Revealing the charge generation is a crucial step to understand the organic photovoltaics. Recent development in non-fullerene organic solar cells （OSCs） indicates efficient charge separation even with negligible en...Revealing the charge generation is a crucial step to understand the organic photovoltaics. Recent development in non-fullerene organic solar cells （OSCs） indicates efficient charge separation even with negligible energetic offset between the donor and acceptor materials. These new findings trigger a critical question concerning the charge separation mechanism in OSCs, traditionally believed to result from sufficient energetic offset between the polymer donor and fullerene acceptor. We propose a new mechanism, which involves the molecular electrostatic potential, to explain efficient charge separation in non-fullerene OSCs. Together with the new mechanism, we demonstrate a record efficiency of -12% for systems with negligible energetic offset between donor and acceptor materials. Our analysis also rationalizes different requirement of the energetic offset between fullerene-based and non-fullerene OSCs, and paves the way for further design of OSC materials with both high photocurrent and high photovottage at the same time.展开更多
Exciton(or spin)statistics is a physical principle based on the statistics of spin multiplicity.In electroluminescence,injected electrons and holes have randomized spin states,and usually form singlet or triplet excit...Exciton(or spin)statistics is a physical principle based on the statistics of spin multiplicity.In electroluminescence,injected electrons and holes have randomized spin states,and usually form singlet or triplet excitons in the ratio of 1:3.Exciton statistics determines that the upper limit of internal quantum efficiency is 25%in fluorescent devices,since only singlet exciton can decay radiatively.However,both experimental and theoretical evidence indicate that the actual efficiency can exceed the exciton statistics limit of 25%by utilizing materials with special electronic structure and optimized device structures.These results bring light to break through the exciton statistics limit and develop new-generation fluorescent materials with low cost and high efficiency.Recently,the exciton statistics,which has attracted great attention in the past decade,is being rejuvenated due to the discovery of some fluorescent materials with abnormally high efficiencies.In view of their significance in theoretical research of organic semiconductors and developing new-generation OLED materials,such materials are widely investigated in both academic institutions and industry.Several key issues still require further clarification for this kind of materials,such as the molecular design concepts.Herein,we review the progress of the materials with efficiency exceeding the exciton statistics limit,and the routes to improve exciton utilization efficiency.In the end,we present an innovative pathway to fully harvest the excitons in fluorescent devices,namely,"hot exciton"model and relevant fluorescence material with hybridized local and charge-transfer(HLCT)excited state.展开更多
The DCM dye doped organic electroluminescence devices with structure of ITO/NPB/Alq 3 : DCM/Alq 3 /LiF/Al were fabricated. From 15 K to room temperature, the magnetic field dependent of electroluminescence (MEL) of de...The DCM dye doped organic electroluminescence devices with structure of ITO/NPB/Alq 3 : DCM/Alq 3 /LiF/Al were fabricated. From 15 K to room temperature, the magnetic field dependent of electroluminescence (MEL) of devices was investigated. Our observations indicated that the MEL is composed of two effects in different regimes: a low field (0≤B≤40 mT) effect and a high field (B 】 40 mT) effect. For undoped devices, the low field effect exhibits a rapid rising with the increasing field, and the high field effect shows a slow increase and gradually saturates at room temperature. For doped devices, the low field rapid increase is also present, whereas the high field effect displays a decrease with the increasing field. The larger the injection current is, the more apparent the high field decrease is. In addition, the doped device demonstrates less temperature dependence of the high field effect than undoped device, although the undoped devices also present high field decrease of electroluminescence at low temperatures (T≤150 K). Based on the energy level trapping effect due to dye doping and magnetic field modulated triplet exciton annihilation, the experimental results are carefully explained.展开更多
Optical selection rules fundamentally determine the optical transitions between energy states in a variety of physical systems,from hydrogen atoms to bulk crystals such as gallium arsenide.These rules are important fo...Optical selection rules fundamentally determine the optical transitions between energy states in a variety of physical systems,from hydrogen atoms to bulk crystals such as gallium arsenide.These rules are important for optoelectronic applications such as lasers,energy-dispersive X-ray spectroscopy,and quantum computation.Recently,single-layer transition metal dichalcogenides have been found to exhibit valleys in momentum space with nontrivial Berry curvature and excitons with large binding energy.However,there has been little study of how the unique valley degree of freedom combined with the strong excitonic effect influences the nonlinear optical excitation.Here,we report the discovery of nonlinear optical selection rules in monolayer WS2,an important candidate for visible 2D optoelectronics because of its high quantum yield and large direct bandgap.We experimentally demonstrated this principle for second-harmonic generation and two-photon luminescence(TPL).Moreover,the circularly polarized TPL and the study of its dynamics evince a sub-ps interexciton relaxation(2p R 1s).The discovery of this new optical selection rule in a valleytronic 2D system not only considerably enhances knowledge in this area but also establishes a foundation for the control of optical transitions that will be crucial for valley optoelectronic device applications such as 2D valley-polarized THz sources with 2p–1s transitions,optical switches,and coherent control for quantum computing.展开更多
