Understanding the Photovoltaic Behavior of A–D–A Molecular Semiconductors through a Permutation of End Groups

Crystalline and Amorphous Interface Simulations of Donor-Acceptor Blends

In an organic solar cell, exciton dissociation and charge transport to generate current depend on interface and bulk morphology, respectively, and their rates dictate device performance. Blend miscibility and processing determine the final morphology. We investigate the blend miscibility of P3HT and O-IDTBR, employing our recently developed "push-pull" computational technique, and explore its effect on nanoscale morphology. The resulting Flory-Huggins χ parameter is primarily enthalpic, and the blend exhibits UCST behavior. To directly simulate an equilibrated morphology in such a blend requires large (30 nm) systems and long (10 μs) simulations, which we attain with our virtual site coarse graining technique. In the resulting amorphous phase-separated state, O-IDTBR swells P3HT by about 15%, but does not percolate within the P3HT-rich phase. The interfacial profile depends on crystallinity: the amorphous interface is several nanometers wide, whereas an acceptor crystal induces order in the adjacent donor polymer, forming a sharp interface quite similar to a crystal-crystal interface. The wide amorphous interface promotes more donor-acceptor contacts with a wide range of relative orientations; however, the acceptors do not form a percolating network, which will likely lead to charge recombination. A crystal interface exhibits fewer donor-acceptor contacts with primarily face-on orientation that are more likely to charge-separate because of instant access to the preferred domain.

Progress and Future Potential of All-Small-Molecule Organic Solar Cells Based on the Benzodithiophene Donor Material

Organic solar cells have obtained a prodigious amount of attention in photovoltaic research due to their unique features of light weight, low cost, eco-friendliness, and semitransparency. A rising trend in this field is the development of all-small-molecules organic solar cells (ASM-OSCs) due to their merits of excellent batch-to-batch reproducibility, well-defined structures, and simple purification. Among the numerous organic photovoltaic (OPV) materials, benzodithiophene (BDT)-based small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking the 17% efficiency barrier in single-junction OPV devices, indicating the significant potential of this class of materials in commercial photovoltaic applications. This review specially focuses on up-to-date information about improvements in BDT-based ASM-OSCs since 2011 and provides an outlook on the most significant challenges that remain in the field. We believe there will be more exciting BDT-based photovoltaic materials and devices developed in the near future.

Covalently linked benzothiadiazole-fullerene adducts for organic optoelectronic devices: synthesis and characterization

Fullerene adducts have attracted attention in a variety of applications including organic optoelectronic devices. In this regard, we have designed a covalently linked donor-acceptor dyad comprising a fluorobenzothiadiazole-thiophene (BTF₂-Th) unit with the electron acceptor fullerene in an Acceptor-Donor-Acceptor (A-D-A) molecular arrangement. We synthesized and characterized two new covalently bonded benzothiadiazole-based fullerene molecules, mono-adduct, 7 (benzothiadiazole : PC₆₁BM = 1 : 1, anchored terminally via esterification reaction) and multi-adduct, 10-I (benzothiadiazole : PC₆₁BM = n : 1, where n ≥ 1, attached directly to the fullerene core via the Prato reaction) using different synthetic strategies. A broadening of the UV-visible spectra of the modified fullerene derivative with strong absorption from 350 to 500 nm and at low wavelengths is observed as compared to PC₆₁BM. A suitable bandgap, good electronic conductivity, and appreciable solubility in solvents suggest their utility in optoelectronic devices. The mono-adduct 7 showed two-order higher electron mobility as compared to bis-adduct 10-I due to retention of extended conjugation in fullerene, as in the case of PC₆₁BM. Experimentally determined optical properties and energy levels of the fullerene adducts were found to be in good agreement and supported by theoretical calculations. The presence of BTF₂ affects the ground state dipole moments as well as the absorption strengths, most noticeable in the case of two attached BTF₂ moieties. The HOMO and LUMO levels are found to be localized on the fullerene cage with the extension of the HOMO to the BTF₂ unit more and the same is noticed in ground state dipole moment in the side-chain functionalized structure. Such structural arrangement provides easy charge transfer between acceptor and donor units to allow a concomitant effect of favorable optoelectronic properties, energy levels of the frontier orbitals, effective exciton dissociation, and charge transport which may reduce processing complexity to advance single material-based future optoelectronic devices.

