Metodología para la determinación del somato tipo

Figure 6.8: SEM cross-section micro-graph showing HBC nanowires and dense supporting HBC barrier layer. The sample was fabricated by a two-step polymerization as described in the text.

A little modication in the fabrication process facilitates a precise control over the for- mation of a dense HBC layer underneath the patterned lm (Figure 6.8). In a rst step a thin layer of the organic material is deposited by spin coating on the ITO support. This at lm is polymerized and immobilized by thermal annealing at170◦Cresulting

in the formation of a dense barrier layer. Subsequently, a second layer of the organic material was deposited from solution followed by an imprinting step as described pre- viously (Section 6.3.1). Only the material in the second layer consisting of monomeric

6.3 Nanostructuring discotic molecules on ITO support

HBCacrylate molecules can be reallocated by the imprinting step. Varying the spin coat-

ing parameters for the two deposition steps allows to individually and precisely control the thickness of the non-structured dense barrier layer and the formation of the nano- pillars. Figure 6.8 shows a sample with an array of 180 nm long wires grafted on a 30 nm thick barrier layer produced using the two-step imprinting procedure described above.

Accurate control over the growth of a barrier layer is very instructive for the assembly of ecient photovoltaic devices; the thin and compact layer at the bottom in combination with a uniform pillar height as shown in Figure 6.5 c) ensure that donor and acceptor compounds will be only in contact with their respective electrodes. As such, exciton and charge carrier recombination at the device contacts can be suppressed eectively. Upon polymerization the molecules can be frozen in crystalline or liquid crystalline state depending on the cross-linking procedure: If acrylate moieties are polymerized at elevated temperatures as reported here the resulting lms show liquid crystalline order [250]. This allows minimizing grain boundaries while maintaining the π −π

stacking of neighboring discotic molecules [17]. On the other hand the polymerization of the molecule could also be induced by UV-curing at lower temperatures resulting in amorphous or crystalline lms which might be advantageous for some applications [250]. Nanostructured organic lms have been produced previously with other organic donor materials yielding a comparable aspect ratio as presented herein [24, 254]. However, in these studies the nanowires are produced by direct (over)-lling of commercial AAO templates and a subsequent lamination step on ITO via special silanes, e.g. vinyl- Si(OMe)3 [24], or directly on the ITO support using siloxane-derivatized organic ma- terials [254]. In contrast, the technique presented here allows for a production of fully organic nanowires directly on the ITO support with precise control of a barrier layer. Intimate contact and complete grafting to the ITO substrate with no air encapsulation is ensured. Furthermore, no siloxane is necessary for ITO grafting which might alter the electronic properties of the organic compound or introduce traps in the bulk material. After cross-linking of the donor material an acceptor compound can be directly pro- cessed onto the imprinted layer from the solution phase while maintaining the interfacial patterns. Furthermore, the cross-linking assisted patterning does not rely on distinct melting temperatures of the two organic materials as it is the case for the so called double imprinting presented recently [255].

Photovoltaic devices have been fabricated using the nanostructured HBCacrylate lms

and PDI or PC61BM as an acceptor. For all devices using the thermally polymerized

HBC material as a donor compound only very low conductivity and almost negligible current generation was observed.

On the contrary, very high charge carrier mobility has been shown for other HBC derivatives [20]. Despite careful processing and subsequent rinsing of the HBC layers a decrease in conductivity could result from adhesion of siloxane used for the AAO surface modication or other solvents used throughout the AAO removal procedure. Furthermore, optimization of the AAO surface treatment, the nanowire diameter and the polymerization procedure might be necessary to yield an improved molecular align- ment [241, 244, 256, 257]. However, in the present study the low conductivity of the polymerized HBCacrylate lms is attributed mainly to charge trapping in the residual

groups attached to the HBC core. In previous studies a strong dependence of the residues attached to HBC on conductivity and charge extraction yield was observed (Chapter 4), well in accordance with literature [194]. Furthermore, it has been shown, that on average only four out of the six acrylate moieties of the monomer will be cross-linked during the polymerization procedure [250]. Non-linked acrylate moieties present in the polymerized lms will most likely act as traps, hinder defect free face- to-face stacking of the HBC cores and strongly limit charge carrier transport through the organic layer. A reduction of cross-linker binding sites per molecule and conjugated linkage units will certainly help reduce this conductivity issue.

It is worth mentioning here, that nitrene mediated cross-linking of polymers allows the formation of highly stabilized structures and does not negatively aect the conductivity of the conducting polymers. In fact, a remarkably high performance has been achieved using the cross-linked polymers as donor materials in OPV devices [258] as will be dis- cussed in more detail in the outlook of this thesis (Section 7.3).

6.4 Nanostructuring solution processable fullerenes

The imprinting techniques shown in the previous Section appears suitable for the pat- terning of other organic semiconducting materials also. PC61BM is an especially attrac-

tive candidate which has been very well studied for application in OPV devices [110]. The molecule can be processed from the solution phase and does not tend to crystallize, unlike PDI [117]. As such, nanometer-sized patterning of the material seems feasible using the AAO template approach presented (Section 6.2).

However, in contrast to the polymerization based structuring method presented for the cross-linkable HBC derivative a selective etching of the AAO template cannot be accomplished without dissolution of the fullerene molecules. Great care must be taken when mechanically removing the AAO template form the soft organic material.

6.4 Nanostructuring solution processable fullerenes

PC61BM molecules are not grafted to the substrate support making delamination and

disruption of the nanometer sized structures very likely.

After carefully adapting the imprinting parameters and by using a suitable silanization techniques we succeeded to fabricate nano-patterned lms of PC61BM on a substrate

support.