Authors: M.Pazoki, M. J. Wolf, T. Edvinsson and J.Kullgren
Ion migration has recently been suggested to play critical roles in the operation of lead halide perovskite solar cells. However, so far there has been no systematic investigation of how the monovalent cation affects the vacancy formation, ion migration and the associated hysteresis effect. Here, we present density functional theory calculations on all possible ion migration barriers in the perovskite materials with different cations i.e. CH3NH3PbI3, CH(NH2)2PbI3 and CsPbI3 in the tetragonal phase and investigate vacancy monovalent-cation interactions within the framework of the possible ion migrations. The most relevant ion movement (iodide) is investigated in greater detail and corresponding local structural changes, the relationships with the local ionic dielectric response, Stark effect and current-voltage hysteresis are discussed. We observe a correlation between the energy barrier for iodine migration and the magnitude of the dipole of the monovalent cation. From the data, we suggest a vacancy-dipole interaction mechanism by which the larger dipole of the monovalent cation can respond to and screen the local electric fields more effectively. The stronger response of the high dipolar monovalent cation to the vacancy electrostatic potential in turn leads to a lower local structural changes within the neighbouring octahedra. The presented data reveal a detailed picture of the ion movement, vacancy dipole interactions and the consequent local structural changes, which contain fundamental information about the photo-physics, and dielectric response of the material.
Nano Energy, 38, 2017, pp. 537-543
Authors: M. Pazoki, A. Röckert, M. J. Wolf, R. Imani, T. Edvinsson, and J. Kullgren.
The emergence of highly efficient lead halide perovskite solar cell materials makes the exploration and engineering of new lead free compounds very interesting both from a fundamental perspective as well as for potential use as new materials in solar cell devices. Herein we present the electronic structure of several lanthanide (La) based materials in the metalorganic halide perovskite family not explored before. Our estimated bandgaps for the lanthanide (Eu, Dy, Tm, Yb) perovskite compounds are in the range of 2.0–3.2 eV showing the possibility for implementation as photo-absorbers in tandem solar cell configurations or charge separating materials. We have estimated the typical effective masses of the electrons and holes for MALaI3 (La= Eu, Dy, Tm, Yb) to be in the range of 0.3–0.5 and 0.97–4.0 units of the free electron mass, respectively. We have shown that the localized f-electrons within our DFT+U approach, make the dominant electronic contribution to the states at the top of the valence band and thus have a strong impact on the photo-physical properties of the lanthanide perovskites. Therefore, the main valence to conduction band electronic transition for MAEuI3 is based on inner shell f-electron localized states within a periodic framework of perovskite crystal by which the optical absorption onset would be rather inert with respect to quantum confinement effects. The very similar crystal structure and lattice constant of the lanthanide perovskites to the widely studied CH3NH3PbI3 perovskite, are prominent advantages for implementation of these compounds in tandem or charge selective contacts in PV applications together with lead iodide perovskite devices.
J. Mater. Chem. A, 5, 2017, pp. 23131-23138
Photon Energy-Dependent Hysteresis Effects in Lead Halide Perovskite Materials
Authors: Meysam Pazoki, T. Jacobsson, Silver H. Jesper and Cruz, Malin Johansson, Roghayeh Imani, Jolla Kullgren, Anders Hagfeldt, Erik M. J. Johansson, Tomas Edvinsson and Gerrit Boschloo.
Lead halide perovskites have a range of spectacular properties and interesting phenomena and are a serious candidate for the next generation of photovoltaics with high efficiencies and low fabrication costs. An interesting phenomenon is the anomalous hysteresis often seen in current–voltage scans, which complicates accurate performance measurements but has also been explored to obtain a more comprehensive understanding of the device physics. Herein, we demonstrate a wavelength and illumination intensity dependency of the hysteresis in state-of-the-art perovskite solar cells with 18\% power conversion efficiency (PCE), which gives new insights into ion migration. The perovskite devices show lower hysteresis under illumination with near band edge (red) wavelengths compared to more energetic (blue) excitation. This can be rationalized with thermalization-assisted ion movement or thermalization-assisted vacancy generation. These explanations are supported by the dependency of the photovoltage decay with illumination time and excitation wavelength, as well as by impedance spectroscopy. The suggested mechanism is that high-energy photons create hot charge carriers that either through thermalization can create additional vacancies or by release of more energetic phonons play a role in overcoming the activation energy for ion movement. The excitation wavelength dependency of the hysteresis presented here gives valuable insights into the photophysics of the lead halide perovskite solar cells.
J. Phys. Chem. C, 121, 2017, pp. 26180-26187
Authors: Meysam Pazoki, Malin B. Johansson, Huimin Zhu, Peter Broqvist, Tomas Edvinsson, Gerrit Boschloo, and Erik M. J. Johansson
Cesium and bismuth iodides (Cs3Bi2I9 and MA3Bi2I9) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs3Bi2I9(2.17 and 2.0 eV) and MA3Bi2I9 (2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs3Bi2I9 and MA3Bi2I9, respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors are therefore also important. Band structure calculations show high effective masses of the charge carriers along the c-axis but on the other hand lower electron effective mass in the a–b planes, revealing the interesting possibility for a directional charge transport. Calculations of dielectric constants, absorption coefficients, carrier effective masses, and exciton binding energies emphasize the fundamental differences between the lead and bismuth iodide perovskites and clarify the reasons behind the lower power conversion efficiency of bismuth iodide perovskite solar cells. Also the calculations show that the orientational disorder of the MA dipoles in the lattice has meaningful impacts on the near valence and conduction band edge of the electronic structure.
J. Phys. Chem. C, 2016, 120 (51), pp 29039–29046