{"id":361,"date":"2024-10-25T16:54:57","date_gmt":"2024-10-25T16:54:57","guid":{"rendered":"https:\/\/seegroup.mit.edu\/?page_id=361"},"modified":"2025-06-07T18:21:35","modified_gmt":"2025-06-07T18:21:35","slug":"charge-injection","status":"publish","type":"page","link":"https:\/\/seegroup.mit.edu\/index.php\/charge-injection\/","title":{"rendered":"Charge Injection"},"content":{"rendered":"

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Research: Interface Disorder and Charge Injection into Organic Semiconductors<\/span><\/strong><\/h2>\n

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B.N. Limketkai, M.A. Baldo<\/p>\n

Sponsorship: MARCO Materials Structures and Devices Center<\/p>\n

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Figure 1: Data of Peisert, et al.[2] showing significant differences in the width of the HOMO of CuPC on flat and rough Au substrates.<\/i><\/em><\/p>\n

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Main Research Areas<\/p>\n

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Solar Cells<\/a><\/p>\n

Light Emitting Devices<\/a><\/p>\n

Algorithms for Engineering System Design<\/a><\/p>\n

Magnetic Domain Wall Devices<\/a><\/p>\n<\/div>\n<\/div>\n

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Calculations of interfacial disorder<\/strong><\/h4>\n

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For metal electrodes, interfacial energetic disorder is due primarily to variation in the image charge effect on a rough metal surface. Modeling the metal surface as self-affine, i.e. fractal over a proscribed range of length scales, we find that the standard deviation in the energy levels of the semiconductor is:[3]<\/span><\/p>\n

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where\u00a0<\/span>z<\/i>\u00a0is the distance of the charge from the metal interface,\u00a0<\/span>w<\/i>\u00a0is the global rms roughness of the metal interface,\u00a0<\/span>q<\/i>\u00a0is the electron charge and\u00a0<\/span>\u03b5r<\/sub>\u03b50<\/sub><\/i>\u00a0is the permittivity. Most notably, for equally spaced molecular layers, the 1\/<\/span>z<\/i>2<\/sup>\u00a0decay of the energetic disorder yields the ratio of standard deviations of energy levels in the first and second molecular layers:[3]<\/span><\/p>\n

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\u03c3<\/i>1<\/sub>\u00a0and\u00a0<\/span>\u03c3<\/i>2<\/sub>\u00a0are the standard deviation of transport states in the first and second layers, respectively.<\/span><\/p>\n

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This result is independent of material parameters such as the surface roughness. Since the image charge effect at an ideal flat interface scales as 1\/<\/span>z<\/i>, the 1\/<\/span>z<\/i>2<\/sup>\u00a0dependence of energetic disorder at a rough interface may be understood as the first perturbation of the image charge effect.<\/span><\/p>\n

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Figure 2: A simulation of energetic disorder at a rough interface.<\/i><\/em><\/p>\n

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This result is independent of material parameters such as the surface roughness. Since the image charge effect at an ideal flat interface scales as 1\/<\/span>z<\/i>, the 1\/<\/span>z<\/i>2<\/sup>\u00a0dependence of energetic disorder at a rough interface may be understood as the first perturbation of the image charge effect.<\/span><\/p>\n

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There are other causes of energetic disorder at interfaces in addition to the image charge effect. For example, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonate) (PEDOT:PSS), a densely charged polymer used frequently as an injection interface, is not expected to exhibit strong image charge effects, but the charge distribution on the PEDOT surface is expected to be disordered. Recently, we have generalized the interfacial disorder calculation to include non-metallic interfaces such as PEDOT. We found that the 1\/<\/span>z<\/i>2<\/sup>\u00a0dependence is also expected at these highly charged polymer interfaces.<\/span><\/p>\n

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Figure 3: The charged surface of PEDOT is disordered.<\/i><\/em><\/p>\n

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Analytic model of charge injection at a disordered interface<\/strong><\/h4>\n

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When electrical bias is applied to a disordered metal-organic interface, the voltage determines the quantity of charge trapped in the interface states. The current density is determined by the rate of charge hopping from the interfacial layer to less disordered sites in the second molecular layer.[4] Using the Marcus expression for charge hopping between Gaussian distributions gives a master equation:[3]<\/span><\/p>\n

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where \u0394<\/span>V<\/i>\u00a0is the doping-dependent voltage shift, and\u00a0<\/span>J<\/i>0<\/sub>\u00a0and\u00a0<\/span>V<\/i>0<\/sub>\u00a0are constants. \u0394<\/span>V<\/i>\u00a0contains the entire cathode dependence of injection; it is equivalent to the additional voltage required to inject an amount of charge equal to the doped charge present in the boundary layer. The power law slope,<\/span><\/p>\n

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where \u03bb is the reorganization energy of the molecule. At low temperatures, the decay of energetic disorder gives<\/span><\/p>\n

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Figure 4: Charge injection dominated by interfacial traps.<\/i><\/em><\/p>\n

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Experimental Data<\/strong><\/h4>\n

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The low temperature IV characteristics of CuPC on flat gold and rough gold are shown in Fig. 5. CuPC on rough gold is a highly resistive contact, exhibiting a power law slope of m = 15. CuPC on flat gold, however, is significantly more conductive, and its power law is only m = 7. Thus, CuPC on rough gold behaves as expected given the large density of traps at its structurally disordered injection interfaces.<\/span><\/p>\n

