Photocurrent Enhancement from Diketopyrrolopyrrole Polymer Solar Cells through Alkyl-Chain Branching Point Manipulation

Abstract

Systematically moving the alkyl-chain branching position away from the polymer backbone afforded two new thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTT-T) polymers. When used as donor materials in polymer:fullerene solar cells, efficiencies exceeding 7% were achieved without the use of processing additives. The effect of the position of the alkyl-chain branching point on the thin-film morphology was investigated using X-ray scattering techniques and the effects on the photovoltaic and charge-transport properties were also studied. For both solar cell and transistor devices, moving the branching point further from the backbone was beneficial. This is the first time that this effect has been shown to improve solar cell performance. Strong evidence is presented for changes in microstructure across the series, which is most likely the cause for the photocurrent enhancement.

Exploring the origin of high optical absorption in conjugated polymers

“…S2.1). A previous study reporting an effect of branching point on device performance had not resolved molecular weight from side chain structure 21 .…”

“…This polymer is interesting on account of the high field-effect transistor mobilities, very promising performance achieved as the donor in solar cells, 19 and high photostability 20 . Moreover the solar cell performance using this polymer has been correlated with the position of the branching point on the polymer side chains 21 and with the molecular weight of the polymer 22 but without any convincing mechanism for the trends. Here, we set out to establish the impact of these structural parameters of the polymer on its optical absorption.…”

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Instead, a less ordered stacking of side-chains arising from zigzag backbones ( Figure 9IV) [86] or substitution of side-chains on the same side of fused aromatic rings [121] (Figure 13d) increased the lamellar period and destroyed the ordered stacking of lamella. Recently, introducing longer side-chains with branching point further from backbones has been found to promote a stronger interchain π-π stacking indicated by the decreased d π-π , for the steric effect of bulky branching side-chains on the aggregation of adjacent backbones has been relieved [17,132]. Accordingly, the carrier mobility has been largely enhanced due to the smaller d π-π [17,132] ) [17].…”

“…Recently, introducing longer side-chains with branching point further from backbones has been found to promote a stronger interchain π-π stacking indicated by the decreased d π-π , for the steric effect of bulky branching side-chains on the aggregation of adjacent backbones has been relieved [17,132]. Accordingly, the carrier mobility has been largely enhanced due to the smaller d π-π [17,132] ) [17]. Especially for some existing groups of high-mobility conjugated polymers, this strategy about side-chains is very promising to reach an extremely high device mobility.…”

Alkyl side-chain branching point effects in thieno[3,4-c]pyrrole-4,6-dione copolymers

“…[16,17] More recently, a number of studies have begun to elucidate the effect of precision alkyl-chain engineering by shifting the branching point position, which has resulted in both improved photovoltaic efficiency and charge carrier mobility in field-effect transistors. [18][19][20] However, the preferred side-chain is largely dependent on the exact polymer system. Our approach herein is to conduct a systematic study of the alkyl side-chain branching point position, investigating the effects of overall steric bulk on solid-state properties in a promising class of semiconducting polymers.…”