Harvard Group Creates Nanophotovoltaic With Macro Potential
Alexander E. Braun, Senior Editor -- Semiconductor International, 4/15/2008
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A Harvard University (Cambridge, Mass.) research group headed by professor Charles Lieber has developed a ~300 nm-thick coaxial silicon nanowire that could form the basis for a photovoltaic (PV) cell used to power small circuits and nanomachines.
The experimental silicon cell has demonstrated an efficiency of 3.5%, which is considered adequate for an experimental device of this kind. High-current densities have been attained on the order of 24 or more mA/cm2, which is better than what can be done in organics or most hybrids. The power is up to ~200 pW, although occasionally 1 nW has been observed, and the group’s early work indicates that the voltage or current could be doubled by hooking two nanophotovoltaic devices in series or parallel.
Resembling a three-layer silicon coaxial cable, the nanowire PV consists of a positive core, thin intrinsic (neutrally charged) intermediate sheath, and a negatively charged exterior shell (Fig. 1). While common in generic flat solar cells, Lieber said this p-i-n structure has not yet been applied to a coaxial wire.
| One end of the nanowire photovoltaic is etched so that the first metal contact can be attached. The second contact is made on the outermost of the three layers. (Source: Harvard University) |
“We considered many schemes, including several nanostructured materials,” Lieber said. “We began by exploring how to improve charge collection by changing the traditional planar configuration photocell geometry and settled on a coaxial geometry.” Because the layers are radially aligned, a coaxial format shortens the collection lengths, providing higher efficiency. In the circular cross-section, electrons and holes must move across considerably shorter distances. Also, it becomes possible to use lower-quality materials without suffering losses. Overall, the process lends itself to making PVs on plastics or almost anything else.
Fabrication begins with the growth of a p silicon nanowire core, a single-crystal structure grown by using a metal nanocluster that nucleates the growth via vapor/liquid/solid growth. The diameter is controlled by the size of the metal catalyst particle used to nucleate. Once the core is done, nanowires are sequentially deposited, forming an intrinsic layer of various thicknesses and then creating an n shell to cap it, resulting in the basic p-i-n core/shell/shell structure. The device can be optimized by varying the thicknesses and doping of the layers.
The nanowires are randomly grown on a substrate that can be something as simple as glass. Then a piece of the substrate is put into ethanol and sonicated for five seconds to shake the wires suspended in a solution. The wires are dropped onto the chip where the devices will be built.
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| The nanophotovoltaic has a coaxial configuration made up of three differently doped silicon regions, which enable it to function as a regular solar cell. Seen here is the positive core, already etched and ready for the first contact. (Source: Harvard University) |
Electron-beam (e-beam) lithography is used to fabricate contacts to the n shell and p core. A mask and wet etch are used to expose the p core, creating what appears like a stinger-like end coming out of a thicker diameter core (Fig. 2). This fine rod is what is left after one of the nanowire’s ends has been etched, and single or multiple contacts can be made to it afterward to determine whether efficient radial collection is obtained. Only two contacts, one on each end, are necessary to complete the device. It is also possible to just etch the end of many of these nanowire elements and put a single p contact to the core to link any number of them and produce an array of parallel devices.
“This is a very robust, completely inorganic system, and like other inorganic PVs, very stable,” Lieber said. “We have operated devices for about a year without any degradation, as well as at a 10-sun illumination with very stable behavior. Organics or polymers don’t tolerate concentrated sunlight well.” With proper packaging, these nanophotovoltaic cells could last decades, he added.
At present, this proof-of-concept effort has produced devices capable of powering a biosensor or small logic gate. The main focus now is to raise device efficiency to 10% and beyond.
While there is no expectation that this work will solve large-scale power-generation problems, it is aimed at nanosystems that need power to function. Assembling them with nanophotovoltaics could offer a way to fabricate integrated, self-powered nanoscale devices. If efficiency can be raised, it becomes possible to think in terms of moderate-scale applications, especially because production costs would not be any higher than for amorphous silicon structures.
Allied to the ongoing effort to increase the nanophotovoltaic’s efficiency is an investigation to determine whether these new devices can be processed on any substrate. “We’re looking at other materials while still maintaining the coaxial geometry model. III-Vs are a possibility, because they have higher absorbtivity in the solar spectrum. It is also possible to directly create tandem cells, but we haven’t yet done this in coaxial. An axial p-i-n/p-i-n structure with double the voltage output from the cell seems possible. However, my goal is to get something that approaches single-crystal-like efficiency,” Lieber said.
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