Optoelectronic and Energy Level Exploration of Bismuth and Antimony-Based Materials for Lead-Free Solar Cells

Bismuth- and antimony-based materials, such as A(3)M(2)X(9) and AMSX(2) (A = cation, M = Bi, Sb, S = sulfur, X = halogen), are promising candidates as the counterpart to lead halide perovskite. However, the large number of different compositions and crystal structures (dimer, perovskite, etc.) has made these materials largely overlooked; thus, an intuitive evaluation strategy is required. Here, we present a comprehensive study of the energy levels (bandgap, valence band maximum, etc.) and optoelectronics (photoconductivity and charge transfer to charge transport material) of the Bi - and Sb-based materials, which include 6 crystal categories with 44 compositions, by using time-resolved microwave conductivity (TRMC). Importantly, we found an efficient hole transfer from the Sb-based materials to the hole transport materials with the inclusion of the thiophene component, leading to an improved power conversion efficiency of 2.91% for Sb2S3-containing SbSI, prepared by a novel one-step method. Our study establishes a key rule for exploring active layer compositions and designing device structures, which would accelerate the evolution of Bi- and Sb-based lead-free solar cells.

Approches numériques du transport de charges dans les semiconducteurs organiques

Hocine Houili

Using plastics or dyes like molecular semiconductors to make optoelectronic devices at low cost is a project in industry that is of great interest to the "Information Community" of this beginning of the 21st century. Some applications of this technology have already been realized including flexible flat-panel displays made with organic light-emitting diodes, organic transistors for active matrices, organic solar cells and sensors. In all these applications it is essential to deduce and understand the laws that determine the charge transport and the light emission in these materials. The goal of this thesis is to contribute to the theoretical models that describe these transport processes. In order to predict the behaviour of charge carriers in a given device, one can choose two methods. One can either numerically solve the master equations that describe the general behaviour of charges and currents in each part of the device, or one can simulate the detailed behaviour of each charge in the device by fixing the conditions of its movement using a procedure known as "Monte Carlo". In this thesis I addressed the two approaches. I have been interested successively in the prediction of the electric properties of multilayer light-emitting devices, considered as a whole. Then I was able to understand in detail the processes accruing at the organic-organic interfaces by using the Monte Carlo method. My last project involved the study of the channel of an organic transistor. In each one of these cases I took account of the characteristic features of small molecule organic materials in order to develop models for charge transport for the study and optimization of two important types of organic devices : organic light-emitting diodes and field-effect transistors. Amorphous organic semiconductors are mainly used for the fabrication of organic diodes. They are characterized by an energetically disordered density of states that is assumed to be Gaussian. The transport of charges in these materials occurs via hopping from one molecule to another in this density of states. The correlations between the energies of the molecular sites have important effects on transport ; the dependence of mobility on the applied electric field in particular. If the energetic disorder is due to the random orientations of the permanent dipoles of the molecules, then the correlations between the orientations of these dipoles can profoundly change the spatial configuration of the energetic disorder and consequently the charge transport. Due to the fact that organic diodes often comprise of multiple organic layers, understanding device behaviour at the organic-organic interfaces is of critical importance. Electrons and holes accumulate close to these interfaces and they are therefore more likely to recombine and emit light. The Coulomb interactions between the charge carriers and the energy disorder are at the origin of complex processes taking place at these accumulation regions. I studied these processes through a 3D multi-particle Monte Carlo simulation. The short-range Coulomb repulsion forbids the double-occupancy of the sites and shifts the energy level for injection of carriers across the interface upward, thus increasing the electric current of the diode. The long-range Coulomb interactions increase the electric field and the current at the interface although this increase is less important than suggested by 1D models. Effects related to the hopping process such as the effect of the exact form of the hopping law and backward or return hop at the interface are clearly discussed. The Monte Carlo simulations are cross-compared with an analytical model for hopping transport across the interface. This comparison made it possible to understand other aspects of the physical processes at organic-organic interface and supports the Monte Carlo simulations. The study of the electron-hole recombination showed a decrease of the cross-section with an increase of the applied electric field, but this decrease is less than what can be expected by the classical Langevin theory. In spite of their efficiency, Monte Carlo simulations are not attractive when modelling complex multi-layer diodes. The 1D models are better suited in this case. A 1D model called MOLED, which was developed at Laboratoire d'Optoélectronique des Matériaux Moléculaires (LOMM) for the simulation of the organic diodes, is used and described in detail in this thesis. We used MOLED to explore the effect of the energy correlations on the electric field dependence of the mobility. In fact we simulated the time-of-flight experiment, a common method used to determine of the carrier mobility. The crystalline organic semiconductors achieve mobility performances close or even better than amorphous silicon. Their use for the fabrication of field-effect transistors is thus a good choice. The models for charge transport in crystalline organic materials differ much from their counterparts for amorphous organic materials. Indeed band models are more appropriate to describe charge transport in crystalline organic materials. However it is very important to take into account the effects of electronic polarization in these models. I developed a method of calculation of these effects of polarization in the bulk of the organic semiconductor and I extended it to the case of the interface with a dielectric insulator, a situation encountered in the channel of the organic transistor. These calculations showed that when the polarity of the dielectric increases the inter-molecular transfer integral decreases. This is confirmed by experimental measurements showing increasingly low values of mobility when the permittivity of the insulator increases.

