Blom, P.W.M.

Wildeman, J.

Woudenbergh, T. van

English

This work describes the current over an interface between two different polymeric semiconductors. The interface barrier amounts to 0.7 to 1.0 eV, which is orders of magnitude larger than the thermal energy. It is demonstrated that the current across the interface is injection limited. Furthermore, it is observed that the electrical characteristics for such an organic organic interface are completely different from those for a metal organic interface.</p

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   University of GroningenInjection-limited current in a polymeric heterojunctionWoudenbergh, T. van; Wildeman, J.; Blom, P.W.M.Published in:ESSDERC 2004: PROCEEDINGS OF THE 34TH EUROPEAN SOLID-STATE DEVICE RESEARCHCONFERENCEIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Woudenbergh, T. V., Wildeman, J., & Blom, P. W. M. (2004). Injection-limited current in a polymericheterojunction. In RP. Mertens, & CL. Claeys (Eds.), ESSDERC 2004: PROCEEDINGS OF THE 34THEUROPEAN SOLID-STATE DEVICE RESEARCH CONFERENCE (pp. 425-427). University of Groningen,The Zernike Institute for Advanced Materials.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 13-02-2023Injection-limited current in a polymeric heterojunction T.van Woudenbergh, J. Wildeman and P.W.M. Blom Molecular Electronics, University of Groningen. Nijenhorgh 4, NL-9747 AG, Groningen, The Netherlands. e-mail: T.van.Woudenhergh@phys.rug.nl Abstract: This work describes the current over an interface between two different polymeric semic0nductor.s. The interface harrier amounts to 0.7 to 1.0 eV, which is orders of magnitude larger than the thermal energy. It is demonstrated that the current acro.ss the interface is injection limited. Furthermore, it is observed that the electrical characterisrics for such an organic organic interface are completely differentfrom those for a metal organic interface. 1. Introduction The organic light-emitting diode (OLED) area is rapidly growing. Several companies have already introduced their products on the market. The characteristic properties are low weight and cheap manufacturing. Moreover, OLEDs have a wide viewing angle and intense pure colors. In addition, polymer light emitting diodes (PLEDs) have a large flexibility, which make them ideal candidates for portable applications. The PLEDs have a large difference in hole and electron mobility, shifting the recombination towards the cathode interface. Losses occur due to exciton quenching to the metal cathode, and a substantial amount of efficiency is lost.' Already for a long time solutions have been suggested, and one of the most efficient is to confine charges in the semiconductor away from the interface.' This can he achieved by separating the transport of the charge carriers from the recombination. The most simple example of such a confinement is an organic organic heterojunction with a hand-offset to prevent the charges from transport over the interface. In this way, the recombination zone is more or less confined to the small interface area itself. The organic-organic interface (001) has the remarkable property of a defect free heterojuntion? This makes it an ideal device to investigate the electrical properties, such as the charge transport and diffusion over such an interface, and the charge injection for interfaces with a band-offset. The charge injection over an 001 will he different from charge injection over a metal ~ organic interface (MOI) for two main reasonx4 425 First of all, the charge reservoir in the injecting layer cannot be thought to be infinite, and will certainly influence the injection rate. Secondly, the 001 banier is not subject to image force lowering, as the charge density in the injecting layer is too low to induce an image charge, while also the dielectric relaxation time of an organic semiconductor is much too long.' This has consequences for the electric field dependence of the charge injection. The back-flow of injected charge carriers over a MO1 is governed by the electric field close to the metal that counteracts the injection of charges.' This will reduce the charge injection at low electric fields due to a large back flow of carriers by the image force, which is described by the Onsager escape probability. However, for a 001 this back-flow of camers cannot be due to the image force, and must he taken into account by the backward