title: Elucidating the impact of molecular packing and device architecture on the performance of nanostructured perylene diimide solar cells.
abstract: The performance of organic photovoltaic devices (OPV) with nanostructured polymer:perylene diimide (PDI) photoactive layers approaches the levels of the corresponding polymer:fullerene systems. Nevertheless, a coherent understanding of the difficulty for PDI-based OPV devices to deliver high power conversion efficiencies remains elusive. Here we perform a comparative study of a set of four different polymer:PDI OPV model systems. The different device performances observed are attributed to differences in the nanostructural motif of these composites, as determined by wide-angle X-ray scattering (WAXS) measurements. Long-range structural order in the PDI domain dictates (i) the stabilization energy and (ii) the concentration of the PDI excimers in the composites. The quenching of the PDI excimer photoluminescence (PL) is found to be insensitive to the former, but it depends on the latter. High PL quenching occurs for the low concentration of PDI excimers that are formed in PDI columns with a length comparable to the PDI excimer diffusion length. The stabilization of the PDI excimer state increases as the long-range order in the PDI domains improves. The structural order of the PDI domains primarily affects charge transport. Electron mobility reduces as the size of the PDI domain increases, suggesting that well-ordered PDI domains suffer from poor electronic connectivity. WAXS further reveals the presence of additional intermolecular PDI interactions, other than the direct face-to-face intermolecular coupling, that introduce a substantial energetic disorder in the polymer:PDI composites. Conventional device architectures with hole-collecting ITO/PEDOT:PSS bottom electrodes are compared with inverted device architectures bearing bottom electron-collecting electrodes of ITO/ZnO. In all cases the ZnO-functionalized devices surpass the performance of the conventional device analogues. X-ray photoelectron spectroscopy explains that in
  PSS-functionalized devices, the PDI component preferentially segregates closer to the hydrophilic
  PSS electrode, thus impeding the efficient charge extraction and limiting device photocurrent.
authors.affiliation: †Fondazione Istituto Italiano di Tecnologia, Centre for Nanoscience and Technology@PoliMi, Via Giovanni Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Eduardo
authors.initials: E
authors.lastname: Aluicio-Sarduy
authors.affiliation: †Fondazione Istituto Italiano di Tecnologia, Centre for Nanoscience and Technology@PoliMi, Via Giovanni Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Ranbir
authors.initials: R
authors.lastname: Singh
authors.affiliation: †Fondazione Istituto Italiano di Tecnologia, Centre for Nanoscience and Technology@PoliMi, Via Giovanni Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Zhipeng
authors.initials: Z
authors.lastname: Kan
authors.affiliation: †Fondazione Istituto Italiano di Tecnologia, Centre for Nanoscience and Technology@PoliMi, Via Giovanni Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Tengling
authors.initials: T
authors.lastname: Ye
authors.affiliation: †Fondazione Istituto Italiano di Tecnologia, Centre for Nanoscience and Technology@PoliMi, Via Giovanni Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Aliaksandr
authors.initials: A
authors.lastname: Baidak
authors.affiliation: ‡Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133, Milano, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Calloni
authors.affiliation: ‡Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133, Milano, Italy.
authors.forename: Giulia
authors.initials: G
authors.lastname: Berti
authors.affiliation: ‡Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133, Milano, Italy.
authors.forename: Lamberto
authors.initials: L
authors.lastname: Duò
authors.affiliation: ⊥University of Ioannina, Department of Physics, 451 10 Ioannina, Greece.
authors.forename: Agathaggelos
authors.initials: A
authors.lastname: Iosifidis
authors.affiliation: §Département de Chimie, Université Laval, Pavillon A-Vachon 1045, Avenue de la Médecine, Québec City, Québec, Canada G1 V 0A6.
authors.forename: Serge
authors.initials: S
authors.lastname: Beaupré
authors.affiliation: §Département de Chimie, Université Laval, Pavillon A-Vachon 1045, Avenue de la Médecine, Québec City, Québec, Canada G1 V 0A6.
authors.forename: Mario
authors.initials: M
authors.lastname: Leclerc
authors.affiliation: ∥Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany.
authors.forename: Hans-Jürgen
authors.initials: HJ
authors.lastname: Butt
authors.affiliation: ⊥University of Ioannina, Department of Physics, 451 10 Ioannina, Greece.
authors.forename: George
authors.initials: G
authors.lastname: Floudas
authors.affiliation: #Cyprus University of Technology, Department of Mechanical Engineering and Materials Science and Engineering, Dorothea Bldg 511, 45 Kitiou Kyprianou Street, 3041 Limassol, Cyprus.
authors.forename: Panagiotis E
authors.initials: PE
authors.lastname: Keivanidis
keywords: charge extraction; non-fullerene acceptors; organic photovoltaics; perylene diimide; vertical phase separation