Torsional Angle Tuning of Naphthalenediimide-based PSCs for Efficient Ambipolar and Unipolar Transport.

Poster Session: 

B

Presenter: 

Jack Ly

Authors: 

Jack Ly, Edmund K. Burnett, Lei Zhang, Yao Liu, Bradley Rose, Areej Aljarb, Todd Emrick, Christopher R. McNeill, Thomas P. Russell, Jean-Luc Brédas, and Alejandro L. Briseno

Author Affiliation: 

Department of Polymer Science and Engineering, University of Massachusetts Amherst Solar and Photovoltaics Engineering Research Center & Division of Physical Science and Engineering, King Abdullah University of Science and Technology College of Energy, Beijing University of Chemical Technology Department of Materials Science and Engineering, Monash University

Abstract: 

We demonstrate a new method to reverse the polarity and charge transport behavior of naphthalenediimide (NDI)-based copolymers by inserting a vinylene linker between the donor and acceptor units. The vinylene linkers minimize the intrinsic steric congestion between the NDI and thiophene moieties to prompt backbone planarity. The polymers with vinylene linkers exhibit electron n-channel transport characteristics under vacuum, similar to the benchmark polymer, P(NDI2OD-T2). To our surprise, when the polymers are measured in air, the dominant carrier type switches from n- to p-type and yield hole mobilities up to 0.45 cm2 V–1 s–1 with hole to electron mobility ratio of three (μh/μe, ∼3), which indicates that the hole density in the active layer can be significantly increased by exposure to air. This increase is consistent with the intrinsic more delocalized nature of the highest occupied molecular orbital of the charged vinylene polymer, as estimated by density functional theory (DFT) calculations, which facilitates hole transport within the polymer chains. This is the first demonstration of an efficient NDI-based hole semiconducting polymer, which will enable new developments in all-polymer solar cells, complementary circuits, and dopable polymers for use in thermoelectrics. To the opposite end, an all-acceptor napthalenediimide-bithiazole based copolymer, P(NDI2OD-BiTz), was also synthesized and characterized for application in thin film transistors. DFT calculations revealed a perpendicular dihedral angle of 90 degrees between acceptor units, presumably due to steric congestion of imine/imide lone pair electrons, yet optimized transistors yield electron mobilities of 0.11 cm2/Vs with use of a zwitterionic naphthalenediimide interlayer. GIXD measurements of annealed films reveal P(NDI2OD-BiTz) adopts an edge-on orientation. This report highlights the first NDI and thiazole all-acceptor polymer, and demonstrates high electron mobility despite large backbone torsion.