arXiv subjects

Zahra Andaji-Garmaroudi

Publications and source records attributed to Zahra Andaji-Garmaroudi.

9 recordsLinked to original sources

arXiv2023

Fast near-infrared photodetectors based on nontoxic and solution-processable AgBiS2

Solution-processable near-infrared (NIR) photodetectors are urgently needed for a wide range of next-generation electronics, including sensors, optical communications and bioimaging. However, there is currently a compromise between low toxicity and slow (<300 kHz cut-off frequency) organic materials versus faster detectors (>300 kHz cut-off frequency) based on compounds containing toxic lead or cadmium. Herein, we circumvent this trade-off by developing solution-processed AgBiS2 photodetectors with high cut-off frequencies under both white light (>1 MHz) and NIR (approaching 500 kHz) illumination. These high cut-off frequencies are due to the short transit distances of charge-carriers in the AgBiS2 photodetectors, which arise from the strong light absorption of these materials, such that film thicknesses well below 120 nm are adequate to absorb >65% of near-infrared to visible light. By finely controlling the thickness of the photoactive layer, we can modulate the charge-collection efficiency, achieve low dark current densities, and minimize the effects of ion migration to realize fast photodetectors that are stable in air. These outstanding characteristics enable real-time heartbeat sensors based on NIR AgBiS2 photodetectors. # equal contribution, * corresponding authors

cond-mat.mtrl-sci↗

arXiv2023

Generalised Framework for Controlling and Understanding Ion Dynamics with Passivated Lead Halide Perovskites

Metal halide perovskite solar cells have gained widespread attention due to their high efficiency and high defect tolerance. The absorbing perovskite layer is as a mixed electron-ion conductor that supports high rates of ion and charge transport at room temperature, but the migration of mobile defects can lead to degradation pathways. We combine experimental observations and drift-diffusion modelling to demonstrate a new framework to interpret surface photovoltage (SPV) measurements in perovskite systems and mixed electronic ionic conductors more generally. We conclude that the SPV in mixed electronic ionic conductors can be understood in terms of the change in electric potential at the surface associated with changes in the net charge within the semiconductor system. We show that by modifying the interfaces of perovskite bilayers, we may control defect migration behaviour throughout the perovskite bulk. Our new framework for SPV has broad implications for developing strategies to improve the stability of perovskite devices by controlling defect accumulation at interfaces. More generally, in mixed electronic conductors our framework provides new insights into the behaviour of mobile defects and their interaction with photoinduced charges, which are foundational to physical mechanisms in memristivity, logic, impedance, sensors and energy storage.

cond-mat.mtrl-sci↗

arXiv2022

Charge Transport in Mixed Metal Halide Perovskite Semiconductors

Investigation of the inherent field-driven charge transport behaviour of 3D lead halide perovskites has largely remained a challenging task, owing primarily to undesirable ionic migration effects near room temperature. In addition, the presence of methylammonium in many high performing 3D perovskite compositions introduces additional instabilities, which limit reliable room temperature optoelectronic device operation. Here, we address both these challenges and demonstrate that field-effect transistors (FETs) based on methylammonium-free, mixed-metal (Pb/Sn) perovskite compositions, that are widely studied for solar cell and light-emitting diode applications, do not suffer from ion migration effects as their pure Pb counterparts and reliably exhibit hysteresis free p-type transport with high mobility reaching 5.4 $cm^2/Vs$, ON/OFF ratio approaching $10^6$, and normalized channel conductance of 3 S/m. The reduced ion migration is also manifested in an activated temperature dependence of the field-effect mobility with low activation energy, which reflects a significant density of shallow electronic defects. We visualize the suppressed in-plane ionic migration in Sn-containing perovskites compared to their pure-Pb counterparts using photoluminescence microscopy under bias and demonstrate promising voltage and current-stress device operational stabilities. Our work establishes FETs as an excellent platform for providing fundamental insights into the doping, defect and charge transport physics of mixed-metal halide perovskite semiconductors to advance their applications in optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Local Nanoscale Defective Phase Impurities Are the Sites of Degradation in Halide Perovskite Devices

