Charge carriers at organic heterojunction interface Exciplex emissio

Charge carriers at organic heterojunction interface:Exciplex emission or electroplex emission?Shengyi Yang,a͒Xiulong Zhang,Yanbing Hou,Zhenbo Deng,and Xurong XuKey Laboratory of Luminescence and Optical Information,Ministry of Education,Institute of OptoelectronicTechnology,Beijing Jiaotong University,Beijing100044,People’s Republic of China͑Received31October2006;accepted22January2007;published online1May2007͒We report the electroluminescence͑EL͒of organic heterojunction devices based on N,NЈ-diphenyl-N,NЈ-bis͑3-methylphenyl͒-͑1,1Ј-biphenyl͒-4,4Ј,-diamine͑TPD͒and2-͑4Ј-biphenyl͒-5-͑4Љ-tert-butylphenyl͒-1,3,4-oxadiazole͑PBD͒.Besides monomolecular emissions from TPD,there are twoadditional EL peaks at around460and480nm from the bilayer device indium tin oxide͑ITO͒/TPD͑100nm͒/PBD͑45nm͒/Al.Our experimental data confirmed that the EL emission maximizedat around460nm is from electroplex as the result of charge carriers cross recombination at theTPD/PBD interface and the EL emission maximized at around480nm originates from͑TPD*PBD͒-type exciplex.©2007American Institute of Physics.͓DOI:10.1063/1.2713947͔Over the last decade,extensive research into organic light-emitting diodes͑OLEDs͒has been made with the aim of improving the out-coupling efficiency and brightness as a way of implementing efficient blue,green,and red OLEDs in passive and active matrix displays.In both layered and single-film structures,the key device processes,charge re-combination and charge separation,occur at the heterojunc-tion between the two components.1,2Recently,a number of papers appeared showing the electroluminescence͑EL͒spec-tra of OLEDs to differ substantially from the photolumines-cence͑PL͒spectra of their component materials.3–11The EL spectra often show up as redshifted broadbands,which have been mostly assigned as emission from exciplexes formed at the solid interface between a hole-transporting layer͑HTL͓͒as molecular electron donors͑D͔͒and an electron-transporting layer͑ETL͓͒as molecular electron acceptors ͑A͔͒.Exciplex formation is favored where there is a signifi-cant spatial overlap between the lowest unoccupied molecu-lar orbitals͑LUMOs͒of the constituent species,12which is clearly the case for conjugated organics,whose LUMOs are highly delocalized␲orbitals.On the other hand,an elec-troplex refers to a particular emission species,and is distin-guished from an exciplex in that an electroplex only occurs under high electricfields,but an exciplex occurs under both photoexcitation and high electricfields.3Thompson et al.13reported that all possible binary combinations of molecules from four different families of organics—N,NЈ-diphenyl-N,NЈ-bis͑3-methylphenyl͒-͑1,1Ј-biphenyl͒-4,4Ј,-diamine͑TPD͒,2-͑4Ј-biphenyl͒-5-͑4Љ-tert-butylphenyl͒-1,3,4-oxadiazole͑PBD͒,2,5-bis͑trimethylsilyl thiophene͒-1,1-dioxide,and poly͑9-vinylcarbazole͒—produce white or near-white emission due to exciplex for-mation between two components.Giro et al.10investigated the EL spectra of double-layer͑DL͒light-emitting diodes ͑LEDs͒based on TPD and PBD incorporated in bisphenol-A-polycarbonate͑PC͒polymer matrix and believed that the EL spectra of DL LEDs͓͑TPD+PC͒/PBD͔are composed of four components:␭1=400nm͑emission of TPD͒,␭3 =480nm͑exciplex emission at the TPD/PBD interface͒,␭2 =550nm͑electroplex emission in strongly disordered re-gions͒,and␭4=690nm͑from the trapping of the e-h hetero-pairs in a disordered environment͒.However,in their another later paper based on the same experimental data,Kalinowski et al.14believed that the broad EL spectra of DL LEDs ͓͑TPD+PC͒/PBD͔can be decomposed into four different Gaussian emission bands.As we know,the detailed process of charge carriers at the TPD/PBD interface is not clearly reported until now.Furthermore,the EL spectra of