- Email: [email protected]
Ultraviolet-illuminated fluoropolymer indium–tin-oxide buffer layers for improved power conversion in organic photovoltaics
Organic Electronics 10 (2009) 1178–1181
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Ultraviolet-illuminated fluoropolymer indium–tin-oxide buffer layers for improved power conversion in organic photovoltaics Bonan Kang a,b, L.W. Tan a, S.R.P. Silva a,* a b
Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom State Key Laboratory on Integrated Optoelectronics, Jilin University Region, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 19 February 2009 Received in revised form 7 May 2009 Accepted 13 May 2009 Available online 18 May 2009 PACS: 72.80Le Keywords: Organic photovoltaic Fluoropolymer UV treatment PTFE
a b s t r a c t We demonstrate that the charge carrier extraction in double heterojunction organic photovoltaic(OPV) devices can be enhanced by inserting an UV-illuminated fluoropolymer polytetrafluoroethylene(PTFE) layer between indium–tin-oxide and the thermal evaporated copper–phthalocyanine(CuPc)/buckyball(C60) organic active layers. In this work, we show that the anode work function influences the photocarrier collection characteristics, where the short-circuit current and open-circuit voltage increase from 1.6 to 4.8 mA/cm2 and 0.41 to 0.48 V, respectively after the buffer layer insertion associated primary with the barrier decrease in the ITO/CuPc interface. This result shows the potential of UV-illuminated PTFE as a low-cost stable buffer layer for OPV devices. Ó 2009 Published by Elsevier B.V.
Organic photovoltaics(OPVs) have steadily improved in power conversion efficiency over the last decade and have attracted interest due to their advantages in low-cost, large-area, mechanically flexible substrates [1–3]. The progress of small-molecule based OPV cells is chiefly attributable to the introduction of the donor–acceptor heterojunction that functions as dissociation sites for the strongly bound photogenerated excitons [4–6]. Material choice and device processing techniques have also been a means of increasing power conversion efficiency [4]. According to Forrest et al., direct contact between the deposited electrode and the active organics leads to quenching of excitons [7]. As a consequence, the double heterojunction cells are developed to confine the excitons within the active layers, and subsequently allowing substantially high internal efficiencies to be achieved [7]. Re* Corresponding author. Tel.: +44 0 1483689825; fax: +44 0 1483300803. E-mail addresses: [email protected] (B. Kang), s.silva@ surrey.ac.uk (S.R.P. Silva). 1566-1199/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.orgel.2009.05.011
search also shows that the photocarrier collection characteristics are significantly influenced by the anode work function [4]. Several studies have shown the improvement of organic light-emitting diodes(OLEDs) and OPVs by inserting fluorocarbon based materials such as CFx and polytetrafluoroethylene(PTFE) in between ITO anode and organic layer [8–10]. In this work, we studied the influence of the PTFE thickness and the effect of UV-illumination on PTFE on the device performance parameter in term of short-circuit voltage(Jsc), open-circuit voltage(Voc), fill factor(FF) and power conversion efficiencies(PCE). The improved results are primary a consequence of the formation of an artificial dipole layer resulting from the rich, negatively charged fluorine that facilitates the hole extraction process. The UV-illumination process further enhances the effect of this artificial dipole layer and subsequently reduces the barrier height between the ITO/copper phthalocyanine interface, which can be concluded as a result of measurement of the work function on UV-illuminated ITO/PTFE samples using Kelvin probe.
