A characterization on hybrid lead halide perovskite solar cells with Ti02 mesoporous scaffold

Treball Final de Màster Universitari en Física Aplicada. Codi: SIN019. Curs acadèmic 2014-2015The booming perovskite solar cells (PSCs) field has emerged from the solid-state dye-sensitized photovoltaic cells with significant advances in solar to electric power conversion efficiency in a relatively short time. However, a lot a research is currently trying to explain many poorly understood aspects of its operating modes. Current density-voltage ( ) solar cells response has always been a fundamental photovoltaics characterization, and specifically on PSCs it presents an anomalous hysteretic behavior whose origin has been mainly addressed to ferroelectric or ionic effects. Here we show, on the basis of a CH3NH3PbI3-xClx with a mesoporous TiO2 scaffold based solar cells characterization, that such anomalous hysteresis can be better explained in terms of interface capacitive ionic slow dynamic processes. Furthermore, we present a methodological review for the proper performance of Mott-Schottky analysis in PSCs stressing on the importance of complementary impedance spectroscopy characterization. On this issue we highlight our finds of an analogous hysteresis and an exponential capacitance excess on capacitance-voltage characteristics that can be associated with an exponential density of localized states below the conduction band. Our calculus of the corresponding total density of localized states supports our stands for interface processes

Light Capacitances in Silicon and Perovskite Solar Cells

The framework on which the physics of silicon solar cells (SiSCs) is based robustly predicts dependences of capacitance on light intensity and voltage, even when most recent innovations are considered as the incorporation of transition metal oxide/Si heterojunctions. However, perovskite solar cells (PSCs) challenge most of the established paradigms, claiming for rethinking of known theories and devising novel models. Here we tackle this scenario by probing and comparing light-induced capacitance responses yielded by these two major exponents in the field of photovoltaic research. SiSCs light capacitances can be easily interpreted in the framework of the so-called chemical capacitance. Current approaches addressing the intriguing low-frequency capacitive features of PSCs are outlined and compared. Here, apparent similarities and differences between both photovoltaic technologies are highlighted, concerning the observation of light capacitances of chemical origin. It is concluded that, contrary to that occurring in SiSCs, bulk electronic chemical capacitances are not straightforwardly observed in PSCs. As capacitive features exhibited by PSCs are believed to be critically connected to performance degradation and device instability, future research and explanation directions are proposed here for advancing in the understanding of this challenging photovoltaic technology

Ionic charging by local imbalance at interfaces in hybrid lead halide perovskites

Identification of specific operating mechanisms becomes particularly challenging when mixed ionic-electronic conductors are used in optoelectronic devices. Ionic effects in perovskitesolar cells are believed to distort operation curves and possess serious doubts about their long term stability. Current hysteresis and switchable photovoltaic characteristics have been connected to the kinetics of ion migration. However, the nature of the specific ionic mechanism (or mechanisms) able to explain the operation distortions is still poorly understood. It is observed here that the local rearrangement of ions at the electrodeinterfaces gives rise to commonly observed capacitive effects. Charging transients in response to step voltage stimuli using thick CH3NH3PbI3 samples show two main polarization processes and reveal the structure of the ionic double-layer at the interface with the non-reacting contacts. It is observed that ionic charging, with a typical response time of 10 s, is a local effect confined in the vicinity of the electrode, which entails absence of net mobile ionic concentration (space-charge) in the material bulk.We thank financial support from MINECO of Spain under Project (MAT2013-47192-C3-1-R), and Generalitat Valenciana (Prometeo/2014/020). O.A. acknowledges Generalitat Valenciana for a Grant (GRISOLIAP2014/035)

Do Capacitance Measurements Reveal Light-Induced Bulk Dielectric Changes in Photovoltaic Perovskites?

