Quantum dots with built-in charge boost solar cell efficiency by 50%

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(Left) A diagram of a quantum dot structure. (Center) A comparison of solar cells with different levels of doping. (Right) The 3D potential profile in quantum dot structures. Image credit: Sablon, et al. 2011 American Chemical Society.

For the past few years, researchers have been using quantum dots to increase the light absorption and overall efficiency of solar cells. Now, researchers have taken a step further, demonstrating that quantum dots with a built-in electric charge can increase the efficiency of InAs/GaAs quantum dot solar cells by 50% or more.

The researchers, Kimberly Sablon and John W. Little (US Army Research Laboratory in Adelphi, Maryland), Vladimir Mitin, Andrei Sergeev, and Nizami Vagidov (University of Buffalo in Buffalo, New York), and Kitt Reinhardt (AFOSR/NE in Arlington, Virginia) have published their study on the increased solar cell efficiency in a recent issue of Nano Letters.

In their study, the researchers studied heterostructure solar cells with InAs/GaAs quantum dots. As photovoltaic materials, the quantum dots allow for harvesting of the infrared radiation to convert it into electric energy. However, the quantum dots also enhance the recombination of photocarriers and decrease the photocurrent. For this reason, up to now the improvement of photovoltaic efficiency due to quantum dots has been limited by several percent.

Here, the researchers have proposed to charge quantum dots by using selective interdot doping. In their experiments, the researchers compared doping levels of 2, 3, and 6 additional electrons per quantum dot, which resulted in photovoltaic efficiency increases of 4.5%, 30%, and 50%, respectively, compared to an undoped solar cell. For the 6-electron doping level, that 50% increase corresponds to an overall efficiency increase from 9.3% (for undoped solar cells) to 14%.

The researchers attributed this radical improvement of the photovoltaic efficiency to two basic effects. First, the built-in-dot charge induces various transitions of the electrons and enhances harvesting of the infrared radiation. Second, the built-in-dot charge creates potential barriers around dots and these barriers suppress capture processes for electrons and do not allow them to return back into the dots. The effect of potential barriers has been previously used by the researchers to improve the sensitivity of infrared detectors.

In addition, the researchers predict that further increasing the doping level will lead to an even stronger efficiency enhancement, since there was no evidence of saturation. In the future, the researchers plan to further investigate how these effects influence each other at higher doping levels. They predict that further increasing the doping level and radiation intensity will lead to an even stronger efficiency enhancement, since there was no evidence of saturation.

?The methodology and principles developed during this research are applicable to a number of photovoltaic devices with quantum dots and nanocrystals, such as polymer plastic cells and dye-sensitized porous metal oxide Gratzel cells,? Dr. Sergeev told PhysOrg.com. ?Effective harvesting and conversion of infrared radiation due to optimized electron-hole kinetics in structures with quantum dots and nanocrystals will lead to potential breakthroughs in the area of solar energy conversion.?

Pairing quantum dots with fullerenes for nanoscale photovoltaics

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Left: Photoinduced electron transfer occurring in quantum dot-bridge-fullerene hererodimers and observed with single molecule microscopy. Right: Control of electron transfer (ET) rate by variation of interparticle distance (R, upper panel) and quantum dot size (D, lower panel).

In a step toward engineering ever-smaller electronic devices, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have assembled nanoscale pairings of particles that show promise as miniaturized power sources. Composed of light-absorbing, colloidal quantum dots linked to carbon-based fullerene nanoparticles, these tiny two-particle systems can convert light to electricity in a precisely controlled way.

"This is the first demonstration of a hybrid inorganic/organic, dimeric (two-particle) material that acts as an electron donor-bridge-acceptor system for converting light to electrical current," said Brookhaven physical chemist Mircea Cotlet, lead author of a paper describing the dimers and their assembly method in Angewandte Chemie.

By varying the length of the linker molecules and the size of the quantum dots, the scientists can control the rate and the magnitude of fluctuations in light-induced electron transfer at the level of the individual dimer. "This control makes these dimers promising power-generating units for molecular electronics or more efficient photovoltaic solar cells," said Cotlet, who conducted this research with materials scientist Zhihua Xu at Brookhaven's Center for Functional Nanomaterials.

