Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We report on a concept for organic solar cells where the layer sequence is inverted compared to the conventional setup. In such a configuration a conducting polymer layer is used as the transparent anode which is able to transport the photocurrent laterally to a metal grid. For the anode a low sheet resistance and a work function matching approximately the chemical potential of the holes of the illuminated photoactive layer is required. We showed that the poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) fits to these requirements. In our setup an aluminium cathode was used. It turned out that for the inverted setup the interface between aluminium and the photoactive layer has to be protected against oxidation. Our investigations show that with a thin layer (20 nm) of electron-beam deposited titanium between aluminium and the photoactive layer the electrical contact is drastically improved. A solar cell efficiency of (1.4 ± 0.3)% was reached in this case. © 2005 Elsevier B.V. All rights reserved.

Abstract:

Carbon nanomaterials are a promising class of materials for optoelectronics with low environmental impact. However, dark states limit their photoluminescence (PL) efficiency and thus applicability. In this thesis, hitherto-unrecognised twilight states in graphene nanoribbons are identified, where inter-valley mixing produced by periodic edge modulation and strong electron-phonon coupling are key mechanisms that make conventionally-dark states at least four times brighter, resulting in a 7.7% PL quantum yield. Strong vibron-electron coupling is revealed by the high spectral definition, with the bandgap modulated by coupling to the radial breathing-like mode. Unexpectedly, the predominant phonon modes affect absorption and emission differently, due to the simultaneous presence of Herzberg-Teller and Franck-Condon couplings. The achievement of efficient light emission from carbon nanostructures, both in solutions and thin films, opens the path to their integration into electro-optical circuits, and to the optical investigation and manipulation of topological states in graphenoids. Further, a detailed study of carbon nanotube-ethylene-vinyl acetate (CNT-EVA) transparent conductive films (TCFs) and the connection between processing and optoelectronic film properties is presented, leading to a 100-fold improvement of the electrical conductivity at the same transmittance. The dispersion technique and polymer removal steps are established as crucial steps for improving TCF performance. Changing the composition of the EVA copolymer allows tuning the mechanical and chemical resilience of CNT-EVA films and the films perform well in flexible applications. CNT-EVA films are chemically p-doped with halogenated metals and low-cost dopants identified. The improved CNT-EVA films perform well in transparent touch-sensitive devices and as transport layer in pervoskite photovoltaic (PV) devices, highlighting the potential of hybrid transport layers and opening a pathway for the large-scale application of low-cost CNT-EVA conductive films. Finding ways to stabilise perovskites under ambient conditions is the main challenge to overcome before they can be applied on a large scale. Substantial stability improvement is demonstrated for the pervoskite formamidinium lead iodide (FAPbI3) in single crystals by the addition of methylenediammonium dichloride (MDACl2). This is quantified by Raman spectroscopy which is established as a powerful tool for the characterisation of FAPbI3 by devising the defocused Raman spectroscopy method. The MDACl2 additive reduced the trap density and increased the excited carrier lifetime significantly, whereas the band gap at room temperature appears unaffected by the addition of MDACl2.

Abstract:

The electronic wavefunctions of an atom or molecule are affected by its interactions with its environment. These interactions dictate electronic and optical processes at interfaces, and is especially relevant in the case of thin film optoelectronic devices such as organic solar cells. In these devices, charge transport and interfaces between multiple layers occur along the thickness or vertical direction, and thus such electronic interactions are crucial in determining the device properties. Here, we introduce a new in-situ spectroscopic ellipsometry data analysis method called DART with the ability to directly probe electronic coupling due to intermolecular interactions along the thickness direction using vacuum-deposited organic semiconductor thin films as a model system. The analysis, which does not require any model fitting, reveals direct observations of electronic coupling between frontier orbitals under optical excitations leading to delocalization of the corresponding electronic wavefunctions with thickness or, equivalently, number of molecules away from the interface in C60 and MeO-TPD deposited on an insulating substrate (SiO2). Applying the same methodology for C60 deposited on phthalocyanine thin films, the analyses shows strong, anomalous features - in comparison to C60 deposited on SiO2 - of the electronic wavefunctions corresponding to specific excitation energies in C60 and phthalocyanines. Translation of such interactions in terms of dielectric constants reveals plasmonic type resonance absorptions resulting from oscillations of the excited state wavefunctions between the two materials across the interface. Finally, reproducibility, angstrom-level sensitivity and simplicity of the method are highlighted showcasing its applicability for studying electronic coupling between any vapor-deposited material systems where real-time measurements during deposition are possible.