Exploring Electronic Properties of Carbon Nanoflake-based Charge Transport Materials for Perovskite Solar Cells: A Computational Study

Abstract

Carbon-based materials, in particular carbon nanoflakes (CNF) and carbon quantum dots (CQD), have been increasingly used in charge transport layers and electrodes for perovskite solar cells (PSC). There are practically limitless possibilities of designing such materials with different sizes, shapes and functional groups, which allows modulating their properties such as band alignment and charge transport. Solid state packing further modifies these properties. However, there is still limited insight into electronic properties of this type of materials as a function of their chemical composition, structure, and packing. Here, we compute the dependence of band alignment and charge transport characteristics on size, chemical composition, and structure of commonly accessible types of nanoflakes and functional groups and further consider the effect of their packing. We use a combination of density functional theory (DFT) and density functional-based tight binding (DFTB) to get electronic structure level of insight at length scales (nanoflake sizes) relevant to the experiment. We find that CNFs must have sizes as small as 1.3 nm to provide band alignments suitable for their use as hole transport materials with methylammonium lead iodide commonly used in PSCs. We show that both shape and functionalization can significantly modify band alignment of the CNF, by more than half an eV. Inter-flake interactions further modify the band alignment, in some cases by about half an eV. CNFs of such small sizes possess sufficient inter-flake electronic coupling for efficient hole transport. In contrast, no shape or size of CNF produces band alignment suitable for use as electron transport materials.