The Study Of Stimulated Emission Properties Of Organic Semiconductors

Organic semiconductor laser is promising to fulfil the requirement of high quality light source on scientific researches and practical applications due to the advantages of higher stimulated absorption and emission cross section,minor temperature dependence,solution processibility,low cost and flexibility.Despite organic optically-pumped laser has already been widely and deeply studied,electrically-pumped laser has not been realized yet.The study of organic electrically-pumped laser is a challenging project,which is of high demands for new gain materials with high stimulated emission performance as well as sophisticated design of device structure.An important foundation is to understand the stimumlated emission properties of gain materials.Spectral properties of organic semiconductors reflect information about the molecular structure,intermolecular interaction and responses to ambient environment;therefore they become a window to understand the behaviours of materials.In this thesis,based on the comprehension of stimulated emission properties of organic semiconductors,sereval key issues in realizing organic electrically-pumped laser,including to develop low loss materials and to study the origin of the loss,and functional interfaces for optimizing stimulated emission performance of gain materials,are investigated by multiple spectral techniques.In Chapter 2,it describes the producing mechanism and loss mechanism of organic lasing process,including the definition of population inversion and bimolecular annihilation as well as photo-induced absorption.It also introduces the amplified spontaneous emission?ASE?effect and transient absorption spectroscopy,of which are used in characterizing gain materials.In Chapter 3,considering the optical loss of gain materials,a series of organic conjugated molecules with low threshold and low optical loss are designed?OCBzC,OCPC,OCNzC,DF-Bz-DF?.Taking the OCBzC molecule as an example,ASE effect is systematically characterized on it.A gain threshold of 39.27?J/cm²,a gain coefficient of 49.33 cm^-1 and a loss coefficient of 1.91 cm^-1 are achieved in this molecule.Ultrafast spectral studies reveal that the relaxation behaviour of the excited state is simple for OCBzC.Excited state loss only originates from the small overlap between singlet exciton absorption and stimulated emission.In addition,OCBzC microring resonator is successfully fabricated on a silicon fibre by solution processed method,obtaining a single mode lasing with a quality factor of 2700 and a full width at half maximum?FWHM?of 0.2 nm.Based on the understanding of OCBzC,influences of emission core and side group structure on stimulated emission are discussed,which leads to a conclusion that the excited state behaviours dominate the formation and quality of stimulated emission.In Chapter 4,the impact of carzole-based defect on stimulated emission is investigated by comparing the ASE performance between OCBzC and DF-Bz-DF,two model molecules of the conjugated polymer F8BT.Through simulation of frontier molecular orbitals and spectral characterizations,defects of charge transfer?CT?couple between carbazole and benzothiazole are evidenced in OCBzC film.Defect lifetime is approximately 1.47?s,the density is about4.57×10¹⁶ cm^-3.Notably,the excited state behaviour of defects competes with ASE process.It may induce extra absorption to weaken the stimulated emission.Further considering the common points between OCBzC and F8BT,one can figure out that CT defect,as a kind of structural defect,is easy to become a hindrance of stimulated emission under high energy injection.Thus,it is necessary to avoid its formation when designing gain materials.In Chapter 5,aiming at reducing the high demand of designing gain materials with excellent performance,thermal crosslinked spacer of PMMA that containing gold nanoparticles?PMMA:Au NP?is utilized as a functional interface to optimize the ASE properties of gain materials.For the MEH-PPV system that fulfils the spectral selection rule,the emission intensity increases by 11.8 folds,the gain threshold reduces by 40%and the FWHM decreased by 16%.It is attributed to an interfacial multiple localized surface plasmon resonance?LSPR?enhancement between MEH-PPV layer and PMMA:Au NP spacer.It can not only increase the absorption possibility of MEH-PPV,decreasing the non-radiative loss,but also add an extra enhancement for ASE while light reflects repeatedly at interface.Besides,PMMA:Au NP spacer also works in OCBzC system that unfulfils the spectral selection rule.By comparing the effect of every possible component contained in PMMA:Au NP spacer,reduced refractive index of PMMA induced by holes in the spacer is responsible for the enhancement.The arisen holes can increase the refractive index difference between the gain layer and the spacer,thus reduce optical loss for light propagation.With this functionalized interface,it provides a new idea to optimize the stimulated emission properties of gain materials.