title: Solution-processed zinc oxide/polyethylenimine nanocomposites as tunable electron transport layers for highly efficient bulk heterojunction polymer solar cells.
abstract: In this study, we employed polyethylenimine-doped sol-gel-processed zinc oxide composites (ZnO:PEI) as efficient electron transport layers (ETL) for facilitating electron extraction in inverted polymer solar cells. Using ultraviolet photoelectron spectroscopy, synchrotron grazing-incidence small-angle X-ray scattering and transmission electron microscopy, we observed that ZnO:PEI composite films' energy bands could be tuned considerably by varying the content of PEI up to 7 wt %-the conduction band ranged from 4.32 to 4.0 eV-and the structural order of ZnO in the ZnO:PEI thin films would be enhanced to align perpendicular to the ITO electrode, particularly at 7 wt % PEI, facilitating electron transport vertically. We then prepared two types of bulk heterojunction systems-based on poly(3-hexylthiophene) (P3HT):phenyl-C61-butryric acid methyl ester (PC61BM) and benzo[1,2-b:4,5-b́]dithiophene-thiophene-2,1,3-benzooxadiazole (PBDTTBO):phenyl-C71-butryric acid methyl ester (PC71BM)-that incorporated the ZnO:PEI composite layers. When using a composite of ZnO:PEI (93:7, w/w) as the ETL, the power conversion efficiency (PCE) of the P3HT:PC61BM (1:1, w/w) device improved to 4.6% from a value of 3.7% for the corresponding device that incorporated pristine ZnO as the ETL-a relative increase of 24%. For the PBDTTBO:PC71BM (1:2, w/w) device featuring the same amount of PEI blended in the ETL, the PCE improved to 8.7% from a value of 7.3% for the corresponding device that featured pure ZnO as its ETL-a relative increase of 20%. Accordingly, ZnO:PEI composites can be effective ETLs within organic photovoltaics.
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan.
authors.forename: Hsiu-Cheng
authors.initials: HC
authors.lastname: Chen
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan.
authors.forename: Shu-Wei
authors.initials: SW
authors.lastname: Lin
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan.
authors.forename: Jian-Ming
authors.initials: JM
authors.lastname: Jiang
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan.
authors.forename: Yu-Wei
authors.initials: YW
authors.lastname: Su
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan.
authors.forename: Kung-Hwa
authors.initials: KH
authors.lastname: Wei
keywords: conduction band; electron transport layer; grazing-incidence small-angle X-ray scattering; photovoltaics; zinc oxide: polyethylenimine nanocomposites