Confinement-induced reduction in phase segregation and interchain disorder in bulk heterojunction films.

The effects of thin-film confinement on the material properties of ultrathin polymer (electron donor):fullerene (electron acceptor) bulk heterojunction films can be important for both fundamental understanding and device applications such as thin-film photovoltaics. We use variable angle spectroscopic ellipsometry and near edge X-ray absorption fine structure spectroscopy to measure the optical constants, donor-acceptor volume fraction profile, and the degree of interchain order as a function of the thickness of a poly(3-hexythiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester bulk heterojunction film. We find that as the thickness of the bulk heterojunction film is decreased from 200 nm to the thickness confinement regime (less than 20 nm), the vertical phase segregation gradient of the donor and acceptor phases becomes less pronounced. In addition, observing the change in exciton bandwidth and the shift of absorption resonances (0-0 and 0-1) relative to neat donor and acceptor films, we find that the conjugation length and disorder in ultrathin films (20 nm) are less affected than thicker (200 nm) films by the addition of fullerene into the polymer. We believe that these findings could be important for discovering methods of precisely controlling the properties of bulk heterojunction films with crucial implications for designing more efficient organic-based photovoltaics.

Directed assembly of P3HT:PCBM blend films using a chemical template with sub-300 nm features.

Surface energy has been demonstrated as a means to direct interfacial-layer composition in polymer:fullerene blends utilized as active layers in organic photovoltaic devices. Combined with recent materials advances in the preparation of nanoscale chemical patterns, surface energy control of nanophase separation presents an opportunity to employ patterned surface energy templates to control the 3D blend morphology of polymer:fullerene blends. This report details the directed assembly of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends atop linear grating patterns with domains of alternating high and low surface energy of 50 to 600 nm in width prepared by nanoscale oxidative lithography of alkyl-terminated self-assembled monolayers on SiO2 and SiH surfaces. Tapping-, contact-, and current-sensing AFM studies demonstrated that chemical patterns were effective at directing the 3D morphology of P3HT:PCBM blends at dimensions of >200 nm. As the dimensionality of domains approached 100 nm, the chemical patterns were no longer able to direct phase segregation, evidence that a directed spinodal decomposition mechanism was responsible for the observed morphology. Surprisingly, the low surface energy component (P3HT) was found to be atop the high surface energy domains of the template, in conflict with current understanding of the role of surface energy directed assembly in polymer blends. These results suggest that the directed spinodal decomposition mechanism applies to conjugated polymer:fullerene blends, but that additional parameters unique to these types of systems will require refinement of the theory to adequately describe and predict the behavior of these scientifically and industrially interesting materials.

Chemically enhancing block copolymers for block-selective synthesis of self-assembled metal oxide nanostructures.

We report chemical modification of self-assembled block copolymer thin films by ultraviolet light that enhances the block-selective affinity of organometallic precursors otherwise lacking preference for either copolymer block. Sequential precursor loading and reaction facilitate formation of zinc oxide, titanium dioxide, and aluminum oxide nanostructures within the polystyrene domains of both lamellar- and cylindrical-phase modified polystyrene-block-poly(methyl methacrylate) thin film templates. Near-edge X-ray absorption fine structure measurements and Fourier transform infrared spectroscopy show that photo-oxidation by ultraviolet light creates Lewis basic groups within polystyrene, resulting in an increased Lewis base-acid interaction with the organometallic precursors. The approach provides a method for generating both aluminum oxide patterns and their corresponding inverses using the same block copolymer template.

TOF-SIMS investigation of degradation pathways occurring in a variety of organic photovoltaic devices--the ISOS-3 inter-laboratory collaboration.

The present work is the fourth (and final) contribution to an inter-laboratory collaboration that was planned at the 3rd International Summit on Organic Photovoltaic Stability (ISOS-3). The collaboration involved six laboratories capable of producing seven distinct sets of OPV devices that were degraded under well-defined conditions in accordance with the ISOS-3 protocols. The degradation experiments lasted up to 1830 hours and involved more than 300 cells on more than 100 devices. The devices were analyzed and characterized at different points of their lifetimes by a large number of non-destructive and destructive techniques in order to identify specific degradation mechanisms responsible for the deterioration of the photovoltaic response. Work presented herein involves time-of-flight secondary ion mass spectrometry (TOF-SIMS) in order to study chemical degradation in-plane as well as in-depth in the organic solar cells. Various degradation mechanisms were investigated and correlated with cell performance. For example, photo-oxidation of the active material was quantitatively studied as a function of cell performance. The large variety of cell architectures used (some with and some without encapsulation) enabled valuable comparisons and important conclusions to be drawn on degradation behaviour. This comprehensive investigation of OPV stability has significantly advanced the understanding of degradation behaviour in OPV devices, which is an important step towards large scale application of organic solar cells.

