Publication date: 1st July 2014
The use of graphene as an acceptor material represents an interesting alternative to fullerene derivatives and CNTs in bulk heterojunction OPV devices [1]. Although excellent progress has been made in this direction, the efficiency of devices made with solution-processable functionalized graphene (SPFGraphene) remain low (~1.1 %) [1]. In this context, several important questions remain unanswered, including what is the maximum achievable open-circuit voltage (VOC) and what are the effects of the oxygen functional groups on VOC.
In this work we address these questions using a first-principles approach, combining DFT with hybrid functionals [2], and considering large interface models including over 700 atoms. We study the atomic structure and energetics of ideal graphene/P3HT interfaces and determine a maximum ideal VOC of ~0.7-0.9 eV, in excellent agreement with reduced (annealed) SPFGraphene/P3HT devices [3]. Additionally, we find that the presence of oxygen functional groups can raise the VOC of the interface by several tenths of an eV (in good agreement with the corresponding non-annealed devices [3]), owing to a distortion of the polymer layer and a simultaneous rise in the energy of the highest occupied graphene state.
Our results indicate that, while annealed devices already operate at or near their theorectical maximum VOC, the dispersity of the polymer matrix and the density of functional groups on reduced graphene oxide can be used to control the interfacial energy-level alignment so as to increase the open-circuit voltage above 1 V. Taken together our present findings represent the first step towards the rational design of graphene/polymer bulk heterojunction solar cells at the nanoscale.
[1] X. Wan, et al., Adv. Mater. 2011, 23, 5342.
[2] K. Noori, F. Giustino, Adv. Funct. Mater. 2012, 22, 5089.
[3] Q. Liu, et al., Adv. Funct. Mater. 2009, 19, 894.
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