EPFL2006

La0.5Sr0.5Fe1-yMyO3-[delta] (M = Ti, Ta) perovskite oxides for oxygen separation membranes

Defne Bayraktar

Mixed ionic electronic conducting perovskite materials have been receiving considerable attention for the application of oxygen separation membranes. These membranes have potential to be integrated in industrial processes that require pure oxygen and to provide advantages considering economical and environmental aspects. The used perovskite materials can accommodate a high concentration of disordered oxygen vacancies and provide high oxygen permeation flux in the presence of an oxygen partial pressure gradient. However, the materials with high oxygen flux are observed to have low chemical and mechanical stability, originating from oxygen vacancy formation and reduction of the B-site transition metal ion. Based on the fact that the stability of the materials at reducing pO2 is dependent on the redox stability of the B-site cation, a partial substitution of the B-site transition metal with a more stable cation is considered in this thesis to improve the stability of the base perovskite oxide. The approach used for the identification of the suitable material based on the host composition La0.5Sr0.5FeO3-δ (LSF) was to first explore the B-site substituting elements in terms of their effect on the thermal expansion and the isothermal expansion measured during the atmosphere change from air to argon. The elements Al, Cr, Zr, Ga, Ti, Sn, Ta, In, V, and Mg were used for 10 or 20% substitution of Fe. The isothermal expansion of the host material was decreased by substitution, which was attributed to decreased amount of oxygen vacancy formation as it was evidenced later by TGA of chosen compositions. As a result, the compositions substituted with higher valence ions (particularly, 20% Ti4+, LSFTi2, and 10% Ta5+, LSFTa1) were identified as the least expanding materials in isothermal conditions. The effect of substitution was further characterized extensively including structural, mechanical, and oxygen transport properties. The oxygen vacancy (VO••) concentration changes induced by substitution of Fe were reflected in unit cell sizes of heat-treated samples under argon atmosphere. The increase in the unit cell size was attributed to the isothermal expansion due to B-site reduction and lowered to half in case of substituted LSFTi2 and LSFTa1 compared to the host material LSF. Similar trends were observed for interrelated properties such as the coefficient of thermal expansion, isothermal expansion, and the actual amount of oxygen vacancies formed under argon at 900°C, all of which were higher for LSF. The mechanical properties of LSF, LSFTi2, and LSFTa1 were characterized at room temperature. The Young's modulus and the bending strength of the materials were in the same range (141-147 GPa and ∼120 MPa, respectively) while the fracture toughness of the LSF sample was improved by Ti and especially Ta substitution. The fractography of the samples provided evidence that the several LSF samples showed extended cracks possibly related to residual stresses forming during cooling, due to non-equilibrated oxygen stoichiometry across the sample. The potential step measurements showed that the chemical diffusion and the surface exchange coefficients measured during oxidation of the samples were higher than the ones measured during reduction of the samples. The tendency was reversed at lowered pO2 along with the decrease of both coefficients. The influence of B-site substitution on both coefficients was not substantial. The oxygen permeation flux of LSF, LSFTi2, and LSFTa1 was measured under air/argon gradient on planar membranes. The permeation rates of the membranes with thicknesses close to 1 mm were considered to be limited by surface exchange kinetics at temperatures above 875°C. The permeation rate of LSF was decreased (from 0.2 µmolcm-2s-1) to its third by Ti and Ta additions. The permeation rate of Ti and Ta-substituted samples showed a drop around 875°C, keeping a similar activation energy at lower and higher temperature regions. Oxygen vacancy ordering in the perovskite structure was given as a possible explanation, which provided explanation for the slow equilibrium in the lower temperature region. Tubular membranes of LSF and LSFTi2 (0.47 mm and 0.36 mm thick, respectively) and a planar LSFTa1 membrane were characterized using an air/(Ar-CH4) gradient. The measurements conducted using pure Ar on the lean-side provided the possibility to compare the thickness dependence of the permeation. In agreement with the previous observations, the permeation of the materials at and above 900°C was independent of the thickness, therefore, surface exchange limited. Stability under reducing atmospheres was increased by substitution. The LSF membrane failed shortly after the introduction of CH4 to the system, while LSFTi2 survived 5% CH4 and its oxygen permeation was improved substantially by a factor of 14. LSFTa1 membrane was measured with pure CH4 and the permeation was increased by a factor of 9, reaching 0.7 µmolcm-2s-1 at 900°C. The membrane operated stably over 2000h with pure CH4, 1000h. A slow degradation of 3.7% per 1000h was observed at 1000°C, most probably due to cation migration. The compositions identified as a result of B-site substitution screening were effective in improving the stability of the materials without extensive loss of the oxygen permeation rate. Ta-substituted material was shown to be promising with its long-term stability as a partial oxidation membrane.

