3351wileyonlinelibrary.com for the monolayer. Hence, the monolayer, contrary to bi-and multilayers, behaves like a direct gap semiconductor and shows signifi cant fl uorescence. [ 11,12 ] The exciton binding energy for bulk MoS 2 has been determined to be 45 and 130 meV for the A and B excitons, respectively. [ 13 ] Both exciton binding energies increase upon decreasing the sample thickness, with estimates for monolayer [14][15][16] ranging from 0.4 to 0.9 eV. Despite this high exciton binding energy, monolayer MoS 2 shows a strong photovoltaic effect [ 17 ] and potential for high sensitivity photodetectors. [ 18 ] Both these functionalities require effi cient charge carrier photogeneration (CPG), either via direct excitation of mobile carriers or via exciton dissociation.The spectral signature of charge carriers has been identifi ed by absorption and fl uorescence spectroscopy of MoS 2 , where the charge concentration varies either via the gate voltage in an FET geometry [ 19 ] or via adsorption, [ 20 ] or substrate doping. [ 21 ] The absorption peaks of charges are red-shifted by about 40 meV compared with the ground-state absorption into the A and B excitons and have been attributed to optical transitions from a charged ground state to a charged exciton (trion). The possibility of alternative interpretations, such as polarons [ 22,23 ] or Stark effect in the local electric fi eld of the charges, [24][25][26] does The 2D semiconductor MoS 2 in its mono-and few-layer form is expected to have a signifi cant exciton binding energy of several 100 meV, suggesting excitons as the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating effi cient charge carrier photogeneration. Here, modulation spectroscopy in the sub-ps and ms time scales is used to study the photoexcitation dynamics in few-layer MoS 2 . The results suggest that the primary photoexcitations are excitons that effi ciently dissociate into charges with a characteristic time of 700 fs. Based on these fi ndings, simple suggestions for the design of effi cient MoS 2 photovoltaic and photodetector devices are made.

Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better separation of the spectral features compared to MoS2 allows us to resolve a previously undetected process: these charges diffuse through the samples and get trapped at defects, such as flake edges or grain boundaries, causing an appreciable change of their transient absorption spectra. This finding opens the way to further studies of traps in TMD samples with different defect contents.

Many of the best performing solar cells based on perovskite-halide light absorbers use TiO2 as an electron selective contact layer. However, TiO2 usually requires high temperature sintering, is related to electrical instabilities in perovskite solar cells, and causes cell performance degradation under full solar spectrum illumination. Here we demonstrate an alternative approach based on the modification of transparent conductive oxide electrodes with self-assembled siloxane-functionalized fullerene molecules, eliminating TiO2 or any other additional electron transporting layer. We demonstrate that these molecules spontaneously form a homogenous monolayer acting as an electron selective layer on top of the fluorine doped tin oxide (FTO) electrode, minimizing material consumption. We find that the fullerene-modified FTO is a robust, chemically inert charge selective contact for perovskite based solar cells, which can reach 15% of stabilised power conversion efficiency in a flat junction device architecture using a scalable, low temperature, and reliable process. In contrast to TiO2, devices employing a molecularly thin functionalized fullerene layer show unaffected performance after 67 h of UV light exposure

This work reports a straightforward and scalable low-temperature synthesis of methylammonium lead tri-iodide (MAPbI 3 ) perovskite particles. Inks are formulated in benign solvents and are printed in ambient conditions for the fabrication of a light detector with high detectivity. SUMMARYHere, we propose a simple and low-temperature approach for the synthesis of methylammonium lead halide perovskite inks based on sub-micrometer-sized particles with tunable band gap. The particles allow the formulation of inks in benign solvents, such as propan-2-ol, and the printing of the photoactive layers of planar photoconductors with scalable, large-area coating techniques under ambient conditions. When surface traps are passivated with [6,6]-phenyl-C61-butyric acid methyl ester fullerene (PCBM), a high photoconductive gain (exceeding 200 in the blue) and reduced noise produce record-high specific detectivities exceeding 7 3 10 13 cmHz 0.5 /W and gain-bandwidth product values of 7.5 3 10 6 Hz. Given the extreme simplicity of the presented device architecture and the straightforward processing, yielding printable light detectors rivalling established technologies, the present work promises a short-term deployment of printed perovskite detectors in a multitude of opto-electronic applications.

The development of novel hole transporting materials (HTMs) for perovskite solar cells (PSCs) that can enhance device's reproducibility is a largely pursued goal, even to the detriment of a very high efficiency, since it paves the way to an effective industrialization of this technology. In this work, we study the covalent functionalization of reduced graphene oxide (RGO) flakes with different organic functional groups with the aim of increasing the stability and homogeneity of their dispersion within a poly(3‐hexylthiophene) (P3HT) HTM. The selected functional groups are indeed those recalling the two characteristic moieties present in P3HT, i.e., the thienyl and alkyl residues. After preparation and characterization of a number of functionalized RGO@P3HT blends, we test the two containing the highest percentage of dispersed RGO as HTMs in PSCs and compare their performance with that of pristine P3HT and of the standard Spiro‐OMeTAD HTM. Results reveal the big influence of the morphology adopted by the single RGO flakes contained in the composite HTM in driving the final device performance and allow to distinguish one of these blends as a promising material for the fabrication of highly reproducible PSCs.

No abstract

Abstract. Ionic surfactants, which are widely used to stabilize nanomaterials in dispersions, can drastically alter the nanomaterial's photophysical properties. Here, we use femtosecond optical spectroscopy to study the dynamics of excitons and charges in few-layer flakes of the twodimensional semiconductor MoS 2 . We compare samples obtained via exfoliation in water with different amounts of adsorbed sodium cholate, obtained by repeated washing of the dried flakes. We find that the femtosecond dynamics is remarkably stable against the surfactant adsorption, with a slight increase of the initial exciton quenching occurring during the first few picoseconds as the only appreciable effect.

The work function W of Mo(6)S(3)I(6) molecular nanowires is determined by Kelvin probe (KP) measurements, UV photoelectron spectroscopy (UPS), and cyclic voltammetry (CV). The values obtained by all three methods agree well, giving W = 4.8 ± 0.1 eV. CV measurements also give a gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of E(g) = 1.2 ± 0.1 eV, in agreement with recent optical measurements, but in disagreement with theoretical calculations, which predict the material to be a metal. The electronic structure of Mo(6)S(3)I(6) suggests use of the material in applications such as bulk heterostructure photovoltaics and transparent electrodes and for molecular electronics devices.