Mixed side-chain geometries for aggregation control of poly(fluorene-alt-bithiophene) and their effects on photophysics and charge transport

TL;DR

This study demonstrates how mixed side-chain geometries on poly(fluorene-alt-bithiophene) can control molecular organization, affecting photophysical properties and charge transport, with implications for organic photovoltaic device performance.

Contribution

It introduces a novel approach of combining two different side-chains on F8T2 to tune morphology and charge transport without altering core properties.

Findings

Mixed side-chains influence polymer crystallization and morphology.

Modified polymers show different thermal and photophysical behaviors.

Charge transport and photovoltaic efficiency are affected by side-chain composition.

Abstract

In organic optoelectronics, order of conjugated molecules is required for good charge transport, but strong aggregation behavior may generate grain boundaries and trapping, opposing those benefits. Side chains on a polymer's backbone are major reason for and also tool to modify its morphological characteristics. In this report, we show on the example poly(9,9-dioctylfluorenyl-co-bithiophene) (F8T2) that by a combination of two types of side-chains on the backbone of equal number of carbons, one promoting crystallization, another hindering it, organization of the main chains can be controlled, without changing its major properties. We compare the traditional F8T2 derivative with octyl substituent with two modified species, one containing solely 2-ethylhexyl side-chains and another with both types randomly distributed. Thermal characteristics, photophysics and morphology are compared and effects on film formation and charge transport in bulk-heterojunction blends demonstrated on photovoltaic devices utilizing F8T2s as donor and the fullerene derivative ICBA as acceptor material.