title: Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation.
abstract: The role of molecular orientation of a polar conjugated polymer in polymer-fullerene organic photovoltaic (OPV) cells is investigated. A planar heterojunction (PHJ) OPV cell composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a model system to isolate the effect of the interfacial orientation on the photovoltaic properties. The molecular orientation of the aggregate P3HT relative to the PCBM layer is varied from highly edge-on (conjugated ring plane perpendicular to the interface plane) to appreciably face-on (ring plane parallel to the interface). It is found that as the P3HT stacking becomes more face-on there is a positive correlation to the OPV open-circuit voltage (V(OC)), attributed to a shift in the highest occupied molecular orbital (HOMO) energy level of P3HT. In addition, the PHJ OPV cell with a broad P3HT stacking orientation distribution has a V(OC) comparable to an archetypal bulk heterojunction (BHJ) device. These results suggest that, in the BHJ OPV cell, the hole energy level in the charge transfer state is defined in part by the orientation distribution of the P3HT at the interface with PCBM. Finally, the photoresponses of the devices are also shown to have a dependence on P3HT stacking orientation.
authors.affiliation: †Department of Mechanical, Aerospace Engineering, North Carolina State University, 911 Oval Drive, Raleigh, North Carolina 27695, United States.
authors.forename: Brent
authors.initials: B
authors.lastname: Kitchen
authors.affiliation: †Department of Mechanical, Aerospace Engineering, North Carolina State University, 911 Oval Drive, Raleigh, North Carolina 27695, United States.
authors.forename: Omar
authors.initials: O
authors.lastname: Awartani
authors.affiliation: ‡Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States.
authors.forename: R Joseph
authors.initials: RJ
authors.lastname: Kline
authors.affiliation: §Department of Physics, North Carolina State University, 2401 Stinson Drive, Raleigh, North Carolina 27695, United States.
authors.forename: Terry
authors.initials: T
authors.lastname: McAfee
authors.affiliation: §Department of Physics, North Carolina State University, 2401 Stinson Drive, Raleigh, North Carolina 27695, United States.
authors.forename: Harald
authors.initials: H
authors.lastname: Ade
authors.affiliation: †Department of Mechanical, Aerospace Engineering, North Carolina State University, 911 Oval Drive, Raleigh, North Carolina 27695, United States.
authors.forename: Brendan T
authors.initials: BT
authors.lastname: O'Connor
keywords: molecular orientation; open-circuit voltage; organic electronics; solar cells