ABSTRACTUsing molecular dynamics simulations we study blends of oligomers of 2,5‐bis(3‐alkylthiophen‐2‐yl)thieno[3,2‐b]thiophene, BTTT, and fullerene derivative based acceptors to understand the role of oligomer length and alkyl side chain (SC) length on the morphology of their blends. We use a validated coarse‐grained model of BTTT and fullerene derivatives presented in recent work along with direct comparison of morphology between simulations and experiments. In this article, we predict computationally that short alkyl SCs (6 alkyl groups) decrease the propensity of fullerene derivative acceptors to intercalate between SCs on the BTTT backbone compared to longer alkyl SCs (9 or 12 alkyl groups), and as a result increase acceptor aggregation. The decreasing acceptor intercalation and increasing acceptor aggregation do not significantly impact the positional or orientational order of the BTTT backbones. However, the BTTT oligomer backbones order better with increasing SC length in both neat systems and in blends, with the blends exhibiting higher positional order than neat systems. While these qualitative trends are similar both in 2mer blends and 4mer blends, we see a larger extent of acceptor intercalation and as a result, smaller acceptor cluster sizes in the 4mer blends. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 89–97

With increasing knowledge of the role of the different phases in the bulk heterojunction organic solar cell, the primary site for charge generation is now considered to be the mixed phase, and not the clean interface between neat polymer and neat fullerene. To gain a better understanding of the primary charge generating and recombination steps in this region of the system, we focus our studies on the role of the solid-state microstructure of neat polymers and light-doping of these polymers with a variety of electron-accepting dopants at low concentration. This presentation will describe some recent work on the doping of polythiophene and polyfluorene derivatives with fullerenes, phthalocyanines and perylenes, which provide a range of reduction potentials that serve to control the driving force for electron transfer processes. Results from flash photolysis, time-resolved microwave conductivity (fp-TRMC), femtosecond transient absorption spectroscopy (fTA) and photoluminescence spectroscopy will be presented.

ABSTRACTSmall molecule additives have been shown to increase the device efficiency of conjugated polymer (donor) and fullerene derivative (acceptor) based organic solar cells by modifying the morphology of the device active layer. In this paper we conduct a systematic study of how additives affect the donor‐acceptor morphology using molecular dynamics simulations of blends of thiophene‐based oligomers, mimicking poly(3‐dodecylthiophene) (P3DDT) or poly(2,2′:5′,2”‐3,3”‐didocyl‐terthiophene) (PTTT), and fullerene derivatives with additives of varying length and chemical functionalization, mimicking experimentally used additives like methyl ester additives, diiodooctane, and alkanedithiols. We find that functionalization of additives with end groups that are attracted to acceptor molecules are necessary to induce increased donor‐acceptor macrophase separation. In blends where acceptors intercalate between oligomer alkyl side chains, functionalized additives decrease acceptor intercalation. Functionalized additives with shorter alkyl segments increase acceptor macrophase separation more than additives with same chemical functionalization but longer alkyl segments. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1046–1057

We have performed coarse-grained molecular dynamics simulations of thiophene-based conjugated oligomers to elucidate how the oligomer architecture, specifically the orientation and density of alkyl side chains extending from the thiophene backbones, impacts the order-disorder temperatures and the various ordered morphologies that the oligomers form. We find that the orientation of side chains along the oligomer backbone plays a more significant role than side chain density, side chain-side chain interactions, or side chain length in determining the thermodynamically stable morphologies and the phase transition temperatures. Oligomers with side chains oriented on both sides of the backbone ("anti") form lamellae, while oligomers with side chains oriented on one side of the backbone ("syn") assemble into hexagonally packed cylinders that can undergo a second, lower temperature transition to lamellae or ribbons depending on side chain-side chain interaction strength. The strength of side chain-side chain interactions affects the order-disorder temperature, with oligomers having moderately attractive side chains exhibiting higher transition temperatures than those with weakly attractive side chains. Side chain length modulates the spacing between morphological features, such as cylinders and lamellae, and affects the order-disorder temperature differently depending on oligomer architecture.

