The dependence of the current-voltage curve on voltage sweep direction, sweep rate, and the situation (pre-bias, illumination) a CH3NH3PbI3 perovskite solar cell has faced before a scan, makes a simple determination of the efficiency more difficult and results in what is called a rate-dependent hysteresis in the current-voltage relation. In this work we show that the rate-dependent hysteresis is related to a slow field-induced process that tends to cancel the electric field in the device at each applied bias voltage. It is attributed to the built-up of space charge close to the contacts, independent of illumination and most likely due to ionic movement, which is enhanced when the device undergoes aging. This process can also lead to a reduction of the steady-state photocurrent and does not directly correlate with the development of the hysteresis if it is measured at a fixed voltage sweep rate. Consequently, investigating the hysteresis itself at a given voltage sweep rate is not sufficient to understand the effects causing the hysteresis. We show that the difference between the photocurrent when scanning from positive to negative bias and the other way around is not related to a displacement current, but to a modified charge-carrier collection efficiency. Our experimental approach allows to discriminate between slow and fast processes, where we compare planar architectures with devices based on a mesoscopic scaffold. We apply a device model to assist the analysis of the experimental data.

Presently, LIBs have got tremendous attention due to their high energy densities and have been considered as promising power source for future EV. In this regard, metallic Si, Ge and Sn are considered as potential substitute to the conventional graphite (372 mAh/g) due to their high theoretical capacities 4200, 1600 and 992 mAh/g, respectively and thermal stability. However, structural disintegration, limited access to redox sites and loss of electrical contact have long been identified as primary reasons for capacity loss and poor cyclic life of these materials. Although nanotechnology plays critical role by developing nanostructures but simple reduction in size introduce new fundamental issues like side reactions and thermally less stable. Furthermore, formations of unstable SEI film due to the decomposition of the organic electrolyte at ?0.5 V vs. Li/Li+. Thus, a careful design that can inhibit the side reaction by surface protection, make all redox sites accessible by increasing the intrinsic conductivity, maintain a continues network for ionic and electronic flow and keeps the structural integrity, resulting improved performance and excellent capacity retention with long cyclic life to meet the requirements set by USABC for LIBs use in EVs. Here, we have developed hybrid structures of Si and Sn using two strategies to overcome the aforementioned problems. The encapsulation of the NPs/NTs in the shell of inactive metal and dual protection was provided by the overcoat of NG. The second strategy accounts the surface protection by C shell and dual protection by soft matrix of porous carbon (PC). The intrinsic conductivity is increased by the backbone of highly conductive metal that efficiently transfers the electron to all redox sites inside the nanostructure and acts as stress relaxer due to its hard nature. Furthermore, these hybrids also took the advantages of Li+ storage at the grain boundaries that brings additional capacity. The high performance of the composite based on the synergistic effect of several components in the nanodesign. Moreover, NG/PC increases the contact area between electrolyte and electrode for better performance because of their high surface area. In addition, due to the high conductivity and fast ions transfer mobility, graphene/porous carbon maintains the fast electrical flow of the composites. As a result these hybrids possess extraordinary performance with capacity retention of ~100% after long cyclic life of 2000 cycles. These strategies to combine the different property enhancing factors in one composite with engineered structures will bring the realization of the LIBs in EVs. Li, Q.; Mahmood, N.; Jinghan, Z.; Hou, Y., Sun, S., Nonotoday, 2014, DOI:10.1016/j.nantod.2014.09.002. Mahmood, N.; Zhang, C.; Liu, F.; Jinghan, Z.; Hou, Y., ACS Nano 2013, 7, 10307-10318. Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y., Adv Mater 2013, 25, 4932-4937. Mahmood, N.; Zhang, C.; Hou, Y., Small 2013, 9, 1321-1328.

