Switchable polarity in polymer solar cells using conjugated polyelectrolyte

Synthetic Metals 188 (2014) 1–5

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Switchable polarity in polymer solar cells using conjugated polyelectrolyte Insoo Shin a , Jihoon Lee a , Seung-Hwan Oh b , Phil Hyun Kang b , Yun Kyung Jung c , Sung Heum Park a,∗ a

Department of Physics, Pukyong National University, Busan 608-739, South Korea Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, 508-185, South Korea c Division for Green Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea b

a r t i c l e

i n f o

Article history: Received 30 September 2013 Accepted 29 October 2013 Available online 14 December 2013 Keywords: Polymer solar cell Polyelectrolyte Organic device Solar cell

a b s t r a c t We report polarity-switchable polymer solar cells that use poly [(9,9-bis((6 -(N,N,Ntrimethylammonium)hexyl)-2,7-fluorene)-alt-(9,9-bis(2-(2-(2-methoxy-ethoxy)ethoxy)ethyl)9-fluorene))dibromide polyelectrolyte (WPF-6-oxy-F). By introducing WPF-6-oxy-F as a polarity-controlling layer, we selectively achieved polarity switching in the device operation. When we deposited the WPF-6-oxy-F film on the top of an active polymer layer, the device operated conventionally; holes moved to the transparent indium tin oxide (ITO) electrode. However, the device showed switched polarity when we changed the position of insertion of the WPF-6-oxy-F film to the top of the transparent ITO electrode. Then, the electrons moved to the transparent ITO electrode, leading to an inverted device. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Plastic solar cells (PSCs) made from semiconducting and metallic polymers have a number of potential advantages, including lightweight, flexibility, and fabrication by printing/coating methods that enable low-cost manufacturing [1–3]. Although the device performances have steadily improved, further improvements in efficiency are required for large-scale commercialization [3,4]. Since conventional PSCs are based on a sandwich structure that uses a metal–insulator–metal (MIM) configuration [5–7], the device performances are very sensitive to the electrical properties of the metal electrodes [8,9]. Specifically, the built-in fields (BIFs) arising from the Fermi-level difference between the anode metal and the cathode metal in the PSCs plays an important role in charge collection [10]. The BIFs accelerate free charges separated at the interfaces of the electron donor and the electron acceptor, to the electrodes, and consequently, it directly affects the internal quantum efficiency (IQE) of the devices [2]. In addition, since the positive charges must move to the electrode with the higher work function (WF) because of the direction of the applied field, the BIF determines the polarity of the device [8]. Controlling the device polarity provides several useful advantages in device fabrication. It enables the conversion of the device

∗ Corresponding author. Tel.: +82 516295574. E-mail address: [email protected] (S.H. Park). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.10.035

structure from a conventional structure consisting of the transparent electrode as the anode to an inverted structure with the transparent electrode as the cathode, which is reversible. While the conventional device shows higher device efficiency, the inverted device leads to an enhancement in the device stability [11,12]. Moreover, it is possible to select better transport channels for electrons and holes in anisotropic bulk heterojunctions (BHJ) of an electron-donating conjugated polymer and electron-accepting fullerene derivatives in PSCs by controlling the polarity [13,14]. In principle, one can simply alter an electrode to modulate the BIF properties. However, it is difficult to secure suitable electrode materials. Most of the transparent electrodes used as anodes, such as indium tin oxide (ITO), indium zinc oxide (IZO), and aluminumdoped zinc oxide (AZO) show almost the same WF values of 4.7–4.8 eV, and low-WF metals used as cathodes such as Ca, Ba, and Li are unstable in air and H2 O [15,16]. Therefore, it is crucial to develop alternative approaches to BIF modification without replacing the electrode materials. In this work, we report a simple, printable, and efficient approach to modify the BIF of the device without changing the metal, leading to polarity variation of the device using the water-soluble poly[(9,9-bis((6 -(N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-alt-(9,9-bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-9-fluorene))dibromide polyelectrolyte (WPF-6-oxy-F) as the polarity-controlling layer. Recently, several materials have been investigated for modifying the interface between the polymer and the metal electrode [17–21]. In particular, conjugated polyelectrolytes (CPEs) with ionic

2

I. Shin et al. / Synthetic Metals 188 (2014) 1–5

Fig. 1. Molecular structure (a) and absorption spectrum (b) of poly [(9,9-bis((6 -(N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-alt-(9,9-bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-9-fluorene))dibromide polyelectrolyte (WPF-6-oxy-F).