Exciton energies as a function of radii of quantum dots in the range of 5–35 ? are calculated based on effective mass approximation model with the B-spline technique and compared with experimental and other theoretic...Exciton energies as a function of radii of quantum dots in the range of 5–35 ? are calculated based on effective mass approximation model with the B-spline technique and compared with experimental and other theoretical data for the CdS dots. This method leads to accurate and fast convergent exciton energy, which are in good agreement with experimental data in the whole confinement regime. The effect of penetration of wave function from the inside to the outside of the dots and the effect of dielectric constants are taken into account. The magnitudes of dynamical parameters are discussed. It is found that the different materials surrounding the CdS quantum dot affect not only the potential energy and Coulomb interaction energy of the system, but also the effective masses. The comparison shows that the effective mass approximation model can describe very well the quantum size effects observed experimentally on the exciton ground state energy.展开更多
Monolayer phosphorene provides a unique two-dimensional(2D)platform to investigate the fundamental dynamics of excitons and trions(charged excitons)in reduced dimensions.However,owing to its high instability,unambiguo...Monolayer phosphorene provides a unique two-dimensional(2D)platform to investigate the fundamental dynamics of excitons and trions(charged excitons)in reduced dimensions.However,owing to its high instability,unambiguous identification of monolayer phosphorene has been elusive.Consequently,many important fundamental properties,such as exciton dynamics,remain underexplored.We report a rapid,noninvasive,and highly accurate approach based on optical interferometry to determine the layer number of phosphorene,and confirm the results with reliable photoluminescence measurements.Furthermore,we successfully probed the dynamics of excitons and trions in monolayer phosphorene by controlling the photo-carrier injection in a relatively low excitation power range.Based on our measured optical gap and the previously measured electronic energy gap,we determined the exciton binding energy to be~0.3 eV for the monolayer phosphorene on SiO_(2)/Si substrate,which agrees well with theoretical predictions.A huge trion binding energy of~100 meV was first observed in monolayer phosphorene,which is around five times higher than that in transition metal dichalcogenide(TMD)monolayer semiconductor,such as MoS_(2).The carrier lifetime of exciton emission in monolayer phosphorene was measured to be,220 ps,which is comparable to those in other 2D TMD semiconductors.Our results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using monolayer phosphorene.展开更多
Perovskite nanocrystal(PNC)solids are promising materials for optoelectronic applications.Recent studies have shown that exciton diffusion in PNC solids occurs via alternate exciton hopping(EH)and photon recycling(PR)...Perovskite nanocrystal(PNC)solids are promising materials for optoelectronic applications.Recent studies have shown that exciton diffusion in PNC solids occurs via alternate exciton hopping(EH)and photon recycling(PR).The energy disorder induced by the size distribution is a common factor in PNC solids,and the impact of this energy disorder on the exciton diffusion remains unclear.Here,we investigated the exciton diffusion in CsPbBr3 NC solids with a Gaussian size distribution of 11.2±6.8 nm via steady and time-resolved photoluminescence(PL)spectroscopy with multiple detection bands in transmission mode.Our results indicated that exciton diffusion was controlled by a downhill transfer among the different energy sites through the disordered energy landscape,as confirmed by the accompanying low-temperature PL analysis.A detailed examination revealed that the acceptor distribution in tandem with the reabsorption coefficient determined the contribution of EH and PR to exciton transfer between different energy sites.Consequently,the exciton diffusion mechanism varied in PNC solids of different thicknesses:in a thin solid with a thickness of several hundred nanometers,the exciton transfer was dominated by efficient EH and PR from the high-energy sites to the lower-energy sites;in a few-micrometer-thick solid,transfer from the medium-energy sites toward the lower-energy sites also became prominent and occurred mainly through PR.These findings enhance the understanding of the vital role that the acceptor distribution plays in the exciton diffusion process in PNC solids,providing important insights for optoelectronic applications based on PNC solids.Our work also exploits the use of commonly available tools for in-depth exciton diffusion studies,which reveals the interior diffusion information that is usually hidden in surface sensitive PL imaging methods.展开更多
基金J. Hou acknowledge financial support from National Natural Science Foundation of China （91633301, 51673201, 91333204）, the Ministry of Science and Technology of China （2014CB643501） and the Chinese Academy of Sciences （XDB12030200）. F. Gao and D. Qian would like to acknowledge the Swedish Research Council VR （Grant No. 2017-007444 the Swedish Energy Agency Ener- gimyndigheten （2016-010174）, the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University （Faculty Grant No. SFO-Mat-LiU #2009- 00971）.
