Theoretical Study of Non-Fullerene Acceptors Using End-Capped Groups with Different Electron-Withdrawing Abilities toward Efficient Organic Solar Cells

Structural modification of A-C-A configured X-PCIC acceptor molecule for efficient photovoltaic properties with low energy loss in organic solar cells

Modification of terminal acceptors of non-fullerene organic solar cell molecule with different terminal acceptors can help in screening of molecules to develop organic photovoltaic cells with improved performance. Thus, in this work, seven new molecules with an unfused core have been designed and thoroughly investigated. DFT/TD-DFT simulations were performed on studied molecules to explore the ground and excited state characteristics. UV-Visible analysis revealed the red shift in the absorption spectrum (reaching 781 nm) owing to their smaller energy gap up to 1.94 eV. Furthermore, transition density matrix analysis demonstrated that peripheral acceptors extract the electron density from the core effectively. The effectiveness of our investigated molecules as materials for high-performing organic photovoltaic cells has been shown by an examination of their electron and hole mobilities for fast charge transfer. When combined with PTB7-Th, all molecules displayed high open circuit voltage. XP5 molecule exhibited highest open circuit voltage (1.70 eV) and lowest energy loss of 0.30 eV. All designed molecules exhibit the improved aforementioned parameters, which shows that these molecules can be used to develop competent solar devices in future.

Designing Thieno[3,4-c]pyrrole-4,6-dione Core-Based, A2-D-A1-D-A2-Type Acceptor Molecules for Promising Photovoltaic Parameters in Organic Photovoltaic Cells

Nonfullerene-based organic solar cells can be utilized as favorable photovoltaic and optoelectronic devices due to their enhanced life span and efficiency. In this research, seven new molecules were designed to improve the working efficiency of organic solar cells by utilizing a terminal acceptor modification approach. The perceived A2-D-A1-D-A2 configuration-based molecules possess a lower band gap ranging from 1.95 to 2.21 eV compared to the pre-existing reference molecule (RW), which has a band gap of 2.23 eV. The modified molecules also exhibit higher λmax values ranging from 672 to 768 nm in the gaseous and 715-839 nm in solvent phases, respectively, as compared to the (RW) molecule, which has λmax values at 673 and 719 nm in gas and chloroform medium, respectively. The ground state geometries, molecular planarity parameter, and span of deviation from the plane were analyzed to study the planarity of all of the molecules. The natural transition orbitals, the density of state, molecular electrostatic potential, noncovalent interactions, frontier molecular orbitals, and transition density matrix analysis of all studied molecules were executed to validate the optoelectronic properties of these molecules. Improved charge mobilities and dipole moments were observed, as newly designed molecules possessed lower internal reorganization energies. The open circuit voltage (Voc) of W4, W5, W6, and W7 among newly designed molecules was improved as compared to the reference molecule. These results elaborate on the superiority of these novel-designed molecules over the pre-existing (RW) molecule as potential blocks for better organic solar cell applications.

A DFT study for improving the photovoltaic performance of organic solar cells by designing symmetric non-fullerene acceptors by quantum chemical modification on pre-existed LC81 molecule

Minimizing the energy loss and improving the open circuit voltage of organic solar cells is still a primary concern for scientists working in this field. With the aim to enhance the photovoltaic performance of organic solar cells by minimizing energy loss and improving open circuit voltage, seven new acceptor molecules (LC1-LC7) are presented in this work. These molecules are designed by modifying the terminal acceptors of pre-existed "LC81" molecule based on an indacinodithiophene (IDT) fused core. The end-group modification approach is very fruitful in ameliorating the efficacy and optoelectric behavior of OSCs. The newly developed molecules presented remarkable improvements in performance-related parameters and optoelectronic properties. Among all designed molecules, LC7 exhibited the highest absorption maxima (λmax = 869 nm) with the lowest band-gap (1.79 eV), lowest excitation energy (Ex = 1.42 eV), lowest binding energy, and highest excited state lifetime (0.41 ns). The newly designed molecules LC2, LC3, and LC4 exhibited remarkably improved Voc that was 1.84 eV, 1.82 eV, and 1.79 eV accordingly, compared to the LC81 molecule with Voc of 1.74 eV LC2 molecule showed significant improvement in fill factor compared to the previously presented LC81 molecule. LC2, LC6, and LC7 showed a remarkable reduction in energy loss by showing Eloss values of 0.26 eV, 0.18 eV, and 0.25 eV than LC81 molecule (0.37 eV). These findings validate the supremacy of these developed molecules (especially LC2) as potential components of future OSCs.