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B. Kang et al. / Organic Electronics 10 (2009) 1178–1181
The double heterostructure OPV cells were fabricated on pre-cleaned glass substrates pre-coated with an ITO anode. ITO glass is pre-cleaned following an oxygen plasma treatment for 3 min prior to use for device fabrication. Films were grown at room temperature with a base pressure of 2 10 5 Torr in the following sequence: thermal evaporation of PTFE layer varied between 0.3, 0.5 and 1.0 nm; 20 nm thick film of the donor-like copper phthalocyanine(CuPc); followed by a 40 nm thick film of the acceptor-like buckyball C60. A 12 nm thick large band-gap material, bathocuproine(BCP) was deposited as an exciton-blocking layer(EBL) because a previous report has found that BCP can be used to transport electrons to the cathode from the adjoining acceptor layer through the cathode-induced defect states in EBL energy gap, at the same time as effectively blocking excitons in the lower-energy-gap acceptor layer from recombining at the cathode [4]. Subsequently, an 100 nm thick Al cathode was deposited by thermal evaporation through a shadow mask. A Honle UV America Inc. UV light source SOL 500 I was used, with a stabilization period of at least 10 min prior to UV exposure on PTFE films. The work function of ITO/PTFE layers before and after UV exposure were determined by examining the contact potential difference(CDP) of the sample using a Kelvin probe arrangement (SKP5050, KP Technologies). All the current–voltage (I–V) measurements were made with a Keithley 2400 source meter in air ambient, in dark and under 82 mW/cm2 solar irradiance (Oriel light source). The active area for all the devices was in excess of 0.1 cm2 for this study. The consistency of the device performance was obtained by measuring a minimum of 5 samples for each individual devices structure. In Fig. 1, the Jsc of the devices are plotted as a function of the PTFE thicknesses. It is clear that the device with 0.5 nm of PTFE as the buffer layer has better performances in term of Jsc, Voc and FF compare to reference device with and without PEDOT:PSS. By inserting a 0.3 nm of PTFE, the Jsc increase 75% from 1.6 to 2.8 mA/cm2, while the Voc remained constant at 0.41 V (Table 1). The Voc are significantly increased from 0.41 to 0.49 V when the PTFE layer thickness increased to 1.0 nm. However, the Jsc is de-
Table 1 List of device performance of OPV devices without PTFE, with PTFE and with UV treated PTFE. Device structure ITO/CuPc/C60/BCP/Al ITO/PEDOT:PSS/CuPc/ C60/BCP/Al ITO/PTFE/CuPc/C60/ BCP/Al
Teflon thickness (nm)
Jsc (mA/ cm2)
Voc (V)
FF (%)
g
– –
1.6 2.5
0.41 0.44
54.9 52.4
0.44 0.70
0.3 0.5 1.0
2.8 2.5 2.1
0.4 0.45 0.47
60.9 59.1 27.9
0.83 0.81 0.34
ITO buffer layers 3.1 0.41 4.8 0.48 4.3 0.49
58.5 56.3 42.5
0.91 1.58 1.09
After ultraviolet-illuminated fluoropolymer 0.3 ITO/PTFE/CuPc/C60/ BCP/Al 0.5 1.0
(%)
creased when the PTFE thickness increase. Since PTFE is an insulating material, with an extremely high resistivity of 1018 X/cm for its bulk properties, and a large value for its ionization potential of 9.8 eV [11], it is reasonable to expect that the short-circuit current densities of OPVs decrease with increasing thickness of the PTFE layer. To study the effect UV treatment on the ITO/PTFE layers, Kelvin probe measurements were conducted on ITO, ITO/ PTFE(0.5 nm) and ITO/PTFE(0.5 nm) with different UV exposure times with the resultant the work functions listed in Table 2. The work function of the anode was shown increase to from 4.83 eV (bare ITO) to 5.00 eV after a 0.5 nm PTFE layer was deposited on the ITO. For the UVilluminated ITO/PTFE samples, the work function increased approximately to 5.17 eV after 5 min of UV-illumination. However, further increase of the UV exposure time decreased the work function of ITO/UV-illuminated PTFE layer as shown in Table 2. A similar treatment on the ITO buffer layer was used to make organic photovoltaic devices. The I–V characteristics of these OPV devices under solar simulator illumination are shown in Fig. 3. The Jsc and Voc of the devices improved in parallel with the increased UV-illumination time for the initial 5 min. However, further increase of the UV-illumination time beyond 5 min saw a decrease in the Jsc and the Voc. This observation is in agreement with the results from work function measurements where the work function increased for the first 5 min, with further increase in the illumination time reducing the work function of the ITO/PTFE anode.
