Conductivity, work function, and environmental stability of PEDOT:PSS thin films treated with sorbitol

Nardes, AM Alexandre; Kemerink, Martijn; Kok, MM de; Vinken, E Esther; Maturová, K Klara; Janssen, Raj René

doi:10.1016/j.orgel.2008.05.006

Cited by 614 publications

(453 citation statements)

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“…[ 24 ] Processing of PEDOT:PSS from aqueous dispersions with different amounts of added sorbitol was found to lead to a reduction in the work function, which has been interpreted as being the result of removal of the H-PSS top layer. [ 25 ] In this work we present results which indicate that work function modifi cation by anionic or cationic (poly)electrolytes and by tertiary aliphatic amines originates from a single mechanism: the formation of a dipole at the interface. This dipole formation is driven by the energetic stabilization by its image charges in the contacting (semi-)conductor.…”

Section: Introductionmentioning

confidence: 92%

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

Reenen

Kouijzer

Janssen

et al. 2014

Adv Materials Inter

130

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: Introductionmentioning

confidence: 92%

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

Reenen

Kouijzer

Janssen

et al. 2014

Adv Materials Inter

130

138

View full text Add to dashboard Cite

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“…The literature values for the WF of this conductive polymer range between 4.8 and 5.2 eV [26,27]. With many semiconducting polymers and small molecules having their highest occupied molecular orbital (HOMO) levels lying much deeper than 5.2 eV below the vacuum level, this interlayer material will not form a perfectly injecting contact with such materials.…”

Section: Built-in Voltagesmentioning

confidence: 99%

Charge Transport in Spiro-OMeTAD Investigated through Space-Charge-Limited Current Measurements

et al. 2018

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Extracting charge-carrier mobilities for organic semiconductors from space-charge-limited conduction measurements is complicated in practice by nonideal factors such as trapping in defects and injection barriers. Here, we show that by allowing the bandlike charge-carrier mobility, trap characteristics, injection barrier heights, and the shunt resistance to vary in a multiple-trapping drift-diffusion model, a numerical fit can be obtained to the entire current density-voltage curve from experimental space-charge-limited current measurements on both symmetric and asymmetric 2; 2 0 ; 7; 7 0 -tetrakis(N; N-di-4-methoxyphenylamine)-9; 9 0 -spirobifluorene (spiro-OMeTAD) single-carrier devices. This approach yields a bandlike mobility that is more than an order of magnitude higher than the effective mobility obtained using analytical approximations, such as the Mott-Gurney law and the moving-electrode equation. It is also shown that where these analytical approximations require a temperature-dependent effective mobility to achieve fits, the numerical model can yield a temperature-, electric-field-, and charge-carrier-density-independent mobility. Finally, we present an analytical model describing trap-limited current flow through a semiconductor in a symmetric single-carrier device. We compare the obtained charge-carrier mobility and trap characteristics from this analytical model to the results from the numerical model, showing excellent agreement. This work shows the importance of accounting for traps and injection barriers explicitly when analyzing current density-voltage curves from space-charge-limited current measurements.

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“…Addition of a co-solvent reduced the workfunction of spin coated PEDOT:PSS [22], while doping of PEDOT:PSS with Cl -ions to 33 at.% increased the workfunction to 5.4 eV [23].…”

Section: Introductionmentioning

confidence: 99%

“…Nardes et al [11,16,22,24] investigated the conduction mechanisms of spin cast PEDOT:PSS in both in-plane and transverse directions. Their work demonstrated how the action of solvent evaporation in spin coating leads to charge conduction anisotropy generating a disparity between in-plane and transverse conduction models, 3D M-VRH and nearest-neighbour hopping, respectively.…”

Section: Introductionmentioning

confidence: 99%