Engineering topics

Nienhaus, Lea

Publications and source records attributed to Nienhaus, Lea.

6 records

DOE OSTI2024

Upconversion on the Micrometer Scale: Impact of Local Heterogeneity

The properties of perovskite/naphtho[2,3-a]pyrene (NaPy) upconversion devices are investigated by a combination of atomic force microscopy and photoluminescence mapping to understand the role of microscopic heterogeneity in the ensemble device properties. The results emphasize strong microscopic inhomogeneity across the perovskite/NaPy upconversion device due to local formation of NaPy microcrystals. NaPy shows emission from three distinct states in the solid state: S 1 ′ emission at 520 nm, excimer emission at 560 nm, and S 1 ″ emission at 620 nm. Clear spatial differences in the emission spectrum under 405 nm excitation are found, highlighting that there is a strong microcrystal-to-microcrystal variation in the optical properties─emphasizing a need for multimodal measurements. Furthermore, our results indicate that microcrystals with strong emission from the strongly coupled low-energy state S 1 ″ (J-dimer) show much higher upconversion intensity than those with dominant emission from the high-energy S 1 ′ state (I-aggregate). Hence, our results suggest that microcrystals with strong emission from the low-energy state S 1 ″ act as isolated hotspots for upconversion.

Crystals↗

DOE OSTI2024

Advances in Spectro‐Microscopy Methods and their Applications in the Characterization of Perovskite Materials

Perovskite materials are promising contenders as the active layer in light-harvesting and light-emitting applications if their long-term stability can be sufficiently increased. Chemical and structural engineering are shown to enhance long-term stability, but the increased complexity of the material system also leads to inhomogeneous functional properties across various length scales. Thus, scanning probe and high-resolution microscopy characterization techniques are needed to reveal the role of local defects and the results promise to act as the foundation for future device improvements. Here, a look at the parameter space: technique-specific sample penetration depth versus probe size highlights a gap in current methods. High spatial resolution combined with a deep penetration depth is not yet achievable. However, multimodal measurement technique may be the key to covering this parameter space. In this perspective, current advanced spectro-microscopy methods which have been applied to perovskite materials are highlighted.

Luo, Yanqi↗

DOE OSTI2024

Cooling Mixed A-Site Halide Perovskites: Impact of Temperature on Optical and Structural Properties

The widespread utilization of perovskite-based photovoltaics requires probing both the structural and optical properties under extreme operating conditions to gain a holistic understanding of the material behavior under stressors. Here, in this study, we investigate the temperature-dependent behavior of mixed A-site cation lead triiodide perovskite thin films (85% methylammonium and 15% formamidinium) in the range from 300 to 20 K. Through a combination of optical and structural techniques, we find that the tetragonal-to-orthorhombic phase transition occurs at similar to 110 K for this perovskite composition, as indicated by the change in the diffraction pattern. With decreasing temperature, the quantum yield increases with a concurrent elongation of the carrier lifetimes, indicating suppression of nonradiative recombination pathways. Interestingly, in contrast to single A-site cation perovskites, an additional optical transition appears in the absorption spectrum when the phase transition is approached, which is also reflected in the emission spectrum. We propose that the splitting of the optical absorption and emission is due to local segregation of the mixed cation perovskite during the phase transition.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Cool carriers: triplet diffusion dominates upconversion yield

Perovskites have gained popularity both as the active material in photovoltaics and as bulk triplet sensitizers for solid-state triplet–triplet annihilation upconversion (TTA-UC). Prior to widespread implementation into commercial photovoltaics, an in-depth understanding of the environmental influences on device performance is required. To this point, the temperature-dependent structure–function properties of TTA-UC within methylammonium formamidinium lead triiodide (MAFA)/rubrene UC devices are explored. A strong temperature dependence of the underlying UC dynamics is observed, where the maximum UC efficiency is achieved at 170 K, reflecting the competition between triplet diffusion length, diffusion rate, and triplet–triplet encounter events. Furthermore, a combination of spectroscopic and structural methods and theoretical modelling illustrates that despite the significantly increased carrier lifetime of the perovskite at low temperatures, the TTA-UC dynamics are not governed by the underlying sensitizer properties but rather limited by the underlying triplet diffusion.

Sullivan, Colette M.↗

DOE OSTI2023

Perovskite-Sensitized Upconversion under Operando Conditions

Perovskite-sensitized upconversion (UC) has resulted in near-infrared-to-visible UC at solar-relevant fluxes. However, the successful implementation of UC devices into operating solar cells will result in exposure to similar environmental stressors as for the commercial photovoltaics (PVs), mainly elevated temperatures, and continuous irradiation. In this article, we investigated the effects of these two stressors, heat and light, on the triplet generation process at the perovskite/rubrene interface. Following exposure to both stressors, local discrepancies across the upconversion device were discovered. Here, the first region showed changes to the morphology, and no detectable upconverted emission was observed. Through the combination of optical microscopy and spectroscopy, crystallization of the organic semiconductor layer, degradation of dibenzotetraphenylperiflanthene, and concurrent degradation of the perovskite sensitizer were found. These effects culminate in a reduction in both triplet generation and triplet–triplet annihilation. In the second region, no changes to the morphology were present and visible UC emission was observed following exposure to both stressors. To probe the triplet sensitization process at elevated temperatures, transient absorption spectroscopy was performed. The presence of the excited spin-triplet state of rubrene at 60 °C highlighted successful triplet generation even at elevated temperatures. This work emphasizes the challenges and continued potential for the integration of perovskite-sensitized UC into commercial photovoltaic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Viewing Optical Processes at the Nanoscale: Combining Scanning Tunneling Microscopy and Optical Spectroscopy

The combination of optical spectroscopy and scanning tunneling microscopy techniques has the potential to yield the unambiguous single-molecule level insight required to fully understand the complex structure–property relationship of emerging material systems. In this Perspective, we highlight the recent progress of single-molecule absorption detected by scanning tunneling microscopy (SMA-STM) to investigate light–matter interactions of supported (nano)particles, quantum dots, and molecular and thin films at the nanoscale. We further show how the SMA-STM technique has been recently extended to expand the classes of materials that can be investigated as well as recent design modifications that enable both time-resolved imaging and tomography. Finally, we hypothesize how these modifications can be further leveraged to advance our current knowledge of light–matter interactions and how the resulting nanoscale insight can be implemented for future material applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