Table 2 Work function of ITO, ITO/PTFE and ITO/PTFE with different UV exposure time measured by Kelvin probe.
Fig. 1. I–V characteristics of OPV devices with and without PEDOT:PSS and with the different PTFE thicknesses.
Sample
UV exposure time
Work function eV (±0.01)
Oxygen plasma cleaned ITO ITO/PTFE(0.5 nm)
–
4.84
– 1 min 4 min 5 min 7 min 10 min
5.00 5.13 5.15 5.17 5.17 5.02
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B. Kang et al. / Organic Electronics 10 (2009) 1178–1181
The high Jsc in OPV devices with PTFE layer are mainly due to the reduction in the barrier height between ITO and organic matrix [11] and subsequently improved hole extraction processes. The improvement in current extraction from CuPc layer to anode is mainly due to the smaller barrier of charge extraction and, hence, an increased photo-generated current under forward bias was observed with the increment in Jsc. As shown in Fig. 1, the Voc of the devices are strongly dependent on the PTFE thicknesses. For the reference devices, with the structure of ITO/CuPc/ C60/BCP/Al and ITO/PEDOT:PSS/CuPc/C60/BCP/Al, the typical Voc were found to be 0.41 V and 0.44 V, respectively (Table 1). However, the Voc is found to be improved up to 0.55 V for the device with 1 nm of UV treated PTFE buffer layer (Fig. 2). The work function of the ITO surface typically varies from 3.90 to 4.80 eV depending on the surface treatment [12,13]. After inserting the untreated PTFE layer and the UV treated PTFE layer, the Voc is improved from 0.41 to 0.49 V, because of the strong dipole layer created by the negatively charged fluorine rich untreated PTFE layer and UV treated PTFE layer that has modified the surface and further increased the effective work function of the ITO. As show in Table 1, the devices with UV treated PTFE buffer
Fig. 2. I–V characteristic of OPV devices consist of ITO/PTFE with different thicknesses exposed to 5 min UV treatment.
layer exhibited at least a factor of 2 increases in term of power conversion efficiency, compared to the devices with untreated PTFE buffer layer. Among the device with PTFE, a device with a 0.5 nm UV treated PTFE buffer layer exhibited the best performance with the PCE of 1.58%, which is attributed to the good coverage of 0.5 nm UV treated PTFE on the ITO to modify the ITO surface without increase in the surface resistance of ITO/UV treated PTFE anode. It can be seen from Fig. 2 that Jsc of the structure ITO/UV treated PTFE(0.5 nm)/CuPc/C60/BCP/Al is the highest because of the smallest hole extraction barrier. Interestingly, UV treatment of PTFE buffer layer can significantly change the contact properties [11]. Our results show that while the pristine PTFE layer is useful for reducing the contact barrier, its effect is considerably enhanced by UVillumination. Modification of polymers by exposure to UV irradiation has been reported previously [14,15]. Radiation energy can be absorbed via ionization, phonon excitation and atomic displacement. This causes bond breaking, followed by scissoring and subsequent release of volatile fragments, which may result in the cross-linking through C-C bonding. In our case, clusters of sp2 bonding may also be formed, leading to an increased in conductivity [8]. At present, there is no clear explanation with regard to the interaction of UV light with PTFE properties and PTFE is found to be highly resistant to UV exposure [16]. However, UV-illumination on a few atomic layers of PTFE may have a very different interactions compared to UV radiation and impact on bulk properties of PTFE. In 2004, Tong and his co-workers have shown that UV-illumination on fluorocarbon coatings CFx (the basic structure of PTFE), created the graphitic regions identified by X-ray photoelectron spectroscopy results. This leads to the higher conductivity of the CFx layer and further improved OLED performance [8]. The same explanation may apply to the UV-illuminated PTFE layer where the Jsc of the OPV is improved due to the relatively higher conductivity of PTFE after UV-illumination. In summary, insertion of UV treated PTFE buffer layer at the anode of ITO/organic interface can significantly improve the Jsc, Voc and PCE of bulk-heterojunction OPV. The improved performance in the UV treated PTFE-coated ITO contact are consistent with its small hole extraction barrier. This fluoro-material also offers a significant advantage in that the film can be simply prepared by thermal evaporation. Acknowledgement The authors gratefully acknowledge the financial support received from EPSRC in the form of a Portfolio Partnership Award and China Scholarship Council to partially fund this research. References
Fig. 3. The I–V characteristics of OPV devices consisting of PTFE (0.5 nm) as a buffer layer on ITO, which is exposed to different UV exposure times.
