Singlet Fission And Polaron Formation From Upper Excited States Of Organic Films

With the burning and utilization of fossil fuels(coal,oil,natural gas,etc.),environmental problems such as air pollution,scarcity of resources,and global warming have become increasingly serious.It is urgent to solve these problems,therefore,the development and utilization of clean and renewable energy such as solar energy has become particularly important.Photoelectric devices such as solar cells convert solar energy into electrical energy,which is a research hotspot in the world today.Due to the limitation of Shockley-Queisser theory,the efficiency of traditional single junction solar cells can reach only 32%.However,the introduction of singlet fission materials can theoretically double the efficiency,so it has attracted wide attention from scientists.The thesis mainly studies the singlet fission process in organic thin films,and understands the specific photophysical and chemical processes that occur with singlet fission,such as intersystem crossing,internal conversion,charge separation,etc.,the work can help people to understand singlet fission more deeply.The thesis used several ultrafast time-resolved spectroscopy techniques,such as Time-Correlated Single Photon Counting(TCSPC),up conversion,and pump-probe transient absorption technique.In addition,we also used several characterization techniques to detect the surface morphology of organic thin films,namely:Atomic Force Microscope(AFM),Scanning Electron Microscope(SEM),and X-ray diffraction(XRD).By use of Labview and Glotaran,we performed fittings on the obtained data.Gassuian is used to do DFT calculation,optimize molecular configuration,and calculate energy levels of excited states.By using the above several testing techniques and fitting methods,we have obtained a series of results,as follows:1.After excited by 250 nm(4.96 e V)excitation wavelength,the rubrene molecule reaches the high-level singlet excited state S₂,and then bypassing the S₁ state to undergo singlet fission(within 30 fs),thus breaking the classical Kasha rule.From the transient absorption results,we also detected the formation of polarons within 30 fs.Whether it is polaron formation or singlet fission,when they occur from the upper excited state,the rate will be higher than when they started from the lowest singlet excited state.Our results show that rubrene films with low crystallinity can also undergo effective singlet fission,especially when excited to a high-level excited state.This result can greatly expand the application of rubrene in organic electronic products,and will provide a new direction for the synthesis of new materials with optimized excited state properties for use in organic photovoltaic applications.2.We have studied the excited state dynamics of LA17a solutions and films under various excitation conditions by use of femtosecond pump probe set up and fluorescence spectroscopy.In solution,we observed both singlet and triplet excited states.The formation of triplet excited states is attributed to the intersystem crossing(ISC).While in films,triplet states mainly come from singlet fission.In addition,we have also observed the formation of polarons,which are faster when molecules are excited to S_N states.We have observed singlet fission in LA17a film.Compared with known molecules which can undergo singlet fission,the molecular structure of LA17a is significantly different.Our results will provide a reference for scientists who are committed to designing and synthesizing new singlet fission molecules.