Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics

Jailaubekov, Askat E.; Willard, Adam P.; Tritsch, John R.; Chan, Wai‐Lun; Sai, Na; Gearba, Raluca I.; Kaake, Loren G.; Williams, Kenrick J.; Leung, Kevin; Rossky, Peter J.; Zhu, Xiaolei

doi:10.1038/nmat3500

Cited by 611 publications

(818 citation statements)

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“…Such non-thermalized 'hot exciton' states have been implicated recently as important precursors in the generation of photocurrent in organic photovoltaics since they undergo fission within the first 50 fs following excitation, creating both interfacial charge-transfer states and polaron species in low bandgap polymer systems 6,7 . In essence, as the energy offset between donor and acceptor materials is increased, an increasing density of long-range charge-transfer states (that is, polarons) becomes isoenergetic with the photoexcited excitonic state.…”

Section: Resultsmentioning

confidence: 99%

“…A detailed mechanistic understanding of primary charge generation dynamics is of key fundamental importance in the development of organic solar cells, and we propose it to be generally important in photoinduced charge-transfer processes in condensed matter. Recent spectroscopic measurements on organic photovoltaic systems have reported that charged photo excitations can be generated on r100-fs time scales [2][3][4][5][6][7][8][9][10] ; however, full charge separation to produce photocarriers is expected to be energetically expensive given strong Coulombic barriers due to the low dielectric constant in molecular semiconductors. Nonetheless, experiments by Gélinas et al 11 in which Starkeffect signatures in transient absorption spectra were analysed to probe the local electric field as charge separation proceeds, indicate that electrons and holes separate by B40 Å over the first 100 fs and evolve further on picosecond time scales to produce unbound charge pairs.…”

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confidence: 99%

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Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions

2014

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Here we report on an exciton/lattice model of the electronic dynamics of primary photo excitations in a polymeric semiconductor heterojunction that includes both polymer p-stacking, energetic disorder and phonon relaxation. Our model indicates that that in polymer/fulerene heterojunction systems, resonant tunnelling processes brought about by environmental fluctuations couple photo excitations directly to photocurrent producing charge-transfer states on o100 fs time scales. Moreover, we find this process to be independent of the location of energetic disorder in the system, and hence we expect exciton fission via resonant tunnelling to polarons to be a ubiquitous feature of these systems.

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Section: Resultsmentioning

confidence: 99%

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confidence: 99%

Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions

2014

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“…Design rules targeting enhanced exciton and charge diffusion or exciton splitting are, however, hard to come by, since the underlying microscopic mechanisms are not well understood. Efficient exciton dissociation, for example, has been attributed to the assistance of charge separation by a gradient in the free-energy landscape [132,133], structural heterogeneity as a function of distance to the interface [134], doping and charged defects [135], increase in entropy as the electron and hole move away from the interface [136], formation of hot CT states [137], or long-range tunneling [138]. Tuning optical absorption profiles, by contrast, is a more manageable approach to enhance the external quantum efficiency of single-junction devices.…”