[1] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [2] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617. [3] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4 (2005) 864. [4] P. Peumans, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 126.
B. Kang et al. / Organic Electronics 10 (2009) 1178–1181 [5] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 58. [6] S. Uchida, J. Xue, B.P. Rand, S.R. Forrest, Appl. Phys. Lett. 84 (2001) 4218. [7] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [8] S.W. Tong, C.S. Lee, Y. Lifshitz, D.Q. Gao, S.T. Lee, Appl. Phys. Lett. 84 (2004) 4032. [9] Y.D. Gao, L.D. Wang, D.Q. Zhang, L. Duan, G.F. Dong, Appl. Phys. Lett. 82 (2003) 155. [10] B. Kang, L.W. Tan, S.R.P. Silva, Appl. Phys. Lett. 93 (2008) 133302. [11] S.W. Tong, K.M. Lau, M.K. Fung, C.S. Lee, Y. Lifshitz, S.T. Lee, Appl. Surf. Sci. 252 (2006) 3806.
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[12] F. Nüesch, L.J. Rothberg, E.W. Forsythe, Q.T. Le, G. Yongli, Appl. Phys. Lett. 74 (1999) 880. [13] J.S. Kim, M. Granström, R.H. Friend, N. Johansson, W.R. Salaneck, R. Daik, W.J. Feast, F. Cacialli, J. Appl. Phys. 84 (1998) 6859. [14] J. Heitz, V. Svorcik, L. Bacakova, K. Rockova, E. Ratajova, T. Gumpenberger, D. Bauerle, B. Dvorankova, H. Kahr, I. Graz, C. Romanin, J. Biomed. Mater. Res. Part A 67A (2003) 130. [15] V. Svorcik, K. Rockova, E. Ratajova, J. Heitz, N. Huber, D. Bauerle, L. Bacakova, B. Dvorankova, V. Hnatowicz, Nucl. Inst. Meth. Phys. Res. Sec. B-Beam Inter. Mater. Atoms 217 (2004) 307. [16] K. Gotoh, Y. Nakata, M. Tagawa, M. Tagawa, Colloids Surf. A-PhysicoChem. Eng. Aspects 224 (2003) 165.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Ultraviolet-illuminated fluoropolymer indium–tin-oxide buffer layers for improved power conversion in organic photovoltaics Bonan Kang a,b, L.W. Tan a, S.R.P. Silva a,* a b
Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom State Key Laboratory on Integrated Optoelectronics, Jilin University Region, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 19 February 2009 Received in revised form 7 May 2009 Accepted 13 May 2009 Available online 18 May 2009 PACS: 72.80Le Keywords: Organic photovoltaic Fluoropolymer UV treatment PTFE
a b s t r a c t We demonstrate that the charge carrier extraction in double heterojunction organic photovoltaic(OPV) devices can be enhanced by inserting an UV-illuminated fluoropolymer polytetrafluoroethylene(PTFE) layer between indium–tin-oxide and the thermal evaporated copper–phthalocyanine(CuPc)/buckyball(C60) organic active layers. In this work, we show that the anode work function influences the photocarrier collection characteristics, where the short-circuit current and open-circuit voltage increase from 1.6 to 4.8 mA/cm2 and 0.41 to 0.48 V, respectively after the buffer layer insertion associated primary with the barrier decrease in the ITO/CuPc interface. This result shows the potential of UV-illuminated PTFE as a low-cost stable buffer layer for OPV devices. Ó 2009 Published by Elsevier B.V.
