arXiv subjects

A. Di Carlo

Publications and source records attributed to A. Di Carlo.

16 recordsLinked to original sources

arXiv2026

Effect of FABr Over-Stoichiometry on the Morphology and Optoelectronic Properties of Wide-Bandgap FAPbBr_3 Films

In this study, we investigate the impact of formamidinium bromide (FABr) over-stoichiometry in the precursor solution on the optoelectronic properties and morphology of the resulting films of formamidinium lead bromide (FAPbBr_3). Optical characterization, including steady-state absorption, photoluminescence (PL), and femtosecond transient absorption spectroscopy, reveals a systematic blueshift in emission energy with increasing FABr content, attributed to the passivation of bromine vacancies and to the reduction of defect-assisted recombination. Power-dependent PL confirms this interpretation: the stoichiometric film exhibits a PL band due to donor-acceptor pair (DAP) recombination as identified by the typical excitation-dependent blueshift, whereas FABr-enriched samples show no evidence of DAP emission, indicating effective defect passivation. Additionally, morphological characterization shows a reduction in grain size with increasing FABr excess, indicating a trade-off between improved electronic quality and enhanced structural disorder. The film synthesized with a 5% excess of FABr provides the optimal balance, yielding the highest power conversion efficiency (6.26%), average visible transmittance (61.6%), and light utilization efficiency (3.85%). These results demonstrate that fine-tuning the precursor stoichiometry through controlled FABr addition represents a simple yet effective strategy to enhance the optoelectronic quality and performance of semitransparent perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2025

Electron-Hole Separation Dynamics and Optoelectronic Properties of a PCE10:FOIC Blend

Understanding charge separation dynamics in organic semiconductor blends is crucial for optimizing the performance of organic photovoltaic solar cells. In this study, we explored the optoelectronic properties and charge separation dynamics of a PCE10:FOIC blend, by combining steady-state and time-resolved spectroscopies with high-level DFT calculations. Femtosecond transient absorption spectroscopy revealed a significant reduction of the exciton-exciton annihilation recombination rate in the acceptor when incorporated into the blend, compared to its pristine form. This reduction was attributed to a decrease in exciton density within the acceptor, driven by an efficient hole-separation process that was characterized by following the temporal evolution of the transient signals associated with the excited states of the donor when the acceptor was selectively excited within the blend. The analysis of these dynamics enabled the estimation of the hole separation time constant from the acceptor to the donor, yielding a time constant of (1.3 +- 0.3) ps. Additionally, this study allowed the quantification of exciton diffusion and revealed a charge separation efficiency of approximately 60%, providing valuable insights for the design of next-generation organic photovoltaic materials with enhanced charge separation and improved device efficiency.

cond-mat.mtrl-sci↗

arXiv2023

Triphenylamine-based interlayer with carboxyl anchoring group for tuning of charge collection interface in stabilized p-i-n perovskite solar cells and modules

A novel triphenylamine-based hole transport material (HTM) with a carboxyl anchoring group (TPATC) was developed for tuning the interface between nanocrystalline NiO and double cation CsCH3(NH2)2PbI3-xClx absorber in p-i-n device architectures. We present a unique comprehensive confinement study of PSCs with TPATC as the self-assembled HTM, including analysis of numerical defect parameters, phase composition evolution, and up-scaling capabilities. Our investigation shows that the ultrathin TPATC interlayer effectively passivates traps, increases work-function of HTL by about ~0.2 eV, and enhances the charge carrier extraction efficiency. Advanced transient spectroscopy measurements revealed that modification of the NiO surface with TPATC in perovskite solar cells (PSCs) reduces the concentration of ionic defects by an order of magnitude. Interface engineering with TPATC allowed to reach power conversion efficiency of 20.58% for small area devices (0.15 cm2) under standard AM 1.5 G conditions. Using TPATC interlayer also provided stabilized performance of PSCs under operation conditions and improved sustainability of the perovskite absorber to decomposition. After continuous light-soaking (1000 h, ISOS-L-2 protocol), NiO/TPATC devices showed a slight decrease of 2% in maximum power. In contrast, NiO PSCs demonstrated decrease in power output (>20%) after 400 h. We explored the potential of TPATC to modify interfaces in large-area perovskite solar modules (PSM, active area-64.8 cm2, 12 sub-cells). By applying slot-die-coated TPATC, the PCE at AM 1.5 G conditions increased from 13.22% for NiO PSM to 15.64% for NiO/TPATC ones. This study provides new insights into the interface engineering for p-i-n perovskite solar cells, behavior of the ionic defects and their contribution to the long-term stability.