bilayer or multilayer devices usually depend both on the thickness of each organic layer and the applied voltage.15Then,what is the particular emission species at the TPD/PBD interface?Single-layer devices werefirstly fabricated and the ac-tive layer͑TPD or PBD͒was thermally evaporated under high vacuum of2ϫ10−6Torr onto the ITO substrates͑sheet resistance of10⍀/sq͒which were thoroughly cleaned by scrubbing,ultrasonication,and irradiation in an UV-ozone chamber,and then cathode Al was evaporated to complete the devices.For the bilayer device ITO/TPD/PBD/Al,TPD and PBDfilms were sequentially deposited by thermal evaporation at a rate of0.1nm/s.Each thickness of TPD or PBD layer was measured with an XP-2surface profilometer. The EL and PL spectra were measured with Spex Fluorolog-3spectrometer at room temperature in air.For comparison,the EL spectra of the device ITO/TPD͑105nm͒/Al at different applied voltages are shown in Fig.1and the inset shows the normalized EL and PL spectra of TPDfilm.From here,one can see that the EL of TPD maximizes at420nm with a shoulder peak at 400nm,but its PL maximizes at400nm with a shoulder peak at420nm.It is not easy to observe the EL of PBD because of its easy crystallization,and the surface morphol-ogy of the PBD layer is always rougher.16Also the normal-ized EL and PL spectra of the single-layer device ITO/PBD͑110nm͒/Al are shown in the inset of Fig.1.Ob-viously,the EL of PBD maximizes at385nm,and its PL maximizes at389nm with a shoulder peak at373nm.Then,a͒FAX:86-10-51683933;electronic mail:syyang@JOURNAL OF APPLIED PHYSICS101,096101͑2007͒0021-8979/2007/101͑9͒/096101/3/$23.00©2007American Institute of Physics101,096101-1we fabricated the bilayer device ITO/TPD ͑100nm ͒/PBD ͑45nm ͒/Al and its normalized EL spectra at different applied voltages are shown in Fig.2.From here,one can see that its EL spectra maximize at around 480nm with two shoulder peaks at around 400and 420nm under lower bi-ases,and these two shoulder peaks get more and more in-tense with increasing applied voltages.To a certain voltage,another shoulder peak at ϳ460nm appears and the emission peak at ϳ480nm gets less intense.Finally,the EL spectra maximize at ϳ400nm and other emission peaks become shoulder peaks.Undoubtedly,the emission peaks at ϳ400and ϳ420nm are from TPD,and the emission maximized at ϳ480nm is designed as exciplex emission 10,14since there is not such an emission from both single-layer devices ITO/TPD/Al and ITO/PBD/Al ͑see Fig.1and its inset ͒.Then what is the origin of the emission maximized at 460nm?Is it also an exciplex emission?The PL spectrum of the bilayer device ITO/TPD ͑100nm ͒/PBD ͑45nm ͒/Al under the excitation wavelength of 325nm is shown by the inset in Fig.2.Ob-viously,no obvious emission peaks at ϳ460and/or ϳ480nm can be observed in its PL spectrum because the monomolecular emission from TPD is more efficient under photoexcitation.Therefore,only monomolecular emissions from TPD ͑i.e.peak at ϳ400and ϳ420nm ͒can be observed although another weak emission peak at around 450nm can be seen in its EL spectra at a low temperature of 4.2K.17Here,it is worthy to note that its PL emission peak at ϳ420nm is more intense than the emission peak at ϳ400nm,which is different from the PL spectrum of pure TPD film but similar to the EL spectrum of ITO/TPD/Al ͑see Fig.1͒and it shows that the vibration levels of TPD play a dominant role in its EL spectra.17Further,the absorption spectra ͑not shown here ͒of TPD film,PBD film,layered TPD/PBD film,and coevaporated TPB:PBD film ͑1:1in weight ͒on quartz plates confirm that the emission peak at 460nm does not belong to charge-transfer transition since there is no new absorption that occurs in their absorption spectra.As we know,an exciplex is formed when the excited state of a very polarizable species participates in