The Role of Graphene Nanostructures on Colloidal Quantum Dot Photovoltaic Devices

PbS quantum dots (QDs) have received considerable attention in the past decade due to their low-cast solution processing, bandgap tenability by control the size, and multiexciton generation make them particularly attractive as an active absorbing layer in optoelectronic devices such as solar cells. Recently, combining semiconductor quantum dots and graphene are a new strategy to improve the optoelectronic properties of photovoltaics devices. Herein, we have reviewed our recent works which are expolred the role of graphene as a passivation agent on the surface of PbS QDs and an interface layer between the electron transfer layer and absorber layer. First, we have demonstrated that the graphene can wrap the surface of PbS QDs and passivate the deep trap states by using a novel solution proces. Simulation by density functional theory of a prototypical model of a non-stoichiometric Pb-rich core (400 atoms) coated by graphene (20 atoms for each graphene sheet) indicates the possibility of surface passivation of (111) planes of PbS with graphene resulting in a decrease in trap states and recombination sites. The graphene coating of the PbS quantum dots decrease the exciton lifetime up to 0.78 µs as compared to 1.2 µs for the oleic acid passivated PbS quantum dots due to the fast extraction of carriers. We have employed PbS/graphene as well as Cd-doped PbS/graphene quantum dots as active layers of bulk heterojunction solar cells, and achieved solar power conversion efficiencies of 3.6% and 4.1%, respectively. In addition to the surface passivation role, a thin layer of graphene as an interface layer can increase the performance of PbS QDs device due to the improvement of carriers transfer and enhancement of current density. In our work, a simple and novel method is employed to synthesize a new structure of graphene, i.e. hollow graphene. In this regards., ZnO-Graphene QDs nanocomposite synthesized by a solution method and then ZnO QDs were dissolved from the structure using an acidic solution to obtain hollow graphene. Afterward, this structure was utilized in PbS QDs solar cell in order to improve the carriers transport. It is shown that holow graphene electrodes enhance electron extraction, yielding 30% improvement in performance compared with conventional device. Figure 1