Balancing Light Absorptivity and Carrier Conductivity of Graphene Quantum Dots for High-Efficiency Bulk Heterojunction Solar Cells

Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDsgraphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDswere tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%)

Strain-Assisted Wafer-Scale Nanoperforation of Single-Layer Graphene by Arrayed Pt Nanoparticles

We demonstrate the large-area lithography-free ordered perforation of reduced graphene oxide (rGO) and graphene grown by chemical vapor deposition (CVD) with arrayed Pt nanoparticles (NPs) prepared by using self-patterning diblock copolymer micelles. The rGO layers were perforated by Pt NPs formed either on top or bottom surface. On the other hand, CVD graphene was perforated only when the Pt NPs were placed under the graphene layer. Various control experiments confirm that the perforation reaction of CVD graphene was catalyzed by Pt NPs, where the mechanical strain as well as the chemical reactivity of Pt lowered the activation energy barriers for the oxidation reaction of CC bonds in graphene. Systematic atomic force microscopy and Raman analyses revealed the detailed perforation mechanism. The pore size and spacing can be controlled, and thus our present work may open a new direction in the development of ordered nanopatterns on graphene using metal NPs