Fig. 2. Device structure (a), current–voltage characteristics under dark condition (b) and under AM1.5G irradiation condition (c). (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

side groups have been successfully used as an interfacial layer in PSCs. The overall efficiencies, including the open-circuit voltage (Voc ) [22,23], short-circuit current (Jsc ), and the fill factor (FF), have been enhanced by the introduction of the CPE interfacial layer. Because the CPE can induce interfacial dipoles because of its ionic group, the BIF in the device can be expected to be modified, leading to the switching of the polarity. Here, we have successfully demonstrated polarity-switchable PSCs by using the water-soluble WPF-6-oxy-F CPE as the polarity-controlling layer. By introducing the WPF-6-oxy-F into the conventional PSC with the structure of ITO electrode/BHJ layer/Al, we selectively achieved polarity switching during device operation. When we deposited the WPF-6-oxy-F film on top of the BHJ active layer, the device operated conventionally and the holes moved to the transparent ITO electrode. However, the device switched polarity when we cast the WPF-6-oxy-F film on top of the transparent ITO electrode and the electrons moved to the transparent ITO electrode.

2. Results and discussion Fig. 1 shows the molecular structure and the optical properties of WPF-6-oxy-F. The WPF-6-oxy-F CPE was synthesized in our group. Although the backbone structure of the WPF-6-oxy-F is identical to that of blue-emitting polyfluorene (PF) [20,21,24], WPF-6-oxyF consists of unique ionic or polar side groups that can induce interfacial dipoles, as shown in Fig. 1(a). WPF-6-oxy-F has several advantages as an interfacial layer material. WPF-6-oxy-F exhibits a relatively large Stokes shift: it emits blue color during photoluminescence (PL) after absorbing ultraviolet (UV) (see the Fig. 1(b)) [25–27]. Therefore, WPF-6-oxy-F can lead to further enhancement in the absorption of the active layer with absorbance in the visible range. Further, because WPF-6-oxy-F is soluble in water, it can prevent damage of the underlying organic soluble active layer during film casting. This enables the fabrication of a multilayer device [28–30]. In addition, since electrons and holes are delocalized along

I. Shin et al. / Synthetic Metals 188 (2014) 1–5

3

Fig. 3. Energy band structure variation of the pristine (a), ITO-CPE (b) and Al-CPE (c) device by introducing polyelectrolyte layer.

the pi-conjugation, the conjugated main chain of WPF-6-oxy-F can provide good conductivity [8,17]. Further, the ionic side-groups of WPF-6-oxy-F induce strong interfacial dipoles between the active layer and the metal electrode leading to the modification of the BIF of the device [8,20,24,31]. To investigate the variation of the device polarity by introducing WPF-6-oxy-F, we prepared PSC devices with and without the WPF-6-oxy-F layer and characterized the current–voltage (J–V) characteristics of the devices. Fig. 2(a) shows the device structure of the PSC together with the molecular structure of the BHJ active material. In particular, we fabricated three kinds of devices i.e., the pristine device, the ITO-CPE device, and the Al-CPE device. The structures of the pristine, the ITO-CPE, and the Al-CPE device were ITO/BHJ/Al, ITO/WPF6-oxy-F/BHJ/Al, and ITO/BHJ/WPF-6-oxy-F/Al, respectively. The BHJ was a composite of poly[N-9 -hepta-decanyl-2,7-carbazolealt-5,5-(4 ,7 -di-2-thienyl-2 ,1 ,3 -benzothiadiazole)] (PCDTBT), an

electron donor, and [6,6]-phenyl C71 butyric acid methyl ester (PC70 BM), an electron acceptor. Fig. 2(b) shows the J–V characteristics of the three devices under darkness. As we expected, the pristine device without the WPF-6oxy-F layer showed conventional J–V curves of a diode with a high rectification ratio ranging from −3 V to +3 V (black curve). However, when we inserted the WPF-6-oxy-F layer between the ITO and the BHJ layer, the ITO-CPE device exhibited a totally different J–V behavior. The ITO-CPE device exhibited a high current density of −7 mA/cm2 at −2 V, in contrast to the zero current shown by the pristine device. Further, this current density was much higher than the current density of 1 mA/cm2 at +2 V (blue curve). These results clearly indicate that the polarity of the device switched. Moreover, this polarity switched again by changing the position of the WPF-6-oxy-F layer. When we deposited the WPF-6-oxyF layer on top of the BHJ layer, the Al-CPE device showed a J–V curve similar to that of the pristine device with a high rectification

