Investigation of Hierarchical Structure Formation in Merocyanine Photovoltaics

Abstract

Merocyanines (MCs) are a versatile class of small-molecule dyes. Their optoelectronic properties are easily tunable by chemically controlling their donor−acceptor strength, and their structural properties can be tuned by simple side-chain substitution. This manuscript demonstrates a novel series of MCs featuring an indoline donor with varying hydrocarbon side-chain length (from 6 to 12 carbons) and a tertbutyl-thiazole acceptor, labeled InTBT. Bulk heterojunction organic photovoltaics are fabricated with a [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) acceptor and characterized. Films composed of I8TBT:PCBM and I9TBT:PCBM produced the highest power conversion efficiency of 4.5%, which suggests that the morphology is optimized by controlling the side-chain length. Hierarchical structure formation in InTBT:PCBM films is studied using grazing incidence X-ray diffraction (GIXRD), small-angle neutron scattering (SANS), and atomic force microscopy (AFM). When mixed with PCBM, InTBTs with ≤8 side-chain carbons form pure crystalline domains, while InTBTs with ≥9 side-chain carbons mix well with PCBM. SANS demonstrates that increasing side-chain length increases the InTBT-rich domain size. In addition, a branched hexyl−dodecyl side-chain IHDTBT:PCBM film was studied and found to exhibit the worst-performance organic photovoltaic (OPV) device. The large-branched side chain inhibited mixing between IHDTBT and PCBM resulting in large segregated phases.

Merocyanine with Hole-Transporting Ability and Efficient Defect Passivation Effect for Perovskite Solar Cells

“…Merocyanine dyes are an important class of polymethines with a dipolar donor–acceptor (D–A) structure. These dyes not only can be easily prepared via a facile synthesis but also exhibit desired optical and electronic properties, making them attractive material candidates for optoelectric applications such as p-type materials used in organic field-effect transistors (OFETs) and organic solar cells (OSCs). − Moreover, as shown in Figure a, one interesting structural feature for merocyanines is that their electronic structure can change from the neutral polyene form (N-form) to the zwitterionic polyene form (Z-form) via an ideal polymethine form. , In this context, merocyanines with zwitterionic structure character could have the potential to passivate both positively charged and negatively charged defects of perovskites. Thus, combining with their advantages of facile synthesis and p-type semiconducting property, it would be meaningful to develop merocyanine-based passivators, which however have been less reported to date.…”