Doctoral Student, University of California at Berkeley
New solar materials cannot wait for guess and check creation. We use super computers to help guess.
Silicon currently holds 90% of the world's market share for solar energy modules. So far, no alternative material has been able to surpass the combination of long term material stability and solar energy conversion efficiency that Silicon demonstrates. Since these solar modules need to be carefully manufactured with super high quality wafers, many gains in manufacturing costs could be achieved through the use of cheaper materials with comparable device efficiencies.
One class of solution processable materials in materials science, known as Halide Perovskites, has quickly risen to fame in the solar cell community. In the span of less than a decade of academic research, these materials have already achieved >22% device efficiency (comparable to the maximum efficiencies of Silicon based solar cells). While these materials are very exciting for future solar cell applications, two major issues remain: (1) the materials tend to be thermodynamically unstable (they fall apart unless they are held in ideal environmental conditions), and (2) some of the best Halide Perovskite systems contain lead, a toxic element. This motivates a search for other halide perovskite materials with similar device efficiencies but with better stability and without toxic elements.
Materials discovery (the search for new ways of synthesizing materials, which may not naturally occur in abundance on earth previously) has historically taken the approach of "guess and check" experimental synthesis, wherein an experimentalist uses their own intuition based on simple physical models to try and guess the magic solution for a system that would work well for their application. In the modern era we have the ability to improve on materials discovery by using the world's largest super computers to run massively parallel codes which can numerically approximate complicated quantum mechanical properties. These properties can then be screened over and speed up the discovery process.
For my research, I am automating and implementing a computational workflow which can produce information about point defects (atomic scale imperfections) in new solar cell materials. These properties are essential for solar cell performance as they can either be beneficial or be detrimental to solar cell performance. Point defect formation is extremely system dependent and therefore using computational screening to avoid incorrect assumptions about defect formation will aid in the search for new perovskite solar cells.
Abstract: Author(s): Christoph Freysoldt, Blazej Grabowski, Tilmann Hickel, Jörg Neugebauer, Georg Kresse, Anderson Janotti, and Chris G. Van de WallePoint defects affect the performance of functional and structural materials in crucial ways. Recent advances in computational techniques have led to first-principles calculations that allow one to model, understand, and predict the effects of defects in solids. This review provides an overview of th...[Rev. Mod. Phys. 86, 253] Published Fri Mar 28, 2014Point defects affect the performance of functional and structural materials in crucial ways. Recent advances in computational techniques have led to first-principles calculations that allow one to model, understand, and predict the effects of defects in solids. This review provides an overview of th...
Pub.: 28 Mar '14, Pinned: 30 Jun '17
Abstract: Point defects have a strong impact on the performance of semiconductor and insulator materials used in technological applications, spanning microelectronics to energy conversion and storage. The nature of the dominant defect types, how they vary with processing conditions, and their impact on materials properties are central aspects that determine the performance of a material in a certain application. This information is, however, difficult to access directly from experimental measurements. Consequently, computational methods, based on electronic density functional theory (DFT), have found widespread use in the calculation of point-defect properties. Here we have developed the Python Charged Defect Toolkit (PyCDT) to expedite the setup and post-processing of defect calculations with widely used DFT software. PyCDT has a user-friendly command-line interface and provides a direct interface with the Materials Project database. This allows for setting up many charged defect calculations for any material of interest, as well as post-processing and applying state-of-the-art electrostatic correction terms. Our paper serves as a documentation for PyCDT, and demonstrates its use in an application to the well-studied GaAs compound semiconductor. We anticipate that the PyCDT code will be useful as a framework for undertaking readily reproducible calculations of charged point-defect properties, and that it will provide a foundation for automated, high-throughput calculations.
