| Organic electroluminescence?EL?diodes can emit light from the organic active-layre when they are applied a electric field.They are generally called OLEDs for short.Since the sandwich-type OLED were invented by doctor C.W.Tang and his colleagues,the organic electroluminescent materials and devices were researched on a large scale and made a great progress.Using organic materials for electronics offers many advantages.They are cheap and easy to process,and also offer new possibilities as they are chemically tunable and can allow for flexible or transparent applications.Many different applications,except for OLEDs,have been developed like organic solar cells?OSC?,organic spin valve?OSV?,and organic field-effect transistor?OFET?.In the recent twenties,a rapid development has been gained in these organic applications.This leads to a new type of cross subject dubbed organic spintronics which involves chemistry and physics such as organic chemistry,photochemistry,high polymer chemistry surface physics,solid physics,and semiconductor physics.Organic spintronics deals with the electronic effects of spins in organicmaterials,it can help us to understand the properties and micro-processes in organic devices and materials like electron transfer,spin transportation and spin mixings of e-h pairs.Hence,organic spintronics can guide us to optimize the organic applications.However,the chemical bond in carbon-hydrogen based organic semiconductors is Van der Waals force,which is an order of magnitude lower than that in inorganic one.On the other hand,the binding force in Frenkel or charge transfer?CT?excitons in organic materials is much larger than the one in Wannier exciton in inorganic device.Morevoer,the mobolity of carriers in organic is several orders of magnitude lower than inorganic much,leading to a strong locality.It is due to the complexity of these of organic semiconductor materials and devices,we can't research them completely in accordance with the method of inorganic semiconductor.Hence,the internal mechanisms in organic semiconductors are not perfectly understanding.For example,the knowledge on the e-h pair involed spin-mixing processes in organic device are not perfect,part of proposed views conflict each other.This need us to further explore them.The spin-mixing processes of e-h pair can be analyzed via various methodes,and the commonly used are the measurements of absorption spectrum and emission spectrum,transient spectra,and bright-voltage-current?B-I-V?characteristics,etc.The possible internal mechanisms can be determined by these measurements by changing the device structure,working temperature,applied voltage or functional layer,and so on.However,part of spin-mixing processes can hardly be distinguished by these methods.For example,both the triplet-triplet annihilation?TTA?and reverse intersystem crossing?RISC?have a similar characteristic in transient spectra.Hence,we can not distinguish them with transient spectra,nor can B-I-V characteristics.Fortunately,both the conductance and EL can change substantially when applying an external magnetic field in organic diodes.This effect was dubbed “organic magnetic field effect”?OMFE?.In OLED,the OMFE on electroluminescence and current are magneto-electroluminescence?MEL?and magneto-conductance?MC?,respectively.The curves of MEL and MC of different spin-mixing processes always have different characteristics which can be used as the identity tags or fingerprints of spin-mixing processes by us to analyze the internal mechanisms non-contactly.By virtue of the MEL,the TTA and RISC can easily distinguished.Additonally,by changing the device structure,working temperature,organic-metal interface and functional layer doping,and so on,various spin-mixing processes of e-h pairs can be modulated before they are analyzed and quantizated by relative change in the multiple magnetic fingerprints.By now,except for OLED,the OMFE was also discovered in OSC,OSV and OFET.While the OMFE in the latter three types of device is mainly MC which involves in the magnetic suppressed mobolity and interactions between carriers or polarons.Differently,the e and h can meet to form e-h pair.If the e and h belong to the e-h pair lie on two organic molecules,the e-h pair called polaron pair?PP?.If e and h locate on a same molecule,the e-h pair dubbed exciton.The PP is precursor of exciton.Based on the spin selected rules,the PP is spin paired and the ratio of singlet to tripet is 1:3,both in PP and exciton.In the flurescent OLED,the radiation of triplet e-h pair is spin prohibition,only singlet exciton can contribute the light.This means 75% triplet exciton is useless.On the other hand,the lifetime of triplet e-h pair ranges from 0-6 to 102 s,which is several orders larger than the 10-9s of singlet.Thus the triplet e-h pairs have enough time to proceed spin mixing.The