Towards Efficient Dye-Sensitized Solar Cells: An economical Strategy for Prototypical Organic Dyes with Tailored Frontier Orbitals

TL;DR

Problem: designing efficient CT dyes for DSSCs requires accurate predictions of frontier orbital energies and low-lying excitations across large chemical spaces with manageable cost. Approach: evaluate an effective RS tuning, $ω_{eff}$, for the LC-$ω$PBE functional, compare to IP-based tuning, and apply to a 27-dye library of mono-/di-/tri-doped D–π–A dyes using KS-DFT and TDDFT/TDA to predict HOMO/LUMO and SS/ST energies. Findings: $ω_{eff}$ reproduces vertical IPs within $\approx$0.1 eV of CCSD(T) benchmarks and provides trends consistent with higher-level methods; boron doping lowers HOMO-LUMO gaps and excitation energies, whereas nitrogen/oxygen doping raises them, with BBN offering the lowest SS/ST energies and some diradical states. Significance: the cheap $ω_{eff}$-tuning approach enables rapid, rational design and screening of CT materials for DSSCs, generating a curated benchmark library and guiding experimental efforts; future work will incorporate solvent, interfacial effects, and dye–TiO$_2$ interactions to connect molecular design with device performance.

Abstract

The strategic incorporation of heteroatoms (N, O, and B) into organic dyes is a versatile and effective approach to enhance molecular properties. This approach is highly attractive for tailoring organic solar cells, as it allows for precise control over the HOMO and LUMO energy levels, enabling the design and customization of organic molecules with desired optical and electronic properties. In this work, we aim to contribute to this pursuit by exploring novel charge transfer materials with Time-Dependent Density Functional Theory (TDDFT), specifically using the Tamm-Dancoff Approximation (TDA). This study evaluates two distinct parameter-tuning strategies for the range-separated hybrid (RSH) functional. The first uses a simplified scheme $ω_{eff}$, while the second implements a more intricate protocol $ω_{IP}$ designed to reproduce the exact ionization potential. The accuracy of the effective-tuning ($ω_{eff}$) method was tested against experimental values of ionization potentials for BN-doped organic molecules. A comparative analysis of our data reveals that the accuracy of the ($ω_{eff}$) approach is superior to that of the more complicated ($ω_{IP}$) method and comparable to wave function theory (WFT).

Figures (5)

• Figure 1: Schematic representation of a library that consists of a series of 27 mono-, di-, and tri-doped prototypical organic dyes with common donor and acceptor moieties.
• Figure 2: The HOMO and LUMO energy levels (eV) of N- and B-doped (A) and O- and B-doped (B) organic dyes, analyzing the performance with the LC-$\omega$PBE functional and def2-TZVPD basis set using the effective tuning parameter ($\omega_{eff}$). The complete data is available in the SI Table S4. For comparison to IP tuning ($\omega_{IP}$), refer to SI Figure S1 and S2.
• Figure 3: The HOMO-LUMO gap trends for the doped system, analyzing the performance with the LC-$\omega$PBE functional and def2-TZVPD basis set using the effective tuning parameter ($\omega_{eff}$). The complete data is available in the SI Table S4. For comparison to IP tuning ($\omega_{IP}$), refer to SI Figure S3.
• Figure 4: The spatial distributions of the frontier molecular orbitals (HOMO and LUMO) for O-, B-, and N-doped organic dyes, calculated using the LC-$\omega_{eff}$PBE functional and def2-TZVPD basis set.
• Figure 5: The singlet-singlet (A) and singlet-triplet (B) excitation energies (eV) of N-, O-, and B-doped organic dyes, analyzing the performance with LC-$\omega$PBE functional and def2-TZVPD basis set using the effective tuning parameter ($\omega_{eff}$). The complete data is available in the SI Table S5. For comparison to IP tuning ($\omega_{IP}$), refer to SI Figure S4.