Emitting plastics are useful for solar cells

Nov. 1, 2002
Cambridge Display Technology (CDT; Cambridge, England), a developer of light-emitting polymer (LEP) technology, is receiving funding from England's Department of Trade and Industry...

Cambridge Display Technology (CDT; Cambridge, England), a developer of light-emitting polymer (LEP) technology, is receiving funding from England's Department of Trade and Industry (DTI) to create commercially viable solar cells and light detectors based on its plastic-semiconductor technology. Flexible solar cells could be used in applications in which solar power has previously been too expensive or technically unfeasible, such as on disposable packaging and clothing and in nonplanar products, says Karl Heeks, who is responsible for strategic technology assets at CDT.

Polymer solar cells have very similar device architecture to CDT's LEP displays. Whereas LEP displays emit light when an electrical charge is applied, CDT researchers have been able to reverse the process and generate electricity when light shines on a polymer-based cell. The company has recently demonstrated polymer-based solar cells that power digital clocks (see figure).

Polymer solar cells power digital clocks in an initial demonstration of Cambridge Display Technology's alternate use of light-emitting polymers.
Click here to enlarge image

Scientists at CDT have discovered much about the polymers they are working with through their research on LEPs. Conjugated polymers are plastic materials for which metallic and semiconductor characteristics can be observed. These polymers possess a delocalized pi-electron system along the polymer backbone; this electron system gives the polymer semiconducting properties and the ability to support positive and negative charge carriers with high mobilities along the polymer chain.

The mechanism for charge transport in these materials is different from that in more traditional inorganic semiconductors. The amorphous chain morphology results in an inhomogeneous broadening of the energies of the chain segments and leads to "hopping" type transport. A secondary effect is the distortion of the chain around a charge carrier, with the result that the charged excitations are usually best described as polarons in these materials. Nevertheless, device engineering of the materials can take advantage of many of the lessons previously learned for classical semiconductors.

The company can now use this knowledge in a new application. Although some of the first solar cells were constructed from red polymer materials used for displays, those that work optimally in the solar-cell systems will be different. Highly efficient photoexcitable materials have lower bandgap energy levels, so they are different from those needed for light emission. Blends of materials are formulated to maximize the dissociation of electrons and holes from excitons generated by the incident radiation.

The device design is also different from light-emitting systems. The internal structure is modified to optimize absorption of the incident radiation, energy conversion, and charge transfer through the internal layers to the electrodes.

The new technology can be used for photodetector systems as well as for solar cells. For simple, single-element photodetectors, a current-to-voltage conversion input circuit can provide a calibrated signal proportional to the light level, which can be used to activate a switch or control some device function. For a multi-element array of detectors configured in a row-and-column matrix, each detector element at the junction of each row and column can be sequentially sampled through a timing circuit, providing an imaging system.

The development of inexpensive flexible plastic solar cells that could be manufactured using low-cost roll-to-roll production is of strong commercial interest. Cambridge Display Technology expects that plastic-solar-cell applications could ultimately range from rechargeable handheld electronic devices, wearable electronics, and large outdoor displays to secondary power sources for homes and factories. The company will collaborate with others on these prospects.

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