Reducing exciton-polaron annihilation in organic planar heterojunction solar cells

Substrates can significantly affect the electronic properties of organic semiconductors. In this paper, we report the effects of contact-induced doping, arising from charge transfer between a high work function hole extraction layer (HEL) and the organic active layer, on organic photovoltaic device performance. Employing a high work function HEL is found to increase doping in the active layer and decrease photocurrent. Combined experimental and modeling investigations reveal that higher doping increases polaron-exciton quenching and carrier recombination within the field-free region. Consequently, there exists an optimal HEL work function that enables a large built-in field while keeping the active layer doping low. This value is found to be ~0.4 eV larger than the pinning level of the active layer material. These understandings establish a criterion for optimal design of the HEL when adapting a new active layer system and can shed light on optimizing performance in other organic electronic devices.

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“…Additionally, polaron-exciton quenching has been attributed to a higher J-V slope under reverse bias in bilayer OPV systems 35,36. …”

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Effects of Contact-Induced Doping on the Behaviors of Organic Photovoltaic Devices

Wang

Lee

et al. 2015

Nano Lett.

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“…For phosphorescent OLEDs, within which the emission originates from relatively long-lived excitons with predominantly triplet character, triplet-polaron quenching (TPQ) can contribute significantly to the efficiency rolloff at high luminance levels [1]. For donor-acceptor blends with nanomorphologies which are optimal for photovoltaic operation, exciton-polaronquenching reactions have also been found to be a relevant loss process [2][3][4].…”

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confidence: 99%

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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“…Excitons then diffuse to the dissociating donor-acceptor (D-A) interface with a diffusion efficiency η D . [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. [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.…”

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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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Reducing exciton-polaron annihilation in organic planar heterojunction solar cells

Cited by 18 publications

References 43 publications

Effects of Contact-Induced Doping on the Behaviors of Organic Photovoltaic Devices

Effects of Contact-Induced Doping on the Behaviors of Organic Photovoltaic Devices

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

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