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Figure 5: A comparison between the IV characteristics of rough and atomically flat gold injecting contacts to CuPC at T=10K.<\/i><\/em><\/p>\n

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The IV characteristics at T = 10K for\u00a0<\/span><\/i>(a)\u00a0<\/span>Alq3<\/sub>\u00a0<\/span>interfaces, and\u00a0<\/span><\/i>(b), a comparison of Al\/LiF contacts to Alq3<\/sub>, BCP, TAZ, CBP, and CuPC. All cathodes exhibit similar power law behavior, i.e.\u00a0<\/span>J ~ Vm<\/sup>, where\u00a0<\/span>m\u00a0<\/span>= (20\u00b11).<\/i><\/p>\n

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Figure 6: A comparison between the IV characteristics of rough and atomically flat gold injecting contacts to CuPC at T=10K.<\/i><\/em><\/p>\n

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In Fig. 7 we plot the temperature dependence of the IV characteristics for a variety of cathodes on Alq3<\/sub>. If we assume that \u0394V<\/i>\u00a0<\/span>contains the cathode dependence, then we expect that all the IV characteristics should be related by rigid shifts in voltage. This is confirmed in Fig. 7.<\/p>\n

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Figure 7: The temperature dependence of the IV characteristics of Alq3<\/sub>\u00a0<\/span>interfaces. A rigid voltage shift was applied to the Alq3<\/sub>\u00a0<\/span>characteristics to overlap with the Mg:Ag\/Alq3<\/sub>\u00a0<\/span>data. The \u0394V=0 fit is shown in dotted lines for each organic material.<\/i><\/em><\/p>\n

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Finally, in Fig. 8, we plot the temperature dependence of \u0394<\/span>V<\/i>\u00a0for different cathodes. The temperature dependence of the rigid voltage shift \u0394<\/span>V<\/i>\u00a0is observed to fit:[3]<\/span><\/p>\n

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where EA is the activation energy, and \u0394<\/span>V<\/i>0<\/sub>\u00a0is a temperature-independent constant determined by the equilibrium density of charge in the LUMO states at zero temperature. Assuming that \u0394<\/span>V<\/i>\u00a0is due to cathode-induced doping of the interfacial states,\u00a0<\/span>ND<\/sub><\/i>\u00a0is assigned as the effective doping density.<\/span><\/p>\n

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Figure 8: The temperature dependence of doping for\u00a0<\/span><\/i>(a)<\/span>\u00a0<\/span>Alq3<\/sub>\u00a0<\/span>interfaces, and\u00a0<\/span><\/i>(b)<\/span>, a comparison of Al\/LiF contacts to Alq3<\/sub>, BCP, TAZ, CBP, and CuPC. In all cases, the temperature dependence of doping is found to follow Arrhenius behavior.<\/i><\/em><\/p>\n

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Conclusion<\/strong><\/h4>\n

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We have demonstrated that interfacial disorder is an important determinant of the IV characteristics of organic semiconductor injection contacts. Interfacial disorder creates traps, which may be recognized by power law IV characteristics. Since very few practical organic semiconductor devices employ atomically flat injection contacts, we expect that the theory is generally applicable to organic devices.<\/span><\/p>\n

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Relevant Publications:<\/strong><\/p>\n

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1. B.N. Limketkai and M.A. Baldo. \u2018Charge injection into cathode-doped amorphous organic semiconductors.\u2019 Physical Review B. 71. 085207 (2005)<\/p>\n

2. M.A. Baldo and S.R. Forrest. \u2018Interface limited injection in amorphous organic semiconductors.\u2019 Physical Review B. 64. 085201 (2001)<\/p>\n

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References:<\/strong><\/p>\n

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1. J.C. Scott. \u2018Metal-organic interface and charge injection in organic electronic devices.\u2019 Journal of Vacuum Science and Technology A. 21. 521-531 (2003).<\/p>\n

2. H. Peisert, M. Knupfer, T. Schwieger, J.M. Auerhammer, M.S. Golden and J. Fink. \u2018Full characterization of the interface between the organic semiconductor copper phthalocyanine and gold.\u2019 Journal of Applied Physics. 91. 4872-4878 (2002).<\/p>\n

3. B.N. Limketkai and M.A. Baldo. \u2018Charge injection into cathode-doped amorphous organic semiconductors.\u2019 Physical Review B. 71. 085207 (2005).<\/p>\n

4. M.A. Baldo and S.R. Forrest. \u2018Interface limited injection in amorphous organic semiconductors.\u2019 Physical Review B. 64. 085201 (2001).<\/p>\n

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Research: Interface Disorder and Charge Injection into Organic SemiconductorsB.N. Limketkai, M.A. Baldo Sponsorship: MARCO Materials Structures and Devices CenterFigure 1: Data of Peisert, et al.[2] showing significant differences in the width of the HOMO of CuPC on flat and rough Au substrates.Main Research Areas Solar Cells Light Emitting Devices Algorithms for Engineering System Design Magnetic […]<\/p>\n","protected":false},"author":3,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_et_pb_use_builder":"on","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"class_list":["post-361","page","type-page","status-publish","hentry"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/pages\/361","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/comments?post=361"}],"version-history":[{"count":7,"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/pages\/361\/revisions"}],"predecessor-version":[{"id":951,"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/pages\/361\/revisions\/951"}],"wp:attachment":[{"href":"https:\/\/seegroup.mit.edu\/index.php\/wp-json\/wp\/v2\/media?parent=361"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}