I. Shin et al. / Synthetic Metals 188 (2014) 1–5

ratio from −3 V to +3 V (red curve). Since these were photovoltaic cells, each device generated a photocurrent under conditions of light irradiation. The J–V characteristics of each device under air mass 1.5 global (AM 1.5G) irradiation from a calibrated solar simulator with an irradiation intensity of 100 mW/cm2 is shown in Fig. 2(c). The pristine device reproducibly yielded Jsc = 7.3 mA/cm2 , Voc = 0.86 V, FF = 0.5, and power conversion efficiency (e ) = 3.2%. e of 3.2% was comparable to that of the conventional PCDTBT:PCBM device reported previously [32]. However, the polarity of the ITOCPE device switched when the WPF-6-oxy-F layer was inserted between the ITO and the BHJ layer. As shown by the blue curve in Fig. 2(c), the ITO-CPE device exhibited a positive current at 0 V in contrast to the negative current shown by the pristine device, which indicates that holes moved to the transparent ITO electrode. However, the device switched its polarity again when we deposited the WPF-6-oxy-F film on top of the transparent Al electrode, indicating the motion of electrons to the transparent ITO electrode. Moreover the Al-CPE device demonstrated significantly enhanced performances with improved FF, Voc , and Jsc . The polarity switch in the J–V curves shown in Fig. 2(b) and (c) can be understood by modification of the BIF in the device via the introduction of the WPF-6-oxy-F layer. Fig. 3(a)–(c) show the energy band structure of the BIF for the pristine, ITO-CPE, and AlCPE devices, respectively. Fig. 3(a) exhibits the conventional band structure of the PSC under dark. Since BHJ materials are electrical insulators, the BIF entirely originates from the difference in the WFs between the ITO and Al electrodes. In principle, the WF of the ITO electrode should be matched with that of the Al electrode when connected, which consequently generates BIFs in the device. As shown in Fig. 3(a), the generated BIF accelerated the electrons to the Al electrode and the holes to the ITO electrode. However, this tendency was altered by inserting the WPF-6-oxy-F CPE to the device. Fig. 3(b) and (c) present the modified energy band structure for the ITO-CPE and Al-CPE devices, respectively caused by introducing the WPF-6-oxy-F layer. Although the ITO-CPE devices consisted of the identical ITO and Al electrodes as that in the pristine device, the BIF was rearranged and consequently, the polarity of device was switched. Hence, the electrons moved to the Al electrode and the holes moved to the ITO electrode [3]. However, a switched polarity was recovered when we changed the position of the WPF-6-oxy-F CPE layer to the interface of the BHJ layer and the Al electrode. Since the WPF-6-oxy-F CPE consists of ionic side-groups, the introduction of the WPF-6-oxy-F layer onto the metallic ITO and Al electrodes can raise their WFs because of the generation of strong interfacial dipoles between the active layer and each electrode; consequently, the polarity of the device was changed by the rearrangement of the BIF. Earlier, ultraviolet photoelectron spectroscopy (UPS) studies demonstrated that the insertion of the CPE layer between the conjugated polymer layer and the metal electrode can alter the WF of the electrode because of the existence of a strong interfacial dipole at the surface of the CPE and electrodes [33]. To clarify the WF shifting of the electrode by the introduction of the WPF-6-oxy-F layer, we directly measured the variation in the WF of ITO and Al electrodes with and without the WPF-6oxy-F layer using the KP method (KP 6500 Digital Kelvin probe, McAllister Technical Services. Co. Ltd.). The KP method is useful for investigating the WF of metals and semiconductors in non-contact, non-destructive, and less environmentally dependent ways. When the probe and the sample are electrically connected, the Fermi level of both materials should match and consequently, a CPD should be generated between the probe and the sample. Then, we can obtain the WF value of the sample by measuring the external voltage corresponding to the CPD at zero current (Iac = 0). Fig. 4 shows the variation of the WF of ITO and Al with the deposition of the WPF-6-oxy-F layer. The WF value of the fresh ITO and Al layer was

