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Influence on the Performance of Cupc/C60 Solar Cells Author(S) View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by HKU Scholars Hub Indium-tin-oxide surface treatments: Influence on the Title performance of CuPc/C60 solar cells Author(s) Djuriši, AB; Kwong, CY; Chui, PC; Chan, WK Citation Journal Of Applied Physics, 2003, v. 93 n. 9, p. 5472-5479 Issued Date 2003 URL http://hdl.handle.net/10722/42357 Rights Creative Commons: Attribution 3.0 Hong Kong License JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 9 1 MAY 2003 Indium–tin–oxide surface treatments: Influence on the performance Õ of CuPc C60 solar cells A. B. Djurisˇic´a) Department of Electrical and Electronic Engineering and Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong C. Y. Kwong and P. C. Chui Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong W. K. Chan Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong ͑Received 2 October 2002; accepted 14 February 2003͒ In this work, we investigate the influence of different indium tin oxide ͑ITO͒ surface treatments on the performance of organic solar cells. ITO substrates have been characterized by Hall measurements, Seebeck coefficient measurements, surface sheet resistance measurements, and surface probe microscopy. Single layer ͑ITO/copper phthalocyanine ͑CuPc͒/Al͒ and double layer (ITO/CuPc/C60 /Al) solar cells were fabricated. It was found that the surface treatments changed the parameters of the ITO ͑work function, carrier concentration, sheet resistance, surface roughness͒ and significantly influenced the solar cell performance. The AM1 power conversion efficiency of the ϳ ITO/CuPc/C60 /Al cell with optimal surface treatment ( 0.1%) is 1 order of magnitude larger than the power conversion efficiency of the solar cell fabricated on untreated ITO substrate (ϳ0.01%). The AM1 power conversion efficiency can be further enhanced with improved device structures. Obtained AM1 power conversion efficiency for a three layer structure ITO/CuPc/CuPc:C60 (1:1)/C60 /Al was measured to be 0.16%. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1565824͔ I. INTRODUCTION oxygen plasma followed by aquaregia.1 The best OLED per- formance in their work was obtained with oxygen plasma Indium–tin–oxide ͑ITO͒ is frequently used as an elec- treatment.1–4 The proposed reason for the observed improve- trode in flat panel displays, solar cells, and organic light ments in the device performance with oxygen plasma treat- emitting diodes ͑OLEDs͒ due to its high conductivity and transparency in the visible spectral region. ITO is a highly ments in their work is increased work function and hence degenerate n type semiconductor with a wide band gap and improved hole injection. It was also reported that plasma relatively high work function. The effect of various surface treatments remove an insulating overlayer from ITO treatments ͓plasma, chemical, ultraviolet ͑UV͒ ozone, etc.͔ to surface.14 This assumption was confirmed by conducting the ITO properties and the OLED performance was exten- atomic force microscopy study of local conductance of ITO sively studied.1–22 Surface treatments have an effect on ITO films before and after oxygen plasma treatment.23 The thin parameters such as the work function, surface roughness, insulating layer is most likely organic hydrocarbons.23 Local carrier concentration, mobility, and surface sheet resistance, variations in surface potential on ITO surface were also so that with appropriate surface treatment significant im- reported.24 This observation can possibly explain the ob- provement in the OLED performance can be achieved. Kim served influence of the ITO morphology to the performance et al.1–4 studied the influence of the oxygen plasma, aquar- of organic devices. It was found that the devices fabricated egia, and combinations of these two treatments to the param- on ITO substrates with similar work functions but different eters of the ITO surface. They have found that the oxygen plasma treatment results in increased work function,1 in- surface morphology exhibit markedly different 16,17 creased carrier concentration and slightly decreased mobility performance. The lowest turn on voltage was obtained resulting in overall decrease in sheet resistance,1–4 and de- with ITO exhibiting granular structure with very rough creased surface roughness.1 On the other hand, aquaregia surface.16,17 Contrary to that result, improvement with me- treatment and combination treatments including aquaregia chanical polishing of ITO which resulted in smoother surface produced only slight increase in the work function,1 increase was reported.19 The relationship between oxidative surface in the sheet resistance due to decrease in carrier treatments ͑oxygen plasma, UV ozone͒ and ITO parameters 1–4 concentration, and increased surface roughness except for ͑work function, sheet resistance, carrier concentration and mobility, surface roughness͒ and OLED performance is not a͒Author to whom correspondence should be addressed; electronic mail: well understood. It was established that oxidative treatments [email protected], fax: ϩ852 2559 8738 0021-8979/2003/93(9)/5472/8/$20.005472 © 2003 American Institute of Physics J. Appl. Phys., Vol. 93, No. 9, 1 May 2003 Djurisˇic´ et al. 5473 of ITO surface improve the OLED performance. Exact na- materials can serve as a model devices for studying the in- ture of physical processes contributing to this improvement fluence of the ITO surface treatments to the organic solar cell still requires further study. performance. The article is organized as follows. In the fol- Unlike oxidative surface treatments, reported results lowing section, experimental details are given. In section III, with acid treatments1–3,5–7,20 are somewhat contradictory. obtained results are presented and discussed. Finally, conclu- Kim et al.1–3 studied the influence of aquaregia treatment sions are drawn. and combined oxygen plasma and aquaregia in different or- der. Oxygen plasma treatment alone was recommended as the best treatment in their study as the best compromise be- II. EXPERIMENTAL DETAILS 1 tween device efficiency and stability. On the other hand, Li The devices were made using high purity CuPc powder et al.20 and Nuesch et al.5–7 found significant improvement which had been purchased from Strem Chemicals and C60 in OLED performance with acid treatments, which was at- which had been purchased from Materials and Electrochemi- 20 tributed to improved injection due to rougher surface and cal Research Corp. ͑Tucson, AZ͒. ITO glass substrates with 5–7 increased work function of the ITO. It was also shown surface sheet resistance ϳ10 ⍀/square were supplied by that the acid treatment of the ITO surface ͑performed after China Southern Glass Holding Co. Ltd., Shenzhen, China, oxygen plasma cleaning͒ changes the growth mode of while ITO glass substrates with surface sheet resistance N, NЈ-bis-(1-naphtyl)-N,NЈ-diphenyl-1,1Ј-biphenyl-4,4Ј ϳ23.5 ⍀/square were supplied by Varitronix Limited, Hong -diamine from island growth on untreated substrate to layer- Kong. Unless otherwise specified, ITO glass substrates with 25,26 by-layer growth on the treated one. The differences be- surface sheet resistance ϳ10 ⍀/square were used. Prior to tween reported works are possibly due to different acids evaporation, ITO substrates were cleaned, first by rubbing used, different acid concentrations, different treatment times, with cotton and acetone, cotton and ethanol, then in ultra- and different device structures. Different acids result in dif- sonic bath for 10 min in acetone, ethanol, and de-ionized ferent work function and sheet resistance values, as well as water consecutively and blow dried in nitrogen. If no surface 18 different rms surface roughness and surface morphologies. treatment other than cleaning with organic solvents as de- It was also demonstrated that optimal surface treatment/ITO scribed above is performed, ITO substrates will be referred parameters are dependent on the material used.1,20 Therefore, to as untreated or as-cleaned substrates. Acid treatment was it is difficult to make any general recommendations concern- performed by dipping the substrate into 4% aqueous acid ing ITO surface treatments other than that oxidative ͑oxygen solution for 1 min. The acid used was HCl since it produced plasma or UV ozone͒ treatment will result in better OLED the best results for organic light emitting diodes.18 UV ozone performance compared to OLEDs fabricated on untreated treatment was done for 10 min, with 12 cm distance between ITO. Optimal treatment, however, may depend on the mate- 20 W UV light source and the sample. Mechanical treatment rials used and it is also likely to depend on the starting prop- has been performed using clean room wiper rubbing. The erties of ITO. Exact nature of the physical processes respon- samples were ultrasonically cleaned with deionized water af- sible for the influence of surface treatments to the OLED ter mechanical treatment. The films were evaporated in high performance is still not entirely clear. vacuum. Pressure during evaporation was of the order In spite of recognized critical importance of the ITO 10Ϫ4 Pa. The evaporation rate was 1–2 Å/s. The distance surface in performance improvement of OLEDs, studies of from source to film was about 23 cm to ensure uniformity of the relationship between ITO surface properties and organic film thickness, and the substrate holder was rotating. The solar cell performance have been scarce. At
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