Halide perovskites excel in the pursuit of highly efficient thin film photovoltaics, with power conversion efficiencies reaching 25.5% in single junction and 29.5% in tandem halide perovskite/silicon solar cell configurations. Operational stability of perovskite solar cells remains a barrier to their commercialisation, yet a fundamental understanding of degradation processes, including the specific sites at which failure mechanisms occur, is lacking. Recently, we reported that performance-limiting deep sub-bandgap states appear in nanoscale clusters at particular grain boundaries in state-of-the-art $Cs_{0.05}FA_{0.78}MA_{0.17}Pb(I_{0.83}Br_{0.17})_{3}$ (MA=methylammonium, FA=formamidinium) perovskite films. Here, we combine multimodal microscopy to show that these very nanoscale defect clusters, which go otherwise undetected with bulk measurements, are sites at which degradation seeds. We use photoemission electron microscopy to visualise trap clusters and observe that these specific sites grow in defect density over time under illumination, leading to local reductions in performance parameters. Scanning electron diffraction measurements reveal concomitant structural changes at phase impurities associated with trap clusters, with rapid conversion to metallic lead through iodine depletion, eventually resulting in pinhole formation. By contrast, illumination in the presence of oxygen reduces defect densities and reverses performance degradation at these local clusters, where phase impurities instead convert to amorphous and electronically benign lead oxide. Our work shows that the trapping of charge carriers at sites associated with phase impurities, itself reducing performance, catalyses redox reactions that compromise device longevity. Importantly, we reveal that both performance losses and intrinsic degradation can be mitigated by eliminating these defective clusters.

cond-mat.mtrl-sci↗

arXiv2021

Efficient Energy Transport in an Organic Semiconductor Mediated by Transient Exciton Delocalization

Efficient energy transport is highly desirable for organic semiconductor (OSC) devices such as photovoltaics, photodetectors, and photocatalytic systems. However, photo-generated excitons in OSC films mostly occupy highly localized states over their lifetime. Energy transport is hence thought to be mainly mediated by the site-to-site hopping of localized excitons, limiting exciton diffusion coefficients to below ~10^{-2} cm^2/s with corresponding diffusion lengths below ~50 nm. Here, using ultrafast optical microscopy combined with non-adiabatic molecular dynamics simulations, we present evidence for a new highly-efficient energy transport regime: transient exciton delocalization, where energy exchange with vibrational modes allows excitons to temporarily re-access spatially extended states under equilibrium conditions. In films of highly-ordered poly(3-hexylthiophene) nanofibers, prepared using living crystallization-driven self-assembly, we show that this enables exciton diffusion constants up to 1.1+-0.1 cm^2/s and diffusion lengths of 300+-50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of the basic picture of exciton dynamics. This establishes new design rules to engineer efficient energy transport in OSC films, which will enable new devices architectures not based on restrictive bulk heterojunctions.

cond-mat.mtrl-sci↗

arXiv2021

Optical and electronic properties of colloidal CdSe Quantum Rings

Luminescent colloidal CdSe nanorings are a new type of semiconductor structure that have attracted interest due to the potential for unique physics arising from their non-trivial toroidal shape. However, the exciton properties and dynamics of these materials with complex topology are not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and single particle measurements to study these materials. We find that on transformation of CdSe nanoplatelets to nanorings, by perforating the center of platelets, the emission lifetime decreases and the emission spectrum broadens due to ensemble variations in the ring size and thickness. The reduced PL quantum yield of nanorings (~10%) compared to platelets (~30%) is attributed to an enhanced coupling between: (i) excitons and CdSe LO-phonons at 200 cm-1 and (ii) negatively charged selenium-rich traps which give nanorings a high surface charge (~-50 mV). Population of these weakly emissive trap sites dominates the emission properties with an increased trap emission at low temperatures relative to excitonic emission. Our results provide a detailed picture of the nature of excitons in nanorings and the influence of phonons and surface charge in explaining the broad shape of the PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that the excitonic properties of nanorings are not solely a consequence of the toroidal shape but are also a result of traps introduced by puncturing the platelet center.

