Research On Surface-plasmon-polariton-enhanced Efficiency In Organic Optoelectronic Devices

During the past decades, the feld of organic optoelectronics has been rapidlydeveloped. The widespread interest in organic semiconductor-based devices is notonly due to their potential in portable application, low cost and large-area manufacture,but also in transparent and fexible applications compared to inorganic devices.However, their eifficiency and stability are still need to be enhanced for thecommercial applications. In this thesis, microstructure has been fabricated and appliedin TOLEDs to excite surface plasmon polaritons （SPPs）. The SPP-induced fieldenhancement has explored in top-emitting organic light-emitting devices （TOLEDs）to solve the tradeoff between device stability and efficiency persisted in theconventional TOLEDs, and improve the power conversion efficiency in organicphotovoltaic devices （OPVs）. The work in this thesis can be summarized as follows:（1） Resolving the tradeoff between stability and efficiency in the TOLEDs byusing the cross coupling between SPPs and microcavity modes to enhance thetransmittance of the thick metallic cathodeGenerally, the thickness of transparent metallic top electrode is less than20nmin TOLED. Pinholes are formed in electrode due to poor film continuity for such athin film, which results in easy diffusion of water and oxygen from air into thedevices and accelerates the degradation of the devices. Thicker film offers highercontinuity, however, the higher device stability comes at the expense of the lowerefficiency due to the decreased optical transmittance. Obviously, there exists afundamental tradeoff between device stability and efficiency in the conventionalTOLEDs. Here, the efficiency-stability tradeoff existed in the TOLEDs has beeneffectively released by employing a periodic microstructure in the device. Theintroduction of the periodic corrugation has allowed a much enhanced lighttransmission through a thick Ag cathode through the grating-induced cross couplingbetween the SPPs associated with top interface of the cathode and microcavity modes within the device cavity. The efficiency of the TOLED was improved by40%and thelife time was prolonged from90to165hours at the initial luniance of1000cd m^-2due to the increased thickness of the transparent electrode.（2） Enhanced optical absorption of the photovoltaic layers by excitingpropagating SPP at the metallic electrode/organic interface.The OPVs are an attractive alternative to Si-based solar cells due to theiradvantages of low cost, light weight, simple fabrication process and flexibility.However, the power conversion efficiency of the OPVs is still low for commercialapplications. The tradeoff between the efficiency of photon absorption and excitonharvesting is one of the main limitation factors. The active layer is generally less than100nm due to the short exciton diffusion length, which limits the efficiency ofincident light absorption. A thicker active layer offers higher light absorption,however, it comes at the expense of lowered exciton harvesting. Here, we employed aperiodical corrugation into the OPVs. The optical absorption of the photovoltaiclayers was enhanced by exciting propagating SPPs at the metallic electrode/organicinterface. Through tuning the SPP resonances to the intrinsic absorption region, theshort circuit current of the corrugated device with appropriate period has beenincreased from4.1mA/cm²for planar device to5.5mA/cm². As a result, the powerconversion efficiency was improved by35%. Then, we employed periodic grating inthe multi-junction organic solar cells. Using the field enhancements of SPPsresonances at the interface between iner-layer cell and metallic electrode, increasedabsorption of the iner-layer cell, and therefore, absorption matching in each sub-cellhas been obtained. Meanwhile, the cross coupling between the microcavity mode andthe SPP mode increases the optical absorption in the whole device. As a result, thephotocurrent of the whole double-junction OPVs was improved by10%, and the PCEof6.1%has achieved, which exhibits12%increment.（3） Expending the absorption spectral coverage by using SPPs-mediated energytransfer from antenna layer to the active lyer in the OPVsNormally, the absorption bands of semiconducting organic molecules andpolymers are narrow, which limits the OPVs to function efficiency. Here, we enhance the absorption spectral coverage in OPVs by employing antenna layer withcomplementary absorption to the active layers. The energy transfer from the antennato the active layer was enhanced through the mediation of the the SPPs associated atthe metallic electrode/organic interface. As a result, compared to traditional OPVs, thephoto current of the antenna–based OPV was increased from6.07mA cm^-2to6.98mA cm^-2and the power conversion efficiency was enhanced by16%.In summary, we study the SPPs excitation by the introduction of the periodiccorrugation to the metallic electrodes, and its applications in both OLEDs and OPVs.The tradeoff between stability and efficiency in the TOLEDs has been resolved tbyusing the cross coupling between SPPs and microcavity modes. Increased efficiencyof the OPVs has been obtained by SPP-induced field enhancement. Meanwhile, thefield enhancements of the SPPs can also be used in other organic optoelectronicdevices, which will create a new way to improve the performance of the organicoptoelectric device.