[精品论文]Effects of ultraviolet light on electrical properties of.doc

精品论文Effects of ultraviolet light on electrical properties ofPEDOT:PSS film5Xing Yingjie, Qian Minfang, Wang Guiwei, Zhang Gengmin, Guo Dengzhu(Department of Electronics, Peking University, Beijing 100871)Abstract: The performance of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film is highly dependent on its in organic optoelectronic devices Conductivity and work function. Significant improvements of conductivity and work function are achieved by irradiation of10PEDOT:PSS film in vacuum under 254 nm ultraviolet light. The mechanism for such an improvementis investigated by current-voltage measurement, X-ray photo electron energy spectrum and atomic force microscopy. A surface degradation and the composition change are found after ultraviolet irradiation. The height of the injection barrier in a hole-only device fabricated with UV-treatedPEDOT:PSS film is decreased effectively. Our results reveal that UV treatment is capable of modifying15the conductivity and work function of PEDOT:PSS film thus allowing them be tuned to the device application.Keywords: Physical electronics; conductivity; work function0Introduction20Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)isacommon material for electrode or hole transport layer in various organic optoelectronic devices.1 The PEDOT:PSS colloidal composite combines insoluble conjugated polymer PEDOT and charge-balancing counterion polyelectrolyte PSS in water, and thermally stable PEDOT:PSS layer can be formed from commercial aqueous PEDOT:PSS solution by spin coating on hard substrate25or printing technique on large area flexible film. The structure model of as-prepared PEDOT:PSS film is described as a lot of PEDOT-rich clusters separated by insulating PSS lamella.2 Although comparable-good conductivity and high transparency in PEDOT:PSS film are easily obtained, an organic optoelectronic device for practical application requires better property of PEDOT:PSSfilm, for example, higher efficiency and longer life time of polymer solar cell after proper30treatment of PEDOT:PSS film.3-5Conductivity and work function are two most important properties of PEDOT:PSS. A PEDOT:PSS layer with high conductivity is highly desired in most fields in organic electronics and many methods have been used to improve the conductivity of PEDOT:PSS film. Anisotropic conductivities,2 10-3 S/cm for parallel direction (parallel to the film surface) and 10-6 S/cm for35perpendicular direction (perpendicular to the film surface), were measured in as-prepared PEDOT:PSS film prepared from commercial PEDOT:PSS solution (Baytron P VP AI 4083), which is frequently used as a hole transport layer in devices of organic light emitting diode and organic solar cell. A common technique to enhance the conductivity of PEDOT:PSS film is by addition of small amount of polar solvent or high boiling point alcohol into the commercial40PEDOT:PSS solution, such as dimethylsulfoxide,3 glycerol,6 or sorbitol.7 The formation of auniformly conductive network was believed as the reason for the conductivity improvement by three order of magnitude because morphology change and aggregation of PEDOT-rich colloidal particles were observed in PEDOT:PSS film.3,7 Recently, a different way to improve the conductivity of PEDOT:PSS film is developed, which employs a wet post-treatment to selective45removal of PSS from the as-prepared PEDOT:PSS film. Kim et al reported a large increase in theFoundations: MOST of China (Grant Nos 2012CB932701, 2011CB933001), National Natural Science Foundation of China (Grant Nos 61076057, 61171023), the Specialized Research Fund for the Doctoral Program of High Education (No. 20090001120024).Brief author introduction:Xing Yingjie, (1970-), Male, Assiciate Professor, Physical electronics. E-mail:xingyj- 13 -conductivity after immersing PEDOT:PSS film in an ethylene glycol bath.8 The highest conductivity (3065 S/cm) was observed from H2SO4 post-treated and water-rinsed PEDOT:PSS film (with Clevios PH1000 for electrode application).9 Effectively replacement of PSS- ions withHSO4- caused phase separation between PSS and PEDOT chains, resulting in an indium tin oxide50(ITO)-comparable conductivity.5 In this case, much less content of PSS in post-treatedPEDOT:PSS film means that much less transport barrier surroundingPEDOT-rich clusters.As a widely used hole injection layer in organic optoelectronic device, PEDOT:PSS film with high work function is needed to match the highest occupied molecular orbital (HOMO) energy of the hole transport material. The value of work function of PEDOT:PSS film varies