Revealing the Fate of Photo-Generated Charges in Metal Halide Perovskites

In this thesis, we have investigated the optoelectronic properties of metal halide perovskites with a special focus on their application in solar cells. In less than a decade of development, metal halide perovskites have yielded solar cells with efficiencies comparable to commercialized technologies. However, there has been limited knowledge about the fundamental properties of these materials. As mentioned in the introduction, the efficiency of perovskite-based solar cells is still not at its theoretical limit. In order to rationally design solar cells with maximized efficiencies, we need to understand which factors are currently limiting the performance of perovskite-based solar cells. In general, one of the first important processes in a solar cell is the absorption of light. For metal halide perovskites based on lead iodide, a thickness of 0.3 micrometer is already sufficient to absorb a substantial amount of visible (sun-)light, which makes these materials very suitable for solar cells. Furthermore, it is crucial that this absorbed light is converted into a current of moving charges, also known as electricity. Semiconductor materials such as silicon or metal halide perovskites have the ideal properties to generate a current of charges from light. In order to use this current however, the charges need to be collected. The efficiency with which charges are collected in a solar cell is closely related to its power conversion efficiency.ChemE/Opto-electronic Material

Thermally Activated Second-Order Recombination Hints toward Indirect Recombination in Fully Inorganic CsPbI₃ Perovskites

The relationship between the dipole moment of the methylammonium cation and the optoelectronic properties of lead halide perovskites remains under debate. We show that both the temperature-dependent charge carrier mobility and recombination kinetics are identical for methylammonium and cesium lead iodide, indicating that the role of the monovalent cation is subordinate to the lead iodide framework. From the observation that for both perovskites the electron-hole recombination is thermally activated, we speculate that the bandgap is slightly indirect.ChemE/Opto-electronic Material

Comparing the Calculated Fermi Level Splitting with the Open-Circuit Voltage in Various Perovskite Cells

While the power conversion efficiency of metal halide perovskite (MHP) solar cells has increased enormously, the open-circuit voltage, V oc , is still below the conceivable limit. Here, we derive the Fermi level splitting, μ F , for various types of noncontacted MHPs, which sets a limit for their achievable V oc , using rate constants and mobilities obtained from time-resolved photoconductivity measurements. Interestingly, we find that for vacuum-evaporated MAPbI 3 and K + -doped (MA,FA,Cs)Pb(I/Br) 3 , the μ F /e values are close to the reported V oc values. This implies that for an improvement of the V oc , charge carrier recombination within the bare perovskite has to be reduced. On the other hand, for MHPs with Cs + and/or Rb + addition, the experimental V oc is still below μ F /e, suggesting that higher voltages are feasible by optimizing the transport layers. The presented approach will help to select which techniques and transport layers are beneficial to improve the efficiency of MHP solar cells. ChemE/Opto-electronic Material

Band-Like Charge Transport in Cs₂AgBiBr₆ and Mixed Antimony-Bismuth Cs₂AgBi_{1- x}Sb_xBr₆ Halide Double Perovskites

Recently, halide double perovskites (HDPs), such as Cs2AgBiBr6, have been reported as promising nontoxic alternatives to lead halide perovskites. However, it remains unclear whether the charge-transport properties of these materials are as favorable as for lead-based perovskites. In this work, we study the mobilities of charges in Cs2AgBiBr6 and in mixed antimony-bismuth Cs2AgBi1-xSbxBr6, in which the band gap is tunable from 2.0 to 1.6 eV. Using temperature-dependent time-resolved microwave conductivity techniques, we find that the mobility is proportional to T-p (with p ≈ 1.5). Importantly, this indicates that phonon scattering is the dominant scattering mechanism determining the charge carrier mobility in these HDPs similar to the state-of-the-art lead-based perovskites. Finally, we show that wet chemical processing of Cs2AgBi1-xSbxBr6 powders is a successful route to prepare thin films of these materials, which paves the way toward photovoltaic devices based on nontoxic HDPs with tunable band gaps.ChemE/Opto-electronic Material

Photoluminescence from Radiative Surface States and Excitons in Methylammonium Lead Bromide Perovskites

In view of its band gap of 2.2 eV and its stability, methylammonium lead bromide (MAPbBr3) is a possible candidate to serve as a light absorber in a subcell of a multijunction solar cell. Using complementary temperature-dependent time-resolved microwave conductance (TRMC) and photoluminescence (TRPL) measurements, we demonstrate that the exciton yield increases with lower temperature at the expense of the charge carrier generation yield. The low-energy emission at around 580 nm in the cubic phase and the second broad emission peak at 622 nm in the orthorhombic phase originate from radiative recombination of charges trapped in defects with mobile countercharges. We present a kinetic model describing both the decay in conductance as well as the slow ingrowth of the TRPL. Knowledge of defect states at the surface of various crystal phases is of interest to reach higher open-circuit voltages in MAPbBr3-based cells.ChemE/Opto-electronic Material