cond-mat.mtrl-sci↗

arXiv2023

Ion-beam sputtering of NiO hole transporting layers for p-i-n halide perovskite solar cells

Ion-beam sputtering offers significant benefits in terms of deposition uniformity and pinhole-free thin-films without limiting the scalability of the process. In this work, the reactive ion-beam sputtering of nickel oxide has been developed for the hole transporting layer of a p-i-n perovskite solar cells (PCSs). The process is carried out by oxidation of the scattered Ni particles with additional post-treatment annealing regimes. Using deposition rate of 1.2 nm/min allowed growth of very uniform NiO coating with the roughness below 0.5 nm on polished Si wafer (15x15 cm2). We performed a complex investigation of structural, optical, surface and electrical properties of the NiO thin-films. The post-treatment annealing (150-300C) was considered as an essential process for improvement of the optical transparency, decrease of defects concentration and gain of the charge carrier mobility. As result, the annealed ion-beam sputtered NiO films delivered a power conversion efficiency (PCE) up to 20.14%, while device without post-treatment reached the value of 11.84%. The improvement of the output performance originated from an increase of the short-circuit current density (Jsc), open circuit voltage (Voc), shunt and contact properties in the devices. We also demonstrate that the ion-beam sputtering of NiO can be successfully implemented for the fabrication of large area modules (54.5 cm2) and PSCs on a flexible plastic substrate (125 microns).

physics.app-ph↗

arXiv2022

Deep-level transient spectroscopy of the charged defects in p-i-n perovskite solar cells induced by light-soaking

The long-term stability of halide perovskite solar cells (PSCs) remains the critical problem of this photovoltaic technology. Different structural defects formed in the thin-film perovskite films were considered as a main trigger for the decomposition of the absorber and corrosion of the interfaces in the device structure. The changes in the stability performance of the PSCs require a detailed analysis of the defects generated under external stress (light and heat). Using admittance, deep-level transient spectroscopy (DLTS) and reverse DLTS we determined the evolution of the defect energy levels in p-i-n PCS under continuous light soaking stress. We compared the impact of the charged defects on the performance and long-term stability of the CsFAPbI3 based devices with and without Cl-doping. Despite the gain in the output performance of the PCSs, the devices with CsFAPbI3-xClx showed improved light soaking stability. The T80 (time required to reduce initial efficiency by 20%) for Cl-doped PSCs was 1280h, while for pure CsFAPbI3 based devices only 650h. Three different defect energy levels were determined for different device configurations. We found that Cl-doping suppressed the formation of the antisite defects (IPb, IFA) and iodine interstitials (Ii). The changes in the defect's energy levels after continuous light soaking stress were analyzed and discussed. The present work provides new insights for the defect behavior of PSCs under continuous external stress, revealing the physical-chemical impact of the Cl-additive strategy.

cond-mat.mtrl-sci↗

arXiv2018

Resonant Silicon Nanoparticles for Enhanced Light Harvesting in Halide Perovskite Solar Cells

Implementation of resonant colloidal nanoparticles for improving performance of organometal halide perovskites solar cells is highly prospective approach, because it is compatible with the solution processing techniques used for any organic materials. Previously, resonant metallic nanoparticles have been incorporated into perovskite solar cells for better light absorption and charge separation. However, high inherent optical losses and high reactivity of noble metals with halides in perovskites are main limiting factors for this approach. In turn, low-loss and chemically inert resonant silicon nanoparticles allow for light trapping and enhancement at nanoscale, being suitable for thin film photovoltaics. Here photocurrent and fill-factor enhancements in meso-superstructured organometal halide perovskite solar cells, incorporating resonant silicon nanoparticles between mesoporous TiO2 transport and active layers, are demonstrated. This results in a boost of the device efficiency up to 18.8\% and fill factor up to 79\%, being a record among the previously reported values on nanoparticles incorporation into CH3NH3PbI3 (MAPbI3) perovskites based solar cells. Theoretical modeling and optical characterization reveal the significant role of Si nanoparticles for increased light absorption in the active layer rather than for better charge separation. The proposed strategy is universal and can be applied in perovskite solar cells with various compositions, as well as in other optoelectronic devices.

