battery and memory rolled into one
TRANSCRIPT
Double role for nanocomposite in solar cells
Cordelia Sealy
Nanoscale dye-sensitized solar cells (DSCs) could offer a routeto cheaper photovoltaic devices. Promising photoelectric con-version efficiencies (PCEs) have been reported for DSCs usingliquid electrolytes, but despite the development of goodsealants, completely solid-state devices would be preferable.
Now researchers from Switzerland and Korea have reported alow-cost, solution-processed, high efficiency solar cell based ona nanocomposite with a perovskite light harvester and apolymeric hole conductor [J.H. Heo et al., Nature Photonics(2013), http://dx.doi.org/10.1038/nphoton.2013.80]. The inorganic–organic heterojunction solar cells created by the teamled by Michael Grätzel at the Swiss Federal Institute ofTechnology (EPFL) and Sang Il Seok of Korea Research Instituteof Chemical Technology and Sungkyunkwan University in Korea,along with colleagues from Kyung Hee University, can achievepower conversion efficiencies (PCEs) of up to 12% understandard illumination conditions (AM 1.5G).
The sandwich-like device relies on a three-dimensionalnanocomposite of mesoporous TiO2 impregnated withCH3NH3PbI3, which acts as a light harvester, and poly-triarylamine (PTAA) as a hole-transporting (and electron-blocking) layer (Figure 1). The CH3NH3PbI3 absorbs light togenerate electron–hole pairs, which mainly dissociate at theTiO2/CH3NH3PbI3 interface, with holes traveling across theperovskite to the hole-transporting layer.
“In contrast to conventional dye- or quantum dot-sensitizedsolar cells, the distinguishing feature of these inorganic–organichybrid heterojunction solar cells is that CH3NH3PbI3-generatedholes are transported to the PTAA layer through itself,” explainsSeok. “That means CH3NH3PbI3 is not only used as a lightabsorber but also as a hole transport material.”
The new platform combines the advantages of solutionprocessing, with inorganic–organic materials and nanostructureto create high-efficiency solar cells at low cost, he says. Theinorganic–organic hybrid heterojunction solar cells reach max-imum PCEs of 12%, with over 30% of the devices tested showinga PCE of over 10% and a majority exceeding 9%.
“The proposed solar cells have the potential to show a highenergy conversion efficiency of around 18% theoretically. Webelieve that an efficiency of 15% will soon be achieved byoptimizing fabrication conditions,” Seok told Nano Energy.
The researchers believe that the pillared structure of themesoporous TiO2/CH3NH3PbI3 nanocomposite and a verythin (30 nm) PTAA layer are crucial to achieving highperformance. They are now working on the device archi-tecture and materials to improve stability and efficiencyprior to potential scale-up for commercialization.
Mercouri G. Kanatzidis of Northwestern University agreesthat the approach could be a practical one for realizing low-cost, easy-to-fabricate all-solid state solar cells. “This typeof architecture is superior to the liquid electrolyte-basedarchitecture that has dominated the field for over 20years,” he says. “[While] the 12% mark is not a record[it is] close to it [and] I anticipate greater efficienciessoon,” he told Nano Energy.
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Battery and memory rolled into one
Cordelia Sealy
Future IT systems could rely on resistive memory cells(ReRAM) to improve performance while driving down energy
consumption. But researchers from the Jülich AachenResearch Alliance (JARA) in Germany have confirmed an
Figure 1 Schematic of inorganic–organic hybrid heterojunc-tion solar cell architecture. [Credit: Sang Il Seok, KoreaResearch Institute of Chemical Technology.]
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additional benefit; these novel devices also act as tinybatteries [I. Valov et al., Nature Communications (2013)http://dx.doi.org/10.1038/ncomms2784].
In conventional memory devices, electrons are used tostore information but they are small and difficult to control.Ions, by comparison, are easier to manage and though muchlarger than electrons can enable smaller memory devices tobe fabricated. Like conventional devices, resistive switchingmemory cells have two electrodes at which ions dissolve andprecipitate again. The researchers from RWTH AachenUniversity and Research Centre Jülich looked specificallyat a redox-based resistive switching memory cell consistingof a SiO2 thin film solid electrolyte and two electrodesfabricated from Ag and Pt (Figure 1).
When a positive voltage is applied to the electrodes, anelectrochemical (oxidation) reaction creates a dissolutionprocess at the active electrode and deposition of a tinyfilament at the counter electrode, which short-circuits thecell creating a low resistive ‘ON’ state. A voltage of theopposite polarity reverses the process (reduction), dissol-ving the filament and resetting the cell to a high resistive‘OFF’ state. But as well as exploiting the change inelectrical resistance to store data, the reduction andoxidation processes also generate an electric voltage.
The combined memory and battery cell has a number ofadvantages including fast operation, low power consump-tion, and scalability down to the near atomic level enablingvery high information storage density.
“In the light of this new knowledge, it is possible tospecifically optimize the design of the ReRAM cells, and itmay be possible to discover new ways of exploiting the cells'battery voltage for completely new applications, whichwere previously beyond the reach of technical possibili-ties,” says group leader Rainer Waser.
The induction of an electromotive force (emf) was alsoconfirmed in a number of other ReRAM cells tested and theresearchers are now exploring the idea of using the batteryvoltage to improve data readout of the memory cells, aswell as the performance and reliability of devices.
“The recognition of emf within these memory cells willbe used for materials/systems selection criteria to improvedevice operation and stability,” adds first author Ilia Valov.
But the finding also has implications for the design of suchdevices, which are currently often considered as memristorsin circuit design.
“The demonstrated internal battery voltage of ReRAMelements clearly violates the mathematical construct of thememristor theory. This theory must be expanded to a wholenew theory—to properly describe the ReRAM elements,”explains Eike Linn of RWTH Aachen University.
Cordelia Sealy has many years' experienceas a scientific journalist and editor in areasspanning nanotechnology, energy, materialsscience and engineering, physics, chemistryand the environment. She is currently afreelance science writer for her own com-pany, Oxford Science Writing, and serves asNews and Opinions Editor of Nano Energyand Nano Today. She also writes on energypolicy and business issues. In the past,Cordelia served as Editor of Materials Today
and Nano Today and as Managing Editor of both titles. She also hasexperience in academic publishing as a books acquisitions editorand in business-to-business publishing as a journalist on EuropeanSemiconductor. She has a First in Physical Sciences (BSc) fromUniversity College London and a DPhil in Materials Science andEngineering from the University of Oxford, and is a Member of theInstitute of Physics.
E-mail address: [email protected]
2211-2855/$ - see front matterhttp://dx.doi.org/10.1016/j.nanoen.2013.05.006
Figure 1 Configuration of a resistive storage cell (ReRAM): anelectric voltage is built up between the two electrodes so thatthe storage cells can be regarded as tiny batteries. Filamentsformed by deposits during operation may modify the battery'sproperties. [Credit: Jülich Aachen Research Alliance (JARA).]
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