Cesare Soci

Lead iodide perovskite light-emitting field-effect transistor

Despite the widespread use of solution-processable hybrid organic–inorganic perovskites in photovoltaic and light-emitting applications, determination of their intrinsic charge transport parameters has been elusive due to the variability of film preparation and history-dependent device performance. Here we show that screening effects associated to ionic transport can be effectively eliminated by lowering the operating temperature of methylammonium lead iodide perovskite (CH3NH3PbI3) field-effect transistors. Field-effect carrier mobility is found to increase by almost two orders of magnitude below 200 K, consistent with phonon scattering-limited transport. Under balanced ambipolar carrier injection, gate-dependent electroluminescence is also observed from the transistor channel, with spectra revealing the tetragonal to orthorhombic phase transition. This demonstration of CH3NH3PbI3 light-emitting field-effect transistors provides intrinsic transport parameters to guide materials and solar cell optimization, and will drive the development of new electro-optic device concepts, such as gated light-emitting diodes and lasers operating at room temperature.

Direct Heteroepitaxy of Vertical InAs Nanowires on Si Substrates for Broad Band Photovoltaics and Photodetection

Catalyst-free, direct heteroepitaxial growth of vertical InAs nanowires on Si(111) substrates was accomplished over a large area by metal-organic chemical vapor deposition. Nanowires showed very uniform diameters and a zinc blende crystal structure. The heterojunctions formed at the interface between the n-type InAs nanowires and the p-type Si substrate were exploited to fabricate vertical array photodiode devices which showed an excellent rectification ratio and low reverse leakage current. Temperature-dependent current transport across the heterojunctions was studied theoretically and experimentally in the dark and under AM 1.5 illumination. When operated in photovoltaic mode, the open-circuit voltage was found to increase linearly with decreasing temperature while the energy conversion efficiency changed nonmonotonically with a maximum of 2.5% at 110 K. Modeling of the nanowire/substrate heterojunctions showed good agreement with the experimental observations, and allowed determining the conduction band offset between the InAs nanowires and Si to be 0.10-0.15 eV. The external quantum efficiency and photoresponsivity profiles of the device showed a broad spectral response from the visible to the infrared region, indicating potential applications as a broad band photovoltaic cell or a visible-infrared dual-band photodetector.

Method for increasing the photoconductive response in conjugated polymer/fullerene composites

The authors poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester present a technique for modifying the internal structure of bulk heterojunction films which yields significantly improved photoconductivity. This method comprises the addition of alkyl thiol molecules to the polymer/fullerene solution prior to spin coating. Based on the steady-state and transient photoconductivity measurements, the photoresponsivity is an order of magnitude greater than that in films spun from pristine solvent; additionally, the carrier mobility measurements indicate significant increase in the hole mobility, consistent with the enhanced structural order inferred from x-ray diffraction and optical absorption measurements.

Nanoporous walls on macroporous foam : rational design of electrodes to push areal pseudocapacitance

Supercapacitors, also known as electrochemical capacitors, are considered the most promising energy storage devices owing to their high power densities and long lifespan. [ 3–5 ] The fast charge and discharge capability make supercapacitors favorable for applications in hybrid vehicles, portable electronics, and backup energy systems. [ 6–10 ] Carbonaceous materials, including carbon nanotubes and graphene, are being widely studied as alternatives to conventional graphites. [ 2 , 11–14 ] However, carbon-based materials usually show low energy density as they store charges electrostatically at their surfaces, so they have intrinsically low specifi c areal capacitance ( C a ) in the range of 10 − 40 μ F cm − 2 . Transition metal oxides/hydroxides store charges with surface faradaic (redox) reactions, which enable higher energy density compared to carbon. Metal oxides/hydroxides such as MnO 2 , NiO, Ni(OH) 2 , CoO x and their compounds have recently come into focus in the design of high-energy-density charge-storage materials. [ 15–27 ] Despite these efforts, practical energy storage applications still require higher specifi c capacitance. One way out is to design nanometer-scale electrode materials with very large surface areas and structural stability. In this context, porous nanostructures are of great interest because they can reduce ionic and electronic diffusion distance and provide large electrode/electrolyte contact area. For example, porous Ni and Au electrodes as current collectors have recently been reported, which signifi cantly improve the specifi c capacitance when covered with the pseudoactive material MnO 2 . [ 28 , 29 ] Also, nanoporous graphene electrodes with ∼ 4 nm pores drastically enhance the specifi c capacitance up to 166 F g − 1 . [ 30 ] For metal oxides,

Light emission from an ambipolar semiconducting polymer field-effect transistor

Ambipolar light-emitting field-effect transistors are fabricated with two different metals for the top-contact source and drain electrodes; a low-work-function metal defining the channel for the source electrode and a high-work-function metal defining the channel for the drain electrode. A thin film of polypropylene-co-1-butene on SiNx is used as the gate dielectric on an n++-Si wafer, which functioned as the substrate and the gate electrode. Transport data show ambipolar behavior. Recombination of electrons and holes results in a narrow zone of light emission within the channel. The location of the emission zone is controlled by the gate bias.

