Theoretical investigation into molecular level encapsulation and active layer strategies with carbon dot integration for micro-sized photovoltaic cell

Abstract

Context

This s tudy explores the potential of functionalized carbon dots (CDs) for integration into the encapsulation and active layers of micro-sized photovoltaic cells (PVCs), with a focus on applications in renewable energy, biomedical devices, optoelectronics, and environmental sensors. Traditional photovoltaic materials such as perovskites and organic polymers, while efficient, pose challenges related to toxicity and environmental instability. In contrast, CDs derived from biocompatible sources offer enhanced safety and sustainability due to their high quantum yield, photostability, and tunable optical properties. Among the variants studied, amide-functionalized carbon dots (CD-CONH₂) were found optimal for encapsulation when paired with graphene (Gr-CD-CONH₂), while nitrogen-doped CD-CONH₂ (CD-CONH₂/N), integrated with silicon quantum dots (Si-CD-CONH₂/N), demonstrated superior characteristics for active layer performance. Key results include high HOMO–LUMO energy gaps of 3.50 eV (vacuum) and 3.48 eV (water) for CD-CONH₂, and 2.11–2.15 eV for Si-(CD-CONH₂/N), along with strong dipole moments and negative formation energies, indicating stability and efficiency across environments. These findings suggest that Gr-(CD-CONH₂) and Si-(CD-CONH₂/N) composites are promising materials for enhancing the environmental sustainability and performance of next-generation micro-sized PVCs.

Methods

All theoretical simulations were performed using Gaussian 09W. Geometry optimizations and frequency analyses of functionalized CDs (-OH, -NO₂, -NH₂, -COOH, -CONH₂, -CHO) were conducted using the B3LYP-D3BJ functional with the 6-311G(d) basis set. Frequency analyses confirmed convergence by the absence of imaginary frequencies. Time-dependent DFT (TD-DFT) calculations were employed for UV–Vis spectral analysis using the TD-SCF method, with solvent effects modeled via the SMD approach (water as solvent). Water was selected as the solvent because our theoretical PVC model is intended to have potential biomedical relevance in non-invasive photodynamic treatments. In this context, water serves as a representation of the physiological environment of the human body. All TD-SCF calculations were done using default Gaussian settings for 10 excited states. Visualization and input generation were performed using GaussView 6.0. Encapsulation studies involving CD-CONH₂ and graphene were initially modeled using AutoDock Vina (SwissDock) to determine optimal binding poses, followed by full DFT optimization in Gaussian 09W. Adsorption and solvation energies were computed using standard DFT protocols. For active layer modeling, the Si-(CD-CONH₂/N) composite underwent similar optimization and TD-DFT analysis. Reactivity descriptors (chemical hardness, softness, chemical potential, and electrophilicity) were derived from HOMO–LUMO energies. Additional analyses such as transition density matrix (TDM), electrostatic potential (ESP) maps, and electron localization function (ELF) were performed using Multiwfn (version 3.8) to further evaluate electronic structure and stability of the composites.

Graphical Abstract