Showing papers by "Dean M. DeLongchamp published in 2013"
TL;DR: It is argued that charge transport in high-mobility semiconducting polymers is quasi one-dimensional, that is, predominantly occurring along the backbone, and requires only occasional intermolecular hopping through short π-stacking bridges.
Abstract: Some of the best-performing semiconducting polymers for electronic devices show a surprising lack of long-range order to support their electrical conductivity. Here Zhang et al. find a common alignment of the structural backbones of these polymers, explaining their superior charge transport.
439 citations
TL;DR: In this paper, two critical mechanical properties, stiffness and ductility, of a widely studied organic solar cell active layer, a blend film composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), are reported.
Abstract: The development of flexible and physically robust organic solar cells requires detailed knowledge of the mechanical behavior of the heterogeneous material stack. However, in these devices there has been limited research on the mechanical properties of the active organic layer. Here, two critical mechanical properties, stiffness and ductility, of a widely studied organic solar cell active layer, a blend film composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are reported. Processing conditions are varied to produce films with differing morphology and correlations are developed between the film morphology, mechanical properties and photovoltaic device performance. The morphology is characterized by fitting the absorption of the P3HT:PCBM films to a weakly interacting H-aggregate model. The elastic modulus is determined using a buckling metrology approach and the crack onset strain is determined by observing the film under tensile strain using optical microscopy. Both the elastic modulus and crack onset strain are found to vary significantly with processing conditions. Processing methods that result in improved device performance are shown to decrease both the compliance and ductility of the film.
132 citations
TL;DR: In this paper, a comprehensive structure and performance study of thin blend films of the small-molecule semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT), with various insulating binder polymers in organic thin-film transistors is reported.
Abstract: A comprehensive structure and performance study of thin blend films of the small-molecule semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT), with various insulating binder polymers in organic thin-film transistors is reported. The vertically segregated composition profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X-ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X-ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly(α-methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composition profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution-processed bottom-gate/bottom-contact thin-film transistors.
104 citations
TL;DR: In this article, the authors studied the time evolution of structure in polymer-fullerene films blade-coated from additive containing solutions using in-situ spectroscopic ellipsometry and UV-vis transmission.
Abstract: The use of processing additives has emerged as a powerful approach for the optimization of active layer performance in organic photovoltaic devices. However, definitive physical mechanisms explaining the impact of additives have not yet been determined. To elucidate the role of additives, we have studied the time evolution of structure in polymer-fullerene films blade-coated from additive containing solutions using in-situ spectroscopic ellipsometry and UV–vis transmission. Additives that are poor solvents for poly(3-hexylthiophene) (P3HT), such as 1,8-octanedithiol, and additives that are good solvents for P3HT, such as 1-chloronapthalene, both promote improved polymer order, phase segregation, and device performance. Regardless of the presence or type of additive, the polymer order develops under conditions of extreme supersaturation. Additives, regardless of whether they are solvents for P3HT, promote earlier polymer aggregation compared to additive - free solutions presumably by degrading the solvent quality. We find evidence that the details of the final film morphology may be linked to the influence of the substrate and long-time film plasticization in the cases of the non-solvent and solvent respectively.
101 citations
TL;DR: In this paper, the π-stacking direction in poly(3-hexylthiophene) (P3HT) films can be made to orient strongly out-of plane by uniaxially straining films in orthogonal directions, providing a valuable opportunity to evaluate charge transport in a very unusual microstructure for this material.
Abstract: We report that the π-stacking direction in poly(3-hexylthiophene) (P3HT) films can be made to orient strongly out-of plane by uniaxially straining films in orthogonal directions, providing a valuable opportunity to evaluate charge transport in a very unusual microstructure for this material. The structure of the films was characterized using UV–visible spectroscopy, X-ray diffraction, and near-edge X-ray absorption fine structure spectroscopy, showing that unstrained films have a weakly edge-on stacking character with a large orientation distribution, whereas films strained biaxially by 100% in orthogonal directions have highly face-on stacking. In the biaxially strained films the face-on packing occurs while the P3HT long axis orientation is found to be only weakly anisotropic in-plane. Charge transport is characterized in an organic thin-film transistor (OTFT) configuration, showing that the saturated field effect mobility in the biaxially strained films is greater than that for unstrained films for cha...
99 citations
TL;DR: In this article, the authors compared the electronic coupling, assembly, and charge transfer rates at the interface between C 60 acceptors and contorted-hexabenzocorone (HBC) donors.
Abstract: While the demonstrated power conversion eff i ciency of organic photovoltaics (OPVs) now exceeds 10%, new design rules are required to tailor interfaces at the molecular level for optimal exciton dissociation and charge transport in higher effi ciency devices. We show that molecular shape-complementarity between donors and acceptors can drive performance in OPV devices. Using core hole clock (CHC) X-ray spectroscopy and density functional theory (DFT), we compare the electronic coupling, assembly, and charge transfer rates at the interface between C 60 acceptors and fl at- or contorted-hexabenzocorone (HBC) donors. The HBC donors have similar optoelectronic properties but differ in molecular contortion and shape matching to the fullerene acceptors. We show that shape-complementarity drives self-assembly of an intermixed morphology with a donor/acceptor (D/A) ball-and-socket interface, which enables faster electron transfer from HBC to C 60 . The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved effi ciency of organic photovoltaic devices that incorporate these concave/convex D/A materials.
47 citations
TL;DR: A series of well-defined perfluoroalkyl end-functionalized poly(3-hexylthiophenes) (P3HT) were synthesized by Stille coupling of stannylated 2-perfluoralkyl thiophene with the bromine end of P3HT.
Abstract: A series of well-defined perfluoroalkyl end-functionalized poly(3-hexylthiophenes) (P3HT) were synthesized by Stille coupling of stannylated 2-perfluoralkylthiophene with the bromine end of P3HT. The length of the perfluoroalkyl end group was varied from −C4F13 to −C8F17. These polymers were fully characterized and tested in bulk heterojunction solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. Performance of the solar cells was highest for the unmodified P3HT and decreased as the length of the perfluoroalkyl end increased. The most affected device parameters were the short-circuit current density (Jsc) and series resistance, pointing to lower charge carrier mobility and poor morphology as the cause for the lower performance. While the morphology of blends did not significantly change with perfluoroalkyl end modification, analysis of blended films by energy-filtered transmission electron microscopy (EF-TEM) revealed wider P3HT domains, consistent with the perfluorinated end grou...
36 citations
TL;DR: The effects of fullerene crystallinity during heat treatment in model structures consisting of a layer of poly(3-hexylthiophene) (P3HT) sandwiched between two layers of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are studied.
Abstract: The morphology of the active layer in an organic photovoltaic bulk-heterojunction device is controlled by the extent and nature of phase separation during processing. We have studied the effects of fullerene crystallinity during heat treatment in model structures consisting of a layer of poly(3-hexylthiophene) (P3HT) sandwiched between two layers of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Utilizing a combination of focused ion-beam milling and energy-filtered transmission electron microscopy, we monitored the local changes in phase distribution as a function of annealing time at 140 °C. In both cases, dissolution of PCBM within the surrounding P3HT was directly visualized and quantitatively described. In the absence of crystalline PCBM, the overall phase distribution remained stable after intermediate annealing times up to 60 s, whereas microscale PCBM aggregates were observed after annealing for 300 s. Aggregate growth proceeded vertically from the substrate interface via uptake of PCBM from t...
23 citations