EPFL2008

Room Temperature as a Goldilocks Environment for CH3NH3PbI3 Perovskite Solar Cells: The Importance of Temperature on Device Performance

Juan-Pablo Correa-Baena, Ulf Anders Hagfeldt, Tor Jesper Jacobsson, Wolfgang Richard Tress

Terrestrial applications of solar cells during day-night cycling as well as operation in winter and summer involve substantial temperature variations, which influence the photophysics as well as the charge separation and transport properties in the various materials employed in a device. In this study, the optical absorption of methylammonium lead iodide (MAPbI(3)) and the device performance of MAPbI(3) solar cells have been investigated in an extended temperature range between -190 and 80 degrees C. The optical properties were found to change by only a small amount in that temperature range. The device performance did, however, show more dramatic changes and decreased in a reversible manner for temperatures both higher and lower than room temperature. For temperatures up to 80 degrees C and down to -80 degrees C, the drop in performance was up to 25% compared to the room temperature value. Given thermal stability and reversible device performance, this is probably not a showstopper for terrestrial applications of perovskite solar cells but should be considered when evaluating the total energy yield under outdoor operations. At temperatures of 100 degrees C and below, which are relevant for outer atmosphere and space applications, the performance decreases rather dramatically and approaches zero at even lower temperature. Irreversible changes set in for temperatures above 50 degrees C. In addition, the hysteresis decreases at reduced temperatures. As the effects for the absorption properties are minor, the decrease in performance can be attributed to a temperature induced limitation in the transport and extraction of the photogenerated charge carriers which is seen as a strong increase of the series resistance at reduced temperature. The drop of the photovoltage for temperatures below -100 degrees C might be related to reduced charge carrier separation in the perovskite due to excitonic effects and a lower dielectric constant.

American Chemical Society2016