Section: The Acceptor-donor-acceptor Puzzlementioning

confidence: 99%

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

Poelking

2018

Springer Theses

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The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors The rational design of organic semiconductors for optoelectronic devices relies on a detailed understanding of how their molecular and morphological structure condition the energetics and dynamics of charged and excitonic states. Investigating the role of molecular architecture, conformation, orientation and packing, this work reveals mechanisms that shape the spatially resolved densities of states in organic, small-molecular and polymeric heterostructures and mesophases. The underlying computational framework combines multiscale simulations of the material morphology at atomistic and coarse-grained resolution with a long-range-polarized embedding technique to resolve the electronic structure of the molecular solid. We show that longrange electrostatic interactions tie the energetics of microscopic states to the mesoscopic structure, with a qualitative and quantitative impact on charge-carrier level profiles across organic interfaces. The computational approach provides quantitative access to the charge-density-dependent opencircuit voltage of planar heterojunctions. The derived and experimentally verified relationships between molecular orientation, architecture, level profiles and open-circuit voltage rationalize the acceptor-donor-acceptor pattern for donor materials in high-performing solar cells. Proposing a pathway for barrier-less dissociation of charge transfer states, we highlight how mesoscale fields generate charge splitting and detrapping forces in systems with finite interface roughness. The associated design rules reflect the dominant role played by lowest-energy configurations at the interface. Acknowledgements 121 Die (nicht-)lokale Zustandsdichte elektronischer Anregungen in organischen Halbleitern List of Publications 123Bibliography 125 Chapter 1 Organic Electronics in a NutshellThe discovery of conductive polymers in the 1970s [1,2] -rewarded with the Nobel prize in chemistry in the year 2000 -and subsequent development of the first polymeric electronic devices in the 1980s [3-5] have marked the beginnings of a vast interdisciplinary research field referred to as organic electronics. Drawing from both polymeric and small-molecular semiconductors, organic thin-film transistors, solar cells, light-emitting diodes, photodetectors and sensors name just a few out of many applications that make use of the supreme chemical versatility and mechanical flexibility of the underlying "soft" molecular materials. Furthermore enabling low-temperature solution-based processing, printing and spray coating, organic electronics is set to complement the conventional silicon-based electronics in diverse waysadding functionality and improving sustainability. This introductory chapter will provide a short review of the materials and device physics, as well as of new trends and challenges that emerged in the field over the past decade. MaterialsOrganic semiconductors are molecular materials that exist at the interface between orga...

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“…For processes that require this interconversion, such as electroluminescence in organic light emitting diodes (OLEDs) and photocurrent generation in organic photovoltaics (OPVs), low-energy (thermalized) CT states are often implicated as a precursor to efficiency loss pathways [1][2][3][4][5][6][7][8][9][10][11][12]. Despite this, much remains to be understood about the properties of CT states and how they contribute to various energy loss mechanisms.…”

Section: Apr 2016mentioning

confidence: 99%

A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects

Lee

Shi

Willard

2016

J. Phys. Chem. Lett.

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In this letter we explore how the microscopic dynamics of charge transfer (CT) excitons are influenced by the presence of an external magnetic field in disordered molecular semiconductors. This influence is driven by the dynamic interplay between the spin and spatial degrees of freedom of the electron-hole pair. To account for this interplay we have developed a numerical framework that combines a traditional model of quantum spin dynamics with a stochastic coarse-grained model of charge transport. This combination provides a general and efficient methodology for simulating the effects of magnetic field on CT state dynamics, therefore providing a basis for revealing the microscopic origin of experimentally observed magnetic field effects. We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials.1 arXiv:1603.08160v2 [physics.chem-ph] Apr 2016Charge transfer (CT) states play a fundamental role in mediating interconversion between bound electronic excitations and free charge carriers in organic electronic materials.For processes that require this interconversion, such as electroluminescence in organic light emitting diodes (OLEDs) and photocurrent generation in organic photovoltaics (OPVs), low-energy (thermalized) CT states are often implicated as a precursor to efficiency loss pathways [1][2][3][4][5][6][7][8][9][10][11][12]. Despite this, much remains to be understood about the properties of CT states and how they contribute to various energy loss mechanisms. Due to their short lifetime and low optical activity, attempts to interrogate CT states directly have brought limited success. Notably, however, recent experiments that probe CT states indirectly via their response to an applied magnetic field have demonstrated the potential to reveal new information about this elusive class of excited states [13][14][15][16][17][18][19][20][21]. Unfortunately, extracting this information is challenging because it is encoded by a complex interplay of electronic and nuclear spin dynamics [15,22,23]. This interplay is further complicated when the dynamics of the electron-hole spin state (or the specific experimental observable) is coupled to a source of fluctuating microscopic disorder such as charge transport or molecular conformational dynamics [21]. In this letter we focus on disentangling this interplay.The dependence of an experimental observable on an applied magnetic field is generically referred to as the magnetic field effect (MFE). For CT-mediated processes, MFEs require that the observed physical property depends either directly or indirectly on the spin state of the electron-hole pair. For instance, spin selection rules for radiative electron-hole recombination can give rise to a magnetic field-dependent electroluminescence yield [20,[24][25][26]. To understand specifically how CT state properties are influenced by the presence of a magnetic field it is na...

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Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics

Cited by 611 publications

References 45 publications

Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions

Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects

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