possible triplet-involved spin mixing process include ISC,one singlet convert into two triplet through fission?SF or STT?,two triplet e-h pairs fuse into one singlet by annihilation?TF or TTA?,one triplet e-h pair interacts with a carrier or a polaron?TCA or TPI?,etc.These processes will change the singlet/triplet ratio and,then bring a substantial influnce both on EL and conductance.Under the external magnetic field,the influnce on EL and conductance can be modulated by field leading to MEL and MC.Hence,the possible spin mixings in OLED are much more complicated and diversiform than those in other organic devices.Moerover,they can interact and transform with each other.The good news is we can use MEL and MC simutaneously to investigate the possible spin mixings in OLEDs.Additonally,we can modulate the spin mixings in OLEDs via changing the device structure,conditional temperature,interface and doping.After nessesary contrastive analysis and formula fitting with the MEL and MC fingerprints of the known spin mixings,we can determine the unknown fingerprints to enrich the fingerprint database of spin mixings involve e-h pair.In this thesis,the spin mixings of e-h pair in OLED were modulated and analyzed with MEL and MC via the following several aspects.First,the cathode and cathode deposition methodes were changed to modify the organic-metal inerface of the device with electron transform material acts as active layer.Secondly,the the cathode of device with hole transform material acts as active layer was changed to modulate the electon injection and traps.Thirdly,the impurities such as Fe3O4 and structual defect were introduced in active layer.Finally,the molecular spacing of exciton energy resonance rubrene was changed and then realized the conversion of STT to TTA,wich can increase the efficiency of rubrene-based OLEDs.The details were divided into the following chapters:In the first introductory chapter dealing with the contemporary knowlede and notions regarding the organic spinelectronics,especially the formation of e-h pair and the spin mixing between e-h pair and excitons and carriers such as hyperfine interaction?HFI?,spin-orbital coupling?SOC?,“?g” model,bipolaron,TTA,STT and TCA.We cover the experimental methods and sample analysis techniques used in this thesis.Chapter 2 contains a detailed description of the methods used in OLED fabrication and sample characterization such as the vacuum deposition methods for organic and metallic cathode,EL spectra and magnetic effect analysis.In chapter 3 we explored mechanisms for the high-field?|B| > 50 mT?decay of organic MEL by modifying the organic/metal interface in pristine tris?8-hydroxyquinolinato?aluminum-based OLEDs.The rganic/metal interface was modified by changing the metal cathodes and their deposition methods.The metals investigated were Al,Au,and Cu and the methods used include molecular beam deposition?MBD?,thermal resistive evaporation?TRE?,and electron beam evaporation?EBE?,respectively.Experimental results revealed that the high-field decay can be observed at room temperature when the cathode is:?i?Cu deposited by EBE or?ii?Au deposited by any of the three deposition methods.Furthermore,this decay is different from the previously reported high-field decay that originates from triplet-triplet annihilation,triplet-charge reaction processes or ?g mechanism.We suggest that the magnetic field can increase the extent of overlap between the electron-hole recombination zone and the organic/metal interface by suppressing electron mobility.The spin-orbital coupling at the organic/metal interface consequently induces intersystem crossing to increase with magnetic field leading to the observed high-field decay.In chapter 4,we fabricated the rubrene-based organic emitting devices with Al,LiF/Al and Ca acting as metallic cathodes to analyze the mechanism for magneto-conductance?MC?induced by triplet-charge interaction?TCI?.The MC in Al-cathode device decreases monotonously with increasing external magnetic field,which is obviously different from the one in LiF/Al-or Ca-cathode device.Using the energy resonance of singlet?S?and triplet?T?excitons in rubrene,the singlet fission and triplet fusion was changed to alter the ratio of T.the charge?C?was also changed by metallic cathode modulated electronic injection barrier.Consequently,the TCI was substantially modulated.The results suggested that the negative MC curves were due to the quenching of T exciton by trapped charges via detrapped channel?T+Ct?S0+C?,rather than by dissociation or scattering of excessive charges.It was also suggested that the MC of balanced injected LiF/Al-and Ca-cathode devices is an order of magnitude smaller than the one of Al-cathode device.Additionally,it decreases in low-field region?