancement originates from “far-field” effect, not LSPR effect. Different from LSPR, the “far-field” effect mainly originates from the interaction between origin emission and mirror-reflected emission, resulting in the increased irradiative rate of chromophores on the mirror-type substrate. The “far-field” effect of metal NPs, when chromophores localized nearby metal NPs(typically the distance>λ/10), is an important optical effect to enhance emission in photoluminescence. The optimized distance range, between the NPs and chromophores with visible light emission ranging from 400 nm to 700 nm, is 60 nm-120 nm. Thus the scope of the “far-field” may overlap the light-emitting profile very well to enhance the efficiency of optoelectronic devices. Through the theoretical simulation, no matter of the original fluorescence efficiency, the “far-field” effect could improve the fluorescence efficiency as percentage, which is also the best advantage of “far-field” in devices. Based on this consideration, we made full use of “far-field” effect of Au NPs to enhance the performance of the full visible(red, green, blue) light and white cPLEDs. This is the first time to improve the full color light-emitting diodes by “far-field” effect of Au NPs, and it is significant to boarden the application of metal NPs in enhancing the performance of OLEDs.Secondly, we study the Au NPs/ZnO cathode interface in enhancing the performance of the iPLEDs devices systematically. Comparing to c PLEDs, the i PLEDs have been developed to enhance the stability and rectification ratio, which are important for the application displays. However, the energy barrier between the work function of cathode and the LUMO of emissive layer is too large to induce the low efficiency. On one hand, mono-Au NPs/ZnO cathode has been modified to iPLEDs, the devices exhibited improved brightness from 5900 cd·m-2 to 15000 cd·m-2(150% enhancement), enhanced luminous efficiency from 4.4 cd·A-1 to 10.5 cd·A-1(140% enhancement) and improved power efficiency from 1.1 lm·W-1 to 2.6 lm·W-1(140% enhancement), when P-PPV applied as the emitting layer. The enhancement is one of the best results compared to literatures reported before. Both the experimental and theoretical results show that it is mainly attributed to effective overlapping between LSPR induced by Au NPs and excitons quenching region at ZnO/P-PPV interface, which makes originally electrode-quenched excitons emissive and increases excitons efficiency. We proposed that the most thing in improving the performance of devices is not only the coupling between plasma resonance and emission wavelength of emitting layer, but also the electromagnetic field of LSPR should mostly cover the light emitting profile in emitting layer. Then the exciton utilization could be improved mostly, and which is the critical thing to get the high enhancement in device performance. On the other hand, coupled-Au NPs/ZnO cathode has been modified to iPLEDs. The “hot spot” effect of coupled-Au NPs could improve the local electromagnetic intensity and broden the LSPR scope. So the coupled surface plasmons of Au NPs have been used to improve the performance of doped red-inverted PLEDs via the enhanced f?rster resonance energy transfer efficiency and the improved metal-enhanced fluorescence dual effect. The iPLEDs devices with the coupled-Au NPs have exhibited the 230% enhancement in brightness and the 130% enhancement in luminous efficiency, comparing to the devices modified by the mono-Au NPs.Finally, we study the Au NPs/PFN cathode interface in enhancing the performance of iPLEDs/cPLEDs devices systematically. We demonstrated that when the Au NPs/PFN act as the cathode interface, the performance of iPELDs devices exhibited the improved brightness from 17000 cd·m-2 to 33000 cd·m-2(94% enhancement) and the enhanced luminous efficiency from 9.4 cd·A-1 to 18.9 cd·A-1(100% enhancement), the performance of cPELDs devices exhibited the improved brightness from 27000 cd·m-2 to 39000 cd·m-2(44% enhancement) and the enhanced luminous efficiency from 14.1 cd·A-1 to 20.2 cd·A-1(43% enhancement), when a greenish emissive polymer P-PPV was applied as an emissive layer. The devices performance would be one of the best results based on the P-PPV emitting layer. The enhancement mechanism could be attributed to the improved conductivity of Au NPs/PFN cathode interface in enhancing the electrons injection and the Au NPs/PFN could trap the more holes in P-PPV layer, resulting in the more balanced electron-hole recombination. The results demonstrate that Au NPs/PFN constitute a feasible and effective route for achieving high-performance polymer optoelectronic devices through their electric properties.