The Study On Magnetic Field Effects In Organic Semiconductors And Hybrid Organic/collodial Quantum Dot Devices

Organic magnetic field effects(MFEs)refer to the phenomena that the current and/or light intensity of the organic semiconductor devices(containing no magnetic components)are changed when the devives are submitted to an external magnetic field.Recent studies have shown that the magnitude of organic MFEs can reach to as high as tens of percent under magnetic field with tens of milliTesla(mT)at room temperature.The organic MFEs are usually independent on the direction of magnetic field.These amazing properities of organic MFEs render their potential appliactions in data storages,sensors,and hand-writing inputing.On the other hand,organic MFEs can be also used as a powerful tool in revealing the nature of the generated elementary excitation and their relaxation dynamics during the operation of the devices.Based on the lineshape,magnitude,and sign changes of organic MFEs in the devices,the charge reaction and excitons behavior can be well revealed.Recently,the MFEs in organic semiconductor devices have received widespread interest.Theoretical and experimental studies of the organic MFEs have been reported.It is observed that the organic MFEs show different lineshapes and/or sign changes.Several mechanisms have been proposed to explain these phenomena such as the electron-hole pair model,triplet-charge interaction,bipolaron formation,and triplet-triplet annihilation.Although these theoretical models can explain well the organic MFEs under certain conditions,the details in these mechanisms are still under debate,especially the origins of the negative organic MFEs and the sign changes in organic MFEs.In most of the reports,the sign of organic MFEs is positive,that is,the light intensity and current increase as the strength of magnetic field increases.Negative organic MFEs are relatively rare.Importantly,the simultaneous sign changes of magneto-electroluminescence(MEL)and magneto-conductance(MC)from positive to negative in a device have never been reported before.The hybrid organic/colloidal quantum dot light emitting diodes(QD-LEDs)are an emerging field in recent years.Since the hybrid organic/inorganic quantum dot systems can easily integrate the solution-processibility of organic semiconductors and tunable bandgap of inorganic quantum dots,the hybrid QD-LEDs are among the hot topics in the community of Light-emitting.Although remarkable achievements in light-emitting efficiency have been achieved in the hybrid QD-LEDs,the underlying mechanisms for the light-emitting are still debated.Specifically,the fundamental processes for energy transfer,charge injection,and Auger recombination in the hybrid organic/colloidal QD-LEDs are still not very clear.Thus,it is very important to introduce the MFEs into the hybrid QD-LEDs.The charge reaction and exciton dynamics show different magnetic field responses in the organic semiconductors and inorganic quantum dots.Investigations on the MFEs in the hybrid organic/colloidal QD-LEDs would not only reveal the underlying mechanisms for light-emitting,but are expected to obtain more information on MFEs.In this thesis,organic semiconductor-based single layer devices and hybrid polymer: colloidal quantum dot nanocomposites-based devices were intentionally designed.The MC and MEL effects in these devices were systematically investigated,and the origins of the negative MC in organic semiconductor single layer devices and simultaneous sign changes of MEL and MC in the hybrid polymer: quantum dot nanocomposites were well studied.Furthermore,we also successfully demonstrated the feasibility of MEL as an in-situ fingerprint for energy transfer process in the hybrid organic/colloidal QD-LEDs.The main contents and results in this work are summarized as follows:1.Negative MC effects in amorphous copper phthalocyanine thin filmMost of the reported organic MFEs are obtained from organic light-emitting diodes and organic solar cells with multiple-layer architectures.Due to the disordering structure and the complicated electron distribution in the organic semiconductor thin films,the interface and energy barriers caused by the multiple-layer structures have significant influences on the organic MFEs.To exclude these unfavorable factors,the simpliest device structure with copper phthalocyanine(CuPc)single layer sandwiched between the Indium Tin Oxide(ITO)anode and Aluminum(Al)cathode,that is,ITO/CuPc/Al was adopted in this work.The CuPc was selected as active layer since the CuPc is a very common p-type organic semiconductor with high hole mobility and high density hole traps in its amorphous thin films.These traps would play important roles in organic MFEs.Our results revealed that the current of the CuPc-based single layer devices decreases with increasing the strength of applied magnetic field,showing negative MC effects in a large temperature range from room temperature to 20 K.The magnitude of negative MC increases when the temperature decreases,and it scales with current density for nearly three orders at 20 K.The corresponding current density–voltage characteristics of the device at different temperatures reveal that this negative MC is related to the presence of