The 2015 MRS Spring Meeting and Exhibit was held in San Francisco, California. Technical symposia were held within the following topical clusters: Energy, Nanomaterials, Electronics and Photonics, Soft and Biomaterials, and General – Fabrication and Characterization. Access to all of the content below is free – simply login with your MRS username and password for complete access. Don’t have a username and password? Create a free account.
Using calorimetric techniques, we have shown that differences on surface energies of various phases lead to large changes in thermodynamic stability among phase assemblages, indeed sometimes startlingly, as small particles are stable in structures that are metastable in the bulk. These changes impact geochemical and biogeochemical processes, the transport of heavy metals in the environment, catalysis, and the optimization of materials for energy applications. Such effects were illustrated in three families of materials: iron and manganese oxides, lithium battery cathode materials and cluster compounds of elements ranging from aluminum to uranium.
Pb halide based perovskite materials are important for photovoltaic devices. In this paper, we systematically discuss the influence of different processing conditions of perovskite devices for both p-i-n and n-i-p configurations. In a p-i-n device, light enters from the p side whereas in the n-i-p device, light enters from the n side. We discuss properties of materials and devices produced using three processing conditions : complete solution growth, partial solution growth, and complete vapor growth where no liquids are used. The properties measured include grain size, structure using x-ray spectrum, deep defects using capacitance-frequency-temperature spectroscopy, drift mobility of holes, type of doping ( p or n), which carrier controls transport and minority carrier diffusion length measured directly using electronic measurements. We show that the complete vapor grown device has the lowest deep defect density, the largest grains size, and a high photovoltaic efficiency of ~15%. We also discuss the differences in properties of materials devices fabricated using either methyl-ammonium iodide (MAI) or formamidinium iodide (FAI). We show that the materials and devices prepared using the vapor process for FAI are far more stable physically and allow a much larger processing range, including processing in vacuum at higher temperatures. In contrast, the devices produced with MAI decompose in vacuum at elevated temperatures. The differences in physical properties between these two classes of materials lead to much better stability for the devices prepared using FAI compared to devices prepared using MAI. We also investigate systematically the influence of Chlorine (eg use of PbCl2 instead of PbI2)during the vapor deposition process for devices. We do not find that Cl has any influence on the electronic properties of the material when the grain size is significant. We find that the material is n type independently of the substrate on which it is deposited, and that the transport is unambiguously controlled by holes and not electrons. Therefore it is inaccurate to say that the device is controlled by ambipolar transport. We also find that the drift mobility of holes, measured using time of flight techniques, is ~10-1 cm2/V-s. The Urbach energy of valence band tails is~16 meV.Speaker(s):
Operational lifetime is a crucial performance feature for many electronic devices such as organic light emitting diodes (OLEDs). One of the many failure mechanisms which limit OLED lifetime is the electromigration of metallic components of the electrode materials, forming short circuits through the device architecture. Metals such as silver, copper or aluminium are e. g. used in the highly conductive shunting lines which are necessary to reduce resistive losses in the transparent electrodes of large area OLED panels. A direct study of the effects of metal electromigration in devices, however, is complicated by the interference with other failure mechanisms, as well as by the very localised formation of short circuits, which renders the application of imaging techniques highly challenging. In this contribution, we present a study of silver and copper electromigration through PEDOT:PSS films, a conductive polymer which is frequently used as replacement for indium tin oxide as transparent electrode material in OLEDs. Due to the experimental setup, electromigration can be studied isolated from other phenomena, and in addition, the formation of short circuits by metal dendrite growth can be easily monitored using electrical current monitoring and optical microscopy. We have studied both experimentally and theoretically the influence of various parameters such as the electric field strength, electrode material and intrinsic electrical conductivity of the PEDOT:PSS on the speed of short circuit formation and on the appearance of the formed metal dendrites. Silver was found to migrate at a much higher rate than copper, and a high current density through the PEDOT:PSS was found to increase the migration rate, which is consistent with the assumption of metal migration being induced by the force exerted on the metal atoms by the moving charge carriers. The results from our study have been correlated with lifetime studies in functional OLED devices. Currently, our experimental setup is used to develop strategies to suppress or slow down metal electromigration and thus improve OLED lifetime.Speaker(s):