Bilayer order in a polycarbazole-conjugated polymer.

One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymer's molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.

Molecular order in high-efficiency polymer/fullerene bulk heterojunction solar cells.

We report quantitative measurements of ordering, molecular orientation, and nanoscale morphology in the active layer of bulk heterojunction (BHJ) organic photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which has been shown to yield very high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC(71)BM). A surprisingly low degree of order was found in the polymer-far lower in the bulk heterojunction than in pure PTB7. X-ray diffraction data yielded a nearly full orientation distribution for the polymer π-stacking direction within well-ordered regions, revealing a moderate preference for π-stacking in the vertical direction ("face-on"). By combining molecular orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, we propose a model describing the PTB7 molecular orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (≈20%) of the polymer in the PTB7/PC(71)BM blend is ordered. Energy-filtered transmission electron microscopy shows that the morphology of PTB7/PC(71)BM is composed of nanoscale fullerene-rich aggregates separated by polymer-rich regions. The addition of diiodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphology, relative to blend films cast without DIO.

Controlling the orientation of terraced nanoscale "ribbons" of a poly(thiophene) semiconductor.

The large-scale manufacture of organic electronics devices becomes more feasible if the molecular orientation and morphology of the semiconductor can be controlled. Here, we report on a previously unidentified crystal shape of terraced nanoscale "ribbons" in thin films of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT). The ribbons form after a pBTTT film is heated above its highest temperature phase transition. In contrast to the wide terrace crystal shape previously reported, terraced ribbons have lateral widths of approximately 60 nm and lengths greater than 10 microm, with a common orientation between adjacent ribbons. Further, we report a simple and scalable flow coating process that can control the ribbon orientation without requiring special substrates or external fields. The degree of molecular orientation is small after coating but increases dramatically after the terraced ribbons are formed, indicating that an oriented minority templates the whole film structure. The large extent of orientation obtained in these polythiophene crystallites provides potential opportunities to exploit anisotropic electrical properties and to obtain detailed information about the structure of organic semiconductor thin films.

A versatile method for tuning the chemistry and size of nanoscopic features by living free radical polymerization.

A novel approach is presented for manipulating the size and chemistry of nanoscopic features using a combination of contact molding and living free radical polymerization. In this approach a highly cross-linked photopolymer, based on a methacrylate/acrylate mixture, was patterned into submicrometer-sized features on a silicon wafer using a contact-molding technique. A critical component of the monomer mixture was the incorporation of an initiator containing monomer into the network structure, which provides sites for functional group amplification. Features ranging in size from 5 microm to <60 nm were accurately replicated by this process and living free radical polymerizations, both atom transfer radical and nitroxide-mediated polymerization (NMP), could be conducted from these initiating sites to yield polymer brushes which represent a grafted layer of linear chains attached to the original network polymer. Grafts consisting of polystyrene, poly(methyl methacrylate), and poly(2-hydroxyethyl)methacrylate were grown with controlled thicknesses ranging from 10 to 143 nm and graft molecular weights of between 18 000 to 290 000 amu. As a result of this secondary graft process, feature sizes could be tuned from the original 100 nm down to 20 nm, and the surface chemistry varied from hydrophilic to hydrophobic starting from the same initial master pattern. The thin films and patterned features were characterized by contact angle, ellipsometry, optical, and atomic force microscopies.

A facile approach to architecturally defined nanoparticles via intramolecular chain collapse.

A novel approach is presented for the controlled intramolecular collapse of linear polymer chains to give well-defined single-molecule nanoparticles whose structure is directly related to the original linear polymer. By employing a combination of living free radical polymerization and benzocyclobutene (BCB) chemistry, nanoparticles can be routinely prepared in multigram quantities with the size being accurately controlled by either the initial degree of polymerization of the linear chain or the level of incorporation of the BCB coupling groups. The latter also allows the cross-link density of the final nanoparticles to be manipulated. In analogy with dendritic macromolecules, a significant reduction of up to 75% in the hydrodynamic volume is observed on going from the starting random coil linear chains to the corresponding nanoparticles. The facile nature of the living free radical process also permits wide variation in monomer selection and functional group incorporation and allows novel macromolecular architectures to be prepared. Furthermore, the use of block copolymers functionalized with benzocyclobutene groups in only one of the blocks gives, after intramolecular collapse, a hybrid architecture in which a single linear polymer chain is attached to the globular nanoparticle.