Carbon Nanoflake Based Materials for Charge Transport Layers of Perovskite Solar Cells: Insight from Atomistic Modeling into Nanosizing and Functionalizations Suitable for Electron and Hole Transport

A key bottleneck on the way to practical deployment of perovskite solar cells (PSC) are charge transport materials. Charge transport layers 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. Another key bottleneck, low cell stability, is related to the issue of charge transport layers, as high-performance HTMs typically require the use of dopants that are detrimental to cell stability.<br/>Recently, the use of carbon-based materials for both HTM and ETM has been gaining attention, in particular due to the possibility of synthesizing such materials from biomass or plastic wastes. This promises combining large-scale, low cost production with environmental remediation. Much of such research is empirical, without detailed insight into electronic properties of the synthesized materials. Such insight is needed to understand the possibility of forming correct band alignment and to ensure adequate charge transfer properties and for rational design of carbon-based HTMs and ETMs.<br/>Carbon nanoflakes are one promising type of carbon based materials. Their size and functionalization/doping can be controlled, albeit imperfectly. We perform ab initio and semiempirical calculations to understand what sizes, shapes, and functionalizations can allow carbon nanoflakes to achieve band alignment and charge transport properties suitable for their use as either HTM or ETM. We include the effects of solid packing in the calculations and show that they substantially affect electronic properties. We show, in particular, that nanoflake sizes required for HTM capability should be as small as 1.5 nm, which is much smaller than nanoparticle sizes typically used in the literature. We also show that different functionalizations can make the same nanoflakes work as either ETM or HTM, and can also be used to control charge transport properties. Conversely, our results indicate that tight control of the presence of functional groups is required during synthesis to preserve HTM or ETM capability of such materials.