High-performance light-soaking-free polymer solar cells based on a LiF modified ZnO electron extraction layer

Abstract

In this work, we investigated the light-soaking effect in inverted polymer solar cells (PSCs) employing ZnO as an electron extraction layer (EEL) and demonstrated highly efficient light-soaking-free PSCs with a LiF modified ZnO (ZnO:LiF) EEL. A blend of poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) was applied as the active layer. The PSCs fabricated with the LiF doped ZnO EEL showed a 21.2% increase in power conversion efficiency (PCE) from 3.35% to 4.06% as compared with the pristine ZnO-based devices. In contrast to the devices with a lithium chloride (LiCl) or lithium bromide (LiBr) modified ZnO EEL, the ZnO:LiF-based PSC devices show excellent light-soaking-free characteristics with high fill factor (FF) and short circuit current density (J_sc). More importantly, the photovoltaic devices employing a poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) blend system as the active layer have been also fabricated, which achieved a high PCE of 8.85%. Our results indicate that the inclusion of additional LiF made for reduced oxygen vacancy defects at the metal-oxide/organic interface, which were confirmed by X-ray photoelectron spectroscopy and charge extraction analysis. It is shown that LiF modified ZnO as an efficient and cost-effective EEL material has the potential to open a new avenue for high performance optoelectronic devices.