Operating Modes of Sandwiched Light‐Emitting Electrochemical Cells

Lenes, Martijn; García-Belmonte, Germà; Tordera, Daniel; Pertegás, Antonio; Bisquert, Juan; Bolink, Henk J.

doi:10.1002/adfm.201002587

Cited by 167 publications

(170 citation statements)

References 37 publications

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“…the basis of light-emitting electrochemical cells. [29][30][31] In the polyelectrolytes studied here, mobile ions are present that can be used for electrochemical doping. Electrochemical doping is, however, not evident in the semiconductor PFN.…”

Section: Considered Mechanismsmentioning

confidence: 99%

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Reenen

Kouijzer

Janssen

et al. 2014

Adv Materials Inter

128

138

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can evolve into a successful thin fi lm photovoltaic technology. To this end, further improvements in the sunlight-toelectricity conversion effi ciency are still needed. To do so, not only the optoelectronic, active layer should be considered, but also the connection between this layer and the electrodes. This connection is facilitated by so-called work function modifi cation layers (WMLs) that serve to align the transport levels in the organic semiconductor and the conductor by modifi cation of the work function. Materials that are often used are poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) for improving hole collection and injection in combination with an indium tin oxide (ITO) electrode and LiF for the electron contact in combination with an Al metal electrode. A new generation of WMLs fi rst introduced by Cao et al. is based on polyelectrolytes or tertiary aliphatic amines, such as poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN) and the corresponding ethyl ammonium bromide. [ 1 ] Although these materials improve carrier injection or extraction in different organic diode devices, [1][2][3][4][5][6][7] the mechanism behind this improvement is not completely clear. This ambiguity hinders design, selection and optimization of new WMLs.Mechanisms that are generally considered to result in electric fi elds and concomitant work function shifts at metal-organic interfaces are doping, charge transfer, and dipole formation. [8][9][10] Regarding the work function modifi cation of amine side-groups, Lindell et al. [ 11 ] found by photoelectron spectroscopy and density functional calculations that the electron donor para-phenylenediamine chemisorbs onto atomically clean Ni in vacuum. [ 12 ] Due to partial electron transfer from the amine unit, a work function reduction of 1.55 eV is achieved, resulting in a less deep Fermi level. This explanation is not likely to hold for the technologically more relevant procedure on which we shall focus, being the deposition of WMLs from solution on a metal at atmospheric pressure: the metal is not atomically clean, which likely inhibits chemisorption. In other work by Zhou et al. [ 13 ] it is shown that the work function reduction, at atmospheric pressure, by the amine groups in poly(ethylenimine ethoxylated) (PEIE), is generally the same on different conductors. Work function modifi cation by polyelectrolytes and tertiary aliphatic amines is found to be due to the formation of a net dipole at the electrode interface, induced by interaction with its own image dipole in the electrode. In polyelectrolytes differences in size and side groups between the moving ions lead to differences in approach distance towards the surface. These differences determine magnitude and direction of the resulting dipole. In tertiary aliphatic amines the lone pairs of electrons are anticipated to shift towards their image when close to the interface rather than the nitrogen nuclei, which are sterically hindered by the alkyl side chains. ...

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Section: Considered Mechanismsmentioning

confidence: 99%

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Reenen

Kouijzer

Janssen

et al. 2014

Adv Materials Inter

128

138

View full text Add to dashboard Cite

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“…-Organic light-emitting devices constitute an important branch in organic optoelectronics due to their great potential to be used in a wide range of applications, from dot-pixels for color displays to large-area panels for ambient illumination [1][2][3][4][5][6]. Differently from organic/polymeric light-emitting diodes (OLEDs/PLEDs), whose technology is already quite well developed, polymeric light-emitting electrochemical cells, LECs [7], are devices whose performance is still not satisfactory for commercial applications, but had presented a growing interest in recent years [8][9][10][11][12]. The main feature of a LEC is that the active layer comprises a blend of a conjugated electroluminescent polymer (EP) and a polymer electrolyte, which confers to them advantageous characteristics like bipolar operation (in forward or in reverse bias) and low operating voltages, regardless of the work function of the electrodes and of the thickness of the active layer [7].…”

Section: Copyright C Epla 2012mentioning

confidence: 99%

Transient and d.c. analysis of the operation mechanism of light-emitting electrochemical cells

Gozzi

Santos

Faria

2012

EPL

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Light-emitting electrochemical cells (LECs) made of electroluminescent polymers were studied by d.c. and transient current-voltage and luminance-voltage measurements to elucidate the operation mechanisms of this kind of device. The time and external voltage necessary to form electrical double layers (EDLs) at the electrode interfaces could be determined from the results. In the low- and intermediate-voltage ranges (below 1.1 V), the ionic transport and the electronic diffusion dominate the current, being the device operation better described by an electrodynamic model. For higher voltages, electrochemical doping occurs, giving rise to the formation of a p-i-n junction, according to an electrochemical doping model.

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“…There is one example of an optoelectronic device in which the interplay between electronic and ionic properties is determinant: in electrochemical light-emitting diodes, the ionic charge kinetics is essential to explain time evolution of the light power emission as functional materials are mixed ionicelectronic conductors. 2 Also, in photovoltaics, perovskitebased solar cells constitute a special type of light-harvesting devices because, in addition to electronic mechanisms, the presence of moving ionic species is believed to modify current-voltage curves. 3,4 Among other explanations related to either ferroelectricity [5][6][7] or electronic trapping properties of perovskites, 8 there is growing consensus that the ionic kinetics lies behind the commonly observed current-voltage hysteretic response, [9][10][11] and switchable photovoltaic characteristics.…”

mentioning

confidence: 99%

Ionic charging by local imbalance at interfaces in hybrid lead halide perovskites

Almora

Guerrero

García-Belmonte

2016

Applied Physics Letters

Self Cite

125

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Identification of specific operating mechanisms becomes particularly challenging when mixed ionic-electronic conductors are used in optoelectronic devices. Ionic effects in perovskite solar cells are believed to distort operation curves and possess serious doubts about their long term stability. Current hysteresis and switchable photovoltaic characteristics have been connected to the kinetics of ion migration. However, the nature of the specific ionic mechanism (or mechanisms) able to explain the operation distortions is still poorly understood. It is observed here that the local rearrangement of ions at the electrode interfaces gives rise to commonly observed capacitive effects. Charging transients in response to step voltage stimuli using thick CH 3 NH 3 PbI 3 samples show two main polarization processes and reveal the structure of the ionic double-layer at the interface with the non-reacting contacts. It is observed that ionic charging, with a typical response time of 10 s, is a local effect confined in the vicinity of the electrode, which entails absence of net mobile ionic concentration (space-charge) in the material bulk. V C 2016 AIP Publishing LLC.

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Operating Modes of Sandwiched Light‐Emitting Electrochemical Cells

Cited by 167 publications

References 37 publications

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Transient and d.c. analysis of the operation mechanism of light-emitting electrochemical cells

Ionic charging by local imbalance at interfaces in hybrid lead halide perovskites

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