5-nm LiF as an Efficient Cathode Buffer Layer in Polymer Solar Cells Through Simply Introducing a C60 Interlayer

Xiaodong Liu,Yonghao Zheng,L Jay Guo

doi:10.1186/s11671-017-2299-y

Abstract

Lithium fluoride (LiF) is an efficient and widely used cathode buffer layer (CBL) in bulk heterojunction polymer solar cells (PSCs). The LiF thickness is normally limited to 1 nm due to its insulting property. Such small thickness is difficult to precise control during thermal deposition, and more importantly, 1-nm-thick LiF cannot provide sufficient protection for the underlying active layer. Herein, we demonstrated the application of a very thick LiF as CBL without sacrificing the device efficiency by simply inserting a C60 layer between the active layer and LiF layer. The devices with the C60/LiF (5 nm) double CBLs exhibit a peak power conversion efficiency (PCE) of 3.65%, which is twofold higher than that (1.79%) of LiF (5 nm)-only device. The superior performance of the C60/LiF (5 nm)-based devices is mainly attributed to the good electrical conductivity of the C60/LiF (5 nm) bilayer, arising from the intermixing occurred at the C60/LiF interface. Besides, the formation of a P3HT/C60 subcell and the optical spacer effect of C60 also contribute to the increase in short-circuit current density (Jsc) of the device. With further increase of LiF thickness to 8 nm, a PCE of 1.10% is attained for the C60/LiF-based device, while the negligible photovoltaic performance is observed for the LiF-only device. All in all, our results show that C60/LiF bilayer is a promising alternative to LiF single layer due to its high tolerance to the LiF thickness variations.

Highlights

Solution-processed bulk heterojunction polymer solar cells (PSCs) have received increasing attention in recent decades because of their potential advantages such as low cost, light weight, and possibility to fabricate large-scale, flexible, and semitransparent devices [1,2,3,4,5]
In summary, we have demonstrated that a thick lithium fluoride (LiF) can be used as cathode buffer layer (CBL) in P3HT:PCBM-based PSCs by introducing a C60 layer between the active layer and the LiF layer
The improved device performance mainly results from the high fill factor (FF) due to the good electrical conductivity of the C60/LiF bilayer

Summary

Introduction

Solution-processed bulk heterojunction polymer solar cells (PSCs) have received increasing attention in recent decades because of their potential advantages such as low cost, light weight, and possibility to fabricate large-scale, flexible, and semitransparent devices [1,2,3,4,5]. Ca is oxidized when exposed to air, resulting in the poor stability of the devices. Another widely used CBL in PSCs is lithium fluoride (LiF), which has been demonstrated to enhance the device performance through the formation of an interfacial dipole at the cathode interface [22]. The thickness of LiF is limited to less than 2 nm (generally ~ 1 nm) due to its insulating property [23, 24]. Such a small thickness is very difficult to be controlled via thermal deposition.

Methods

Results

Conclusion