Abstract
Achieving high performance and reliable organic solar cells hinges on the development of stable and energetically suitable hole transporting buffer layers in tune with the electrode and photoactive materials of the solar cell stack. Here we have identified solution-processed copper(I) iodide (Cul) thin films with low-temperature processing conditions as an effective hole-transporting layer (HTL) for a wide range of polymer:fullerene bulk heterojunction (BHJ) systems. The solar cells using Cul NIL show higher power conversion efficiency (PCE) in standard device structure for polymer blends, up to PCE of 8.8%, as compared with poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL, for a broad range of polymerfullerene systems. The Cul layer properties and solar cell device behavior are shown to be remarkably robust and insensitive to a wide range of processing conditions of the NIL, including processing solvent, annealing temperature (room temperature up to 200 degrees C), and film thickness. Cul is also shown to improve the overall lifetime of solar cells in the standard architecture as compared to PEDOT:PSS. We further demonstrate promising solar cell performance when using Cul as top HTL in inverted device architecture. The observation of uncommon properties, such as photoconductivity of Cul and templating effects on the BHJ layer formation, is also discussed. This study points to Cul as being a good candidate to replace PEDOT:PSS in solution-processed solar cells thanks to the facile implementation and demonstrated robustness of Cul thin films. (C) 2015 Elsevier Ltd. All rights reserved.