title: Unraveling the Fundamental Mechanisms of Solvent-Additive-Induced Optimization of Power Conversion Efficiencies in Organic Photovoltaic Devices.
abstract: The realization of controllable morphologies of bulk heterojunctions (BHJ) in organic photovoltaics (OPVs) is one of the key factors enabling high-efficiency devices. We provide new insights into the fundamental mechanisms essential for the optimization of power conversion efficiencies (PCEs) with additive processing to PBDTTT-CF:PC71BM system. We have studied the underlying mechanisms by monitoring the 3D nanostructural modifications in BHJs and correlated the modifications with the optical analysis and theoretical modeling of charge transport. Our results demonstrate profound effects of diiodooctane (DIO) on morphology and charge transport in the active layers. For small amounts of DIO (<3 vol %), DIO promotes the formation of a well-mixed donor-acceptor compact film and augments charge transfer and PCE. In contrast, for large amounts of DIO (>3 vol %), DIO facilitates a loosely packed mixed morphology with large clusters of PC71BM, leading to deterioration in PCE. Theoretical modeling of charge transport reveals that DIO increases the mobility of electrons and holes (the charge carriers) by affecting the energetic disorder and electric field dependence of the mobility. Our findings show the implications of phase separation and carrier transport pathways to achieve optimal device performances.
authors.forename: Nuradhika
authors.initials: N
authors.lastname: Herath
authors.affiliation: Department of Electrical Engineering and Computer Science, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: Sanjib
authors.initials: S
authors.lastname: Das
authors.forename: Jiahua
authors.initials: J
authors.lastname: Zhu
authors.forename: Rajeev
authors.initials: R
authors.lastname: Kumar
authors.forename: Jihua
authors.initials: J
authors.lastname: Chen
authors.forename: Kai
authors.initials: K
authors.lastname: Xiao
authors.affiliation: Department of Electrical Engineering and Computer Science, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: Gong
authors.initials: G
authors.lastname: Gu
authors.forename: James F
authors.initials: JF
authors.lastname: Browning
authors.forename: Bobby G
authors.initials: BG
authors.lastname: Sumpter
authors.forename: Ilia N
authors.initials: IN
authors.lastname: Ivanov
authors.forename: Valeria
authors.initials: V
authors.lastname: Lauter
keywords: device efficiencies; morphology; neutron reflectometry; organic photovoltaics; solvent additives; theoretical modeling