between 4.9 eV55to 5.5 eV 3,4,6,7 depending on the preparation method and measuring technique. It is found that athin PSS-rich layer (thickness about 3.5 nm) always locates at the surface of as-prepared PEDOT:PSS film because of vertical phase separation during the period of film formation.10 Because PSS has a higher the work function than PEDOT, the PEDOT:PSS films treated upon above additive methods suffer a small decrease in work function (0.1 0.3 eV),3,7 which will60increase the injection barrier height. Higher work function could be obtained by other methods introducing more vertical phase separation in PEDOT:PSS films, such as by addition of perfluorinated ionomer 11 or by polar-solvent vapor annealing,12 resulting in a perfluorinated ionomer-rich or PSS surface layer.Contrast to above “wet” methods, treatment by ultraviolet (UV)-ozone exposure or UV light65irradiation is effective and “dry” to modify the work function of PEDOT:PSS film. The increase in work function of PEDOT:PSS film (0.2 - 0.4 eV) after UV-ozone exposure 13 or 365 nm UV irradiation 14 was reported, and enhanced performance in organic optoelectronic devices was achieved with the help of UV-treated PEDOT:PSS, which proved the importance of higher work function of PEDOT:PSS film.13,15 Unfortunately, worse conductivity or little changed70conductance were observed from PEDOT:PSS film after UV-ozone exposure or 365 nm UV irradiation.14,16 Recently, the study of the effect of UV treatment on PEDOT:PSS film mainly focuses on whether the better hole injection in organic light emitting diodes comes from a lower injection barrier due to the increased work function of PEDOT:PSS film. Two mechanisms are proposed to explain the improvement of hole injection. Anzenbachers groups suggested a smaller75injectionbarrierbetweenN,N¢-diphenyl-N,N¢-bis-(1-naphthyl)-1-1¢-biphenyl-4,4¢-diamine (-NPD) and PEDOT:PSS after UV-Ozone treatment.14,17 A different model was given by Helander et al. with results measured via in situ photoelectron spectroscopy.18 They proposed that because of a surface dipole generated between UV-Ozone treated PEDOT:PSS and -NPD, the work function improvement of PEDOT:PSS did not bring a decrease of injection barrier as80expected. Understanding and better controlling the property of interface between UV-treated PEDOT:PSS and organic semiconductor will be beneficial to improve the device performance in the future.It is known that UV illumination may break some organic compounds and these broke bonds will be oxidized quickly in air. Specifically, no decomposition occurs under 365 nm UV85irradiation, whereas the 254 nm UV light can break chemical bonds, such as C-C, C-H, and C-OH.20 For PEDOT:PSS film under 254 nm UV irradiation, PSS is more detrimental than PEDOT because p-conjugated part in PEDOT chain has a larger bonding energy than a single bond in PSS chain.16 Therefore, selective inhibition of PSS barrier in PEDOT:PSS film may be realized by weak irradiation of 254 nm UV. In this work, we separated the processes of 254 nm90UV irradiation and oxidation. We obtained the increases in both conductivity and work function, which could be concluded as a result of a more conductive network after UV irradiation and the95100105110115120125130135oxidation of the film surface in air. We suggest that, by combining this UV treatment with above “wet” methods, PEDOT:PSS film with both high conductivity and high work function may be prepared in the future.1ExperimentalITO glasses or Si plates covered with SiO2 layer (300 nm) were used as the substrates for perpendicular or parallel conductivity measurements, respectively. PEDOT:PSS film was deposited on UV Ozone-cleaned substrates by spin coating using commercial PEDOT:PSS (Clevios P VP AI 4083). The PEDOT:PSS films were baked at 160 °C in air for 20 min. All samples in present work were prepared by this process and had a thickness of about 45 nm. Then the samples were placed in a stainless steel vacuum chamber. The base pressure of the chamberwas 10-4 Pa. A UV light (SunMonde Noblelight, 8 W) with emission centered at 254 nmirradiated the sample through a quartz window. The distance between the light source and sample was about 23 cm. The current-voltage measurements were conducted during the irradiation period.The parallel conductivity was measured by the van der Pauw four-point probe technique. Small In pads were used as the contacts at four corners of a PEDOT:PSS/SiO2/Si substrate (1´1 cm2). The calculation method can be found in Ref. 9. Square gold electrodes (2 ´ 2 mm2) were deposited on PEDOT:PSS/ITO by thermal evaporation for perpendicular conductivity