The Australian Centre for Advanced Photovoltaics (ACAP) co-ordinates the activities of the Australian partners in the Australia-US Institute for Advanced Photovoltaics (AUSIAPV), supported by the Australian Renewable Energy Agency, to develop the next generations of photovoltaic technology and to provide a pipeline of opportunities for performance increase and cost reduction. The Australian partners in ACAP are UNSW, ANU, University of Melbourne, Monash University, University of Queensland and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) with industrial partners Suntech R&D Australia (now Wuxi Suntech), Trina Solar, Bluescope Steel and BT Imaging. AUSIAPV links ACAP with US-based partners, specifically the NSF/DOE Energy Research Center for Quantum Energy and Sustainable Technologies (QESST), the National Renewable Energy Laboratory, Sandia National Laboratories, Lawrence Berkeley National Laboratories, Stanford University, Georgia Institute of Technology and University of California Santa Barbara. These national and international research collaborations provide a pathway for highly visible, structured photovoltaic research collaboration between Australian and US researchers, institutes and agencies with significant joint programs based on the clear synergies between the participating organizations. The research program is organised under five collaborative Program Packages (PP1-PP5). PP1 deals with silicon wafer-based cells, focussing on three main areas: cells made from solar grade silicon, rear contact and silicon-based tandem cells. Program Package 2 (PP2) involves research into a range of organic solar cells, organic/inorganic hybrid cells, "earth abundant" thin-film materials, including Si and CZTS, and more futuristic "third generation" approaches, with the overall goal of demonstrating efficiency above 15% during the program for cells of above 1cm2 area and of demonstrating the feasibility of costs below the US SunShot targets. PP3, concerned with optics and characterisation, targets experimental demonstration that theoretical conversion limits can be increased by the use of structures that have a high local density of optical states, with particular emphasis on thin film organic and inorganic solar cells. PP4, manufacturing issues, will deliver a substantiated methodology for assessing manufacturing costs of the different technologies under investigation by the ACAP/AUSIAPV partnership. The overall cost target is to exceed the SunShot targets, for one or more of the technologies, in at least one major SunShot targeted application, as deduced by a substantiated costing methodology. PP5 involves education, training and outreach. ACAP/AUSIAPV began in February 2013 and will have an eight-year life. National and international partnerships have been and are being formed and significant results have been generated already. The main research topics, results and plans for the future will be presented.Speaker(s):
Computer keyboard is one of the most common, reliable, accessible and effective approaches used for human-machine interfacing and information exchange. Accessing the information provided by computer from internet dictates the quality, efficiency and happiness of our work and life. A keyboard, an indispensable component of the system, is the only means for information input and control for many purposes such as information recording/outputting, financial management, bill payment, personal communications and many more. With this regard, the heavy reliance on computer incurs a major concern for its security issue.Although keyboard has been used for hundreds of years for advancing human civilization, studying human behavior by keystroke dynamics using smart keyboard remains a great challenge. Here we report the first intelligent, self-powered, non-mechanical-punching keyboard enabled by contact electrification between human fingers and keys, which converts mechanical stimuli applied onto the keyboard into local electronic signals without applying an external power. The intelligent keyboard (IKB) can not only sensitively trigger a wireless alarm system once gentle finger tapping occurs but also be capable of tracing and recording typing contents by detecting both the dynamic time intervals between and during inputting letters and the force used for