Session F33: Energy Conversion and Storage Materials

Shift current bulk photovaltaic effect influenced by quasiparticle and exciton

We compute the shift current bulk photovoltaic effect (BPVE) in bulk BaTiO3 and two dimensional monochalcogenide SnSe considering quasi-particle corrections and exciton effects. We explore changes in shift current peak position and magnitude reduction due to band renormalization. For BaTiO3, we demonstrate that shift current is dramatically reduced near the band edge but enhanced at high energies due to exciton effect. Additionally, we reveal that the shift current is reduced in two dimensional monochalcogenide SnSe due to the enhancement of optical reflectivity from many-body corrections. Comparison of these results with experiments on BaTiO3 indicate that mechanisms other than shift current may be contributing to BPVE. These results suggest that many-body corrections are important for accurate assessments of bulk photovoltaic materials and to understand the mechanisms behind the BPVE.   

Bulk photovoltaic effects in the presence of a static electric field

Irradiated crystalline insulators in the presence of a static electric field exhibit three new types of nonlinear photocurrents. They represent physical singularities of the third order free electron polarization susceptibility and hence generalize the standard second order bulk photovoltaic effects. In the absence of momentum relaxation and saturation effects they grow as tⁿ (n=2,1,0) with illumination time and are dubbed jerk, modified injection, and modified shift current, respectively. The presence of a static electric field gives rise to new processes which are described in detail. Experimental signatures and extensions to higher order susceptibilities are also discussed.   

Core-shell colloidal quantum dots for photovoltaics with improved open circuit voltage

High carrier recombination rates in lead sulfide (PbS) colloidal quantum dot (CQD) photovoltaics (PV) can result in a reduction in open circuit voltage (Voc). Surface states and sub-gap states are thought to increase recombination which leads to Voc loss. Here we present a method of forming an oxide shell on the CQD surface capped with native oleic acid ligands prior to the deposition of the CQD film and ligand swap. The core-shell QDs exhibit a narrowing in size distribution and the resulting devices yield improved Voc. In addition, films of core-shell QDs are more resilient to damage incurred during sputter deposition of overlaying films. Because the thickness and uniformity of the oxide shell can be precisely controlled, a natural balance between trap passivation and charge transport can be achieved.   

Si24: a next-generation semiconductor with growing promise for future solar energy

The recently discovered allotrope of silicon, Si₂₄, is an exciting material for the future of solar energy due to a quasi-direct bandgap near 1.3 eV [1]. Synthesized via precursor Na₄Si₂₄ at high temperature and pressure (~850 °C, 9 GPa), free standing single crystals of on the 1 mm scale are now achievable [2]. An epitaxial relationship between Na₄Si₂₄ and diamond cubic silicon (DC-Si), observed through high-resolution transmission electron microscopy (HRTEM), is proposed to facilitate the growth of these high-quality Na₄Si₂₄ crystals from DC-Si wafers mixed with metallic Na. These observations illuminate a path toward scaling of Na₄Si₂₄ and Si₂₄. Removal of Na from Na₄Si₂₄ on this length scale is shown to be effective, revealing intrinsic optical and electronic properties of Si_24. Our results encourage the pursuit of Si₂₄ for future solar energy conversion and efficient optoelectronics.
[1] D. Y. Kim, et al., Nat. Mater., vol. 14, no. 2, pp. 169–173, Feb. 2015.
[2] Guerette, M.; Ward, M.; Lokshin, K.; Wong, A.; Zhang, H.; Stefanoski, S.; Kurakevych, O.; LeGodec, Y.; Juhl, S.; Alem, N.; et al. Synthesis and Properties of Single-Crystalline Na₄Si₂₄. Crystal Growth & Design 2018.   

Tailoring the structure and defects of non-toxic nanocrystalline Bi2S3solar cells

Solar cells can satisfy the increasing demand for energy worldwide, but the toxicity of semiconductors used in solar cells can overshadow their utility as a renewable source of energy. Bi₂S₃, with a desirable band gap of 1.3ev, and as a non-toxic n-type semiconductor can be a favorable replacement for toxic semiconductors containing Pb, Cd or Te. However, nanocrystalline Bi₂S₃films synthesized by various techniques such as successive ionic layer adsorption and reaction (SILAR) have not reached high solar energy conversion efficiencies hitherto, and have primarily been studied as sensitizers for photoelectrochemical applications. Here, we report the synthesis and characterization of non-toxic all-inorganic solid-state Bi₂S₃photovoltaic solar cells. We enhanced the solar energy conversion efficiency of the nanocrystalline Bi₂S₃solar cells by optimizing the structure of the electron and hole transport layers, and by tailored annealing treatments that modify the size of the Bi₂S₃nanocrystals and decrease their defect concentrations.
  

