title: Effect of Polymer Side Chains on Charge Generation and Disorder in PBDTTPD Solar Cells.
abstract: The effect of polymer side chains on device performance was investigated for PBDT(EtHex)-TPD(Oct):PC70BM and PBDT(EtHex)-TPD(EtHex):PC70BM BHJ solar cells. Going from a linear side chain on the polymer's acceptor moiety to a branched side chain was determined to have a negative impact on the overall device efficiency, because of significantly reduced short-circuit current (J(sc)) and fill factor (FF) values. Sub-bandgap external quantum efficiency (EQE) and transient photoluminescence (PL) measurements showed more-efficient carrier generation for the polymer with linear side chains, because of a higher degree of charge-transfer (CT) state delocalization, leading to more-efficient exciton dissociation. Furthermore, the increase in π-π stacking distance and disorder for the bulkier ethylhexyl side chain were shown to result in a lower hole mobility, a higher bimolecular recombination, and a higher energetic disorder. The use of linear side chains on the polymer's acceptor moiety was shown to promote photogeneration, because of more-effective CT states and favorable carrier transport resulting in improved solar cell performance.
authors.affiliation: Department of Materials Science and Engineering, University of Florida , Gainesville, Florida 32611, United States.
authors.forename: Iordania
authors.initials: I
authors.lastname: Constantinou
authors.affiliation: Department of Materials Science and Engineering, University of Florida , Gainesville, Florida 32611, United States.
authors.forename: Tzung-Han
authors.initials: TH
authors.lastname: Lai
authors.affiliation: Department of Materials Science and Engineering, University of Florida , Gainesville, Florida 32611, United States.
authors.forename: Erik D
authors.initials: ED
authors.lastname: Klump
authors.affiliation: Department of Chemistry, University of Florida , Gainesville, Florida 32611, United States.
authors.forename: Subhadip
authors.initials: S
authors.lastname: Goswami
authors.affiliation: Department of Chemistry, University of Florida , Gainesville, Florida 32611, United States.
authors.forename: Kirk S
authors.initials: KS
authors.lastname: Schanze
authors.affiliation: Department of Materials Science and Engineering, University of Florida , Gainesville, Florida 32611, United States.
authors.forename: Franky
authors.initials: F
authors.lastname: So
keywords: carrier recombination; charge transfer states; energetic disorder; organic photovoltaics; organic solar cells