Organic photovoltaics(OPVs) have steadily improved in power conversion efficiency over the last decade and have attracted interest due to their advantages in low-cost, large-area, mechanically flexible substrates [1–3]. The progress of small-molecule based OPV cells is chiefly attributable to the introduction of the donor–acceptor heterojunction that functions as dissociation sites for the strongly bound photogenerated excitons [4–6]. Material choice and device processing techniques have also been a means of increasing power conversion efficiency [4]. According to Forrest et al., direct contact between the deposited electrode and the active organics leads to quenching of excitons [7]. As a consequence, the double heterojunction cells are developed to confine the excitons within the active layers, and subsequently allowing substantially high internal efficiencies to be achieved [7]. Re* Corresponding author. Tel.: +44 0 1483689825; fax: +44 0 1483300803. E-mail addresses: [email protected] (B. Kang), s.silva@ surrey.ac.uk (S.R.P. Silva). 1566-1199/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.orgel.2009.05.011
search also shows that the photocarrier collection characteristics are significantly influenced by the anode work function [4]. Several studies have shown the improvement of organic light-emitting diodes(OLEDs) and OPVs by inserting fluorocarbon based materials such as CFx and polytetrafluoroethylene(PTFE) in between ITO anode and organic layer [8–10]. In this work, we studied the influence of the PTFE thickness and the effect of UV-illumination on PTFE on the device performance parameter in term of short-circuit voltage(Jsc), open-circuit voltage(Voc), fill factor(FF) and power conversion efficiencies(PCE). The improved results are primary a consequence of the formation of an artificial dipole layer resulting from the rich, negatively charged fluorine that facilitates the hole extraction process. The UV-illumination process further enhances the effect of this artificial dipole layer and subsequently reduces the barrier height between the ITO/copper phthalocyanine interface, which can be concluded as a result of measurement of the work function on UV-illuminated ITO/PTFE samples using Kelvin probe.
1179
B. Kang et al. / Organic Electronics 10 (2009) 1178–1181
The double heterostructure OPV cells were fabricated on pre-cleaned glass substrates pre-coated with an ITO anode. ITO glass is pre-cleaned following an oxygen plasma treatment for 3 min prior to use for device fabrication. Films were grown at room temperature with a base pressure of 2 10 5 Torr in the following sequence: thermal evaporation of PTFE layer varied between 0.3, 0.5 and 1.0 nm; 20 nm thick film of the donor-like copper phthalocyanine(CuPc); followed by a 40 nm thick film of the acceptor-like buckyball C60. A 12 nm thick large band-gap material, bathocuproine(BCP) was deposited as an exciton-blocking layer(EBL) because a previous report has found that BCP can be used to transport electrons to the cathode from the adjoining acceptor layer through the cathode-induced defect states in EBL energy gap, at the same time as effectively blocking excitons in the lower-energy-gap acceptor layer from recombining at the cathode [4]. Subsequently, an 100 nm thick Al cathode was deposited by thermal evaporation through a shadow mask. A Honle UV America Inc. UV light source SOL 500 I was used, with a stabilization period of at least 10 min prior to UV exposure on PTFE films. The work function of ITO/PTFE layers before and after UV exposure were determined by examining the contact potential difference(CDP) of the sample using a Kelvin probe arrangement (SKP5050, KP Technologies). All the current–voltage (I–V) measurements were made with a Keithley 2400 source meter in air ambient, in dark and under 82 mW/cm2 solar irradiance (Oriel light source). The active area for all the devices was in excess of 0.1 cm2 for this study. The consistency of the device performance was obtained by measuring a minimum of 5 samples for each individual devices structure. In Fig. 1, the Jsc of the devices are plotted as a function of the PTFE thicknesses. It is clear that the device with 0.5 nm of PTFE as the buffer layer has better performances in term of Jsc, Voc and FF compare to reference device with and without PEDOT:PSS. By inserting a 0.3 nm of PTFE, the Jsc increase 75% from 1.6 to 2.8 mA/cm2, while the Voc remained constant at 0.41 V (Table 1). The Voc are significantly increased from 0.41 to 0.49 V when the PTFE layer thickness increased to 1.0 nm. However, the Jsc is de-