Scientists seeking to develop molecular electronics have been very interested in organic donor-bridge-acceptor systems because they have a wide range of charge transport mechanisms and because their charge-transfer properties can be controlled by varying their chemistry. Recently, quantum dots have been combined with electron-accepting materials such as dyes, fullerenes, and titanium oxide to produce dye-sensitized and hybrid solar cells in the hope that the light-absorbing and size-dependent emission properties of quantum dots would boost the efficiency of such devices. But so far, the power conversion rates of these systems have remained quite low.

"Efforts to understand the processes involved so as to engineer improved systems have generally looked at averaged behavior in blended or layer-by-layer structures rather than the response of individual, well-controlled hybrid donor-acceptor architectures," said Xu.

The precision fabrication method developed by the Brookhaven scientists allows them to carefully control particle size and interparticle distance so they can explore conditions for light-induced electron transfer between individual quantum dots and electron-accepting fullerenes at the single molecule level.

The entire assembly process takes place on a surface and in a stepwise fashion to limit the interactions of the components (particles), which could otherwise combine in a number of ways if assembled by solution-based methods. This surface-based assembly also achieves controlled, one-to-one nanoparticle pairing.

To identify the optimal architectural arrangement for the particles, the scientists strategically varied the size of the quantum dots - which absorb and emit light at different frequencies according to their size - and the length of the bridge molecules connecting the nanoparticles. For each arrangement, they measured the electron transfer rate using single molecule spectroscopy.

"This method removes ensemble averaging and reveals a system's heterogeneity - for example fluctuating electron transfer rates - which is something that conventional spectroscopic methods cannot always do," Cotlet said.

The scientists found that reducing quantum dot size and the length of the linker molecules led to enhancements in the electron transfer rate and suppression of electron transfer fluctuations.

"This suppression of electron transfer fluctuation in dimers with smaller quantum dot size leads to a stable charge generation rate, which can have a positive impact on the application of these dimers in molecular electronics, including potentially in miniature and large-area photovoltaics," Cotlet said.

"Studying the charge separation and recombination processes in these simplified and well-controlled dimer structures helps us to understand the more complicated photon-to-electron conversion processes in large-area solar cells, and eventually improve their photovoltaic efficiency," Xu added.

A U.S. patent application is pending on the method and the materials resulting from using the technique, and the technology is available for licensing.

More information: Quantum Dot-Bridge-Fullerene Heterodimers with Controlled Photoinduced Electron Transfer, Dr. Zhihua Xu, Dr. Mircea Cotlet, Article first published online: 10 MAY 2011 DOI: 10.1002/anie.201007270

Abstract
A series of donor?bridge?acceptor systems in the form of core/shell CdSe/ZnS quantum dot?bridge?fullerene heterodimers (see picture) with varying bridge length and varying quantum dot size were self-assembled by a surface-based stepwise method to demonstrate control of the rate and of the magnitude of fluctuations of photoinduced electron transfer at the single-molecule level.

Quantum sensor tracked in human cells could aid drug discovery

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The image illustrates the quantum measurement carried out on a single atom quantum sensor in a living human HeLa cell. The atom sensor is encased in a nanodiamond particle and is controlled by external microwaves and laser light, and tracked by its emission of red light. The information gleaned is of a quantum nature, where the states of the atom exist in two quantum states at the same time prior to measurement. The measurement and control of the atom sensor provides information about the nanoscale environment of the cell and the motion and orientation of the nanoparticle, which could be used in the development of new drugs and delivery systems for nanomedicine. Credit: David Haworth

Groundbreaking research has shown a quantum atom has been tracked inside a living human cell and may lead to improvements in the testing and development of new drugs.

Professor Lloyd Hollenberg from the University of Melbourne?s School of Physics who led the research said it is the first time a single atom encased in nanodiamond has been used as a sensor to explore the nanoscale environment inside a living human cell.

?It is exciting to see how the atom experiences the biological environment at the nanoscale,? he said.

?This research paves the way towards a new class of quantum sensors used for biological research into the development of new drugs and nanomedicine.?

The sensor is capable of detecting biological processes at a molecular level, such as the regulation of chemicals in and out of the cell, which is critical in understanding how drugs work.

The paper has been published in the journal Nature Nanotechnology.

Funded by the ARC Centre of Excellence for Quantum Computation and Communication Technology, the research was conducted by a cross-disciplinary team from the University of Melbourne?s Physics, Chemistry, and Chemical and Biomolecular Engineering departments.

The researchers developed state of the art technology to control and manipulate the atom in the nanodiamond before inserting it into the human cells in the lab.

