Synthetic Biology + Quantum Dots = Solar Cells
Biotechnology and nanotechnology are converging to create a new way of manufacturing photovoltaics. Microorganisms can now be engineered to produce functional structures suitable for use in silicon substrates. Specifically, researchers have bioengineered the intestinal bacterium Escherichia coli (E. coli) to synthesize quantum dots (QDs): semiconducting nanoscale crystals whose excitons (electron-hole pairs that can transport energy without transporting net electric charge) are confined in all three spatial dimensions. QDs have unique optical properties that make them suitable for many applications, including medical imaging, enhanced LEDs, biocompatible medical devices, solid-state quantum computation, and photovoltaics (PVs). When used in solar cells, quantum dots can potentially boost photon-to-electron conversion efficiency to levels higher than can be achieved with current technology., Using synthetic biology to produce quantum dots for photovoltaic applications may lead to a new biomanufacturing component in the production of high-efficiency solar cells.
Modifying living cells allows for the production of functional, environmentally friendly and relatively low-cost structures with a wide range of applications. While quantum dots can be chemically manufactured, these processes are toxic, energy intensive, and yield dots that are challenging to use for biological applications. Biologically-created quantum dots require less energy to produce, are believed to be more compatible with biological systems, and also display more uniformity in quality and colour.
This last quality makes them well-suited for used in photovoltaics, where current commercial solar cell conversion efficiencies range from 20-30%, with a theoretical limit (known as the Shockley–Queisser, or SQ, limit) of ~33.5% for single p-n junction crystalline silicon devices. To improve conversion efficiency and increase its theoretical limit, scientists are focusing on basic research into so-called third-generation solar energy strategies. These include colloidal quantum dots, multiple exciton generation, thermionic (direct heat-to-energy) conversion, quantum-dot intermediate band solar cells (QD-IBSCs), and singlet exciton fission and research suggests they may raise the theoretical solar cell conversion efficiency limit to 42-63%.
Who, Where, and When?
The E. coli quantum dot research, Environmentally-Friendly Manufacture of Quantum Dots in E. coli, was led by several senior researchers and performed by a team of students – both undergraduates and high school students – who in 2011 won the prestigious International Genetically Engineered Machine (iGEM) synthetic biology competition for their work. Student teams participating in iGEM are given a kit of biological parts at the beginning of the summer from the BioBricks Registry of Standard Biological Parts. Working at their own schools (in this case, Columbia University and Cooper Union), the teams use these bioparts – and new bioparts of their own design – to build biological systems and operate them in living cells.
The iGEM team based their project, in part, on earlier research demonstrating the production of quantum dot nanowires by engineered viruses conducted at the at the University of Texas and Angela Belcher’s group at the Massachusetts Institute of Technology.
Ted Sargent and his group at the Department of Electrical and Computer Engineering at the University of Toronto have been focussing on the role of colloidal quantum dots in improving solar cell conversion efficiency. At the same time, scientists lead by Arthur Nozik and Matthew Beard of the National Renewable Energy Laboratory have been working to improve efficiency through the use of multiple exciton generation in quantum dot-based solar cells. At the Optoelectronics Group at the Cavendish Laboratory at University of Cambridge, Neil Greenham and his colleagues have been developing an alternate exciton technology – singlet exciton fission – to generate improved efficiencies.
The iGEM Columbia-Cooper team engineered E. coli bacteria to express several different peptides that bind to and nucleate salts of heavy metals, crystallizing the metal ions into quantum dots. In addition, toxicity was reduced by using environmentally-friendly metals, such as zinc. Since the spectrum of light emitted by a quantum dot is related to its size, and QD size is a key factor in nanoscale structural design, the team also implemented a sensor-based selection process in which quantum dots growing in long-wave UV light activate a light-sensitive promoter sensitive to the emission spectrum of the required QD size. The promoter, in turn, is coupled to the expression of antibiotic resistance only in those E. coli emitting the desired frequency of light, allowing the survival of only the cells producing the desired wavelength of light.
The iGEM research was informed by earlier work in biologically-based quantum dot production, where scientists bioengineered the M13 bacteriophage virus and exposed them to semiconductor precursor solutions. The process caused nanocrystals to form along the viral surfaces, which then formed nanowires. High-resolution analytical electron microscopy and photoluminescence showed electron diffraction patterns suggesting that the nanocrystals were preferentially oriented with their crystal axis perpendicular to the viral surface.
In June 2011, researchers reported that colloidal quantum dots (CQDs) can be tuned by changing their size, allowing them to be customized for photovoltaic applications, a feature that – if CQDs were used in tandem or multi-junction configurations – might increase the theoretical limit to 42% (tandem) and 49% (multiple junction). Moreover, CQD photovoltaics are amenable to significant scaling: even in the R&D lab, say the researchers, the team can synthesize enough colloidal quantum dots in each run to cover a square meter of surface with a complete light absorber.
In December 2011 another report described a third-generation solar energy conversion technology in which photonic quanta – whose energy is typically lost as heat – can be retained and used to improve quantum conversion efficiency. Moreover, quantum dot confinement can increase the efficiency of the primary conversion step from a high energy photon to multiple charge carriers. Since quantum efficiency is a function of wavelength, if all photons of a certain wavelength are absorbed and the resulting minority carriers (e.g., electrons in a p-type material) are collected, the quantum efficiency at that particular wavelength typically has a value of one; the quantum efficiency for photons with energy below the bandgap is zero. In this study, researchers increased quantum efficiency by employing quantum dots in a process known as multiple exciton generation (MEG), where absorption of a photon bearing at least twice the bandgap energy produces two or more excitons. The researchers observed an external quantum efficiency (the spectrally resolved ratio of collected charge carriers to incident photons) that peaked at 114% and an associated internal quantum efficiency (corrected for reflection and absorption losses) of 130%.