each typing action. Such features promise its use as a smart security system that can realize detection, alert, recording, and identification. Moreover, the IKB is able to identify personal characteristics from different individuals if assisted by behavioral biometric of keystroke dynamics. Furthermore, the IKB can effectively harness typing motions for electricity to charge commercial electronics at arbitrary typing speed larger than 100 characters per min, with an area power density of 69.6 mWcm-2. Given the above features, the IKB can be potentially applied not only to self-powered electronics but also to artificial intelligence, cyber security, and computer or network access control. The justified concepts and demonstrations in this work can be immediately and extensively adopted in a variety of applications, and come into effect of improving the way of our living. References: (* indicate co-first author). 1.J. Chen*, G. Zhu*, J. Yang*, Q. Jing, P. Bai, W. Yang, X. Qi, Y.Su and Z.L. Wang.Science, under review. 2.J. Chen*, G. Zhu*, W. Yang, Q. Jing, P. Bai, Y. Yang, T. C. Hou and Z. L. Wang. Adv. Mater. 25 (2013), 6094ï¿½6099 3.W. Yang*, J. Chen*, G. Zhu, J. Yang, P. Bai, Y. Su, Q. Jing and Z. L. Wang.ACS Nano 7 (2013),11317-11324 4.G. Zhu*, J. Chen*, Y Liu, P Bai, Y Zhou, Q Jing, C Pan, ZL Wang. Nano letters 13 (2013), 2282-2289 5.G. Zhu*, J. Chen*, T. Zhang, Q. Jing and Z. L. Wang. Nat. Commun. 5 (2014), 3426 6.J. Yang*, J. Chen*, Y. Yang, H. Zhang, W. Yang, P. Bai, Y. Su and Z. L. Wang. Adv. Energy Mater. 4 (2014), 1301322Speaker(s):
Organometal halide perovskites have recently attracted enormous attention since CH3NH3PbX3 can be successfully applied as photoactive material in photovoltaic devices, yielding solar cells with an efficiency exceeding 15%. Surprisingly, the exact mechanism how charges are generated and extracted so well is unclear. In this paper, we report the dynamics of electron injection from CH3NH3PbI3 into PCBM and hole injection into Spiro-OMeTAD, using time resolved microwave conductivity measurements. For intrinsic CH3NH3PbI3 deposited on an inert substrate we observed fast formation of microsecond lived charge carriers. At low laser fluences a maximum charge carrier mobility of about 5 cm2/Vs, yielding charge carrier diffusion lengths well above 5 ?m is found. In a CH3NH3PbI3/PCBM bilayer electron injection into PCBM occurs on a sub-ns timescale, similar as has been found for TiO2. In contrast in a CH3NH3PbI3/Spiro-OMeTAD bilayer, hole injection into Spiro-OMeTAD is much slower, extending up to a few hundreds of ns. This large difference in dynamics is related to the type of junction formed at both interfaces. Furthermore, the low conductivity of Spiro-OMeTAD causes the hole to be essentially immobile at the interface enabling fast recombination with electrons residing in the perovskite. These results highlight the need to optimize the conductivity of hole transporting materials in order to enhance the hole extraction to push the overall power conversion efficiency further.Speaker(s):
The generally short lifetimes of organic light-emitting diodes (OLEDs) presents a challenge to their widespread acceptance for use in large-area displays and solid-state lighting. A greater understanding of the degradation mechanisms would help to further improve the reliability of OLEDs particularly at high brightness levels by optimizing the material selection and structural design, and pave the way for their broader applications as lighting sources. In this work, we studied the stability of green phosphorescent OLEDs with different structures under constant-current (20-50 mA/cm2) stressing. Through the modifications of the ITO anode by different plasma treatments and the hole transport layer (HTL) by incorporating inorganic component or dopants, we proved that energy level misalignment at the ITO/HTL interface leads to localized joule heating, accelerating defect generation and luminescence decay. Pulsed current stressing was then employed to suppress the joule-heating effect so as to differentiate the thermal and nonthermal factors governing the device degradation. The luminance evolution comprised an initial rapid decay regime and a subsequent slow decay regime, and only the latter was governed predominantly by electrical excitation. In OLEDs with an appropriate energy level alignment at the ITO/HTL interface, pulsed stressing with 10% duty cycle only improved the effective half life by ~15% as compared to continuous-wave stressing, indicating a minor role played by joule heating.Speaker(s):
