Unified analysis of transient and steady-state electrophosphorescence using exciton and polaron dynamics modeling

Hershey, Kyle W.; Holmes, Russell J.

doi:10.1063/1.4966615

Cited by 25 publications

(30 citation statements)

References 37 publications

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“…The full curves show an empirical fit [Eq. (13)] to the high-field part of the data, and the dashed lines give the linear fit [Eq. (16)] to the low-field data shown already in Fig.…”

Section: B Electric-field Dependence Of the Epq Ratementioning

confidence: 99%

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Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

et al. 2017

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Exciton-polaron quenching (EPQ) is a major efficiency loss process in organic optoelectronic devices, in particular at high excitation densities. Within commonly used models, the rate is assumed to be given by the product of the exciton density, the polaron density, and a constant EPQ rate coefficient, which is proportional to the polaron diffusion coefficient and an EPQ capture radius. In this work, we study the effects of polaron diffusion on the EPQ rate in energetically disordered materials with a Gaussian density of states using kinetic Monte Carlo simulations, and show that the effective rate coefficient can depend strongly on the polaron concentration and on the electric field. We furthermore find that under realistic conditions, the effective value of the capture radius can exceed the expected value of ∼1 nm by up to two orders of magnitude. To a first approximation, the simulation results can be understood from macroscopic diffusion theory, adapted at finite electric fields to include the observed "polaron wind" effect. However, for strongly disordered systems we find distinct deviations from that theory, related to the very small time and spatial scales involved in the capture process.

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“…The full curves show an empirical fit [Eq. (13)] to the high-field part of the data, and the dashed lines give the linear fit [Eq. (16)] to the low-field data shown already in Fig.…”

Section: B Electric-field Dependence Of the Epq Ratementioning

confidence: 99%

“…Experimental values obtained for k EPQ range from typically (0.01−1) × 10 −18 m 3 /s for TPQ in phosphorescent small-molecule host-guest systems used in OLEDs [7][8][9][10][11][12][13] to values which can be three orders of magnitude larger for singlet-polaron quenching (SPQ) in fluorescent polymer materials [5].…”

Section: Introductionmentioning

confidence: 99%

Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

et al. 2017

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“…[33,[36][37][38] In these devices, no PL quenching was observed at the polaron densities relevant to device operation. As the voltage is increased to 700 mV, the spatially averaged polaron density increases from 2.6 × 10 16 cm −3 and 1.6 × 10 16 cm −3 near 0 mV to 2.2 × 10 18 cm −3 and 1.1 × 10 18 cm −3 for the SubPc and C 60 layers, respectively.…”

Section: Excitonic Loss Mechanismsmentioning

confidence: 99%

“…[33,[36][37][38] External Quantum Efficiency Measurements: External quantum efficiency measurements were taken under monochromatic illumination from a 300 W xenon arc lamp coupled into a Cornerstone 130 1/8 meter monochromator and chopped with an SRS SR540 optical chopper. Hole-only devices consisted of 89.6 to 176.3 nm of SubPc deposited on the ITO-coated substrate, capped with an 11.4 nm thick MoO x electronblocking layer and a 100 nm thick Al cathode.…”

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confidence: 99%

Decoupling Photocurrent Loss Mechanisms in Photovoltaic Cells Using Complementary Measurements of Exciton Diffusion

Curtin

Holmes

2018

Advanced Energy Materials

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techniques which have been previously applied to extract L D in organic semiconductors. Each of these measurements probes a different stage of the photoconversion process, and by combining these techniques we quantitatively decouple the associated exciton and charge carrier losses as a function of forward bias, providing insight into materials and device properties which limit OPV performance.Photoconversion in OPVs relies on a heterojunction between electron donating and accepting materials and occurs via a series of steps, each characterized by its own efficiency. [1,4] Incident photons are absorbed by the active material with efficiency η A leading to exciton generation. Excitons then diffuse to the dissociating donor-acceptor (D-A) interface with a diffusion efficiency η D . The exciton diffusion length is typically on the order of tens of nanometers, limiting active layer thickness and consequently η A . [2,4,5] At the D-A interface, the offset in molecular orbital energy levels drives exciton dissociation by charge transfer with efficiency η CT , leading to the formation of a Coulombically bound charge transfer (CT) state across the interface. The CT state may be dissociated into a pair of charge carriers with efficiency η CS , or undergo geminate recombination at the interface. Generated free charge carriers may be collected at the electrodes as photocurrent (η FC ) or undergo non-geminate recombination with a carrier of opposite polarity. [6,7] The product of these efficiencies determines the device external quantum efficiency (η EQE ).As a function of forward bias, the values of η D , η CS , and η FC can be reduced due to exciton-and polaron-driven losses. These parasitic loss pathways often result in a steep reduction in the photocurrent (J photo ) as a function of forward bias voltage, leading to low device fill factors that limit the maximum operating power. [6,8] Reductions in η D may result from excitonic losses by exciton-polaron quenching. [9][10][11][12] It has been previously established that there is a direct relationship between the operating voltage and the number of polarons present in an OPV. [13][14][15][16] Therefore, it follows that as the forward bias voltage increases in a device, so does the number of polarons, increasing the likelihood of an exciton being quenched prior to dissociation. [9] Dissociation of the interfacial CT state is assisted by the device built-in field. [8,[17][18][19] However, as the Significant work has been directed at measuring the exciton diffusion length (L D ) in organic semiconductors due to its significance in determining the performance of photovoltaic cells. Several techniques have been developed to measure L D , often probing photoluminescence or charge carrier generation. Interestingly, in this study it is shown that when different techniques are compared, both the diffusive behavior of the exciton and active carrier recombination loss pathways can be decoupled. Here, a planar heterojunction device based on the donor-acceptor pairing of boron subphthal...

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“…and, (4) where J is the injected current density and w is the width of the exciton recombination zone, taken to be the emissive layer width. See Reference [20] for a more detailed discussion of each generation term.…”

Section: Theorymentioning

confidence: 99%

10‐2: Invited Paper: Unified Analysis of Transient and Steady‐State Electroluminescence –Establishing an Analytical Formalism for OLED Charge Balance

Hershey

Holmes

2017

Symp Digest of Tech Papers

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Organic light-emitting devices (OLEDs) can exhibit an efficiency roll-off under high current excitation that has been previously modeled using bimolecular quenching. The corresponding transient electroluminescence behavior has not been as effectively treated . Here, we reproduce both the steady-state and transient regimes by introducing into the analysis a dynamic polaron population. This approach permits a rigorous definition of OLED charge balance in terms of dynamics as the exciton formation efficiency.

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Unified analysis of transient and steady-state electrophosphorescence using exciton and polaron dynamics modeling

Cited by 25 publications

References 37 publications

Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

Decoupling Photocurrent Loss Mechanisms in Photovoltaic Cells Using Complementary Measurements of Exciton Diffusion

10‐2: Invited Paper: Unified Analysis of Transient and Steady‐State Electroluminescence –Establishing an Analytical Formalism for OLED Charge Balance

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