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.
Highlights
Despite of the disadvantages described above, progresses towards ultrafast spectroscopic characterizations of neat polymer films have evidenced simultaneous ultrafast generation of excitons and polarons upon photoexcitation[5,6,7]
Coherent band-like carrier transport has been validated by experimental observations, such as mobility increasing with decreasing temperature[11], free hole absorption based on the Drude model[12] and Hall effects[13,14], in small-molecular organic thin films, where the carriers were accumulated through electrode injection or heterojunction effect
Two kinds of zinc phthalocyanine (ZnPc) films were prepared, one by direct vacuum deposition, while the other by applying weak epitaxy growth (WEG) method in vacuum deposition. Both films were determined to be in the α -phase by X-ray diffraction (XRD) and selected area electron diffraction (SAED)[22]
Summary
Xiaochuan He1, Gangbei Zhu[1], Jianbing Yang[2], Hao Chang[2], Qingyu Meng[3], Hongwu Zhao[3], Xin Zhou[4], Shuai Yue[1], Zhuan Wang[1], Jinan Shi[5], Lin Gu5, Donghang Yan2 & Yuxiang Weng[1]. 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. Our results suggest that delocalized free carriers photogeneration can be achieved in organic semiconductors when the molecules are packed properly Organic semiconductors, such as π -conjugated polymers and small-molecule crystals, are promising materials for future applications ranging from flexible electronics to photovoltaics[1,2]. To address the above issue, noncontact steady-state and ultrafast time-resolved spectroscopic methods were employed to characterize the intrinsic photoexcitation properties of a typical highly crystalline small-molecule organic semiconductor, i.e. zinc phthalocyanine (ZnPc) films prepared by weak epitaxy growth (WEG) method. While time-resolved THz spectroscopy measurements of the ZnPc/C60 blending and multilayer films show significant THz absorption attributed to charge generation by the dissociation of excitons at the ZnPc/C60 heterojunction interface, the time-resolved THz signal for vacuum deposited ZnPc films are negligible, which suggests negligible photogenerated free carriers in vacuum deposited ZnPc films[19,20,21]
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