Ultrafast Processes In Perylene And Rubrene Systems:Singlet Fission,Excimer Formation And Charge Transfer

As the world moves to a more sustainable economy,photovoltaics（PVs）,which convert sunlight into electrical current,are considered as the most promising technology.There are fundamental Shockley-Queisser limits beyond which these devices cannot be pushed by conventional methods.Singlet fission（SF）,a spin-allowed conversion of a spin-singlet exciton to two spin-triplet excitons,was introduced to overcome these limits and boost the efficiency of PV devices to 44%in theory.However,to realize the SF based PVs,the understanding of comprehensive SF mechanisms is still missing.Therefore,a dissertation is providing fundamental investigation on photophysical mechanisms of perylene and rubrene systems,where ultrafast phenomena of SF,excimer formation,triplet-triplet annihilation and charge transfer are taking place.Moreover,for the first time we demonstrated anti-Kasha SF in these materials,resulting in enhancement of SF efficiency and reduction in heat dissipation for solar cells.Employing femtosecond pump-probe technique,assisted by steady-state and time-resolved fluorescence spectroscopy,systematic study of excited state relaxation processes of perylene dimer,thin film,perylene-TCNQ crystal,rubrene nanoaggregates and rubrene/perovskite bilayer was performed.Insight into the detailed mechanisms will help to guide the development of SF molecular structure and material fabrication by manipulating the excited state properties towards efficient photovoltaics.Few important results of Ph D thesis are summarized below:（1）Singlet Fission from Upper Excited Electronic States in Cofacial Perylene DimerA relatively long 0.9 ps lifetime of S₂ in perylene is resolved by femtosecond pump probe spectroscopy and further validated by theoretical simulations.This finding prompts exploration of ultrafast hot SF process from upper excited states（anti-Kasha photochemistry）in new-designed cofacial perylene dimers.When the higher excited states above the SF threshold（3.06e V）are populated,SF takes place within 30 fs,competing well with excimer formation（～1 ps）,vibrational cooling in S₁（4.7-7.0 ps）and S₂→S₁ internal conversion（380 fs）.（2）Enhancement of SF Yield by Hindering Excimer Formation in Perylene Thin FilmIn thermally-deposited perylene thin film,excimer forms efficiently within 30 fs and radiatively relaxes within 19 ns.One-photon induced SF atλ_ex=250 nm is demonstrated to occur from higher excited states,reaching a triplet formation quantum yield of 108%,while two-quantum absorption induced SF atλ_ex=450 nm runs within 170 fs.In order to improve SF efficiency,a film of perylene derivative,2,5,8,11-Tetra-tert-butylperylene（TBPe）,is fabricated.Here,the steric hindrance of the tert-butyl groups organizes slip-stacking spatial arrangement resulting in absence of excimer fluorescence.On the contrary,triplet quantum yield via SF is enhanced to 185%due to blocking of excimer generation channel.（3）Ultrafast Tuning of Various Photochemical Pathways in Perylene-TCNQ Charge Transfer CrystalUltrafast spectroscopic methods were applied to investigate the unique charge-transfer（perylene）₃-7,7,8,8-tetracyanoquinodimethane co-crystal system（P3T1）.The excited state dynamics in P3T1 crystal was found to depend on excitation photon energies.At higher pump energy（4.96 e V）,charge transfer between perylene and TCNQ moieties was observed:it proceeds within 0.6-1.2 ps and subsequently recombines with time 6.6-12 ps.SF was detected directly from upper excited singlet state of perylene.Moreover,excitation into low ion pair states（1.91 e V）results in enhancement of triplet formation via spin-orbit charge transfer intersystem crossing.（4）Rubrene Nanoaggregate-Integrated CH₃NH₃Pb I₃ Bilayer Film:Role of Singlet Fission and Photon Up-ConversionRubrene molecules were fabricated into spherical nanoaggregates（NAs）of diameter～250nm by re-precipitation method.Quenching of fluorescence unveiled competing pathways in NAs:hot and thermally activated SF,triplet-triplet annihilation（TTA）as well as energy transfer between inhomogeneously broadened NAs ensemble.SF of NAs runs directly from upper excited electronic states within 340 fs（vs.200 fs in crystal;30 fs in film）,contributing to triplet quantum yield of 162%.With aim to apply rubrene in photovoltaics,rubrene NA-integrated layer was deposited on the top of CH₃NH₃Pb I₃ film.Triplet formation via SF within 25 ps in this bilayer system was well documented.Subsequently,the triplet excitons are accumulated and further up-converted via triplet-triplet annihilation to populate conduction band of perovskite within 39 ps（80%）and>>10 ns（20%）.