Low-voltage organic transistors are sought for implementation in high volume low-power portable electronics of the future. Here we assess the suitability of three phosphonic acid based self-assembling molecules for use as ultra-thin gate dielectrics in low-voltage solution processable organic field-effect transistors. In particular, monolayers of phosphonohexadecanoic acid in metal-monolayer-metal type sandwich devices are shown to exhibit low leakage currents and high geometrical capacitance comparable to previously demonstrated self-assembled monolayer (SAM) type dielectrics but with a higher surface energy. The improved surface energy characteristics enable processing of a wider range of organic semiconductors from solution. Transistors based on a number of solution-processed organic semiconductors with operating voltages below 2 V are also demonstrated.

Nanostructured phase-separated blends, or bulk heterojunctions, of conjugated polymers and fullerene derivatives form a very attractive approach to large-area, solid-state organic solar cells. The key feature of these cells is that they combine easy processing from solution on a variety of substrates with good performance. Efficiencies of up to 5% in solar light have been achieved, and lifetimes are increasing to thousands of hours. Further improvements can be expected and some of the promising strategies towards that goal are presented in this article.

We report on an approach towards integrated complementary-like circuits based on organic ambipolar transistors. In particular, we show that ambipolar transport can be achieved within a single transistor channel using gold electrodes and a solution processable polymer-small molecule blend as the electroactive material. To demonstrate the suitability of these devices for practical utilisation in logic circuits we realise complementary-like voltage inverters comprised entirely of ambipolar transistors. Moreover, by integrating several such inverters we are able to demonstrate more complex circuits such as ring oscillators.

Regioregular polythiophenes baring 3-(p-methoxyethoxyethoxy)-phenyl substituents (PEOPT) show high photoluminescence efficiencies. Exposing thin films of this polymer to vapors of chloroform or annealing them by heat treatment results in a red shift of the absorption maximum due to solvent or heat induced ordering which gives rise to more planar conformations. The fact, that annealed thin films of PEOPT exhibit absorption edges at relatively low energies and thus have an enhanced spectral range makes them suitable for use in photodiodes / solar cells. The photoinduced charge generation efficiency in PEOPT is significantly enhanced by the addition of a strong electron acceptor like fullerene, as observed by quenching of the luminescence and by photoinduced absorption measurements in the infrared and UV-Vis regime. The efficiency of the photoinduced charge transfer from PEOPT to a methanofullerene is found to depend on the ordering of PEOPT in thin films.

Using the ultrafast photoinduced electron transfer with long-living charge separation in the conjugated polymer/fullerene thin films, photovoltaic devices have been fabricated. The photoinduced charge separation happens with internal quantum efficiency near unity. The performance of such “bulk heterojunction” photovoltaic devices is critically dependent on the transport properties of the interpenetrating network. The influence of the variation of different donor / acceptor materials on the sample morphology is monitored by atomic force microscopy (AFM), while I/V characteristics have been studied to evaluate the conversion efficiency.

Homogenous blends of a processable methanofullerene, [6,6]-Phenyl C61 - butyric acid methyl ester (PCBM), with poly (p-phenylene vinylene) (PPV) made from a novel nonionic precursor route were investigated by optical spectroscopy and by photocurrent measurements. The conversion process of the precursor polymer to the PPV in the blends with PCBM was monitored by IR absorption. In composites of PPV/PCBM strong quenching of the PPV luminescence was observed. LESR (light induced electron spin resonance) and PIA (photoinduced absorption) studies confirmed the occurrence of photoinduced electron transfer from the PPV to PCBM. Photovoltaic devices made from PPV/PCBM blends showed monochromatic power conversion efficiencies of app. 0.3%. The spectral photocurrent was observed to follow the absorption profile of the PPV.