Pub.: 22 Nov '16, Pinned: 30 Jun '17
Abstract: Instability of hybrid organic–inorganic halide perovskites hinders their development for photovoltaic applications. First-principles calculations are used for evaluation of a decomposition reaction enthalpy of hybrid halide perovskites, which is linked to experimentally observed degradation of device characteristics. However, simple criteria for predicting the intrinsic stability of halide perovskites are lacking since Goldschmidt’s tolerance and octahedral geometrical factors do not fully capture formability of those perovskites. In this paper, we extend the Born–Haber cycle to partition the reaction enthalpy of various perovskite structures into lattice, ionization, and molecularization energy components. The analysis of various contributions to the reaction enthalpy points to an ionization energy of an organic molecule and an inorganic complex ion as an additional criterion for predicting chemical trends in stability of hybrid halide perovskites. The ionization energy equal to or less than that for cesium and the size comparable to that of methylammonium define the design space for cations A+ in the search for new perovskite structures APbI3 with improved chemical stability that are suitable for photovoltaic applications.
Pub.: 12 May '17, Pinned: 30 Jun '17
Abstract: Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)2PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.
Pub.: 05 Jun '17, Pinned: 30 Jun '17
Abstract: While hybrid organic-inorganic perovskites are now taking center stage in photovoltaic research, thermally unstable nature of organic components will be an ultimate obstacle to commercialization. As a potential alternative, all-inorganic cesium lead halide perovskites, especially CsPbI2Br, have emerged as thermally stable and efficient photovoltaic light absorbers. However, the fundamental properties of this material have not been studied in detail. Here, the crystal formation behavior of cesium lead mixed-halide perovskites is investigated in terms of the surface morphology, crystal structure and chemical state. We discover a previously uncharacterized feature that the crystal growth and morphology evolution of CsPbI2Br are significantly influenced by annealing temperature. By evaluating perovskite films and solar cells, we confirm that optimally crystallized perovskites exhibit superior photovoltaic performance and high phase stability against humid atmosphere.
Pub.: 14 Jun '17, Pinned: 30 Jun '17
Abstract: The recent meteoric rise in the field of photovoltaics with the discovery of highly efficient solar-cell devices is inspired by solution-processed organic–inorganic lead halide perovskites that exhibit unprecedented light-to-electricity conversion efficiencies. The stunning performance of perovskites is attributed to their strong photoresponsive properties that are thoroughly utilized in designing excellent perovskite solar cells, light-emitting diodes, infrared lasers, and ultrafast photodetectors. However, optoelectronic application of halide perovskites in realizing highly efficient subwavelength photonic devices has remained a challenge. Here, the remarkable photoconductivity of organic–inorganic lead halide perovskites is exploited to demonstrate a hybrid perovskite–metamaterial device that shows extremely low power photoswitching of the metamaterial resonances in the terahertz part of the electromagnetic spectrum. Furthermore, a signature of a coupled phonon–metamaterial resonance is observed at higher pump powers, where the Fano resonance amplitude is extremely weak. In addition, a low threshold, dynamic control of the highly confined electric field intensity is also observed in the system, which could tremendously benefit the new generation of subwavelength photonic devices as active sensors, low threshold optically controlled lasers, and active nonlinear devices with enhanced functionalities in the infrared, optical, and the terahertz parts of the electromagnetic spectrum.
Pub.: 22 Jun '17, Pinned: 30 Jun '17
Abstract: Mixed cation organic lead halide perovskites attract unfaltering attention owing to their excellent photovoltaic properties. Currently, the best performing perovskite materials contains multiple cations and provide power conversion efficiencies up to around 22%. Here, we report the first quantitative, cation-specific data on cation reorientation dynamics in hybrid mixed-cation formamidinium (FA)/methylammonium (MA) lead halide perovskites. We use 14N, 2H, 13C and 1H solid-state MAS NMR to elucidate cation reorientation dynamics, microscopic phase composition, and the MA/FA ratio, in (MA)x(FA)1-xPbI3 between 100 and 330 K. The reorientation rates correlate in a striking manner with the carrier lifetimes previously reported for these materials and provide evidence of the polaronic nature of charge carriers in PV perovskites.