charge-transfer interactions with other polar or polarizable species.Generally,the exciplex can be described 18by a function ␺ex =c 1͉DA ͘loc *+c 2͉DA ͘CT *,which expresses the quantum me-chanical mixing of the locally ͑LOC ͒excited exciplex con-figuration ͉DA ͘loc *=a 1͉D *A ͘+a 2͉DA *͘,and charge-transfer ͑CT ͒exciplex configuration,͉DA ͘CT *=b 1͉D +A −͘+b 2͉D −A +͘.Whereas the coefficients c 1and c 2determine the contribu-tions of the LOC and CT configurations to the exciplex,the coefficients a 1and a 2are the amplitudes of the two compo-nent state of the LOC configuration emerging as a result of the excitonic resonance between states ͉D *A ͘and ͉DA *͘with the excitation localized on the donor and acceptor,respec-tively.Furthermore,the coefficients b 1and b 2determine the amplitudes of two extreme CT states,͉D +A −͘and ͉D −A +͘.The relation between amplitudes of various components of ␺ex and the exciplex energy ͑E exc ͒depend on the difference between the ionization potential of the donor ͑I D ͒and elec-tron affinity of the acceptor ͑A A ͒of the molecular exciplex components.If the difference between electron affinities ͑LUMO level positions ͒is smaller than that between ioniza-tion potentials ͓highest occupied molecular orbital ͑HOMO ͒level positions ͔of donor and acceptor species,the formation of D *by the excessive electron transfer from the LUMO of A −to the LUMO of D +becomes dominating and exciplexes of the ͉D *A ͘-type are efficiently produced.The cross recom-bination of a hole from a HTL molecule ͑D +͒and an electron from an ETL molecule ͑A −͒results to emission of ͉D +A −͘state in the presence of oppositely charged molecules pro-duced as a rule in OLEDs operating under external electric field,and this special emitting species have been named “electroplex.”3The energy diagram 7,19of TPD/PBD heterojunction and several possible radiation processes are shown by the inset in Fig.3.For the high injection barrier for holes ͑0.7eV ͒at the TPD/PBD interface,some holes will be blocked by PBD and accumulate at the TPD/PBD interface.Similarly,electron in-jection barrier is 0.4eV at the TPD/PBD interface,andpartsFIG.1.͑Color online ͒The EL spectra of the device ITO/TPD ͑105nm ͒/Al at different applied voltages.The inset shows the normalized EL spectra at different applied voltages and the PL spectrum of TPD film under excitation wavelength of 325nm,as well as the EL and PL of PBDfilm.FIG.2.͑Color online ͒The normalized EL spectra of the bilayer device ITO/TPD ͑100nm ͒/PBD ͑45nm ͒/Al at different applied voltages.Note that these are normalized at 470nm.The inset shows its PL spectrum under excitation wavelength of 325nm.of electrons will be blocked by TPD and accumulate at the TPD/PBD interface,while other electrons will tunnel through the interface into TPD layer—that is the reason why monomolecular emissions from TPD ͑especially the emis-sion peak at ϳ400nm ͒and exciplex emission ͑maximized at ϳ480nm ͒can be observed.The amount of holes accumu-lated at the TPD/PBD interface increases with applied volt-age,and sequentially the electric field in the bulk redistrib-utes.As a result,the electric field in the PBD layer gets higher than that in the TPD layer,which will make the elec-tron mobility in the PBD layer increase dramatically.In this way,more electrons tunnel into the TPD layer and exciton radiations from TPD get more intense.At the same time,cross recombination takes place between charge carriers across the TPD/PBD interface ͑which corresponds to process 2͒and it results to 460nm emission band.After tunneling through the TPD/PBD interface,electrons will form excitons with holes in TPD and then follow processes 3and 1:pro-cess 3corresponds to monomolecular emissions of TPD and process 1corresponds to ͑TPD *PBD ͒-type exciplex emission maximized at ϳ480nm ͑ϳ2.6eV ͒.10,14As a further