physics.app-ph↗

arXiv2020

Bandgap Lowering in Mixed Alloys of Cs2Ag(SbxBi1-x)Br6 Double Perovskite Thin Films

Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide bandgaps, which limit photo conversion efficiencies. The bandgap can be reduced through hallowing with Sb3+, but Sb-rich alloys are difficult to synthesise due to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We develop a solution-based route to synthesis phase-pure Cs2Ag(SbxBi1-x)Br6 thin films, with the mixing parameter x continuous varying over the entire composition range. We reveal that the mixed alloys (x between 0.5 and 0.9) demonstrate smaller bandgaps (as low as 2.08 eV) than the pure Sb- (2.18 eV) and Bi-based (2.25 eV) compounds, with strong deviation from Vegard's law. Through in-depth computations, we propose that bandgap lowering arises from the Type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch between the Bi and Sb s and p atomic orbitals, coupled with their non-linear mixing, results in the alloys adopting a smaller bandgap than the pure compounds. Our work demonstrates an approach to achieve bandgap reduction and highlights that bandgap bowing may be found in other double perovskite alloys by pairing together materials forming a Type II band alignment.

cond-mat.mtrl-sci↗

arXiv2018

Static and Dynamic Disorder in Triple-Cation Hybrid Perovskites

A detailed understanding of the carrier dynamics and emission characteristics of organic-inorganic lead halide perovskites is critical for their optoelectronic and energy harvesting applications. In this work, we reveal the impact of the crystal lattice disorder on the photo-generated electron-hole pairs through low-temperature photoluminescence measurements. We provide strong evidence that the intrinsic disorder forms a sub-bandgap tail density of states, which determines the emission properties at low temperature. The PL spectra indicate that the disorder evolves with increasing temperature, changing its character from static to dynamic. This change is accompanied by a rapid drop of the PL efficiency, originating from the increased mobility of excitons/polarons, which enables them to reach deep non-radiative recombination centers more easily.

cond-mat.mtrl-sci↗

arXiv2017

Maximising and Stabilising Luminescence in Metal Halide Perovskite Device Structures

Metal halide perovskites are attracting tremendous interest for a variety of optoelectronic applications. The ability to tune the perovskite bandgap by tweaking the chemical compositions opens up new applications as coloured emitters and as components of tandem photovoltaics. Nevertheless, non-radiative losses are still limiting performance, with luminescence yields in state-of-the-art perovskite solar cells still far from 100% under solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability (bandgaps ~1.7-1.9 eV), photo-induced ion segregation leads to bandgap instabilities. Here, we substantially mitigate both non-radiative losses and photo-induced ion migration in perovskite structures by decorating the surfaces and grain boundaries with passivating potassium-halide interlayers. We demonstrate external photo-luminescence quantum yields of 66%, translating to internal yields exceeding 95%. The high luminescence yields are achieved while maintaining high mobilities over 40 cm2V-1s-1, giving the elusive combination of both high luminescence and excellent charge transport. We find that the external luminescence yield when interfaced with electrodes in a solar cell device stack, a quantity that must be maximized to approach the efficiency limits, remains as high as 15%, indicating very clean interfaces. We also demonstrate the inhibition of photo-induced ion migration processes across a wide range of mixed halide perovskite bandgaps that otherwise show bandgap instabilities. We validate these results in full operating solar cells, highlighting the importance of stabilising luminescence in device structures. Our work represents a critical breakthrough in the construction of tunable halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells and coloured LEDs.

cond-mat.mtrl-sci↗