Designing Benzodithiophene-Based Small Molecule Donors for Organic Solar Cells by Regulation of Halogenation Effects

The donors are key components of organic solar cells (OSCs) and play crucial roles in their photovoltaic performance. Herein, we designed two new donors (BTR-γ-Cl and BTR-γ-F) by finely optimizing small molecule donors (BTR-Cl and BTR-F) with a high performance. The optoelectronic properties of the four donors and their interfacial properties with the well-known acceptor Y6 were studied by density functional theory and time-dependent density functional theory. Our calculations show that the studied four donors have large hole mobility and strong interactions with Y6, where the BTR-γ-Cl/Y6 has the largest binding energy. Importantly, the proportion of charge transfer (CT) states increases at the BTR-γ-Cl/Y6 (50%) and BTR-γ-F/Y6 (45%) interfaces. The newly designed donors are more likely to achieve CT states through intermolecular electric field (IEF) and hot exciton mechanisms than the parent molecules; meanwhile, donors containing Cl atoms are more inclined to produce CT states through the direct excitation mechanism than those containing F atoms. Our results not only provided two promising donors but also shed light on the halogenation effects on donors in OSCs, which might be important to design efficient photovoltaic materials.

Green-Solvent Processed Blade-Coating Organic Solar Cells with an Efficiency Approaching 19% Enabled by Alkyl-Tailored Acceptors

Power-conversion-efficiencies (PCEs) of organic solar cells (OSCs) in laboratory, normally processed by spin-coating technology with toxic halogenated solvents, have reached over 19%. However, there is usually a marked PCE drop when the blade-coating and/or green-solvents toward large-scale printing are used instead, which hampers the practical development of OSCs. Here, a new series of N-alkyl-tailored small molecule acceptors named YR-SeNF with a same molecular main backbone are developed by combining selenium-fused central-core and naphthalene-fused end-group. Thanks to the N-alkyl engineering, NIR-absorbing YR-SeNF series show different crystallinity, packing patterns, and miscibility with polymeric donor. The studies exhibit that the molecular packing, crystallinity, and vertical distribution of active layer morphologies are well optimized by introducing newly designed guest acceptor associated with tailored N-alkyl chains, providing the improved charge transfer dynamics and stability for the PM6:L8-BO:YR-SeNF-based OSCs. As a result, a record-high PCE approaching 19% is achieved in the blade-coating OSCs fabricated from a green-solvent o-xylene with high-boiling point. Notably, ternary OSCs offer robust operating stability under maximum-power-point tracking and well-keep > 80% of the initial PCEs for even over 400 h. Our alkyl-tailored guest acceptor strategy provides a unique approach to develop green-solvent and blade-coating processed high-efficiency and operating stable OSCs, which paves a way for industrial development.

A Theoretical Study on the Underlying Factors of the Difference in Performance of Organic Solar Cells Based on ITIC and Its Isomers

Recently, non-fullerene-based organic solar cells (OSCs) have made great breakthroughs, and small structural differences can have dramatic impacts on the power conversion efficiency (PCE). We take ITIC and its isomers as examples to study their effects on the performance of OSCs. ITIC and NFBDT only differed in the side chain position, and they were used as models with the same donor molecule, PBDB-T, to investigate the main reasons for the difference in their performance in terms of theoretical methods. In this work, a detailed comparative analysis of the electronic structure, absorption spectra, open circuit voltage and interfacial parameters of the ITIC and NFBDT systems was performed mainly by combining the density functional theory/time-dependent density functional theory and molecular dynamics simulations. The results showed that the lowest excited state of the ITIC molecule possessed a larger ∆q and more hybrid FE/CT states, and PBDB-T/ITIC had more charge separation paths as well as a larger kCS and smaller kCR. The reason for the performance difference between PBDB-T/ITIC and PBDB-T/NFBDT was elucidated, suggesting that ITIC is a superior acceptor based on a slight modulation of the side chain and providing a guiding direction for the design of superior-performing small molecule acceptor materials.

