Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K47: Photovoltaics -- Solar Energy Conversion I |
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Chair: Vivian Ferry Room: BCEC 213 |
Wednesday, March 6, 2019 8:00AM - 8:12AM |
K47.00001: Photovoltaics for High Specific Power: Increasing Markets and Decreasing Package Weight Nancy Haegel Emerging markets for lightweight, flexible, and portable power, including unmanned aerial vehicles, portable charging, remote-site power generation, vehicle-integrated and building facades, could benefit from photovoltaic technologies with high specific power (W/kg). Thin-film and emerging technologies offer advantages for lightweight, flexible power, including decreased cost and package weight and the opportunity to utilize new materials and lift-off techniques. We have assessed the role of the substrate, packaging, and interconnects to provide a quantitative assessment of designs to maximize specific power. Including all required components and weight limitations associated with safety and reliability, we estimate a lower bound for a durable lightweight module of ~ 300-500 g/m2. For a thin film device with 15% efficiency, this would yield up to 500 W/kg and up to 1200 W/kg for a 35% for a high efficiency III-V multijunction device [1]. |
Wednesday, March 6, 2019 8:12AM - 8:24AM |
K47.00002: Stability and the Electronic Properties of Silicon-rich Silicon Carbide Structures by First Principle Calculations Noura Alkhaldi, Sajib Barman, Muhammad Nurul Huda Silicon carbide has been used in a variety of applications such as solar cells material due to its high stability. Obtaining silicon-rich silicon carbide materials are necessary to tune the band gap for efficient solar light absorptions. In addition, thermodynamically stable Si-rich SiC materials can be used in solar cell applications without requiring the expensive pure grade silicon or pure grade silicon carbide materials. We have used density functional theory (DFT) calculations to examine different phases of silicon-rich silicon carbide to predict stable structures. Different configurations of silicon and carbon atoms in silicon-rich silicon carbide structures have been considered because the configurations play a significant role in getting stable results. The electronic structures have been studied, and the total energies have been calculated as well as the formation energies. These results will be presented. The results show that higher-order hexagonal-phases are more favorable structures than other silicon-rich silicon carbide structures due to their more covalent nature of bonding compared to the cubic counterpart. |
Wednesday, March 6, 2019 8:24AM - 8:36AM |
K47.00003: Toward the rational design of organic solar photovoltaics: A DFT study of substitutent effects on P3MT David Perry, Sandile Mamba, Guiseppe Pellicane, Mesfin Tsige Organic polymers containing conjugated thiophene rings are among the candidates for the electron-donor materials in organic solar cells. Since material synthesis, device fabrication, and definitive characterization of those devices is tedious and expensive, it is desirable to apply computational methods to a systematic variation of the material chemistry to predict which materials will have the best properties and, potentially, which will yield the highest photon conversion efficiency. Reported here is a model study in which DFT calculations of poly-3-methyl thiophene (P3MT) with a range of 12 different substituents. For each candidate polymer, DFT calculations at the B3LYP-D2/6-31G(d,p) level were extrapolated to the long-chain limit. The following properties relevant to application in a photovoltaic device were estimated from the calculations: (i) the bandgap, (ii) the LUMO energy, and (iii) the steric hindrance to coplanarity of adjacent rings. While (ii) was found to be well correlated with the electron-donating property of the various substituents, (i) and (iii) were much less so indicating a design space in which these three critical properties could, to some extent, be varied independently. |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K47.00004: Quantitative Nanoscale Mapping of Photovoltaic Properties in Hybrid Organic/Inorganic Solar Cells Haian Qiu, Jong Hyun Shim, Junghyun Cho, Jeffrey M. Mativetsky Conductive atomic force microscopy (C-AFM) has been widely used to map local charge transport in functional materials and photovoltaic active layers. Recently, we have developed and employed C-AFM-based point-by-point current-voltage (PPIV) mapping to quantitatively investigate electrical properties such as local charge carrier mobility and to visualize local spatial variations in photovoltaic parameters such as open-circuit voltage, power conversion efficiency, and charge photogeneration rate. In this talk, we will present two examples in which PPIV mapping is employed to elucidate the influence of local morphology on photovoltaic properties in hybrid organic-inorganic systems. In the case of P3HT: ZnO nanorod active layers, photovoltaic properties are strongly dependent on the local P3HT hole transport layer thickness. Charge generation rate and charge collection probability maps reveal that photocurrent is mainly limited by charge collection. For inverted perovskite solar cells, PPIV photovoltaic characteristic maps exhibit an increased open-circuit voltage at crystal grain boundaries, indicating an important role played by these features. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K47.00005: Degradation Mechanisms in Perovskite Solar Cells Probed by Low-Frequency Carrier Kinetics Vinod Sangwan, Menghua Zhu, Sarah Clark, Kyle Luck, Tobin Marks, Mercouri Kanatzidis, Mark Hersam Hybrid organic-inorganic perovskite solar cells have emerged as leading candidates for third-generation solar cell technology. Despite their superlative power conversion efficiencies (PCEs), hysteresis and degradation limit their applications, thus motivating detailed studies of the underlying physical mechanisms. We introduce correlated low-frequency noise and impedance spectroscopy characterization that reveals carrier kinetics in perovskite solar cells. We employ cells with different hole transport layers that also elucidate tradeoffs between solar cell performance metrics and stability. We focus on the technologically relevant planar cell structure using an emerging SnO2 electron transport layer and two widely used hole transport layers: poly(triarylamine) (PTAA) and Spiro-OMe TAD. PTAA and Spiro-OMe TAD cells with moderate PCEs of 5–12% show a Lorentzian feature at ~200 Hz corresponding to a single fluctuator. Spiro-OMe TAD cells with high PCE (>15%) show four orders of magnitude larger 1/f noise amplitude with a distinctive peak, which is indicative of a cyclostationary process that is correlated with an inductive loop in impedance spectra. The observed current fluctuations are consistent with trapping and de-trapping of methylammonium ions near the SnO2 interface. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K47.00006: Domain boundaries in the dipolar order in the perovskite material CH3NH3PbI3 Sahel Ashhab, Marcelo Carignano, Mohamed E. Madjet We investigate the ordering of the CH3NH3 dipoles in the material CH3NH3PbI3. The dipoles are arranged in a simple cubic lattice. We perform numerical simulations in which we set the boundary conditions such that opposite sides of the simulated sample are ordered in different directions, hence simulating a domain boundary. We calculate the lowest energy state under this constraint. We find that at the level of dipole-dipole interactions, the dipole orientations tend to gradually transform between the two orientations at the two ends of the sample. When we take into consideration the finite spatial size of the CH3NH3 molecules and go beyond the point dipole approximation, we find that the domain boundary becomes sharper. For the parameters of CH3NH3PbI3, our results indicate that the optimal energy structure has a boundary region of a width on the order of a single unit cell. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K47.00007: Efficient first-principles calculation of phonon assisted photocurrent in large-scale
solar cell devices Mattias Palsgaard, Kurt Stokbro, Troels Markussen, Tue Gunst, Mads Brandbyge We present a straightforward and computationally cheap method to obtain the phonon-assisted photocurrent in large-scale devices from first-principles transport calculations[1]. The photocurrent is calculated using nonequilibrium Green's functions with light-matter interaction from the first-order Born approximation while electron-phonon coupling (EPC) is included through special thermal displacements (STD). We apply the method to a silicon solar cell device and demonstrate the impact of including EPC in order to properly describe the current due to the indirect band-to-band transitions. The first-principles results are successfully compared to experimental measurements of the temperature and light intensity dependence of the open-circuit voltage of a silicon PhotoVoltaic (PV) module[1]. We use the method to predict the solar cell efficiency of new Janus type 2D devices[2] and show that they outperform the silicon PV module. This work represents a recipe for computational characterization of future PV devices including the combined effects of light-matter interaction, phonon-assisted tunneling and the device potential at finite bias from the level of first-principles simulations. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K47.00008: Highly efficient Non-Fullerene Organic Solar Cell with Fine Tuned Hole Transporting Layer Qin Hu, Zhong Zheng, Wenkai Zhong, jianhui hou, Huiqiong zhou, Feng Liu, Thomas Russell Non-fullerene organic solar cells have experienced rapid development in the past few years with their impressive optoelectronic properties and great potential in practical applications. Here, we present a facile and effective strategy to improve the device performance through hole-transporting layer (HTL) modification. By optimizing Wox (nanoparticles): PEDOT:PSS composition, the surface free energy of the HTL is improved, hence influences the crystallization mechanism of PBDB-TF:IT4F bulk-heterojunction (BHJ) active layer. In-situ grazing incident X-ray diffraction (GIXD) is applied to monitor the crystallization kinetics and morphology formation of the active layer based on different HTLs. The crystal coherence lengths and lamellar stacking distance as well as the phase separation are adjusted by various HTLs. In addition, the optimized HTL can promote more balanced carrier transport ability, leading to reduced non-radiative recombination and higher fill factor. The crystallization to structure, and film property to device performance relationship are established. A power conversion efficiency of 14.2% is achieved based on laboratorial spin-coating process, while an efficiency of 11.9% is obtained via industrial comparable slot-die printing fabrication. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K47.00009: Conduction Band Matching in CdSe – Metal Oxide Quantum Dot Solar Cells: The Competition Between Energetics and Electron Transfer Kinetics Matthew Becker, Sam Ayala Quantum dot solar cells were constructed by pairing CdSe quantum dots with metal oxides whose conduction band is very close to that of the CdSe. We hypothesized that the reduced loss of energy due to the closely matched conduction bands would result in a higher open circuit voltage. It was found that the band-matched solar cells perform more poorly and that the improved electron kinetics resulting from a large difference between CdSe and metal oxide conduction band energies seems to have a larger influence on the efficiency of these quantum dot solar cells. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K47.00010: How did solar energy get so cheap, and how much cheaper can it get? Harry Apostoleris, Marco Stefancich, Matteo Chiesa In the last several years, the cost of electricity from photovoltaics has fallen to the point where it is now the cheapest source of electricity across large parts of the world. Understanding how this happened is essential to guiding future research in solar energy technology. We have conducted a detailed analysis of the technological and economic factors that led to the realization of ultra low solar electricity prices in the United Arab Emirates, Saudi Arabia, Chile, Mexico and the southwestern US. We show that the primary influences are the declining cost of hardware (PV modules, inverters, trackers) and the low cost of capital available to these projects, combined with local factors such as labor costs and reductions in soft costs.In this presentation we will demonstrate the relative impact of these factors in different locations using a bottom-up LCOE model, and discuss their likely future trends (i.e. future evolution of hardware prices, or the expected impact of interest rate variations on the cost of financing). In this way we aim to provide a "big picture" context for today's solar energy research. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K47.00011: Electrostatic Potential Fluctuations in Thin-Film Photovoltaics Harvey Guthrey, John Moseley, Mowafak Al-Jassim Increasing the conversion efficiency of photovoltaics based on thin-film absorbers materials requires minimizing the open circuit voltage (Voc) deficit (difference between the band gap and Voc) and producing films with sufficient charge carrier concentrations. However, inherent to these materials are high concentrations of intrinsic point defects and intentional extrinsic defects that can result in 1) significant charge carrier compensation thus reducing the carrier concentration and 2) fluctuations in the local electrostatic potential that define the band structure resulting in increased charge carrier recombination and low Voc values. Understanding both the magnitude and spatial distribution of such potential fluctuations is necessary in order to refine material and device fabrication processes to achieve the highest photovoltaic conversion efficiency. In this contribution, we demonstrate how cathodoluminescence microscopy can be used to monitor such changes in CdSeTe and CuInGaSe2 thin-films with sub-100nm spatial resolution. In addition we discuss the physical mechanisms behind how atomic-scale compositional variations relate to the magnitude and spatial distribution of electrostatic potential fluctuations in these materials. