title: Photogenerated Intrinsic Free Carriers in Small-molecule Organic Semiconductors Visualized by Ultrafast Spectroscopy.
abstract: Confirmation of direct photogeneration of intrinsic delocalized free carriers in small-molecule organic semiconductors has been a long-sought but unsolved issue, which is of fundamental significance to its application in photo-electric devices. Although the excitonic description of photoexcitation in these materials has been widely accepted, this concept is challenged by recently reported phenomena. Here we report observation of direct delocalized free carrier generation upon interband photoexcitation in highly crystalline zinc phthalocyanine films prepared by the weak epitaxy growth method using ultrafast spectroscopy. Transient absorption spectra spanning the visible to mid-infrared region revealed the existence of short-lived free electrons and holes with a diffusion length estimated to cross at least 11 molecules along the π-π stacking direction that subsequently localize to form charge transfer excitons. The interband transition was evidenced by ultraviolet-visible absorption, photoluminescence and electroluminescence spectroscopy. Our results suggest that delocalized free carriers photogeneration can also be achieved in organic semiconductors when the molecules are packed properly.
authors.affiliation: Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences (CAS), Beijing 100190, China.
authors.forename: Xiaochuan
authors.initials: X
authors.lastname: He
authors.affiliation: Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences (CAS), Beijing 100190, China.
authors.forename: Gangbei
authors.initials: G
authors.lastname: Zhu
authors.affiliation: State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China.
authors.forename: Jianbing
authors.initials: J
authors.lastname: Yang
authors.affiliation: State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China.
authors.forename: Hao
authors.initials: H
authors.lastname: Chang
authors.affiliation: Laboratory of Solid State Quantum Information and Quantum Computation, Institute of Physics, CAS, Beijing 100190, China.
authors.forename: Qingyu
authors.initials: Q
authors.lastname: Meng
authors.affiliation: Laboratory of Solid State Quantum Information and Quantum Computation, Institute of Physics, CAS, Beijing 100190, China.
authors.forename: Hongwu
authors.initials: H
authors.lastname: Zhao
authors.affiliation: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, CAS, Dalian 116023, China.
authors.forename: Xin
authors.initials: X
authors.lastname: Zhou
authors.affiliation: Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences (CAS), Beijing 100190, China.
authors.forename: Shuai
authors.initials: S
authors.lastname: Yue
authors.affiliation: Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences (CAS), Beijing 100190, China.
authors.forename: Zhuan
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Laboratory of Advanced Materials &Electron Microscopy, Institute of Physics, CAS, Beijing 100190, China.
authors.forename: Jinan
authors.initials: J
authors.lastname: Shi
authors.affiliation: Laboratory of Advanced Materials &Electron Microscopy, Institute of Physics, CAS, Beijing 100190, China.
authors.forename: Lin
authors.initials: L
authors.lastname: Gu
authors.affiliation: State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China.
authors.forename: Donghang
authors.initials: D
authors.lastname: Yan
authors.affiliation: Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences (CAS), Beijing 100190, China.
authors.forename: Yuxiang
authors.initials: Y
authors.lastname: Weng