Impact of Terminal End-Group of Acceptor-Donor-Acceptor-type Small Molecules on Molecular Packing and Photovoltaic Properties

Orientational Effects and Molecular-Scale Thermoelectricity Control

The orientational effect concept in a molecular-scale junction is established for asymmetric junctions, which requires the fulfillment of two conditions: (1) design of an asymmetric molecule with strong distinct terminal end groups and (2) construction of a doubly asymmetric junction by placing an asymmetric molecule in an asymmetric junction to form a multicomponent system such as Au/Zn-TPP+M/Au. Here, we demonstrate that molecular-scale junctions that satisfy the conditions of these effects can manifest Seebeck coefficients whose sign fluctuates depending on the orientation of the molecule within the asymmetric junction in a complete theoretical investigation. Three anthracene-based compounds are investigated in three different scenarios, one of which displays a bithermoelectric behavior due to the presence of strong anchor groups, including pyridyl and thioacetate. This bithermoelectricity demonstration implies that if molecules with alternating orientations can be placed between an asymmetric source and drain, they can be potentially utilized for increasing the thermovoltage in molecular-scale thermoelectric energy generators (TEGs).

Tailoring the Weight of Surface and Intralayer Edge States to Control LUMO Energies

The energies of the frontier molecular orbitals determine the optoelectronic properties in organic films, which are crucial for their application, and strongly depend on the morphology and supramolecular structure. The impact of the latter two properties on the electronic energy levels relies primarily on nearest-neighbor interactions, which are difficult to study due to their nanoscale nature and heterogeneity. Here, an automated method is presented for fabricating thin films with a tailored ratio of surface to bulk sites and a controlled extension of domain edges, both of which are used to control nearest-neighbor interactions. This method uses a Langmuir-Schaefer-type rolling transfer of Langmuir layers (rtLL) to minimize flow during the deposition of rigid Langmuir layers composed of π-conjugated molecules. Using UV-vis absorption spectroscopy, atomic force microscopy, and transmission electron microscopy, it is shown that the rtLL method advances the deposition of multi-Langmuir layers and enables the production of films with defined morphology. The variation in nearest-neighbor interactions is thus achieved and the resulting systematically tuned lowest unoccupied molecular orbital (LUMO) energies (determined via square-wave voltammetry) enable the establishment of a model that functionally relates the LUMO energies to a morphological descriptor, allowing for the prediction of the range of accessible LUMO energies.

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.

Modulating Crystal Packing, Film Morphology, and Photovoltaic Performance of Selenophene-Containing Acceptors through a Combination of Skeleton Isomeric and Regioisomeric Strategies

Here, we report a series of acceptor-donor-acceptor (A-D-A) architecture isomeric acceptors (SeCT-IC, CSeT-IC, and CTSe-IC), which have an identical electron-deficient terminal A-group and three different central D-cores with the selenophene at the innermost, relatively outer, and outermost positions of the central core, respectively. From CSeT-IC to the atom regioisomer of CTSe-IC and to the conjugated skeleton isomer of SeCT-IC, the optical band gap of neat films continuously reduced and highest occupied molecular orbitals (HOMO) gradually upshifted with changing the selenophene from relatively outer position to the outermost position and to the innermost position of the central core. More importantly, the single-crystal structure and the GIWAXS measurements revealed that CTSe-IC presents the closest π-π stacking distance, the largest CCL, and the best molecular order and crystallinity, which led to the highest electron mobility in neat films. Furthermore, the J71:CTSe-IC blend film presents a more ordered film morphology with more proper phase separation domain size, more dominant face-on orientation, and relatively higher and more balanced electron-hole mobilities in comparison with that of J71:SeCT-IC and J71:CSeT-IC. Consequently, the J71:CTSe-IC-based organic solar cell gave a superior power conversion efficiency (PCE) of 11.59%, which was obviously higher than those for J71:SeCT-IC (10.89%) and J71:CSeT-IC (8.52%). Our results demonstrate that the acceptor with selenophene in the outermost position led to significantly enhance the PCE. More importantly, rational modulation of the central fused core in combination with the conjugated skeleton isomeric method and the atom regioisomeric method provides an effective way to understand the structure-crystallinity-photovoltaic property relationship of selenophene-based regioisomers.

