An n-Doped Organic Cross-Linked Electron Transport Layer with High Electrical Conductivity for High-Efficiency Tandem Organic Photovoltaics

Abstract

With merits of good solution processability, intrinsic flexibility, etc, organic/organic interconnecting layers (ICLs) are highly desirable for tandem organic photovoltaics (OPVs). Herein, an n-doped cross-linked organic electron transport layer (ETL), named c-NDI-Br:PEI is developed, via a simple in situ quaternization reaction between bromopentyl-substituted naphthalene diimide derivative (NDI-Br) and polyethylenimine (PEI). Due to strong self-doping, c-NDI-Br:PEI films exhibit a high electrical conductivity (0.06 S cm^-1), which is important for efficient hole and electron reombination in ICL of tandem OPVs. In addition, the cross-linked ETLs show strong work function modulation ability, and good solvent-resistance. The above features enable c-NDI-Br:PEI to function as an efficient ETL not only for single-junction OPVs, but also for tandem devices without any metal layer in ICL. Under solar radiation, the single-junction device with c-NDI-Br:PEI as ETL achieves a power conversion efficiency (PCE) of 18.18%, surpassing the ZnO-based device (17.09%). The homo- and hetero-tandem devices with m-PEDOT:PSS:c-NDI-Br:PEI as ICL exhibit remarkable PCEs of 19.06% and 20.06%, respectively. Under 808 nm laser radiation with a photon flux of 57 mW cm^-2, the homo-tandem device presents a superior PCE of 38.5%. This study provides a new ETL for constructing all-solution-processed organic/organic ICL, which can be integrated in flexible and wearable devices.

Keywords: electrical conductivity; electron transport layer; power conversion efficiency; tandem organic photovoltaics; work function.