The influences of buffer interlayer at the donor/acceptor interface on the open circuit voltage (VOC) of typical copper phthalocyanine (CuPc) / C60 organic photovoltaic devices are studied. Six fluorescent dyes with progressively increasing ionization potentials (I P) were used to investigate the factors influencing the VOC. The short-circuit current and fill factor of CuPc/ C60 device incorporating dye interlayer are lower than those of standard bilayer device. On the other hand, the VOC increases linearly with the I P of dye material and falls off when the I P is equal to or greater than 5.6 eV, in which the energy offset between the highest occupied molecular orbitals at the interlayer/ C60 heterojunction is smaller than the C60 exciton binding energy. The findings underscore the importance of energy offsets in photovoltaic responses. © 2009 American Institute of Physics.published_or_final_versio

Chan, MY

Lai, SL

Lee, CS

Lee, ST

Lo, MF

Applied Physics Letters

Crossref

HKU Scholars Hub

Title Impact of dye interlayer on the performance of organicphotovoltaic devicesAuthor(s) Lai, SL; Lo, MF; Chan, MY; Lee, CS; Lee, STCitation Applied Physics Letters, 2009, v. 95 n. 15, article no. 153303Issued Date 2009URL http://hdl.handle.net/10722/69840Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Impact of dye interlayer on the performance of organic photovoltaicdevicesS. L. Lai,1 M. F. Lo,1 M. Y. Chan,2,a C. S. Lee,1,a and S. T. Lee11Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and MaterialsScience, City University of Hong Kong, Hong Kong SAR, China2Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, ChinaReceived 28 July 2009; accepted 15 September 2009; published online 13 October 2009The influences of buffer interlayer at the donor/acceptor interface on the open circuit voltage VOCof typical copper phthalocyanine CuPc /C60 organic photovoltaic devices are studied. Sixfluorescent dyes with progressively increasing ionization potentials IP were used to investigate thefactors influencing the VOC. The short-circuit current and fill factor of CuPc /C60 deviceincorporating dye interlayer are lower than those of standard bilayer device. On the other hand, theVOC increases linearly with the IP of dye material and falls off when the IP is equal to or greater than5.6 eV, in which the energy offset between the highest occupied molecular orbitals at theinterlayer/C60 heterojunction is smaller than the C60 exciton binding energy. The findings underscorethe importance of energy offsets in photovoltaic responses. © 2009 American Institute of Physics.doi:10.1063/1.3243991It is well known that the short-circuit current densityJSC strongly depends on the absorption efficiency and theexciton diffusion length in the photoactive materials, whilethe open-circuit voltage VOC is predominately determinedby the energy difference between the highest occupied mo-lecular orbital HOMO of the donor D and the lowest un-occupied molecular orbital LUMO of the acceptor A, i.e.,HOMOD−LUMOA.1–6Currently, most organic photovoltaic OPV devices em-ploy two photoactive materials to form the donor-acceptorjunction where photogenerated exciton dissociation andcharge transfer take place. To further improve the photovol-taic responses, the use of more complicated device structureswith multiple photoactive layers is one feasible approach.Sista et al.7 recently reported three-component OPV deviceswith a cascade-type energy level structure, in which a thininterfacial layer, copper phthalocyanine CuPc, was insertedbetween the donor/acceptor interface. The requirements forsuch a cascade-type energy band structure is that the ioniza-tion potential IP of the interlayer should be larger than thatof p-type