By using density-functional tight-binding method we have calculated the electronic structure of graphene quantum dot (GQD)-fullerene hybrid systems and explored the efficacy of their use in designing solar cells. We have shown that the electronic energy levels of the nanohybrids can be tuned either by varying the size of the quantum dots or by proper functionalization of the quantum dot (QD). The GQD-fullerene nanohybrids form type-I or type-II band energy alignment depending upon the size of the GQD. Thus, hybrid systems with smaller sized QDs form type-II band energy alignment while those of larger GQDs form type-I alignment. The type-II band alignment confirms the spatial charge separation for the systems and thus the rate of recombination of charge carriers will be low. The value of ΔG i.e. the difference in energy between the LUMO of the donor (GQD) and LUMO of the acceptor (fullerene) which measures the rate of electron injection from the donor to the acceptor is also large for the nanohybrids with smaller GQDs. So, we suggest that GQD-fullerene nanocomposites with smaller GQD will be a suitable system for photovoltaic devices. We also show that the type-II band energy alignment for the nanohybrids with larger QDs can be achieved through the functionalization of the GQD with electron donating group such as -NH2.
This article is dedicated to postdoctoral research supervisor of one of the authors (Pranab Sarkar), Prof. Dr. Michael Springborg, University of Saarlandes, Germany on the happy occasion of his 60th birthday. He is the person who had got me (P. S.) introduced into this fascinating field of research. The financial support from CSIR, New Delhi [01(2744)/13/EMR-II] through research grant is gratefully acknowledged. Chandrima Chakravarty and Bikash Mandal are grateful to CSIR, New Delhi for the award of Junior and Senior Research Fellowships, respectively.
©2016 Walter de Gruyter Berlin/Boston