and renewable or earth abundant materials. [ 1,2 ] For photovoltaic panels, the energy recovery time is the length of operation of a panel required to generate the energy used to manufacture the panel. In thin fi lm inorganic panels, this time has dropped to about one year. [ 3 ] One of the revolutionary appeals of roll-to-roll manufacturing of organic photovoltaics (OPV) is the potential to achieve energy recovery times as low as 10 d. [ 4 ] Tremendous advances have recently been made in OPV effi ciencies, [ 5 ] all based on optimization of the bulk-heterojunction (BHJ) motif. [ 6,7 ] The performance of a BHJ requires the optimization of both its molecular-scale order and its nanometer-scale domain structure. [ 8 ] In general, the optimized structure is not at equilibrium. Many techniques have been employed for BHJ fi lm optimization, [ 9 ] including solvent choice and post deposition thermal [ 10 ] or vapor annealing. [ 11 ] Recently, formulations using low volatility liquid additives to achieve higher effi ciencies without thermal treatment have become popular. [12][13][14] Although additives such as 1,8-octanedithiol (ODT) and 1-chloronaphthalene (CN) have almost ubiquitous benefi cial effects, there is no consensus on either the origin or mechanism of their effi cacy. In some systems, additives cause smaller domains, [ 15,16 ] while in others they cause larger domains. [ 12,17 ] The effi cacy of ODT and diiodooctane has been attributed to selective solvation of the fullerene, enabling improved polymer order. CN, however, is typically a good solvent for both polymer and fullerene and thus must act via a different mechanism.We recently employed real-time optical techniques to study the mechanism by which the additives ODT and CN infl uence the solidifi cation of the poly(3-hexylthiophene):phenyl-C61butyric-acid-methyl-ester (P3HT:PCBM) BHJ. [ 18 ] Both additives promoted a greater than 5× improvement in device effi ciency over the additive-free fi lm in the absence of thermal annealing. Post-deposition characterization with ultraviolet-visible absorption spectroscopy (UV-Vis) and grazing incidence X-ray diffraction (GIXD) established that the additives increased the polymer's local order and crystallinity. Additionally, energy-fi ltered transmission electron microscopy (EF-TEM) revealed that the The most successful active fi lm morphology in organic photovoltaics is the bulk heterojunction (BHJ). The performance of a BHJ arises from a complex interplay of the spatial organization of the segregated donor and acceptor phases and the local order/quality of the respective phases. These critical morphological features develop dynamically during fi lm formation, and it has become common practice to control them by the introduction of processing additives. Here, in situ grazing incidence X-ray diffraction (GIXD) and grazing incidence small angle X-ray scattering (GISAXS) studies of the development of order in BHJ fi lms formed from the donor polymer poly(3-hexylthiophene) and acceptor phenyl-C61-butyric acid methyl e...

A combination of wavelength-, time-, and polarizationresolved photoluminescence imaging on isolated P3HT nanofibers of varying molecular weight (from 10 to 65 kDa) has revealed a transition in dominant exciton coupling from primarily interchain (H-aggregation) for low molecular weight nanofibers, to predominantly intrachain (Jaggregation) coupling for high molecular weight nanofibers. Based on nanofiber width measurement from TEM imaging, the driving force for this transition appears to be folding of individual polymer chains within the lamellae, resulting in enhanced chain planarity and reduced torsional disorder.

Comprehensive studies of the influence of the formulation additive 1,8-diiodooctane on the structural evolution of small molecule BHJ films with GIWAXS and GISAXS are presented.

Self-assembled crystalline organic nanostructures containing electron donor and acceptor materials hold promise as building blocks for photovoltaic devices. We show that coupled crystallization of poly(3-hexyl thiophene) (P3HT) and perylene tetracarboxydiimide (PDI) induced by solvent evaporation, wherein both components modify crystallization of the other, gives rise to donor/acceptor "shish-kebabs" with tunable nanostructures. P3HT kinetically stabilizes supersaturated solutions of PDI and modifies the growth of PDI crystals, leading to formation of extended PDI shish nanowires that in turn serve as heterogeneous nucleation sites for fibrillar P3HT kebabs during solvent casting. The dimensions of these nanostructures can be tailored through variations in donor/acceptor ratio or solvent quality, and the method is shown to be general to several other poly(3-alkyl thiophenes) and perylene derivatives, thus providing a simple and robust route to form highly crystalline nanophase separated organic donor/acceptor assemblies.