traps in CuPc thin film.Based on these results,trap-assisted bipolaron formation,a developed mechanism based on bipolaron,has been proposed.We suggest that traps existing in CuPc thin film can assist the formation of bipolarons through lowering the formation energy.This model is further confirmed by the negative MC responses with light illumination.2.Sign changes of MFEs in polymer: colloidal quantum dot nanocompositesNanocomposites of conjugated polymer and colloidal quantum dots have attracted considerable attention for optoelectronic and photovoltaic applications.The hybrid polymer: inorganic quantum dot systems can not only integrate the advantages of both the polymer semiconductors and inorganic quantum dots,but induce the interactions between the polymer semiconductors and inorganic quantum dots,resulting in some interesting results.We can easily transfer the knowledge on organic MFEs into such a hybrid system to identify the origins of the observed MFE.In the mean while,some interesting MFEs that were not reported before are also expected to be obtained.Here,typical polymer: inorganic quantum dot systems,that is,poly [2(4(3',7' dimethyloctyloxyphenyl)1,4phenylene Vinylene](P-PPV): CdSe-CdS-ZnS quantum dots composites are selected as active layer.The nanocomposite-based devices with the structure of ITO/PEDOT: PSS/P-PPV:CdSe-CdS-ZnS(x %,by weight)/electron transport layer(ETL)/Li F/Al were fabricated.The effects of magnetic field on the electroluminescence and current in such nanocomposite systems were studied.Our results show that by increasing the concentration of CdSe-CdS-ZnS QDs in the hybrid nanocomposites from 0 to 25 wt.%,the polarities of MEL and MC are simultaneously changed from positive to negative.To the best of our knowledge,this is the first time to obtain simultaneously negative MEL and MC in the same device.In addition,the amplitudes of negative MEL and MC are enhanced as the temperature decreases,showing an abnormal temperature dependence.We attribute these magnetic field effects to CdSe-CdS-ZnS QDs-induced direction reverse in spin-mixing of loosely bound polaron pairs in P-PPV matrix prior to energy transfer to QDs.With this study,we demonstrated that incorporating QDs in polymer matrix can strongly influence spin-selective interactions in the hybrid nanocomposites,which may path the way for spin-related applications of these fascinating hybrid nanocomposites.3.In-situ investigation of energy transfer in hybrid organic/colloidal quantum dot light-emitting diodes by magneto-electroluminescenceNotwithstanding dramatic advances in the hybrid organic/colloidal QD-LEDs,the fundamental processes for light-emitting in these devices are still not very clear.In general,energy transfer and charge injection are the main two mechanisms responsible for the light-emitting in the QD-LEDs.To further improve the device performance,it is necessary to clarify which mechanism governs the light-emitting in a specific device.Unfortunately,until now there is not any facile and effective way avaible for such a clarification.Here,we proposed that MFEs might be a useful way to discern these two processes.In the charge injection process the excitons are mainly formed in quantum dots layer.Thus,the charge injections and radiative recombination of excitons in quantum dots cannot be affected by a small magnetic field(< 0.5 T)due to the inorganic nature of quantum dots with strong spin-orbit coupling.In contrast,in energy transfer process the excitons are formed firstly in the organic semiconductor layer.Although the rate of F?rster resonant energy transfer(FRET)is not related to the magnetic field,the singlet/triplet ration of excied states in the organic semiconductor can be changed by the applied magnetic field.In this case,MFEs are expected to be observed based on the fact that singlet excitons can be transferred into quantum dots through the FRET process,while triplet excitons cannot since they have zero oscillatory strength.To verify this assumption,typical hybrid organic/colloidal QD-LEDs were fabricated with the structure of ITO/PEDOT: PSS/poly-TPD/CdSe-ZnS quantum dots/ETL/LiF/Al.Through measuring the MEL and current efficiency,we indicated that energy transfer would be the main mechanism for the light emission in the present hybrid QD-LEDs.To further demonstrate the feasibility of MEL as an in-situ fingerprint for ET process in QD-LEDs,devices with various thickness of TPBi ETL from 0 to 80 nm were fabricated and tested.The dependences of MFEs on the thickness of the ETL confirm the positive relationship between the MFEs and energy transfer.We also studied the MFEs in the device without poly-TPD but with organic and inorganic ETLs.The results revealed that organic ETL could be also involved in energy transfer in the hybrid QD-LEDs under some given conditions,showing obvious MEL.However,with the inorganic ETL of Zn O(the excitation of quantum dots in such a system is suggested to be governed by direct charge injection),no MEL is observable in the devices.Therefore,this study strongly suggested that MEL could be a highly sensitive fingerprint for energy transfer,which provides a facile and efficient method for in-situ investigation on fundamental processes in hybrid organic/colloidal QD-LEDs.