Power/Performance Bits: Dec. 19

Stabilizing perovskites
Scientists at EPFL and the University of Cordoba found a way to improve the stability of perovskite solar cells. While perovskites show promising efficiencies as solar cells, they are soft crystalline materials and prone to problems due to decomposition over time. By introducing the large organic cation guanidinium (CH6N3+) into methylammonium lead iodide perovskites, the team’s solar cell stopped degrading at 19% efficiency.

The guanidinium cation inserts into the crystal structure of the perovskite and enhances the material’s overall thermal and environmental stability, overcoming what is known in the field as the “Goldschmidt tolerance factor limit.” This is an indicator of the stability of a perovskite crystal, which describes how compatible a particular ion is to it. An ideal Goldschmidt tolerance factor should be below or equal to 1; guanidinium’s is just 1.03.

Stability test of the novel MA(1-x)GuaxPbI3 perovskite material under continuous light illumination compared with the state-of-the-art MAPbI3. A schematic of the device architecture and the simulated crystalline structure is also provided (Source: M.K. Nazeeruddin/EPFL)

The study found that the addition of guanidinium significantly improved the material stability of the perovskite while delivering an average power conversion efficiency over 19% (19.2 ± 0.4%) and stabilizing this performance for 1000 hours under continuous light illumination. The scientists estimate that this corresponds to 1333 days (or 3.7 years) of real-world usage, a big boost from what it’s capable of otherwise.

Graphene nanotransistor
Researchers from Empa, the Max Planck Institute for Polymer Research in Mainz, and the University of California at Berkeley constructed graphene nanoribbons capable of being used as transistor components.

Graphene is normally a conductive material, but can become a superconductor in the form of nanoribbons. The band gap of the nanoribbons depends on its particular atomic structure. Since graphene consists of equilateral carbon hexagons, the border may have a zigzag or a so-called armchair shape, depending on the orientation of the ribbons. While bands with a zigzag edge behave like metals, they become semiconductors with the armchair edge.

Creating nanoribbons by cutting a layer of graphene or carbon nanotubes risks irregular edges that don’t have the desired electrical properties. Instead of cutting, the team grew ribbons exactly nine atoms wide with a regular armchair edge from precursor molecules. The specially prepared molecules are evaporated in an ultra-high vacuum for this purpose. After several process steps, they are combined like puzzle pieces on a gold base to form the desired nanoribbons of about one nanometer in width and up to 50 nanometers in length.

The microscopic ribbons lie criss-crossed on the gold substrate. (Source: Empa)

These structures now have a relatively large and, above all, precisely defined energy gap. This enabled the researchers to integrate the graphene ribbons into nanotransistors.

Initially, however, the first attempts were not very successful: Measurements showed that the difference in the current flow between the “ON” state (i.e. with applied voltage) and the “OFF” state (without applied voltage) was far too small. The problem was the dielectric layer of silicon oxide, which connects the semiconducting layers to the electrical switch contact. In order to have the desired properties, it needed to be 50 nanometers thick, which in turn influenced the behavior of the electrons.

However, the researchers subsequently succeeded in massively reducing this layer by using hafnium oxide (HfO2) instead of silicon oxide as the dielectric material, making the layer only 1.5 nanometers thin and the “on”-current orders of magnitudes higher.

The nanoribbons were be located criss-cross on the transistor substrate, but the team says it would be better to align them exactly along the transistor channel to reduce the currently high level of non-functioning nanotransistors.

3-D printed backscatter
Researchers at the University of Washington developed 3-D printed plastic objects and sensors that use backscatter to send information to commercial WiFi recievers, replacing some functions normally performed by electrical components with mechanical motion activated by springs, gears, switches and other parts that can be 3-D printed.

Backscatter systems use an antenna to transmit data by reflecting radio signals emitted by a WiFi router or other device. In this case, the antenna is contained in a 3-D printed object made of conductive printing filament that mixes plastic with copper.

Physical motion triggers gears and springs elsewhere in the object cause a conductive switch to intermittently connect or disconnect with the antenna and change its reflective state.

Information is encoded by the presence or absence of the tooth on a gear. Energy from a coiled spring drives the gear system, and the width and pattern of gear teeth control how long the backscatter switch makes contact with the antenna, creating patterns of reflected signals that can be decoded by a WiFi receiver.

In this backscatter system, an antenna embedded in a 3-D printed object (middle) reflects radio signals emitted by a  WiFi router (left) to encode information that is “read” by the WiFi receiver in a phone, computer or other device (right). (Source: University of Washington)

The team printed several different tools that were able to sense and send information successfully to other connected devices: a wind meter, a water flow meter and a scale. They also printed a flow meter that was used to track and order laundry soap, and a test tube holder that could be used for either managing inventory or measuring the amount of liquid in each test tube.

The team’s CAD models are available to the public.