Organic electronic materials hold promise to revolutionize the manner in which low-cost, lightweight, and mechanically flexible optoelectronic thin film devices are fabricated and utilized. These materials have received explosive research interest in the past decade for their rich science as complex soft matter blends and for their potential in realizing efficient, solution-processable photovoltaic devices. Organic photovoltaics (OPVs), most typically comprised of an electron-donating conjugated polymer and an electron-accepting fullerene-based molecular species, heavily rely on structural order on molecular, nanoscopic, and mesoscopic length scales in order for charge carriers to be efficiently extracted from the device under illumination. In general, following photon absorption, the resulting exciton may produce free holes (electrons) within the donor (acceptor) phase, as long as the excitation is within its diffusion distance of the donor-acceptor interface. Secondly, once free charges are formed following excitonic splitting, they must be efficiently transported to the electrodes for collection. This process adds another structural requirement; the domains must be large enough for transport within and between them, and the coherence of the domains and structural order of crystallites within domains both become relevant.

In this work, these structural details are explored in two OPV formulations and related to thin film fabrication and processing methodologies. Specific emphasis is placed on the interplay of miscibility of components and their crystallization behavior. In early work exploring the concept of donor-acceptor mixing in polymer-fullerene blends, new understanding was developed regarding their miscibility, and it was shown that the idea of pure donor and acceptor domains in polymer-fullerene (P3HT:PC61BM) blends is unrealistic. In an extension, a secondary donor, light-absorbing polymer, PDPP2FT, was added to a blend of P3HT and PC61BM. Using dynamic secondary ion mass spectrometry (DSIMS) bilayer diffusion experiments and grazing incidence wide and small angle X-ray scattering (GIWAXS and GISAXS) investigations of thin film blends, this work highlights that the miscibility of the fullerene with each donor polymer is critical for understanding the distribution of acceptor components within donor-rich domains in ternary OPV blends. The performance of compositionally controlled devices is reconciled based on the described microstructure studies.

Due to the complex, highly mixed nature of these blends, the second segment will focus on work that investigates the structural evolution of an all-polymer formulation developed for generating controlled order in OPV films. An all-conjugated diblock copolymer, P3HT-b-DPPT-T, will be structurally described in the bulk, solution, and thin film states. Thin films were deposited from solution and thermally annealed in the rubbery and melt states. Synchrotron X-ray characterization probes were then used to study the structural order at both crystallite and domain length scales in order to evaluate the ability to form ordered bulk heterojunctions (BHJs). Using GIWAXS and DSC-WAXS, the crystalline order of each block is probed. Both blocks are semicrystalline and form edge-on textured crystallites in the thin film state. Interestingly, as characterized by GISAXS, AFM, and resonant soft X-ray spectroscopic scattering, this crystallization behavior leads to fibril formation that in turn dominates the mesoscale domain ordering. Additionally, it is shown that this fibril formation, although tunable by thermal annealing, has its origins in the solution state, as probed by solution-SAXS. Annealing within the P3HT melt and slow cooling allows for highly ordered BHJ films, where the fibrils formed by crystallites of each block are aligned in lamellar arrays, with a domain spacing of ~50 nm. Such a structure is extremely advantageous for OPVs, and the implications of further design of block chemistry and fullerene compatibility with each segment for generating highly ordered, efficient ternary OPVs will be discussed.