Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.

https://yylab.seas.ucla.edu/papers/yy-pv-7-slow growth-nm-05.pdf

High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends

Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

The path to ubiquitous and low-cost organic electronic appliances on plastic

Over the past 100 years, the global average temperature has increased by approximately 0.6 °C and is projected to continue to rise at a rapid rate1. Although species have responded to climatic changes throughout their evolutionary history2, a primary concern for wild species and their ecosystems is this rapid rate of change3. We gathered information on species and global warming from 143 studies for our meta-analyses. These analyses reveal a consistent temperature-related shift, or ‘fingerprint’, in species ranging from molluscs to mammals and from grasses to trees. Indeed, more than 80% of the species that show changes are shifting in the direction expected on the basis of known physiological constraints of species. Consequently, the balance of evidence from these studies strongly suggests that a significant impact of global warming is already discernible in animal and plant populations. The synergism of rapid temperature rise and other stresses, in particular habitat destruction, could easily disrupt the connectedness among species and lead to a reformulation of species communities, reflecting differential changes in species, and to numerous extirpations and possibly extinctions.

/pdf/fingerprints-of-global-warming-on-wild-animals-and-plants-4y9aiz1zx6.pdf

Fingerprints of global warming on wild animals and plants

Transparent, conductive, and ultrathin graphene films, as an alternative to the ubiquitously employed metal oxides window electrodes for solid-state dye-sensitized solar cells, are demonstrated. These graphene films are fabricated from exfoliated graphite oxide, followed by thermal reduction. The obtained films exhibit a high conductivity of 550 S/cm and a transparency of more than 70% over 1000−3000 nm. Furthermore, they show high chemical and thermal stabilities as well as an ultrasmooth surface with tunable wettability.

Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells

THE recent fabrication of light-emitting diodes (LEDs) from conjugated polymers1,2demonstrates the technological potential of this class of electronic materials. A variety of colours are possible, because the wavelength of luminescence emission can be chemically tuned during synthesis1–4. In addition, the mechanical properties of polymers suggest that light-emitting structures can be made that are more flexible than their inorganic counterparts, provided appropriate materials can be found for the substrate and electrodes. Here we report the fabrication of a fully flexible LED using poly(ethylene terephthalate) as the substrate, soluble poly-aniline as the hole-injecting electrode, a substituted poly(1,4-phenylene-vinylene) as the electroluminescent layer and calcium as the electron-injecting top contract. The structure is mechanically robust and may be sharply bent without failure. The LED is easily visible under room lighting and has an external quantum efficiency of about 1%. With a turn-on voltage for light emission of 2–3 V, the 'plastic' LED demonstrates that this unique combination of optical, electrical and mechanical properties can be used to make novel structures that are compatible with conventional devices.

Flexible light-emitting diodes made from soluble conducting polymers

THE range of materials now available for polymer-based light-emitting diodes (LEDs) is such that electroluminescence can be obtained throughout the visible spectrum(1-12). Here we show that, by ble ...

Light-emitting diodes with variable colours from polymer blends

We report a systematic approach to the control of the conjugation length along the poly(thiophene) backbone. The planarity of the main chain can be permanently modified by altering the pattern of s ...

Electroluminescence from Substituted Poly(thiophenes) : From Blue to Near-Infrared

Low operating voltages for p-channel organic field-effect transistors (OFETs) can be achieved by using an electrolyte as the gate insulator. However, mobile anions in the electrolyte can lead to un ...

Low-Voltage Polymer Field-Effect Transistors Gated via a Proton Conductor

We report visible light emission from diodes made from poly(3‐octylthiophene). Use of a soluble derivative of polythiophene allows fabrication of the light emitting diodes by casting the polymer film from solution with no subsequent processing or heat treatment required. The devices emit dim red‐orange light with relatively low external quantum efficiencies, below 2.5×10−5 photons per electron at room temperature. Electrical characterization reveals diode behavior with rectification ratios greater than 102. The temperature dependence indicates that tunneling phenomena dominate the charge injection.

/pdf/electroluminescence-and-electrical-transport-in-poly-3-1gvyyq4b75.pdf

Göran Gustafsson

Papers

Flexible light-emitting diodes made from soluble conducting polymers

Light-emitting diodes with variable colours from polymer blends

Electroluminescence from Substituted Poly(thiophenes) : From Blue to Near-Infrared

Low-Voltage Polymer Field-Effect Transistors Gated via a Proton Conductor

Electroluminescence and electrical transport in poly(3‐octylthiophene) diodes