Engineering topics

Mundt, Laura E.

Publications and source records attributed to Mundt, Laura E..

9 records

DOE OSTI2022

Accurately Quantifying Stress during Metal Halide Perovskite Thin Film Formation

The role of strain in metal halide perovskite (MHP) solar cells is still under investigation, showing both beneficial and detrimental effects on the device performance and stability. One crucial component to elucidating the impact of strain in the MHP absorber is a robust method of quantifying the amount of strain in the material. In this work, we present a parametric refinement approach based on grazing incidence wide-angle X-ray scattering and demonstrate its use on quantifying strain during thermal annealing and subsequent cooling as a function of substrate and processing route. We use the analysis to reveal the impact of the cubic-to-tetragonal phase transition during cooling on the material's strain and discuss texture formation as a potential strain-relief mechanism. Thereby we present both a robust approach to quantify strain in MHPs and potential mechanisms to control strain in the film, opening the path for further investigations of strain in MHPs.

14 SOLAR ENERGY↗

DOE OSTI2022

Reversible Methanolation of Metal Halide Perovskites

The low formation energies inherent in metal halide perovskites enable the structure to be easily broken and remade with little energy input. In this work, we leverage low formation energy to demonstrate 0D/3D structural transformations induced by reversible intercalation of methanol to yield dramatic control of visible light absorption. We identify a methanolated structure that features sheets of 0D isolated [PbI6]4- octahedra separated by MAI and methanol. Methanol and water reversibly displace each other in the 0D complex by controlling the chemical potential of the system via Le Chatelier's principle. The weaker H-bonding of methanol compared to water effectively reduces the complex dissociation temperature from 70 to 50 degrees C, rendering the methanol complex more desirable for an array of next-generation applications spanning low-power-consumption memory to switchable photovoltaics.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2021

Mixing Matters: Nanoscale Heterogeneity and Stability in Metal Halide Perovskite Solar Cells

The structural stability of the metal halide perovskite (MHP) absorber material is crucial for the long-term solar cell stability in this thin-film photovoltaic technology. Here, we use mixed A-site FA0.83Cs0.17PbI3 to demonstrate that nanoscale compositional heterogeneity can serve as initiation sites for more macroscale, irreversible phase segregation, which causes device performance degradation. Probing compositional heterogeneity on length scales that has not been detected with conventional characterization techniques, we analyze the tetragonal to cubic phase transition behavior to indirectly determine the level of nanoscale compositional heterogeneity in the initial films. Further, we show that the thermal annealing conditions of the MHP layer during film processing influence the initial nanoscale compositional heterogeneity, and changing these processing conditions can be used to improve the device performance stability. The insights into structural degradation mechanisms initiated by nanoscale compositional heterogeneity and the proposed mitigation strategies will help guide the way toward long-term stable MHP solar cells.

14 SOLAR ENERGY↗

DOE OSTI2021

Metastable Dion-Jacobson 2D structure enables efficient and stable perovskite solar cells

The performance of three-dimensional (3D) organic-inorganic halide perovskite solar cells (PSCs) can be enhanced through surface treatment with 2D layered perovskites that have efficient charge transport. We maximized hole transport across the layers of a metastable Dion-Jacobson (DJ) 2D perovskite that tuned the orientational arrangements of asymmetric bulky organic molecules. The reduced energy barrier for hole transport increased out-of-plane transport rates by a factor of 4 to 5, and the power conversion efficiency (PCE) for the 2D PSC was 4.9%. With the metastable DJ 2D surface layer, the PCE of three common 3D PSCs was enhanced by approximately 12 to 16% and could reach approximately 24.7%. For a triple-cation–mixed-halide PSC, 90% of the initial PCE was retained after 1000 hours of 1-sun operation at ~40°C in nitrogen.

14 SOLAR ENERGY↗

DOE OSTI2021

Structural Stability of Tin-Lead Halide Perovskite Solar Cells

One of the key features that makes halide perovskite solar cells such an attractive and intensely researched photovoltaic (PV) technology, is the tunability of the bandgap of these halide perovskite materials [1], [2]. Shortly after it was established that the bandgap could be increased to 1.7-1.8 eV, investigations into the application for silicon based tandem solar cells launched. Within a few years, conversion efficiencies of up to 28 % have been reported [3]. Concurrently, the interest in commercial application of this technology emerged and have rapidly increased. Whereas there are advantages to piggy backing on an established PV technology such as silicon, efforts to develop all-perovskite tandem solar cells are of considerable interest. Partly substituting the metal cation in the ABX3 perovskite enables the low bandgap absorbers required for an all-perovskite tandem solar cell. One common approach is to partially substitute the lead with tin, however these materials often suffered from poor stability. Therefore the development of an efficient, stable mixed tin-lead perovskite is key to enabling all-perovskite tandem solar cells.

