Hydrothermally grown ZnO electrodes for improved organic photovoltaic devices

P Steiger,J Zhang,K Harrabi,I.A Hussein,J.M Downing,M.A Mclachlan

doi:10.1016/j.tsf.2017.11.021

Abstract

Here we report a simple, solution based processing route for the formation of large surface area electrodes resulting in improved organic photovoltaic devices when compared with conventional planar electrodes. The nanostructured electrode arrays are formed using hydrothermally grown ZnO nanorods, subsequently infiltrated with blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (IC60BA) as photoactive materials. This well studied organic photoactive blend allows the composition/processing/performance relationships to be elucidated. Using simple solution based processing the resultant nanostructured devices exhibited a maximum power conversion efficiency (PCE) of 2.5% compared with the best planar analogues having a PCE of around 1%. We provide detailed structural, optical and electrical characterization of the nanorod arrays, active layers and completed devices giving an insight into the influence of composition and processing on performance. Devices were fabricated in the desirable inverse geometry, allowing oxidation resistant high work-function top electrodes to be used and importantly to support the hydrothermal growth of nanorods on the bottom electrode — all processing was carried out under ambient conditions and without the insertion of a hole transport layer below the anode. The nanorods were successfully filled with the active layer materials by carrying out a brief melt processing of a spin-cast top layer followed by a subsequent thermal anneal which was identified as an essential step for the fabrication of operational devices. The growth method used for nanorod fabrication and the active layer processing are both inherently scalable, thus we present a complete and facile route for the formation of nanostructured electron acceptor layers that are suitable for high performance organic active layers.

Highlights

Organic photovoltaics (OPVs) are routinely reported with power conversion efficiencies (PCEs) exceeding 10% [1,2,3], with recent devices showing continued improvements in power conversion efficiency (PCE) to report values above 13% in devices combining polymer absorbers with non-fullerene acceptor species [4]
Unlike previous reports where multistep patterning was required or where non-standard conjugated materials utilized [23] here we describe the preparation of bulk heterojunction photovoltaic devices using the commonly used, widely available active materials poly(3hexylthiophene-2,5-diyl) (P3HT) — indene-C60 bisadduct (IC60BA) using planar and nanostructured ZnO electron extracting layers and report on the device improvements enabled by incorporating nanorod electrodes and the impact these electrodes have on the optimum active layer composition
We have demonstrated the effective implementation of nanostructured ZnO cathodes into organic bulk heterojunction photovoltaic devices using a solution-processing route for electrode fabrication followed by a simple infiltration process using commercially available materials

Summary

Introduction

Organic photovoltaics (OPVs) are routinely reported with power conversion efficiencies (PCEs) exceeding 10% [1,2,3], with recent devices showing continued improvements in PCE to report values above 13% in devices combining polymer absorbers with non-fullerene acceptor species [4] This rapid acceleration of performance has been enabled through a combination of advances in synthesis and the development of new active layer materials, improvements in charge transport and through a greater understanding of the required device morphology and microstructure. While strategies to prepare hybrid photovoltaics i.e. where the oxide acts as both an active material and the electron transport layer have had limited success [19,20,21,22] the incorporation of a nanostructured electron acceptor here, using simple device structures processed in ambient, is shown to improve performance. Unlike previous reports where multistep patterning was required or where non-standard conjugated materials utilized [23] here we describe the preparation of bulk heterojunction photovoltaic devices using the commonly used, widely available active materials poly(3hexylthiophene-2,5-diyl) (P3HT) — indene-C60 bisadduct (IC60BA) using planar and nanostructured ZnO electron extracting layers and report on the device improvements enabled by incorporating nanorod electrodes and the impact these electrodes have on the optimum active layer composition

Experimental details

Results & discussion

Conclusions