Fast, Faithful Simulations of Donor-Acceptor Interface Morphology

The local structure of conjugated polymers governs key optoelectronic properties, such as charge conduction and photogeneration at donor-acceptor interfaces. Because conjugated polymers are large, stiff, and relax slowly, all-atom molecular dynamics simulations are computationally expensive. Here, we describe a coarse-graining method that exploits the stiffness of constituent aromatic moieties by representing each moiety as rigidly bonded clusters of atoms wherein virtual sites replace several atoms. This approach significantly reduces the degrees of freedom while faithfully representing the shape and interactions of the moieties, resulting in 10 times faster simulations than all-atom simulations. Simulation of a donor polymer (P3HT) and a non-fullerene acceptor (O-IDTBR) validates the coarse-graining method by comparing structural properties from experiments, such as the density and persistence length. The fast simulation produces equilibrated systems with realistic morphologies. The simulation results of an equimolar mixture of P3HT, with a molecular weight of 1332 g mol^-1, and an O-IDTBR mixture suggest that the interface width must be larger than 7 nm. Also, we investigate the effect of slow cooling on morphologies, particularly the number of close contacts that facilitates carrier transport. Slow cooling increases close contacts, and the effect is more pronounced in crystal-forming P3HT than in O-IDTBR, where bulky side-groups hinder crystal formation.

Amphipolar, Amphiphilic 2,4-diarylpyrano[2,3-b]indoles as Turn-ON Luminophores in Acidic and Basic Media

A versatile amphiphilic pyrano[2,3-b]indole for halochromic turn-ON luminescence in acidic or basic media is accessed by an insertion-coupling-cycloisomerization and adjusting solubilizing and phenolic functionalities. While almost non-emissive in neutral solutions, treatment with acids or bases like trifluoroacetic acid (TFA) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) reveals distinct luminescence at wavelengths of 540 nm or 630 nm in propan-2-ol, respectively. Turn-ON emission can be detected at pH values as mild as pH = 5.31 or 8.70. Quantum yields in propan-2-ol are substantial for protonated (Φf = 0.058) and deprotonated (Φf = 0.059) species. Photometrically, pKa1 of 3.5 and pKa2 of 10.5 were determined in propan-2-ol. With lipophilic polyether sidechains and hydrophilic protonation and deprotonation sites the molecule can be regarded as amphipolar, which results in good solubility properties for different organic solvents. In aqueous media, an organic co-solvent like propan-2-ol (35%) or tetrahydrofuran (25%) is needed, and the solution can be diluted with pure water without precipitation of the compound. At higher concentrations of water, a turbid solution is formed, which indicates the formation of micellar structures or clusters. With dynamic light scattering we could show that these clusters increase in size with increasing water content.

Effect of the Terminal Acceptor Unit on the Performance of Non-Fullerene Indacenodithiophene Acceptors in Organic Solar Cells

Four acceptor–donor–acceptor (A–D–A)-type molecules bearing indacenodithiophene as donating central core and various end-capping acceptor units have been designed and synthesised as n-type materials suitable for organic solar cells (OSCs). The studied optical and electrochemical properties supported by theoretical calculations revealed that the nature and the strength of the terminal groups exert a decisive influence on the polymer bulk-heterojunction OSC performance.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Computational Identification of Novel Families of Nonfullerene Acceptors by Modification of Known Compounds

We considered a database of tens of thousands of known organic semiconductors and identified those compounds with computed electronic properties (orbital energies, excited state energies, and oscillator strengths) that would make them suitable as nonfullerene electron acceptors in organic solar cells. The range of parameters for the desirable acceptors is determined from a set of experimentally characterized high-efficiency nonfullerene acceptors. This search leads to ∼30 lead compounds never considered before for organic photovoltaic applications. We then proceed to modify these compounds to bring their computed solubility in line with that of the best small-molecule nonfullerene acceptors. A further refinement of the search can be based on additional properties like the reorganization energy for chemical reduction. This simple strategy, which relies on a few easily computable parameters and can be expanded to a larger set of molecules, enables the identification of completely new chemical families to be explored experimentally.

Trifluoromethyl Group-Modified Non-Fullerene Acceptor toward Improved Power Conversion Efficiency over 13% in Polymer Solar Cells

Herein, we report a new molecule structure modification strategy for non-fullerene small-molecule electron acceptors (NFAs) for solar cells through trifluoromethylation of end-capping groups. The synthesized trifluoromethylated acceptor ITCF3 exhibits narrower band gap, stronger light absorption, lower molecular energy levels, and better electron transport property compared to the reference NFA without the trifluoromethyl group (ITIC). Bulk heterojunction solar cells based on ITCF3 combined with the PM6 polymer donor exhibit a significantly improved power conversion efficiency of 13.3% compared with the ITIC-based device (8.4%). This work reveals great potential of trifluoromethylation in the design of efficient photovoltaic acceptor materials.