Inverted organic solar cells with non-clustering bathocuproine (BCP) cathode interlayers obtained by fullerene doping

Fatemeh Jafari,Fatemeh Mohtaram,Bhushan R Patil,André L Fernandes Cauduro,Morten Madsen,Abbas Behjat,Horst-Günter Rubahn

doi:10.1038/s41598-019-46854-w

Abstract

Bathocuproine (BCP) is a well-studied cathode interlayer in organic photovoltaic (OPV) devices, where it for standard device configurations has demonstrated improved electron extraction as well as exciton blocking properties, leading to high device efficiencies. For inverted devices, however, BCP interlayers has shown to lead to device failure, mainly due to the clustering of BCP molecules on indium tin oxide (ITO) surfaces, which is a significant problem during scale-up of the OPV devices. In this work, we introduce C70 doped BCP thin films as cathode interlayers in inverted OPV devices. We demonstrate that the interlayer forms smooth films on ITO surfaces, resulting from the introduction of C70 molecules into the BCP film, and that these films possess both improved electron extraction as well exciton blocking properties, as evidenced by electron-only devices and photoluminescence studies, respectively. Importantly, the improved cathode interlayers leads to well-functioning large area (100 mm2) devices, showing a device yield of 100%. This is in strong contrast to inverted devices based on pure BCP layers. These results are founded by the effective suppression of BCP clustering from C70, along with the electron transport and exciton blocking properties of the two materials, which thus presents a route for its integration as an interlayer material towards up-scaled inverted OPV devices.

Highlights

Organic photovoltaic (OPV) devices based on small molecules and polymers have attracted a great interest in recent decades due to their appealing properties being eco-friendly, potentially low-cost, lightweight, mechanically flexible and semi-transparent[1,2,3,4]
It was observed that while scaling up the organic photovoltaic (OPV) device area, the performance and device yield of the inverted OPV devices decrease significantly compared to standard configuration cells, which was demonstrated to be due to the clustering of BCP on indium tin oxide (ITO) surfaces[33,34]
Incorporation of C70 molecules into the BCP layer suppresses clustering of the BCP molecules, resulting in smooth layers on ITO surfaces, a prerequisite for using them as efficient ETL and exciton blocking (EBL) in inverted OPV device configurations

Summary

Introduction

Organic photovoltaic (OPV) devices based on small molecules and polymers have attracted a great interest in recent decades due to their appealing properties being eco-friendly, potentially low-cost, lightweight, mechanically flexible and semi-transparent[1,2,3,4]. This potentially results in electrical shunting of the inverted OPV devices, which dramatically decrease the device yield for up-scaled cells In recent work, this has been demonstrated to lead to faster degradation of inverted OPV devices based on pure BCP ETL and EBL layers[35]. The integration of Ag doped BCP layers in inverted OPVs as buffer layers has previously been reported on[36,37] These layers provide improved electrical properties, Ag doped BCP may lead to unwanted exciton quenching processes in the fullerene acceptor layer and deteriorate the device performance[37]. Such quenching processes between metals and adsorbed molecules are well-know[38]

Methods

Results

Conclusion