The intrinsic mechanisms of OLED degradation are investigated in this work. An increase of voltage and a decrease of luminance and efficiency of OLED under accelerated degradation conditions (i.e. constant high current) were observed in the experimental analysis. Degradation of OLED can be due to the decrease of charge carrier mobility, the change of electrode contact, or the formation of more traps. In this work an increase in the luminance ideality factor of OLED in diffusion regime has been observed from pristine to degraded device. Such increase of this value can either be interpreted as the deterioration of contact, which will change the current into injection limited. Or it can due to the formation of more traps, which brings the trap-assisted recombination more dominant. Further experiments have been carried out where the dependence of corresponding open-circuit voltage (Voc) on the incident light intensity (I) was investigated. From the 2.0kT/q enhancement of the Voc dependence on the logarithmic incident light intensity in the aged devices we could eliminate the influence of electrode contact change. Furthermore the formation of more traps during degradation is being investigated.Speaker(s):
By placing a wide band gap semitransparent thin film solar cell in front of a high-efficiency crystalline silicon (c-Si) cell, the practical c-Si single junction conversion efficiency limit of about 26% can be surpassed, which is attractive for further reduction of systems costs provided that the thin film cell is inexpensive and does not have too much parasitic optical absorption or shadow losses. We have investigated by computer simulation (using thin film optics and ray-tracing models) the optical behavior of several hybrid junction stacks and have evaluated their relative potential. As the module concept we adopted the 4-terminal approach, as this avoids constraints regarding band gap values and absorber thicknesses, while in later production also constraints regarding processing temperatures and chemicals used can be avoided in this way. The cost of substrates and encapsulants is eliminated by the concept of depositing the thin film solar cell on the inside of the c-Si module cover glass. We considered three types of thin film PV cells: 1. Enlarged-bandgap oxygenated amorphous silicon (a-SiO:H) cells, 2. Wide band gap chalcopyrite (CuGaSe2) cells, and 3. Perovskite (CH3NH3PbI3) cells. In our model we also included the change in electrical output parameters by the color-filtering effect of the top cells. Our modelling showed a few interesting results. Starting with an interdigitated back contact (IBC) c-Si cell with a conversion efficiency of 20%, and adding the top cell while minimizing the reflection and parasitic absorption losses by choosing proper thicknesses of absorber and contacting layers and implementing light scattering by texturing the glass, we find that +2.0% (absolute) conversion efficiency can be gained using state-of-the-art a?SiO:H (band gap of 2.0 eV) and CuGaSe2 (1.8 eV) top cells and +4.75% can be gained using perovskite cells (1.55 eV). The perovskite cell type appears to be particularly suitable as the overall reflection loss is small due to the low refractive index of the absorber layer. Moreover, in the case of a perovskite top cell, light trapping by texturization of surfaces is not needed which helps avoiding parasitic absorption losses.Speaker(s):
Silicon, with a high specific capacity ~ 3,579 mAh/g, has been widely investigated as anode materials in lithium ion batteries . There are few well know issues preventing the commercialization of silicon for this application, such as pulverization induced by volume expansion, low electrical conductivity, unstable solid electrolyte formation upon cycling. These issues have been addresses to some extent, although many of the proposed nanostructures that solve these problems are realized using slow and difficult to scale techniques. We propose a simple, scalable method to produce a metal-silicon nano-structure and we verify its applicability as anode material for lithium-ion batteries. NiO-Si core shell particles are synthesized utilizing a one-step spray pyrolysis method starting from a mixture of silicon nanoparticles and NiCl2.6H2O water based precursor. After coating, the core shell NiO-Si structure is annealed either at low temperature (1240 mAh/g, silicon basis) for 110 cycles at 0.5 C discharge rate. The amorphous carbon coating successfully prevents the silicon nanoparticles inside the shell from directly contacting the electrolyte during cycling. We verified this by testing the battery performance with and without fluoroethylene carbonate (FEC) additive, finding little to no change in capacity and stability. In the case of silicon in direct contact with electrolyte, FEC additive helps forming thinner and more stable SEI on silicon surface improving the cycling performance . In addition to its scalability, another advantage of this technique is its potential applicability to other material systems, such as NiO-Sn, NiO-SnO etc.  M. N. Obrovac and L. Christensen, Electrochemical and Solid State Letters 2004, 7, A93-A96.  C. C. Nguyen and B. L. Lucht, Journal of The Electrochemical Society 2014, 161(12), A1933-A1938.Speaker(s):