title: The critical role of interfacial dynamics in the stability of organic photovoltaic devices.
abstract: Understanding the stability and degradation mechanisms of organic solar materials is required to achieve long device lifetimes. Here we study photodegradation mechanisms of the (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]):[6,6]-phenyl-C61-butyric acid methyl ester (PCPDTBT:PCBM) low band gap-based photovoltaic blend. We apply quasi steady state Photo-induced Absorption Optical Spectroscopy, time-resolved Electron Spin Resonance Spectroscopy and theoretical modeling to investigate the dynamics of long-lived photoexcited species. The role of the interfacial physics in the efficiency and robustness of the photovoltaic blend is clarified. We demonstrate that the polymer triplet state (T), populated through the interfacial charge transfer (CT) state recombination, coexists with charge carriers. However, in contrast to previous suggestions, it has no role in the degradation process caused by air exposure. Instead, the long-lived emissive interfacial CT state is responsible for the blend degradation in air. It mediates direct electron transfer to contaminants, leading to the formation of reactive and harmful species, such as the superoxide.
authors.affiliation: Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy. annamaria.petrozza@iit.it.
authors.forename: G
authors.initials: G
authors.lastname: Grancini
authors.forename: M
authors.initials: M
authors.lastname: De Bastiani
authors.forename: N
authors.initials: N
authors.lastname: Martino
authors.forename: D
authors.initials: D
authors.lastname: Fazzi
authors.forename: H-J
authors.initials: HJ
authors.lastname: Egelhaaf
authors.forename: T
authors.initials: T
authors.lastname: Sauermann
authors.forename: M R
authors.initials: MR
authors.lastname: Antognazza
authors.forename: G
authors.initials: G
authors.lastname: Lanzani
authors.forename: M
authors.initials: M
authors.lastname: Caironi
authors.forename: L
authors.initials: L
authors.lastname: Franco
authors.forename: A
authors.initials: A
authors.lastname: Petrozza