title: Understanding device-structure-induced variations in open-circuit voltage for organic photovoltaics.
abstract: We investigate the structural influences on the device performance, especially on open-circuit voltage (V(OC)) in squaraine (SQ)/fullerene (C60) bilayer cells. Simply changing the SQ thickness could lead to 40% variation in V(OC) from 0.62 to 0.86 V. The ionization potential (IP) of SQ films and recombination at the anode surface as well as donor/acceptor (D/A) interface sensitively vary with film thicknesses, which account for the shifts in V(OC). The anode recombination can be effectively suppressed by preventing direct contact between C60 and the anode with a buffer layer, delivering an elevated V(OC). Through polarized infrared-multiple-angle incidence resolution spectroscopy measurement, the molecular structure of SQ films is found to gradually evolve from lying-down on indium-tin oxide substrates with noncentrosymmetric orientation at low thicknesses to random structure at high thicknesses. The different molecular orientation may yield different strengths of electronic coupling, which affects the charge-carrier recombination and thus V(OC). Moreover, the oriented SQ films would spontaneously compose aligned dipole moments at the D/A interface because of the strong dipolar effects in SQ molecules identified by density functional theory calculations, whereas no aligned interfacial dipole moment exists in the random structure. The resulting interfacial dipole moments would form an electric field at the D/A interface, leading to variations in the IP and thus impacting V(OC). Our findings demonstrate that V(OC) in organic photovoltaic cells is critically associated with the molecular orientation that affects the charge-carrier recombination and interfacial dipole alignment, which should be seriously taken into consideration for the design of organic molecules and optimization of the cell efficiency.
authors.forename: Zhiping
authors.initials: Z
authors.lastname: Wang
authors.affiliation: ‡Faculty of Engineering, Department of Nanoscience, Sojo University, 4-22-1 Ikeda, Nishi-ku, Kumamoto 860-0082, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Uemura
authors.forename: Ying
authors.initials: Y
authors.lastname: Zhou
authors.affiliation: ∥Japan Science and Technology Agency (JST)-PRESTO, 4-1-8 Honcho Kawaguchi, 332-0012 Saitama, Japan.
authors.forename: Tetsuhiko
authors.initials: T
authors.lastname: Miyadera
authors.forename: Reiko
authors.initials: R
authors.lastname: Azumi
authors.forename: Yuji
authors.initials: Y
authors.lastname: Yoshida
authors.forename: Masayuki
authors.initials: M
authors.lastname: Chikamatsu
keywords: molecular orientation; open-circuit voltage; organic photovoltaics; structural control