Experimental Study On Charge-Transfer Model For Intermolecular Singlet Exciton Fission

Over the past few years,the singlet exciton fission process?SF?occurring in the special origin materials has become a scientific hotspot in the field of organic electronics and Organic photovoltaic device?OPV?.The singlet fission process means that an exciton?S₁?transfers its half of energy to another molecule in the ground state?S₀?,and subsequently both of them undergo conversion into a pair of triplet exciton?T₁?.On the one hand,the triplet excitons have much longer lifetime than singlet excitons(S₁:10^-910^-66 s,T₁:10^-620 s),which facilitate the excitons diffusion to the interface and dissociation in devices.On the other hand,after absorbing a photon it can product two triplet excitons,if each triplet exciton can be dissociated into its component electron and hole,thereby enhancing the photocurrent of organic photovoltaic devices.Whereas,in practical applications,the singlet exciton fission process is one of the kind of the attenuation processes,the possible quenching affect between the excitons?including the singlet and triplet excitons?and charges might lead to the failure of the above sensitizing mechanism.So,as a kind of effective sensitizer,exciton fission rate must higher than the other attenuation process.We find that,with the increase of LUMO energy barrier between spacer molecules exciton fission rate gradually decreased by the experiment;Combined with this experimental results,we analyzed exciton fission's the electron-transfer model and the hole-transfer model next,so affirmed the“electron-transfer”model.For the research of intermolecular distance influence on the rate constant of singlet fission,we combined Dexter energy transfer with“electron-transfer”model,once again to verify the the rationality of exciton fission's electron-transfer model.The effective implementation of these work,for the research of exciton fission microcosmic physical mechanism provides a practical and reliable experimental method and train of thought.But to fully understand the phenomenon of principle and working mechanism,is very difficult.Higher LUMO energy barrier induced by spacer molecules led to slower rate constant of singlet fission between rubrene,This thesis will focus on the multiple important problems which are related with the singlet fission rate including molecular distance and energy barrier in rubrene doped organic films,and deeply analysis“electron-transfer”model.Through the careful design of experiment,trying to obtain quantitative results and certainty conclusions,and promoting the progress in the aspects of theoretical explanation and device application of singlet fission.Our results summarizes as the following:?1?Firstly,some theoretical knowledge about exciton fission process was introduced.The current research situations of singlet fission process in the world were analyzed.Secondly introduced the research progress of exciton fission process,finally expounds the main research work of this thesis.?2?The second chapter,mainly from the experiment preparation,thermal evaporation of the doped organic films,this paper introduces the experimental sample preparation process.From steady-state photo-luminescence?PL?to time-resolved florescence decay were measured,and introduces the measurement process.?3?The third chapter introduces the intermolecular charge transfer model of exciton fission process,by doping highly efficient fission molecule?rubrene?in different organic molecular,we explore the influence of molecular distance to singlet exciton fission in rubrence-doped organic films.Experimentally,the five samples with structure of Alq₃:rubrene?x??NPB:rubrene?x??BCP:rubrene?x??TPBi:rubrene?x??mCP:rubrene?x?were fabricated?x=50%?40%?30%?,steady-state photo-luminescence?PL?and time-resolved florescence decay were measured and at room temperature.Through fitting all transient fluorescence decay curves,we find higher LUMO energy barrier induced by spacer molecules led to slower rate constant of singlet fission between rubrene,but don't exhibit any relevance with HOMO energy barrier in the same doping concentration.In this conclusion,we verified the rationality of exciton fission's“electron-transfer”model and denied“hole-transfer”model from the perspective of experiment.?4?The fourth charter induced the Dexter energy transfer,by doping highly efficient fission molecule?rubrene?in different organic materials with different concentration,we explore the influence of molecular distance to singlet exciton fission in rubrence-doped organic films.Experimentally,the four samples with structure of NPB:rubrene?x??BCP:rubrene?x??TPBi:rubrene?x??mCP:rubrene?x?were selected?x=20%?30%?40%?50%?,steady-state photo-luminescence?PL?and time-resolved florescence decay were measured at room temperature.Through fitting all transient fluorescence decay curves,we find that nanoscale exponential distance dependence for intermolecular singlet exciton fission observer in rubrene-doped organic films.This is similar with the well-known Dexter transfer process in the function relation and physical modern.our findings suggest that singlet fission is greatly relater with the intermolecular electron-transfer.We believe that all the results present in this work will offer a glimpse of how the intricate dynamics of singlet fission manages this efficient state transformation.At last,according to the Fermi golden rule,we take about“electron-transfer”model deeply and analysis the direct mechanism or superexchange mediated mechanism.?5?In this paper,the fifth chapter mainly to summarize the above experimental results,and prospects the developing prospects.