In recent work, our group has demonstrated improved cycling performance of silicon (Si) negative electrodes for lithium ion (Li-ion) batteries though the use of a self-healing polymer (SHP) binder. Our work was successful in utilizing commercially available micron sized particles to produce cells that were stable for over 100 cycles1. This work details further investigation into the effects of increased crosslinking on the performance of the SHP in the Si electrodes. Crosslinking was varied using a combination of difunctional and trifunctional molecules as the starting materials for the supramolecular SHPs. Additionally, a covalent crosslinker was used in some samples to examine the effects of static crosslinks as compared to the dynamic hydrogen bonding of the SHP. Standard mechanical tensile measurements were performed in addition to creep and stress relaxation experiments used to determine characteristic relaxation times for each material from viscoelastic theory. Cell cycling performance was correlated to this data and a relationship between mechanical characteristics, Li-ion conductivity, and cycling stability was seen. This work represents a step toward understanding the reasons behind performance improvements seen in the Si electrodes with SHP and will allow us to begin to tailor molecular structures of self-healing polymer binders specifically for battery applications. 1. Wang, C. et al. Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries. Nat Chem 5, 1042�1048 (2013).

Chirality is a geometric property of objects that cannot be superimposed onto their mirror images. Chiral structures also give rise to unique chiroptical effects when interacting with polarized light. These properties are fundamental in biomolecular systems, and chiroptical spectroscopy is often used for their characterization. The role of chiral surfaces of inorganic crystals and their interaction with biomolecular systems has been discussed in many different theories that relate to the generation of homochirality in biomolecules. In this talk a somewhat complementary field will be discussed. In the first type of systems that will be described, induction of chiroptical effects in nanocrystals of achiral semiconductor materials using chiral biomolecules will be described.1 This effect is interesting for fundamental studies of exciton - molecular level interactions, as well as exciton level structure characterization. However, it is generally very weak. More recently we introduced the concept of enantioselective synthesis of intrinsically chiral inorganic nanocrystals, which leads to more pronounced effects.2 Many inorganic materials such as quartz, mercury sulfide and tellurium crystallize in chiral space groups with a chiral lattice. Biomolecules can be used to induce enantioselectivity in the nucleation and growth of nanocrystals of these materials. For the case of tellurium, we show that crystal growth in the presence of the small peptide, glutathione, results in nanocrystals where the atomic scale lattice chirality translates to the overall shape chirality on a 100 nm scale.3 This is a unique example for a colloidal chemistry approach for self assembling inorganic nanocrystals, which exhibit chirality at two size hierarchies. These systems may be useful for applications in metamaterials fabrication, asymmetric catalysis, sensing and optical devices. On a more fundamental level, these are excellent model systems for studies of chiral crystallization and separation, and the interaction of chiral biomolecules with chiral crystals. The possible role of chiral inorganic crystals and surfaces in the evolution of biomolecular homochirality has been considered by many researchers. Here it is demonstrated that the opposite effect, of biomolecules affecting chiral inorganic crystallization, is also intriguing. 1.Ben-Moshe, A.; Swarczman, D.; Markovich, G. " Size Dependence of Chiroptical Activity in Colloidal Semiconductor Quantum Dots" ACS Nano 5, 9034-9043 (2011) 2.Ben-Moshe, A.; Govorov, A. O.; Markovich, G. "Enantioselective Synthesis of Intrinsically Chiral Mercury Sulfide Nanocrystals" Angew. Chem. Int. Ed. 52, 1275-1279 (2013) 3.Ben-Moshe, A.; Wolf, S. G.; Bar-Sadan, M.; Houben, L.; Fan, Z.; Govorov, A. O.; Markovich, G. "Enantioselective control of lattice and shape chirality in inorganic nanocrystals using chiral biomolecules" Nat. Comm. 5, 4302 (2014)

