Energy isn’t free, as many Americans have learned from rising utility and gas bills. Fossil fuels are being expended at an ever increasing rate; once they are gone, they are gone practi forever. The only solution to this impending crisis is to find new sources of energy: solar cells, wind farms, water turbines, and hydrogen cells are all options. Recently, though, a very promising breakthrough has been made for solar power derived from the sun – quantum dots, very tiny nanocrystals. (Quantum-Dot Leap…).

To fully understand how these “dots” are so revolutionary, you first need to realize how inefficient solar energy is now (Quantum-Dot Leap…). When a photon from sunlight hits the solar cell, it strikes the electrons in semiconductor materials that send the electric current. Normal photons free only one electron, which is called an exciton after leaving its atom. This small amount is because the electron freed normally collides with a nearby atom, which should release another electron and so on in a chain reaction in a process called impact ionization. Instead, the electron is more likely to trigger useless atomic vibrations that waste the energy on heat (Quantum-Dot Leap…). Normal cells also have a lower band-gap, so they can only support a certain amount of excitons. The average conversion efficiency of a solar cell has been put at around 20-31%; a relatively bad efficiency (Problems with…).

On the other hand, quantum dots have been tested to raise the level of conversion efficiency to more than 65%. This is because semiconductors, materials that hold electrons until given small energy bursts, made of quantum dots have a lower band-gap energy than normal photovoltaic cells. This smaller band-gap allows quantum dots to absorb higher-energy light, which can trigger more electrons to be released. In tests done on miniscule 8-nanometer-in-diameter lead selenide quantum dots using ultraviolet light, each photon created seven excitons. The theory about the number of electrons unleashed is that to find the number of excitons, you must divide the energy of the photon by the band-gap energy of the solar cell (Quantum-Dot Leap…). This ratio is more efficient in quantum dots. Quantum dots are also more efficient in impact ionization.

The team developing these quantum dots has been working for decades, but has only recently come upon a breakthrough. A National Renewable Energy Laboratories team, or an NREL team, led by Arthur Nozik theorized that the smallness of quantum dots would help with impact ionization because the dots “grip” on the electron would nullify the small things that made the process challenging (Quantum-Dot Leap…).. His team focused on dots made of indium phosphide and and indium arsenide; basically a wasted effort, as these were the wrong type of quantum dots (Quantum-Dot Leap…).

While Nozik was testing, a Los Alamos National Laboratory team with Richard Schaller and Victor Klimov focused on studying lead selenide dots (Quantum-Dot Leap…). When they observed the effects of a high-energy blue light on the quantum dots, they discovered that they produced more than one exciton per photon (Quantum-Dot Leap…). In a 2004 report, the team mentioned that a single photon could produce up to three excitons in lead selenide quantum dots (Quantum-Dot Leap…). The NREL team soon changed gears and confirmed the Los Alamos findings in 2005 (Quantum-Dot Leap…). Further research by the Los Alamos team showed even more possibilities for quantum dots.

“[It's] not just a pipedream to think about this giving you a real benefit in a solar cell device” says Schaller (Quantum-Dot Leap…).

“[A small boost to solar cell efficiency] would be a major, major achievement” Nozrik says (Quantum-Dot Leap…).

Although there is general agreement that quantum dots are going to be extremely helpful, the process in which this happens is still a mystery. Some scientists argue that an unseen type of “quantum mechanical entity” briefly forms in each quantum dot (Quantum-Dot Leap…). Others believe that believe “an already well-understood process can account for the multiple-electron output,” but there is no firm theory yet (Quantum-Dot Leap…).

Regardless of this lack of theory, quantum dots are soon going to become a necessary part of solar technology. If the United States lowered its industrial energy by just 1%, it would save 55 million barrels of oil per year; one billion dollars worth of gas that could be used for better things. Within 15 years, renewable energy sources are predicted to generate enough energy to power 40 million households. Solar energy with quantum dots also has use internationally – with two million villages worldwide that lack basic things like electricity, it’s cheaper and easier to install a photovoltaic grid than an electrical grid. Quantum dots are changing solar energy generation, and soon photovoltaic cells will be viable.