Several studies have identified complex interactions between photogenerated carriers and the crystal lattice in perovskite materials for photovoltaics, which are regarded to have an inherently soft character. Light is known to induce phase segregation in mixed halide perovskites, enhance piezoelectricity, and change dipole moments of the unit cell. Therefore, it is appealing to consider photogenerated variations in the bulk polarizability and dielectric properties. Light-induced bulk polarization changes should be observable by capacitance measurements, preferentially at intermediate frequencies where the geometrical capacitance dominates. However, capacitance spectra are influenced by several capacitive and resistive mechanisms, which are also modulated by light. Capacitances may arise from dielectric bulk polarization, space-charge depletion zones, chemical electronic bulk storage, and interfacial accumulation mechanisms. This variety of capacitive mechanisms may induce wrong interpretations and produce misleading outcomes when uncritically connected to the bulk polarization response. It is shown here how capacitance–voltage analyses performed at a given frequency are influenced by overlapping effects that mask the actual value of the geometrical capacitance. Careful analyses are then needed before attributing the light-induced modulation of the measured capacitance to hybrid perovskite dielectric changes

Utilization of Temperature-Sweeping Capacitive Techniques to Evaluate Band Gap Defect Densities in Photovoltaic Perovskites

Capacitive techniques, routinely used for solar cell parameter extraction, probe the voltage-modulation of the depletion layer capacitance isothermally as well as under varying temperature. In addition, defect states within the semiconductor band gap respond to such stimuli. Although extensively used, capacitive methods have found difficulties when applied to elucidating bulk defect bands in photovoltaic perovskites. This is because perovskite solar cells (PSCs) actually exhibit some intriguing capacitive features hardly connected to electronic defect dynamics. The commonly reported excess capacitance observed at low frequencies is originated by outer interface mechanisms and has a direct repercussion on the evaluation of band gap defect levels. Starting by updating previous observations on Mott–Schottky analysis in PSCs, it is discussed how the thermal admittance spectroscopy and the deep level transient spectroscopy characterization techniques present spectra with overlapping or even “fake” peaks caused by the mobile ion-related, interfacial excess capacitance. These capacitive techniques, when used uncritically, may be misleading and produce wrong outcomes

Celdas Solares de Perovskitas: Una breve Introducción y algunas Consideraciones

The spectacular and unprecedented rise of so-called perovskite solar cells (PSCs) in conversion efficiency with low-cost manufacturing processes has grabbed the attention of the scientific community in the field of photovoltaics during the last four years. The inclusion of perovskite type absorber materials, typically CH 3 NH 3 PbI 3 , has been the key factor for the development of this emerging technology that has created great of expectations. However, many poorly understood aspects of its operating modes still need convincing explanations. This paper provides a brief introduction to the structure, materials and characteristics of PSCs. In addition, some remarks about the stability of these devices are provided and the state-of-the-art of several subjects of interest is discussed, such as the hysteresis phenomenon of current-voltage curves.El espectacular y sin precedentes ascenso de las llamadas celdas solares de perovskitas (PSCs, por sus siglas en inglés) en cuanto a eficiencia de conversión con procesos de fabricación de bajo presupuesto ha acaparado la atención de la comunidad científica en el campo de la fotovoltaica en los últimos cuatro años. La inclusión de materiales absorbedores tipo perovskita, típicamente el CH3NH3PbI3, ha sido el factor clave para el desarrollo de esta tecnología emergente con la que se tienen muchas perspectivas. Sin embargo, no son pocos los aspectos de su funcionamiento que aún faltan por comprender. En este trabajo se brinda una breve introducción a la estructura, materiales y características de las PSCs. Además se comenta especialmente acerca de la estabilidad de estos dispositivos y se discuten varios temas de interés como el fenómeno de la histéresis de las curvas corriente-voltaje