proof,one can see that the relative emission intensities at 400and 420nm increase significantly with in-creasing applied voltages ͑see Fig.2͒,which implies that more electrons tunnel through the TPD/PBD interface and form excitons in the TPD layer with increasing applied volt-ages.It is worthy to note that the ratio of EL emission inten-sity at ϳ400nm to that at ϳ420nm ͑I 400nm /I 420nm ͒of the bilayer device ITO/TPD ͑100nm ͒/PBD ͑45nm ͒/Al in-creases with applied voltages,as shown in Fig.3.However,the ratio I 400nm /I 420nm for the single-layer device ITO/TPD ͑105nm ͒/Al increases at the beginning and then de-creases with increasing applied voltages ͑not shown here ͒.Under higher electric fields,more charge carriers accumulate at the TPD/PBD interface and more TPD excitons will be produced;however,the contact area between TPD and PBD is limited and the quantity of holes in vibration levels gets less—that is the reason why the emission peak at ϳ460nm gets more and more intense but the intensity of the emission peak at ϳ480nm gradually decreases under high electric fields.Excitons get more energy under higher electric fields,so that the intensity of the emission peak at ϳ400nm gets more and more intense than that of the emission peak at ϳ420nm with increasing applied voltages.In conclusion,we have observed exciplex emission ͑maximized at around 480nm ͒and electroplex emission ͑maximized at around 460nm ͒coexisting at the TPD/PBD interface,and they compete with each other under high elec-tric fields.The emission peak at ϳ460nm is from elec-troplex emission as the result of charge carrier cross recom-bination at the TPD/PBD interface,and the emission peak at ϳ480nm originates from ͑TPD *PBD ͒-type exciplex.This project was supported by the National Natural Sci-ence Foundation of China ͑60406006and 10434030͒,the Beijing Natural Science Foundation ͑2062019͒,the Beijing NOV A program ͑2006B20͒,and the “973”National Key Ba-sic Research Special Foundation of China ͑2003CB314707͒.1B.K.Crone,P.S.Davids,I.H.Campbell,and D.L.Smith,J.Appl.Phys.87,1974͑2000͒.2H.J.Snaith,A.C.Arias,A.C.Morteani,C.Silva,and R.H.Friend,Nano Lett.2,1353͑2002͒.3T.Granlund,L.A.A.Petterson,M.R.Anderson,and O.Inganäs,J.Appl.Phys.81,8097͑1997͒.4D.D.Gebler,Y .Z.Wang,D.-K.Fu,T.M.Swager,and A.J.Epstein,J.Appl.Phys.108,7842͑1998͒.5K.Itano,H.Ogawa,and Y .Shirota,Appl.Phys.Lett.72,636͑1998͒.6J.Rommens,A.Vaes,M.Van der Auweraer,F.C.De Schryver,H.Bässler,H.Vestweber,and J.Pommerehne,J.Appl.Phys.84,4487͑1998͒.7H.Ogawa,R.Okuda,and Y .Shirota,Appl.Phys.A:Mater.Sci.Process.67,599͑1998͒.8X.-T.Tao,Y .-D.Zhang,T.Wada,H.Sasabe,H.Suzuki,T.Watanabe,and S.Miyata,Adv.Mater.͑Weinheim,Ger.͒10,226͑1998͒.9H.Cao,X.C.Gao,and C.H.Huang,Appl.Surf.Sci.161,443͑2000͒.10G.Giro,M.Cocchi,J.Kalinowski,P.Di Marco,and V .Fattori,Chem.Phys.Lett.318,137͑2000͒.11B.Hu,Z.Yang,and F.E.Karasz,J.Appl.Phys.76,2419͑1994͒.12A.Horvath and K.L.Stevenson,Coord.Chem.Rev.153,57͑1996͒.13J.Thompson,R.I.R.Blyth,M.Mazzeo,M.Anni,G.Gigli,and R.Cingotani,Appl.Phys.Lett.79,560͑2001͒.14J.Kalinowski,M.Cocchi,P.Di Marco,W.Stampor,G.Giro,and V .Fattori,J.Phys.D 33,2379͑2000͒.15S.Y .Yang,D.Liu,Y .Jiang,F.Teng,Z.Xu,Y .Hou,and X.R.Xu,Eur.Phys.J.B 52,329͑2006͒.16K.-H.Choi,D.-H.Hwang,H.-M.Lee,L.-M.Do,and T.Zyung,Synth.Met.96,123͑1998͒.17A.J.Campbell,D.D.C.Bradley,ubender,and M.Sokolowski,J.Appl.Phys.86,5004͑1999͒.18M.Cocchi,D.Virgili,C.Sabatini,and J.Kalinowski,Chem.Phys.Lett.421,351͑2006͒.19Y .Shirota,Y .Kuwabara,H.lnada,T.Wakimoto,H.Nakada,Y .Yonemoto,S.Kawami,and K.lmai,Appl.Phys.Lett.65,807͑1994͒.FIG.3.͑Color online ͒The ratio of emission intensity at 400nm to that at 420nm ͑I 400nm /I 420nm ͒for the bilayer device ITO/TPD ͑100nm ͒/PBD ͑45nm ͒/Al at different applied voltages.The inset shows the energy diagram of the TPD/PBD heterojunction and several possible radiation processes.。