The generated polaritons in highly ordered organic crystals are known to be highly stable even at room temperature 24, 25 and can propagate up to a millimeter 26. Yet no distinct influence of the exciton diffusion could be estimated, indicating this thickness regime to be smaller than the exciton diffusion length reported for dip. Diffusion length of triplet excitons in organic semiconductors. Probing exciton interaction with a spatially defined charge accumulation layer in the organic semiconductor diindenoperylene. Diffusion of excitons pl efficiency of alq 3 exciton diffusion length 10 nm the four general methods used to measure diffusion of excitons are. Universal description of exciton diffusion length in organic. Macroscopic coherence of a single exciton state in a. Pdf exciton diffusion in organic semiconductors semantic scholar. In this context, the frenkeltype exciton in organic semiconductors is particularly attractive, owing to its high binding energy and long diffusion path length.
Nonthermally activated exciton transport in crystalline. Exciton diffusion lengths of organic semiconductor thin. Furthermore, experimental methods that are used to measure the key parameters of this process as well as strategies to manipulate the exciton diffusion length are summarized. Tailored exciton diffusion in organic photovoltaic cells for. Exciton diffusion length and charge extraction yield in. Oct 10, 2010 one of the important microscopic parameters of organic semiconductors, relevant to organic photovoltaics, is the exciton diffusion length, l ex refs 1,2. Estimation of exciton diffusion lengths of organic. It is shown that the previously reported values for the exciton binding energies in many organic semiconductors, which differ by more than an order of magnitude, can be consistently rationalized within the framework of the charging energy of the molecular units, with a. Finally, an outlook of remaining challenges for future research is provided. Exciton binding energies in organic semiconductors. The exciton diffusion length ld is a characteristic. Relationship between crystalline order and exciton. Exciton diffusion in conjugated polymers american chemical. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength.
A ballistic transport would require an exciton kinetic energy 2 x l m 2 1 e. Here, we measured the exciton diffusion length in poly3hexylthiophene p3ht as a function of the crystalline order using spectrally resolved photoluminescence quenching srplq techniques. Grazing incidence wideangle xray scattering shows that the size of crystalline domains of pffbt4t2od increases to. Engineered exciton diffusion length enhances device. Photoconversion in organic photovoltaic cells, which relies on charge generation at donoracceptor interfaces, is limited by short excitondiffusionlengths.
The temperature dependent exciton transport in the prototypical organic semiconductor diindenoperylene dip is investigated by photoluminescence quenching. Direct measurement of the triplet exciton diffusion length. Key to these studies will be the ability to accurately and reproducibly measure l d. We show that aggregation and crystal orientation influence the. Here, using polarization and wavelengthdependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of. We present a method to measure the exciton diffusion length l d of optically dark triplet excitons in organic semiconductor thin films. The exciton binding energy is one of the key parameters that govern the physics of many optoelectronic organic devices. An exciton can form when a material absorbs a photon of higher energy than its bandgap.
Role of impurities in determining the exciton diffusion length in organic semiconductors ian j. Pdf systematic study of exciton diffusion length in. In amorphous or polycrystalline organic semiconductors, the diffusion length of singlet excitons is often 10nm 5or less as a result of exciton trapping. Systematic study of exciton diffusion length in organic. Exciton diffusion is one of the determining factors of oled efficiency rolloff at high brightness. In turn, opvs based on dilute layers of subpc exhibit a 30% enhancement in power conversion efficiency. Exciton diffusion in organic semiconductors energy. Jul 09, 2015 exciton diffusion is one of the determining factors of oled efficiency rolloff at high brightness. Exciton diffusion length in the organic semiconductor. For excitation power densities 1 mwmm2 triplettriplet annihilation processes can significantly limit the triplet penetration depth into organic semiconductor.
Jun 28, 2015 the optical properties of organic semiconductor materials are primarily determined by excitons, or molecular excited states that are mobile within the solid in which they are generated. Six experimental methods have been used to investigate the exciton diffusion length in materials with systematic chemical modifications. Observation of longrange exciton diffusion in highly. We present a simple and accurate method to determine the singlet diffusion length in an operating organic lightemitting device oled. Here, we investigate ld for the organic small molecule n,n. The optical properties of organic semiconductor materials are primarily determined by excitons, or molecular excited states that are mobile within the solid in which they are generated.
