Water/Ethanol Soluble p-Type Conjugated Polymers for the Use in Organic Photovoltaics

Xun Pan,Renee Kroon,Sait Elmas,Mats R Andersson,Gunther G Andersson,Yanting Yin,David A Lewis,Desta Gedefaw,Anirudh Sharma

doi:10.3389/fmats.2020.00281

Abstract

We have developed two series of p-type conjugated polymers based on poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) polymeric backbone utilizing polar pendant groups, i.e., tertiary amine and pyridine, to achieve switchable solubility in water and ethanol. By balancing the ratio between polar and non-polar side-groups, we could combine green-solvent processability with the manufacturing of functional photovoltaic devices. Due to the unavailability of water/alcohol soluble acceptors, the photovoltaic performance of these new polymers was evaluated using organic solvent by incorporating PC61BM. For water/alcohol soluble partial amine-based polymers, we achieve a maximum power conversion efficiency (PCE) of ∼0.8% whereas alcohol soluble partial pyridine-based polymers show enhanced PCE of ∼1.3% with inverted device structure. We propose that the enhancement in PCE is a result of the reduction in amino-group content and the lower basicity of pyridine, both of which decrease the interaction between functionalized polymers with the anode interface material and reduce the miscibility of the donor and acceptor. Further improvement of the photovoltaic performance, in particular the open-circuit voltage (Voc), was achieved by using an anode buffer layer to mitigate the unfavorable interaction of the amino/pyridine groups with the MoO3 electrode. Our work demonstrated the possibility of substituent modification for conjugated polymers using tertiary amine and pyridine groups to achieve water/alcohol soluble and functional donor materials.

Highlights

Over the past few years, a growing demand for green solvent processable conjugated materials has been witnessed in the field of organic photovoltaic (OPV) research (Xie et al, 2018; Xue et al, 2018; Lee S. et al, 2019) in order to realize the mass production of OPVs using low-cost and high-throughput printing method without the negative environmental effect caused by harmful halogenated solvent (Søndergaard et al, 2012; Angmo et al, 2013; Lee J. et al, 2019)
Water/Ethanol Soluble Donor Polymers tremendous efforts have been put into the molecular design of conjugated polymers to push up the power conversion efficiencies (PCE) of single junction OPVs up to 18% (Liu et al, 2020), harmful, halogenated solvents such as chloroform, chlorobenzene or ortho-dichlorobenzene (o-DCB) are still widely used to dissolve the high-performing active materials for processing (Wang et al, 2017; Li X. et al, 2019; Fan et al, 2020)
Our results show that switchable water/ethanol soluble p-type conjugated polymers can be achieved and using pyridine pendant groups can give decent photovoltaic performance, which will be beneficial for environmentally friendly fabrication of OPVs in the future

Summary

Introduction

Over the past few years, a growing demand for green solvent processable conjugated materials has been witnessed in the field of organic photovoltaic (OPV) research (Xie et al, 2018; Xue et al, 2018; Lee S. et al, 2019) in order to realize the mass production of OPVs using low-cost and high-throughput printing method without the negative environmental effect caused by harmful halogenated solvent (Søndergaard et al, 2012; Angmo et al, 2013; Lee J. et al, 2019). Compared to the reported TQ1 (Wang et al, 2010), tertiary amine or pyridine functionalized polymers in the solid states all exhibit near-identical optical bandgaps and similar energy levels (see Supplementary Table S2 and Supplementary Figure S5), indicating that the modification of substituents attached to the quinoxaline backbone have a negligible influence on the conjugation of the resulting polymers.

Results

Conclusion