Scanning photocurrent and photoluminescence imaging of a frozen polymer p-n junction

Inayeh, Alex; Dorin, Bryce; Gao, Jun

doi:10.1063/1.4773235

Cited by 28 publications

(19 citation statements)

References 28 publications

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“…Polymer light-emitting electrochemical cells (LECs) have attracted great interest in recent years due to their complex and yet fascinating operating mechanism as well as their potential applications in display and lighting. [1][2][3][4][5][6][7][8][9][10][11] LECs are solid-state semiconductor devices based on a mixed ionic/ electronic conductor. 12 The operation of LECs under bias involves dynamic electrochemical p-and n-doping of the luminescent polymer and the formation of a light-emitting p-n junction.…”

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confidence: 99%

Decoupled luminance decay and voltage drift in polymer light-emitting electrochemical cells: Forward bias vs. reverse bias operation

Gao

AlTal

2014

Applied Physics Letters

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Sandwich polymer light-emitting electrochemical cells (LECs) with low electrolyte loading have been investigated. The LECs can be turned on to emit light at low voltages by applying either a forward- or reverse-bias current. The reverse operation, however, displays significantly higher luminance, frozen-junction behavior, and voltage instability. Moreover, the LECs display de-coupled luminance decay and voltage drift. Under forward-current operation, the luminance decreases for most of the test duration, while the operating voltage remains nearly unchanged. Under reverse-current operation, the luminance remains constant for over 200 h while the operating voltage more than doubles. The highly asymmetric device characteristics between forward and reverse operation can be explained by the different doping profiles created when the initial charge injection barriers are different. Under forward-current operation, the LEC film is predominantly p-doped. This leads to heavy luminescence quenching but a stable operating voltage. Under reverse operation, a predominately n-doped film is responsible for the higher luminance, frozen-junction behavior, and voltage drift. The anomalous device characteristics offer additional insights into the complex operating mechanism of LECs and the behaviours of a doped luminescent polymer.

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confidence: 99%

Decoupled luminance decay and voltage drift in polymer light-emitting electrochemical cells: Forward bias vs. reverse bias operation

Gao

AlTal

2014

Applied Physics Letters

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“…27,28 Most of the experiments performed to shed light on the functional principle of LECs were conducted on devices with a planar geometry featuring active layer thicknesses up to the centimeter scale. [29][30][31] The device design is enabled by the formation of the highly conductive, doped regions during operation and allows for the use of analysis techniques like optical imaging via fluorescence microscopy, 21,27,[29][30][31][32][33] scanning Kelvin probe microscopy (SKPM), 20,21,27,32,34 and optical beam induced current (OBIC) measurements. 33,35,36 However, LECs that are relevant for practical use comprise a stacked architecture with an active layer of only a few hundreds of nanometers.…”

Section: Introductionmentioning

confidence: 99%

The dynamic behavior of thin-film ionic transition metal complex-based light-emitting electrochemical cells

Meier

Hartmann

Winnacker

et al. 2014

Journal of Applied Physics

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Light-emitting electrochemical cells (LECs) have received increasing attention during recent years due to their simple architecture, based on solely air-stabile materials, and ease of manufacture in ambient atmosphere, using solution-based technologies. The LEC's active layer offers semiconducting, luminescent as well as ionic functionality resulting in device physical processes fundamentally different as compared with organic light-emitting diodes. During operation, electrical double layers (EDLs) form at the electrode interfaces as a consequence of ion accumulation and electrochemical doping sets in leading to the in situ development of a light-emitting p-i-n junction. In this paper, we comment on the use of impedance spectroscopy in combination with complex nonlinear squares fitting to derive key information about the latter events in thin-film ionic transition metal complex-based light-emitting electrochemical cells based on the model compound bis-2-phenylpyridine 6-phenyl-2,2′-bipyridine iridium(III) hexafluoridophosphate ([Ir(ppy)2(pbpy)][PF6]). At operating voltages below the bandgap potential of the ionic complex used, we obtain the dielectric constant of the active layer, the conductivity of mobile ions, the transference numbers of electrons and ions, and the thickness of the EDLs, whereas the transient thickness of the p-i-n junction is determined at voltages above the bandgap potential. Most importantly, we find that charge transport is dominated by the ions when carrier injection from the electrodes is prohibited, that ion movement is limited by the presence of transverse internal interfaces and that the width of the intrinsic region constitutes almost 60% of the total active layer thickness in steady state at a low operating voltage.

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“…In addition, the frozen‐junction cell can operate as a photovoltaic cell when illuminated with photons of sufficient energy. An important realization is that both EL and photovoltaic functionalities of the LECs are only contributed by the p–n junction . In a planar cell, as shown in Figure , the neutral p and n regions of the cell account for over 90 % of cell area, but are neither electroluminescent nor photovoltaic.…”

Section: Resultsmentioning

confidence: 99%

Solid‐State Bipolar Electrochemistry: Polymer‐Based Light‐Emitting Electrochemical Cells

et al. 2015

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We present solid‐state, polymer‐based, light‐emitting electrochemical cells incorporating bipolar electrodes (BPEs) positioned between the driving electrodes. The dominant processes in these planar electrochemical cells are in situ electrochemical p and n doping of the luminescent conjugated polymer when a sufficient voltage bias is applied between the feeder anode and cathode. The inclusion of floating BPEs causes additional doping at the extremities of the BPE, which has been visualized through in situ fluorescence imaging. We investigate how the applied voltage bias and the BPE size affect electrochemical doping from the BPE. We observe an interesting delay in the appearance of doping from the BPE if the applied voltage bias was initially insufficient. Doping eventually occurs when the potential difference across the BPE/polymer interface becomes sufficient, as doping from the driving electrodes propagates. The inclusion of BPEs can lead to more efficient polymer light‐emitting and photovoltaic cells when a large number of active junctions are formed.

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Scanning photocurrent and photoluminescence imaging of a frozen polymer p-n junction

Cited by 28 publications

References 28 publications

Decoupled luminance decay and voltage drift in polymer light-emitting electrochemical cells: Forward bias vs. reverse bias operation

Decoupled luminance decay and voltage drift in polymer light-emitting electrochemical cells: Forward bias vs. reverse bias operation

The dynamic behavior of thin-film ionic transition metal complex-based light-emitting electrochemical cells

Solid‐State Bipolar Electrochemistry: Polymer‐Based Light‐Emitting Electrochemical Cells

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