In this work, it was demonstrated that either physically or chemically prepared gold nanoparticles being sparsely dispersed on ITO surface could work not only as hole injection layer in OLEDs but also hole extraction layer or anode buffer layer in OSCs.
In particular, the mechanism of improvement is also systematically discussed by analyzing the energy alignment at ITO/NPB interface.
A small surface coverage via gold nanoparticles obtained by HPS-Au (2-3nm), evaporated Au (2-3nm) and APG-Au (1nm and 1.5nm) sparsely dispersed on ITO can act as anode modification layer respectively, the hole only devices perform obviously current density enhancement. The J-V curve of each device indicates that an ohmic or quasi ohmic contact can be formed between ITO and NPB, verifying the energy barrier could be reduced by gold nanoparticles. When these three gold nanoparticles are applied in OLEDs as hole injection layer, higher hole injection also bring higher current density and luminance, however, the EQE has a little decrease due to imbalance between hole and electron. When they are used as anode buffer in OSCs, comparing with device without anode buffer, the efficiency and Voc are improved obviously by these nanoparticles, which is comparative with conventional anode buffer, PEDOT:PSS.
The reduced energy barrier/gap between ITO/NPB and ITO/active layer is ascribed
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to the upward shift of vacuum level owing to an interface electrical dipole, which can be proved by the increase of work function of ITO surface after modification. This interface electrical dipole is originated from that the negatively charges taken by insulated gold nanoparticles on ITO and attracted positive image charges on the inside of ITO surface. According to the theory of interface science, this interface dipole could make the vacuum level of organic layer have an upward shift when anode and organic layers are aligned, then the energy barrier/gap for hole injection or extraction is obviously decrease, which is the reason for current density increased in OLEDs and Voc reduced in OSCs.
During the performance as injection or buffer layer, an insulating layer outside the surface of gold nanoparticles. Although these three kinds of nanoparticles perform similar effect on hole injection and extraction, some differences should be noticed. For chemically synthesized Au nanoparticles like HPS-Au, the insulating layer is realized by the stabilization layer HPS. For the case of physically processed Au particles such as evaporated Au and APG-Au, the HPS insulating layer between ITO and particles is the key point. For evaporated Au, no effective enhancement of current density is observed without HPS layer. In the case of APG-Au, the current density is highly increased without HPS; however, the repeatability is much worse than the device with evaporated
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Au. The reason is thought that the APG-Au is moveable on ITO surface, but evaporated Au could strongly bond with ITO surface.
The negative charge taken by gold is another important issue. It is already known that the negative charges are generated from the absorption property of nanoparticles for HPS-Au. Evaporated Au is probably neutral initially, but an oxide layer (AuOx) can be observed after UV-ozone treatment. One side, it can perform as the outside insulating layer. The other side, the negative charges could be transferred from ITO to evaporated Au or chemical species during UV-ozone treatment. Whereas, for the case of APG-Au, instead observing oxide layer as evaporated Au, another chemical species is thought to be generated after UV-ozone treatment because APG-Au should have higher crystalline content than evaporated Au, which needs to be further investigated.
It is also thought that the level of hole injection enhancement is related with the surface coverage by Au nanoparticles on surface. However, the amount of gold taking negative charge effectively is still a matter to discuss due to the aggregation for chemically synthesized Au nanoparticles at higher concentration. For physically synthesized nanoparticles, higher number density is easily obtained, but how much particles is negatively charged is difficult to characterize. In a result, the quantitative relation between surface coverage and hole injection is needed to be figured out via
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more thorough experiments.
Physically or chemically obtained gold nanoparticles as modification material on ITO are good candidates as anode modification materials at the interface between ITO and organic layer. The discussion about the mechanism is also very helpful to understand the interface science; meanwhile the results of the increase of hole injection in OLEDs and efficiency in OSCs are sufficient to prove the effect of this negatively charged insulated gold nanoparticles on ITO/organic interface. This proposal also provide a general idea directing us to find new modification materials, such as other stabilized nanoparticles taking negatively charges, and it can also be extended to the interfaces of other metal/organic.
However, there are still some disadvantages. First, for chemically obtained nanoparticles, although the procedure of gold nanoparticles synthesis is easy, usually it needs to be very careful to control the particles size. Uniform distribution on metal surface is difficult to be controlled, and aggregation is also easily happened when the solution concentration is increased. For physically obtained nanoparticles, uniform distribution and aggregation on surface is well controlled, but the analysis of the insulating layer is usually not easy. These are the issues to be solved in the future research.
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List of Publications and Presentations
Publications
1. D. Wang, K. Yasui, M. Ozawa, K. Odoi, S. Shimamura and K. Fujita. Hole injection enhancement by sparsely dispersed Au nanoparticles on indium tin oxide electrode in organic light emitting devices, Appl. Phys. Lett., 102, 023302 (2013)
2. D. Wang, N. Yukitake and K. Fujita. Improvement of Organic Solar Cells by Anode Buffer Composed of Evaporated Gold Nanoparticles, J. J. Appl. Phys., 52, 100211 (2013)
3. D. Wang, N. Yukitake and K. Fujita. Anode Buffer for Organic devices Composed of Gold Nanoparticles Prepared by Ark Plasma Deposition, Advanced Materials Research. (being revised)
Presentations
1. Performance improvement in Organic Light-emitting Diodes via Modifying ITO Surface by an Au Layer. The 3rd Asian Symposium on Advanced Materials, 2011, Kyushu University Chikushi campus, Japan.
2. The Performance Improvement in OLEDs by Employing an Hyper-branched Polymer Protecting Au Nanoparticle Layer. KJF International Conference on Organic Materials for Electronics and Photonics, 2011, Gyeongju, Korea.
3. Hole Injection Enhancement in Organic Light-emitting Diodes by Introducing an Au Nanoparticle Layer. 2011 International Conference on Solid State Devices and Materials, Nagoya, Japan.
4. An Approach to enhance the hole injection in organic light emitting diodes by introducing Au nanoparticles on ITO. 11th International Discussion&Conference on Nano Interface Controlled Electronic Devices, 2011, Dazaifu, Japan.