A real-time study of the benefits of co-solvents in polymer solar cell processing.

Jacobus J Van Franeker,Weiwei Li,Martijn M Wienk,Mathieu Turbiez,René A.J Janssen

doi:10.1038/ncomms7229

Abstract

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors. Controlling the degree of phase separation between these components is crucial to reach efficient solar cells. In solution-processed polymer-fullerene solar cells, small amounts of co-solvents are commonly used to avoid the formation of undesired large fullerene domains that reduce performance. There is an ongoing discussion about the origin of this effect. To clarify the role of co-solvents, we combine three optical measurements to investigate layer thickness, phase separation and polymer aggregation in real time during solvent evaporation under realistic processing conditions. Without co-solvent, large fullerene-rich domains form via liquid-liquid phase separation at ~20 vol% solid content. Under such supersaturated conditions, co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains. This rationalizes the formation of intimately mixed films that give high-efficient solar cells for the materials studied.

Highlights

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors
A prominent example is PTB7 that provides a well mixed blend and power conversion efficiencies (PCEs) 1⁄4 9.2% when processed with co-solvents16, but forms large (4100 nm) fullerene-rich domains and correspondingly low PCEs when processed from a single solvent17–19
We show for the first time that without the co-solvent, polymer aggregation occurs after large-scale liquid–liquid phase separation

Summary

Introduction

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors. To clarify the role of co-solvents, we combine three optical measurements to investigate layer thickness, phase separation and polymer aggregation in real time during solvent evaporation under realistic processing conditions. Without co-solvent, large fullerene-rich domains form via liquid–liquid phase separation at B20 vol% solid content Under such supersaturated conditions, co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains. Co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains This rationalizes the formation of intimately mixed films that give high-efficient solar cells for the materials studied. The origin of the decrease of domain size when using a cosolvent has often been attributed to polymer aggregation, either in the casting solution or during solvent evaporation. Enough co-solvent has to be added to ensure that the onset of polymer aggregation occurs before largescale phase separation can occur

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