A pinboard by
Daniel Broberg

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.


PyCDT: A Python toolkit for modeling point defects in semiconductors and insulators

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

Band-Tail Recombination in Hybrid Lead Iodide Perovskite

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

Hybrid Lead Halide Perovskites for Ultrasensitive Photoactive Switching in Terahertz Metamaterial Devices

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

Highly Efficient and Stable Sn-rich Perovskite Solar Cells by Introducing Bromine.

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

Defect-induced local variation of crystal phase transition temperature in metal-halide perovskites.

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