文摘Revealing the charge generation is a crucial step to understand the organic photovoltaics. Recent development in non-fullerene organic solar cells （OSCs） indicates efficient charge separation even with negligible energetic offset between the donor and acceptor materials. These new findings trigger a critical question concerning the charge separation mechanism in OSCs, traditionally believed to result from sufficient energetic offset between the polymer donor and fullerene acceptor. We propose a new mechanism, which involves the molecular electrostatic potential, to explain efficient charge separation in non-fullerene OSCs. Together with the new mechanism, we demonstrate a record efficiency of -12% for systems with negligible energetic offset between donor and acceptor materials. Our analysis also rationalizes different requirement of the energetic offset between fullerene-based and non-fullerene OSCs, and paves the way for further design of OSC materials with both high photocurrent and high photovottage at the same time.
基金financially supported by the National Science Foundation of China(51073069,51273078)the National Basic Research Program of China(2013CB834801)
文摘Exciton(or spin)statistics is a physical principle based on the statistics of spin multiplicity.In electroluminescence,injected electrons and holes have randomized spin states,and usually form singlet or triplet excitons in the ratio of 1:3.Exciton statistics determines that the upper limit of internal quantum efficiency is 25%in fluorescent devices,since only singlet exciton can decay radiatively.However,both experimental and theoretical evidence indicate that the actual efficiency can exceed the exciton statistics limit of 25%by utilizing materials with special electronic structure and optimized device structures.These results bring light to break through the exciton statistics limit and develop new-generation fluorescent materials with low cost and high efficiency.Recently,the exciton statistics,which has attracted great attention in the past decade,is being rejuvenated due to the discovery of some fluorescent materials with abnormally high efficiencies.In view of their significance in theoretical research of organic semiconductors and developing new-generation OLED materials,such materials are widely investigated in both academic institutions and industry.Several key issues still require further clarification for this kind of materials,such as the molecular design concepts.Herein,we review the progress of the materials with efficiency exceeding the exciton statistics limit,and the routes to improve exciton utilization efficiency.In the end,we present an innovative pathway to fully harvest the excitons in fluorescent devices,namely,"hot exciton"model and relevant fluorescence material with hybridized local and charge-transfer(HLCT)excited state.
基金supported by For Ying Tung Education Foundation (Grant No.101006)the National Natural Science Foundation of China (Grant Nos.10974157 and 10974159)
文摘The DCM dye doped organic electroluminescence devices with structure of ITO/NPB/Alq 3 : DCM/Alq 3 /LiF/Al were fabricated. From 15 K to room temperature, the magnetic field dependent of electroluminescence (MEL) of devices was investigated. Our observations indicated that the MEL is composed of two effects in different regimes: a low field (0≤B≤40 mT) effect and a high field (B 】 40 mT) effect. For undoped devices, the low field effect exhibits a rapid rising with the increasing field, and the high field effect shows a slow increase and gradually saturates at room temperature. For doped devices, the low field rapid increase is also present, whereas the high field effect displays a decrease with the increasing field. The larger the injection current is, the more apparent the high field decrease is. In addition, the doped device demonstrates less temperature dependence of the high field effect than undoped device, although the undoped devices also present high field decrease of electroluminescence at low temperatures (T≤150 K). Based on the energy level trapping effect due to dye doping and magnetic field modulated triplet exciton annihilation, the experimental results are carefully explained.
基金This work was supported by the“Light-Material Interactions in Energy Conversion”Energy Frontier Research Center funded by the U.S.Department of Energy,Office of Science,Office of Basic Energy Sciences under Award Number DE-AC02-05CH11231.
