Three photons for the price of one

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Solutions of quantum dots: each colour corresponds to the size of the dots, and the wavelength of light they emit

Researchers have created a new material that can produce three or more free electrons every time it absorbs a single photon. This is unlike conventional semiconductors, which produce just one free electron per photon. Based on tiny semiconductor structures called quantum dots, the new material – developed by researchers at Delft University of Technology in the Netherlands and Toyota Europe in Belgium – could someday be used to make more efficient solar cells.
Solar cells work by absorbing photons, each of which liberates an electron and positively charged hole that travel in opposite directions, thereby creating a voltage and current that can do work. However, when an electron is liberated, a lot of its kinetic energy is lost to the semiconductor as heat, rather than being available as useful electrical energy. Researchers are therefore keen to develop new materials in which some or all of this energy is captured rather than wasted.
One way of capturing this energy is to use thin films of quantum dots in which the energy needed to liberate an electron can be fine-tuned by adjusting the size of the dots. An electron can therefore liberate more electrons as it travels through a dot in a process known as “carrier multiplication”. Unfortunately, this approach does not involve truly free electrons and holes – but rather excitons, which are bound pairs of electrons and holes. Although excitons can be separated into free charges by applying an electric field or connecting the dots to another semiconductor material, both techniques reduce the efficiency of the devices.
Now, Michiel Aerts and colleagues have made a film of quantum dots in which carrier multiplication occurs with free electrons, rather than excitons. The quantum dots are each about 5 nm in diameter and are made from the compound-semiconductor lead selenide. The films themselves are made by dipping quartz substrate into a solution of the dots…..
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Written by physicsgg

October 18, 2011 at 4:39 pm


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