Since July 2014, thirteen European metrology organizations are collaborating to address some of the main metrological challenges faced by the developments of high�efficiency III-V Multi-Junction Solar Cells (MJSC). III�V MJSC structures are made of a high number of layers, which makes a pure experimental optimization difficult and expensive; this also limits the uncertainty of cell calibration due to the complexity of their spectral response. The project is structured in three distinct parts: 1) improved accuracy in materials and transport characterization for existing MJSC; 2) improved accuracy and repeatability in traceable efficiency characterization for MJSC cells with three or more junctions and finally, 3) metrology for advance concept such as coupling with thermoelectric, dilute nitride, quantum dots or growth on Silicon. Each part of the project will be presented, but our contribution will be centered on our initial results for the first part that is developing accurate and spatially resolved metrology to determine traceable and complete III -V material data sets. In particular, the accuracy of existing and emerging methods to measure electrical transport properties of III -V complex heterostructures will be presented in details with an emphasis on experimental determination of band-gap alignment, dopant distribution, carrier density, and series -resistances.

The interface between proteins and, extended or nanostructured, inorganic surfaces, is the key element of several current and potential biotechnological applications. For example, redox enzymes may be supported on electrodes, to create biofuel cells able to use fuel other than hydrogen [1]; peptides able to specifically bind a given surface have the potential to guide the self-assembling of nanosystems [2]. In a more biologically-oriented context, the interaction of amyloidogenic proteins with surfaces and nanoparticles is being investigated to understand how such interaction may affect the misfolding and fibrillation process [3]. Bare and functionalized gold surfaces, and gold nanoparticles, are particularly relevant in these frameworks. Au surfaces are often used for electrodes or to support self-assembled monolayers. Au nanoparticles are relatively cheap, can be synthesized with a good control of shape, size, and functionalization, and present useful optical properties. We have developed and applied tools for the atomistic simulations of the interaction of proteins and peptides with various surfaces of gold [4,5], including functionalized ones, and with gold nanoparticles [6,7]. In this talks, such tools will be introduced and some examples of their applications relevant for material science (e.g., enzymatic biofuel cells [8]) and nanobiotechnology (e.g., interaction of gold nanoparticles with amyloidogenic proteins/peptides [7]) will be presented. [1] Cracknell, J. A. et al. Chem. Rev. 108, 2439 (2008) [2] Sarikaya, M., et al. Nature Mater. 3, 577 (2003) [3] Linse, S., et al. PNAS 104, 8691 (2007) [4] Iori, F. et al. J. Comp. Chem. 30, 1465 (2009) [5] Wright, L. et al. JCTC 9, 1616 (2013); JPC C 117, 24292 (2013) [6] Brancolini, G. et al. ACS Nano 6, 9863 (2012) [7] Brancolini, G. et al. Nanoscale 6, 7903 (2014) [8] Zanetti-Polzi, L. et al. JACS 136, 12929 (2014)

Concentrating photovoltaics (CPV) incorporating high efficiency multijunction photovoltaic cells are a leading candidate for large-scale ground-based solar energy installations. As in other solar technologies, reliability over extended operating lifetimes is an important metric for success. Despite the successful application of multijunction cells in space based systems, the application of multijunction cells with their complex layered structures in terrestrial applications requires an improved understanding of thermomechanical reliability and testing metrologies as a basis for improved lifetime predictions. Of particular concern is the adhesion of the many internal interfaces including those involving backside metal contacts, substrates, active layers, antireflective coatings, and frontside metal gridlines. The effects of stressing parameters that include mechanical stress, temperature, and UV exposure in the presence of humidity from terrestrial environments are also of significant interest. We discuss modified thin-film adhesion testing metrologies together with the first quantitative measurement of adhesion of selected interfaces within state-of-the-art multijunction cells. In particular, we address the adhesion of several 2- and 3-layer antireflective coating systems on multijunction cells along with frontside gridlines and backside metal contacts. Special modifications to previously established techniques were necessitated by the fragility of germanium and gallium arsenide substrates ubiquitously used in these devices. By varying interface chemistry and morphology through processing, we initially demonstrate the marked effects on adhesion and help to develop an understanding of how high adhesion can be achieved, as adhesion values ranging from 2 J/m2 to 12 J/m2 were measured. Furthermore, long-term temperature cycling and high-humidity exposures demonstrate environmental degradation modes and begin to build the basis for physics-based degradation models.

