Publication date: 1st July 2014
Graphene and its derivatives are well known for having remarkable properties such as a very high charge carrier mobility, mechanical flexibility, optical transparency and moreover being compatible with low-cost and efficient solution-based processes. Consequently, over the past few years various graphene derivatives have been examined as possible alternatives for different layers of solar cell (SC) devices. However, the use of graphene in SC layers (e.g. the hole or electron transport layer) still poses some challenges. In all these applications it is important to be able to tune the graphene work function in such a way that the optimum energy level alignment of the SC’s layers is achieved. In this work, we investigate strategies for engineering the work function of graphene derivatives by employing first-principles density functional theory calculations. Here we focus on functionalized graphene nanoribbons, and we consider the case where different species attach to the edges of pristine and oxidized graphene ribbons. After identifying the most stable adsorption geometries we calculate the effects of these functional groups on the work function. Our results reveal that the work function is very sensitive to the chemical and structural properties of the functional groups. Our calculated work function shifts are consistent with recent UPS measurements. In order to rationalize the calculated trends and gain a deeper understanding of the underlying atomic scale mechanisms, we develop a simple electrostatic model, which captures the essential physics at play.
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