Comparing Adsorption of an Electron-Rich Triphenylene Derivative: Metallic vs Graphitic Surfaces

Crucial to the performance of devices based on organic molecules is an understanding of how the substrate-molecule interface influences both structural and electronic properties of the molecular layers. Within this context we studied the self-assembly of an alkoxy-triphenylene derived electron donor (HAT) in the monolayer regime on graphene/Ni(111). The molecules assembled into a close-packed hexagonal network commensurate with the graphene layer. Despite the commensurate structure, the HAT molecules only had a weak, physisorptive interaction with the substrate as pointed out by the photoelectron spectroscopy data. We discuss these findings in view of our recent reports for HAT adsorbed on Ag(111) and graphene/Ir(111). For all three substrates HAT adopts a similar close-packed hexagonal structure commensurate with the substrate while being physisorbed. The ionization potential is equal for all three substrates, supporting weak molecule-substrate interactions. These findings are remarkable, as commensurate overlayers usually only form at strongly interacting interfaces. We discuss potential reasons for this particular behavior of HAT which clearly sets itself apart from most studied molecule-substrate systems. In particular, these are the relatively weak but flexible intermolecular interactions, the molecular symmetry matching that of the substrate, and the comparatively weak but directional molecule-substrate interactions.

Tuning the transport properties of tetracene-based single-molecule junctions with chemical or structural variation of side and anchoring groups

CONTEXT

This study aims to tune the transport properties of tetracene single-molecule junctions with the proper choice and placement of side and anchoring groups. For the operationalization of the molecule that was anchored with thiol or isocyanide groups, two different side groups, amine and nitro, in two different positions, were taken into consideration. For unperturbed tetracene molecule, a prominent negative differential resistance (NDR) feature at 1.8 V was observed with the isocyanide anchoring group while the thiol anchoring group exhibits a plateau region over a bias voltage of 2.2 to 3.2 V. At a bias voltage that is dependent on the chemical or structural change of side or anchoring groups, NDR feature of varying degree was seen in all configurations. Results show that the current flowing through the thiol-anchored molecule perturbed with the amine group at S' position is relatively larger than other configurations because of the smaller HOMO-LUMO gap and broader transmission peaks resulting in a peak to valley current ratio (PVCR) of 1.22. In addition, multiple NDR regions were realized in nitro-perturbed isocyanide-anchored molecule at S position. These results suggest their promising applications in switches, logic cells, and storage devices.

METHODS

The modeling and simulation of side-group mediated anchored tetracene molecule through two electrodic systems were studied using density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) in Virtual NanoLab-AtomistixToolkit (ATK). The electron transport properties were calculated using Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation function. To optimize computing time, gold electrodes were single zeta polarized whereas the molecule, anchor groups, and side groups were double zeta polarized.

Effects of the Selective Alkoxy Side Chain Position in Quinoxaline-Based Polymer Acceptors on the Performance of All-Polymer Solar Cells

The effects of the position of alkoxy side chains in quinoxaline (Qx)-based polymer acceptors (P_As) on the characteristics of materials and the device parameters of all-polymer solar cells (all-PSCs) are investigated. The alkoxy side chains are selectively located at the meta, para, and both positions in pendant benzenes of Qx units, constructing P_As denoted as P(QxCN-T2)-m, P(QxCN-T2)-p, and P(QxCN-T2), respectively. Among them, P(QxCN-T2)-m exhibits the deepest energy levels owing to the enhanced electron-withdrawing effect of meta-positioned alkoxy chains, which is in contrast to P(QxCN-T2)-p where para-positioned alkoxy chains have an electron-donating property. In addition, the meta-positioned alkoxy chains induce good electron-conducting pathways, while the para-positioned ones significantly interrupt crystallization and intermolecular interactions between the conjugated backbones. Thus, when the P_As are applied to all-PSCs, a power conversion efficiency (PCE) of 5.07% is attained in the device using P(QxCN-T2)-m with efficient exciton dissociation and good electron-transporting ability. On the contrary, the P(QxCN-T2)-p-based counterpart has a PCE of only 1.62%. These results demonstrate that introducing alkoxy side chains at a proper location in the Qx-based P_As is crucial for their application to all-PSCs.