The fluid-like nature of solid cubic halide perovskites

X-ray diffraction ‘see’ in the high temperature phase of ABX₃ halide perovskites a macroscopically averaged cubic structure with a single formula unit (FU) per cell. Yet DFT calculations on this structure reveal a number of anomalies: i) It has dynamically unstable phonons, ii) its band gap is lower than both experiment and molecular dynamics (MD) predictions, iii) the trends with A cation show inconsistency with experiment e.g., cubic FASnI₃ has larger gap than the orthorhombic CsSnI₃, and iv) it has higher total energy than the relaxed supercell with many repeated FUs. We find via DFT that the real microscopic structure of cubic halide perovskites is polymorphous, i.e., it contains a dynamic fluid-like distribution of different local motifs, each having differently tilted/rotated/B-atom displaced BX₆ octahedra and differently oriented A molecules, the average of which is the fictitious monomorphous cubic structure. The polymorphous configurations have stable phonons and much larger band gaps, forming the correct description of trends in gaps and structures in the ABX₃ group of materials. We will present the main effects that govern band gaps in perovskites, via carefully constructed static supercell approximants to the dynamic MD structure.   

The Photoluminescence Origination of the Cs4PbBr6 Perovskite Crystals

Due to the unique optical properties and high quantum efficiency, the lead halide perovskites have obtained great attentions in the recent years. Currently, the research focuses not only on their potential applications in solar cell, LED, and laser, but also on several unsolved fundamental issues, such as the origin of the luminescence of Cs₄PbBr₆. The mechanism of the luminescent Cs₄PbBr₆ perovskite is still under debate. Some people think that the pure Cs₄PbBr₆ crystals are non-emissive while the strong green PL (Photoluminescence) originates from the CsPbBr₃ nanocrystals encapsulated in the Cs₄PbBr₆ bulk crystals. The others claim that the strong emissive comes from the structural defects of the Cs₄PbBr₆ crystals. Despite the various reports to support each opinion, no final conclusion has been made up to now. In this work, we plan to controllably synthesize emissive and non-emissive Cs₄PbBr₆ crystals as well as CsPbBr₃ nanocrystals with a strong photoluminescence. By combining various techniques, such as Raman, XRD, (HR)TEM, PL, etc., we attempt to confirm the contribution of CsPbBr₃ dopants to the the photoluminescent Cs₄PbBr₆ crystals.   

Phase Stability and Ordering in Rocksalt-based ABX2 and (Pb/Sn)X-ABX2 Thermoelectrics

Using first-principles density functional theory (DFT) calculations, we study the phase stability and ordering in ABX2 and (Pb/Sn)X-ABX2 (A=Na, Ag; B=Sb; X=S, Se, Te) systems that crystallize in rocksalt-based lattices. We use the cluster expansion method to calculate the T=0 K energies of ordered arrangements, and special quasirandom structures (SQS) to estimate the energetics of structures with A/B and (Pb/Sn)/AB cation disorder. From the DFT-calculated energetics, we: (a) predict the miscibility and ordering tendency in each system, (b) find that phase separation is favored in all (Pb/Sn)X-ABX2 systems while compound-forming is favored in most ABX2 systems, (c) predict solubility boundaries for phase-separating systems, and (d) identify that cation ordering in the L11 structure type is often the lowest-energy one in ABX2 compounds, and discuss why L11 is favored.   

Orientation-Dependent Behavior of Reversible Switching in Li-Ion Gated Devices

Inspired by the neural system, electrolyte-gated devices are expected to provide new computing paradigms for pattern recognition and machine learning. In this study, we demonstrate reversible switching in ionic-polymer gated devices fabricated on epitaxial LiCoO₂ (LCO) films with different crystal orientations. The epitaxial LCO films with different thicknesses (50 nm or 20 nm) were grown on (100), (110), and (111) SrTiO₃ (STO) substrates using the pulsed laser deposition method. After films were patterned into micrometer-scaled bridges, a Li-ion polymer was used to cover patterned films to complete the devices. By controlling the amplitude and the duration of the current or voltage pulse applied to the gate, reversible multi-level switching in the conductance was realized. In particular, we found that current pulses with a relatively low amplitude are sufficient to drive the switching in the conductance. Orientations of epitaxial LCO films are different on different STO substrates which allow for a systematic study of the relation between the diffusion path of Li ions and the crystal structure of the LCO film. The (104) oriented films demonstrate the fastest switching speed (less than 1 s), consistent with the fact that this is known to be the fast-diffusion direction in LCO.   

Theory of mixed ion-electron transfer kinetics in concentrated solutions and solids

Electron transfer (ET) reactions are often successfully described using the phenomenological Butler-Volmer (BV) equation. However, there are cases where its predictions deviate from the experimentally observed rates. To account for these shortcomings, microscopic models of electron transfer, which take into account quantum mechanical and environmental polarization effects, have been developed. The current description of ET is constrained to dilute systems only, where ionic concentrations do not contribute to the activation step of the reaction. In the present study, the classical picture of Marcus ET reactions is generalized to describe the kinetics in concentrated solutions and solids, leading to the formulation of mixed ion-electron transfer kinetics (MIET). The reaction rates that are predicted by the theory are found to be in excellent agreement with recent in-situ experimental measurements in li-ion battery systems. Finally, we investigate the decrease of the maximum capacity with increasing discharge rate under ET-limited conditions for a variety of li-ion intercalation materials.   