Charge Carrier Dynamics upon Sub-bandgap Excitation in Methylammonium Lead Iodide Thin Films: Effects of Urbach Tail, Deep Defects, and Two-Photon Absorption

To further understand the optoelectronic properties of metal halide perovskites, we investigate sub-bandgap absorption in methylammonium lead iodide (MAPbI3) films. Charge carrier dynamics are studied using time-resolved microwave conductivity measurements using sub-bandgap excitation. From changes in the decay dynamics as a function of excitation energy and intensity, we have identified three regimes: (i) Band-like charge transport at photon energies above 1.48 eV; (ii) a transitional regime between 1.48 and 1.40 eV; and (iii) below 1.40 eV localized optically active defects (8 × 1013 cm-3) dominate the absorption at low intensities, while two-photon absorption is observed at high intensities. We determined an Urbach energy of approximately 11.3 meV, indicative of a low structural and/or thermal disorder. Surprisingly, even excitation 120 meV below the bandgap leads to efficient charge transfer into electron (C60) or hole (spiro-OMeTAD) transport layers. Therefore, we conclude that for MAPbI3, the band tail states do not lead to nonradiative losses. ChemE/Opto-electronic Material

The balancing act between high electronic and low ionic transport influenced by perovskite grain boundaries

A better understanding of the materials' fundamental physical processes is necessary to push hybrid perovskite photovoltaic devices towards their theoretical limits. The role of the perovskite grain boundaries is essential to optimise the system thoroughly. The influence of the perovskite grain size and crystal orientation on physical properties and their resulting photovoltaic performance is examined. We develop a novel, straightforward synthesis approach that yields crystals of a similar size but allows the tuning of their orientation to either the (200) or (002) facet alignment parallel to the substrate by manipulating dimethyl sulfoxide (DMSO) and tetrahydrothiophene-1-oxide (THTO) ratios. This decouples crystal orientation from grain size, allowing the study of charge carrier mobility, found to be improved with larger grain sizes, highlighting the importance of minimising crystal disorder to achieve efficient devices. However, devices incorporating crystals with the (200) facet exhibit an s-shape in the current density-voltage curve when standard scan rates are used, which typically signals an energetic interfacial barrier. Using the drift-diffusion simulations, we attribute this to slower-moving ions (mobility of 0.37 × 10-10 cm2 V-1 s-1) in combination with a lower density of mobile ions. This counterintuitive result highlights that reducing ion migration does not necessarily minimise hysteresis.ChemE/Opto-electronic Material

Crystal orientation and grain size: Do they determine optoelectronic properties of MAPbI₃ Perovskite?

Growing large, oriented grains of perovskite often leads to efficient devices, but it is unclear if properties of the grains are responsible for the efficiency. Domains observed in SEM are commonly misidentified with crystallographic grains, but SEM images do not provide diffraction information. We study methylammoinium lead iodide (MAPbI3) films fabricated via flash infrared annealing (FIRA) and the conventional antisolvent (AS) method by measuring grain size and orientation using electron back-scattered diffraction (EBSD) and studying how these affect optoelectronic properties such as local photoluminescence (PL), charge carrier lifetimes, and mobilities. We observe a local enhancement and shift of the PL emission at different regions of the FIRA clusters, but we observe no effect of crystal orientation on the optoelectronic properties. Additionally, despite substantial differences in grain size between the two systems, we find similar optoelectronic properties. These findings show that optoelectronic quality is not necessarily related to the orientation and size of crystalline domains.ChemE/Opto-electronic Material

Interconversion between Free Charges and Bound Excitons in 2D Hybrid Lead Halide Perovskites

The optoelectronic properties of hybrid perovskites can be easily tailored by varying their components. Specifically, mixing the common short organic cation (methylammonium (MA)) with a larger one (e.g., butyl ammonium (BA)) results in 2-dimensional perovskites with varying thicknesses of inorganic layers separated by the large organic cation. In both of these applications, a detailed understanding of the dissociation and recombination of electron-hole pairs is of prime importance. In this work, we give a clear experimental demonstration of the interconversion between bound excitons and free charges as a function of temperature by combining microwave conductivity techniques with photoluminescence measurements. We demonstrate that the exciton binding energy varies strongly (between 80 and 370 meV) with the thickness of the inorganic layers. Additionally, we show that the mobility of charges increases with the layer thickness, in agreement with calculated effective masses from electronic structure calculations.ChemE/Opto-electronic Material