Branched Electron-Donor Core Effect in D-π-A Star-Shaped Small Molecules on Their Properties and Performance in Single-Component and Bulk-Heterojunction Organic Solar Cells †

Alexander N Solodukhin,Sergey A Ponomarenko,Artem V Bakirov,Petr S Savchenko,Maxim A Shcherbina,Dmitry Yu Paraschuk,Yuriy N Luponosov,Artur L Mannanov,Sergei N Chvalun

doi:10.3390/en14123596

Abstract

Star-shaped donor-acceptor molecules are full of promise for organic photovoltaics and electronics. However, the effect of the branching core on physicochemical properties, charge transport and photovoltaic performance of such donor-acceptor materials in single-component (SC) and bulk heterojunction (BHJ) organic solar cells has not been thoroughly addressed. This work shows the comprehensive investigation of six star-shaped donor-acceptor molecules with terminal hexyldicyanovinyl blocks linked through 2,2′-bithiophene π-conjugated bridge to different electron-donating cores such as the pristine and fused triphenylamine, tris(2-methoxyphenyl)amine, carbazole- and benzotriindole-based units. Variation of the branching core strongly impacts on such important properties as the solubility, highest occupied molecular orbital energy, optical absorption, phase behavior, molecular packing and also on the charge-carrier mobility. The performance of SC or BHJ organic solar cells are comprehensively studied and compared. The results obtained provide insight on how to predict and fine-tune photovoltaic performance as well as properties of donor-acceptor star-shaped molecules for organic solar cells.

Highlights

Organic solar cells are attracting considerable attention due to their ability to produce flexible, lightweight devices with solvent and printing technologies [1,2,3,4,5,6]
The concept includes variation of types and numbers of electron-donor and electron-withdrawing groups or their sequence within the molecular structure, which results in a lower bandgap by reason of orbital mixing of the donor and the acceptor blocks and shifts absorption into the long-wave region due to the intramolecular charge transfer (ICT) effects [10,11,12,13]
In our recent reports we have demonstrated that TPA-based donor-acceptor starshaped molecules have a field-assisted mechanism of charge photogeneration and demonstrate small energy losses resulting in high open-circuit voltage up to 1.19 V and the power conversion efficiency over 1% [53,54,55]

Summary

Introduction

Organic solar cells are attracting considerable attention due to their ability to produce flexible, lightweight devices with solvent and printing technologies [1,2,3,4,5,6]. The “double-cable” polymers or similar small molecule dyads consist of donor and acceptor units linked to each other through an aliphatic spacer within one molecule The drawbacks of these concepts are a relatively complex synthesis and the necessity to carry out a posttreatment of functional layers in the devices to tune the miscibility between conjugated backbones and side units. 2,20 -bithiophene π-spacer to the different branching cores: triphenylamine (TPA) [56], tris(2-methoxyphenyl)amine (m-TPA), 4,4,8,8,12,12-hexakis(4-methylphenyl)-4H,8H,12Hbenzo[1,9]quinolizino[3,4,5,6,7-defg]acridine (f-TPA), 9-phenyl-9H-carbazole (s-CBZ), 2,7dibromo-9-(4-bromophenyl)-9H-carbazole (t-CBZ) [35] or 5,10,15-triethyl-10,15-dihydro5H-diindolo[3,2-a:30 ,20 -c]carbazole (BTI) [57] (Figure 1) The comparison of their properties and photovoltaic devices performance allows the impact of the donor branching core type on the solubility, phase behavior, optical, electrochemical properties and photoelectric characteristics of SC and BHJ OSCs to be revealed.

Synthesis

Optical and Electrochemical Properties

Single-Component

Bulk-Heterojunction Solar Cells