physics.optics↗

arXiv2017

Graphene-based electron transport layers in perovskite solar cells: a step-up for an efficient carrier collection

The electron transport layer (ETL) plays a fundamental role in perovskite solar cells. Recently, graphene-based ETLs have been proved to be good candidate for scalable fabrication processes and to achieve higher carrier injection with respect to most commonly used ETLs. In this work we experimentally study the effects of different graphene-based ETLs in sensitized MAPI solar cells. By means of time-integrated and picosecond time-resolved photoluminescence techniques, the carrier recombination dynamics in MAPI films embedded in different ETLs is investigated. Using graphene doped mesoporous TiO2 (G+mTiO2) with the addition of a lithium-neutralized graphene oxide (GO-Li) interlayer as ETL, we find that the carrier collection efficiency is increased by about a factor two with respect to standard mTiO2. Taking advantage of the absorption coefficient dispersion, we probe the MAPI layer morphology, along the thickness, finding that the MAPI embedded in the ETL composed by G+mTiO2 plus GO-Li brings to a very good crystalline quality of the MAPI layer with a trap density about one order of magnitude lower than that found with the other ETLs. In addition, this ETL freezes MAPI at the tetragonal phase, regardless of the temperature. Graphene-based ETLs can open the way to significant improvement of perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2013

Strain evolution in GaN Nanowires: from free-surface objects to coalesced templates

Top-down fabricated GaN nanowires, 250 nm in diameter and with various heights, have been used to experimentally determine the evolution of strain along the vertical direction of 1-dimensional objects. X-ray diffraction and photoluminescence techniques have been used to obtain the strain profile inside the nanowires from their base to their top facet for both initial compressive and tensile strains. The relaxation behaviors derived from optical and structural characterizations perfectly match the numerical results of calculations based on a continuous media approach. By monitoring the elastic relaxation enabled by the lateral free-surfaces, the height from which the nanowires can be considered strain-free has been estimated. Based on this result, NWs sufficiently high to be strain-free have been coalesced to form a continuous GaN layer. X-ray diffraction, photoluminescence and cathodoluminescence clearly show that despite the initial strain-free nanowires template, the final GaN layer is strained.

cond-mat.mtrl-sci↗

arXiv2008

Resonant electron heating and molecular phonon cooling in single C$_{60}$ junctions

We study heating and heat dissipation of a single \c60 molecule in the junction of a scanning tunneling microscope (STM) by measuring the electron current required to thermally decompose the fullerene cage. The power for decomposition varies with electron energy and reflects the molecular resonance structure. When the STM tip contacts the fullerene the molecule can sustain much larger currents. Transport simulations explain these effects by molecular heating due to resonant electron-phonon coupling and molecular cooling by vibrational decay into the tip upon contact formation.

cond-mat.mtrl-sci↗

arXiv2005

DFT modelling of bulk-modulated carbon nanotube field-effect transistors

We report density-functional theory (DFT), atomistic simulations of the non-equilibrium transport properties of carbon nanotube (CNT) field-effect transistors (FETs). Results have been obtained within a self-consistent approach based on the non-equilibrium Green's functions (NEGF) scheme. Our attention has been focused on a new kind of devices, the so called bulk-modulated CNTFETs. Recent experimental realizations \cite{Chen,Lin_condMat} have shown that such devices can exhibit excellent performances, even better than state-of-the-art Schottky barrier (SB)-modulated transistors. Our calculations have been intended to explore, at an atomistic level, the physical mechanisms governing the transport in these new devices. We emphasize the role that one-dimensional screening has on gate- and drain-induced current modulation mechanisms, pointing out, at the same time, the importance of a correct evaluation of the nanotube quantum capacitance. The operative regimes and the performance limits of the device are analysed, pointing out, at the same time, the role played by the quasi-one-dimensional, short channel effects.