Use of LIBS Technology for CO2 Leak Detection in Carbon Sequestration

In this study, the in-situ measurement capability of Laser-Induced Breakdown Spectroscopy (LIBS) to detect CO₂ leakage in geological carbon storage (GCS) was evaluated. As LIBS is an optics-based (contains no electronics) spectroscopic technique, it can perform contamination-free analysis in extreme conditions of underground environment. During the leakage, the interaction of CO₂ with carbonate rocks at elevated pressures can release various metals which can potentially contaminate the drinking water sources above storage sites. Therefore, we evaluated the underwater LIBS technique for studying the carbonate dissolution with increased CO₂ pressure. Dissolution experiments using four carbonates (CaCO₃, SrCO₃, MnCO₃, and MgCO₃) at the elevated CO₂ pressure (ranging from ambient to 250 bars) were carried out by analyzing LIBS spectra obtained from aqueous solutions containing these carbonates. The results indicated the dissolution of all carbonates. However, the rate of dissolution varied from carbonate to carbonate. This study shows that in-situ monitoring of carbonate dissolution by underwater LIBS can be used for CO₂ leak detection in GCS.   

Energy-Saving Meta-Glasses with Embedded Plasmonic Nanoparticles

Plasmons are collective charge carrier excitations. Using different plasmonic materials to fabricate nanocrystals and controlling their size and geometry allows us to obtain sharp resonances from UV to near IR. By embedding ensembles of nanocrystals in dielectric materials, we can design metamaterials with specific transmission profiles that filter out a broad spectrum while remaining transparent to selected frequency bands.¹ We discuss this idea in the specific context of its application in energy-saving windows.² By blocking UV and IR radiation from the solar spectrum they reduce the energy expenditure of active cooling systems in warm climates, but they retain the desirable property of being transparent to visible light. In this talk we will present theoretical results for plasmonic glasses created with different nanoparticle geometries³ and materials, offering alternatives to current industrial fabrication standards, with comparable efficiencies and potentially lower costs.

[1] L.V. Besteiro, K. Gungor, H.V. Demir, A.O. Govorov, J. Phys. Chem. C 121, 2987–2997 (2017).
[2] L.V. Besteiro, X.-T. Kong, Z. Wang, F. Rosei, A.O. Govorov, Nano Lett. 18, 3147-3156 (2018).
[3] Y. Qin, X.-T. Kong, Z. Wang, A.O. Govorov, U.R. Kortshagen, ACS Photonics 4, 2881-2890 (2017).   

Unsteady State of a Solid State Device: Characteristic Cooling Length

Thermoelectric modules (TEM) are solid state devices that can work as a Thermoelectric cooler (TEC). TECs can reach temperatures below that obtained with a steady-state current by applying an electrical current pulse which enables a transitory state in a Peltier couple. This phenomenon is known as supercooling. The objective of this work is to analyze characteristics parameters such as minimum cooling temperature, COP and temperature spatial profile in a TEC operated under current pulses and cooling load (Qc) pulses. We study a numerical model of a one-dimensional thermoelectric cooling system in the unsteady state. In this work, we propose a new parameter called characteristic cooling length to describe the distance in which occurs minimum cooling temperature in the elements of a TEM. Our results show the transient temperature spatial profile along the thermoelements of the TEM and the characteristic cooling length for different materials. We propose a general principle to make a judgment how the temperature profile will occur along the elements due to material properties under current pulse operation.   

Highly-Miscible and Bandgap-Tunable (LixCu1−x)2ZnSnS4 Alloys

Doping or alloying lithium into Cu₂ZnSnS₄ (CZTS) has drawn extensive attention recently because it is crucial for fabricating high-performance Cu₂ZnSnS₄ thin film solar cells. Experiments showed that lithium can be alloyed into CZTS with a high content and even completely substitute copper to form a new compound Li₂ZnSnS₄ (LZTS). Using the first-principles calculations, we found that Li₂ZnSnS₄ adopts wurtzite-kesterite as ground state structure with a band gap around 3.29 eV. The valence band offset between Li₂ZnSnS₄ and Cu₂ZnSnS₄ is 2.20 eV. As the Li content increases, the band gap of (Li_xCu_1−x)₂ZnSnS₄ alloy increases rapidly with a big band gap bowing. The mixing enthalpies of (Li_xCu_1−x)₂ZnSnS₄ alloys in the kersterite and wurtzite-kesterite structures are both small, suggesting that the (Li_xCu_1−x)₂ZnSnS₄ alloys can easily form at a low growth temperature. These results demonstrate that the lithium alloying may be an effective way for adjusting the band structure and the optoelectronic properties of CZTS.