Due to the unique crystallinity of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), it is an excellent model polymer to study the structure-property relationship in organic devices, especially those relying on junctions of electron- and hole-transporting materials. Here, we report the synthesis and characterization of a series of monodisperse PBTTT oligothiophenes (n = 1-5) and systematically examine the evolution of crystalline behavior, morphology, and interaction with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the molecular conjugation length increases. We discovered that fullerene intercalation occurs when there is enough free volume between the side chains to accommodate the fullerene molecule. The intercalation of PCBM is observed beyond BTTT-2 and longer oligomers, likely similar to that of PBTTT. Interestingly, both experiments and molecular simulations show that PCBM intercalation also appears to "catalyze" a more efficient packing of the BTTT-2 dimers. Crystal structure analysis revealed that the straight BTTT-2 side chains form one-dimensional (1D) channels that could perfectly host PCBM but, in the pure material, accommodate the interdigitated side chains from adjacent layers. In the blend with PCBM, these channels are maintained and enable the cocrystallization and intercalation of PCBM. This is the first time the actual sublattice cell of PCBM has been determined from the X-ray data, and demonstration the utility of the oligomers as model systems for their polymer counterparts. Among the organic photovoltaic devices (OPVs) made from the BTTT oligomers and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blends, the ones containing the BTTT-2 dimer exhibit the highest performance.

ABSTRACT The microstructure of neat conjugated polymers is crucial in determining the ultimate morphology and photovoltaic performance of polymer/fullerene blends, yet until recently, little work has focused on controlling the former. Here, we demonstrate that both the long‐range order along the (100)‐direction and the lamellar crystal thickness along the (001)‐direction in neat poly(3‐hexylthiophene) (P3HT) and poly[(3,3″‐didecyl[2,2′:5′,2″‐terthiophene]‐5,5″‐diyl)] (PTTT‐10) thin films can be manipulated by varying crystallization temperature. Changes in crystalline domain size impact the yield and dynamics of photogenerated charge carriers. Time‐resolved microwave conductivity measurements show that neat polymer films composed of larger crystalline domains have longer photoconductance lifetimes and charge carrier yield decreases with increasing crystallite size for P3HT. Our results suggest that the classical polymer science description of temperature‐dependent crystallization of polymers from solution can be used to understand thin‐film formation in neat conjugated polymers, and hence, should be considered when discussing the structural evolution of organic bulk heterojunctions. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52 , 700–707

We use coarse-grained Langevin dynamics simulations of blends of generic conjugated polymers and acceptor molecules to show how architecture (e.g., side chains, backbone flexibility of oligomers) and the pair-wise interactions between the constituents of the blend affect morphology and phase transition. Alkyl side chains on the conjugated oligomer backbones shift the liquid crystal (LC) transition temperature from that of bare conjugated backbones and the direction of the shift depends on backbonebackbone interactions. Rigid backbones and constrained side chains cause a layer-by-layer morphology of conjugated polymers and amorphous acceptors, whereas flexible backbones and unconstrained side chains facilitate highly ordered acceptor arrangement. Strong backbonebackbone attraction shifts LC transition to higher temperatures than weak backbonebackbone attraction, and strong acceptoracceptor attraction increases acceptor aggregation. Pure macro-phase separated domains form when all pair-wise interactions in the blend are strongly attractive, whereas interconnected domains form at intermediate acceptoracceptor attraction and strong polymerpolymer attractions. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

The efficiency of bulk heterojunction solar cells depends strongly on the morphology of the electron donors (conjugated polymers) and electron acceptors (fullerene derivatives) in the active layer. The features of the donor acceptor morphology (e.g., domain shapes, crystalline/amorphous domains, and donor acceptor interface) can in principle be tuned by choosing the chemistry and architecture of the conjugated polymer, fullerene functional group, solvents, and processing conditions (e.g., annealing temperature). Here, we present a high-throughput coarse-grained simulation study that links molecular-level design parameters to features in the assembled morphology in neat polymers and donor-acceptor blends. These models reproduce neat polymer morphologies observed in experiments, such as lamellae, hexagonally packed cylinders, and acceptor intercalation among donor side chains. Furthermore, for blends of conjugated polymers and fullerene derivatives, this study shows how conjugated polymer architecture and acceptor miscibility can be tailored to obtain new blend morphologies, which have features that are known to be optimal for higher efficiency solar cells.