title: Direct Free Carrier Photogeneration in Single Layer and Stacked Organic Photovoltaic Devices.
abstract: High performance organic photovoltaic devices typically rely on type-II P/N junctions for assisting exciton dissociation. Heremans and co-workers recently reported a high efficiency device with a third organic layer which is spatially separated from the active P/N junction; but still contributes to the carrier generation by passing its energy to the P/N junction via a long-range exciton energy transfer mechanism. In this study the authors show that there is an additional mechanism contributing to the high efficiency. Some bipolar materials (e.g., subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc)) are observed to generate free carriers much more effectively than typical organic semiconductors upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than those of reported single-layer devices. In addition, internal quantum efficiencies (IQEs) of bilayer devices with opposite stacking sequences (i.e., SubNc/SubPc vs SubPc/SubNc) are found to be the sum of IQEs of single layer devices. These results confirm that SubNc and SubPc can directly generate free carriers upon photoexcitation without assistance from a P/N junction. These allow them to be stacked onto each other with reversible sequence or simply stacking onto another P/N junction and contribute to the photocarrier generation.
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Hrisheekesh Thachoth
authors.initials: HT
authors.lastname: Chandran
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.|City University of Hong Kong Shenzhen Research Institute, Shenzhen, 518000, P. R. China.
authors.forename: Tsz-Wai
authors.initials: TW
authors.lastname: Ng
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Sciences, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Yishu
authors.initials: Y
authors.lastname: Foo
authors.affiliation: Department of Physics and Material Sciences, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Ho-Wa
authors.initials: HW
authors.lastname: Li
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Jian
authors.initials: J
authors.lastname: Qing
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Xiao-Ke
authors.initials: XK
authors.lastname: Liu
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Chiu-Yee
authors.initials: CY
authors.lastname: Chan
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Fu-Lung
authors.initials: FL
authors.lastname: Wong
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Sciences, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Juan Antonio
authors.initials: JA
authors.lastname: Zapien
authors.affiliation: Department of Physics and Material Sciences, City University of Hong Kong, Hong Kong SAR, P. R. China.
authors.forename: Sai-Wing
authors.initials: SW
authors.lastname: Tsang
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.|City University of Hong Kong Shenzhen Research Institute, Shenzhen, 518000, P. R. China.
authors.forename: Ming-Fai
authors.initials: MF
authors.lastname: Lo
authors.affiliation: Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.|City University of Hong Kong Shenzhen Research Institute, Shenzhen, 518000, P. R. China.
authors.forename: Chun-Sing
authors.initials: CS
authors.lastname: Lee
keywords: free carrier generation; organic photovoltaics; single-layer device