material donor while its electron affinity EAshould be smaller than that of n-type material acceptor.This approach can effectively engineer the VOC of OPV de-vices via choosing interlayers with appropriate energy levels.Kinoshita et al.8 have also demonstrated the use of a thinzinc phthalocyanine interfacial layer 2 nm between pen-tacene and C60 to improve both JSC and VOC. On the otherhand, Rand et al.9 reported that the use of an interfacial layerwould negatively affect the JSC if the interfacial layer thick-ness exceeds 5 nm. While the cascade-type energy bandstructure provides a viable means to improve the photovol-taic response, there is little information on how the deviceperformance is related to the constituent materials. In orderto optimize the performance of such three-component OPVdevices, systematic studies on the influence of the interlayerwould obviously be desirable. In this work, we report the useof six representative fluorescent dyes as interfacial layers toform the cascade-type energy band structure, from which wedetermine the ideal material parameters for the three-component OPV devices. In addition, the relation betweenthe VOC and the offset energies at the donor-acceptor inter-face is discussed.Six representative fluorescent dyes, namely 4-dicyano-methylene-2-tert-butyl-6-1,1,7,7-tetramethyljulolidyl-9-enyl-4H-pyran DCJTB, 4-dicyanomethylene-2-tert-butyl-1,3,3,7,7,-penta-methyljulolidyl-9-enyl-4H-pyran DCJPB,tetrat-butylrubrene TBRB, rubrene RB, 10-2-benzo-thiazolyl-1,1,7,7-tetramethyl-,3,6,7-tetrahydro-1H ,5H ,11H-benzolpyrano6 7 8-ijquinolizin-11-one C545T, and4,4-bis2,2-diphenyl vinyl-1 ,1-biphenyl DPVBi, wereinserted as an interlayer between CuPc donor and C60 ac-ceptor. These six materials were selected because of theirprogressively increasing ionization potentials IP that allowus to study the dependence of VOC on the offset energies atthe organic heterojunctions.10–15 For comparison, a standardbilayer device with the structure of ITO /CuPc /C60 /bathocuproine BCP /Al was also fabricated. Current-voltage characteristics of OPV devices were measured with aprogrammable Keithley model 237 power source. The pho-tocurrent was measured in the dark and under illuminationwith an intensity of 100 mW /cm2 from an Oriel 150 W solarsimulator equipped with AM1.5G AM: air mass, G: globalfilters, and the light intensity was measured with a calibratedsilicon detector with a KG-5 color filter.16 Absorption spectraof organic films on quartz substrates were measured with aPerkin Elmer Lambda 2S UV/VIS spectrometer.Table I summarizes the key cell parameters for deviceswith the structure of ITO/CuPc 34 nm/dye 5 nm /C60 40nm/BCP 5 nm/Al 80 nm, in which DCJTB, DCJPB,TBRB, RB, C545T, and DPVBi were used as dye interlayer,respectively, together with the standard device. Under illumi-nation, the standard device exhibits a JSC of 6.40 mA /cm2, aVOC of 0.46 V, a fill factor FF of 0.52, and a power con-version efficiency P of 1.54%. However, the JSC dropssharply when a thin dye interlayer is inserted between CuPcaAuthors to whom correspondence should be addressed. Electronic ad-dresses: chanmym@hku.hk and apcslee@cityu.edu.hk.APPLIED PHYSICS LETTERS 95, 153303 20090003-6951/2009/9515/153303/3/$25.00 © 2009 American Institute of Physics95, 153303-1Downloaded 07 May 2013 to 147.8.230.100. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsand C60. The poor JSC may be attributed to the absence ofC60 exciton dissociation see discussion below and the rela-tively low absorption coefficients of dye materials. In par-ticular, the absorption coefficients of all dye materials are onthe order of 104 cm−1, which are one order of magnitudelower than those of CuPc and C60.17 The low absorptionwould inevitably lead to a smaller number of photogeneratedexcitons, and thus a lower photocurrent. The low photocur-rent may also be attributed to the relatively low carrier mo-bility in dye materials. Particularly, carrier mobilities in dyematerials on the order of 10−5–10−7 cm2 V−1 s−1 are com-paratively smaller than those in CuPc and C60, provided thatthe hole mobility in CuPc is on the order of10−4 cm2 V−1 s−1 and the electron mobility in C60 is on theorder of 10−2 cm2 V−1 s−1. The low carrier mobility wouldimpede the charge carrier transport to the respective elec-trodes. It should be mentioned that the JSC for the DPVBi-based device is exceptionally low 0.67 mA /cm2, aboutone-fourth that of the devices with other dyes. One reasonfor the low JSC is the extremely low absorption of DPVBi inthe visible spectrum. Figure 1 depicts the normalized absorp-tion spectra of the thin films of the dye materials on quartzsubstrates. Clearly, DPVBi absorbs light in the UV regionthat coincides with the absorption cutoff of the ITO coatedglass substrate, i.e., wavelength 360 nm. This definitelylimits photon harvest in the DPVBi layer.On the other hand, the VOC increases from 0.46 to 0.62 Vand then falls off to 0.48 V when a C545T interlayer is used.To investigate the controlling factors of VOC in the three-component OPV devices, the VOC of devices are plotted as afunction of IP of the dyes in Fig. 2. The VOC of a device withan interlayer of smaller IP i.e., IP5.5 eV is generallylarger than that of a device with an interlayer having IP equalor larger than 5.6 eV. In addition, by subtracting the VOC ofthe former devices by 0.145 V, a linear fit to the data with aslope of 0.24 and a correlation coefficient R2 of 0.96 can bedrawn. In particular, the line represents a linear relation be-tween the VOC and the IP of dye materials. It is interesting tonote that the linear fit intercepts the x-axis at 3.6 eV, whichcorresponds to the LUMO level of C60 measured by inversephotoelectron spectroscopy.18 This is in good agreement withan increase in VOC with increasing the energy difference, thatis, HOMO dye–LUMO C60 as derived.The voltage difference of 0.145 V may be attributed tothe VOC contribution from the presence of C60 exciton disso-ciation. Table II lists the energy levels of organic materials.All energy levels of organic materials are taken from theliteratures,10–15,18 in which the LUMO levels of CuPc andC60 were measured by inverse photoelectron spectroscopy, socalled “true LUMO”, and those of dye materials were esti-mated from the optical absorption spectra, referred as “opti-cal LUMO”, in which the exciton binding energies EB ofthe dye materials have not been considered. EB is the mini-mum energy needed to overcome the Coulomb electric forceto separate the exciton into free electron and hole, and isgiven by the energy difference between the true LUMO andthe optical LUMO, i.e., EB=Etrue LUMO−Eoptical LUMO. Dju-rovich et al.18 recently reported that EB in most molecularorganic semiconductors linearly increases with increasingenergy gap either transport gap, ET, i.e., ET=EHOMOTABLE I. Key photovoltaic responses for devices without and with differ-ent dye interlayers.DyeJSCmA /cm2VOCV FFP%Standard 6.40 0.46 0.52 1.54DCJTB 2.48 0.51 0.42 0.53DCJPB 3.41 0.53 0.43 0.76TBRB 2.90 0.60 0.39 0.67RB 3.47 0.62 0.39 0.83C5454T 3.98 0.48 0.45 0.86DPVBi 0.67 0.54 0.44 0.16FIG. 1. Color online Normalized absorption spectra of thin films of dyematerials on quartz substrates. Inset: Transmission of ITO coated glass sub-strate with reference to air.FIG. 2. Color online VOC of devices with different dye interlayers as afunction of IP of dye materials.TABLE II. Energy levels of dye materials used. Eopt, ET, and