文摘Optical selection rules fundamentally determine the optical transitions between energy states in a variety of physical systems,from hydrogen atoms to bulk crystals such as gallium arsenide.These rules are important for optoelectronic applications such as lasers,energy-dispersive X-ray spectroscopy,and quantum computation.Recently,single-layer transition metal dichalcogenides have been found to exhibit valleys in momentum space with nontrivial Berry curvature and excitons with large binding energy.However,there has been little study of how the unique valley degree of freedom combined with the strong excitonic effect influences the nonlinear optical excitation.Here,we report the discovery of nonlinear optical selection rules in monolayer WS2,an important candidate for visible 2D optoelectronics because of its high quantum yield and large direct bandgap.We experimentally demonstrated this principle for second-harmonic generation and two-photon luminescence(TPL).Moreover,the circularly polarized TPL and the study of its dynamics evince a sub-ps interexciton relaxation(2p R 1s).The discovery of this new optical selection rule in a valleytronic 2D system not only considerably enhances knowledge in this area but also establishes a foundation for the control of optical transitions that will be crucial for valley optoelectronic device applications such as 2D valley-polarized THz sources with 2p–1s transitions,optical switches,and coherent control for quantum computing.
文摘Exciton energies as a function of radii of quantum dots in the range of 5–35 ? are calculated based on effective mass approximation model with the B-spline technique and compared with experimental and other theoretical data for the CdS dots. This method leads to accurate and fast convergent exciton energy, which are in good agreement with experimental data in the whole confinement regime. The effect of penetration of wave function from the inside to the outside of the dots and the effect of dielectric constants are taken into account. The magnitudes of dynamical parameters are discussed. It is found that the different materials surrounding the CdS quantum dot affect not only the potential energy and Coulomb interaction energy of the system, but also the effective masses. The comparison shows that the effective mass approximation model can describe very well the quantum size effects observed experimentally on the exciton ground state energy.
基金financial support from the ANU PhD scholarship,the China Research Council PhD scholarship,the National Science Foundation(USA,grant number ECCS-1405201)the Australian Research Council(grant number DE140100805),and the ANU Major Equipment Committee.
文摘Monolayer phosphorene provides a unique two-dimensional(2D)platform to investigate the fundamental dynamics of excitons and trions(charged excitons)in reduced dimensions.However,owing to its high instability,unambiguous identification of monolayer phosphorene has been elusive.Consequently,many important fundamental properties,such as exciton dynamics,remain underexplored.We report a rapid,noninvasive,and highly accurate approach based on optical interferometry to determine the layer number of phosphorene,and confirm the results with reliable photoluminescence measurements.Furthermore,we successfully probed the dynamics of excitons and trions in monolayer phosphorene by controlling the photo-carrier injection in a relatively low excitation power range.Based on our measured optical gap and the previously measured electronic energy gap,we determined the exciton binding energy to be~0.3 eV for the monolayer phosphorene on SiO_(2)/Si substrate,which agrees well with theoretical predictions.A huge trion binding energy of~100 meV was first observed in monolayer phosphorene,which is around five times higher than that in transition metal dichalcogenide(TMD)monolayer semiconductor,such as MoS_(2).The carrier lifetime of exciton emission in monolayer phosphorene was measured to be,220 ps,which is comparable to those in other 2D TMD semiconductors.Our results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using monolayer phosphorene.
基金financially supported by Shenzhen Fundamental Research Program(JCYJ20200109142425294)the National Natural Science Foundation of China(62034009 and 62104266)the Shenzhen Science and Technology Innovation Program(2022A006)。
文摘Perovskite nanocrystal(PNC)solids are promising materials for optoelectronic applications.Recent studies have shown that exciton diffusion in PNC solids occurs via alternate exciton hopping(EH)and photon recycling(PR).The energy disorder induced by the size distribution is a common factor in PNC solids,and the impact of this energy disorder on the exciton diffusion remains unclear.Here,we investigated the exciton diffusion in CsPbBr3 NC solids with a Gaussian size distribution of 11.2±6.8 nm via steady and time-resolved photoluminescence(PL)spectroscopy with multiple detection bands in transmission mode.Our results indicated that exciton diffusion was controlled by a downhill transfer among the different energy sites through the disordered energy landscape,as confirmed by the accompanying low-temperature PL analysis.A detailed examination revealed that the acceptor distribution in tandem with the reabsorption coefficient determined the contribution of EH and PR to exciton transfer between different energy sites.Consequently,the exciton diffusion mechanism varied in PNC solids of different thicknesses:in a thin solid with a thickness of several hundred nanometers,the exciton transfer was dominated by efficient EH and PR from the high-energy sites to the lower-energy sites;in a few-micrometer-thick solid,transfer from the medium-energy sites toward the lower-energy sites also became prominent and occurred mainly through PR.These findings enhance the understanding of the vital role that the acceptor distribution plays in the exciton diffusion process in PNC solids,providing important insights for optoelectronic applications based on PNC solids.Our work also exploits the use of commonly available tools for in-depth exciton diffusion studies,which reveals the interior diffusion information that is usually hidden in surface sensitive PL imaging methods.
