14 research outputs found
Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells
Strong
Coulomb interactions in organic photovoltaic cells dictate
that charges must separate over relatively long distances in order
to circumvent geminate recombination and produce photocurrent. In
this article, we measure the distance distributions of thermalized
charge pairs by accessing a regime at low temperature where charge
pairs are frozen out following the primary charge separation step
and recombine monomolecularly via tunneling. The exponential attenuation
of tunneling rate with distance provides a sensitive probe of the
distance distribution of primary charge pairs, reminiscent of electron
transfer studies in proteins. By fitting recombination dynamics to
distributions of recombination rates, we identified populations of
charge-transfer states and well-separated charge pairs. For the wide
range of materials we studied, the yield of separated charges in the
tunneling regime is strongly correlated with the yield of free charges
measured via their intensity-dependent bimolecular recombination dynamics
at room temperature. We therefore conclude that populations of free
charges are established via long-range charge separation within the
thermalization time scale, thus invoking early branching between free
and bound charges across an energetic barrier. Subject to assumed
values of the electron tunneling attenuation constant, we estimate
critical charge separation distances of ∼3–4 nm in all
materials. In some blends, large fullerene crystals can enhance charge
separation yields; however, the important role of the polymers is
also highlighted in blends that achieved significant charge separation
with minimal fullerene concentration. We expect that our approach
of isolating the intrinsic properties of primary charge pairs will
be of considerable value in guiding new material development and testing
the validity of proposed mechanisms for long-range charge separation
Transient Grating Photoluminescence Spectroscopy: An Ultrafast Method of Gating Broadband Spectra
Ultrafast photoluminescence (PL) spectroscopy can cleanly resolve excited-state dynamics and coupling to the environment, however, there is a demand for new methods that combine broadband detection and low backgrounds. We present a new method, transient grating photoluminescence spectroscopy (TGPLS), that addresses this challenge by exploiting a focusing geometry where ultrafast broadband spectra are transiently diffracted away from the background PL. We show that TGPLS can resolve the complex spectral relaxation observed in conjugated polymer and oligomer solutions, with an essentially flat spectral response throughout the visible region and potentially beyond. The benefits we demonstrate using TGPLS could expand access to spectral information, particularly for other multichromophoric and heterogeneous materials where complex spectral relaxation is expected
Transient Grating Photoluminescence Spectroscopy: An Ultrafast Method of Gating Broadband Spectra
Ultrafast photoluminescence (PL) spectroscopy can cleanly resolve excited-state dynamics and coupling to the environment, however, there is a demand for new methods that combine broadband detection and low backgrounds. We present a new method, transient grating photoluminescence spectroscopy (TGPLS), that addresses this challenge by exploiting a focusing geometry where ultrafast broadband spectra are transiently diffracted away from the background PL. We show that TGPLS can resolve the complex spectral relaxation observed in conjugated polymer and oligomer solutions, with an essentially flat spectral response throughout the visible region and potentially beyond. The benefits we demonstrate using TGPLS could expand access to spectral information, particularly for other multichromophoric and heterogeneous materials where complex spectral relaxation is expected
Broadband Ultrafast Photoluminescence Spectroscopy Resolves Charge Photogeneration via Delocalized Hot Excitons in Polymer:Fullerene Photovoltaic Blends
Conventional
descriptions of excitons in semiconducting polymers
do not account for several important observations in polymer:fullerene
photovoltaic blends, including the ultrafast time scale of charge
photogeneration in phase separated blends and the intermediate role
of delocalized charge transfer states. We investigate the nature of
excitons in thin films of polymers and polymer:fullerene blends by
using broadband ultrafast photoluminescence spectroscopy. Our technique
enables us to resolve energetic relaxation, as well as the volume
of excitons and population dynamics on ultrafast time scales. We resolve
substantial high-energy emission from hot excitons prior to energetic
relaxation, which occurs predominantly on a subpicosecond time scale.
Consistent with quantum chemical calculations, ultrafast annihilation
measurements show that excitons initially extend along a substantial
chain length prior to localization induced by structural relaxation.
Moreover, we see that hot excitons are initially highly mobile and
the subsequent rapid decay in mobility is correlated with energetic
relaxation. The relevance of these measurements to charge photogeneration
is confirmed by our measurements in blends. We find that charge photogeneration
occurs predominately via these delocalized hot exciton states in competition
with relaxation and independently of temperature. As well as accounting
for the ultrafast time scale of charge generation across large polymer
phases, delocalized hot excitons may also account for the crucial
requirement that primary charge pairs are well separated in efficient
organic photovoltaic blends
Effect of Carrier Thermalization Dynamics on Light Emission and Amplification in Organometal Halide Perovskites
The remarkable rise of organometal halide perovskites as solar
photovoltaic materials has been followed by promising developments
in light-emitting devices, including lasers. Here we present unique
insights into the processes leading to photon emission in these materials.