EB are theoptical bandgap, the transport gap, and the exciton binding energy in theorganic materials, respectively. The electron affinities of CuPc and C60 weremeasured by the inverse photoelectron spectroscopy, in which Etrue LUMOlevels were directly determined.Material CuPc DCJTB DCJPB TBRB RB C545T DPVBi C60EA eV 3.2 3.1 3.2 3.2 3.3 2.8 2.8 3.5IP eV 5.2 5.1 5.3 5.4 5.5 5.6 5.9 6.2Eopt eV ¯ 2.0 2.1 2.2 2.2 2.8 3.1 ¯ET eV 2.0 2.3 2.4 2.6 2.6 3.4 3.9 2.7EB eV 0.3 0.3 0.3 0.4 0.4 0.6 0.8 0.7153303-2 Lai et al. Appl. Phys. Lett. 95, 153303 2009Downloaded 07 May 2013 to 147.8.230.100. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions−Etrue LUMO, or optical bandgap, Eopt, i.e., Eopt=EHOMO−Eoptical LUMO, with a minimum of 0.25 eV for ET=2 eV to1.5 eV for ET=6 eV.For efficient exciton dissociation into free charge carriersat the donor/acceptor interface, the energy level offset for theHOMO or LUMO EHOMO or ELUMO, respectivelyshould be larger than EB.19,20 In particular, EHOMOELUMO should be larger than the acceptor donor excitonbinding energy for efficient exciton dissociation in the accep-tor donor.20 In other words, excitons in C60 would be dis-sociated if the EHOMO at the interlayer/C60 interface islarger than the EB in C60 0.7 eV; while excitons in CuPconly dissociate if the ELUMO at the CuPc/interlayer inter-face is larger than 0.3 eV.18 At the same time, excitons in dyematerial would be dissociated if the EHOMO at the CuPc/interlayer and ELUMO at the interlayer/C60 interface, respec-tively, are larger than the EB in dye material. Table III sum-marizes the conditions for exciton dissociation at differentorganic/organic interfaces, in which the ELUMO is calcu-lated from the difference between the Etrue LUMO CuPc orC60 and Etrue LUMO dye, as shown in Table II. At theinterlayer/C60 interfaces, efficient C60 exciton dissociationtakes place at the interfaces between the dye materials andC60. Except at the C545T/C60 and DPVBi/C60 contacts, theclose proximity of the HOMO levels of C60 and dye materi-als prohibits C60 exciton dissociation; particularly theEHOMO is much smaller than the exciton binding energy inC60 EB=0.7 eV. On the other hand, all excitons in theinterlayer are expected to dissociate at the interlayer/C60 in-terfaces i.e., ELUMOEB dye, especially when the EB indye material has been counted. However, it is not the case atthe CuPc/interlayer interfaces, in which only CuPc excitonscan be efficiently dissociated. The close proximity of theHOMO levels of CuPc and dye materials prohibits the dyeexciton dissociation; particularly the EHOMO is muchsmaller than the exciton binding energy in dye materials.As C60 exciton dissociation only takes place under cer-tain conditions, this may account for the origin of the voltagedifference between devices with interlayer having smaller IPi.e., IP5.5 eV and those having IP equal or greater than5.6 eV. This may also explain the loss of photocurrent whena 5 nm thick tinII phthalocyanine SnPc is inserted be-tween the CuPc and C60 layers, as here the energy leveloffset is not sufficiently large for efficient CuPc exciton dis-sociation and thus results in the lower photocurrent, as re-ported by Rand et al.9 It is worth noting that the presentresults suggest that VOC is related to exciton dissociation inboth the donor and the acceptor. The absence of either donoror acceptor exciton dissociation would result in a lower VOC.The exact mechanism or physical basis for this is still un-clear, and more investigation is definitely needed.In summary, we have used fluorescent dyes as interfaciallayers between CuPc and C60 to form the cascade-type en-ergy level structure. With the insertion of the dye interlayer,photovoltaic responses such as