Exploring the electronic properties of carbon nanoflake-based charge transport materials for perovskite solar cells:...

Carbon-based materials, in particular carbon nanoflakes (CNFs) and carbon quantum dots (CQDs), have been increasingly used in charge transport layers and electrodes for perovskite solar cells (PSCs). There are practically limitless possibilities of designing such materials with different sizes, shapes and functional groups, which allow modulating their properties such as band alignment and charge transport. Solid state packing further modifies these properties. However, there is still limited insight into the electronic properties of these types 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 the 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 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 in PSCs containing the commonly used methylammonium lead iodide perovskite. We show that both shape and functionalization can significantly modify the band alignment of the CNF, by more than half an electron volt. Inter-flake interactions further modify the band alignment, in some cases by about half an electron volt. CNFs having small sizes possess sufficient inter-flake electronic coupling for efficient hole transport. In contrast, no shape or size of CNFs produces band alignment suitable for their use as electron transport materials.