cond-mat.mes-hall↗

arXiv2005

Scaling issues for AlGaN/GaN HEMTs: performance optimization via devices geometry modelling

The potential barrier between source and gate in HEMTs and between source and channel in MOSFET controls the current output and the velocity injection of electrons in the channel [1], [2]. In non self aligned structures the electric field behavior along the channel, for fixed applied voltages, is determined by the contacts positions. Anyway, in GaAs based HEMTs, the geometry of the device appears to be not determinant for the output current due to saturation effects. On the other hand, the GaN based technology still offers the possibility to enhance devices output current handling contacts distances. In this paper we will present Monte Carlo simulations results which show how a downscaling of the Source to Gate distance could improve the device performances inducing an higher potential barrier between source and gate.

cond-mat.other↗

arXiv2005

Vibrational effects in the linear conductance of carbon nanotubes

We study the influence of structural lattice fluctuations on the elastic electron transport in single-wall carbon nanotubes within a density-functional-based scheme. In the linear response regime, the linear conductance is calculated via configurational averages over the distorted lattice. Results obtained from a frozen-phonon approach as well as from molecular dynamics simulations are compared. We further suggest that the effect of structural fluctuations can be qualitatively captured by the Anderson model with bond disorder. The influence of individual vibrational modes on the electronic transport is discussed as well as the role of zero-point fluctuations.

cond-mat.mes-hall↗

arXiv2004

Negative Quantum Capacitance of Carbon Nanotube Field-Effect Transistors

Atomistic density functional theory (DFT) calculations of the capacitance between a metallic cylindric gate and a carbon nanotube (CNT) are reported. Results stressing the predominant effect of quantum capacitance in limiting or even enhancing screening properties of the CNT are shown. Other contributions to the quantum capacitance beyond the electronic density of state (DOS) are pointed out. Negative values of the quantum capacitance are obtained for low-density systems, which correspondingly over-screen the gate field. This unconventional behavior of the quantum capacitance is related to the predominance of the exchange contribution in the total electronic energy of the CNT.

cond-mat.mes-hall↗

arXiv2004

Quasiparticle energies for large molecules: a tight-binding GW approach

We present a tight-binding based GW approach for the calculation of quasiparticle energy levels in confined systems such as molecules. Key quantities in the GW formalism like the microscopic dielectric function or the screened Coulomb interaction are expressed in a minimal basis of spherically averaged atomic orbitals. All necessary integrals are either precalculated or approximated without resorting to empirical data. The method is validated against first principles results for benzene and anthracene, where good agreement is found for levels close to the frontier orbitals. Further, the size dependence of the quasiparticle gap is studied for conformers of the polyacenes ($C_{4n+2}H_{2n+4}$) up to n = 30.

cond-mat.mtrl-sci↗

arXiv1999

Effects of macroscopic polarization in III-V nitride multi-quantum-wells

Huge built-in electric fields have been predicted to exist in wurtzite III-V nitrides thin films and multilayers. Such fields originate from heterointerface discontinuities of the macroscopic bulk polarization of the nitrides. Here we discuss the background theory, the role of spontaneous polarization in this context, and the practical implications of built-in polarization fields in nitride nanostructures. To support our arguments, we present detailed self-consistent tight-binding simulations of typical nitride QW structures in which polarization effects are dominant.

cond-mat.mtrl-sci↗

arXiv1998

Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures

The free-carrier screening of macroscopic polarization fields in wurtzite GaN/InGaN quantum wells lasers is investigated via a self-consistent tight-binding approach. We show that the high carrier concentrations found experimentally in nitride laser structures effectively screen the built-in spontaneous and piezoelectric polarization fields, thus inducing a ``field-free'' band profile. Our results explain some heretofore puzzling experimental data on nitride lasers, such as the unusually high lasing excitation thresholds and emission blue-shifts for increasing excitation levels.

cond-mat.mtrl-sci↗