Table 1 List of device performance of OPV devices without PTFE, with PTFE and with UV treated PTFE. Device structure ITO/CuPc/C60/BCP/Al ITO/PEDOT:PSS/CuPc/ C60/BCP/Al ITO/PTFE/CuPc/C60/ BCP/Al
Teflon thickness (nm)
Jsc (mA/ cm2)
Voc (V)
FF (%)
g
– –
1.6 2.5
0.41 0.44
54.9 52.4
0.44 0.70
0.3 0.5 1.0
2.8 2.5 2.1
0.4 0.45 0.47
60.9 59.1 27.9
0.83 0.81 0.34
ITO buffer layers 3.1 0.41 4.8 0.48 4.3 0.49
58.5 56.3 42.5
0.91 1.58 1.09
After ultraviolet-illuminated fluoropolymer 0.3 ITO/PTFE/CuPc/C60/ BCP/Al 0.5 1.0
(%)
creased when the PTFE thickness increase. Since PTFE is an insulating material, with an extremely high resistivity of 1018 X/cm for its bulk properties, and a large value for its ionization potential of 9.8 eV [11], it is reasonable to expect that the short-circuit current densities of OPVs decrease with increasing thickness of the PTFE layer. To study the effect UV treatment on the ITO/PTFE layers, Kelvin probe measurements were conducted on ITO, ITO/ PTFE(0.5 nm) and ITO/PTFE(0.5 nm) with different UV exposure times with the resultant the work functions listed in Table 2. The work function of the anode was shown increase to from 4.83 eV (bare ITO) to 5.00 eV after a 0.5 nm PTFE layer was deposited on the ITO. For the UVilluminated ITO/PTFE samples, the work function increased approximately to 5.17 eV after 5 min of UV-illumination. However, further increase of the UV exposure time decreased the work function of ITO/UV-illuminated PTFE layer as shown in Table 2. A similar treatment on the ITO buffer layer was used to make organic photovoltaic devices. The I–V characteristics of these OPV devices under solar simulator illumination are shown in Fig. 3. The Jsc and Voc of the devices improved in parallel with the increased UV-illumination time for the initial 5 min. However, further increase of the UV-illumination time beyond 5 min saw a decrease in the Jsc and the Voc. This observation is in agreement with the results from work function measurements where the work function increased for the first 5 min, with further increase in the illumination time reducing the work function of the ITO/PTFE anode.
Table 2 Work function of ITO, ITO/PTFE and ITO/PTFE with different UV exposure time measured by Kelvin probe.
Fig. 1. I–V characteristics of OPV devices with and without PEDOT:PSS and with the different PTFE thicknesses.
Sample
UV exposure time
Work function eV (±0.01)
Oxygen plasma cleaned ITO ITO/PTFE(0.5 nm)
–
4.84
– 1 min 4 min 5 min 7 min 10 min
5.00 5.13 5.15 5.17 5.17 5.02
1180
B. Kang et al. / Organic Electronics 10 (2009) 1178–1181
The high Jsc in OPV devices with PTFE layer are mainly due to the reduction in the barrier height between ITO and organic matrix [11] and subsequently improved hole extraction processes. The improvement in current extraction from CuPc layer to anode is mainly due to the smaller barrier of charge extraction and, hence, an increased photo-generated current under forward bias was observed with the increment in Jsc. As shown in Fig. 1, the Voc of the devices are strongly dependent on the PTFE thicknesses. For the reference devices, with the structure of ITO/CuPc/ C60/BCP/Al and ITO/PEDOT:PSS/CuPc/C60/BCP/Al, the typical Voc were found to be 0.41 V and 0.44 V, respectively (Table 1). However, the Voc is found to be improved up to 0.55 V for the device with 1 nm of UV treated PTFE buffer layer (Fig. 2). The work function of the ITO surface typically varies from 3.90 to 4.80 eV depending on the surface treatment [12,13]. After inserting the untreated PTFE layer and the UV treated PTFE layer, the Voc is improved from 0.41 to 0.49 V, because of the strong dipole layer created by the negatively charged fluorine rich untreated PTFE layer and UV treated PTFE layer that has modified the surface and further increased the effective work function of the ITO. As show in Table 1, the devices with UV treated PTFE buffer
Fig. 2. I–V characteristic of OPV devices consist of ITO/PTFE with different thicknesses exposed to 5 min UV treatment.
