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The photochemical coupling of fullerene molecules into covalently connected oligomeric or polymeric structures can result in drastically lowered solubility in common solvents with retained semiconductor properties. Here, we exploit this combination of properties for the utilization of fullerenes as a negative photoresist material with electronic functionality. Specifically, we develop an easily tunable exposure system, essentially comprising a laser and a computer-controlled spatial light modulator (SLM) featuring >8 million independently controlled pixels, for the spatially selective photochemical transformation of nanometer-thin C60 fullerene films. With a carefully designed laser-SLM-exposure/solvent-development cycle, we are able to realize well-resolved two-dimensional hexagonal or square patterns of circular C60 microdots with a center-to-center distance of 1–5 μm and a maximum thickness of 20–35 nm over several square-millimeter-sized areas on a substrate. The functionality of such a hexagonal C60 pattern was demonstrated by its inclusion in between the transparent electrode and the active material in a light-emitting electrochemical cell, which featured an enhanced light output by >50% in comparison to a reference device void of the patterned C60 layer.

The functionality and application of organic semiconductors are largely dependent on their constituent structure and morphology. This thesis presents a number of functional and novel approaches for the control and tuning of structural and morphological features of a variety of organic semiconductor materials, and also demonstrates that these approaches can be utilized for improved device operation of field-effect transistors, organic solar cells and light-emitting electrochemical cells.

The fullerene family is a particular group of closed-cage organic semiconductors, which can be photochemically coupled into larger dimeric or polymeric structures through the excitation of the fullerene molecules by light emission. In Paper I, we perform a detailed experimental and analytical investigation, which demonstrates that this photochemical monomer-to-dimer transformation requires that both constituent fullerene molecules are photoexcited. The direct consequence is that the initial probability for the photochemical transformation is dependent on the square of the light-emission intensity.

The photochemical coupling of fullerene molecules commonly results in a distinctly lowered solubility in common hydrophobic solvents, which can be utilized for the direct patterning of fullerene films by resist-free lithography. In Paper II, we utilize this patterning opportunity for the fabrication of one-dimensional fullerene nano-stripes using two-beam laser interference lithography. A desired high contrast between the patterned and non-patterned fullerene regions is facilitated by the non-linear response of the photochemical transformation process, as predicted by the findings in Paper I. The patterned fullerene nano-stripes were utilized as the active material in field-effect transistors, which featured high electron mobility and large on-off ratio.

This patterning was in Paper III extended into easy tunable two-dimensional fullerene structures by the design and development of an exposure setup, essentially comprising a laser and a spatial light modulator featuring >8 millions of independently controlled mirrors. With this approach, we could fabricate well-defined fullerene microdots over a several square-millimeter sized area, which was utilized as an internal out-coupling layer in a light-emitting electrochemical cell with significantly enhanced light output.

Paper IV reports on the development of a new “spray-sintering” method for the cost-efficient solution-based deposition of the active material in light-emitting electrochemical cells. This carefully designed approach effectively resolves the issue with phase separation between the hydrophobic organic semiconductor and the hydrophilic electrolyte that results in a sub-par LEC performance, and also allows for the direct fabrication of LEC devices onto complex surfaces, including a stainless-steel fork.

Paper V finally reports on the design and synthesis of a soluble small molecule, featuring a donor-acceptor-donor configuration. It acts as the donor when combined with a soluble fullerene acceptor in the active material of organic solar cells, and such devices with optimized donor/acceptor nanomorphology feature a high open-circuit voltage of ~1.0 V during solar illumination.

A donor-acceptor-donor (D-A-D) molecule has been designed and synthesized for use as the electron donating material in solution-processed small-molecule organic solar cells (OSCs). The D-A-D molecule comprises a central electron-accepting (2Z,2'Z)-2,2'-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(thiophen-2-yl)acry lonitrile) (ZOPTAN) core, which is chemically connected to two peripheral and electron-donating triphenylamine (TPA) units. The ZOPTAN-TPA molecule features a low HOMO level of -5.2 eV and an optical energy gap of 2.1 eV. Champion OSCs based on a solution-processed and non-annealed active material blend of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and ZOPTAN-TPA in a mass ratio of 2:1 exhibits a power conversion efficiency of 1.9% and a high open-circuit voltage of 1.0 V. 

Light-emitting electrochemical cells, featuring uniform and efficient light emission over areas of 200 cm(2), are fabricated under ambient air with a for-the-purpose developed "spray-sintering" process. This fault-tolerant fabrication technique can also produce multicolored emission patterns via sequential deposition of different inks based on identical solvents. Significantly, additive spray-sintering using a mobile airbrush allows a straightforward addition of emissive function onto a wide variety of complex-shaped surfaces, as exemplified by the realization of a light-emitting kitchenware fork.

Experimental findings and associated theoretical insights regarding the photochemical transformation of fullerenes are reported, which challenge the conventional wisdom in the field and point out a viable path towards improved fullerene-based electronic devices. It is shown that the efficiency of the photochemical monomer-to-dimer transformation of the fullerene [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) is strongly dependent on the light intensity, and this is utilized to demonstrate that direct patterning of an electroactive PCBM film can be effectuated by sub-second UV-light exposure followed by development in a tuned developer solution. By straightforward analytical reasoning, it is demonstrated that the observed intensity-dependent monomer-to-dimer transformation dictates that a significant back-reaction to the ground state must be in effect, which presumably originates from the excited-triplet state. By a combination of numerical modeling and analytical argumentation, it is further shown that the final dimer formation must constitute a bi-excited reaction between two neighboring monomers photo-excited to the triplet state.