Asymmetric photon transport in organic semiconductor. Relationship between crystalline order and exciton diffusion. Apr 18, 2016 the design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length ld. We discuss situations in which certain experimental techniques are more appropriate. We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic. Dilution of the organic semiconductor boron subphthalocyanine chloride subpc is found to increase the l d for subpc by 50%. A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. Singlet exciton diffusion length in organic lightemitting diodes. University of groningen direct measurement of the triplet.
However, many challenges remain to optimise efficiency. In order to directly probe only these states, triplets are optically injected into the material of interest via energy transfer from an adjacent phosphorescent thin film. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion ld can be developed. Singlet exciton diffusion length in organic lightemitting. A physical model of exciton diffusion length in organic photovoltaic cell based on percolation theory and forster resonant transfer rate is presented here. Pdf exciton diffusion lengths of organic semiconductor. We studied crystallinity and exciton harvesting in bulk heterojunctions of the semiconducting polymer pffbt4t2od and electron acceptor pc71bm that are used to make highly efficient organic solar cells. Large crystalline domains and an enhanced exciton diffusion. Despite the importance of this parameter, there have been few investigations connecting ld and materials purity. Measurements of exciton diffusion in conjugated polymers. Tailored exciton diffusion in organic photovoltaic cells. Firstprinciples simulations of exciton diffusion in organic.
Measurement of the triplet exciton diffusion length in. Pdf we demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic. We find that exciton diffusion length correlates with molecular ordering. Measurement of exciton diffusion lengths of phthalocyanine. The critical percolation conductance exponent s c is an intermediate parameter which can be numerical solved by eqs. Forrest2,a 1department of chemical engineering, princeton institute for the science and technology of materials prism, princeton university, princeton, new jersey 08544, usa. Probing exciton interaction with a spatially defined charge. In this section, we estimate the exciton diffusion length in an inverse manner with the asymptoticbased method as the forward solver. By using electrical rather than optical excitation, the method ensures that excitons are formed in a tightly confined generation zone, from which they can diffuse towards a quenching material. These changes in diffusion length are connected to changes in the fluorescence quantum yield, which is useful for. One particular challenge lies in our understanding of the factors affecting exciton diffusion and the length scale of this process. As in the case of charge mobility, we show that the exciton transport diffusion length is a monotonic function of the extent of crystalline order. Efficient exciton diffusion and resonanceenergy transfer in.
The design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length l d. Then, the exciton diffusion length can then be easily calculated as 12 l d d. Fits to the data are shown for a diffusion coefficient of 1. Understanding exciton diffusion in organic solar cells mphil amy illingworth organic solar cells are promising candidates for future energy production. Forrest2,a 1department of chemical engineering, princeton institute for the science and technology of materials. Exciton diffusion and interfacial charge separation in. Probing exciton interaction with a spatially defined. Since exciton diffusion depends on the initial energy of the exciton, we will examine three representative initial states labeled as s1, s2, and s3 of the intramolecular excitons in the following. Efficient exciton diffusion and resonanceenergy transfer. Exciton diffusion lengths of organic semiconductor thin films. Here we present a new method to probe triplet exciton diffusion in a prototypical organic semiconductor n,n0di1naphthyl n, 0diphenyl1,10biphenyl4. Exciton diffusion length in the organic semiconductor diindenoperylene article in applied physics letters 92.
The simulation results of exciton diffusion are summarized in table 1. Mar 22, 2010 the effect of crystalline order on the exciton diffusion length l d is shown for the archetypal organic semiconductor 3,4,9,10. Future studies will hopefully lead to our understanding of what limits exciton transport and lead the way to design materials with larger l ds. Dependence of exciton diffusion length on crystalline. These changes in diffusion length are connected to changes in the. Mcgeheea department of materials science and engineering, stanford university, stanford, california 943052205. However, in this work we observed similar diffusion coefficients and diffusion lengths for surface and bulk quenching techniques table 2.