hopping probability. This results in a completely different field dependence.' To investigate the charge injection over an 001, a bi- ethyl hexyl poly-p-phenylene vinylene derivative (BEH- PPV) has been used together with poly(9.9- dioctylfluorene) (PFO). BEH-PPV has a highest occupied molecular orbit (HOMO) of 5.3 eV.' Values for the HOMO of PFO range from 5.8 eV * to 6.1 eV '. Thus the interface energy banier between BEH-PPV and PFO is between 0.7 and 1.0 eV. It is expected that for such a large interface energy barrier the current will he limited by the injection rate. 2 Experimental The devices that have been investigated are of three types, they are all designed to measure the hole transport cq injection through the polymer layers. For that reason, all the devices have a top contact of gold. Gold has a high work-function, and will therefore block the electron injection. First of all, devices have been constructed where a layer of BEH-PPV has been spin-coated on top of an Ohmic indium tin oxide (ITO) bottom contact. Furthermore, devices have been constructed where the IT0  has been covered by the BEH-PPV, and on top of this layer PFO has been spin-coated. Via thickness measurements it has been confirmed that the total thickness equals the thickness of the two separate layers 0-7803-8478-4/04/$20.00 02004 IEEE (BEH-PPV and PFO), prepared under the same spin-coat conditions. The third device type consist of a bottom contact of Pt, on top of which the PFO has been spin- coated. 3 Results In figure 1 the current density voltage (J-V)  characteristic of the ITOIBEH-PPVIAu device has been shown, together with the ITOBEH-PPVPFO/Au device. It is demonstrated that the current through the ITO/PPV/Au device is space charge limited (SCL). The mobility amounts topo=1.5x10~10 m'Ns for low field, whereas the field dependence can he described by poxexp( $E), with F3XIOA ( f l y .  - 1 0 ' ~  I .  I ,  I ,  I I I O '  100 n- E $ 1 0 '  102 1 0 2  '"'I 8 i - : ; ? 5 10 15 20 1 0 9 1  6 ' , J .  ' ' ' ' ' ' v.v, IV) Figure 1, current density as a function of voltage, for two different devices. The ITOIPPVIAu device has a thickness of 6 1 4 0  nm, whereas the ITOIPPVIPFOIAu device has a total thickness of 6 3 7 0  nm, the PFO layer being 6 2 3 0 n m  thick. The dashed line shows the calculated SCL current through the ITO/PPV/Au device, the solid line represents the calculated SCL current through the ITO/PPV/PFO/Au device. It is observed that the ITOPPV/PFO/Au device indeed carries a much lower current. In order to discriminate between a SCL current for this thick device, or an injection limited (IL) current due to the hole harrier at the PPViPFO interface, a device model has been used to calculate the SCL current in such a double layer device without interface barrier. The device model is similar to that for a single layer,' where the physical parameters for both layers have been included. The calculated SCL current for such a device without interface barrier is also plotted in Figure 1 as a solid line. It is observed that the measured ITO/PPV/PFO/Au device current (squares) is indeed much lower. To demonstrate the IL nature of the current through the ITOPPVPFO/Au. the current has been plotted as a function of electric field for different thicknesses. The JV characteristics should fall on top of each other, as an lL current is a function of electric field J=J(E). It is shown in figure 2 that the current as a function of electric field does coincide for the different thicknesses, which confirms the IL nature of the current through the device, in correspondence with the large injection barrier of PPVPFO of 4d.7 eV. .,B OB 0 d=lOOnm 0 d=140nm OR" A d=230nm ." 0.0 5 . 0 ~ 1 0 '  l . O X l 0 ~  E (Vlm) Figure 2, current density as a function of electric field for three different device thickness at room temperature. Now we are sure about the IL naNre of the current through ITO/PPV/PFO/Au, one might ask whether the injection current indeed differs from the injection current over a MOI. For this reason PtmFO/Au devices have been made. It has been measured with Kelvin probe technique that the work -function of Pt under ambient conditions amounts to -5 eV, which is much lower than the value in vacuum. This has also been observed for other metal electrodes." The energy barrier for Pr/PFO is therefore comparable or even larger as for the PPVPFO system. In figure 3, the J-E characteristics of both a ITOIPPV/PFO/Au and a Pt/PFO/Au device are shown, both for a thickness of d= 160 nm of the PFO layer. It is observed that for low voltages the leakage current in the Pt device dominates, while for the ITOPPVPFO/Au device the device current no leakage is observed. This demonstrates the superiority in device performance for a polymer bottom contact layer with respect to metal contacts. E (Vlm) Figure 3, current density as a function of electric field for two different device types, a MO1 and a 001. 