Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P54: Organic Systems for Photovoltaics, Including PerovskitesFocus
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Sponsoring Units: GERA DPOLY FIAP Room: Hilton Baltimore Holiday Ballroom 5 |
Wednesday, March 16, 2016 2:30PM - 3:06PM |
P54.00001: Ovshinsky Sustainable Energy Fellowship: Excitonics for Transparent Photovoltaics Invited Speaker: Richard Lunt Room-temperature excitonic materials offer new opportunities for low-cost photovoltaic (PV) systems and provide prospects for unique solar harvesting science and applications. In the first part of this talk, I will introduce our pioneering work on developing transparent PVs that are creating a new paradigm for seamless solar harvesting around buildings, automobiles, and mobile electronics. These devices are enabled by the manipulation of excitonic semiconductor materials with selective harvesting in the nearinfrared and ultraviolet components of the solar spectrum. I will describe key photophysical properties, outline the thermodynamic and practical limits to these new classes of materials and devices, and briefly discuss their commercial impact for a range of applications. In the second part, I will describe the development of a new series organic salts that allow tunable photoresponse from 900nm to 1600nm, an unprecedented range for smallmolecule semiconductors. These organic salts also enable precise tuning of frontier orbital levels and heterojunction interface gaps through anion alloying that result in voltages near the thermodynamic limit. This design strategy can further enable rapid development of efficient and lowcost multijunction devices (both opaque and transparent) with complimentary response across the solar spectrum. [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P54.00002: The role of molecular layer mixing on the thermal conductance of organic-inorganic heterojunctions Shubhaditya Majumdar, Alan J.H. McGaughey, Jonathan A. Malen The role of interfacial properties in affecting energy transport characteristics is an extensive area of research. Hybrid materials composed of organic-inorganic heterojunctions are gaining popularity as alternatives to conventional semiconductors for various energy-generation devices, thus requiring detailed study of their interfacial properties -- especially thermal transport. Previous works have isolated the organic-inorganic interface thermal properties using self-assembled monolayer (SAM) junctions between two inorganic substrates and characterized them based on interfacial bonding strength, vibrational mismatch and molecule length. Here, we investigate the effect of having a mixed SAM layer on the thermal conductance of the SAM junction. The mixed SAM layers either have molecules of the same length but different end groups (thiols and methyl) or different lengths. This creates a modifiable bonding environment at one interface either through a varying ratio of strong and weakly bonded end groups or a decreasing surface coverage of the molecule. Both these scenarios are investigated to study the cooperative nature of the molecules/interface bonds and their effect on the heat transport across the junction. We follow a combined experimental and computational approach in our investigation -- we fabricate the SAM junctions (alkanethiols between two gold substrates) and measure their thermal conductance using Frequency Domain Thermoreflectance, and use molecular dynamics simulations to get a deeper understanding of the role of intermolecular cross talk. [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P54.00003: MOVED TO X33.011 |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P54.00004: Switchable Solar Window Devices Based on Polymer Dispersed Liquid Crystals Joseph Murray, Dakang Ma, Jeremy Munday Windows are an interesting target for photovoltaics due to the potential for large area of deployment and because glass is already a ubiquitous component of solar cell devices. Many demonstrations of solar windows in recent years have used photovoltaic devices which are semitransparent in the visible region. Much research has focused on enhancing device absorption in the UV and IR ranges as a means to circumvent the basic tradeoff between efficiency and transparency to visible light. Use of switchable solar window is a less investigated alternative approach; these windows utilize the visible spectrum but can toggle between high transparency and high efficiency as needed. We present a novel switchable solar window device based on Polymer Dispersed Liquid Crystals (PDLC). By applying an electric field to the PDLC layer, the device can be switched from an opaque, light diffusing, efficient photovoltaic cell to a clear, transparent window. In the off state (i.e. scattering state), these devices have the added benefits of increased reflectivity for reduced lighting and cooling costs and haze for privacy. Further, we demonstrate that these windows have the potential for self-powering due to the very low power required to maintain the on, or high transparency, state. [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 3:54PM |
