Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation Brent Kitchen, † Omar Awartani, † R. Joseph Kline, ‡ Terry McAfee, § Harald Ade, § and Brendan T. O’Connor* ,† † Department of Mechanical, Aerospace Engineering, North Carolina State University, 911 Oval Drive, Raleigh, North Carolina 27695, United States ‡ Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States § Department of Physics, North Carolina State University, 2401 Stinson Drive, Raleigh, North Carolina 27695, United States * S Supporting Information 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. KEYWORDS: solar cells, organic electronics, open-circuit voltage, molecular orientation 1. INTRODUCTION In organic solar cells the photoactive layer typically consists of an electron donor and an electron acceptor material with offsets in their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Given the low dielectric constant and weak intermolecular coupling character of organic semiconductors, light absorption results in the formation of Coulombically bound electron-hole pairs, or excitons, and the heterojunction provides a driving force to efficiently dissociate the excitons into free charge carriers. The difference in the HOMO of the donor and LUMO of the acceptor is also directly related to the open-circuit voltage (V OC ) of the solar cell. 1-4 Given the critical nature of the heterojunction on organic photovoltaic (OPV) device performance, there has been a significant amount of research considering energy level alignment at the interface, charge transfer states, and dipole interactions. 2-12 Recently, the role of molecular orientation at the heterojunction interface has been investigated in a number of OPV systems, 5,6,9,13 where the relative molecular orientation has been shown to significantly influence device performance including charge transfer dissociation and recombination rates. 5,6 The orientation of polar organic molecules is also expected to alter the molecular energy levels of frontier orbitals, which then vary the output voltage of the solar cell. 12,14 However, there has not been a detailed experimental analysis on the variation in V OC with polar molecule orientation in OPV devices, and in particular in polymer:fullerene systems. 6,11 Here, we consider the effect the out-of-plane stacking orientation of the quadrupolar donor polymer poly(3- hexylthiophene) (P3HT) relative to the acceptor ([6,6]-phenyl C61-butyric acid methyl ester) (PCBM) on OPV V OC . Given the widespread use of P3HT, details of the energy conversion process in P3HT/PCBM solar cells have been considered at length. 1,15,16 However, probing the interface in polymer- fullerene systems such as P3HT/PCBM is difficult due to the typical approach of solution processing with common solvents resulting in the formation of a bulk heterojunction (BHJ). 16 In Received: December 15, 2014 Accepted: June 1, 2015 Published: June 1, 2015 Research Article www.acsami.org © 2015 American Chemical Society 13208 DOI: 10.1021/am508855s ACS Appl. Mater. Interfaces 2015, 7, 13208-13216 Downloaded via AMERICAN UNIV OF BEIRUT on November 24, 2019 at 11:13:44 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.