Biologist Dr Yan Yan of the University?s Department of Chemical and Biomolecular Engineering who works in the field of nanomedicine, said the sensor provides critical information about the movement of the nanodiamonds inside the living cell.

?This is important for the new field of nanomedicine where drug delivery is dependant on the uptake of similar sized nanoparticles into the cell.?

Quantum physicist and PhD student Liam McGuinness from the University?s School of Physics said that monitoring the atomic sensor in a living cell was a significant achievement. ?Previously, these atomic level quantum measurements could only be achieved under carefully controlled conditions of a physics lab,? he said.

It is hoped in the next few years, that following these proof of principle experiments, the researchers will be able to develop the technology and provide a new set of tools for drug discovery and nanomedicine.

Packing electrons in a nano box: Control of many-Body correlation by quantum confinement

Packing electrons in a nano box: Control of many-Body correlation by quantum confinement

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Fig. 1 Atomic force microscope image of GaAs quantum dots used in this research.

A team headed by Dr. Takashi Kuroda, Senior Researcher, and Dr. Marco Abbarchi, Researcher, of the Quantum Dot Research Center, National Institute for Materials Science, in joint research with Hokkaido University, succeeded in controlling the few-particle quantum state of a semiconductor quantum dot, and changing its correlation energies. This research achievement will make it possible to develop semiconductor non-linear devices which enable stable drive with low power consumption.

When an electron and proton are brought into proximity in vacuum, the two particles are mutually attracted by Coulomb force and form a hydrogen atom. If another electron or proton is placed in addition, the many-body effect will result in formation of an ionic hydrogen molecule comprising a total of three particles.

This kind of quantum state also exists in solids. A pair of an electron and hole in a semiconductor form an exciton, analogous to a hydrogen atom. If another electron or hole is added, a complex state of three particles, called a charged exciton, is formed. In a semiconductor, unlike hydrogen in vacuum, it is possible to confine electrons an holes in quantum dots, i.e., an extremely small space on the order of several nanometers, and an increase in the stabilization energy of the multi-electronic state can be expected.

In this research, gallium arsenide (GaAs) quantum dots embedded in aluminum gallium arsenide (AlGaAs), fabricated by the droplet epitaxy method were used. This method was originally developed by NIMS. As a distinctive feature of the quantum dots, the length of the crystal lattice is perfectly matched between the guest and host materials.

As a result, an unprecedented clean quantum structure was realized. We succeeded in observation of charged excitons by measuring the photon emission signals from single quantum dots. In particular, when the stabilization energy of charged excitons was compared with that of a quantum well structure of the same type of material, which was previously known to be ~1 meV, it was found to have a value more than 10 times larger. This increase in many-body energy is due to a remarkable increase in the Coulomb force between in the many-particle system resulting from packing electrons in a 3-dimensional nano-space. This result elucidates for the first time the effect of confinement of a multi-electron state in a nano-space, which had not been known in the past, and thus is a result with extremely large scientific impact.

From the viewpoint of applied technology, because electron correlation is also the source of diverse types of non-linear effect devices such as optical switching devices and lasers, if interaction intensity can be controlled using nanostructures, this can be expected to lead to the development of optical semiconductor devices which enable stable drive with low power consumption.

Single quantum dot nanowire photodetectors

Single quantum dot nanowire photodetectors

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This image shows the photocurrent from the nanowire detector (the yellow spot represents the region where current is generated under illumination) and the electrical contacts are indicated in blue, while the nanowire is indicated in green. Credit: Zwiller

Moving a step closer toward quantum computing, a research team in the Netherlands recently fabricated a photodetector based on a single nanowire, in which the active element is a single quantum dot with a volume of a mere 7,000 cubic nanometers. The device is described in the American Institute of Physics' journal Applied Physics Letters.

Photodetectors based on single quantum dots are expected to find uses in optoelectrical interfaces in future quantum computers, where single photons will carry information over long distances and single electrons will be used for computation.

"Our research team fabricated a very localized light detector with dimensions far below the wavelength of light, which enables detection with very high spatial resolution," explains Val Zwiller of the Kavli Institute of Nanoscience at Delft University of Technology.

"In addition, because quantum dots have discrete energy levels, only light in resonance with the energy levels of the quantum dot will be detected. The indium phosphide (InP) nanowire was grown with standard epitaxial techniques and the quantum dot was obtained by sandwiching a thin segment in between with a lower bandgap. We created the device using electron beam lithography to contact a single nanowire."