Electronic properties and open-circuit voltage enhancement in mixed copper phthalocyanine:fullerene bulk heterojunction photovoltaic devices T. W. Ng, M. F. Lo, M. K. Fung, S. L. Lai, Z. T. Liu et al. Citation: Appl. Phys. Lett. 95, 203303 (2009); doi: 10.1063/1.3264966 View online: http://dx.doi.org/10.1063/1.3264966 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v95/i20 Published by the AIP Publishing LLC.
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APPLIED PHYSICS LETTERS 95, 203303 共2009兲
Electronic properties and open-circuit voltage enhancement in mixed copper phthalocyanine:fullerene bulk heterojunction photovoltaic devices T. W. Ng, M. F. Lo, M. K. Fung, S. L. Lai, Z. T. Liu, C. S. Lee,a兲 and S. T. Leeb兲 Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, Hong Kong
共Received 19 August 2009; accepted 14 October 2009; published online 18 November 2009兲 While bulk heterojunctions 共HJ兲 have been used in organic photovoltaic 共OPV兲 devices, there are few studies on their interface electronics and mechanisms on device performance. Here, we studied the electronic structure of a mixed CuPc: C60 and a discrete CuPc/ C60 junctions using photoemission spectroscopy. The HOMOCuPc-LUMOC60 energy offset, which controls the theoretical maximum open-circuit voltage 共Voc兲, was increased from 0.64 to 1.13 eV by mixing CuPc with C60. This Voc increase is attributed to the underlying substrate work function and charge transfer between two molecules. The results provide an understanding of the Voc enhancement in OPV devices with bulk HJ. © 2009 American Institute of Physics. 关doi:10.1063/1.3264966兴 Organic photovoltaic 共OPV兲 devices have received much attention due to their potential as an alternative source for low-cost renewable energy.1 Extensive research efforts have been devoted to enhancing device performance by adopting smart device configurations. Among them, the employment of a homogeneously mixed donor-acceptor 共D:A兲 layer in a so-called bulk heterojunction 共HJ兲 device is an important advance.2–4 The enhancement is attributed to efficient exciton dissociation arising from the enormous nanometer-sized D:A interfaces, which allow dissociation throughout the whole HJ layer leading to exciton dissociation efficiencies close to 100%.5 Moreover, the thickness of the photoactive film layer is no longer limited by the exciton diffusion length, and thus can be increased suitably to maximize light harvesting. Indeed, using bulk HJ has led to increases in both the short circuit current density 共Jsc兲 and the open circuit voltage 共Voc兲.2,6,7 While the increase in Jsc was attributed to enhanced optical absorption and exciton dissociation, the cause of Voc increase is not yet clear. It is recognized that the Voc is limited by the energy level offset between the highest occupied molecular orbital of the donor 共HOMOD兲 and the lowest unoccupied molecular orbital of the acceptor 共LUMOA兲.8 From the vacuum-aligned model, Voc is equal to the difference between the ionization potential of the donor and the electron affinity of the acceptor. Within this model, the Voc of the OPV device with a D:A structure should be identical to that with a discrete D:A bilayer structure. As the efficiency of OPV device is limited by Voc, it is thus of interest to understand the device physics governing the Voc in bulk HJ OPV device.9 Therefore, we study the electronic energy alignments of OPV based on bulk HJ layer of CuPc: C60 using photoemission spectroscopy. Compared to our previous results on the bilayer of CuPc/ C60,10 the bulk HJ layer of CuPc: C60 shows distinctly different electronic structure with the HOMOD-LUMOA offset significantly varying from 0.64 of 1.13 eV. It is shown that the Voc enhancement is mainly due to two important factors. First, the electronic structure of an organic-organic 共O-O兲 HJ, includElectronic mail:
[email protected] 共C.S. Lee兲. Electronic mail:
[email protected] 共S.T. Lee兲.
