Understanding equivalent circuits in perovskite solar cells. Insights from drift-diffusion simulation.

Perovskite solar cells (PSCs) have reached impressively high efficiencies in a short period of time; however, the optoelectronic properties of halide perovskites are surprisingly complex owing to the coupled ionic-electronic charge carrier dynamics. Electrical impedance spectroscopy (EIS) is a widely used characterization tool to elucidate the mechanisms and kinetics governing the performance of PSCs, as well as of many other semiconductor devices. In general, equivalent circuits are used to evaluate EIS results. Oftentimes these are justified via empirical constructions and the real physical meaning of the elements remains disputed. In this perspective, we use drift-diffusion numerical simulations of typical thin-film, planar PSCs to generate impedance spectra avoiding intrinsic experimental difficulties such as instability and low reproducibility. The ionic and electronic properties of the device, such as ion vacancy density, diffusion coefficients, recombination mechanism, etc., can be changed individually in the simulations, so their effects can be directly observed. We evaluate the resulting EIS spectra by comparing two commonly used equivalent circuits with series and parallel connections respectively, which result in two signals with significantly different time constants. Both circuits can fit the EIS spectra and by extracting the values of the elements of one of the circuits, the values of the elements of the other circuit can be unequivocally obtained. Consequently, both can be used to analyse the EIS of a PSC. However, the physical meaning of each element in each circuit could differ. EIS can produce a broad range of physical information. We analyse the physical interpretation of the elements of each circuit and how to correlate the elements of one circuit with the elements of the other in order to have a direct picture of the physical processes occurring in the device.

Transferable Classical Force Field for Pure and Mixed Metal Halide Perovskites Parameterized from First-Principles.

Many key features in photovoltaic perovskites occur in relatively long time scales and involve mixed compositions. This requires realistic but also numerically simple models. In this work we present a transferable classical force field to describe the mixed hybrid perovskite MAxFA1-xPb(BryI1-y)3 for variable composition (∀x, y ∈ [0, 1]). The model includes Lennard-Jones and Buckingham potentials to describe the interactions between the atoms of the inorganic lattice and the organic molecule, and the AMBER model to describe intramolecular atomic interactions. Most of the parameters of the force field have been obtained by means of a genetic algorithm previously developed to parametrize the CsPb(BrxI1-x)3 perovskite (Balestra et al. J. Mater. Chem. A. 2020, DOI: 10.1039/d0ta03200j). The algorithm finds the best parameter set that simultaneously fits the DFT energies obtained for several crystalline structures with moderate degrees of distortion with respect to the equilibrium configuration. The resulting model reproduces correctly the XRD patterns, the expansion of the lattice upon I/Br substitution, and the thermal expansion coefficients. We use the model to run classical molecular dynamics simulations with up to 8600 atoms and simulation times of up to 40 ns. From the simulations we have extracted the ion diffusion coefficient of the pure and mixed perovskites, presenting for the first time these values obtained by a fully dynamical method using a transferable model fitted to first-principles calculations. The values here reported can be considered as the theoretical upper limit, that is, without grain boundaries or other defects, for ion migration dynamics induced by halide vacancies in photovoltaic perovskite devices under operational conditions.

Study on photovoltaic stability and performance by incorporating tetrabutyl phosphonium iodide into the active layer of a perovskite type photovoltaic cell.

A simple synthesis of an ionic liquid is carried out using a trialkylphosphine and an alkyl halide. The results showed that the quality of perovskite crystals is enhanced by the incorporation of B4PI, when the percentage is 1.5% the PCE of champion PSCs MA98.5(B4PI)1.5PbI3 increases significantly from 15.5%, with a V OC of 0.957 mV, J SC of 23.6 mA cm-2, and an FF of 68.4%. Stability tests show that excess B4PI by 20% has a protective effect against humidity, MA80(B4PI)20PbI3 was more stable towards humidity, losing only 20% efficiency for 200 h.

Correlation between the Effectiveness of the Electron-Selective Contact and Photovoltaic Performance of Perovskite Solar Cells.

Metal halide perovskites (MHPs) are mixed electronic-ionic semiconductors with a remarkable photovoltaic potential that has led to a current world record efficiency surpassing 23%. This good performance stems from the combination of excellent light harvesting and relatively slow nonradiative recombination, which are characteristic of MHPs. However, taking advantage of these properties requires electron and hole transport materials that can efficiently extract charge with minimal photovoltage losses and recombination. It is well-known that n-type anatase TiO2 is a good electron-selective contact (ESC), although the fundamental reasons for its functioning are not completely clear to date. In this Letter, we investigate this issue by preparing perovskite-based solar cells with various n-type metal-oxide electron-selective contacts of different chemical nature and crystal structure. Our main finding is that the open-circuit photovoltage remains essentially independent of the nature of the contact for highly selective electron contacts, a fact that we attribute to a recombination rate that is mainly governed by the bulk of the MHPs. In contrast, replacement of the "standard" TiO2 contact by alternative contacts leads to lower short-circuit photocurrents and more pronounced hysteresis, related to enhanced surface recombination at less effective electron-selective contacts.