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November 2016

ES3: Perovskite Solar Cell Research from Material Properties to Photovoltaic Function

Henk Bolink, University of Valencia

Fully Evaporated High Efficiency Perovskite Based Solar Cells

Written by Xiwen Gong

Perovskite thin film solar cells synthesized by solution process have achieved high power conversion efficiency (PCE) of over 22%. There are certain advantages of the solution process, such as the ease to engineer the material composition, fast film formation, and low cost. However, it is also limited by the fact that this process relies on a toxic solvent, such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), for example. The crystallization dynamic is also difficult to control during the solution process.

Henk Bolink from the University of Valencia introduced an alternative synthetic solution utilizing evaporation techniques. The perovskite evaporation takes place under vacuum condition, which is widely used in the industrial deposition process. Vacuum deposition also shows advantages over solution process in producing materials with high purity, in large area, and with arbitrary thickness.

Through the dual-source evaporation method, researchers can fabricate close-packed uniform perovskite films with low roughness. The solar cell made with p-i-n structure through fully evaporated deposition reached a PCE of around 15%. The highest efficiency of 20.3% was achieved with devices structure with n-i-p structure, with optimized doping level in the electron and hole transport layer.

MB5: Size Effects and Small-Scale Mechanical Behavior of Materials

Keith Dusoe, University of Connecticut

A New Class of Superelastic Materials—ThCr2Si2-Structured Novel Intermetallic Compounds at Small Length Scales

Written by Vineet Venugopal

Keith Dusoe introduced two new materials from the ThCr2Si2 system that show unusual superelastic properties. The compounds CaFe2As2 and LaRu2P2 both undergo structural collapse along the c-axis, producing large strains. The materials were made using solution melt processes that produced high purity faceted single crystals. Micropillars made from these materials were used to study the mechanical properties. Uniaxial compression of LaRu2P2 gives around 8% recoverable strain. These properties are interesting as they resemble the behavior of shape memory alloys albeit with a different mechanism. Dusoe, who emphasized the collaborative nature of his work, pointed out that there are over 400 AB2X2 type compounds that could now be suitable candidates for shape memory alloys.

TC2: Design, Discovery, and Understanding of Materials Guided by Theory, Computation and Data Mining

Alfred Ludwig, Ruhr University, Germany

Thin Film Combinatorial Materials Science for the Design of Materials

Written by Vineet Venugopal

The discovery of new materials with interesting properties has been a largely serendipitous event. By merging high throughput technologies with combinatorial materials science, Alfred Ludwig and his colleagues have taken materials discovery to a new level. They use an innovative combinatorial sputtering technology to make thin films of several binary and ternary compositions, which when subjected to high throughput characterization creates large materials databases. This approach has yielded several new compositions of technological interest. New shape memory alloys have been discovered in the Ni-Ti system, light absorbent materials have been found in the Cu-Si-Ti system and a new ternary phase has been discovered in the Co-Ti-W system, to cite only a few examples. Ludwig also noted the utility of resistivity screening as a characterization tool in combinatorial materials screening.

PM3: Science-Enabled Advances in Materials- and Manufacturing-Technologies

Olaf Dambon, Fraunhofer Institute for Production Technology

Glass Material Modeling and its Molding Behavior

Written by Vineet Venugopal

Glass molding is a technologically important process that has enormous applications in optics manufacturing, lighting optics, infrared optics, wafer optics, and lens arrays, for example. To enable accurate and replicable industrial manufacturing processes, it is essential to have an accurate model of this process and to be able to simulate it. However, glass is a very difficult material to model because in the temperature regime of interest, it is visco-elastic and no one model captures all of its features fully. Olaf Dambon and his colleague Gang Liu have studied these models extensively and made several additions to them. Using FEM simulations the researchers are able to predict density variation and index drop in the structures before and after molding. They also apply Weibull statistics to calculate the fracture probability at each fine point. However, Dambon noted that the current fracture model is not good enough. 

Science communication

In the first day of MRS, I attended the panel discussion on how to use social media as an academic. Not surprisingly, twitter and its role in science communication between academics and to the general public was emphasized over and over again. The same message was hammered again by Dr. Sunshine Menezes in the public speaking and communication session in the second day. 

Not surprising because I know it is true, being on twitter myself (@Kerologist). I barely have any of my "personal friends" on twitter. I almost exclusively have other scientists who I have never met in my life. I also got to meet other academics and administrators of my university through twitter and then got to meet them in person and learn from. My main supervisor is also on twitter and I have got to learn that she responds to twitter much faster than e_mail. Actually, let me rephrase, the response success rate is orders of magnitude higher on twitter than in e_mail- I have quantitative data on that!

However, I also often have my twitter account in my conference presentation but I am sometimes told to remove it (by other mentors) because it is not so "scientific". I wanted to write a post to change that perception but then I realized that if you are reading this post, you are also likely to be active on twitter sci-comm. I wonder if you have ideas on how to change that culture?


