Potentiometry on organic semiconductor devices

Abstract

In this thesis, we are interested in the local electrical potential of operational devices based on organic semiconductors. Potential profiles are measured using advanced scanning probe techniques with the final aim to confront experimental data with charge transport and device models. The motivation to do so is that spatially resolved potentials give a true window on the actual internal operation of the device, rather than on its mere external output. In order to do so in a quantitative manner, the spatial resolution of the used potential probe needs to be understood in detail. Ideally, one should not only be able to predict the experimental response from a known theoretical prediction, but also to recover the underlying ‘true’ potential profile from an experimental (broadened) profile. The most commonly used probe in this respect is scanning Kelvin probe microscopy, or SKPM, which combines the classical Kelvin probe with atomic force microscopy (AFM). Hence, in the first part of this thesis, we have focused on this technique. In a later chapter, the first steps towards an alternative, scanning tunneling microscope (STM) based potential probing technique are set. The last chapter uses AFM, STM and SKPM to investigate a surprising actuation effect of PEDOT:PSS thin films. Non-contact potentiometry or scanning Kelvin probe microscopy is a widely used technique to study charge injection and transport in (in)organic devices by measuring a laterally resolved local potential. This technique suffers from the significant drawback that experimentally obtained curves do generally not reflect the true potential profile in the device due to non-local coupling between the probing tip and the device. In Chapter 2 we quantitatively explain the experimental SKPM response, and by doing so directly link theoretical device models to real observables. In particular, the model quantitatively explains the effects of the tip-sample distance and the dependence on the orientation of the probing tip with respect to the device. This coarse approach requires a very long calculation time. The performance of organic light emitting field effect transistors (LEFET) is strongly influenced by the width of the recombination zone. In Chapter 3, we present an analytical model for the recombination profile. Assuming Langevin recombination, the recombination zone width W is found to be given by with d and ? the gate dielectric and accumulation layer thicknesses respectively. The model compares favorably to both numerical calculations and measured surface potential profiles of an actual LEFET. As explained in Chapter 3, in ambipolar organic field effect transistors (oFET) the shape of the channel potential is intimately related to the recombination zone width W, and hence to the electron-hole recombination strength. However, surface potentials as measured by scanning Kelvin probe microscopy (SKPM) are distorted due to spurious capacitive couplings. In Chapter 4, we develop a (de)convolution method with an experimentally calibrated transfer function to reconstruct the actual surface potential from a measured SKPM response and vice versa. This approach is much faster than the one presented in Chapter 2. Using this scheme, we find W = 0.5 ?m for a nickel dithiolene oFET, which translates into a recombination rate that is two orders of magnitude below the Langevin value. In Chapter 5, we report a new method for investigating the local electrical properties of organic field effect transistors with potentially unprecedented spatial resolution. In particular, we show that it is possible to perform scanning tunneling microscopy on the channel of an operational pentacene oFET, despite the absence of a conducting substrate. Charge transport from the STM tip to the contacts occurs via a three-step process of vacuum tunneling, followed by vertical transport to the accumulation layer and lateral transport in the accumulation layer. In Chapter 6, we demonstrate giant out-of-plane actuation of thin films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) under ambient conditions. The actuation is driven by an electrical bias and resulting current between interdigitated gold electrodes on glass which are placed below the PEDOT:PSS thin film. We combine current measurements with optical, AFM, SKPM and STM measurements on spin cast films. The actuation is found to be independent of film thickness and composition and leads to a maximum actuation in the first cycle of 950% for a 21 nm thick film in ambient. Two actuation regimes are identified. In the first regime, reversible redox reactions occur and actuation is mainly due to osmotic effects brought about by an increasing ion concentration on the anode. In the irreversible regime, secondary oxidation reactions occur and mass transport to the anode becomes also important. The independence of the maximally attainable actuation height on channel width and film thickness and on PEDOT:PSS ratio is explained by screening inside the thin film. From the thickness dependence of the potential screening we concluded that the screening length is of the order of tens of nm. Hence, the screening length rather than the film thickness determines the actuation height. To summarize, we succeeded to resolve the resolution issues associated with scanning Kelvin probe microscopy, thereby enabling a quantitative comparison of measured and modeled surface potentials. The developed tools and insight is applied to the potential in an ambipolar oFET where we used the experimental SKPM response to investigate the recombination zone width W. As a next step, potentially unprecedented spatial resolution on organic semiconductors might be achieved using scanning tunneling potentiometry. The first step in this direction, stable STM operation on the channel of a working oFET, has been established.