a兲
b兲
0003-6951/2009/95共20兲/203303/3/$25.00
ing the molecular level offset and dipole, is significantly influenced by the effective work function of the organic underlayer predeposited on the substrate. More importantly, there is an observable charge transfer between the molecules of CuPc and C60 leading to a considerable shift of electron energy levels across the junction. Photoemission experiments were performed in situ using a VG ESCALAB 220i-XL ultrahigh vacuum 共UHV兲 surface analysis system with a base vacuum of 10−10 Torr. A Hedischarge lamp 共21.22 eV兲 and monochromatic Al K␣ x-ray 共1486.6 eV兲 sources were used, respectively. for ultraviolet photoemission spectroscopy 共UPS兲 and x-ray photoemission spectroscopy 共XPS兲. The instrumental energy resolution was 90 meV as estimated from the Fermi edge of a cleaned Au film. In this letter, indium tin oxide 共ITO兲 coated glass was used as the substrate. CuPc, CuPc: C60 共with doping ratio 1:1兲 and C60 were deposited by thermal evaporation stepwise onto the substrate under a deposition pressure of 5 – 6 ⫻ 10−9 Torr so as to prevent the oxygen doping of C60. The details regarding experimental procedure and equipment were described elsewhere.10 Shown in Fig. 1 are the UPS spectra of the CuPc: C60 mixed films at the secondary electron cut-off and the near the Fermi edge. Upon deposition of CuPc: C60, the high binding
FIG. 1. 共Color online兲 UPS spectra 共He I兲 of ITO/ CuPc: C60 interface showing the onset of the low electron energy cut-off and the HOMO edge near the Fermi level with increasing coverage of CuPc: C60 film. The position marked “0 eV” corresponds to the Fermi energy position.
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© 2009 American Institute of Physics
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FIG. 2. 共Color online兲 UPS spectra 共He I兲 of CuPc/ CuPc: C60 / C60 interface showing the onset of the low electron energy cut-off and the HOMO edge near the Fermi level as a function of increasing thickness of CuPc: C60 and C60 film.
energy cutoff region, which reflects the vacuum level change of the sample show an obvious shift of upto 0.43 eV toward higher binding energy. Concurrently, the highest occupied molecular orbital 共HOMO兲 peaks of C60 and CuPc show similar up-shifts of upto 0.42 and 0.44 eV, respectively. The amount of shift in both cutoff and HOMO edges were saturated when the thickness reaches 100 Å, it is consistent to the XPS results 共not shown兲. The similar shifts of the HOMO and vacuum level indicate no or little interfacial dipole at the ITO/ CuPc: C60 interface; instead the energy level shifts are attributed to the band bending in the CuPc: C60 film. It is noted that new state emerges within the energy gap 共peak position at ⬃0.25 eV兲 in the mixed film, and its intensity increases monotonically with increasing film thickness. The peak separation between the HOMO of CuPc and the gap state is ⬃0.6 eV, which remains the same for all film thickness. Note that this peak was not observed in the pristine CuPc or C60 films in our previous letter.10,11 It suggests that the gap state may probably be generated by the charge transfer between the molecules of CuPc and C60. However, there is no new core level peak in the XPS spectrum 共not shown兲. Unlike the case for metal doping, the interaction between molecules may be too weaker to lead to observable new bonding. To further examine the charge transfer properties, similar experiments were carried out to study the interfaces in the CuPc/ CuPc: C60 / C60 system. Figure 2 shows the UPS spectra of the corresponding interfaces. The bottom UPS spectrum refers to the pristine CuPc underlayer, which undergoes a small shift of 0.14 eV in secondary electron cutoff toward lower binding energy upon CuPc: C60 deposition. On the other hand, a shift of 0.06 eV in the opposite direction occurs upon subsequent deposition of pure C60 on CuPc: C60. Upon deposition of the CuPc: C60 mixed layer, the HOMO peak of CuPc shifted 共0.42 eV兲 toward higher binding energy. It cannot be clearly identified whether the measured shift is coming from signal of the underlying pure CuPc or the mixed CuPc: C60 addlayers, the 0.42 eV shift labeled is the overall shift of the CuPc HOMO. It is noted that the UPS peaks of C60 remained unchanged throughout the deposition process. Based on the results from Figs. 1 and 2, the corresponding energy level diagrams are obtained and shown in Figs. 3共a兲 and 3共b兲. For discussion on electronic differences between the intermixed CuPc: C60 and discrete CuPc/ C60 interfaces, the energy level diagram for the ITO/ CuPc/ C60 bi-
Appl. Phys. Lett. 95, 203303 共2009兲
FIG. 3. 共Color online兲 Schematic energy level diagrams of the 共a兲 ITO/ CuPc: C60 HJ; 共b兲 ITO/ CuPc/ CuPc: C60 / C60 junctions; and 共c兲 discrete ITO/ CuPc/ C60 junction, extracted from our previous results 共see Ref. 6兲. The band structure of CuPc is presented in red while that of C60 in blue. All numbers are in electron volts.
