Asymmetric photon transport in organic semiconductor. Controlling exciton diffusion and fullerene distribution in photovoltaic blends by side chain modification. Pdf exciton diffusion in organic semiconductors researchgate. Photoconversion in organic photovoltaic cells, which relies on charge generation at donoracceptor interfaces, is limited by short excitondiffusionlengths. Exciton diffusion in organic semiconductors is crucial to the performance of organic solar cells.
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. Apr 18, 2016 the design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length ld. May 25, 2018 organic semiconductors typically exhibit exciton diffusion lengths on the order of tens of nanometers. Diffusion of excitons pl efficiency of alq 3 exciton diffusion length 10 nm the four general methods used to measure diffusion of excitons are. Exciton diffusion length in the organic semiconductor. Universal description of exciton diffusion length in organic. Controlling exciton diffusion and fullerene distribution in. 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.
However, many challenges remain to optimise efficiency. For photocurrent spectrum measurement, we also show the equivalence between diffusion equation model and montecarlo method in section 3. Singlet excitons in organic materials are coulombically. Exciton diffusion length and charge extraction yield in. Analysis by an advanced diffusion model including interference and morphological aspects reveals an exciton diffusion length of about 60 nm at room temperature. The critical percolation conductance exponent s c is an intermediate parameter which can be numerical solved by eqs. The exciton diffusion length in organic semiconductors strongly impacts the efficiency of energy harvesting organic optoelectronics. Estimation of exciton diffusion lengths of organic. The design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length l d.
Measurement of the triplet exciton diffusion length in. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion ld can be developed. 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. Here, using polarization and wavelengthdependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of. Effects of optical interference and energy transfer on exciton diffusion length measurements in organic semiconductors shawn r. These changes in diffusion length are connected to changes in the. Bobbert department of applied physics and institute for complex molecular systems, eindhoven university of technology.
Role of impurities in determining the exciton diffusion length in organic semiconductors ian j. Fits to the data are shown for a diffusion coefficient of 1. This excites an electron from the valence band into the conduction band. Exciton diffusion is one of the determining factors of oled efficiency rolloff at high brightness. Pdf exciton diffusion in organic semiconductors semantic scholar. We present a method to measure the exciton diffusion length l d of optically dark triplet excitons in organic semiconductor thin films. Universal description of exciton diffusion length in. Macroscopic coherence of a single exciton state in a. The effect of crystalline order on the exciton diffusion length l d is shown for the archetypal organic semiconductor 3,4,9,10. The temperature dependent exciton transport in the prototypical organic semiconductor diindenoperylene dip is investigated by photoluminescence quenching. Exciton diffusion in organic semiconductors energy. Here, we investigate l d for the organic small molecule n,n.
Excitons and the lifetime of organic semiconductor devices. We find that exciton diffusion length correlates with molecular ordering. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge. We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic. Exciton diffusion in conjugated polymers american chemical. In turn, opvs based on dilute layers of subpc exhibit a 30% enhancement in power conversion efficiency. Modelling of the exciton diffusion length based on the experimental data and equations 1 and 2. The exciton binding energy is one of the key parameters that govern the physics of many optoelectronic organic devices. Exciton diffusion lengths of organic semiconductor. The simulation results of exciton diffusion are summarized in table 1. Probing exciton interaction with a spatially defined charge.
Dependence of exciton diffusion length on crystalline order. Future studies will hopefully lead to our understanding of what limits exciton transport and lead the way to design materials with larger l ds. Measurements of exciton diffusion in conjugated polymers. Here, we investigate ld for the organic small molecule n,n. Special attention is devoted to the temperature dependence of exciton diffusion and its relationship to forster energy transfer rates.
Organic semiconductor agcontact itocovered glass h h 2 2 1. Tailored exciton diffusion in organic photovoltaic cells. For excitation power densities 1 mwmm2 triplettriplet annihilation processes can significantly limit the triplet penetration depth into organic semiconductor. A physical model of exciton diffusion length in organic photovoltaic cell based on percolation theory and forster resonant transfer rate is presented here. In this context, the frenkeltype exciton in organic semiconductors is particularly attractive, owing to its high binding energy and long diffusion path length. The generated polaritons in highly ordered organic crystals are known to be highly stable even at room temperature 24, 25and. That is, the exciton diffusion length, l d, is typically much less than the optical absorption length, 1. Triplet exciton diffusion in pdtppc simulated 15 10 5 0 ipcse % 1ns 10ns 100ns 1s 10s time 2. Exciton diffusion lengths of organic semiconductor thin films.
