Microcavity exciton-polaritons are bosonic quasiparticles that result from the hybridization of excitons and modes of a confined electromagnetic field in a regime known as strong light-matter coupling. Having a low effective mass, polaritons can undergo condensation, the macroscopic occupation of the lowest energy and momentum state. Two-dimensional (2D) perovskites are promising candidates for polariton condensation due to their high exciton binding energies, low non-radiative recombination rates and strong oscillator strengths. However, despite their optimal optoelectronic properties, there are no reports of room temperature polariton condensation in 2D perovskites and only one unreproduced report at low temperature. In this study, we systematically examine the interplay between the emission from the exciton reservoir and the population of the lower polariton. We gain insights on how the spectral features of the emission of 2D perovskites affect polariton relaxation and onto one of the mechanisms making polariton condensation challenging in 2D perovskites.
In situ multimodal microscopic x-ray characterizations demonstrate their unique capabilities in revealing the mechanisms of material degradation and the pathways for mitigation in energy harvesting applications such as halide perovskite solar cells. Despite the excellent device performance exhibited by halide perovskites, their sensitive nature and material interfaces necessitate a precisely controlled and tunable characterization environment to identify the sources of device performance loss. In this work, we designed an in-situ sample chamber that allows the control of various environmental conditions, including heat, illumination, and bias, while simultaneously collecting chemical (X-ray fluorescence, XRF), optical (X-ray Excited Optical Luminescence, XEOL), and performance (X-ray Beam Induced Current, XBIC) measurements on functional devices. The integrated thermoelectric cooler module of the designed chamber enables controlled heating up to 100 °C and rapid cooling back down to room temperature. This allows simultaneous multimodal XRF, XEOL and XBIC signal collections on Cs0.05FA0.95PbI3 perovskite devices at various temperatures. The results show increasing homogeneity in the XBIC maps and continuous reduction in XEOL intensity, with a redshift in XEOL peak positions as sample temperatures increase. The results of the simultaneous multimodal study pave the way for improved in situ sample environments for future photovoltaic device characterizations.
In organic and organic/inorganic hybrid materials, the role of the exciton reservoir (i.e., uncoupled excitons) towards populating exciton-polariton states —emerging when an excitonic transition strongly couples with a microcavity optical mode— is not well understood. Here, we identify many-body processes in the exciton reservoir by probing the time-resolved nonlinear photoluminescence of polaritons in an organic dye (1,6,7,12-bay-substituted perylene-diimide derivative) and a Ruddlesden-Popper (PEA)2PbI4 perovskite. We observe that, in the dye, exciton-exciton annihilation is hindered in the strong light-matter coupling regime and, in (PEA)2PbI4, Auger recombination obstructs the population of exciton-polaritons via radiative pumping by the emission of the exciton reservoir.
Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation, and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells. In this presentation, I will discuss the role of electronic defects and how these can be passivated to improve charge-carrier lifetimes and to achieve high open-circuit voltages. I will discuss the characterization of 2D and 3D defects, such as grain boundaries, crystal surface defects, and precipitate formation within the films, by synchrotron-based techniques. The importance of interfaces and their contribution to detrimental recombination will also be discussed. As a result of these contributions to better understanding 2D and 3D defects, the perovskite solar cell field has been able to improve device performance. Albeit the rapid improvements in performance, there is still a need to improve these defects to push these solar cells beyond the current state-of-the-art.
To mitigate perovskites’ degradation, there have been a pressing need to identify the effects of environmental stressors on material physical behavior and device performance. We implement high-throughput environmental photoluminescence (PL) to interrogate the response of Cs-FA perovskites with a range of chemical composition while exposed to temperature and relative humidity cycles. These measurements are used as input when comparing how machine learning methods can be realized to forecast material response. We quantitatively compare linear regression, Echo State Network (ESN), and Auto-Regressive Integrated Moving Average with eXogenous regressors (ARIMAX).
Perovskite solar cells promise to yield efficiencies beyond 30% by improving the quality of the materials and devices. Electronic defect passivation and suppression of detrimental charge-carrier recombination has been used as a strategy to achieve high performance perovskite solar cells. This strategy often relies on interlayers that may also hinder carrier transfer across interfaces. In addition to interfacial charge transport, very little is known about the role of crystallographic structure on charge carrier transport through the bulk of the material.
In this presentation, I will discuss how different crystallographic phases of the perovskite affect charge carrier transport. Interfacial transport across interfaces will be further studied to understand whether crystalline structures or amorphous phases are able to efficiently allow transport out the device. Synchrotron-based characterization techniques, such as grazing incidence x-ray spectroscopy and x-ray fluorescence will be used to understand the structural and chemical composition of the films, whereas intensity-modulated photocurrent spectroscopy will be used to understand transport processes in the devices. We show that the orthorhombic phase of the methylammonium lead iodide perovskite forms along with the tetragonal phase and hinders carrier transport and thus short circuit currents. We also show that the crystal structure of the 2D perovskite used as an interlayer in the perovskite solar cell is crucial to efficient charge extraction.
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