Ring Substituents Mediate the Morphology of PBDTTPD-PCBM Bulk-Heterojunction Solar Cells

Among π-conjugated polymer donors for efficient bulk-heterojunction (BHJ) solar cell applications, poly(benzo[1,2-b:4,5-b′]dithiophene–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers yield some of the highest open-circuit voltages (VOC, ca. 0.9 V) and fill-factors (FF, ca. 70%) in conventional (single-cell) BHJ devices with PCBM acceptors. In PBDTTPD, side chains of varying size and branching affect polymer self-assembly, nanostructural order, and impact material performance. However, the role of the polymer side-chain pattern in the intimate mixing between polymer donors and PCBM acceptors, and on the development of the BHJ morphology is in general less understood. In this contribution, we show that ring substituents such as furan (F), thiophene (T) and selenophene (S)—incorporated into the side chains of PBDTTPD polymers—can induce significant and, of importance, very different morphological effects in BHJs with PCBM. A combination of experimental and theoretical (via density functional theory) characterizations sheds light on how varying the heteroatom of the ring substituents impacts (i) the preferred side-chain configurations and (ii) the ionization, electronic, and optical properties of the PBDTTPD polymers. In parallel, we find that the PBDT(X)TPD analogs (with X = F, T, or S) span a broad range of power conversion efficiencies (PCEs, 3–6.5%) in optimized devices with improved thin-film morphologies via the use of 1,8-diiodooctane (DIO), and discuss that persistent morphological impediments at the nanoscale can be at the origin of the spread in PCE across optimized PBDT(X)TPD-based devices. With their high VOC ∼1 V, PBDT(X)TPD polymers are promising candidates for use in the high-band gap cell of tandem solar cells

Molecular packing of non-fullerene acceptors for organic solar cells: Distinctive local morphology in Y6 vs. ITIC derivatives

Since a couple of years ago, Y6 has emerged as one of the main non-fullerene acceptors for organic solar cells, as its use leads to superior power conversion efficiencies. It is thus of major interest to investigate the multiscale phenomena that are responsible for Y6's efficacy. Here, we modeled neat films of Y6 and earlier non-fullerene acceptors, IT-4F and ITIC, using a combination of density functional theory calculations and molecular dynamics simulations, to investigate the various factors that control their charge and exciton transport rates. We find that the molecular packing in Y6 is drastically different from that in IT-4F and ITIC. At the nanoscale, the local morphology of Y6 consists of a large number of directional face-on stackings and well-connected transport networks. Y6 also consistently shows higher electronic couplings for LUMOs, HOMOs, and local excitations than ITIC-type acceptors, which results in fast transport rates for electrons, holes, and excitons. Importantly, when considering dimers, their configurations in Y6 are more diverse than in ITIC-type acceptors, with many of those similar to the configurations observed in the Y6 crystal structure reported recently. Most Y6 dimer configurations exhibit strong binding interactions, large electronic couplings, and high transport rates, which when taken together rationalize the better performance of OSCs based on Y6. © 2021 The AuthorsOpen access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Pathways for resonant energy transfer in oligo(phenylenevinylene)-fullerene dyads : an atomistic model

Fast resonant energy transfer (RET) takes place from oligo(phenylenevinylene) (OPVn) segments to C60 in OPVn-C60 dyads as a result of the presence of multiple energy-transfer pathways and of significant electronic couplings (when going beyond the point-dipole model)

Pathways for resonant energy transfer in oligo(phenylenevinylene)-fullerene dyads : an atomistic model

Fast resonant energy transfer (RET) takes place from oligo(phenylenevinylene) (OPVn) segments to C60 in OPVn-C60 dyads as a result of the presence of multiple energy-transfer pathways and of significant electronic couplings (when going beyond the point-dipole model)

THEORETICAL STUDIES OF THE PHYSICS OF CHARGED DEFECT FORMATION IN DOPED ORGANIC POLYMERS : TOWARDS A COHERENT THEORETICAL PICTURE

