Fluorene Small Molecular Organic Light-emitting Exciplex And Its Role In The Device

Organic light-emitting diodes/devcies (OLEDs) is a new emerging display technology with great potential market, which possesses many advantages such as full solid devices, self-emission, high efficiency, wide view-angle, fast response and flexibility. For the practical application of OLEDs, fluorene derivative material is one promising kind of materials to yield high-performance devices. There, characteristics of two novel fluorene derivative materials and devices have been scrutinized in this work. Furthermore, the exciplex and electroplex emission in devices have been systematically investigated.1. OLEDs were fabricated with a novel fluorene derivative material BDHFLYDFLQ as a molecular probe, exciplex emission was observed in devices with emitting peak locates around 542 nm. However, OLEDs have low efficency and brightness because of the existence of exciplex. In order to manipulate the strength of exciplex emission, thin layer of CBP was inserted into the interface of NPB/BDHFLYDFLQ heterojunction. Light emission of exciplex was decreased once CBP was inserted, and emission of electroplex will occur when CBP is not thick enough. When the film thickness of CBP layer is increased, light emission of exciplex and electroplex were both eliminated. Light emittied from the device with 6 nm CBP layer inserted into NPB/BDHFLYDFLQ interface only characteristic light from BDHFLYDFLQ excitons was observed, and this device had higher external quantum efficiency and lower turn-on voltage.2. NPB/BDHFLYDFLQ heterojunction was used to study to the mechanism of exciplex formation. The process of exciplex formation and recombination is not carried out precisely through HOMO and LUMO of materials, since the existence of binding energy of electron-hole pair. By the investigating the energy structure of heterojunction, the electroplex in device was considered to form between NPB molecules and BDHFLYDFLQ molecules. Moreover, the calculation of the quantum yield efficencies of BDHFLYDFLQ exciton and exciplex were simulated, and it was found that the quantum yield efficiency of exciplex is significant lower than that of exciton. 3. A novel fluorene derivative material BDHFLCNPy was used to fabricate OLEDs for optoelectronic study. Exciplex emission was observed in device EL, locating around 657 nm. And electroplex emission occurred as the film thickness of BDHFLCNPy layer grows thicker, emitting peak located around 600 nm. Furthermore, ultrathin layers of CBP with various film thickness were inserted into the interface between NPB/BDHFLCNPy heterojunction to manipulate light-emitting strength of exciplex and electroplex. It showed that, when CBP layer was extremely thin, exciplex emission in device decreased significantly, in contrast, light emission from exciton and electroplex increased. Moreover, when thickness of CBP layer was as thin as 1 nm, exciplex and electroplex emission were terminated, and only characteristic emission from BDHFLCNPy was observed in device, which leads to the promotion of device efficiency.4. The structure of NPB/BDHFLCNPy heterojunction was studied. It was found that exciplex tends to recombine by the effective quasiparticle energies, ascribing to the influence of binding energy. On the other hand, the energy of electroplex exceeds the energy gap of quasiparticle energy, and electroplex in NPB/BDHFLCNPy heterojunction tends to recombine by HOMO and LUMO of materials accordingly.In summary, the optoelectronic properties of two novel fluorene derivative materials and devices were stduied. An effective method has been introduced to manipulate exciplex emission, which is inserting CBP as spacer between electron donor and electron acceptor layers. By altering film thickness of CBP layer, the light emission of exciton, exciplex and electroplex were manipulated. Insertion of CBP layer changed heterojunction property, and in turn influenced the characteristics of charge carrier recombination. Through the study of energy structure of heterojunction and device performance, the mechanism of exciton, exciplex and electroplex formation was discussed, and the interaction of exciton, exciplex and electroplex was also investigated. The results showed that exciplex formation enables charge carrier of involving with barrier-free capture, and this is very benefical to fabricate OLEDs with high efficiency and low turn-on voltage.