A piezoelectric can create enough pressure to drive a piezoresistive film from an insulating to a conductive state. If we combine these components into a monolithic device, the result is a PiezoElectronic Transistor (PET). This talk will focus on the experimental realization of the PET, describing both progress and challenges. I will discuss the high-performance materials needed to achieve optimum device results, as well as strategies for integrating these incompatible substances. For a piezoresistive element, we have used a rare-earth monochalocogenide, samarium selenide. In thin film form, this gives a large dynamic range with pressures of a few GPa (1). Lead titanates were the basis for the piezoelectric films. Initial results have been obtained using commercial PZT films. Capabilities for more advanced piezoelectrics are underway, with a recently developed pathway to wafer-scale fabrication of PMN-PT by chemical solution deposition (2). Of course, combining an oxide piezoelectric with a rare-earth piezoresistor in one device creates an additional level of difficulty. This talk will also report electrical results for preliminary devices. 1) M. Copel et al, Nano Letters 13 (10), 4650-4653 (2013). 2) R. Keech et al, J. Appl. Phys. 115, 234106 (2014).

The organic-inorganic hybrid lead halide perovskite has a bandgap 1.5 eV and VOC of this perovskite-based solar cell was much greater than that of a dye-sensitized solar cell, rendering this photovoltaic system promising for further investigations. In this lecture, both n-type and p-type perovskite solar cells will be introduced based on varied structural configurations of the devices. Typical n-type device has a structural configuration FTO/TiOx/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au whereas that of a p-type device is configured as ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al. For n-type devices, varied mesoporous TiO2 nanostructures were applied to show the morphological effect of the scaffold on the device performance with a mesoscopic heterojuction; for p-type devices, varied additives were applied to control the formation morphology of the perovskite nanocrystals with a planar heterojunction. To understand the relaxation mechanism inside the perovskite solar cells, we carried out femtosecond optical gating (FOG) measurements for perovskite (CH3NH3PbI3) deposited on thin films of nanocrystalline TiO2, NiO and Al2O3 upon excitation at 450 nm. The emission transients of perovskite on semiconductor films were observed in the spectral region 650-810 nm. Measurements of power dependence on emission intensities vs excitation densities were also performed and an Auger-type energy transfer model was utilized to rationalize the observed relaxation dynamics. Photo-induced absorption spectra and nanosecond transient absorption kinetics were also performed to understand the electron-hole recombination rates responsible for the corresponding device performances.

A tandem dye-sensitized solar cell (tandem-DSSC) was synthesized on the basis of thin-film semiconductor electrodes. The nanoporous p-type NiO films were successfully obtained by simultaneous deposition of Al and Ni, followed by selective etching of Al and oxidation. Likewise, the n-type photoanode was made where Ag was etched in nitric acid after the initial formation of Ag/TiO2 nanocomposites. Such dye-sensitized photoelectrodes were combined to construct a tandem solar cell which exhibited an enhanced open-circuit voltage. Also, the tandem devices were subjected to various light fluxes to correlate the experimental cell parameters (open-circuit voltage, short-circuit current, fill factor, recombination shunt resistance, etc.) with the ideal one-diode model. Interestingly, impedance spectra of the tandem cell was well matched with the parameters from each of the n-type or p-type DSSC, indicative of successfully-designed tandem structure. [1] C. Nahm, H. Choi, J. Kim, S. Byun, S. Kang, T. Hwang, H. H. Park, J. Ko, and B. Park, Nanotechnology 24, 365604 (2013). [2] A. Nattestad, A. J. Mozer, M. K. R. Fischer, Y.-B. Cheng, A. Mishra, P. B�uerle, and U. Bach, Nat. Mater. 9, 31 (2010). Corresponding Author: Byungwoo Park: byungwoo@snu.ac.kr