Graphene quantum dots (GQDs), have unique quantum confinement effects, tunable bandgap and luminescence property, with a wide range of potential applications such as optoelectronic and biomedical areas. However, GQDs usually have a strong tendency toward aggregation especially in making solid films, which will degrade their optoelectronic properties, for example, causing undesired fluorescence quenching. Here, we designed a composite film by embedding GQDs in a polyvinyl pyrrolidone (PVP) matrix through hydrogen bonding with well-preserved fluorescence, with a small addition of acid for compensating the poor conductivity of PVP. As a multifunctional solid coating on carbon nanotube/silicon (CNT/Si) solar cells, the photon down-conversion by GQDs and the PVP anti-reflection layer for visible light lead to enhanced external quantum efficiency (by 12.34% in the ultraviolet (UV) range) and cell efficiency (up to 14.94%). Such advanced optical managing enabled by low-cost, carbon-based quantum dots, as demonstrated in our results, can be applied to more versatile optoelectronic and photovoltaic devices based on perovskites, organic and other materials.

Since Akira Yoshino first proposed the usage of the carbonaceous materials as an anode of lithium ion batteries (LIBs) in 1985, carbonaceous materials such as graphite and graphene have been widely considered as LIB anodes. Here, we explored the application of novel carbonaceous LIB anodes incorporating graphene quantum dots (GQDs). We fabricated a freestanding all-carbon electrode based on a porous carbon nanotube (CNT) sponge via a facile in-situ hydrothermal deposition technique, creating coaxial structure of GQD-coated CNTs (GQD@CNTs) through electrostatic interaction and π-π stacking with tunable loading and functionalization. This hybrid structure combined conductive CNTs with highly active GQDs, in which GQDs with predesigned functional groups provided massive storage sites for Li ions and the 3D CNT frameworks avoided the agglomeration of GQDs, together contributing to a high specific capacity (700 mAh·g^-1 at 100 mA·g^-1 after 100 cycles) and rate performance. Even at a high current density of 1,000 mA·g^-1, the reversible specific capacity remained at 483 mAh·g^-1 after 350 cycles. In particular, the mechanism study demonstrated the important role of oxygen functional groups of GQDs in promoting the performance of the LIB anodes by controlled grafting of GQDs onto various porous-carbon and metal-foam based structures.