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 toward several fields in the optoelectronics, such as photo- and ionizing-radiation-detectors. One of the most basic characterization protocols consists of measuring the dark current-voltage ( J - V) curve of symmetrically contacted samples for identifying the different regimes of the space-charge-limited current (SCLC). Customarily, J & PROP; V-n indicates the Mott-Gurney law when n & AP; 2 or the Child-Langmuir ballistic regime of SCLC when n = 3 / 2. The latter has been found in perovskite samples. Herein, we start by discussing the interpretation of J & PROP; 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. Therefore, we introduce the models of quasi-ballistic velocity-dependent dissipation (QvD) and the ballistic-like voltage-dependent mobility (BVM) regimes of SCLC. The QvD model is shown to better describe electronic kinetics, whereas the BVM model results are suitable for describing both electronic and ionic kinetics in halide perovskites as a particular case of the Poole-Frenkel ionized-trap-assisted transport. The proposed formulations can be used as the characterization of effective mobilities, charge carrier concentrations and times-of-flight from J - V curves, and resistance from impedance spectroscopy spectra.The authors acknowledge the financial support from European Union's Horizon 2020 research and innovation program under the Photonics Public Private Partnership (www.photonics21.org) with the project PEROXIS under the Grant Agreement No. 871336. O.A. thanks Dr. Gebhard J. Matt for his feedback on the link to the Poole–Frenkel ionized-trap-assisted transport mechanism. M.G.-B. acknowledges Generalitat Valenciana for Grant No. GRISOLIAP/2018/073

Effective Ion Mobility and Long-Term Dark Current of Metal Halide Perovskites with Different Crystallinities and Compositions

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 case of X-ray detectors, dark current level and instability are regarded to be connected to the ion migration upon bias application. Two compositions (MAPbBr3 and MAPbI3) and structures (single- and microcrystalline) are checked by the analysis of long-term dark current evolution. Electronic current increases with time before reaching a steady-state value within a response time (from 104 down to 10 s) that strongly depends on the applied bias. A coupling between electronic transport and ion kinetics exists that ultimately establishes the time scale of electronic current. Effective ion mobility is extracted for a range of applied electric field ξ. While ion mobility results field-independent in the case of MAPbI3, a clear field enhancement is observed for MAPbBr3 ( ), irrespective of the crystallinity. Both perovskite compounds present effective ion mobility in the range of  ≈ 10−7–10−6 cm−2 V−1 s−1, in accordance with previous analyses. The ξ-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.This work has received funding from the European Union's Horizon 2020 research and innovation program under the Photonics Public Private Partnership (www.photonics21.org) with the project PEROXIS under the grant agreement N° 871336

Device Performance of Emerging Photovoltaic Materials (Version 2)

Following the 1st release of the “Emerging photovoltaic (PV) reports”, the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2020. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application and are put into perspective using, e.g., the detailed balance efficiency limit. The 2nd instalment of the “Emerging PV reports” extends the scope toward tandem solar cells and presents the current state-of-the-art in tandem solar cell performance for various material combinations

Analytical model for light modulating impedance spectroscopy (LIMIS) in all-solid-state p-n junction solar cells at open-circuit

Potentiostatic impedance spectroscopy (IS) is a well-known tool for characterization of materials and electronic devices. It can be complemented by numerical simulation strategies relying on drift-diffusion equations without any equivalent circuit-based assumptions. This implies the time-dependent solutions of the transport equations under small perturbation of the external bias applied as a boundary condition at the electrodes. However, in the case of photosensitive devices, a small light perturbation modulates the generation rate along the absorber bulk. This work then approaches a set of analytical solutions for the signals of IS and intensity modulated photocurrent and photovoltage spectroscopies, intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS), respectively, from one-sided p-n junction solar cells at the open-circuit. Subsequently, a photoimpedance signal named “light intensity modulated impedance spectroscopy” (LIMIS = IMVS/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 the starting point for future more realistic numerical simulations. We acknowledge the funding support from the Ministerio de-Ciencia, Innovación y Universidades of Spain under project (No. MAT2016-76892-C3-1-R). O.A. acknowledges the financial support from the VDI/VD Innovation + Technik GmbH (Project-title: PV-ZUM) and the SAOT funded by the German Research Foundation (DFG) in the framework of the German excellence initiative