Morphological Characterization of a High Open-Circuit Voltage, Narrow Band Gap Semiconducting Polymer

Michael Ford,  Jessica Huang,  Ming Wang,  Gui Bazan
University of California, Santa Barbara


Abstract

Semiconducting polymers have been considered viable candidates for the active material in organic electronic devices due to low processing costs. Solution-processed organic photovoltaic devices (OPVs) implementing semiconducting polymers have not seen widespread adoption due to low power conversion efficiency (PCE) compared to other alternatives. Nevertheless, vast improvements have been made with polymer OPVs now exceeding 10% PCE. Conjugated polymer molecular structure design, most often incorporating an electron-rich fragment (donor, D) and an electron-deficient fragment (acceptor, A) alternating and repeating, has been linked with increased performance. This typically results in intramolecular charge transfer states with narrow optical band gaps. For the D-A polymer PIPCP (containing a backbone comprised of CPDT-PT-IDT-PT repeat units (CPDT = cyclopentadithiophene, PT = pyridyl[2,1,3]thiadiazole, IDT = indacenodithiophene) and strictly organized PT orientations, such that the pyridyl N-atoms point toward the CPDT fragment), we recently reported a Voc of 0.86 V, the highest Voc for a narrow band gap polymer. More recent research has been on investigating the structural characteristics of PIPCP and the bulk heterojunction. Morphological characterization can reveal insight into charge transport by, for example, revealing the stacking distance and clarifying the blend morphology, both of which are important for OPV performance. We examined the paracrystalline structure using grazing incidence X-ray scattering techniques. Through grazing incidence wide-angle X-ray scattering (GIWAXS), PIPCP reflection peaks in the out-of-plane direction were of high intensity at q = 0.24 Å−1 and at q = 1.47 Å−1, values that are typically attributed to alkyl stacking and π−π stacking. These signals extend to the in-plane direction, suggesting that PIPCP assumes both face-on and edge-on orientations in the thin film. The regioirregular analog of PIPCP, PIPC-RA, has also been studied, demonstrating similar reflection q-values but exhibiting an amorphous hump in the in-plane and out-of-plane directions. These results indicate that PIPCP has greater morphological order than its regioirregular counterpart. More surprisingly, PIPCP does not show strong alkyl stacking in the in-plane direction, which is typically cited as necessary for high-performance OPVs. Through X-ray scattering techniques of the polymer and bulk heterojunction, we attempt to unravel the morphology of the polymer and blend thin films.