Advanced Functional Materials and Devices (AFMD) Group

Abstract:

The working principle of organic bulk-heterojunction solar cells is a widely discussed topic. For thin film solar cells it is commonly supposed that the built-in potential Vbi is the driving force for charge separation and determines the open-circuit voltage Voc. In former works, V bi was estimated by measuring Voc in the saturation regime. To check the validity of this model, the direct measurement of the built-in potential is desirable. We have investigated the origin of the open-circuit voltage of organic bulk-heterojunction solar cells by means of electroabsorption spectroscopy. This technique allows measurement of the built-in potential directly and therefore permits an independent measurement of Vbi and Voc. In our experiments on indium tin oxide/poly(3,4-ethylendioxythiophene) : poly(styrene-sulfonate)/poly(2 -methoxy-5-(3′,7′-dimethyloktyloxy)-p-phenylene-vinylene) : 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61/metal bulk-heterojunction solar cells no significant correlation between the open-circuit voltage and the built-in potential was observed. For certain electrode materials, i.e. gold and copper, Voc exceeded V bi which is revealing of semipermeable membranes. © 2005 Elsevier B.V. All rights reserved.

Abstract:

Along with efficiency and lifetime, costs are one of the most important aspects for the commercialization of organic solar cells. Thinking of large scale production of organic solar cells by an efficient reel-to-reel process, the materials are expected to determine the costs of the final product. Our approach is to develop functional substrates for organic solar cells which have the potential for cost effective production. The functionality is obtained by combining periodically microstructured substrates with lamellar electrode structures. Such structured substrates were fabricated by cost effective replication from masterstructures that were generated by large area interference lithography. Two cell architectures were investigated - holographic microprisms and interdigital buried nanoelectrodes. A structure period of 20μm in combination with a 2μm wide metal grid was chosen for the microprism cells based on the results of electrical calculations. Current-voltage curves with reasonable fill factors were measured for these devices. A significant light trapping effect was predicted from optical simulations. Interdigital buried nanoelectrodes are embedded in the photoactive layer of the solar cell. Separated interdigital metal electrodes with a sufficiently high parallel resistance were manufactured despite a small electrode distance below 400 nm. Experimental results on first photovoltaic devices will be presented. We observe an insufficient rectification of the photovoltaic device which we attribute to partial electron injection into the gold anode.

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:

Previous theoretical calculations show azetidinium has the right radial size to form a 3D perovskite with lead halides [1], and has been shown to impart, as the A-site cation of ABX3 unit, beneficial properties to ferroelectric perovskites [2]. However, there has been very limited research into its use as the cation in lead halide perovskites to date. In this communication we report the synthesis and characterization of azetidinium-based lead mixed halide perovskite colloidal nanocrystals. The mixed halide system is iodine and chlorine unlike other reported nanocrystals in the literature where the halide systems are either iodine/bromine or bromine/chlorine. UV-visible absorbance data, complemented with photoluminescence spectroscopy, reveals an indirect-bandgap of about 1.96 eV for our nanocrystals. Structural characterization using TEM shows two distinct interatomic distances (2.98 +/- 0.15 Angstroms and 3.43 +/- 0.16 Angstroms) and non-orthogonal lattice angles (approximately 112 degrees) intrinsic to the nanocrystals with a probable triclinic structure revealed by XRD. The presence of chlorine and iodine within the nanocrystals is confirmed by EDS spectroscopy. Finally, light-induced electron paramagnetic resonance (LEPR) spectroscopy with PCBM confirms the photoinduced charge transfer capabilities of the nanocrystals. The formation of such semiconducting lead mixed halide perovskite using azetidinium as the cation suggests a promising subclass of hybrid perovskites holding potential for optoelectronic applications such as in solar cells and photodetectors.

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.