The Influence of Alkoxy Substitutions on the Properties of Diketopyrrolopyrrole-Phenyl Copolymers for Solar Cells.

Zandra George,Stefan Hellström,Robert Gehlhaar,Angelica Lundin,Yves Geerts,Paul Heremans,Mats Andersson,Christian Müller,Gabin Gbabode,Renee Kroon

doi:10.3390/ma6073022

Abstract

A previously reported diketopyrrolopyrrole (DPP)-phenyl copolymer is modified by adding methoxy or octyloxy side chains on the phenyl spacer. The influence of these alkoxy substitutions on the physical, opto-electronic properties, and photovoltaic performance were investigated. It was found that the altered physical properties correlated with an increase in chain flexibility. Well-defined oligomers were synthesized to verify the observed structure-property relationship. Surprisingly, methoxy substitution on the benzene spacer resulted in higher melting and crystallization temperatures in the synthesized oligomers. This trend is not observed in the polymers, where the improved interactions are most likely counteracted by the larger conformational possibilities in the polymer chain upon alkoxy substitution. The best photovoltaic performance was obtained for the parent polymer: fullerene blends whereas the modifications on the other two polymers result in reduced open-circuit voltage and varying current densities under similar processing conditions. The current densities could be related to different polymer: fullerene blend morphologies. These results show that supposed small structural alterations such as methoxy substitution already significantly altered the physical properties of the parent polymer and also that oligomers and polymers respond divergent to structural alterations made on a parent structure.

Highlights

Polymer based organic photovoltaics have attracted a lot of attention as potential renewable energy technology in the last decades [1,2,3,4]
All polymers and oligomers were synthesized via Suzuki polymerization (Scheme 1), which resulted in rather reasonable molecular weights
O1 and O2 (Scheme 1) were synthesized and analyzed by DSC to investigate the effect of the different phenyl spacer on the observed trend in the polymers’ thermal transitions while excluding molecular length as a factor

Summary

Introduction

Polymer based organic photovoltaics have attracted a lot of attention as potential renewable energy technology in the last decades [1,2,3,4]. Photovoltaic devices based on a blend of the so-called PDPPTPT (hereafter P1) and PC71BM reached good power conversion efficiency of 5.5% after optimization with a processing agent [19]. If this polymer is modified with alkoxy side chains on the phenyl spacer a redshifted absorption will be obtained. In an attempt to ascertain structure-property relationships we synthesized P1 derivatives with short methoxy (P2) and long octyloxy (P3) side groups on the benzene ring, as well as well-defined oligomers based on P1 and P2 This allowed us to attribute several altered physical and optical properties to either the alkoxy substitution or conformational effects in the polymer

Synthesis and Physical Properties of Oligomers and Polymers

Electrochemical and Optical Properties

Experimental Details

Characterization

Conclusions