layer interface is extracted from our previous results and shown in Fig. 3共c兲.10 In Fig. 3共c兲, the ITO/CuPc interface is represented by two dotted lines as the interface formation process has not been traced in detail. Nevertheless, as the ITO substrates in Figs. 3共b兲 and 3共c兲 have similar work functions, it is reasonable to expect that the electronic structure at the ITO/CuPc interface in these two cases should be similar. The LUMO levels of CuPc and C60 are estimated by subtracting the charge transport gaps of 1.90 and 2.30 eV, respectively, form their corresponding HOMOs.12 Figure 3 clearly shows that the electronic structure of the mixed CuPc: C60 layer 关Figs. 3共a兲 and 3共b兲兴 is distinctly different from that of the bilayer CuPc/ C60 关Fig. 3共c兲兴, i.e., the HOMOD-LUMOA or HOMOCuPc-LUMOC60 energy offset increases from 0.64 eV 关Fig. 3共c兲兴 to 1.13 eV 关Fig. 3共a兲兴 in the mixed materials. The interface of ITO/ CuPc: C60 关Fig. 3共a兲兴 can be considered to be comprised of the two following interfaces: CuPc-on-ITO and C60-on-ITO. Upon contact formation, the energy levels of both CuPc and C60 align themselves according to the substrate work function 共⬃5.0 eV兲 of the ITO glass showing a ⬃0.4 eV downward band bending, resulting in a HOMOCuPc-LUMOC60 difference of 1.13 eV. In contrast to the ITO/ CuPc: C60 interface, the C60 layer in the discrete CuPc/ C60 structure 关Fig. 3共c兲兴 aligns its energy with respect to the ITO/CuPc layers with an “effective” substrate work eff 13 eff 兲. This results in a ⌽sub of 4.49 eV on the function 共⌽sub CuPc covered ITO 共ITO/CuPc兲 surface. This value somehow restricts the energy level alignment of addlayers. Upon deposition of C60, the energy levels of C60 aligns by matching the vacuum level of the CuPc yielding a negligible interfacial dipole of ⬃0.1 eV. At the same time, it leads to a flatband electronic structure at the CuPc/ C60 interface and resulting in a HOMOCuPc-LUMOC60 difference of 0.64 eV. The totality of results suggests that the electronic structure of Organiceff of the organic modified Organic HJ is dependent on the ⌽sub substrates. According to the above discussion, the energy level alignment of CuPc/ CuPc: C60 关Fig. 3共b兲兴 interface should be similar to that of CuPc/ C60 关Fig. 3共c兲兴 layers since the system now consists of the CuPc-on-CuPc and C60-on-CuPc interfaces. Indeed, the CuPc in the mixed layer should follow the original energy level alignments of the underlying CuPc layer 关Fig. 3共b兲兴. Meanwhile, the energy level alignment of C60 throughout the interlayer CuPc/ CuPc: C60 / C60 should be similar to that shown in Fig. 3共c兲 共ITO/ CuPc/ C60兲. It is be-
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Ng et al. TABLE I. Summary of OPV device performance Device configuration
Jsc/mA cm−2
Voc/V
FF
p/%
2.88
0.38
0.44
0.48
8.83 11.4
0.45 0.47
0.33 0.41
1.29 2.19
Discrete bilayer heterojunction cells 共a兲 ITO/CuPc共34nm兲/C60共40nm兲/Al Mixed-layer heterojunction cells 共b兲 ITO/CuPc:C60共74nm兲/Al 共c兲 ITO/CuPc共5nm兲/CuPc:C60共64nm兲/C60共5nm兲/Al
cause in both cases, CuPc and C60 share the same effective eff of ⬃4.5 eV. However, in substrate ITO/CuPc with an ⌽sub Fig. 3共b兲, the HOMO of CuPc in the mixed layer shows an extra 0.42 eV downward banding upon deposition on the CuPc pristine film, whereas the HOMO of C60 关Fig. 3共c兲兴 remains unchanged throughout the entire deposition process. In the case of Fig. 3共b兲, C60 in the mixed layer behaves as a p-type dopant with a strong electron accepting capability that exerts a profound effect in shifting the position of the Fermi level within the energy band gap of the CuPc in a direction toward the LUMO. This results in the observed 0.42 eV downward shift in the HOMO level of the CuPc. The mixing induced charge transfer in the CuPc: C60 layer is obviously an important cause for the increase in HOMOD-LUMOA or HOMOCuPc-LUMOC60 energy gap from 0.64 