degradation↗

DOE OSTI2020

Reversible multicolor chromism in layered formamidinium metal halide perovskites

Abstract Metal halide perovskites feature crystalline-like electronic band structures and liquid-like physical properties. The crystal–liquid duality enables optoelectronic devices with unprecedented performance and a unique opportunity to chemically manipulate the structure with low energy input. In this work, we leverage the low formation energy of metal halide perovskites to demonstrate multicolor reversible chromism. We synthesized layered Ruddlesden-Popper FA n+1 Pb n X 3n+1 (FA = formamidinium, X = I, Br; n = number of layers = 1, 2, 3 … ∞) and reversibly tune the dimensionality ( n ) by modulating the strength and number of H-bonds in the system. H-bonding was controlled by exposure to solvent vapor (solvatochromism) or temperature change (thermochromism), which shuttles FAX salt pairs between the FA n+1 Pb n X 3n+1 domains and adjacent FAX “reservoir” domains. Unlike traditional chromic materials that only offer a single-color transition, FA n+1 Pb n X 3n+1 films reversibly switch between multiple colors including yellow, orange, red, brown, and white/colorless. Each colored phase exhibits distinct optoelectronic properties characteristic of 2D superlattice materials with tunable quantum well thickness.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Surface-Activated Corrosion in Tin–Lead Halide Perovskite Solar Cells

Mixed tin–lead halide perovskite solar cells have promising power conversion efficiencies, but long-term stability is still a challenge. Here, we examine the stability of a 60:40 tin–lead perovskite to better understand diminished device performance upon thermal treatment, both in ambient and inert atmosphere. Operando X-ray diffraction shows a stable bulk structure of the perovskite absorber, leading to the hypothesis that surface chemistry dominates the degradation mechanism. X-ray photoelectron spectroscopy reveals two new observations post-thermal annealing that accompany previously reported Sn 4+ evolution: (i) the formation of I 3 – intermediates preceding I 2 loss at the surface and (ii) evidence of under-coordinated tin and lead surface sites (Sn δ<2+ and Pb δ<2+ , respectively) in inert and ambient conditions. These two species indicate an activated corrosion (i.e., both oxidation and reduction) process at the surface as a possible chemical pathway for degradation, which is expected to be accelerated under operando voltage and light biases.

14 SOLAR ENERGY↗

DOE OSTI2020

The Role of Dimethylammonium in Bandgap Modulation for Stable Halide Perovskites

Halide perovskites with bandgaps of 1.70–1.85 eV are of interest for multijunction photovoltaics. Mixing halides on the X site of the ABX3-structured perovskite system is a common way to reach these bandgaps, but this method introduces phase segregation pathways, limiting photovoltage. Recently, a new strategy for increasing the bandgap has been introduced, where cations normally too large to fit into the lattice, but compensated by smaller cations, are substituted on the A site. The mechanism underlying the increase of the bandgap with this strategy remained an open question. Here, we show that by partial substitution of the large dimethylammonium (DMA) cation at the A site of FA x Cs 1–x PbI y Br 3–y perovskites, a bandgap increase is observed not only when DMA is compensated by smaller Cs cations but also when only DMA is added, which is accompanied by an expansion of the crystal lattice. Our experimental findings suggest that adding DMA is causing an unexpected tilt in the perovskite octahedra, increasing the bandgap. Efficient solar cells based on 1.73 eV DMA-incorporated materials are extremely stable, retaining 96% of their original efficiency over 2200 h at 85 °C in the dark and 92% of their original efficiency after operation at 60 °C for 500 h. Finally, this octahedral tilting strategy is a promising route for attaining efficient and stable wide bandgap perovskite solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Spatially Resolved Performance Analysis for Perovskite Solar Cells

This review explores the current state of the art in spatially resolved characterization of mixed-halide perovskite solar cells. As the size of perovskite cells and modules continues to grow, quantification of the spatial distribution of key cell parameters will become increasingly valuable in predicting ultimate cell-level performance and tracking process homogeneity. Here, both high resolution microscopic approaches using scanning techniques and camera-based methods for full-area cell and/or module analysis are discussed. The value of this local data in predicting performance losses at the cell level is particularly emphasized. Measurable physical parameters sensitive to losses of voltage, current, fill factor, and efficiency are discussed together with selected experimental results. It is demonstrated that a combination of spatially resolved cell parameter mapping/imaging can be used to quantitatively discriminate various loss contributions at high resolution. The impact and control of inhomogeneities become particularly important when upscaling from small devices to large formats compatible with industrial mass production.

14 SOLAR ENERGY↗