Most recently, researchers demonstrated an organic/inorganic hybrid photovoltaic architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. Absorbed infrared and visible photons create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets that can be ionized at the organic/inorganic heterointerface. The scientists report internal quantum efficiencies of >50% and power conversion efficiencies approaching 1%, making the device yet another way to exceed the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells by as much as 25% to an estimated 44%. Moreover, the materials can be handled by roll-to-roll printing, potentially making the cost of a hybrid solar panel lower than that of its conventional silicon counterpart.
These technologies, while promising, are in the early research phase. Firstly, the iGEM results need further review. Secondly, in colloidal quantum dot research, work remains to be done in developing thin-film-processing approaches that are compatible with large-area roll-to-roll processing. Thirdly, there have been doubts about the existence and significance of MEG. Although these have mostly been resolved the phenomenon is only a first step toward breaking the single junction Shockley-Queisser limit of present-day first and second generation solar cells. In addition, singlet exciton fission researchers need another few years to determine the commercial viability of their devices.
Matthew C. Beard et al, Third Generation Photovoltaics: Multiple Exciton Generation in Colloidal Quantum Dots, Quantum Dot Arrays, and Quantum Dot Solar Cells, 35th IEEE Photovoltaic Specialists Conference (PVSC), June 2010.
Matthew C. Bear et al, Comparing Multiple Exciton Generation in Quantum Dots to Impact Ionization in Bulk Semiconductors: Implications for Enhancement of Solar Energy Conversion, Nano Letters, July 2010.
Bruno Ehrler et al, Singlet Exciton Fission-Sensitized Infrared Quantum Dot Solar Cells, Nano Letters, January 2012.
Ellen Jorgensen, Oliver Medvedik, David Orbach, Dionne Lutz, David Benjamin, et al, Environmentally-Friendly Manufacture of Quantum Dots in E. coli, iGEM Project Website, 2011 (results not published in a peer-reviewed journal).
N. López et al, Experimental Analysis of the Operation of Quantum Dot Intermediate Band Solar Cells, Journal of Solar Energy Engineering, August 2007.
Chuanbin Mao, Angela M. Belcher, et al, Viral assembly of oriented quantum dot nanowires, PNAS, June 10, 2003.
Jared W. Schwede, Nicholas A. Melosh, et al, Photon-enhanced thermionic emission for solar concentrator systems, Nature Materials, September 2010 (Published online 01 August 2010).
Octavi E. Semonin, Matthew C. Beard, et al, Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell, Science, 16 December 2011.
Xihua Wang, Edward H. Sargent, et al, Tandem colloidal quantum dot solar cells employing a graded recombination layer, Nature Photonics, August 2011 (Published online 26 June 2011).
Systems Engineering for Microscale and Nanoscale Technologies, M. Ann Garrison Darrin (Editor), December 13, 2011.
Quantum Dot Devices (Lecture Notes in Nanoscale Science and Technology), Zhiming M. Wang, June 15, 2012 (pre-publication).
- Enviromentally-Friendly Manufacture of Quantum Dots in E. coli, iGEM Project Website, 2011 (results not published in a peer-reviewed journal).
- Xihua Wang, Ghada I. Koleilat, Jiang Tang, Huan Liu, Illan J. Kramer, Ratan Debnath, Lukasz Brzozowski, D. Aaron R. Barkhouse, Larissa Levina, Sjoerd Hoogland, and Edward H. Sargent, Tandem colloidal quantum dot solar cells employing a graded recombination layer, Nature Photon., p. 480–484, 2011.
- Octavi E. Semonin, Joseph M. Luther1, Sukgeun Choi1, Hsiang-Yu Chen1, Jianbo Gao1,3, Arthur J. Nozik1,4, Matthew C. Beard1, Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell, Science 334, pp. 1530–1533, 2011.
- M.C. Beard, J.M. Luther, A.G. Midgett, O.E. Semonin, J.C. Johnson, A.J. Nozik, Third generation photovoltaics: Multiple Exciton Generation in colloidal quantum dots, quantum dot arrays, and quantum dot solar cells, 35th IEEE Photovoltaic Specialists Conference (PVSC), June 2010.
- Jared W. Schwede, Igor Bargatin, Daniel C. Riley, Brian E. Hardin, Samuel J. Rosenthal, Yun Sun, Felix Schmitt, Piero Pianetta, Roger T. Howe, Zhi-Xun Shen & Nicholas A. Melosh, Photon-enhanced thermionic emission for solar concentrator systems, Nat. Mater. 9, p. 762–767, 2010.
- N. López, A. Martí, and A. Luque, Experimental analysis of the operation of quantum dot intermediate band solar cells, Nat. Mater. 9, p. 762–767, 2010.
- Bruno Ehrler, Mark W. B. Wilson, Akshay Rao, Richard H. Friend, and Neil C. Greenham, Singlet exciton fission-sensitized infrared quantum dot solar cells, Nano Lett. 12, p. 1053–1057, 2012.
- Chuanbin Mao, Christine E. Flynn, Andrew Hayhurst, Rozamond Sweeney, Jifa Qi, George Georgiou, Brent Iverson, and Angela M. Belcher, Viral assembly of oriented quantum dot nanowires, PNAS 100, p. 6946–6951, 2003.
- Tiny tech, big results: Quantum dot solar cells increase solar conversion efficiency, 2011.