Unidirectional Sidechain Engineering to Construct Dual-Asymmetric Acceptors for 19.23 % Efficiency Organic Solar Cells with Low Energy Loss and Efficient Charge Transfer

Achieving both high open-circuit voltage (Voc ) and short-circuit current density (Jsc ) to boost power-conversion efficiency (PCE) is a major challenge for organic solar cells (OSCs), wherein high energy loss (Eloss ) and inefficient charge transfer usually take place. Here, three new Y-series acceptors of mono-asymmetric asy-YC11 and dual-asymmetric bi-asy-YC9 and bi-asy-YC12 are developed. They share the same asymmetric D1 AD2 (D1 =thieno[3,2-b]thiophene and D2 =selenopheno[3,2-b]thiophene) fused-core but have different unidirectional sidechain on D1 side, allowing fine-tuned molecular properties, such as intermolecular interaction, packing pattern, and crystallinity. Among the binary blends, the PM6 : bi-asy-YC12 one has better morphology with appropriate phase separation and higher order packing than the PM6 : asy-YC9 and PM6 : bi-asy-YC11 ones. Therefore, the PM6 : bi-asy-YC12-based OSCs offer a higher PCE of 17.16 % with both high Voc and Jsc , due to the reduced Eloss and efficient charge transfer properties. Inspired by the high Voc and strong NIR-absorption, bi-asy-YC12 is introduced into efficient binary PM6 : L8-BO to construct ternary OSCs. Thanks to the broadened absorption, optimized morphology, and furtherly minimized Eloss , the PM6 : L8-BO : bi-asy-YC12-based OSCs achieve a champion PCE of 19.23 %, which is one of the highest efficiencies among these annealing-free devices. Our developed unidirectional sidechain engineering for constructing bi-asymmetric Y-series acceptors provides an approach to boost PCE of OSCs.

Molecular engineering of Pyran‐fused acceptor–donor–acceptor‐type non‐fullerene acceptors for highly efficient organic solar cells—A density functional theory approach

The end‐capped modification proves that it is an excellent attempt to improve the solar cells performances. Therefore, nowadays, many researchers are working to design new molecules for potential use in organic photovoltaics. Herein, we have modified new molecules (SA1–SA5) from the reference (R) for fullerene‐free solar cells. These novel molecules have lower excitation energy levels that make the easier excitation in the excited state. Additionally, SA1 to SA5 molecules exhibit excellent charge mobility due to the modification of an efficient core units. Geometric and physiochemical investigations indicate that the modeled molecules are beneficial for efficient organic solar cells. The estimation of frontier molecular orbitals analysis, reorganizational energy, photovoltaic characteristics, and charge transmission calculations was done using density functional theory calculations with B3LYP/6‐31G (d, p) basis set. Among all designed molecules, SA3 has emerged as the preferred choice because of its outstanding photovoltaic characteristics, which include a minimal bandgap of 2.03 eV and reorganization energy of electron and holes of 0.0095 and 0.0077 eV, correspondingly. The designed materials (SA1–SA5) displayed a high λ max values, that is, 693.54 nm (in gas) and 679.63 nm (in chloroform). This theoretical framework suggests that the required photovoltaic properties may be efficiently obtained by remodeling the new molecules.

Rational design of non-fullerene acceptors via side-chain and terminal group engineering: a computational study

We investigated the optoelectronic and photovoltaic properties of three types of acceptor-donor-acceptor-based non-fullerene acceptor (NFA) molecules for organic solar cell (OSC) applications. Density functional theory and its time-dependent variant were employed to compute the quadrupole moment perpendicular to the π-system (Q₂₀), open circuit voltage (V_OC), and other relevant solar cell parameters. The role of functionalization in the acceptor unit on the overall device performance was explored by incorporating halogen and methoxy-based electron-withdrawing groups. The electronegativity differences between the halogen atoms and the methoxy group demonstrated contrasting effects on the energy levels, molecular orbitals, and absorption maximum. We observed a trade-off between short-circuit current (J_SC) and V_OC, which was further substantiated by an inverse correlation between Q₂₀ and V_OC. We found an optimum value of Q₂₀ in the range of 80 to 130 ea₀² to achieve an optimized solar cell performance. Among the designed systems, Se-derived NFAs with a small band gap, red-shifted absorption maximum, high-oscillator strength, small exciton binding energy, and optimum Q₂₀ turned out to be potential candidates for future applications. These criteria can be generalized to design and screen next-generation non-fullerene acceptors to achieve improved OSC performance.