Cyano-Functionalized Quinoxaline-Based Polymer Acceptors for All-Polymer Solar Cells and Organic Transistors

Quinoxaline (Qx) derivatives are promising building units for efficient photovoltaic polymers owing to their strong light absorption and high charge-transport abilities, but they have been used exclusively in the construction of polymer donors. Herein, for the first time, Qx-based polymer acceptors (P_A s) were developed by introducing electron-withdrawing cyano (CN) groups into the Qx moiety (QxCN). A series of QxCN-based P_A s, P(QxCN-T2), P(QxCN-TVT), and P(QxCN-T3), were synthesized by copolymerizing the QxCN unit with bithiophene, (E)-1,2-di(thiophene-2-yl)ethene, and terthiophene, respectively. All of the P_A s exhibited unipolar n-type characteristics with organic field-effect transistor (OFET) mobilities of around 10^-2  cm²  V^-1  s^-1 . In space-charge-limited current devices, P(QxCN-T2) and P(QxCN-TVT) exhibited electron mobilities greater than 1.0×10^-4  cm²  V^-1  s^-1 , due to the well-ordered structure with tight π-π stacking. When the P_A s were applied in all-polymer solar cells (all-PSCs), the highest performance of 5.32 % was achieved in the P(QxCN-T2)-based device. These results demonstrate the significant potential of Qx-based P_A s for high-performance all-PSCs and OFETs.

Side-group-mediated thermoelectric properties of anthracene single-molecule junction with anchoring groups

Charge transfer characteristics of single-molecule junctions at the nanoscale, and consequently, their thermoelectric properties can be dramatically tuned by chemical or conformational modification of side groups or anchoring groups. In this study, we used density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) formalism in the linear response regime to examine the thermoelectric properties of a side-group-mediated anthracene molecule coupled to gold (Au) electrodes via anchoring groups. In order to provide a comparative inspection three different side groups, i.e. amine, nitro and methyl, in two different positions were considered for the functionalization of the molecule terminated with thiol or isocyanide anchoring groups. We showed that when the anchored molecule is perturbed with side group, the peaks of the transmission spectrum were shifted relative to the Fermi energy in comparison to the unperturbed molecule (i.e. without side group) leading to modified thermoelectric properties of the system. Particularly, in the thiol-terminated molecule the amine side group showed the greatest figure of merit in both positions which was suppressed by the change of side group position. However, in the isocyanide-terminated molecule the methyl side group attained the greatest thermoelectric efficiency where its magnitude was relatively robust to the change of side group position. In this way, different combinations of side groups and anchoring groups can improve or suppress thermopower and the figure of merit of the molecular junction depending on the interplay between charge donating/accepting nature of the functionals or their position.

A New Electron Acceptor with meta-Alkoxyphenyl Side Chain for Fullerene-Free Polymer Solar Cells with 9.3% Efficiency

A new small molecule acceptor, m-ITIC-OR, based on indacenodithieno[3,2-b]thiophene core with meta-alkoxyphenyl side chains, is designed and synthesized. The m-ITIC-OR film shows broader and redshift absorption compared to its solution and matched energy levels with a hexafluoroquinoxaline-based polymer donor-HFQx-T. Here, polymer solar cells (PSCs) by blending an HFQx-T donor and an m-ITIC-OR acceptor as an active layer deliver the power conversion efficiency (PCE) of 6.36% without any posttreatment. The investigations demonstrate that the HFQx-T:m-ITIC-OR blend films possess higher and more balanced charge mobility, negligible bimolecular recombination, and nanoscale interpenetrating morphology after thermal annealing (TA) treatment. Through a simple TA treatment at 150 °C for 5 min, an impressive PCE of 9.3% is obtained. This efficiency is among one of the highest PCEs for additive free PSCs. This is the first time alkoxyphenyl side chain is introduced into nonfullerene electron acceptor; more interestingly, the new electron acceptor (m-ITIC-OR) in this work shows a great potential for highly efficient photovoltaic properties.

Understanding the colorimetric properties of quinoxaline-based pi-conjugated copolymers by tuning their acceptor strength: a joint theoretical and experimental approach

We report the synthesis and characterization of donor–acceptor π-conjugated copolymers leading to blue-to-green coloration according to acceptor strength.

