Charge accumulation at organic semiconductor interfaces due to a permanent dipole moment and its orientational order in bilayer devices

Noguchi, Yutaka; Miyazaki, Yukimasa; Tanaka, Yuya; Sato, Naoki; Nakayama, Yasuo; Schmidt, Tobias D.; Brütting, Wolfgang; Ishii, Hisao

doi:10.1063/1.4724349

Cited by 169 publications

(180 citation statements)

References 36 publications

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“…Interface charge was first detected by Brütting et al at the a-NPD/Alq3 interface in a bilayer OLED. Furthermore, Ishii et al reported clear GSP behavior with a slope of 48 mV/nm, which corresponds to an interface charge density of 1.36 mC/m 2 [14]. In our study, the carrier accumulation at the TPD/C 60 interface was observed by the DMP analysis, which is similar to previous reports.…”

Section: Dmp Of P-n Heterostructured Opv Cellssupporting

confidence: 91%

Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy

Maeda¹,

Tokairin²,

Ikeda³

et al. 2015

AMPC

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The carrier dynamics in organic photovoltaic (OPV) cells were investigated by impedance spectroscopy. We introduced a novel impedance spectrum representation called dynamic modulus plot (DMP), which allowed us to observe the layer-to-layer carrier injection behavior graphically. In this work, the impedance responses were characterized in the N,N'-diphenyl-N,N'-di-m-tolyl-4,4'-diaminobiphenyl (TPD)/C 60 p-n heterostructured OPV cells against applied voltages. The dependence of impedance responses on the layer thickness revealed a constant internal electric field that disturbed the carrier transport within the OPV cells. We applied this technique to new donor materials, in which thiophene units were inserted to the center of TPD. By increasing the number of thiophene units in TPD the fill-factor (FF) improved from 33% to 59%, which increased the power conversion efficiency (PCE). Based on the DMP analysis, we assigned the improvement in device performance to the reduction of the internal electric field.

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Section: Dmp Of P-n Heterostructured Opv Cellssupporting

confidence: 91%

Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy

Maeda¹,

Tokairin²,

Ikeda³

et al. 2015

AMPC

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“…Other polar electron transport materials exhibiting orientation polarization are TPBi (À1.1 mC/m . 36 There are two key advantages of the presented approach compared to other measurement techniques. First, the active layers to be investigated can be prepared and deposited in the same way and especially with the same thickness as in a regular OLED or solar cell stack; thus, the extracted mobility values can be assumed to be the same in the regular device.…”

Section: Discussionmentioning

confidence: 99%

“…35 Radiative recombination takes place in the emissive layer at the HTL/ETL interface, as soon as the applied voltage is large enough (>V bi ) to inject both charge carrier species into their respective transport layers. The most commonly used electron transport materials for OLEDs, like Alq 3 , TPBi, BCP, and BPhen, 36,37 exhibit a permanent macroscopic polarization due to spontaneous orientation of molecular dipoles upon layer deposition, forming an effective sheet charge density on both sides of the layer. 38 It has been shown that this polarization effect leads to a reduction of the hole injection voltage V t , dependent on the thickness of the ETL and on the interface charge density, according to the following equation: 33,34 …”

Section: Introductionmentioning

confidence: 99%

The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes

Züfle

Altazin

Hofmann

et al. 2017

Journal of Applied Physics

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We demonstrate the application of the CELIV (charge carrier extraction by linearly increasing voltage) technique to bilayer organic light-emitting devices (OLEDs) in order to selectively determine the hole mobility in N,N0-bis(1-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine (α-NPD). In the CELIV technique, mobile charges in the active layer are extracted by applying a negative voltage ramp, leading to a peak superimposed to the measured displacement current whose temporal position is related to the charge carrier mobility. In fully operating devices, however, bipolar carrier transport and recombination complicate the analysis of CELIV transients as well as the assignment of the extracted mobility value to one charge carrier species. This has motivated a new approach of fabricating dedicated metal-insulator-semiconductor (MIS) devices, where the extraction current contains signatures of only one charge carrier type. In this work, we show that the MIS-CELIV concept can be employed in bilayer polar OLEDs as well, which are easy to fabricate using most common electron transport layers (ETLs), like Tris-(8-hydroxyquinoline)aluminum (Alq3). Due to the macroscopic polarization of the ETL, holes are already injected into the hole transport layer below the built-in voltage and accumulate at the internal interface with the ETL. This way, by a standard CELIV experiment only holes will be extracted, allowing us to determine their mobility. The approach can be established as a powerful way of selectively measuring charge mobilities in new materials in a standard device configuration.

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“…However, this value can be modified by different effects, such as Fermi-level pinning, spontaneous orientation polarization or band-bending phenomena. There are light-irradiation techniques to estimate V bi [53,54] or the method given by Mantri et al [55]. We follow this last method, in which an initial value of V bi is extracted from the transition voltage at which the slope of the J − V AC curve changes from exponential (diffusion regime) to power-law (space-charge-limited regime) (point D in Fig.…”

Section: Parameter Extraction and Verification -Darknessmentioning

confidence: 99%

Boundary condition model for the simulation of organic solar cells

López-Varo

Jiménez-Tejada

Marinov

et al. 2017

Organic Electronics

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Organic solar cells (OSCs) are promising photovoltaic devices to convert solar energy into electrical energy. Their many advantages such as lightweight, flexibility and low manufacturing costs are intrinsic to the organic/polymeric technology. However, because the performance of OSCs is still not competitive with inorganic solar cells, there is urgent need to improve the device performance using better designs, technologies and models. In this work, we focus on the developing an accurate physics-based model that relates the charge carrier density at the metal-organic boundaries with the current density in OSCs using our previous studies on singlecarrier and bipolar diodes. The model for the boundary condition of the charge carrier density at the interfaces of OSCs follows a power-law function with the current density, both in dark and under illumination, and simulated current-voltage characteristics are verified with experimental results. The numerical simulations of the current-voltage characteristics of OSCs consider wellestablished models for the main physical and optical processes that take place in the device: light absorption and generation of excitons, dissociation of excitons into free charge carriers, charge transport, recombination and injection-extraction of free carriers.Our analysis provides important insights on the influence of the metal-organic interfaces on the overall performance of OSCs. The model is also used to explain the anomalous S-shape current-voltage curves found in some experimental data.

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Charge accumulation at organic semiconductor interfaces due to a permanent dipole moment and its orientational order in bilayer devices

Abstract: Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

Cited by 169 publications

References 36 publications

Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy

Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy

The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes

Boundary condition model for the simulation of organic solar cells

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