Keroles B. Riad (@Kerologist)

EM11.6 : Life time Killer and Life time Control

The opening thirty minute talk by Tsunenobu Kimoto from Kyoto University, Kyoto, Japan in the Symposium SiC Power Electronics covered points which involved applications, life time control techniques and breakdown analysis of SiC power devices.

The remarkable energy saving ability of SiC power device finds application in air conditioner/ PV converters, trains and four wheeler etc, however, the life time of a power device basically regulates its life performance. And carbon vacancy acts as a life time killer

The question which arises is - How to eliminate Carbon Vacancy ? 

Solutions : 

1.) Carbon implantation + Ar Annealing. 

 2.) Thermal Oxidation (+Ar Annealing)

So, eliminating carbon vacancies will enhance the life time of device. 

The second question which follows is how to have lift time control ? 

Dr. Kimoto pointed out two important solutions to the follow up question which are -

1.) By C - vacancy elimination 

2.) By intentional generation of C- vacancy. 


Children of the flame

Today, I attended a talk by Dr Albert Dato from Harvey Mudd College in NM6 symposium (Nanoscale Materials and Devices by High-Temperature Gas-Phase Processes). He talked about synthesizing pristine graphene using plasma. His talk was extremely interesting to me given my PhD project on trying to 3D-print graphene oxide. I am sure I wont be alone in despising the yield of the Hummer’s method in making graphene. We, children of the flame, are hardly ever enthusiastic about having to do wet-chemistry synthesis.

But, this is not what I found most impressive about the talk- and it was very impressive. What I really admired is what he pointed out at the very beginning. Dr Dato belongs to an entirely undergraduate college and the research is conducted by undergraduate students. I relate to this myself as I started my journey in research as an undergraduate student who joined a research lab by getting an “undergraduate student research award” that the Canadian government provides to undergraduate students to work in a research lab. Based on those internships, my master and my PhD projects were conceived. Having spent some time collaborating with research labs in ETH Zürich, I have always admired the fact that there are so many undergraduate students working in the lab. I dream of a future that as a community, we proactively engage and recruit undergraduate students in graduate level research.

Keroles B. Riad (@Kerologist)

ES4: Thermoelectric Polymers and Composites—Nontraditional Routes to High Efficiency

Alex Barker, Center for Nano Science and Technology, Italy

Simple Thermal Conductivity Measurements and Data Modelling of Thin-Film Polymers through Pump-Probe Spectroscopy

Written by Xiwen Gong

Thermoelectric (TE) effect describes the process of the direct conversion of thermal energy (heat) to electricity. Devices made based on thermoelectricity provide a clean method for electricity generation by either directly converting heat from solar radiation or recovering the waste heat generated by machines. The ideal materials for TE would have high ZT value, which requires high electrical conductivity with low thermal conductivity. Polymers have shown desirable properties for TE, such as low thermal conductivity, low cost, and solution processability.

There are a few methods that can measure thermal conductivity (), including transient line source, 3 , and laser flash method. Alex Barker presented a simple measurement based on a standard pump-probe setup that was developed to measure the  of polymers.

The sample is first heated up with a modulated pump laser in order to get a small increase of temperature. The reflectivity at this pump frequency will then change, due to the increased temperature, which can be monitored by using the probe beam. The data collected from different materials thicknesses were then fit with a one-dimensional diffusion model in order to achieve thermal conductivity. The results of thermal properties of poly (methyl methacrylate) (PMMA) obtained from this method agrees well with a previous report. The researchers further applied this technique to study the thermal properties of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), the polymers of great promise to achieve high ZT value for their high electrical conductivity.

ES3: Perovskite Solar Cell Research from Material Properties to Photovoltaic Function

Olga Ovchinnikova, Oak Ridge National Laboratory

Ionic Migration in Grain Boundaries of Organometallic Halide Perovskite Films Enhanced by Chlorine

Written by Xiwen Gong

In the field of perovskite photovoltaic research, the rationale design of materials is based on the in-depth understanding of the material fundamental properties. Ion migration is one of the interesting properties of perovskite, but its mechanism and the role in determining the device performance remain unknown. Several imaging techniques are commonly used to study the structural properties of perovskite, including standard atomic force microscopy (AFM) and Kelvin probe force microscope (KPFM). However, the standard AFM and KPFM take all the behaviors and the complicated interactions between the perovskite and the tip into account, making the measurement more complex.

Olga Ovchinnikova from Oak Ridge National Laboratory provides a method of band excitation contact Kelvin probe force microscopy (BE-KPFM) to study the ionic mobility and surface charge of the perovskite systems. By fitting the data collected with different applied DC voltages, it is possible to get the data of surface charge and capacitance gradient separate from one another, which is crucial for the understanding of what is occuring during the ion migration. Their study shows chloride ions segregate at the grain boundaries and facilitate the ion migration there. Therefore, the improved ionic mobility of mixed iodide/chloride perovskite may come from the migrated chloride ions at the grain boundaries.