to 1.05 eV and a higher Voc in the OPV cell with bulk HJ. It is noteworthy that in Fig. 3共b兲, the downward band bending induced by the charge transfer between CuPc and C60 molecules provides an efficient conduction pathway for hole by mean of cascade-energy structure. The photogenerated exciton first dissociates in the CuPc: C60 layer with a large HOMOD-LUMOA energy gap 共1.05 eV兲, resulting in a large Voc. After dissociation, hole created in the HOMO of the CuPc and electron generated in LUMO of the C60 can flow spontaneously crossing the interfacial layers with small transport barriers toward the two electrodes. The cascadeenergy structure facilitates the charge collection at two electrodes. These results suggest that OPV devices with the mixed C60 / CuPc layer should have a higher Voc and short circuit current 共Jsc兲. To corroborate the above analysis, we fabricated OPV devices with configurations shown in Fig. 3. To eliminate other possible side-effects, the exciton blocking layer typically used in other devices is not used here. Table I summarizes the key operation parameters of the OPV devices. All measurements were performed under illumination with an intensity of 100 mW/ cm2 with AM 1.5 G filter. Under illumination, devices b and c consisting of a CuPc: C60 mixed layer show comparable Voc at ⬃0.46 V, which is larger than that 共0.38 V兲 of device a with a discrete CuPc/ C60 layer. Moreover, the OPV device c with a CuPc/ CuPc: C60 / C60 structure shows the highest power conversion efficiency of
2.19%, which is attributed to the cascade energy level alignment. The results shown in Table I are consistent with photoemission spectroscopy results that OPV devices with a mixed CuPc: C60 layer would show better performance than those with a discrete CuPc/ C60 bilayer. In conclusion, we have studied and contrasted the electronic structures of the intermixed CuPc: C60 layer and the discrete CuPc/ C60 bilayer. The HOMOD-LUMOA or HOMOCuPc-LUMOC60 offset is significantly enhanced from 0.64 to 1.13 eV by inserting a CuPc: C60 mixed layer. The enhancement is attributed to the influence of the effective substrate work function on the energy level alignments at the O-O HJ, as a result of, doping-induced charge transfer which modifies the position of the Fermi level within the band gap of the CuPc and thus the energy level alignments of the CuPc: C60 layer. The present work provides an understanding on the D-A energy level alignment and the operation enhancement mechanism in the OPV with a bulk HJ. The project is supported by the Research Grants Council 共RGC兲 of Hong Kong, Hong Kong SAR 共Project code: CityU 101707兲. C. W. Tang, Appl. Phys. Lett. 48, 183 共1986兲. J. Xue, S. Uchida, B. P. Rand, and S. R. Forrest, Appl. Phys. Lett. 85, 5757 共2004兲. 3 P. Peumans, S. Uchida, and S. R. Forrest, Nature 共London兲 425, 158 共2003兲. 4 P. Sullivan, S. Heutz, S. M. Schultes, and T. S. Jones, Appl. Phys. Lett. 84, 1210 共2004兲. 5 J. Xue, B. P. Rand, S. Uchida, and S. R. Forrest, Adv. Mater. 共Weinheim, Ger.兲 17, 66 共2005兲. 6 J. Drechsel, B. Männig, F. Kozlowshi, M. Pfeiffer, K. Leo, and H. Hoppe, Appl. Phys. Lett. 86, 244102 共2005兲. 7 W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, Adv. Funct. Mater. 15, 1617 共2005兲. 8 B. P. Rand, D. P. Burk, and S. R. Forrest, Phys. Rev. B 75, 115327 共2007兲. 9 Y. Kinoshita, R. Takenaka, and H. Murata, Appl. Phys. Lett. 92, 243309 共2008兲. 10 T. W. Ng, M. F. Lo, Y. C. Zhou, Z. T. Liu, C. S. Lee, O. Kwon, and S. T. Lee, Appl. Phys. Lett. 94, 193304 共2009兲. 11 Y. C. Zhou, Z. T. Liu, J. X. Tang, C. S. Lee, and S. T. Lee, J. Electron Spectrosc. Relat. Phenom. 174, 35 共2009兲. 12 X. Tong, R. F. Bailey-Salzman, G. Wei, and S. R. Forrest, Appl. Phys. Lett. 93, 173304 共2008兲. 13 J. X. Tang, C. S. Lee, and S. T. Lee, Appl. Surf. Sci. 252, 3948 共2006兲. 1 2
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