The effect of crystalline order on the exciton diffusion length l d is shown for the archetypal organic semiconductor 3,4,9,10. The optimized cs 2 annealed devices consistently show a pce between 7. Effects of optical interference and energy transfer on. Special attention is devoted to the temperature dependence of exciton diffusion and its relationship to forster energy transfer rates. Firstprinciples simulations of exciton diffusion in. Role of impurities in determining the exciton diffusion. Since only limited photoluminescence data from experiments are available, we solve the forward model to generate data in our numerical tests. For an absorbing organic layer of thickness d, we have a 1 e d, 4 whereas, assuming that the arrival of excitons at a dissociation site is. Despite the importance of this parameter, there have been few investigations connecting l d and materials purity. Pdf exciton diffusion lengths of organic semiconductor thin films. Interdiffusion would enhance the quenching efficiency and lead to an overestimation of the exciton diffusion length. Modelling of the exciton diffusion length based on the experimental data and equations 1 and 2. Nov 11, 2012 photoconversion in organic photovoltaic cells, which relies on charge generation at donoracceptor interfaces, is limited by short exciton diffusion lengths.
Excitons and the lifetime of organic semiconductor devices. Pdf systematic study of exciton diffusion length in organic. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuumdeposited polycrystalline films. Analysis by an advanced diffusion model including interference and morphological aspects reveals an exciton diffusion length of about 60 nm at room temperature. The generated polaritons in highly ordered organic crystals are known to be highly stable even at room temperature 24, 25and. Effect of polaron diffusion on excitonpolaron quenching. Triplet exciton diffusion in pdtppc simulated 15 10 5 0 ipcse % 1ns 10ns 100ns 1s 10s time 2. Here, we investigate l d for the organic small molecule n,n. Organic semiconductors typically exhibit exciton diffusion lengths on the order of tens of nanometers. Dependence of exciton diffusion length on crystalline order. Controlling exciton diffusion and fullerene distribution. Studies of the exciton diffusion length in longrange. Understanding exciton diffusion in organic solar cells amy. Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching.
Pdf exciton diffusion in organic semiconductors researchgate. Controlling exciton diffusion and fullerene distribution in. We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. Exciton diffusion in organic semiconductors is crucial to the performance of organic solar cells.
May 25, 2018 organic semiconductors typically exhibit exciton diffusion lengths on the order of tens of nanometers. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge. For photocurrent spectrum measurement, we also show the equivalence between diffusion equation model and montecarlo method in section 3. The crystalline order in the p3ht films, characterized according to the mean crystal size and normalized. Dilution of the organic semiconductor boron subphthalocyanine chloride subpc is found to increase the ld for subpc by 50%. Controlling exciton diffusion and fullerene distribution in photovoltaic blends by side chain modification. Exciton diffusion the quenching of excitons in the p3ht film by the tio 2 results in a faster decay of the photoluminescence when compared to a film on a nonquenching substrate such as fused silica. Observation of longrange exciton diffusion in highly ordered organic semiconductors. We study the exciton diffusion in organic semiconductors from a macroscopic viewpoint. This excites an electron from the valence band into the conduction band. Excitons generated in the polyfluorene cannot enter the polyethylene glycol layer and so diffuse more. Bobbert department of applied physics and institute for complex molecular systems, eindhoven university of technology.
Singlet excitons in organic materials are coulombically. The exciton diffusion length in organic semiconductors strongly impacts the efficiency of energy harvesting organic optoelectronics. The calculated results show a good agreement with monte carlo simulation and experimental data, indicating that the proposed model can well describe the temperature and material disorder. Observation of longrange exciton diffusion in highly ordered. Inter diffusion would enhance the quenching efficiency and lead to an overestimation of the exciton diffusion length. Direct measurement of the triplet exciton diffusion length in. An extensive table of more than a hundred measurements of the exciton diffusion length in various organic semiconductors is presented. Injected triplets migrate through the full thickness of the material before undergoing. Universal description of exciton diffusion length in. Mar 11, 2009 we demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. Effects of optical interference and energy transfer on exciton diffusion length measurements in organic semiconductors shawn r. Organic semiconductor agcontact itocovered glass h h 2 2 1.
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