89 research outputs found
The role of a dark exciton reservoir in the luminescence efficiency of two-dimensional tin iodide perovskites
No full textWe report on the excitonic luminescence of polycrystalline films composed of two-dimensional (2D) tin iodide perovskites. By combining steady-state and time-resolved spectroscopies between room temperature and 5 K, we identify an optically inactive, dark excitonic state within the spectral fine structure at the exciton energy, ubiquitous in the optical spectra of 2D perovskites. Lying at about 10 meV below the bright exciton that is responsible for luminescence, the dark state mediates non-radiative decay of the photo-excited population at temperatures below 100 K. However, at about 100–120 K, we observe a thermally activated transfer of population from the dark state back to the bright state which results in an increase in luminescence efficiency. Based on quantitative analysis of the observed exciton dynamics, we argue that the dark state acts as a reservoir of photo-excitations at ambient temperature. By feeding the bright state at a rate that is slower than the radiative rate, the dark state mitigates the loss of photo-excited population to other non-radiative pathways. Our work provides insights into the dynamics of the inevitable dark states in 2D perovskites and their relevance in enhancing the emissivity of the technologically relevant lead-free material architectures
Charge generation at polymer/metal oxide interface: From molecular scale dynamics to mesoscopic effects
No full textPlurality of excitons in Ruddlesden–Popper metal halides and the role of the B-site metal cation
No full textWe investigate the effect of metal cation substition on the excitonic structure and dynamics in a prototypical Ruddlesden-Popper metal halide. Through an in-depth spectroscopic and theoretical analysis, we identify the presence of multiple resonances in the optical spectra of a phenethyl ammonium tin iodide, a tin-based RPMH. Based on ab initio calculations, we assign these resonances to distinct exciton series that originate from the splitting of the conduction band due to spin-orbit coupling. While the splitting energy in the tin based system is low enough to enable the observation of the higher lying exciton in the visible-range spectrum of the material, the higher splitting energy in the lead counterpart prevents the emergence of such a feature. We elucidate the critical role played by the higher lying excitonic state in the ultrafast carrier thermalization dynamics
Identifying incoherent mixing effects in the coherent two-dimensional photocurrent excitation spectra of semiconductors
No full text: We have previously demonstrated that in the context of two-dimensional (2D) coherent electronic spectroscopy measured by phase modulation and phase-sensitive detection, an incoherent nonlinear response due to pairs of photoexcitations produced via linear excitation pathways contributes to the measured signal as an unexpected background [Grégoire et al., J. Chem. Phys. 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 the crosstalk between the linear amplitudes 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 modeling of a simple system
Photoinduced Emissive Trap States in Lead Halide Perovskite Semiconductors
No full textThe 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
Probing femtosecond lattice displacement upon photo-carrier generation in lead halide perovskite
No full textElectronic properties and lattice vibrations are expected to be strongly correlated in metal-halide perovskites, due to the soft fluctuating nature of their crystal lattice. Thus, unveiling electron-phonon coupling dynamics upon ultrafast photoexcitation is necessary for understanding the optoelectronic behavior of the semiconductor. Here, we use impulsive vibrational spectroscopy to reveal vibrational modes of methylammonium lead-bromide perovskite under electronically resonant and non-resonant conditions. We identify two excited state coherent phonons at 89 and 106 cm-1, whose phases reveal a shift of the potential energy minimum upon ultrafast photocarrier generation. This indicates the transition to a new geometry, reached after approximately 90 fs, and fully equilibrated within the phonons lifetime of about 1 ps. Our results unambiguously prove that these modes drive the crystalline distortion occurring upon photo-excitation, demonstrating the presence of polaronic effects
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