battery and memory rolled into one

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Double role for nanocomposite in solar cells Cordelia Sealy Nanoscale dye-sensitized solar cells (DSCs) could offer a route to cheaper photovoltaic devices. Promising photoelectric con- version efciencies (PCEs) have been reported for DSCs using liquid electrolytes, but despite the development of good sealants, completely solid-state devices would be preferable. Now researchers from Switzerland and Korea have reported a low-cost, solution-processed, high efciency solar cell based on a nanocomposite with a perovskite light harvester and a polymeric hole conductor [J.H. Heo et al., Nature Photonics (2013), http://dx.doi.org/10.1038/nphoton.2013.80]. The inor ganicorganic heterojunction solar cells created by the team led by Michael Grätzel at the Swiss Federal Institute of Technology (EPFL) and Sang Il Seok of Korea Research Institute of Chemical Technology and Sungkyunkwan University in Korea, along with colleagues from Kyung Hee University, can achieve power conversion efciencies (PCEs) of up to 12% under standard illumination conditions (AM 1.5G). The sandwich-like device relies on a three-dimensional nanocomposite of mesoporous TiO 2 impregnated with CH 3 NH 3 PbI 3 , which acts as a light harvester, and poly- triarylamine (PTAA) as a hole-transporting (and electron- blocking) layer (Figure 1). The CH 3 NH 3 PbI 3 absorbs light to generate electronhole pairs, which mainly dissociate at the TiO 2 /CH 3 NH 3 PbI 3 interface, with holes traveling across the perovskite to the hole-transporting layer. In contrast to conventional dye- or quantum dot-sensitized solar cells, the distinguishing feature of these inorganicorganic hybrid heterojunction solar cells is that CH 3 NH 3 PbI 3 -generated holes are transported to the PTAA layer through itself,explains Seok. That means CH 3 NH 3 PbI 3 is not only used as a light absorber but also as a hole transport material.The new platform combines the advantages of solution processing, with inorganicorganic materials and nanostructure to create high-efciency solar cells at low cost, he says. The inorganicorganic hybrid heterojunction solar cells reach max- imum PCEs of 12%, with over 30% of the devices tested showing a PCE of over 10% and a majority exceeding 9%. The proposed solar cells have the potential to show a high energy conversion efciency of around 18% theoretically. We believe that an efciency of 15% will soon be achieved by optimizing fabrication conditions,Seok told Nano Energy. The researchers believe that the pillared structure of the mesoporous TiO 2 /CH 3 NH 3 PbI 3 nanocomposite and a very thin (30 nm) PTAA layer are crucial to achieving high performance. They are now working on the device archi- tecture and materials to improve stability and efciency prior to potential scale-up for commercialization. Mercouri G. Kanatzidis of Northwestern University agrees that the approach could be a practical one for realizing low- cost, easy-to-fabricate all-solid state solar cells. This type of architecture is superior to the liquid electrolyte-based architecture that has dominated the eld for over 20 years,he says. [While] the 12% mark is not a record [it is] close to it [and] I anticipate greater efciencies soon,he told Nano Energy. E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2013.05.010 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 Aachen Research Alliance (JARA) in Germany have conrmed an Figure 1 Schematic of inorganicorganic hybrid heterojunc- tion solar cell architecture. [Credit: Sang Il Seok, Korea Research Institute of Chemical Technology.] 437 News and Opinions

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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.

E-mail address: [email protected]

2211-2855/$ - see front matterhttp://dx.doi.org/10.1016/j.nanoen.2013.05.010

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.]

437News and Opinions

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).]

C. Sealy438