Pub.: 24 Jun '17, Pinned: 30 Jun '17
Abstract: Metal halide perovskites are promising candidates for many classes of different optoelectronic devices. Apart from being a semiconductor, they additionally show ionic conductivity. It expresses itself in slow response times, reversible degradation, and hysteresis in the current–voltage characteristics of solar cells. This Perspective gives a condensed overview about experiments and theory on ion migration in metal halide perovskites focusing on its effects in solar cells. Apart from being a potential stability concern for photovoltaics, ion migration paired with the excellent optoelectronic properties of this material offers opportunities for novel devices such as optically controlled memristors and switchable diodes.
Pub.: 22 Jun '17, Pinned: 30 Jun '17
Abstract: Compositional engineering of recently-arising methylammonium (MA) lead (Pb) halide based perovskites is an essential approach for finding better perovskite compositions to resolve still remaining issues of toxic Pb, and long-term instability, etc. In this work, we carried out crystallographic, morphological, optical, and photovoltaic characterization of compositional MASn0.6Pb0.4I3-xBrx by gradually introducing bromine (Br) into parental Pb-Sn binary perovskite (MASn0.6Pb0.4I3) to elucidate its function in Sn-rich (Sn : Pb = 6 : 4) perovskites. We found significant advances in crystallinity and dense coverage of the perovskite films by inserting the Br into Sn-rich perovskite lattice. Furthermore, light-intensity-dependent open circuit voltage (Voc) measurement revealed much suppressed trap-assisted recombination for proper Br-added (x=0.4) device. These contributed to attain unprecedented power conversion efficiency of 12.1% and Voc of 0.78 V, which are, to the best of our knowledge, the highest performance in the Sn-rich (≥60%) perovskite solar cells reported so far. In addition, impressive enhancement of photocurrent-output stability and little hysteresis were found, which paves the way for the development of environmentally-benign (Pb-reduction), stable monolithic tandem cells using the developed low bandgap (1.24-1.26 eV) MASn0.6Pb0.4I3-xBrx with suggested composition (x=0.2-0.4).
Pub.: 27 Jun '17, Pinned: 30 Jun '17
Abstract: Solution-processed organometal halide perovskites are hybrid crystalline semiconductors highly interesting for low-cost and efficient optoelectronics. Their properties are dependent on the crystal structure. Literature shows a variety of crystal phase transition temperatures and often a spread of the transition over tens of degrees Kelvin. We explain this inconsistency by demonstrating that the temperature of the tetragonal-to-orthorhombic phase transition in methylammonium lead triiodide depends on the concentration and nature of local defects. Phase transition in individual nanowires was studied by photoluminescence microspectroscopy and super-resolution imaging. We propose that upon cooling from 160 to 140 K, domains of the crystal containing fewer defects stay in the tetragonal phase longer than highly defected domains that readily transform to the high bandgap orthorhombic phase at higher temperatures. The existence of relatively pure tetragonal domains during the phase transition leads to drastic photoluminescence enhancement, which is inhomogeneously distributed across perovskite microcrystals.Understanding crystal phase transition in materials is of fundamental importance. Using luminescence spectroscopy and super-resolution imaging, Dobrovolsky et al. study the transition from the tetragonal to orthorhombic crystal phase in methylammonium lead triiodide nanowires at low temperature.
Pub.: 28 Jun '17, Pinned: 30 Jun '17
Abstract: Halide perovskites are promising semiconductor materials for solution-processed optoelectronic devices. Their strong ionic bonding nature results in highly dynamic crystal lattices, inherently allowing rapid ion exchange at the solid-vapor and solid-liquid interface. Here, we show that the anion-exchange chemistry can be precisely controlled in single-crystalline halide perovskite nanomaterials when combined with nanofabrication techniques. We demonstrate spatially resolved multicolor CsPbX3 (X = Cl, Br, I, or alloy of two halides) nanowire heterojunctions with a pixel size down to 500 nm with the photoluminescence tunable over the entire visible spectrum. In addition, the heterojunctions show distinct electronic states across the interface, as revealed by Kelvin probe force microscopy. These perovskite heterojunctions represent key building blocks for high-resolution multicolor displays beyond current state-of-the-art technology as well as high-density diode/transistor arrays.
