High-performance Organic Light-emitting Devices Based On Organic Single Crystals

Organic single crystals（OSCs）have drawn significant attention in the field of optoelectronics due to their highly ordered structure,low impurity,high carrier mobility and high thermal stability.Progress has been made in the OSCs-based organic light-emitting devices（OLEDs）including surface-emitting devices with three primary color and white emission.However,the luminance and current efficiency of OSCs-based OLEDs are far from the requirements for practical applications.Numerous efforts are still needed for high-performance OSCs-based OLEDs,such as rational design principle and growth strategy for OSCs and device fabrication and optimization for OSCs-based OLEDs.One of the major problems is the unipolar properties of OSCs with unbalanced hole and electron mobilities,resulting in unbalanced carrier transport.In particular,the highly aligned molecular orientation of OSCs endows them with anisotropic properties that are promising for realizing polarized OLEDs.The lack of researches on the OSCs-based polarized OLEDs makes it a challenge to realize high polarized emission.In this thesis,we focus on the topic of high-performance OSCs-based OLEDs.The device efficiency has been promoted by manipulating the optical and electrical properties of OSCs through controllable molecular doping.Meanwhile,OSCs are employed in the OLEDs for polarized electroluminescence（EL）emission.The EL polarization can be dramatically amplified by constructing a microcavity structure in OSCs-based OLEDs to effectively couple microcavity resonance to polarized emission.The research details of this thesis are shown as follows:1.The host BSB-Me OSCs were doped with P2TCF₃ that was used as the dopant in the molecular doping.Both tunable light emission and enhanced electron transport were achieved in the host OSCs.The OLED performance based on the doped OSCs is three times over that of the undoped OSCs.The maximum luminance and current efficiency of the doped OSCs-based OLEDs are 423 cd m^-2 and 0.48 cd A^-1,respectively.2.The ambipolar OSCs with balanced carrier mobility can be realized by the growth of the mixed n-type BTPB and p-type BSB-Me molecules.Then,these ambipolar OSCs were applied to OLEDs with a threefold enhancement in device efficiency.The pentacene molecule as fluorescent dye was further doped into the ambipolar OSCs to act as the exciton confinement center with efficient energy transfer.The TAPC layer was employed in the OLED structure as the hole-transport layer to reduce hole-transporting barrier.As a result,the maximum luminance and current efficiency were obtained to be 5467 cd m^-2 and 2.82 cd A^-1,respectively,which were record performances for the OSCs-based OLEDs.3.The BP1T-CN OSCs with anisotropic properties have been employed in polarized OLEDs to obtain polarized EL.The polarization of EL from BP1T-CN OLEDs has been dramatically amplified to be 176 by constructing a microcavity structure in OLEDs to effectively couple microcavity resonance to polarized light.Both experimental results and theoretical simulations confirmed that the high polarization was attributed to the high anisotropy of BP1T-CN OSCs as well as the Purcell effect induced by microcavity.In addition,BP1T-CN OLEDs exhibited high efficiency because the orientation of transition dipole moment was nearly parallel to the crystal surface in the BP1T-CN OSCs,which reduced the coupling efficiency to surface plasmon-polariton modes.The maximum luminance and current efficiency obtained from polarized BP1T-CN OLEDs were 6122 cd m^-2 and 1.86 cd A^-1,respectively.In summary,systematic researches have been made for the growth and tunable properties of OSCs as well as the design and fabrication of OSCs-based OLEDs.High efficiency and high polarization OSCs-based OLEDs have been realized.In this thesis,the development of OSCs-based OLEDs will pave the way for their applications in the field of electronics and optoelectronics.