arXiv subjects

David A. Valverde-Chávez

Publications and source records attributed to David A. Valverde-Chávez.

4 recordsLinked to original sources

arXiv2022

Identifying incoherent mixing effects in the coherent two-dimensional photocurrent excitation spectra of semiconductors

We have previously demonstrated that in the context of two-dimensional (2D) coherent electronic spectroscopy measured by phase modulation and phase-sensitive detection, an \emph{incoherent} nonlinear response, due to pairs of photoexcitations produced via linear excitation pathways, contribute to the measured signal as unexpected background [Grégoire et al., J.\ Chem.\ Phys. \textbf{147}, 114201 (2017)]. Here, we simulate the effect of such incoherent population mixing in the photocurrent signal collected from a GaAs solar cell by acting externally on the transimpedance amplifier circuit used for phase-sensitive detection, and we identify an effective strategy to recognize the presence of incoherent population mixing in 2D data. While we find that incoherent mixing is reflected by cross-talk between the linear amplitude at the two time-delay variables in the four-pulse excitation sequence, we do not observe any strict phase correlations between the coherent and incoherent contributions, as expected from modelling of a simple system.

cond-mat.mtrl-sci↗

arXiv2021

Frenkel biexcitons in hybrid HJ photophysical aggregates

Frenkel excitons, the primary photoexcitations in organic semiconductors that are unequivocally responsible for the optical properties of this materials class, are predicted to form \emph{bound} exciton pairs, i.e., biexcitons. These are key intermediates, ubiquitous in many relevant photophysical processes; for example, they determine the exciton bimolecular annihilation dynamics in such systems. Deciphering the details of biexciton correlations is, thus, of utmost importance to understand the optical processes in these semiconductors. To date, however, due to their spectral ambiguity, there has been only scant direct evidence of bound biexcitons, limiting the insights that can be gained. Moreover, a quantum-mechanical basis describing biexciton correlation/stability has so far been lacking. By employing nonlinear coherent spectroscopy, we identify here bound biexcitons in a model polymeric semiconductor. We find, unexpectedly, that excitons with \emph{interchain} vibronic dispersion reveal \emph{intrachain} biexciton correlations and vice versa. Moreover, using a Frenkel exciton model, we can relate the biexciton binding energy to molecular parameters quantified by quantum chemistry, including the magnitude and sign of the exciton-exciton interaction the inter-site hopping energies. Therefore, our work promises a window towards general insights into the many-body electronic structure in polymeric semiconductors and beyond; e.g., other excitonic systems such as organic semiconductor crystals, molecular aggregates, photosynthetic light-harvesting complexes, or DNA.

cond-mat.mtrl-sci↗

arXiv2019

Electron-phonon couplings inherent in polarons drive exciton dynamics in two-dimensional metal-halide perovskites

We report on the exciton formation and relaxation dynamics following photocarrier injection in a single-layer two-dimensional lead-iodide perovskite. We probe the time evolution of four distinct exciton resonances by means of time-resolved photoluminescence and transient absorption spectroscopies, and find that at 5\,K a subset of excitons form on a $\lesssim$ 1-ps timescale, and that these relax subsequently to lower-energy excitons on $\sim$ 5--10\,ps with a marked temperature dependence over $<$ 100\,K. We implement a mode projection analysis that determines the relative contribution of all observed phonons with frequency $\leq$50\,cm$^{-1}$ to inter-exciton nonadiabatic coupling, which in turn determines the rate of exciton relaxation. This analysis ranks the relative contribution of the phonons that participate in polaronic lattice distortions to the exciton inter-conversion dynamics and thus establishes their role in the nonadiabatic mixing of exciton states, and this in the exciton relaxation rate.

cond-mat.mtrl-sci↗

arXiv2015

Intrinsic femtosecond charge generation dynamics in a single crystal organometal halide perovskite

Hybrid metal-organic perovskite solar cells have astounded the solar cell community with their rapid rise in efficiency over the past three years. Despite this success, the basic processes governing the photogeneration of free charges, particularly their dynamics and efficiency, remain unknown. Here we use ultrabroadband pulses of THz frequency light to see the intrinsic photophysical properties of single crystal lead halide perovskite just femtoseconds after a photon is first absorbed. Our spectra reveal the dynamics and efficiencies of free charge creation, the remarkable ease in which they move through the lattice and the complicated interplay between free and bound charges in these materials.

cond-mat.mtrl-sci↗