Pub.: 28 Jun '17, Pinned: 30 Jun '17
Abstract: Organic cation dynamics in organic–inorganic hybrid perovskite such as CH3NH3PbI3 have been reported to play an important role in the charge carrier lifetime and ferroelectricity but have not been fully investigated by theoretical approach. We have compared the rotational energy barriers and relaxation times of the methylammonium cation (CH3NH3, MA) in the cubic phases of MABX3 (B = Pb or Sn, X = Cl, Br, or I) by considering full relaxation of the PbX6 or SnX6 inorganic framework from first principles. We successfully reproduced the experimental rotational barrier for the 4-fold rotational symmetry (C4) of the C—N axis of MA. Our calculations suggest that the MA can rotate relatively freely because the flexible inorganic framework exhibits liquid-like behavior, which induces cooperative displacement of the metal cation (B) and the halogen (X) via a hydrogen bond and lowers the rotational barrier height. We also demonstrate that the rotational barrier of MA (6–11 kJ mol–1), which is closely correlated to the hardness of the PbX6 or SnX6 inorganic framework, can be controlled by choosing the constituent metal cations and halogens. In particular, the rotation barrier heights increase with lighter halogen atoms, which is an important insight for the design of ferroelectric materials.
Pub.: 16 Jun '17, Pinned: 30 Jun '17
Abstract: The electron-transport layer (ETL) plays an important role in the photovoltaic performance of perovskite solar cells (PSCs), and the energy level match of functional layers can appreciably improve the power conversion efficiency (PCE) of PSCs. Herein, we choose the rare-earth europium ion Eu3+ to dope a mesoporous TiO2 ETL. X-ray diffraction, field-emission scanning electron microscopy, ultraviolet photoelectron spectroscopy, time-resolved photoluminescence, and incident-photo-to-current conversion efficiency experiments were performed to characterize the material. It is found that the Eu doping does not change the morphology, grain size, and crystallinity of TiO2. However, Eu doping upshifts the Fermi level of TiO2 ETL by scavenging oxygen atoms and introducing oxygen vacancies on the surface of the layer, which results in low series resistance and fast charge transport in the ETLs, which in turn increases the power conversion efficiency of the device from 15.85 % (undoped PSC) to 17.90 % (Eu-doped PSC). This work demonstrates an effective approach for enhancing the performance of PSCs by rare-earth doping.
Pub.: 27 Jun '17, Pinned: 30 Jun '17
Abstract: Colloidal perovskite nanocrystals based on formamidinium lead halide (FAPbX3) have been synthesized by the ligand-assisted reprecipitation method using PbX2–dimethyl sulfoxide complexes as precursors at room temperature. Well-defined cubic-shaped FAPbX3 nanocrystals have been obtained with a size d of ∼10 nm. The synthesized FAPbX3 nanocrystals show bright photoluminescence with a high photoluminescence quantum yield (75% for FAPbBr3). The lifetimes of FAPbBr3 nanocrystals were measured for the samples isolated at several different centrifugal speeds. The photoluminescence can be tuned from the blue to near-infrared region (λpeak = 408–784 nm) by changing either the amount of oleylamine or the composition of X. The color expression range is 135% of the NTSC standard. The bandwidth of the photoluminescence spectra of FAPbX3 nanocrystals is narrow (full width at half-maximum of 18–48 nm). FAPbX3 nanocrystals show thermal stability that is better than that of MAPbBr3 nanocrystals.
Pub.: 13 Jun '17, Pinned: 30 Jun '17