Imaging Electron Motion in Organic Semiconductors Using Femtosecond Photoemission Electron Microscopy

Electronic and optoelectronic devices, solar cells, LED, and transistor and so on, work by the motion of charge carriers. In particular, the carrier transport properties at interfaces between two materials play a role for device performance. To clarify the properties at the interface (e.g. organic heterointerface), we have performed femtosecond photoemission electron microscopy (fs-PEEM) experiments to image the dynamics of conducting electrons with time, space, and energy resolution [1, 2]. In this contribution, we will present our recent achievements on investigating organic semiconductor materials and devices using fs-PEEM.<br/>A first example is a result of tracing the electron motion across an organic heterostructure composed of pentacene and fullerene, which is a typical structure of organic solar cells. Recombination times of photogenerated electrons in pentacene and in fullerene were individually estimated. Furthermore, the electron motion across the interface was visualized. The estimated transporting time across the interface was as short as 8 ps [3].<br/>Organic light emitting diode (OLED) using thermally activated delayed fluorescence is a candidate of next generation high efficiency OLED. However, the efficiency degradation occurs when it is in the solid states. Mount of loss was estimated by comparing the recombination dynamics of photogenerated electrons measured by fs-PEEM with one by time-resolved photo luminescence experiments.<br/>Finally, we are going to present operando observation of organic transistors. Antiambipolar transistors (AAT) with high on/off current ratio at room temperature has been demonstrated by fabricating defect-free and direct-contact organic heterointerfaces [5,6]. The AAT was consisted of a partially stacked p- (α-6T) and n-type (PTCDI) layers at around the middle of the transistor channel, source and drain electrodes on opposite side of the channel, respectively, and a gate electrode on the back side. Although, the AAT shown Λ-shape I-V curves, the mechanism of charge transport is not clear. Operando observation of AAT was performed by taking PEEM images by sweeping the gate voltage, while constant source-drain voltage was applied. We visualized that influence of depletion layer created at the interface works as a valve for the current.<br/>References:<br/>[1] Keiki Fukumoto, Yuki Yamada, Ken Onda, and Shin-ya Koshihara, Appl. Phys. Lett. 104, 053117 (2014).<br/>[2] Shin-ya Koshihara, and Keiki Fukumoto, WO2018/159272, published on 2018.09.07.<br/>[3] Masato Iwasawa, Ryohei Tsuruta, Yasuo Nakayama, Masahiro Sasaki, Takuya Hosokai, Sunghee Lee, Keiki Fukumoto, Yoichi Yamada, J. Phys. Chem. C 124, 13572 (2020).<br/>[4] Yusuke Fukami, Masato Iwasawa, Masahiro Sasaki, Takuya Hosokai, Hajime Nakanotani, Chihaya Adachi, Keiki Fukumoto and Yoichi Yamada, Adv. Opt. Mater. 2100619 (2021).<br/>[5] Yutaka Wakayama, and Ryoma Hayakawa, Avd. Func. Mater. 30, 1903724 (2020).<br/>[6] Kazuyoshi Kobashi, Ryoma Hayakawa, Toyohiro Chikyow, and Yutaka Wakayama, Nanoletters 18, 4355 (2018).