Synthesis and electronic investigation of mono- and di-substituted 4-nitro- and 4-amino-pyrazol-1-yl bis(pyrazol-1-yl)pyridine-type ligands and luminescent Eu(iii) derivatives

The synthesis of nitro- and amino-bis(pyrazol-1-yl)pyridines was achieved, allowing for tuning of frontier orbital energies and Eu(iii) complex spectroscopic investigations.

Donor–acceptor conjugated polymers based on cyclic imide substituted quinoxaline or dibenzo[a,c]phenazine for polymer solar cells

A series of donor–acceptor type of conjugated polymers based on cyclic imide substituted quinoxaline or dibenzo[a,c]phenazine were synthesized, where the copolymers comprising dibenzo[a,c]phenazine showed much higher photovoltaic performances.

Two-dimensional quinoxaline based low bandgap conjugated polymers for bulk-heterojunction solar cells

Photovoltaic cells based on the two-dimensional quinoxaline-based polymers,P2andP3, exhibited PCEs of over 3% compared to the PCE of less than 2% in theP1-based device.

Theoretical study on the torsional potential of alkyl, donor, and acceptor substituted bithiophene: the hidden role of noncovalent interaction and backbone conjugation

The torsional potential of substituted bithiophene is influenced not only by steric repulsion, but also by backbone conjugation and noncovalent interactions.

Molecular design toward highly efficient photovoltaic polymers based on two-dimensional conjugated benzodithiophene

As researchers continue to develop new organic materials for solar cells, benzo[1,2-b:4,5-b']dithiophene (BDT)-based polymers have come to the fore. To improve the photovoltaic properties of BDT-based polymers, researchers have developed and applied various strategies leading to the successful molecular design of highly efficient photovoltaic polymers. Novel polymer materials composed of two-dimensional conjugated BDT (2D-conjugated BDT) have boosted the power conversion efficiency of polymer solar cells (PSCs) to levels that exceed 9%. In this Account, we summarize recent progress related to the design and synthesis of 2D-conjugated BDT-based polymers and discuss their applications in highly efficient photovoltaic devices. We introduce the basic considerations for the construction of 2D-conjugated BDT-based polymers and systematic molecular design guidelines. For example, simply modifying an alkoxyl-substituted BDT to form an alkylthienyl-substituted BDT can improve the polymer hole mobilities substantially with little effect on their molecular energy level. Secondly, the addition of a variety of chemical moieties to the polymer can produce a 2D-conjugated BDT unit with more functions. For example, the introduction of a conjugated side chain with electron deficient groups (such as para-alkyl-phenyl, meta-alkoxyl-phenyl, and 2-alkyl-3-fluoro-thienyl) allowed us to modulate the molecular energy levels of 2D-conjugated BDT-based polymers. Through the rational design of BDT analogues such as dithienobenzodithiophene (DTBDT) or the insertion of larger π bridges, we can tune the backbone conformations of these polymers and modulate their photovoltaic properties. We also discuss the influence of 2D-conjugated BDT on polymer morphology and the blends of these polymers with phenyl-C61 (or C71)-butyric acid methyl ester (PCBM). Finally, we summarize the various applications of the 2D-conjugated BDT-based polymers in highly efficient PSC devices. Overall, this Account correlates the molecular structures of the 2D-conjugated BDT-based polymers with their photovoltaic properties. As a result, this Account can guide the molecular design of organic photovoltaic materials and the development of organic materials for other types of optoelectronic devices.

Synthesis characterization and bulk-heterojunction photovoltaic applications of new naphtho[1,2-b:5,6-b′]dithiophene–quinoxaline containing narrow band gap D–A conjugated polymers

New naphtho[1,2-b:5,6-b′]dithiophene–quinoxaline containing donor–acceptor conjugated copolymers with alkyl/alkoxy side chains positioned differently were developed and investigated for polymer solar cells.

Improving the ordering and photovoltaic properties by extending π-conjugated area of electron-donating units in polymers with D-A structure

A systematic molecular design process from PBDTTT-S to PBDTDTTT-S-T, a high-performance semiconducting polymer for organic photovoltaics, has been achieved by enhancing structural order, self-assembly and carrier mobility. Solar cells made from PBDTDTTT-S-T blended with PC(70) BM show a power conversion efficiency (PCE) of 7.81%, which is 25% higher than that of the parent PBDTTT-S.