Computational Design of Crescent Shaped Promising Nonfullerene Acceptors with 1,4-Dihydro-2,3-quinoxalinedione Core and Different Electron-withdrawing Terminal Units for Photovoltaic Applications

This study aims to design a series of nonfullerene acceptors (NFAs) for photovoltaic applications having 1,4-dihydro-2,3-quinoxalinedione fused thiophene derivative as the core unit and 1,1-dicyanomethylene-3-indanone (IC) derivatives and different π-conjugated molecules other than IC as terminal acceptor units. All the investigated NFAs are found air-stable as the computed highest occupied molecular orbitals (HOMOs) are below the air oxidation threshold (ca. -5.27 eV vs saturated calomel electrode). The studied NFAs can act as potential nonfullerene acceptor candidates as they are found to have sufficient open-circuit voltage (V_oc) and fill factor (FF) ranging from 0.62 to 1.41 V and 83%-91%, respectively. From the anisotropic mobility analysis, it is noticed that the studied NFAs except dicyano-rhodanine terminal unit containing NFA, exhibit better electron mobility than the hole mobility, and therefore, they can be more promising electron transporting acceptor materials in the active layer of an organic photovoltaic cell. From the optical absorption analysis, it is noted that all the designed NFAs have the maximum absorption spectra ranging from 597 nm-730 nm, which lies in the visible region and near-infrared (IR) region of the solar spectrum. The computed light-harvesting efficiencies for the PM6 (thiophene derivative donor selected in our study):NFA blends are found to lie in the range of 0.96-0.99, which indicates efficient light-harvesting by the PM6:NFA blends during photovoltaic device operation.

Promoting the Efficiency and Stability of Nonfullerene Organic Photovoltaics by Incorporating Open-Cage [60]Fullerenes in the Nonfullerene Nanocrystallites

The device efficiency of PM6:Y6-based nonfullerene organic solar cells is fast advanced recently. To maintain organic solar cells (OSCs) with high power conversion efficiency over 16% in long-term operation, however, remains a challenge. Here, a novel non-volatile additive, an open-cage [60]fullerene (8OC₆₀Me), is incorporated into PM6:Y6-based OSCs for high-performance with high durability. With optimized addition of 1.0 wt % 8OC₆₀Me, the PCE value of PM6:Y6/8OC₆₀Me OSCs can be promoted to 16.5% from 15.0%. Most strikingly, such a high PCE performance can maintain nearly 100% for over 500 h at room temperature; at an elevated operation temperature of 80 °C, the PCE can be stabilized above 15.0% after 45 h of operation. Grazing incidence small- and wide- angle X-ray scattering studies reveal improved orientation and crystallinity of Y6 in a fractal-like network structure of PM6 in PM6:Y6/8OC₆₀Me films under in situ annealing, parallel to the enhanced electron mobility. Analysis of charge distributions lines up possible van der Waals interaction between the thienyl/carbonyl moiety of 8OC₆₀Me and difluorophenyl-based FIC-end groups of Y6. This result is of great contrast to those devices with the best-selling PC₆₁BM as the additives─8OC₆₀Me might be of interest to be incorporated into future Y6-based OSCs for concomitantly improved PCE and excellent stability.

Theoretical investigation of fused N-methyl-dithieno-pyrrole derivatives in the context of acceptor-donor-acceptor approach

In this work we have theoretically investigated the optoelectronic properties of a series of acceptor-donor-acceptor type molecules by employing density functional theory formalism. We have used 1,1-dicyano-methylene-3-indanone as the acceptor unit and a fused N-methyl-dithieno-pyrrole as the donor unit. We have calculated the values of dihedral angle, inter-ring bond length, bond length alteration parameters, HOMO-LUMO gap, ionization potential, electron affinity, partial density of states, reorganization energies for holes and electrons, charge transfer rate for holes and electrons of the seven types of compounds designed via molecular engineering. Calculated IP and EA values manifest that PBDB-C2 shows excellent charge transportation compared to others. Absorption spectra of the designed compounds have been studied using the time-dependent density functional theory method. From the calculation of reorganization energy it is confirmed that our designed molecules behave more likely as donor materials. Our calculated results also reveal that compounds with electron donating substituents at the acceptor units show higher value of λ max. Absorption spectra of donor/acceptor blends show similar trends with the isolated compounds. Observed lower exciton binding energy values for all the compounds indicate facile charge carrier separation at the donor/acceptor interface. Moreover, the negative values of Gibb's free energy change also indicate the ease of exciton dissociation of all the designed compounds. The photovoltaic characteristics of the studied compounds infer that all the designed compounds have the potential to become suitable candidate for the fabrication of organic semiconductors. However, PBDB-C2 and PBDB-C4 with the highest PCE of 18.25% can become the best candidate for application in photovoltaics.