Oct 19, 2011
Three electrons for the price of one
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.
Stable, yet conducting
One challenge for Aerts was to make sure that electrons can move easily between individual quantum dots. This is normally a problem because the nanoparticles have to be coated with an electrically insulating organic layer to prevent them from degrading while the film is being made. So, what Aerts and colleagues did was to work out a way to remove the organic layer of the dots in the film so that conduction can occur.
The carrier-multiplication process begins when a photon is absorbed by a quantum dot, which liberates an electron and hole that can then travel into adjacent dots to liberate further electron and holes. Using a technique called time-resolved microwave conductivity (TRMC) to measure the conductivity of the films, the team was able to show that – on average – about three free electrons are created per photon when the films are illuminated with 400 nm ultraviolet light. This wavelength is right on the edge of the visible spectrum and therefore abundant in sunlight.
Aerts told physicsworld.com that the team now wants to try to make solar cells from the films. In theory, such solar cells could achieve efficiencies of 44%, compared with the theoretical limit of 35% on conventional silicon cells. Although the quantum-dot films are relatively cheap and easy to produce, making devices out of them is not easy. Apart from lead selenide being a toxic material, the quantum dots deteriorate quickly when exposed to air.
The research is described in Nano Letters 10.1021/nl202915p.
About the author
Hamish Johnston is editor of physicsworld.com