We employ ultrafast broadband photoluminescence (PL) and transient
absorption spectroscopies to directly link density dependent ultrafast
charge dynamics to PL. We find that exceptionally strong PL at the
band edge is preceded by thermalization of free charge carriers. Short-lived
PL above the band gap is clear evidence of nonexcitonic emission from
hot carriers, and ultrafast PL depolarization confirms that uncorrelated
charge pairs are precursors to photon emission. Carrier thermalization
has a profound effect on amplified stimulated emission at high fluence;
the delayed onset of optical gain we resolve within the first 10 ps
and the unusual oscillatory behavior are both consequences of the
kinetic interplay between carrier thermalization and optical gain
Photoinduced Emissive Trap States in Lead Halide Perovskite Semiconductors
The recent success of lead halide
perovskites is given by their optimal primary optoelectronic properties
relevant for photovoltaic and, more in general, for optoelectronic
applications. However, a lack of knowledge about the nature of instabilities
currently represents a major challenge for the development of such
materials. Here we investigate the luminescence properties of polycrystalline
thin films of lead halide perovskites as a function of the excitation
density and the environment. First we demonstrate that in an inert
environment photoinduced formation of emissive sub-band gap defect
states happens, independently of the chemical composition of the lead
halide semiconductor, which quenches the band-to-band radiative emission.
Carrier trapping occurs in the subnanosecond time regime, while trapped
carriers recombine in a few microseconds. Then, we show that the presence
of oxygen, even in a very small amount, is able to compensate such
an effect
Altered Right Ventricular Kinetic Energy Work Density and Viscous Energy Dissipation in Patients with Pulmonary Arterial Hypertension: A Pilot Study Using 4D Flow MRI
<div><p>Introduction</p><p>Right ventricular (RV) function has increasingly being recognized as an important predictor for morbidity and mortality in patients with pulmonary arterial hypertension (PAH). The increased RV after-load increase RV work in PAH. We used time-resolved 3D phase contrast MRI (4D flow MRI) to derive RV kinetic energy (KE) work density and energy loss in the pulmonary artery (PA) to better characterize RV work in PAH patients.</p><p>Methods</p><p>4D flow and standard cardiac cine images were obtained in ten functional class I/II patients with PAH and nine healthy subjects. For each individual, we calculated the RV KE work density and the amount of viscous dissipation in the PA.</p><p>Results</p><p>PAH patients had alterations in flow patterns in both the RV and the PA compared to healthy subjects. PAH subjects had significantly higher RV KE work density than healthy subjects (94.7±33.7 mJ/mL vs. 61.7±14.8 mJ/mL, p = 0.007) as well as a much greater percent PA energy loss (21.1±6.4% vs. 2.2±1.3%, p = 0.0001) throughout the cardiac cycle. RV KE work density and percent PA energy loss had mild and moderate correlations with RV ejection fraction.</p><p>Conclusion</p><p>This study has quantified two kinetic energy metrics to assess RV function using 4D flow. RV KE work density and PA viscous energy loss not only distinguished healthy subjects from patients, but also provided distinction amongst PAH patients. These metrics hold promise as imaging markers for RV function.</p></div
RV KE density and percent viscous energy loss together separated the PAH subjects from the healthy controls.
<p>In addition, PAH patients within the same functional classes had a wide distribution.</p
Comparing viscous energy loss in healthy subjects and patients with pulmonary arterial hypertension.
<p>(A) The amount of viscous energy loss is higher in patients throughout the cardiac cycle. (B) The percent viscous energy loss is significantly greater in PAH patients than in healthy subjects (21.1±6.4% vs. 2.2±1.3%, p = 0.0001).</p
Comparison of kinetic energy density and viscous energy loss in PAH and healthy subjects.
<p>A. Healthy subjects showed significantly lower RV KE density than PAH patients (61.7±14.8 mJ/mL vs. 94.7±33.7 mJ/mL, p = 0.007). Linear regression of RV KE density against RVEF for both populations gives result: y = -1.47x + 152.00, R² = 0.11. B. Healthy subjects showed significantly lower percent viscous energy loss than PAH patients (2.2±1.3% vs. 21.1±6.4%, p = 0.0001). Linear regression of percent viscous energy loss against RVEF for both populations gives result: y = -1.05x + 63.82, R² = 0.41.</p