JSC and FF are reduced com-pared to those of the standard device. The decreased photo-current may be attributed to the absence of C60 exciton dis-sociation, given that the absorption coefficients in CuPc andC60 are an order of magnitude higher than those in dye ma-terials, as well as the relatively low carrier mobility in dyematerials. On the other hand, the VOC linearly increases withthe IP of the dye interlayer, provided that the energy offset ofHOMO levels at the interlayer/C60 interfaces is larger thanthe C60 exciton binding energy. In particular, the VOC falls offwhen the IP of dye materials is equal to or greater than5.6 eV, which may be attributed to the absence of C60 excitondissociation. Our findings provide guideline in material anddevice design with respect to improving the photovoltaic re-sponses of trilayer OPV devices.This work was supported by the Research Grants Coun-cil of Hong Kong Project No. CityU 101707.1P. Peumans, S. Uchida, and S. R. Forrest, Nature London 425, 1582003.2S. Uchida, J. Xue, B. P. Rand, and S. R. Forrest, Appl. Phys. Lett. 84,4218 2004.3P. Sullivan, S. Heutz, S. M. Schultes, and T. S. Jones, Appl. Phys. Lett.84, 1210 2004.4T. Taima, M. Chikamatsu, Y. Yoshida, K. Saito, and K. Yase, Appl. Phys.Lett. 85, 6412 2004.5M. C. Scharber et al., Adv. Mater. 18, 789 2006.6B. P. Rand, D. P. Burk, and S. R. Forrest, Phys. Rev. B 75, 115327 2007.7S. Sista, Y. Yao, Y. Yang, M. L. Tang, and Z. Bao, Appl. Phys. Lett. 91,223508 2007.8Y. Kinoshita, T. Hasobe, and H. Murata, Appl. Phys. Lett. 91, 0835182007.9B. P. Rand, J. Xue, F. Yang, and S. R. Forrest, Appl. Phys. Lett. 87,233508 2005.10Y. M. Wang, F. Teng, Z. Xu, Y. B. Hou, S. Y. Yang, and X. R. Xu, Mater.Chem. Phys. 92, 291 2005.11M. J. Chang, W. Y. Huang, and W. C. Huang, U.S. Patent No. 6,649,08918 November 2003.12Y. S. Wu, T. H. Liu, H. H. Chen, and C. H. Chen, Thin Solid Films 496,626 2006.13H. Kanno, Y. Hamada, and H. Takahashi, IEEE J. Sel. Top. QuantumElectron. 10, 30 2004.14K. Okumoto, H. Kanno, Y. Hamaa, H. Takahashi, and K. Shibata, Appl.Phys. Lett. 89, 063504 2006.15W. C. Shen, Y. K. Su, and L. W. Ji, J. Cryst. Growth 293, 48 2006.16V. Shrotriya, G. Li, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Adv.Funct. Mater. 16, 2016 2006.17P. Peumans, A. Yakimov, and S. R. Forrest, J. Appl. Phys. 93, 36932003.18P. I. Djurovich, E. I. Mayo, S. R. Forrest, and M. E. Thompson, Org.Electron. 10, 515 2009.19S. S. Sun, Mater. Sci. Eng., B 116, 251 2005.20P. Peumans and S. R. Forrest, Chem. Phys. Lett. 398, 27 2004.TABLE III. Conditions for exciton dissociation at the organic/organic inter-faces, given that EB of CuPc and C60 are 0.3 and 0.7 eV, respectively.ELUMO is calculated from the difference between the Etrue LUMO CuPc orC60 and Etrue LUMO dye, in which Etrue LUMO dye is estimated accordingto the equation EB=Etrue LUMO−Eoptical LUMO. 3 and ✗ show whether exci-tons can or cannot be dissociated in the particular layers at various inter-faces.Interface ELUMO EHOMO CuPc Dye C60CuPc /C60 0.3 1.0 3 ¯ 3CuPc/DCJTB 0.4 0.1 3 ✗ ¯CuPc/DCJPB 0.3 0.1 3 ✗ ¯CuPc/TBRB 0.4 0.2 3 ✗ ¯CuPc/RB 0.3 0.3 3 ✗ ¯CuPc/C545T 1.0 0.4 3 ✗ ¯CuPc/DPVBi 1.2 0.7 3 ✗ ¯DCJTB/C60 0.7 1.1 ¯ 3 3DCJPB/C60 0.6 0.9 ¯ 3 3TBRB/C60 0.7 0.8 ¯ 3 3RB/C60 0.6 0.7 ¯ 3 3C545T/C60 1.3 0.6 ¯ 3 ✗DPVBi/C60 1.5 0.3 ¯ 3 ✗153303-3 Lai et al. Appl. Phys. Lett. 95, 153303 2009Downloaded 07 May 2013 to 147.8.230.100. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

Impact of dye interlayer on the performance of organic photovoltaic devices

http://hub.hku.hk/bitstream/10722/69840/1/content.pdf

Impact of dye interlayer on the performance of organic photovoltaic devices

Abstract

Similar works

Full text

Available Versions

Crossref

HKU Scholars Hub