The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Electron and ambipolar transport in organic field-effect transistors.

In this Progress Report we provide an update on recent developments in inkjet printing technology and its applications, which include organic thin-film transistors, light-emitting diodes, solar cells, conductive structures, memory devices, sensors, and biological/pharmaceutical tasks. Various classes of materials and device types are in turn examined and an opinion is offered about the nature of the progress that has been achieved.

Inkjet Printing—Process and Its Applications

Flexible pressure sensors are essential parts of an electronic skin to allow future biomedical prostheses and robots to naturally interact with humans and the environment. Mobile biomonitoring in long-term medical diagnostics is another attractive application for these sensors. Here we report the fabrication of flexible pressure-sensitive organic thin film transistors with a maximum sensitivity of 8.4 kPa(-1), a fast response time of 15,000 cycles and a low power consumption of <1 mW. The combination of a microstructured polydimethylsiloxane dielectric and the high-mobility semiconducting polyisoindigobithiophene-siloxane in a monolithic transistor design enabled us to operate the devices in the subthreshold regime, where the capacitance change upon compression of the dielectric is strongly amplified. We demonstrate that our sensors can be used for non-invasive, high fidelity, continuous radial artery pulse wave monitoring, which may lead to the use of flexible pressure sensors in mobile health monitoring and remote diagnostics in cardiovascular medicine.

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Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring

2.3. Medical Devices and Sensors 9 2.4. Radio Frequency Applications 10 3. Materials 12 3.1. Organic Electronics Materials 12 3.2. Semiconducting Polymer Design 13 3.3. Poly(3-alkylthiophenes) 14 3.4. Poly(thieno(3,2-b)thiophenes 15 3.5. Benchmark Polymer Semiconductors 15 3.6. High Performance Polymer Semiconductors 15 4. Device Stability 16 4.1. Bias Stress in Organic Transistors 17 4.1.1. Bias Stress Characterization 17 4.1.2. Bias Stress Mechanism 18 4.2. Short Channel Effects in Organic Transistors 19 5. Materials Patterning and Integration 20 6. Conclusions 22 7. Acknowledgments 22 8. References 22

Materials and applications for large area electronics: solution-based approaches.

Surface-Induced Self-Encapsulation of Polymer Thin-Film Transistors†

The long term bias stress stability of poly(quaterthiophene) thin film transistors is described. Transistors are subject to repetitive pulsed gate bias stress for periods of up to five months, and the threshold voltage and mobility are monitored. The samples are unencapsulated bottom gate transistors held in either clean dry air or dry nitrogen. The threshold voltage shift increases as a power law with time for the first 1–2days, and the magnitude of the shift is a strong function of gate voltage. After a few days, threshold voltage shift stabilizes to an approximately constant value of 2–10V for gate voltages of 20–40V. When the stress is removed, the threshold voltage recovers with a time constant of 2–4days, and the recovery is the origin of the stabilization. The gate bias stress also induces a reduction in mobility of 10%–30% over about ten days, and there are further slow changes in mobility, which may be unrelated to electrical stress.

Extended time bias stress effects in polymer transistors

The effects of humidity on unencapsulated polymeric thin-film transistors (TFTs) of poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’-bithiophene] (PQT-12) were investigated. The field effect mobility of PQT-12 TFTs decreases and the rate of trapping of charge carriers increases under increasing humidity. The amount of water absorbed by the PQT-12 films was measured using a quartz crystal microbalance. Thin films of PQT-12 absorb comparable amounts of water to the carrier concentration in TFTs under routine operating conditions (humidity of 30% relative humidity and gate voltage of −30V); the changes in electrical characteristics under humid atmospheres are attributed to the interaction of absorbed water with the carriers in the film.

Effects of humidity on unencapsulated poly(thiophene) thin-film transistors

The effects of the roughness of the gate dielectric on the performance of thin film transistors (TFTs) fabricated with poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b] thiophene) (PBTTT-C16) were examined. The field effect mobility of coplanar TFTs made with PBTTT-C16 and plasma-enhanced chemical vapor deposited SiO2∕SiNx dielectrics decreased nearly exponentially with surface roughness. Films of PBTTT-C16 have similar crystalline structure on smooth and rough surfaces, but the domain size decreases with increasing roughness. Surface roughness was found to have less of an impact on field effect mobility than the chemical treatment of the dielectric.

Effects of the surface roughness of plastic-compatible inorganic dielectrics on polymeric thin film transistors

Exposure of unencapsulated bottom gate polythiophene thin film transistors (TFTs) to several organic and inorganic volatile chemicals induces large changes in the TFT properties. Various neutral molecules cause the mobility of the TFT to decrease and enhance the bias stress effect, with the changes being reversible upon annealing. The effect on the mobility and stress are characteristically different for each compound and are attributed to different preferred locations in the host. Some molecules cause electronic doping of the semiconductor with corresponding changes in threshold voltage. In the case of the donor, ammonia, a gate-induced incorporation rate is observed. The distribution of acceptor energies of the recently found ozone acceptor is analyzed.

F. J. Endicott

Papers

Surface-Induced Self-Encapsulation of Polymer Thin-Film Transistors†

Extended time bias stress effects in polymer transistors

Effects of humidity on unencapsulated poly(thiophene) thin-film transistors

Effects of the surface roughness of plastic-compatible inorganic dielectrics on polymeric thin film transistors

Chemical impurity effects on transport in polymer transistors