Design and Synthesis of Carbon Quantum Dots with Enhanced Optoelectronic Performance for Photovoltaics

Abstract

Carbon Quantum Dots (CQDs) are competing with existing metals based functional materials, for their winning advantages like abundance, low cost, and versatility in design. Particularly, the properties of carbon materials are highly dependent on the size and their microstructure orientation. For instance, quantum sized-carbon offers both catalytic properties and have size-dependent quantum confinement and associated photoluminescence. Similarly, the electrocatalytic properties of graphene sheets are highly sensitive to the porous or stacked orientation. Based on the above considerations, the size and orientation-dependent optical and electrochemical properties of CQDs are studied here. The optimized design of carbon-based optical and catalyst coating was comparable with the existing metals based optical tuning and electrocatalyst layers of DSSC. Optical properties of CQDs were utilized for the energy downshift layer of Si solar cell and luminescent solar concentrator (LSC). Electrocatalytic activity of CQDs was used for the counter electrode of dye sensitized solar cell (DSSC). In the first stage, we studied the optical properties of carbon quantum dots for energy down-shift application. Enhancing the efficiency of crystalline silicon solar cell (c-Si SC) by coating the energy shifting layer of quantum dots (QDs) is a recent approach to efficiently utilize the high energy spectrum of light. Carbon QDs are an attractive candidate for such applications, however, low Stokes shift and non-uniform coating due to high aggregation are the bottlenecks to fully utilize their potential. For the purpose, here we propose a layer by layer self-assembled uniform coating of ecofriendly red-emissive hollow nitrogen-doped carbon QDs (NR-CQDs), as an efficient energy-down shifting layer. A unique hollow and conjugated structure of NR-CQDs was designed to achieve a large Stokes shift (UV excited - red emission), with a quantum yield (QY) comparable to Cd/Pb QDs. Highly uniform coating of intrinsically negatively charged NR-CQDs on c-Si SCs was achieved by cationizing the c-Si SC by Bovine serum albumin (BSA), under mildly acidic conditions. By opposite charge assisted self-assembled over-layer, the short-circuit current density (Jsc) and power-conversion efficiency was increased by 5.8%, which is attributed to the large Stokes shift (255 nm) and high QY. Blue-emissive undoped-carbon QDs were synthesized for comparison with the proposed NR-CQDs, to elucidate the significance of the novel proposed structure. Similarly, optical properties were also used for the LSC application. In this study, we demonstrate the simultaneous use of carbon quantum dots and organic dyes as a highly emissive luminescent material in high-performance LSC. The top LSC layer is based on CQDs, while the bottom one is based on organic dye. The role of förster resonance energy transfer (FRET) is demonstrated in organic dyes in a bottom LSC. Moreover, the CQDs layer which has the capability to harvest ultraviolet (UV) and near-UV photons acts as a protective layer to improve the photo-stability of the organic dyes contained in bottom waveguide. The electrical measurements showed that the optical conversion efficiency (ηopt) and power conversion efficiency (ηPCE) of CQDs-based single LSC are 5.62% and 1.03% respectively. While, ηopt, and ηPCE the dye-LSC are 13.42% and 2.72% respectively. However, in the tandem LSC, overall ηopt and ηPCE are 16.32% and 3.2% respectively. Our results showed that tandem structured CQDs and dye-based luminescent solar concentrators make the practical use of LSCs more feasible because of the unique properties such as good photostability and high efficiency. In the second stage, we used the highly reactive surface of graphene quantum dots for improving the electrocatalytic activity of carbon composites. Complex electrolyte diffusion through the stacked graphene nanosheets limits their electrochemical performance. As a potential solution, this study explored the potential of nitrogen-doped graphene quantum dots (NGQDs) to induce 3D porous orientation of holey graphene oxide (hGO) nanosheets. The sizes of NGQDs and antisolvent for phase separation assisted assembly were optimized to achieve a 3D nanoporous network. This nano-network serves as a soft template for the porous orientation of hGO, forming a 3D hierarchically porous carbon architecture. Benefiting from the porosity of the 3D framework, π-π restacking was radically avoided, providing high electrolyte transport rates. In addition, doped nitrogen and J-type aggregation of NGQDs effectively tuned the band structure to realize charge transfer at low overpotential. The enhanced electrocatalytic activity and exceptionally low charge transfer resistance of the composite structure were attributed to the enhanced electrode/electrolyte interface and multidimensional charge & electrolyte transport. Porous composite structure-based counter electrode showed 78% enhanced photovoltaic performance (compared to unmodified graphene) in the DSSC, which is comparable to the performance of Pt electrode. The proposed 3D porous orientation can be utilized in emerging electrocatalytic applications, such as supercapacitors, water splitting, and battery electrodes. Similarly, the electrocatalytic activity of CQDs was studied for the counter electrode application. Multiwalled carbon nanotubes (MWCNTs) are at the forefront of metal-free electrocatalysts, however, the performance is still limited due to lack of functionality and dispersion. Coupling of MWCNTs with nitrogen doped carbon quantum dots (NCQDs) can impart the required active sites and dispersion. For the purpose, NCQDs are generally attached to MWCNTs by multistep processing, such as NCQDs synthesis, followed by their complex purification, surface activation, and crosslinking with MWCNT. The scalability of such a multistep process is limited, which is addressed by direct microwave-assisted growth of NCQDs on MWCNT. The concentration of reactants of NCQDs synthesis was optimized (with respect to MWCNTs), to achieve controlled direct growth of NCQDs on MWCNTs. The proposed strategy significantly reduced time and energy consumption, along with providing an overlapped interface for the fast charge transfer. Moreover, NCQDs' growth effectively modulated the surface reactivity and internal band structure of the MWCNTs. In response, dye-sensitized solar cells employing NCQDs modified MWCNT as a counter electrode showed 50% higher photovoltaic performance as compared to bare MWCNTs. We believe that the understanding provided by this study of the sustainable alternative will compete with the existing metals-based materials in energy applications. In particular, multifunctional applications of CQDs, i.e. tunable optical properties, functional interface, and excellent catalytic properties can integrate diverse functions in a single material.