arXiv subjects

Germà Garcia-Belmonte

Publications and source records attributed to Germà Garcia-Belmonte.

4 recordsLinked to original sources

arXiv2022

Effective Ion Mobility and Long-Time Dark Current of Metal-Halide Perovskites of Different Crystallinity and Composition

Ion transport properties in metal-halide perovskite still constitute a subject of intense research because of the evident connection between mobile defects and device performance and operation degradation. In the specific case of X-ray detectors, dark current level and instability is regarded to be connected to the ion migration upon bias application. Different compositions (MAPbBr3 and MAPbI3) and structures (single- and micro-crystalline) are checked by the analysis of long-time dark current evolution. In all cases, electronic current increases with time before reaching a steady-state value within a response time (from 10.000 s down to 10 s) that strongly depends on the applied bias. Our findings corroborate the existence of a coupling between electronic transport and ion kinetics that ultimately establishes the time scale of electronic current. Effective ion mobility mui is extracted that exhibits applied electrical field E dependence that varies on the perovskite composition. While ion mobility results field-independent in the case of MAPbI3, a clear field-enhancement is observed for MAPbBr3 (dmui/dE>0), irrespective of the crystallinity. Both perovskite compounds present effective ion mobility in the range of mui = 10-7-10-6 cm-2 V-1 s-1, in accordance with previous analyses. The E-dependence of the ion mobility is related to the lower ionic concentration of the bromide compound. Slower-migrating defect drift is suppressed in the case of MAPbBr3, in opposition to that observed here for MAPbI3.

cond-mat.mtrl-sci↗

arXiv2021

Ballistic-like Space-charge-limited Currents in Halide Perovskites at Room Temperature

The emergence of halide perovskites in photovoltaics has diversified the research on this material family and extended their application towards several fields in the optoelectronics, such as photo- and ionizing-radiation-detectors. One of the most basic characterization protocols consist on measuring the dark current-voltage (J-V) curve of symmetrically contacted samples for identifying the different regimes of space-charge-limited current (SCLC). Customarily, J=C*V^n curves indicate the Mott-Gurney law when n=2, or the Child-Langmuir ballistic regime of SCLC when n=3/2. The latter can be often found in perovskite samples. In this work, we start by discussing the interpretation of currents proportional to V^(3/2) in relation to the masking effect of the dual electronic-ionic conductivity in halide perovskites. However, we do not discard the actual occurrence of SCLC transport with ballistic-like trends. For those cases, we introduce the models of: quasi-ballistic velocity-dependent dissipation (QvD) and the ballistic-like voltage-dependent mobility (BVM) regime of SCLC. The QvD model is shown to better describe electronic kinetics, whereas the BVM model is revealed as suitable for describing electronic or ionic kinetics in halide perovskites. The proposed formulations can be used as characterization tools for the evaluation of effective mobilities, charge carrier concentrations and times-of-flight from J-V curves and impedance spectroscopy spectra.

cond-mat.mtrl-sci↗

arXiv2019

Light Intensity Modulated Impedance Spectroscopy (LIMIS) in All-Solid-State Solar Cells at Open Circuit

Potentiostatic impedance spectroscopy (IS) is a well stablished characterization technique for elucidating the electric resistivity and capacitive features of materials and devices. In the case of solar cells, by applying a small voltage perturbation the current signal is recorded and the recombination processes and defect distributions are among the typical outcomes in IS studies. In this work a photo-impedance approach, named light intensity modulated impedance spectroscopy (LIMIS), is first tested in all-solid-state photovoltaic cells by recording the individual photocurrent (IMPS) and photovoltage (IMVS) responsivity signals due to a small light perturbation at open-circuit (OC), and combining them: LIMIS=IMVS/IMPS. The experimental LIMIS spectra from silicon, organic, and perovskite solar cells are presented and compared with IS. An analysis of the equivalent circuit numerical models for total resistive and capacitive features is discussed. Our theoretical findings show a correction to the lifetimes evaluations by obtaining the total differential resistances and capacitances combining IS and LIMIS measurements. This correction addresses the discrepancies among different techniques, as shown with transient photovoltage. The experimental differences between IS and LIMIS (i) proves the unviability of the superposition principle, (ii) suggest a bias-dependent photo-current correction to the empirical Shockley equation of the steady-state current at different illumination intensities around OC and (iii) are proposed as a potential figure of merit for characterizing performance and stability of solar cells. In addition, new features are reported for the low-frequency capacitance of perovskite solar cells, measured by IS and LIMIS.

physics.app-ph↗

arXiv2019

Analytical Model for Light Modulating Impedance Spectroscopy (LIMIS) in All-Solid-State p-n Junction Solar Cells at Open-Circuit

Non-circuit theory drift-diffusion numerical simulation of standard potentiostatic impedance spectroscopy (IS) is a well-known strategy for characterization of materials and electronic devices. It implies the time-dependent solutions from the continuity and Poisson's equations under small perturbation of the bias boundary condition at the electrodes. But in the case of photo-sensitive devices a small light perturbation can be also taken modulating the generation rate along the absorber bulk. In that focus, this work approaches a set of analytical solutions for the signals of IS and intensity modulated photocurrent and photovoltage spectroscopies, IMPS and IMVS respectively, from one-sided p-n junction solar cells at open-circuit. Subsequently, a photo-impedance signal named light intensity modulated impedance spectroscopy (LIMIS equals IMVS over IMPS) is analytically simulated and its difference with respect to IS suggests a correlation with the surface charge carrier recombination velocity. This is an illustrative result and starting point for future more realistic numerical simulations.

cond-mat.mtrl-sci↗