Donor-Acceptor Polymer Based on Planar Structure of Alkylidene-Fluorene Derivative: Correlation of Power Conversion Efficiency among Polymer and Various Acceptor Units.

Eui Jin Lee,Ho Jun Song

doi:10.3390/polym12122859

Abstract

This study synthesized a novel polymer, poly(alkylidene fluorene-alt-diphenylquinoxaline) (PAFDQ), based on a planar alkylidene-fluorene and a highly soluble quinoxaline derivative through the Suzuki coupling reaction. We designed a novel molecular structure based on alkylidene fluorene and quinoxaline derivatives due to compact packing property by the planar structure of alkyidene fluorene and efficient intra-molecular charge transfer by quinoxaline derivatives. The polymer was largely dissolved in organic solvents, with a number average molecular weight and polydispersity index of 13.2 kg/mol and 2.74, respectively. PAFDQ showed higher thermal stability compared with the general fluorene structure owing to its rigid alkylidene-fluorene structure. The highest occupied and lowest unoccupied molecular orbital levels of PAFDQ were −5.37 eV and −3.42 eV, respectively. According to X-ray diffraction measurements, PAFDQ exhibited the formation of an ordered lamellar structure and conventional edge-on π-stacking. The device based on PAFDQ/Y6-BO-4Cl showed the best performance in terms of short circuit current (9.86 mA/cm2), open-circuit voltage (0.76 V), fill factor (44.23%), and power conversion efficiency (3.32%). Moreover, in the PAFDQ/Y6-BO-4Cl-based film, the phase separation of donor-rich and acceptor-rich phases, and the connected dark domains, was observed.

Highlights

TGA of poly(alkylidene fluorene-alt-diphenylquinoxaline) (PAFDQ) showed a 5% weight loss at 385 ◦ C (Table 1 & Figure A1), indicating higher thermal stability compared to the general donor structure, which can be attributed to the rigid alkylidene-fluorene structure
According to X-ray diffraction (XRD) measurements, PAFDQ exhibited a formation of an ordered lamellar structure
According to XRD measurements, PAFDQ exhibited a formation of an ordered lamellar structure and conventional edge-on π-stacking

Summary

Introduction

Conducting molecules have long been utilized for diverse applications such as organic light-emitting diodes [1,2,3], organic solar cells (OSCs) [4,5,6,7,8,9,10,11,12,13,14,15], and organic thin-film transistors [16,17].In particular, OSCs have received considerable attention because of the focus on economic viability and environmental friendliness. The low power conversion efficiency (PCE) of conjugated polymers has primarily hindered their use in OSCs [8]. The donor-acceptor (D-A)-type conjugated polymer has received significant attention for several decades because its electronic characteristics can be tuned owing to the unique synergy between its donor and acceptor units. It can extend its absorption range for wavelengths in the infrared region [8]. In order to improve the efficiency of D-A-type conjugated polymers, the following characteristics are required [18]: (1) band gap property to absorb a wide range of wavelengths,

Methods

Results

Conclusion