layer exhibited at least a factor of 2 increases in term of power conversion efficiency, compared to the devices with untreated PTFE buffer layer. Among the device with PTFE, a device with a 0.5 nm UV treated PTFE buffer layer exhibited the best performance with the PCE of 1.58%, which is attributed to the good coverage of 0.5 nm UV treated PTFE on the ITO to modify the ITO surface without increase in the surface resistance of ITO/UV treated PTFE anode. It can be seen from Fig. 2 that Jsc of the structure ITO/UV treated PTFE(0.5 nm)/CuPc/C60/BCP/Al is the highest because of the smallest hole extraction barrier. Interestingly, UV treatment of PTFE buffer layer can significantly change the contact properties [11]. Our results show that while the pristine PTFE layer is useful for reducing the contact barrier, its effect is considerably enhanced by UVillumination. Modification of polymers by exposure to UV irradiation has been reported previously [14,15]. Radiation energy can be absorbed via ionization, phonon excitation and atomic displacement. This causes bond breaking, followed by scissoring and subsequent release of volatile fragments, which may result in the cross-linking through C-C bonding. In our case, clusters of sp2 bonding may also be formed, leading to an increased in conductivity [8]. At present, there is no clear explanation with regard to the interaction of UV light with PTFE properties and PTFE is found to be highly resistant to UV exposure [16]. However, UV-illumination on a few atomic layers of PTFE may have a very different interactions compared to UV radiation and impact on bulk properties of PTFE. In 2004, Tong and his co-workers have shown that UV-illumination on fluorocarbon coatings CFx (the basic structure of PTFE), created the graphitic regions identified by X-ray photoelectron spectroscopy results. This leads to the higher conductivity of the CFx layer and further improved OLED performance [8]. The same explanation may apply to the UV-illuminated PTFE layer where the Jsc of the OPV is improved due to the relatively higher conductivity of PTFE after UV-illumination. In summary, insertion of UV treated PTFE buffer layer at the anode of ITO/organic interface can significantly improve the Jsc, Voc and PCE of bulk-heterojunction OPV. The improved performance in the UV treated PTFE-coated ITO contact are consistent with its small hole extraction barrier. This fluoro-material also offers a significant advantage in that the film can be simply prepared by thermal evaporation. Acknowledgement The authors gratefully acknowledge the financial support received from EPSRC in the form of a Portfolio Partnership Award and China Scholarship Council to partially fund this research. References
Fig. 3. The I–V characteristics of OPV devices consisting of PTFE (0.5 nm) as a buffer layer on ITO, which is exposed to different UV exposure times.
[1] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [2] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617. [3] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4 (2005) 864. [4] P. Peumans, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 126.
B. Kang et al. / Organic Electronics 10 (2009) 1178–1181 [5] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 58. [6] S. Uchida, J. Xue, B.P. Rand, S.R. Forrest, Appl. Phys. Lett. 84 (2001) 4218. [7] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [8] S.W. Tong, C.S. Lee, Y. Lifshitz, D.Q. Gao, S.T. Lee, Appl. Phys. Lett. 84 (2004) 4032. [9] Y.D. Gao, L.D. Wang, D.Q. Zhang, L. Duan, G.F. Dong, Appl. Phys. Lett. 82 (2003) 155. [10] B. Kang, L.W. Tan, S.R.P. Silva, Appl. Phys. Lett. 93 (2008) 133302. [11] S.W. Tong, K.M. Lau, M.K. Fung, C.S. Lee, Y. Lifshitz, S.T. Lee, Appl. Surf. Sci. 252 (2006) 3806.
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