A partir de calculs Hartree-Fock en méthode du champ auto-cohérent sur le polyacétylène trans, le polyparaphénylène et le polypyrrole, nous voyons émerger une représentation théorique cohérente des mécanismes de conduction dans les polymères organiques dopés. Nous mettons en évidence l'importance primordiale des modifications géométriques survenant sur les chaînes polymériques suite au transfert de charge. Nous démontrons que ces modifications font apparaître des états électroniques nouveaux dans la bande interdite qui jouent un rôle essentiel dans le mécanisme de conduction. Des calculs MNDO sur des chaînes de polyacétylène trans indiquent que les géométries des défauts de type soliton et polaron dépendent fortement de leur état de charge.From Hartree-Fock self-consistent-field calculations on trans-polyacetylene, polyparaphenylene, and polypyrrole, a coherent theoretical description emerges for the physics of conducting doped organic polymers. The importance of the geometric modifications that occur on the polymer chains upon charge transfer is stressed. These modifications are shown to lead to the appearance of electronic states in the gap that play a major role in the conductivity mechanism. MNDO calculations on trans-polyacetylene chains indicate that the geometries of soliton or polaron defects vary significantly with their charge state

Electronic structure and optical properties of mixed phenylene vinylene/phenylene ethynylene conjugated oligomers

We present a combined experimental and theoretical investigation of the photophysical properties of four pi-conjugated oligomers with varying chemical constitutions. Three of these contain alkoxy-substituted p-phenylene vinylene/ethynylene units with a phenylene, anthracene, or p-dialkoxyphenylene moiety as the central aromatic ring; the fourth oligomer contains only p-dialkoxyphenylene vinylene units. The electronic structure and optical properties are investigated by combining UV-vis, fluorescence, and electrochemical techniques with quantum-chemical semiempirical calculations. The simulated absorption spectra are in excellent agreement with the experimental data and illustrate the role of the central aromatic ring on the nature of the lowest excited stat

Exciton coupling in oligothiophenes: A combined experimental/theoretical study

The influence of exciton coupling on the optical properties of oligothiophenes is studied by means of optical and CD (CD) spectroscopy. The CD spectrum of a model compd., formed by two terthienyl chromophores arranged in a chiral orientation, displays two bands with opposite signs that can be related to the two Davydov components of the first optical transition. From correlated quantum-chem. calcns., the Davydov splitting is estd. to be on the order of 0.15-0.2 e

Molecular Fluorescence Lifetime Fluctuations: On the Possible Role of Conformational Effects

The radiative lifetime of single 1,1'-dioctadecyl-3,3,3',3'- etramethylindodicarbocyanine molecules, embedded in a polymer thin film, has been characterized. At room temperature the chemically identical molecules exhibit strong fluctuations in their fluorescence lifetime. The possible conformational origin of these fluctuations has been addressed by semi-empirical quantum-chemical calculations. Specifically, the impact of conformational effects induced by thermal motion o­n the transition energy and the value of the transition dipole moment has been studied. The results indicate that the strong fluctuations cannot be explained by pure thermally activated conformational changes. Since quenching mechanisms are not responsible for the observed fluctuations, we suggest that molecular segmental dynamics of the surrounding polymer play a key role in determining the lifetime fluctuation

Investigation of exciton coupling in oligothiophenes by circular dichroism spectroscopy

The solid-state photophysical properties of semiconducting polymers result from the interplay between the intrinsic features of the polymer chains and interchain interactions. Exciton coupling, which in principle can be estimated from circular dichroism (CD) spectra, is one of the major interchain interactions affecting the optical properties of semiconducting polymers. Here the authors present a combined experimental and theoretical investigation of the chiroptical properties of oligothiophenes. The results confirm that the CD spectrum provides an independent estimate of the exciton coupling