Role of Ferroelectric Nanodomains in the Transport Properties of Perovskite Solar Cells.

Metropolis Monte Carlo simulations are used to construct minimal energy configurations by electrostatic coupling of rotating dipoles associated with each unit cell of a perovskite CH3NH3PbI3 crystal. Short-range antiferroelectric order is found, whereas at scales of 8-10 nm, we observe the formation of nanodomains, strongly influencing the electrostatics of the device. The models are coupled to drift-diffusion simulations to study the actual role of nanodomains in the I-V characteristics, especially focusing on charge separation and recombination losses. We demonstrate that holes and electrons separate into different nanodomains following different current pathways. From our analysis we can conclude that even antiferroelectric ordering can ultimately lead to an increase of photoconversion efficiencies thanks to a decrease of trap-assisted recombination losses and the formation of good current percolation patterns along domain edges.

Scanning probe microscopy investigation of self-organized perylenetetracarboxdiimide nanostructures at surfaces: structural and electronic properties.

A scanning probe microscopy investigation of the self-organization and local electronic properties of spin-coated ultrathin films of N-alkyl substituted perylenetetracarboxdiimide (PDI) is described. By carefully balancing the interplay between molecule-molecule and molecule-substrate interactions, PDI is able to form highly ordered supramolecular architectures on flat surfaces from solution. On an electrically insulating yet highly polar surface (mica) PDI forms strongly anisotropic architectures with needlelike structures with lengths of up to a few micrometers. On a conductive yet apolar surface (highly oriented pyrolytic graphite), the competition between the strong molecule-substrate interactions and the intermolecular forces leads to the generation of more disordered structures. The local electronic properties of these architectures are studied by Kelvin probe force microscopy by estimating their surface potential (SP). Quantitative measurements of the SP are obtained by analyzing the experimentally estimated SP data with a computational model, which discriminates between the intrinsic SP and the effect of long-range tip-surface interactions. The SP of PDI aggregates depends on the structural order at the supramolecular level. Narrow needles of constant width reveal identical SPs independent of length. Wider needles with a polydisperse width distribution exhibit a greater SP.

The real TiO2/HTM interface of solid-state dye solar cells: role of trapped states from a multiscale modelling perspective.

In this paper we present a multiscale simulation of charge transport in a solid-state dye-sensitized solar cell, where the real morphology between TiO2 and the hole transport material is included. The geometry of the interface is obtained from an electron tomography measurement and imported in a simulation software. Charge distribution, electric field and current densities are computed using the drift-diffusion model. We use this approach to investigate the electrostatic effect of trap states at the interface between the electron and hole transport materials. The simulations show that when the trapped electrons are not screened by external additives, the dynamics of holes is perturbed. Holes accumulate at the interface, enhancing recombination and reducing cell performance.