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K47.00012: Detection of shot noise in perovskite solar cells and related devices Kevin Davenport, Mark Hayward, Logan Draper, Andrey Rogachev Hybrid organic-inorganic perovskite solar cells are one of the most promising emerging technologies with the capability to compete with established silicon devices. The effective commercial rollout of perovskite-based devices, however, requires a fundamental understanding of the material’s electrical transport properties. We have performed current noise spectroscopy on a series of methylammonium lead triiodide perovskite solar cells. Under illumination, the noise power spectrum exhibits two main components: a 1/fa flicker noise and a frequency-independent white noise. Our main finding is that this white noise is associated with photo-generated shot noise. The extracted Fano factor (0.6 < F < 1) indicates that the observed shot noise is full scale and that the photo-generated carriers have no significant barriers to escape the device. We have performed a similar series of measurements on amorphous silicon solar cells as well as super yellow PPV polymer-based LEDs, the results of which will also be presented. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K47.00013: Spectral mapping of open circuit voltage of Cu(In,Ga)Se2 thin-film solar cells via surface photo-voltage and its implications for hole carrier transport Juran Kim, Kihwan Kim, Jihye Gwak, Jae Ho Yun, William Jo Due to its suitable physical properties, Cu(In,Ga)Se2 (CIGS) thin-film solar cells are commercialized and expanding its application fields. Thus, the needs for bendable CIGS solar cells are also rising. Up to date, flexible CIGS solar cells have reached power conversion efficiency (PCE) over 20%, but their performance is still limited for low open-circuit voltage. It is related to electron-hole (e-h) carrier transport. Therefore, we need to comprehend carrier behavior in the solar cells and their band structure by analyzing surface photovoltage (SPV) of CIGS thin-films on flexible substrates. By 3-step process and NaF post-deposition treatment we obtained 4 different PCE of 0, 6, 12, and 17%, altering substrate temperatures. Confocal micro-Raman spectroscopy provides phase distribution inside of the materials and the surface. Kelvin probe force microscopy (KPFM) displayed upward surface potential barrier near grain boundaries, helping e-h separation. SPV results was obtained by photo-assisted KPFM under three different wavelength lasers (640, 532, and 405 nm) with different skin depths. These optoelectrical properties from confocal micro-Raman and SPV variation induced by the phase difference can elucidate how non-uniformity affects carrier transport in the solar cell materials. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K47.00014: Modeling realistic grain boundaries in CdTe Eric Schwenker, Arun Kumar Mannodi Kanakkithodi, Fatih Sen, Li An Chen, Spencer Hills, Jinglong Guo, Moon Kim, Robert Klie, Maria Chan Grain boundaries (GBs) are performance-limiting in CdTe photovoltaics. Towards understanding and improving GBs, especially in the presence of impurities or alloying elements such as Se and Cl, it is important to perform first principles density functional theory (DFT) modeling on realistic structural models. We develop and use a code FANTASTX (Fully Automated Nanoscale To Atomistic Structure from Theory and eXperiment) which aims at creating atomistic structures which are consistent with scanning transmission electron microscopy (STEM) images as well as are energetically reasonable. We will discuss the electronic structures of realistic CdTe GBs, the effects of impurities on the electronic structure, and the implications for photovoltaic performance. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K47.00015: Quasiparticle Band Structure of Iron Pyrite Gabe Lopez-Candales, Weiyi Xia, Yiyang Sun, Peihong Zhang Being a non-toxic and abundant material, iron pyrite (FeS2) is an attractive material for photovoltaic applications with a very high quantum efficiency and absorption coefficient. Despite much research effort, the fundamental band gap of FeS2 is still not accurately determined. The measured fundamental band gap of FeS2 ranges from 0.84 to 1.2 eV. Surprisingly, straightforward density functional theory calculations within the generalized gradient approximation (GGA) predict a band gap of 0.46 eV, whereas the supposedly more accurate Heyd-Scuseria-Ernzerhof (HSE) hybrid functional predicts a band gap of 2.6 eV. Perhaps more intriguing is that it was reported [1] that quasiparticle calculations within the GW approximation predicts a band gap of 0.3 eV for FeS2. In this work, we report fully converged G0W0 quasiparticle band structure of FeS2 using a recently developed accelerated method [2]. Contrary to the previous claim, our results predict a 0.81 eV (dipole forbidden) band gap at the Gamma point and a dipole allowed transition energy of about 1.07 eV. Our work illustrates the importance of the convergence issue in GW calculations . |
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