Exploring Carbon Nanoflake Based Materials for Charge Transport Layers of Perovskite Solar Cells: A Combined DFT-DFTB Study Including Effects of Solid-State Packing

One of the biggest difficulties in the commercialization of perovskite solar cells (PSCs) lies in their charge transport layers. Charge transport materials are needed to achieve high performance with PSCs, but hole transporting materials (HTM) and electron transporting materials (ETM) used in high-performance lab-scale PSCs are relatively high-cost. Meanwhile, high-performance HTMs typically require the use of dopants that are detrimental to cell stability, while the commonly used metal oxide ETLs suffer from low electron mobility and photo-generated oxygen vacancies, which result in photocurrent hysteresis and serious interface recombination that affects the performance of the device.<br/>Carbon-based materials, in particular carbon nanoflakes (CNF) and carbon quantum dots (CQD), have been increasingly used in charge transport layers and electrodes for PSCs. 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.<br/>Here, we study the dependence of band alignment and charge transport characteristics on chemical composition and structure of commonly accessible types of nanoflakes and functional groups and further consider the effect of their solid-state 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 scales relevant to experiments. 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 the commonly used MAPbI₃, certain functional groups are able to change the band alignment so strongly that the same carbon nanoflake might act as either hole transport material or electron transport material. We show that both the shape of the CNF and inter-flake interactions can significantly modify band alignment by more than half an eV; solid-state packing has a moderate effect on HOMO of less than half an eV, whereas the effect on LUMO can be on the order of one eV.