N-type semiconducting perylene diimide based molecules for organic solar cells
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N-type semiconducting perylene diimide based molecules for organic solar cells D. Kotowski, S. Luzzati, E. Kozma and M. Catellani Istituto per lo Studio delle Macromolecole, CNR, Via Bassini 15, 20133 Milano, Italy ABSTRACT Core substituted perylene diimides (PDIs) are promising candidates as non-fullerene acceptor materials for organic solar cells. The functionalization of PDIs in the bay positions using chemical groups with different electron donating abilities and with steric hindrance is a versatile tool to modify both the optoelectronic properties and the morphology in the solid state. Herein we present two new PDI based molecules having bulky aromatic substituents linked into the bay positions: PDI-SF with spirobifluorene group and PDI-BSF with bithienylspirobifluorene moieties. The high steric hindrance of spirobifluorene reduce the tendency to form aggregates that has been identified as a limiting factor for the photovoltaic performances in PDI based solar cells. The PDI molecules were tested as electron acceptors in bulk heterojunction solar cells with P3HT as electron donor. Power conversion efficiencies (PCE) of 1.58% and 1.18% were obtained for PDI-SF and PDI-BSF devices. INTRODUCTION Organic solar cells have recently reached power conversion efficiencies exceeding 10% and the reason of these improvements is due mostly to the development of novel electron donor materials, rather than electron acceptors one. Fullerene derivatives are the most commonly used organic acceptors due to their outstanding electronic properties [1]; however, the chemical modification of the fullerene is limited and the production costs are relevant [2]. In order to expand the library of electron acceptor materials, certain requirements must be met: excellent molecular optoelectronic characteristics, such as light harvesting ability, good electronaccepting and transport properties, good solution processability, distinctive supramolecular features like long-range ordered π-π stacking in combination with small aggregate domains, and suitable compatibility with the electron donor material in solar cells [3]. PDI derivatives received attention as substitute of fullerenes because they posses strong light harvesting ability with tunable absorption and good electron mobility, in addition to robustness, thermal stability and low cost [4-6]. PDI optoelectronic properties and supramolecular selfassembly characteristics can be finely tuned by chemical substitution of the perylene core. The PDI bay functionalization affords out-of-plane structures, which helps to reduce the π−π intermolecular interactions in the solid state [7,8]. In this paper we report the results obtained with two perylene diimides containing spirobifluorene moiety as core substituent: directly connected in PDI-SF or with bithiophene πlinkers in PDI-BSF (see Figure 1). The PDI molecules were tested as acceptor materials in solar cells with P3HT as donor. The optical properties of the active layers, the charge transport and photovoltaic characteristics are herein presented.
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