title: Effect of copper oxide oxidation state on the polymer-based solar cell buffer layers.
abstract: Transporting buffer layers are important components of polymer-based organic photovoltaic devices. In this study, we have investigated the effects of the oxidation state in copper oxide based buffer layer in conjunction to its role in device performance. We have shown that variation in the oxidation state affects the band alignment and built-in voltage of the device, therefore leading to variation in device performance. Specifically, the fully oxidized copper oxide buffer layer has a valence band position at 5.12 eV, much closer to the highest occupied molecular orbital of poly(3-hexylthiophene-2,5-diyl) (P3HT) (∼5.2 eV), giving a best fill factor and efficiency at 57% and 4.06%, respectively. Lastly, we also demonstrate significant enhancement in device stability, with power conversion efficiency maintained at 75% of the original value even after 40 days, and propose a strategy for recovering the device performance based on the observed property of the oxide buffer layer.
authors.affiliation: Center for Condensed Matter Sciences, National Taiwan University , Taipei 10617, Taiwan, R.O.C.
authors.forename: Hsiang-Ting
authors.initials: HT
authors.lastname: Lien
authors.forename: Deniz P
authors.initials: DP
authors.lastname: Wong
authors.forename: Nai-Hung
authors.initials: NH
authors.lastname: Tsao
authors.forename: Ching-I
authors.initials: CI
authors.lastname: Huang
authors.forename: Chaochin
authors.initials: C
authors.lastname: Su
authors.forename: Kuei-Hsien
authors.initials: KH
authors.lastname: Chen
authors.forename: Li-Chyong
authors.initials: LC
authors.lastname: Chen
keywords: X-ray absorption; buffer layer; copper oxide; photovoltaics