<100 mT?and then turns to increase with increasing B field.The small magnitude of MC in balanced devices does not contributed to the weakness of TCI,but to the assumption that weak influence detrapping and trapping channels of TCI on electronic current.We believe this is because the traps in active layer of rubrene are fully occupied by minority electrons that are easier injected from cathode.Hence,the carrier traps in quenching way paly a primary role in the MC response of TCI.Consequently,producing enough un-occupied carrier traps in organic functional layer is a promising approach to utilize the MC of TCI.In the previous chapter,the modulation of organic-metal interface indicates that the minority electron in prinstine Rubrene is the origin of MC for TCI.We go deeper into the MEL and MC induced by trap related spin mixings of e-h pairs in chapter 5.The traps were introduced intentionally into active layer via two approaches.In the first approach we produce structure defect in crystal anthracene-based OLED.The effects of a magnetic field on the dissociation of triplet excitons by free charges?TCI?are well understood.However,the magneto-conductance?MC?characteristics of trapped triplet-polaron interactions?TtPI?and triplet-trapped polaron interactions?TPtI?within organic light emitting diodes?OLEDs?are not well understood.We have studied these interactions in an anthracene-based OLED.The electroluminescence spectra,current-voltage characteristics and magneto-electroluminescence indicated that the anthracene layer contained many defects that could trap either triplet excitons or polarons,which led to TPtI and TtPI.The MC curves at low temperature exhibited a complex line shape,which indicated that intersystem crossing,TPtI,TtPI,and TCI were occurring simultaneously in the device.The individual MC characteristics of TPtI and TtPI were extracted from temperature dependant MC curves by fitting them to three empirical Lorentzain and one non-Lorentzain functions.The MC of TPtI exhibited a negative sign,while that of TtPI exhibited a positive sign,with characteristic magnetic fields?B0?of 10.5 and 15 mT,respectively.Both processes were prominent below 150 K and weakened with increasing temperature.TPtI was neglected above 200 K while TtPI was observed even at ambient temperature.These results add significant insight to magnetic field effects on triplet-polaron interactions.In the second approach we doped Fe3O4 nanoparticle into polymer of SY-PPV to act as non-radiation trap sites.Moreover,the ageing treatment was performed on the OLEDs by means of large current to analyze the magnetic field effects of large amount of traps.The results indicate that MEL lineshapes changed significantly: in absence of doping,the MEL increase with field quikly and then saturated at higher field that can characterized by ISC.When the device doped with Fe3O4 nanoparticle,the EL became much weaker and the device showed a large leak current.Additonally,its MEL is similar to that of device without doping.However,the MEL shape become negative and opposite after the ageing treatment.This case has never been reported.The AFM of the surface of Fe3O4 doped SY-PPV indicated that large number of crystallization structure trap caused by high temperature.These results further demonstrated that the traps can play a crucial role in the MEL,they can even change the direction of spin mixings of excitonic e-h pair leading to c complete opposite magnetic response.In chapter 6 we have studied the influence of molecular spacing d on the singlet fission?SF??yriplet fusion?TF?convesion in rubrene-based OLEDs using MEL.The TF and SF are two important spin-coupled exciton interactions that occur in rubrene-based organic light emitting diodes?OLEDs?.TF produces additional singlets,which increases fluorescence efficiency while SF consumes singlets and lowers the fluorescence efficiency.In an effort to adjust the SF and TF processes in rubrene-based OLEDs,the d was changed by doping rubrene at varying concentrations in the high triplet energy material 1,3-bis?9-carbazolyl?benzene?mCP?.Using MEL,we observed that TF increased while SF decreased at ambient temperature as d was increased from 1.8 nm to 5.0 nm.This was further confirmed using MEL at different temperatures and current intensities.We found that the efficiency of rubrene-based OLEDs was improved by altering the value of d,with the highest efficiency being observed at d = 3.8 nm because of complete SF?TF.This work presents a promising approach to improve the efficiency of rubrene-based OLEDs at room temperature.In addition,based on the Merrifield theory on MEL induced by TF,we proposed a quantitative formula for the relative intensity of TF and SF via the high-field decay of MEL.This formula provide a approach for the quantitation of spin mixings of excitonic e-h pair in OLEDs by MEL. |
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