P54.00005: Discovery of Novel Perovskites for Solar Thermochemical Water Splitting from High-Throughput First-Principles Calculations Antoine Emery, Chris Wolverton Among the several possible routes of hydrogen synthesis, thermochemical water splitting (TWS) cycles is a promising method for large scale production of hydrogen. The choice of metal oxide used in a TWS cycle is critical since it governs the rate and efficiency of the gas splitting process. In this work, we present a high-throughput density functional theory (HT-DFT) study of ABO$_3$ perovskite compounds to screen for thermodynamically favorable two-step thermochemical water splitting materials. We demonstrate the use of two screens, based on thermodynamic stability and oxygen vacancy formation energy, on 5,329 different compositions to predict 139 stable potential candidate materials for water splitting applications. Several of these compounds have not been experimentally explored yet and present promising avenues for further research. Additionally, the large dataset of compounds and stability in our possession allowed us to revisit the structural maps for perovskites. This study shows the benefit of using first-principles calculations to efficiently screen an exhaustively large number of compounds at once. It provides a baseline for further studies involving more detailed exploration of a restricted number of those compounds. [Preview Abstract] |
Wednesday, March 16, 2016 3:54PM - 4:06PM |
P54.00006: Super-ion inspired colorful hybrid perovskite solar cells. Hong Fang, Puru Jena Organic-inorganic hybrid perovskites, with the general formula AMX$_{\mathrm{3}}$ (A$=$cation; M$=$metal; X$=$halogen), have emerged as a new generation of efficient yet inexpensive photovoltaic cells. These materials show record high conversion efficiency as solar cells and have excellent light-emission properties that can also be used in other optoelectronic devices. They can be processed easily from solution with optic band gaps, tunable from visible to infrared regions and are considered to be ``the next big thing in photovoltaics''. However, several important issues such as the relationship between their photoexcitation properties and the chemical structures, their stability under ambient conditions, as well as the possibility to invent their environment-friendly analogues remain unsolved. In this work, our aim is not only to gain a fundamental understanding of the structure-property relationship of organic-inorganic hybrid perovskites, but also to rationally design a new class of hybrid perovskites that have desired electronic band gaps for solar cell applications. This is accomplished by using super-ions that can mimic the properties of elementary alkali and halogen ions as building blocks. These super-ions include superalkalis -- CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$, HC(NH$_{\mathrm{2}})_{\mathrm{2}}^{\mathrm{+}}$, and Li$_{\mathrm{3}}$O$^{\mathrm{+}}$ as cations and hyperhalogens -- Ge(BH$_{\mathrm{4}})_{\mathrm{3}}^{\mathrm{-}}$ and Sn(BH$_{\mathrm{4}})_{\mathrm{3}}^{\mathrm{-}}$ as anions. The results are compared with perovskites composed of GeCl$_{\mathrm{3}}^{\mathrm{-}}$, GeBr$_{\mathrm{3}}^{\mathrm{-}}$, GeI$_{\mathrm{3}}^{\mathrm{-}}$, SnCl$_{\mathrm{3}}^{\mathrm{-}}$, SnBr$_{\mathrm{3}}^{\mathrm{-}}$, and SnI$_{\mathrm{3}}^{\mathrm{-}}$ superhalogen anions. We develop a strategy to assemble these super-ions to form environment-friendly solar cells with adjustable band gaps (covering the visible range and beyond) and with improved resistance to moisture. [Preview Abstract] |
Wednesday, March 16, 2016 4:06PM - 4:18PM |
P54.00007: High efficiency graded band gap perovskite solar cells Onur Ergen, Sally Demaio-turner, Thang thoan pham, Mark Tian Zhi Tan, Jongmin Yuk, Alex Zettl We report high efficiency graded band gap perovskite solar cells with very large current output and high power conversion efficiencies (PCE) by using simultaneously mixed halides (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$SnI3 and CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$-$_{\mathrm{x}}$Br$_{\mathrm{x}})$ perovskite absorber layers. An analysis of the experimental data yields a high fill factor (FF) of \textasciitilde 75{\%} and high short circuit current density (J$_{\mathrm{sc}})$ of up to 46.2 mA/cm$^{\mathrm{2}}$. These devices provide the highest current output aiming above 20{\%} PCE. [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P54.00008: Efficient organic-inorganic hybrid perovskites and doped metal oxide heterojunction solar cells. Xiaojuan Fan Organic-Inorganic hybrid perovskite CH3NH3PbI3 has recently attracted much attention for its high efficient solar energy conversion. This semiconducting pigment with a direct bandgap of 1.55 eV has made it an interesting optical and electronic material over the whole visible solar emission spectrum. The role of hole conducting has been found in this semiconductor that allows perovskite solar cell (PSC) to be formed by CH3NH3PbI3/TiO2 heterojunctions that use TiO2 as scaffold, and carbon as a back contact. We will report a double layer metal doped TiO2/Al2O3 mesoporous scaffold covered by the p-type semiconducting pigment to form a high efficient PSC through solution method. TiO2 and Al2O3 are both large band gap semiconductors that affect conducting and recombination rate in solar cells. One improvement work is doping other metal