Fabrication And Characteristics Of Organic Light-emitting Devices Based On Organic Semiconductor Crystals

Organic semiconductor crystals have attracted their extended interests as activelayer material in application of optoelectronic and electronic devices. Their flexibility,high efficiency, and availability in the low cost and low temperature fabricatingprocess are the main motivation for research on the organic semiconductor crystals.Meanwhile organic single crystals are attractive candidates for optically pumpedlasers, organic field-effect transistors (OFETs), and organic light-emitting diodes(OLEDs) because of their advantages such as the high order molecular packing, highchemical purity and high carrier mobility. Organic semiconductor crystals using forelectroluminescent devices are currently being studied since Pope et al., who openedup a new application in single crystals. However, the amount of reports on OLEDsbased on organic single crystals is scarce. A significant problem is the contactproblem which caused by the lamination of between the weak van der Waals forcebottom electrode and crystal. Meanwhile, the lack of material with balanced injectionof electronics and holes at high current density could be a drawback when one tries toachieve uniform surface emission from the single crystal-based OLEDs. Thus, there isa pressing need a simple, non-destructive and mechanically flexible method to resolvethe contact problem which is the great obstacles to make high throughput in devicefabrication.In this work, a simple and non-destructive method template stripping, has beenapplied into the single crystal-based OLEDs to improve the contact problem andrealize its bright emission. Template stripping technique has been demonstrated to produce smooth metal films as electrodes on uniform large-area polymer substrates.This method has been employed to realize flexible OLEDs and organic photovoltaic(OPV) cells with high flexibility and efficiency. On the basis of this technique,single-crystal-based OLEDs with better contact between the crystals and electrodesare expectable. Both anodes and electrodes of the crystal OLEDs can be depositedonto the opposite surface of the organic single crystals by thermal evaporation on bothsides, so that a much improved contact between the crystals and the electrodes can berealized, which enhance the carrier injection into the active layer of organic singlecrystals.The main research work:1. Based on the template stripping method, organic light-emitting devices(OLEDs) based on BP2T organic single crystals have successfully prepared by usingthis simple and non-destructive method. By comparing the performance of OLEDsprepared by laminating method and template stripping method, we found that theOLEDs based on template stripping method show uniform electroluminscence. Bothanodes and electrodes of the crystal OLEDs can be deposited onto the oppositesurface of the organic single crystals by thermal evaporation on both sides, so that amuch improved contact between the crystals and the electrodes can be realized toincrease the carrier injection. Because of the intrinsic property of organic singlecrystals, the self-waveguide effect will lead the electroluminescence from the OLEDstransfer to the edge of the OLEDs. Also the devices fabrication rate and efficiency ofOLEDs can be increased based on the template stripping method compare to theprevious reports.2. The OLEDs based on thiophene/phenylene co-oligomers single crystals areideal candidate for the polarized electroluminescence due to their intrinsic highlyordered moleculer arrangement and highly uniaxial alignment. Because of nearlyupright configuration of the transition dipole moments in crystals, the light will beconfined within the crystal and transfer parallel to the crystal plane. With theanisotropic microcavity formed by the thiophene/phenylene co-oligomers crystals,there is a large birefringence difference between a-axis and b-axis. There are two modes of EL from the crystal-based OLEDs which are TM and TE modes. Bychanging the direction of polarizer, the EL spectra will be split and describe as TE andTM polarization. Then the emission color from the crystal-based OLEDs can bechanged.3. The blue color surface light-emitting OLEDs can be achieved by theBSB-Me single crystals. The EL spectra from the OLEDs depend on the thichness ofsingle crytals used in the devices which will influence the microcavity of the OLEDsformed by the metal electrodes and the active layer of single crystal. Based on thedye-doping technique, we can change the blue color of the pure BSB-Me singlecrystals to the green and red by doping Pentacene and Tetracene materials. By takinguse of these RGB three color crystals, the basic colors for display by these RGBcrystal-based OLEDs which will expand the application of crystalline materials.4. By changing the dosage concentration of Pentacene in the doping crystals,the white light crystals can be prepared for the white light-emitting devices bycontrolling the proper energy transfer efficiency which containing either blue lightand red light in the EL and PL spectra. The effiency of crystal-based OLEDs wasincreased significantly by the doping materials which will confine the holes andelectrons for the recombination luminescence and decrease the carrier mobility. Inorder to explain the mechanism of the doping materials which is substitutional doping,we analyzed the polarized PL spectral obtained from the doped crystals.5. The DFB configuration of active waveguide grating structures for crystallaser, where a typical light emitting crystal material is used as the active medium, anda photoresist grating is fabricated on top of the active waveguide by using inerferencelithography. And by taking advantages of a periodic change in refrative index, theDFB mechanisms reflect light in a waveguide to provide optical feedback. Theavoidance of micromachining in the crystals may greatly maintain the intrinsicproperties of crystal and improve the cyrstal device performace which will be expectgreat applications in crystal electric pumped lasers.In summary, by taking advantages of template stripping method, this thesissystematically study the optical and electrical properties of the crystal-based OLEDs. The OLEDs show different characteristics by chaning different types of crystalswhich will make great sense of application of crystal materials. We realized edgelight-emitting, surface light-emitting, polarized EL light-emitting, RGB light-emittingand even white light-emitting by crystal-based OLEDs. In addition, by analysis ofpolarized PL spectra from doping crystals, we prove the doping molecules aresubstitutional doped in the doping crystal.