Computer-Designed Naphthalene Molecules Point to Cheaper, More Efficient Organic Solar Cells

Organic solar cells have long promised a future of lightweight, flexible and cheaply manufactured photovoltaics, but the hunt for the right molecules to make that promise pay off is relentless. Now, a team of computational chemists has designed a family of naphthalene-based molecules that, according to detailed quantum chemical simulations, could push the performance of non-fullerene acceptors even further. The study, published in the Journal of the Saudi Chemical Society, used density functional theory (DFT) and time-dependent DFT (TD-DFT) to screen a series of newly designed chromophores before a single one is ever synthesized in a laboratory.

The researchers, led by Mashal Khan and Misbah Azhar of Khwaja Fareed University of Engineering and Information Technology in Pakistan, together with collaborators at King Khalid University in Saudi Arabia and Southwest Medical University in China, started from a parent molecule called NDNA. They built a reference compound, NDNR, and six derivatives, NDND1 through NDND6, by swapping in increasingly powerful electron-withdrawing groups at the two terminal ends of the molecule. Each compound follows an A–π–D–π–A architecture: an electron-rich donor core at the center, flanked by thiophene π-spacers and capped with electron-poor acceptor units. This push-pull arrangement is the workhorse of modern organic photovoltaics because it drives intramolecular charge transfer, the process that turns absorbed sunlight into separated electrical charges.

The choice of a naphthalene core is no accident. Naphthalene offers extended π-conjugation, a high optical absorption coefficient and rich chemical tunability, making it a versatile scaffold for solar cell materials. By modifying only the end-capped acceptors while keeping the backbone constant, the team could isolate exactly how each acceptor group reshapes the electronic structure. The acceptors ranged from singly fluorinated variants to dinitro and dicyanomethylene-substituted cyclopenta[c]thiophene-dione units, each chosen for its distinct electron-withdrawing strength.

The headline result belongs to NDND6, the derivative bearing the strongest acceptor, 5-methylene-1,3-dinitro-4H-cyclopenta[c]thiophene-4,6(5H)-dione. It recorded the lowest HOMO–LUMO energy gap of the series at 2.414 electronvolts, the highest absorption maximum at 587.74 nanometers, and the lowest exciton binding energy at 0.304 electronvolts. In practical terms, a narrow gap means the molecule can harvest more of the visible spectrum, while a low exciton binding energy means the electron-hole pairs created by absorbed photons can be pulled apart into free charges more easily, which is the critical bottleneck in organic photovoltaics.

The frontier molecular orbital analysis revealed a textbook charge-transfer picture. In every compound, the highest occupied molecular orbital, the HOMO, concentrated its electron density over the donor core and the π-spacers, while the lowest unoccupied orbital, the LUMO, piled up on the terminal acceptor groups. As the acceptors grew stronger, the LUMO energies dropped steadily, from −2.896 electronvolts in the reference compound to −4.025 electronvolts in NDND6, dragging the energy gap down from 3.262 to 2.414 electronvolts. The ordering of the gaps, NDND6 < NDND4 < NDND5 < NDND2 < NDND3 < NDND1 < NDNR, tracked the electron-withdrawing power of the attached acceptors almost perfectly.

Global reactivity parameters told the same story from a different angle. NDND6 showed the highest global softness, 0.414 inverse electronvolts, and the lowest chemical hardness, 1.207 electronvolts, marking it as the most polarizable and reactive molecule of the set, properties that favor high charge mobility. It also posted the largest electrophilicity index, 11.339 electronvolts, and the maximum electronic charge transfer, 4.334. The team also computed dipole moments and found that NDND4 and NDND6 carried the largest values, 8.920 and 9.871 debyes respectively, which previous studies associate with more efficient exciton dissociation and charge separation.

Excited-state calculations reinforced the trend. TD-DFT spectra showed absorption maxima spanning 438.63 to 587.74 nanometers, with NDND6 and NDND4 red-shifted the furthest, at 587.738 and 520.851 nanometers. Interestingly, these long-wavelength absorbers showed relatively low oscillator strengths, 0.295 and 0.503, a well-known signature of charge-transfer transitions in which the electron and hole end up on different parts of the molecule, reducing orbital overlap. Density of states analysis quantified the redistribution: in NDND6, the donor fragment contributed 93.5 percent of the HOMO but only 2.7 percent of the LUMO, while the acceptor fragments accounted for 88.3 percent of the LUMO, confirming that photoexcitation pushes charge decisively from donor to acceptor ends.

Transition density matrix heat maps and hole-electron maps generated with the Multiwfn software added spatial detail. The maps showed electron-hole coherence concentrated along the donor diagonal, with density flowing through the π-linker and pooling on the acceptor termini. NDND6 and NDND4 displayed weaker hole-electron coupling than their siblings, meaning their excitons dissociate more readily in the excited state, accelerating charge transmission. All seven exciton binding energies fell below 0.5 electronvolts, the threshold commonly cited for photovoltaic relevance, ranging from 0.243 electronvolts in NDND2 to 0.435 electronvolts in the reference compound.

The team also benchmarked the designed molecules against O-IDTBR, a well-studied non-fullerene acceptor with reported HOMO and LUMO energies of −5.56 and −3.93 electronvolts. The designed chromophores showed favorable open-circuit voltage values and good agreement with the reference, suggesting they could hold their own in real devices. The context makes the stakes clear: non-fullerene acceptors have already lifted organic solar cell efficiencies past 18 percent, with recent single-junction reports exceeding 20 percent, far outstripping the roughly 12 percent ceiling of fullerene-based bulk heterojunction cells, which suffered from expensive purification, limited visible absorption and non-tunable energy levels.

The authors are careful to note the limits of the approach. The calculations were performed in the gas phase, and solvent and solid-state effects can shift absolute values of the energy gap, absorption maximum and exciton binding energy, even if the relative trends hold. Still, the study delivers exactly what computational screening is meant to provide: a ranked shortlist of molecules whose electronic properties justify the cost of synthesis and device fabrication. If experimentalists confirm the predicted behavior of NDND6 and its cousins, the humble naphthalene ring, best known as the smell of mothballs, may find itself at the heart of the next generation of printable, flexible solar panels.

Subject of Research: DFT design of naphthalene-based non-fullerene acceptors for organic solar cells

Article Title: Unraveling the photovoltaic insights of naphthalene-based functionalized chromophores with modified peripheral acceptors for organic solar cells: A DFT/TD-DFT approach

Article References: Unraveling the photovoltaic insights of naphthalene-based functionalized chromophores with modified peripheral acceptors for organic solar cells: A DFT/TD-DFT approach. (n.d.). https://doi.org/10.1007/s44442-026-00066-w

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00066-w

Keywords: organic solar cells, non-fullerene acceptors, DFT, TD-DFT, naphthalene chromophores, exciton binding energy, open-circuit voltage, intramolecular charge transfer, end-capped acceptors, photovoltaics, computational chemistry, molecular engineering