Highly Efficient Non-Fused-Ring Electron Acceptors Enabled by the Conformational Lock and Structural Isomerization Effects

Two novel nonfused-ring electron acceptors (N-FREAs) namely DTP-out-F and DTP-in-F, containing 2,5-difluorophenylene central core flanked with DTP blocks and end-capped with IC-2F terminals were designed and synthesized. The C-H···F noncovalent interactions between F atom of 2,5-difluorophenylene and H-3 and H-6 from DTP moiety (for DTP-in-F and DTP-out-F, respectively) locked the molecular conformation within a planar geometry. Benefiting from asymmetric nature of DTP block, the two different connection positions (2- or 7-position) of DTP to 2,5-difluorophenylene afforded the structural isomers of DTP-in-F and DTP-out-F, which affected the overall properties of these N-FREAs, especially the molecular packing behaviors. The more preferred J-aggregation and face-on packing of DTP-in-F shifted the absorption to slightly longer wavelength and provided a polymer-like extended crystal transport channels for improving the charge transport. Therefore, the power conversion efficiency (PCE) was significantly improved from 3.97% of DTP-out-F-based devices to 10.66% of DTP-in-F-based devices. These results reveal the great potential of isomerization strategy to develop high-performance N-FREAs.

Alcohol-Soluble Zwitterionic 4-(Dimethyl(pyridin-2-yl)ammonio)butane-1-sulfonate Small Molecule as a Cathode Modifier for Nonfullerene Acceptor-Based Organic Solar Cells

A new zwitterionic small molecule 4-(dimethyl(pyridin-2-yl)ammonio)butane-1-sulfonate (PAS), synthesized from 2-dimethylaminopyrindine (2-DMAP), was developed for the ITO cathode modifier. PAS and 2-DMAP dissolved in methanol can form a thin layer on ITO cathode by a simple spin-coating process. The heteroatom moieties in 2-DMAP (sp² and sp³ nitrogen) and PAS (sp² nitrogen and sulfonate ion) can coordinate to the ITO surface and decrease the ITO work function by the induced surface dipoles. The fullerene-based (PBDTT-FTTE:PC₇₁BM) inverted OSCs using PAS and 2-DMAP interlayer can achieve PCEs of 8.95 and 8.26%, respectively, which are superior to the devices without a modifier (PCE = 3.25%) and comparable to the corresponding ZnO-based device (PCE = 8.57%). Nevertheless, 2-DMAP, like other nitrogen-containing polymer interlayer materials, turns out to be not applicable to inverted organic solar cells (I-OSCs) with IT-4F as the n-type electron acceptor because the amino group of 2-DMAP can act as a nucleophile to attack the end-group of IT-4F at the interface. The decomposition of IT-4F by 2-DMAP was carefully proved to be via retro-aldol condensation. As a result, the device (PBDBT-F:IT-4F) modified with 2-DMAP displayed a low PCE of 7.34%. The zwitterionic PAS with reduced nucleophilicity and basicity can modify the ITO surface without decomposing IT-4F. The PBDBT-F:IT-4F-based device modified with PAS maintained a high PCE of 11.41%. Most importantly, the PAS-based device using the well-known Y6 acceptor (PBDBT-F:Y6) can achieve a PCE of 13.82%. This new interfacial material can be universally applied to I-OSCs employing various A-D-A-type acceptors installed with the electrophilic 1,1-dicyanamethylene-5,6-difluoro-3-indanone (FIC) end-group.

Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells

The introduction of the IDIC/ITIC families of non-fullerene acceptors has boosted the photovoltaic performances of bulk-heterojunction organic solar cells. The fine tuning of the photophysical, morphological and processability properties with the aim of reaching higher and higher photocurrent efficiencies has prompted uninterrupted worldwide research on these peculiar families of organic compounds. The main strategies for the modification of IDIC/ITIC compounds, described in several contributions published in the past few years, can be summarized and classified into core modification strategies and end-capping group modification strategies. In this review, we analyze the more recent advances in this field (last two years), and we focus our attention on the molecular design proposed to increase photovoltaic performance with the aim of rationalizing the general properties of these families of non-fullerene acceptors.