Apart from broadband absorption of solar radiation, the performance of photovoltaic devices is governed by the density and mobility of photogenerated charge carriers. The latter parameters indicate how many free carriers move away from their origin, and how fast, before loss mechanisms such as carrier recombination occur. However, only lower bounds of these parameters are usually obtained. Here we independently determine both density and mobility of charge carriers in a perovskite film by the use of time-resolved terahertz spectroscopy. Our data reveal the modification of the free carrier response by strong backscattering expected from these heavily disordered perovskite films. The results for different phases and different temperatures show a change of kinetics from two-body recombination at room temperature to three-body recombination at low temperatures. Our results suggest that perovskite-based solar cells can perform well even at low temperatures as long as the three-body recombination has not become predominant.

1 tex file (6 pages), 4 (eps) figuresThe beam spin asymmetries in the hard exclusive electroproduction of photons on the proton (ep -> epg) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (x_B from 0.11 to 0.58, Q^2 from 1 to 4.8 GeV^2, -t from 0.09 to 1.8 GeV^2), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A = a*sin(phi)/[1+c*cos(phi)]. This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector

Strong spin–orbit coupling, resulting in the formation of spin‐momentum‐locked surface states, endows topological insulators with superior spin‐to‐charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, an all‐optical method is presented, which enables unprecedented tracking of the ultrafast dynamics of spin‐to‐charge conversion in a prototypical topological insulator Bi2Se3/ferromagnetic Co heterostructure, down to the sub‐picosecond timescale. Compared to pure Bi2Se3 or Co, a giant terahertz emission is observed in the heterostructure that originates from spin‐to‐charge conversion, in which the topological surface states play a crucial role. A 0.12 ps timescale is identified that sets a technological speed limit of spin‐to‐charge conversion processes in topological insulators. In addition, it is shown that the spin‐to‐charge conversion efficiency is temperature independent in Bi2Se3 as expected from the nature of the surface states, paving the way for designing next‐generation high‐speed optospintronic devices based on topological insulators at room temperature.

Farout-of-equilibrium spin populations trigger giant spin injection into atomically thin MoS2.

Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ1(ω)] of twisted bilayer graphene was obtained at different temperatures (10 -300 K) in the frequency range 0.3 -3 THz. On top of a Drude-like response, we see a strong peak in σ1(ω) at ∼2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, that is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers.Compared to single-layer graphene (SLG), where there are two non-equivalent lattice sites (A and B), bilayer graphene (BLG) has two SLGs stacked in the third direction. In the most common Bernal (AB) stacking of BLG, adjacent layers are rotated by 60 • , so that the B atoms of layer 2 (B ′ ) sits directly on top of A atoms in layer 1 (A), and B and A ′ atoms are in the center of the hexagons of the opposing layer. Electrons can then hop between these two A sites with a hopping energy t ⊥ . In the undoped case, though both SLG and BLG are gapless semi-metals, carriers in SLG exhibit linear dispersion, while those in BLG show quadratic dispersion. An energy gap in SLG opens up due to finite geometry effects, but its control has proven to be unreliable [1]. On the other hand, the electronic gap in BLG can be reliably opened and controlled by an applied electric field, shown theoretically and demonstrated experimentally [2][3][4][5], and promises interesting applications. Both SLG and BLG however, are sensitive to disorder. Hence, to realize graphene-based optoelectronic devices, an understanding of the temperature and disorder effects in the transport and spectroscopic properties of BLG is needed. Temperature and disorder-dependent conductivity of BLG have been derived theoretically [1,6]. Experimentally, spectroscopies (from terahertz (THz) to visible) and ultrafast dynamics of various flavors of graphene have been reported, such as SLG, few and many-layer graphene, and graphite [7][8][9][10][11]. For example, Fourier-transform infrared spectroscopy (FTIR) on large-area SLG grown by chemical vapor deposition (CVD) revealed a Drude-like frequency dependence of the spectral density from THz to mid-infrared at different carrier concentrations [12]. In addition, graphene plasmons, which lie in the THz range, are strongly coupled to the interband electronic transitions and decay by exciting interband electron-hole pairs [13]. Hence knowledge of graphene's electromagnetic response, as a function of disorder, in the THz frequency range is critical for applications such as graphene-based THz oscillators [14].