Conjugated Thiophene Dendrimer with an Electron-Withdrawing Core and Electron-Rich Dendrons: How the Molecular Structure Affects the Morphology and Performance of Dendrimer:Fullerene Photovoltaic Devices William L. Rance,* ,† Benjamin L. Rupert, ‡ William J. Mitchell, ‡ Muhammet E. Ko ¨se, § David S. Ginley, ‡ Sean E. Shaheen, | Garry Rumbles, ‡ and Nikos Kopidakis* ,‡ Department of Physics, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, United States, National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado 80401, United States, Department of Chemistry and Biochemistry, North Dakota State UniVersity, Fargo, North Dakota 58108, United States, and Department of Physics and Astronomy, UniVersity of DenVer, DenVer, Colorado 80208, United States ReceiVed: July 22, 2010; ReVised Manuscript ReceiVed: October 5, 2010 The combination of electron-rich and electron-poor moieties in conjugated molecules is frequently utilized in order to red shift the absorption spectrum and improve photon harvesting in bulk heterojunction photovoltaic devices. In this study we characterize a conjugated thiophene dendrimer that has an electron-withdrawing core and electron-rich dendrons in order to investigate the effects of this design approach on the salient properties that influence the performance of photovoltaic devices with this dendrimer donor. Beside the absorption onset, these properties are the morphology of dendrimer:fullerene films and the dynamics of photoinduced carrier generation and loss. For comparison we also characterize a control dendrimer with the same structure but without the electron-withdrawing core. In addition to lowering the band gap by ca. 0.5 eV, the electron-withdrawing core also planarizes the dendrimer resulting in enhanced order in bulk heterojunction films. We observe longer photocarrier lifetimes in this ordered structure compared to the films of the predominantly amorphous control. The characterization of dendrimer:fullerene bulk heterojunction photovoltaic devices shows no voltage loss despite the decreased absorption onset. The properties of the device are consistent with the improved photocarrier lifetimes, but they are limited by a low short-circuit photocurrent density. We attribute this to electron confinement in the core that hinders transfer to the fullerene acceptor. 1. Introduction Organic photovoltaics (OPV) have attracted much interest recently, with power conversion efficiencies exceeding 7% in polymer:fullerene bulk heterojunctions. 1 Extensive research in photoactive polymers has led to macromolecular designs that offer control of important properties that pertain to the perfor- mance of polymer:fullerene-based OPV. Alternating donor- acceptor copolymers, also termed “push-pull” structures, have been designed to have low absorption onset, thereby improving the harvesting of solar photons. 2,3 Design of the backbone and side chains 4-6 as well as processing additives 7 has offered control of the morphology of films that has led to improved charge carrier collection. It has also been recently recognized that the energetics at the polymer:fullerene interface, measured by the free energy difference for free-carrier production following exciton dissociation, is an important factor that may need to be considered in the design of next-generation materials for OPV. 8 While polymers have been at the forefront of solution-based OPV, solution processable small molecules are increasingly successful as donors, demonstrating device efficiencies up to 4.4% when blended with fullerene acceptors. 9 Small molecules are attractive because of their well-defined structure that allows for the in-depth understanding of their structure-property correlations that is crucial to the design of molecules tailored for OPV applications. 10 π-Conjugated dendrimers are a particular class of molecules with a highly branched structure that offers the possibility to “fill space” with electronically active groups without obstruction from the solubilizing groups. 11 Previously we have reported on the synthesis, 12,13 spectros- copy, 14,15 charge transport, 16,17 and photovoltaic properties 18 of conjugated dendrimers composed of a phenyl core with con- jugated thiophene dendrons (“arms”) attached at ortho, para, and meta positions on the core. These dendrimers were primarily first generation, i.e., the dendron only had one branching point, with the number of R-linked thiophenes in the arm ranging from three to five, making these structures two-dimensional. Due to the high band gaps (>2 eV) and the poor morphology of these early version of dendrimers, device efficiency of bulk hetero- junction blends using phenyl-C 61 -butyric acid methyl ester (PCBM) as the acceptor was limited to 1.3% under 1 sun. 18 In an effort to improve overlap with the solar spectrum, we recently reported on a series of thiophene dendrimers that were modified with electron-donating or -withdrawing groups in order to lower the band gap. 13 Shown in Figure 1, the dendrimer 3G1- 2S-CN (2) uses an electron-withdrawing cyanobenzene core and thiophene arms to create a push-pull structure that lowers the lowest unoccupied molecular orbital (LUMO). A similar den- drimer 3G1-2S-Ac (1), also shown in Figure 1, was synthesized without the electron-withdrawing cyano groups on the core but is otherwise identical to 2. Compared to the control molecule 1, the optical absorption onset of 2 was ca. 0.5 eV lower. Cyclic voltammetry measurements showed this change to be primarily * To whom correspondence should be addressed. E-mail: wrance@ mines.edu (W.L.R); nikos.kopidakis@nrel.gov (N.K.). † Colorado School of Mines. ‡ National Renewable Energy Laboratory. § North Dakota State University. | University of Denver. J. Phys. Chem. C 2010, 114, 22269–22276 22269 10.1021/jp106850f  2010 American Chemical Society Published on Web 11/30/2010