measurements. A pair of parallel Ti/Au electrodes (length 5 mm) with the distance of 5 mm weredeposited on Si/SiO2 for addition planar IV measurements before PEDOT:PSS deposition. Electrode thicknesses of 50 nm and 75 nm were chose for perpendicular and planar conductance measurements,respectively.Forhole-onlydevices, N,N¢-diphenyl-N,N¢-bis-(1-naphthyl)-1-1¢-biphenyl-4,4¢-diamine(-NPD)(Luminescence Technology) was deposited on PEDOT:PSS film as the hole-transport material. All IV measurements were performed with a digital source meter (Keithley 2602). The work function of PEDOT:PSS film was measured by photoelectron yield spectroscopy (Riken Keiki, AC-2). The chemical bonding states at the film surface were evaluated with X-ray photo electron energy spectrum (XPS, Kratos Axis Ultra). The surface morphology was observed by atomic force microscopy (DI Nanoscpoe SII).2Results and discussionFirst, the effect of UV irradiation on the parallel conductivity of PEDOT:PSS film was studied. The untreated sample was measured in air and in vacuum for comparison and similar conductivity was obtained. Then we used a UV light to irradiate the sample surface. After one hour of irradiation, the quartz window was covered by a light barrier and the IV measurement was conducted in the dark. This procedure was repeated by more than 20 times. No obvious resistance change was observed for the first several hours. Usually, a slight increase of conductivity could be detected after 10 hour of irradiation. Longer irradiation caused more increase until the conductivity increase tended to become saturated after 21 hour of irradiation. The treatment time dependence of the parallel conductivity is summarized in Fig. 1(a). The conductivity of as-prepared PEDOT:PSS film is 1.73´10-3 S/cm, which is in accordance with the value in Ref. 2. After 23 hour of irradiation, the conductivity becomes 50.47 S/cm, which means a dramatically improvement by four orders. This value is higher than the best result (10 S/cm) obtained bysorbitol addition.7 After UV treatment, we measured the parallel conductivity again in air and noobvious change was found. To further confirm the influence of air exposure on the improved conductivity, we performed IV measurement with several additional planar devices. After 23 hour140of UV irradiation, we measured the samples in the vacuum chamber firstly and then placed them in air for several hours. No apparent conductance difference due to environment could be detected under small bias. Only a slight conductance decrease was found under the bias voltage of 4 V (in air 2.65´10-5 A, in vacuum 2.70´10-5 A). We noted that by naked eye, no color or transparencychange in treated PEDOT:PSS film was observed.145150155160Fig. 1 Conductivity improvement after UV irradiation. (a) Treatment time dependence of parallel conductivity of PEDOT:PSS film, (b) IV curves of PEDOT:PSS film (untreated: diamond, after 23 hour of irradiation: square) measured in vertical device structure. Inset: device structure for perpendicular conductivity measurement.We also examined the perpendicular conductivity of PEDOT:PSS film in vacuum before and after UV irradiation. The device structure of a sample for vertical IV measurement is schematically drawn in the inset of Fig. 1(b). The measured results of PEDOT:PSS film sandwiched between gold and ITO are shown in Fig. 1(b). Although the improvement of perpendicular conductivity by UV illumination induced was observed, its increasing rate is much lower than that of parallel conductivity. The perpendicular conductivity of untreated PEDOT:PSS film is 1.98´10-5 S/cm, which is a little higher than the reported value in Ref. 2. After 23 hour of irradiation, the perpendicular conductivity becomes 5.70´10-5 S/cm, which means that the anisotropic conductivity still remains in treated PEDOT:PSS film.The mechanism for conductivity improvement induced by 254 nm UV irradiation is interesting. For UV-ozone exposure or 365 nm UV irradiation, no beneficial effect on conductivity was observed from PEDOT:PSS film.14,16 Many efficient techniques to modify the conductivity of PEDOT:PSS film were solution-involved methods for PSS redistribution/removal. However, no PSS redistribution/removal could happen inside a vacuum environment in our experiments.Ouyang et al. indicted a mechanism for conductivity enhancement due to the structure change of165170175180185190195200PEDOT chains,20 in which the formation of quinoid conformation in PEDOT chains was ascribed as the reason for the conductivity increase and a shift of Raman peak around 1440 cm-1 was used as the pointer for the conformation of quinoid