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Acta Phys. -Chim. Sin.  2017, Vol. 33 Issue (10): 1934-1943    DOI: 10.3866/PKU.WHXB201715185
FEATURE ARTICLE     
Visualization of Energy Band Alignment in Thin-Film Optoelectronic Devices with Scanning Kelvin Probe Microscopy
Ji-Chong LIU1,2,Feng TANG1,2,Feng-Ye YE1,3,Qi CHEN1,*(),Li-Wei CHEN1,*()
1 i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, Jiangsu Province, P. R. China
2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3 Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China
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Abstract  

Understanding the energy band alignment across multiple layers in thin-film optoelectronic devices is extremely important because it governs elementary optoelectronic processes, such as charge carrier generation, separation, transport, recombination and collection. This monograph summarizes recent progress in visualization of energy band alignment in thin-film optoelectronic devices, such as organic solar cells (OSCs) and organic-inorganic perovskite photodetectors from our group by using scanning Kelvin probe microscopy (SKPM). Since active layers are enclosed by the top and bottom electrodes in vertically stacked devices, it is highly challenging to study the energy band alignment under operando conditions. Thus, cross-sectional SKPM has been developed to resolve this challenge. The results demonstrated that the interlayer was one of the most important factors for adjusting energy band alignment, determining device polarity and improving device performance. The characterization methods described in this monograph are poised to be widely applied to research in various thin-film optoelectronic devices, such as photovoltaic devices, photodetectors and light-emitting diodes (LEDs), especially those devices with tandem structures.



Key wordsScanning Kelvin probe microscopy      Energy band alignment      Cross-section      Interlayer Organic solar cells      Organic-inorganic perovskite photodetectors     
Received: 13 April 2017      Published: 18 May 2017
MSC2000:  O647  
Fund:  the National Natural Science Foundation of China(21625304);the National Natural Science Foundation of China(51473184);the National Natural Science Foundation of China(11504408);Ministry of Science and Technology of China(2016YFA0200703);the CAS Research Equipment Development Program(YZ201654)
Corresponding Authors: Qi CHEN,Li-Wei CHEN     E-mail: qchen2011@sinano.ac.cn;lwchen2008@sinano.ac.cn
Cite this article:

Ji-Chong LIU,Feng TANG,Feng-Ye YE,Qi CHEN,Li-Wei CHEN. Visualization of Energy Band Alignment in Thin-Film Optoelectronic Devices with Scanning Kelvin Probe Microscopy. Acta Phys. -Chim. Sin., 2017, 33(10): 1934-1943.

URL:

http://www.whxb.pku.edu.cn/10.3866/PKU.WHXB201715185     OR     http://www.whxb.pku.edu.cn/Y2017/V33/I10/1934

Fig 1 Schematic illustration of scanning Kelvin probe microscopy (SKPM).
Fig 2 PFN interlayer/active layer interface probed by SKPM 19. (a, b) Topographic image (a) and surface potential image (b) of device active layer area partially covered with the PFN (scale bar: 10 μm); (c) schematic illustration of the SKPM setup; (d) horizontal profiles of the topographic (a) and surface potential (b) images; (e, f) energy band diagram of the devices without (e) and with (f) the interlayer under light illumination in short-circuit condition.
Fig 3 Effect of K2CO3 interlayer on tuning ITO work function probed by SKPM 26. (a–d) Surface potential images of ITO modified with K2CO3 at different solution concentrations from 0–12 mmol?L?1 (scale bar: 1 μm). (e, f) energy band alignment between active layer and ITO without (e) and with (f) K2CO3 modification.
Fig 4 PMMA : PCBM Mixed interlayer measured by SKPM 34. (a–d) Surface potential images of PCBM mixed with different ratio PMMA from 0%-25% (scale bar: 1 μm); (e, f) energy band diagram of the devices in dark before (e) and after (f) PCBM mixed with PMMA.
Fig 5 Organic solar cell cross-section preparation and measurement 40. (a) Schematic illustration of device cross-section prepared by ion-milling; (b) schematic illustration of device cross-section measured by SKPM; (c) scanning electron microscopy image of device cross-section with structure of ITO/MoOx/P3HT:PCBM/LiF/Al (scale bar: 250 nm); (d, e) topography image (d) and phase image (e) of the device cross-section acquired by atomic force microscopy (scale bar: 200 nm).
Fig 6 Visualization of energy band alignment in conventional structure OSC by cross-sectional SKPM 40. (a, b) Surface potential images and the extracted profiles (c) of device with structure of ITO/MoOx/P3HT:PCBM/LiF/Al in open-circuit condition in dark (a) and under AM 1.5G illumination (b) (scale bar: 250 nm); (d–f) energy band diagram under different condition: VL alignment of device before contact (d); Fermi-level alignment after contact in the dark (e) and quasi-Fermi level splitting under illumination (f).
Fig 7 Quantification of energy level offset in conventional structure OSCs by bias compensation method 40. (a) Schematic illustration of the tip-induced averaging effect in SKPM measurements; (b) an illustration of the measured SP profile resulted from the convolution of the true profile with the transfer function of the SKPM tip; (c) surface potential profiles of device with structure of ITO/MoOx/P3HT:PCBM/LiF/Al in open-circuit condition in dark, under AM 1.5G illumination and at bias voltage that equals to Voc; (d) surface potential profiles of device under +0.8, +1.0 and +1.3 V bias voltages; (e–g) energy band diagram under different bias voltage: bias voltage that equals to Voc (e), active layer flat-band condition (f), cathode-anode VL alignment condition (g).
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