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Darling, S B

Publications and source records attributed to Darling, S B.

8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Hierarchical nanomorphologies promote exciton dissociation in polymer/fullerene bulk heterojunction solar cells.

PTB7 semiconducting copolymer comprising thieno[3,4-b]thiophene and benzodithiophene alternating repeat units set a historic record of solar energy conversion efficiency (7.4%) in polymer/fullerene bulk heterojunction solar cells. To further improve solar cell performance, a thorough understanding of structure-property relationships associated with PTB7/fullerene and related organic photovoltaic (OPV) devices is crucial. Traditionally, OPV active layers are viewed as an interpenetrating network of pure polymers and fullerenes with discrete interfaces. Here we show that the active layer of PTB7/fullerene OPV devices in fact involves hierarchical nanomorphologies ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains, themselves hundreds of nanometers in size. These hierarchical nanomorphologies are coupled to significantly enhanced exciton dissociation, which consequently contribute to photocurrent, indicating that the nanostructural characteristics at multiple length scales is one of the key factors determining the performance of PTB7 copolymer, and likely most polymer/fullerene systems, in OPV devices.

14 SOLAR ENERGY↗
DOE OSTI2011

Mesoscale morphologies in polymer thin films.

In the midst of an exciting era of polymer nanoscience, where the development of materials and understanding of properties at the nanoscale remain a major R&D endeavor, there are several exciting phenomena that have been reported at the mesoscale (approximately an order of magnitude larger than the nanoscale). In this review article, we focus on mesoscale morphologies in polymer thin films from the viewpoint of origination of structure formation, structure development and the interaction forces that govern these morphologies. Mesoscale morphologies, including dendrites, holes, spherulites, fractals and honeycomb structures have been observed in thin films of homopolymer, copolymer, blends and composites. Following a largely phenomenological level of description, we review the kinetic and thermodynamic aspects of mesostructure formation outlining some of the key mechanisms at play. We also discuss various strategies to direct, limit, or inhibit the appearance of mesostructures in polymer thin films as well as an outlook toward potential areas of growth in this field of research.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Optoelectronic properties and charge transfer in donor-acceptor all-conjugated diblock copolymers.

All-conjugated block copolymers, which can self-assemble into well-ordered morphologies, provide exciting opportunities to rationally design and control the nanoscale organization of electron-donor and electron-acceptor moieties in optoelectronic active layers. Here we report on the steady-state and time-resolved optical characterization of block copolymer films and solutions containing poly(3-hexylthiophene) as the donor block and poly(9,9-dioctylfluorene) with and without copolymerization with benzothiadiazole as the acceptor block. Transient absorption measurements suggest rapid charge transfer occurs in both systems, with higher efficiency observed in the latter composition. These results indicate that this class of materials has promise in preparing highly ordered bulk heterojunction all-polymer organic photovoltaic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Tetrathienoanthracene-based copolymers for efficient solar cells.

A series of semiconducting copolymers (PTAT-x) containing extended {pi}-conjugated tetrathienoanthracene units have been synthesized. It was shown that the extended conjugation system enhanced the {pi}-{pi} stacking in the polymer/PC{sub 61}BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC{sub 61}BM.

14 SOLAR ENERGY↗
DOE OSTI2010

Polythiophene-block-polyfluorene and Polythiophene-blockpoly(fluorene-co-benzothiadiazole): Insights into the Self-Assembly of All-Conjugated Block Copolymers

All-conjugated block copolymers have significant potential for solution-processed optoelectronic applications, in particular those relying on a p/n junction. Herein, we report the synthesis and structure of all-conjugated diblock copolymers poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene) and poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene-co-benzothiadiazole) in thin films and in the bulk. The diblock copolymers are prepared using a combination of Grignard metathesis polymerization and Suzuki polycondensation and characterized with NMR spectroscopy, size-exclusion chromatography, multiangle laser light scattering, and UV/vis spectroscopy. Structure in thin films and in the bulk is characterized using differential scanning calorimetry, X-ray diffraction, small-angle X-ray scattering, and atomic force microscopy. Diblock copolymer thin films self-assemble into a crystalline nanostructure with some long-range order after extended solvent annealing, and X-ray scattering measurements show that powder samples exhibit crystallinity throughout the bulk. By temperature dependent X-ray scattering measurements, we find that diblock copolymers self-assemble into crystalline nanowires with phase segregated block copolymer domains. These measurements show all-conjugated diblock copolymers may be useful for achieving solution-processed active layers in organic photovoltaics and light-emitting diodes with optimized structural and photophysical characteristics.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Nanopatterning of ultrananocrystalline diamond thin films via block copolymer lithography.

Nanopatterning of diamond surfaces is critical for the development of diamond-based microelectromechanical system/nanoelectromechanical system (MEMS/NEMS), such as resonators or switches. Micro-/nanopatterning of diamond materials is typically done using photolithography or electron beam lithography combined with reactive ion etching (RIE). In this work, we demonstrate a simple process, block copolymer (BCP) lithography, for nanopatterning of ultrananocrystalline diamond (UNCD) films to produce nanostructures suitable for the fabrication of NEMS based on UNCD. In BCP lithography, nanoscale self-assembled polymeric domains serve as an etch mask for pattern transfer. The authors used thin films of a cylinder-forming organic-inorganic BCP, poly(styrene-block-ferrocenyldimethylsilane), PS-b-PFS, as an etch mask on the surface of UNCD films. Orientational control of the etch masking cylindrical PFS blocks is achieved by manipulating the polymer film thickness in concert with the annealing treatment. We have observed that the surface roughness of UNCD layers plays an important role in transferring the pattern. Oxygen RIE was used to etch the exposed areas of the UNCD film underneath the BCP. Arrays of both UNCD posts and wirelike structures have been created using the same starting polymeric materials as the etch mask.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Optoelectronics using block copolymers.

Block copolymers, either as semiconductors themselves or as structure directors, are emerging as a promising class of materials for understanding and controlling processes associated with both photovoltaic energy conversion and light emitting devices.

36 MATERIALS SCIENCE↗