Voxtel’s Innovation Powers a Next-Generation Solar Cell

July 18, 2012

First-ever practical demonstration of advanced solar collection technique published in Science

Voxtel has demonstrated solar cell devices with the first measured signals from signal amplification due to multiple exciton generation (MEG) in quantum dot structures. This is the first practical verification of the MEG approach for improving the efficiency of solar cells, a ‘third-generation’ solar energy technique. The approach offers the potential for highly efficient, inexpensive photovoltaics that could be printed directly onto surfaces. This groundbreaking finding was published in the prestigious journal Science by a partnership between researchers at Voxtel Inc. and the University of Wyoming. Voxtel is headquartered in Beaverton, Oregon, and Voxtel’s photovoltaic research team is based in Eugene, Oregon.

Voxtel’s approach promises to overcome the Shockley–Queisser limit, the well-known performance ceiling of about 34% efficiency for conventional ‘first-generation’ silicon cells. To overcome this limit, Voxtel developed an approach using quantum dots — semiconductor materials about one-billionth of a meter in diameter. The response of these custom-made materials can be tuned to match the sun’s light — including the infrared portion of the spectrum that silicon cells can not harness.

The engineered use of such quantum dots offers a maximum of about 66% efficiency, but in Voxtel’s MEG approach, the fundamental efficiency limit is raised to approximately 75%. For most photovoltaic technologies, a photon of solar energy can produce only one excited electron in the solar cell, but the MEG design allows multiple excited electrons to be produced and collected when a single photon is absorbed. This effectively multiplies the electrical current that can be produced from the absorption of energy from the sun. Although previous experiments showed that MEG was possible, today’s Science report demonstrated the process in an actual photovoltaic device, using Voxtel’s quantum dots to double the collection of electrons from high-energy photons.

Says George Williams, Voxtel’s president and founder, “Harnessing solar energy using MEG has profound implications for the next generation of solar cells. Today, a typical domestic rooftop installation can power at most a dozen light bulbs, but the potential efficiency of quantum dot solar cells would make solar power a much more practical alternative to fossil fuels.”

The quantum dot approach also has significant benefits in terms of cost. Says Mr. Williams, “Quantum dot solar cells can be fabricated directly from chemicals, and the quantum dot inks can be directly deposited on flexible substrates using roll-to-roll printing techniques, including ink jet printing. This is a major departure from conventional silicon solar cell manufacturing, which relies on costly infrastructure and intensive processing, and also generates a considerable amount of waste.” Both efficiency and cost are crucial in the pursuit of practical photovoltaic systems; for example, a solar cell that is only 15% efficient would have to be supplied at no cost in order to be financially practical when installed.

Regarding the Science report of the first demonstration of MEG in a working device, Mr. Williams says, “in the laboratory, we and others have see evidence of two, three, and more excitons using laboratory equipment, and this data has shown that, in order to extract the extra signal generated in the quantum dots, we needed to extract the carriers from the quantum dot in less than one picosecond — one millionth of one millionth of a second — or else they would recombine with each other. Voxtel used chemical coatings on the quantum dots to induce an electric dipole, which allowed us to capture the amplified signal before the carriers were annihilated.”

This result is a major step in a years-long effort to advance the technology to where it can be manufactured in commercial devices. “This is an extraordinary achievement, but there is also a lot of work remaining to realize the full benefits of quantum dot solar cells. The maximum efficiency of quantum dot cells has been about 7% so far, and despite the potential benefits of MEG, it will be several years before quantum dot solar cells exceed the efficiency of silicon, and several more years more before we realize the cost benefits of printed solar cells.”

Initial press coverage of this development has been brisk:

Upping the Limit on Solar Cell Efficiency — MIT Technology Review
…Two major hurdles remain before the trick can be used to make ultraefficient solar cells. Parkinson used lead-sulfide quantum dots with a crystalline titanium-dioxide electrode. Researchers need to try other combinations of quantum dots and electrode materials to find ones that can convert more photons into multiple electrons. Parkinson says his new methods for making quantum dot solar cells will help them directly test these other combinations. Researchers also need to increase the total amount of light that the quantum dot solar cells can absorb…

Solar cells get two electrons for the price of one, efficiency bonus — Ars Technica
…The technology demonstrated in this paper is particularly interesting for several reasons. First, it is a true “nanomaterial” application where the size of the semiconductor particles enable truly unique properties by confining the excitons to quantum length scales. During my daily abstract scan, it is all too common to find “nano-” papers that simply involve small particles rather than truly novel properties enabled by the scale of the materials.
The work also concentrated on extracting electrons from the nanoparticles rather than just trying to break efficiency records for electron generation…Finally, the experimental setup for this study is largely consistent with dye sensitized solar cells, which are easy to manufacture compared to silicon technologies…

New technology that captures “exciton” particles could replace today’s solar cells —
…This offers a chance for solar cells to trap excitons in a similar way. As long as the cells are coupled with the appropriate electrodes, they too can capture these quasiparticles before they degrade, which means they would save most of the heat and hang onto it as useful energy. It would greatly improve the efficiency of solar cells, all without even having to do anything to the basic photon capture technology.

Work light twice as hard to make cheap solar cells — New Scientist
…Now Bruce Parkinson and Justin Sambur at the University of Wyoming in Laramie, and Thomas Novet of Voxtel in Beaverton, Oregon, have taken the first steps along another route to super-efficient solar cells. Their approach involves harnessing particularly energetic photons – those with more than twice the energy needed to free an electron – and using them to free two electrons rather than one, potentially doubling the current generated…

September 30, 2010, Beaverton, Ore.