elements in TiO2 to raise the mobility while extend the recombination time. It has suggested that optimal amounts of doped metals such as Cu, Co, Mn can suppress the reduction of Ti4$+$ resulting better transportation. TiO2 thin films doped with metals are subjected to the EPR analysis and the results will be correlated with measurements of electronic-optical properties. [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P54.00009: Modeling morphology dependence of the power generation in bulk heterojunction organic photovoltaics Timothy Schlittenhardt, Selman Hershfield Bulk heterojunctions are mixtures of differently doped organic semiconducting materials that provide for a highly interconnected and complex morphology. A three dimensional simulation is conducted of these systems, where the junctions are modeled by diodes with a given j-V characteristic and the transport within a particular material is treated as ohmic. The current and potential profile are calculated throughout the sample with an iterative method that allows us to readily treat systems with ~10$^4$ sites for a full range of applied voltage biases. Visualizations of the current flow and voltage profile are given. It is found that power is not generated uniformly throughout the sample, but is concentrated near the edges. As has been observed experimentally, this leads to an optimal thickness for power generation. A simple analytical model is presented which reproduces and provides understanding of our simulation results. [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P54.00010: Reliable thermal processing of organic perovskite films deposited on ZnO Alex Zakhidov, Chris Manspeaker, Dmitry Lyashenko Zinc oxide (ZnO) is a promising semiconducting material to serve as an electron transport layer (ETL) for solar cell devices based on organo-halide lead perovskites. ZnO ETL for perovskite photovoltaics has a combination of attractive electronic and optical properties: i) the electron affinity of ZnO is well aligned with valence band edge of the CH$_{3}$NH$_{3}$PbI$_{3}$, ii) electron mobility of ZnO is \textgreater 1 cm$^{2}$/(Vs), which is a few orders of magnitude higher than that of TiO$_{2}$ (another popular choice of ETL for perovskite photovoltaic devices), and iii) ZnO has a large of band gap of 3.3 eV, which ensures optical transparency and large barrier for the hole injection. Moreover, ZnO nanostructures can be printed on flexible substrates at room temperatures in cost effective manner. However, it was recently found that organic perovskites deposited on ZnO are unstable and readily decompose at \textgreater 90\textdegree C. In this work, we further investigate the mechanism of decomposition of CH$_{3}$NH$_{3}$PbI$_{3}$ film deposited on ZnO and reveal the role of the solvent in the film during the annealing process. We also develop a restricted volume solvent annealing (RVSA) process for post annealing of the perovskite film on ZnO without decomposition. We demonstrate that RVSA enables reliable perovskite solar cell fabrication. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:06PM |
P54.00011: Charge Generation Dynamics in Efficient All-Polymer Solar Cells: Influence of Polymer Packing and Morphology Bhoj Gautam, Changyeon Lee, Robert Younts, Wonho Lee, Evgeny Danilov, Bumjoon Kim, Kenan Gundogdu All-polymer solar cells exhibit rapid progress in power conversion efficiency (PCE) from 2 to 7.7{\%} over the last few years. While this improvement is primarily attributed to efficient charge transport and balanced mobility between the carriers, not much is known about the charge generation dynamics in these systems. Here we measured exciton relaxation and charge separation dynamics using ultrafast spectroscopy in polymer/polymer blends with different molecular packing and morphology. These measurements indicate that preferential face on configuration with intermixed nanomorphology increases the charge generation efficiency.~ In fact there is a direct quantitative correlation between the free charge population in the ultrafast time scales and the external quantum efficiency, suggesting not only the transport but also charge generation is key for the design of high performance all polymer solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 5:06PM - 5:18PM |
P54.00012: Phonon Mode Transformation across the Orthohombic-Tetragonal Phase Transition in a Lead-Iodide Perovskite CH$_{3}$NH$_{3}$PbI$_{3}$: a Terahertz Time-Domain Spectroscopy Approach Elbert E. M. Chia, Chan La-o-vorakiat, Jeannette Kadro, Teddy Salim, Daming Zhao, Towfiq Ahmed, Yeng Ming Lam, Jian-Xin Zhu, Rudolph Marcus, Maria-Elisabeth Michel-Beyerle Using terahertz time-domain spectroscopy (THz-TDS), we study the temperature-dependent phonon modes of the organometallic lead iodide perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ thin film across the terahertz (0.5-3 THz) and temperature (20-300 K) ranges. These modes are related to the vibration of the Pb-I bonds. We found that two phonon modes in the tetragonal phase at room temperature split into four modes in the low-temperature orthorhombic phase. By use of the Lorentz model fitting, we analyze the critical behavior of this phase transition. [Preview Abstract] |
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