Hollow core–shell nanostructure supercapacitor electrodes: gap matters

Hollow core–shell nanorods with a nanogap are designed and constructed with the assistance of atomic layer deposition (ALD) for energy storage applications. As a demonstration, CoO nanorods and NiO nanowalls are enclosed by a TiO2 nanotube shell, forming the “wire in tube” and “wall in box” structures, respectively. A thin sacrificial layer of Al2O3 is deposited by ALD and removed eventually, forming a nanogap between the CoO core (or the NiO nanowall) and the TiO2 shell. When they are tested as supercapacitor electrodes, an evident difference between the solid core–shell nanostructure and hollow ones can be found; for example, the hollow structure shows ∼2 to 4 times the capacitance compared to the solid wires. The electrochemical properties are also superior compared to the bare nanorods without the nanotube shell. The enhancement is ascribed to the conformal hollow design which provides enlarged specific surface areas and a shorter ion transport path. It is prospected that such a positive nanogap effect may also exist in other electrochemical cell electrodes such as lithium ion batteries and fuel cells.

Novel hole transporting materials based on triptycene core for high efficiency mesoscopic perovskite solar cells

Three novel hole-conducting molecules (T101, T102 and T103) based on a triptycene core have been synthesized using short routes with high yields. The optical and electrochemical properties were tuned by modifying the functional groups, through linking the triptycene to diphenylamines via phenyl and/or thienyl groups. The mesoporous perovskite solar cells fabricated using T102 and T103 as the hole transporting material (HTM) showed a power conversion efficiency (PCE) of 12.24% and 12.38%, respectively, which is comparable to that obtained using the best performing HTM spiro-OMeTAD. The T102 based device showed higher fill factor (69.1%) and Voc (1.03 V) than the spiro-OMeTAD based device (FF = 63.4%, Voc = 0.976 V) whereas the T103 based device showed comparable Jsc (20.3 mA cm−2) and higher Voc (0.985 V) than the spiro-OMeTAD (Jsc = 20.8 mA cm−2) based cell.

Lead-Free MA2CuClxBr4–x Hybrid Perovskites

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)2CuCl(x)Br(4-x) was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu(+) ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the materials properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.

Influence of surface states on the extraction of transport parameters from InAs nanowire field effect transistors

The capacitive effects of interface trap states in top-gated InAs nanowire field effect transistors and their influence on the experimental extraction of transport parameters are discussed. Time resolved transfer characteristics exhibit transient behavior indicating surface state trapping and detrapping with long characteristic time constants of 45s. Varying gate voltage sweep rate results in a time-dependent extrinsic transconductance; a reduced gate voltage sweep rate leads to a charge neutral interface, reduced interface state capacitance, higher measured transconductance, and minimal hysteresis. These results demonstrate that measurements with a charge neutralized or passivated surface are key to extract intrinsic nanowire transport parameters.

GaAs/AlGaAs Nanowire Photodetector

We demonstrate an efficient core-shell GaAs/AlGaAs nanowire photodetector operating at room temperature. The design of this nanoscale detector is based on a type-I heterostructure combined with a metal-semiconductor-metal (MSM) radial architecture, in which built-in electric fields at the semiconductor heterointerface and at the metal/semiconductor Schottky contact promote photogenerated charge separation, enhancing photosensitivity. The spectral photoconductive response shows that the nanowire supports resonant optical modes in the near-infrared region, which lead to large photocurrent density in agreement with the predictions of electromagnetic and transport computational models. The single nanowire photodetector shows a remarkable peak photoresponsivity of 0.57 A/W, comparable to large-area planar GaAs photodetectors on the market, and a high detectivity of 7.2 × 10(10) cm·Hz(1/2)/W at λ = 855 nm. This is promising for the design of a new generation of highly sensitive single nanowire photodetectors by controlling the optical mode confinement, bandgap, density of states, and electrode engineering.