Pdf exciton diffusion lengths of organic semiconductor thin films. One particular challenge lies in our understanding of the factors affecting exciton diffusion and the length scale of this process. Exciton binding energies in organic semiconductors. Forrest2,a 1department of chemical engineering, princeton institute for the science and technology of materials. The crystalline order in the p3ht films, characterized according to the mean crystal size and normalized. Pdf exciton diffusion lengths of organic semiconductor. Firstprinciples simulations of exciton diffusion in organic. 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. Pdf systematic study of exciton diffusion length in organic. Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching. Since only limited photoluminescence data from experiments are available, we solve the forward model to generate data in our numerical tests. Despite the importance of this parameter, there have been few investigations connecting l d and materials purity. Exciton diffusion lengths of organic semiconductor thin. Excitons generated in the polyfluorene cannot enter the polyethylene glycol layer and so diffuse more.
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. Observation of longrange exciton diffusion in highly. Direct measurement of the triplet exciton diffusion length. 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. 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 prism, princeton university, princeton, new jersey 08544, usa. Nov 11, 2012 photoconversion in organic photovoltaic cells, which relies on charge generation at donoracceptor interfaces, is limited by short exciton diffusion lengths. Probing exciton interaction with a spatially defined. Pdf we demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic. These changes in diffusion length are connected to changes in the fluorescence quantum yield, which is useful for.
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. Effects of optical interference and energy transfer on. Pdf systematic study of exciton diffusion length in. 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. Measurement of exciton diffusion lengths of phthalocyanine. Exciton diffusion and interfacial charge separation in. Relationship between crystalline order and exciton. However, in this work we observed similar diffusion coefficients and diffusion lengths for surface and bulk quenching techniques table 2. Relationship between crystalline order and exciton diffusion.
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. Here we present a new method to probe triplet exciton diffusion in a prototypical organic semiconductor n,n0di1naphthyl n, 0diphenyl1,10biphenyl4. Injected triplets migrate through the full thickness of the material before undergoing. Firstprinciples simulations of exciton diffusion in. Singlet exciton diffusion length in organic lightemitting. Mcgeheea department of materials science and engineering, stanford university, stanford, california 943052205. Understanding exciton diffusion in organic solar cells amy. 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. Here, we measured the exciton diffusion length in poly3hexylthiophene p3ht as a function of the crystalline order using spectrally resolved photoluminescence quenching srplq techniques. Then, the exciton diffusion length can then be easily calculated as 12 l d d. 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. 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.
Studies of the exciton diffusion length in longrange. The exciton diffusion length ld is a characteristic. Exciton diffusion length in the organic semiconductor diindenoperylene article in applied physics letters 92. Dilution of the organic semiconductor boron subphthalocyanine chloride subpc is found to increase the l d for subpc by 50%. Direct measurement of the triplet exciton diffusion length in. Role of impurities in determining the exciton diffusion. Singlet exciton diffusion length in organic lightemitting diodes. An extensive table of more than a hundred measurements of the exciton diffusion length in various organic semiconductors is presented.
Observation of longrange exciton diffusion in highly ordered organic semiconductors. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuumdeposited polycrystalline films. 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. Inter diffusion would enhance the quenching efficiency and lead to an overestimation of the exciton diffusion length. We present a simple and accurate method to determine the singlet diffusion length in an operating organic lightemitting device oled.
The optimized cs 2 annealed devices consistently show a pce between 7. 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. 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. Diffusion length of triplet excitons in organic semiconductors. We discuss situations in which certain experimental techniques are more appropriate. 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. Jul 09, 2015 exciton diffusion is one of the determining factors of oled efficiency rolloff at high brightness. Nonthermally activated exciton transport in crystalline. We study the exciton diffusion in organic semiconductors from a macroscopic viewpoint. Organic semiconductors typically exhibit exciton diffusion lengths on the order of tens of nanometers.
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. Interdiffusion would enhance the quenching efficiency and lead to an overestimation of the exciton diffusion length. Large crystalline domains and an enhanced exciton diffusion. Dilution of the organic semiconductor boron subphthalocyanine chloride subpc is found to increase the ld for subpc by 50%. Systematic study of exciton diffusion length in organic.
Six experimental methods have been used to investigate the exciton diffusion length in materials with systematic chemical modifications. Probing exciton interaction with a spatially defined charge accumulation layer in the organic semiconductor diindenoperylene. Efficient exciton diffusion and resonanceenergy transfer. Grazing incidence wideangle xray scattering shows that the size of crystalline domains of pffbt4t2od increases to. Dependence of exciton diffusion length on crystalline. Effect of polaron diffusion on excitonpolaron quenching. Despite the importance of this parameter, there have been few investigations connecting ld and materials purity. A ballistic transport would require an exciton kinetic energy 2 x l m 2 1 e.
In this section, we estimate the exciton diffusion length in an inverse manner with the asymptoticbased method as the forward solver. 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. An exciton can form when a material absorbs a photon of higher energy than its bandgap. Key to these studies will be the ability to accurately and reproducibly measure l d. Controlling exciton diffusion and fullerene distribution.
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