426 Furthermore, it is observed from Figure 3 that the IL current of the ITOIPPVIPFOIAu device (001) increases strongly with electric field at low applied biases, whereas the IL current through the Pt device (MOI) only comes up at much higher fields. The current in the 001 is larger at not too high electric field, which is an indication that the injection barrier for the P f l F O  contact is indeed larger compared with the 001. It is also observed that at high fields the field dependence of the IL current of the MO1 is larger. This indicates that for the MO1 the image force lowering is present, which reduces the injection harrier for increasing electric field. The strong increase of the injection current at low fields for the 001 is not yet clear and will he subject to further study. 3. Conclusion The current in a double layer device with a large interface barrier (&=0.7-1.0 eV) is strongly reduced with respect to the space charge limited current for such a device. It is found that the current-density scales with electric field, indicating that the current across the 001 is injection limited. Moreover, it is found that the current density voltage characteristics are completely different for an 001 and a MOI. indicating the different nature of the organic-organic and the metal-organic contact. The strong rise with electric field for a MO1 can be explained by the image force lowering, while the difference in injection efficiency at moderate fields can be attributed to the absence of image force in the 001, resulting in a smaller hack-flow of carriers. 4. Acknowledgement We gratefully acknowledge J. Wildeman, J.J.A.M. Bastiaansen, and B.M.W. Langeveld-Voss for providing the polymers used in this work. This work is pan of the research programme of the Dutch Polymer Institute. 5. References 1. P.W.M.Blom and M.J.M. De Jong; "Electrical characterization of polymer light-emitting diodes"; lEEE Journal of Selected Topics in Quantum Electronics vol4, pp. 105-112(1998). 2. D.R.Baigent er al; "Light-Emitting-Diodes Fabricated with Conjugated Polymers ~ Recent Progress."; Synthetic Metals vol67, pp 3-10 (1994). 3. A.C. Morteani er al; "Barrier-free electron-hole capture in polymer blend heterojunction light-emitting diodes."; Advanced Materials 15, pp 1708-+ (2003). 4. V.I.Arkhipov, E.V. Emelianova, and H. Bassler; "Charge canier transport and recombination at the interface between disordered organic dielectrics." Journal of Applied Physics vol90, pp. 2352-2357 (2001). 5 .  K.C. Kao and W. Hwang; "Electrical transport in solids"; Pergamon Press, Oxford (1981). 6. V.1 Arkhipov, E.V. Emelianova, Y.H. Tak, and H. Bassler; "Charge injection into light-emitting diodes: Theory and experiment." Journal of Applied Physics 84, 848-856 (1998). 7. I. H. Campbell, T.W. Hagler, D.L. Smith, and J.P. Ferraris; "Direct measurement of conjugated polymer electronic excitation energies using metaUpolymer1metal structures."; Physical Review Letters vol76, pp 1900- 1903 (1996). 8. A.J. Campbell, D.D.C. Bradley, and H. Antoniadis; "Quantifying the efficiency of electrodes for positive carrier injection into poly(9.9-dioctylfluorene) and representative copolymers." Journal of Applied Physics vol89, pp 3343-3351 (2001). 9. L.S. Liao et al; "metal-polyfluorene interface and surface" in: "Conjugated polymer and molecular interfaces" Eds W.R. Salaneck et al, Marcel Dekker, New York (2002). 10. K. Besocke, and S.  Berger; "Piezoelectric Driven Kelvin Probe for Contact Potential Difference Studies."; Review of Scientific Instruments vol47, pp 840-842 (1976). 11. W.N. Hansen and K.B. Johnson; "Work Function Measurements in Gas Ambient."; Surface Science vol 316, pp 373-382 (1994). 421 

Injection-limited current in a polymeric heterojunction

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Injection-limited current in a polymeric heterojunction

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