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. ...

The operational mechanism of polymer light-emitting electrochemical cells (LECs) in sandwich geometry is studied by admittance spectroscopy in combination with numerical modeling. At bias voltages below the bandgap of the semiconducting polymer, this allows the determination of the dielectric constant of the active layer, the conductivity of mobile ions, and the thickness of the electric double layers. At bias voltages above the bandgap, p-n junction formation gives rise to an increase in capacitance at intermediate frequencies ( ≈ 10 kHz). The time and voltage dependence of this junction are successfully studied and modeled. It is shown that impedance measurements cannot be used to determine the junction width. Instead, the capacitance at intermediate biases corresponds to a low-conductivity region that can be signifi cantly wider than the recombination zone. Finally, the long settling time of sandwich polymer LECs is shown to be due to a slow process of dissociation of salt molecules that continues after the light-emitting p-n junction has formed. This implies that in order to signifi cantly decrease the response-time of LECs an electrolyte/salt combination with a minimal ion binding energy must be used.

The characteristic doping process in polymer light-emitting electrochemical cells (LECs) causes a tradeoff between luminescence intensity and efficiency. Experiments and numerical modeling on thin film polymer LECs show that on the one hand carrier injection and transport benefit from electrochemical doping, leading to increased electron-hole recombination. On the other hand the radiative recombination efficiency is reduced by exciton quenching by polarons involved in the doping. Consequently the quasi-steady-state luminescent efficiency decreases with increasing ion concentration. The transient of the luminescent efficiency shows a characteristic roll-off while the current continuously increases, attributed to ongoing electrochemical doping and the associated exciton quenching. Both effects can be modeled by exciton polaron-quenching via diffusion-assisted Förster resonance energy transfer. These results indicate that the tradeoff between efficiency and intensity is fundamental, suggesting that the application realm of future LECs should be sought in high-brightness, low-production cost devices, rather than in high-efficiency devices.2

Driven by promising recent results, there has been a revived interest in the thermoelectric properties of organic (semi) conductors. Concomitantly, there is a need to probe the Seebeck coefficient S of modestly conducting materials in thin film geometry. Here we show that geometries that seem desirable from a signal-to-noise perspective may induce systematic errors in the measured value of S, S-m, by a factor 3 or more. The enhancement of S-m by the device geometry is related to competing conduction paths outside the region between the electrodes. We derive a universal scaling curve that allows correcting for this and show that structuring the semiconductor is not needed for the optimal electrode configuration, being a set of narrow, parallel strips.

Funding Agencies|Dutch program NanoNextNL

Evidence for spacecharge-limited conduction in organic photovoltaic cells at open-circuit conditions. Physical Review B, 87(8), 085207-1/11. [085207].