-3.6

Effective WF (eV)

4

-4.0

-4.4

-4.8

-5.2

Al

Al/PEI

ITO

ITO/PEI

Electrode Fig. 4. Effective work–function values of modified electrodes measured by Kelvin probe method.

estimated as ∼4.45 eV, which is in good agreement with previous findings. However, when we introduced the WPF-6-oxy-F layer on top of the ITO and Al, the WF increased dramatically and reached a saturated CPD value of ∼4.2 eV and ∼4.3 eV for ITO-CPE and Al-CPE, respectively. This clearly indicates that the WPF-6-oxy-F layer disrupted the original band structure of the components of the pristine PSC and served as a polarity-controlling layer. 3. Conclusion In conclusion, we have investigated polarity variations in PSCs by introducing a water-soluble WPF-6-oxy-F layer in various positions and have demonstrated a polarity-switchable PSC. By introducing WPF-6-oxy-F as a polarity-controlling layer, we selectively achieved a polarity switch during device operation. When we deposited the WPF-6-oxy-F film on top of an active polymer layer, the device operated conventionally, and the holes moved to the transparent ITO electrode. However, the device showed switched polarity when we changed the position of the WPF-6-oxy-F film and placed it on top of the transparent ITO electrode. Then, the electrons moved to the transparent ITO electrode. In particular, the performance of the PSC significantly enhanced when we inserted the WPF-6-oxy-F layer between the BHJ and the Al electrode. The introduction of the WPF-6-oxy-F layer onto the metallic ITO and Al electrode in PSC could raise the WF of the ITO and Al electrode because of the generation of strong interfacial dipoles between the active layer and each of the electrodes, leading to changes in the polarity of the device. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (NRF-2013R1A2A2A04014576 and C-D-2013-0674). References [1] S.R. Forrest, The path to ubiquitous and low-cost organic electronic appliances on plastic, Nature 428 (2004) 911–918. [2] S.H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J.S. Moon, D. Moses, M. Leclerc, K. Lee, A.J. Heeger, Bulk heterojunction solar cells with internal quantum efficiency approaching 100, Nat. Photon. 3 (2009) 297–302. [3] G. Li, R. Zhu, Y. Yang, Polymer solar cells, Nat. Photon. 6 (2012) 153–161. [4] G. Dennler, M.C. Scharber, C.J. Brabec, Polymer-fullerene bulk-heterojunction solar cells, Adv. Mater. 21 (2009) 1323–1338. [5] R.E. Holmlin, R. Haag, M.L. Chabinyc, R.F. Ismagilov, A.E. Cohen, A. Terfort, M.A. Rampi, G.M. Whitesides, Electron transport through thin organic films in metal–insulator–metal junctions based on self-assembled monolayers, J. Am. Chem. Soc. 123 (2001) 5075–5085. [6] M.L. Chabinyc, X. Chen, R.E. Holmlin, H. Jacobs, H. Skulason, C.D. Frisbie, V. Mujica, M.A. Ratner, M.A. Rampi, G.M. Whitesides, Molecular rectification in

I. Shin et al. / Synthetic Metals 188 (2014) 1–5

[7]

[8] [9] [10]

[11]

[12]

[13]

[14]

[15]

[16] [17]

[18]

[19]

a metal–insulator–metal junction based on self-assembled monolayers, J. Am. Chem. Soc. 124 (2002) 11730–11736. P.W.M. Blom, V.D. Mihailetchi, L.J.A. Koster, D.E. Markov, Device physics of polymer:fullerene bulk heterojunction solar cells, Adv. Mater. 19 (2007) 1551–1566. A.J. Heeger, N.S. Sariciftci, E.B. Namdas, Semiconducting and Metallic Polymers, Oxford University Press, New York, USA, 2010. F. Zhang, M. Ceder, O. Inganäs, Enhancing the photovoltage of polymer solar cells by using a modified cathode, Adv. Mater. 19 (2007) 1835–1838. A. Armin, G. Juska, B.W. Philippa, P.L. Burn, P. Meredith, R.D. White, A. Pivrikas, Doping-induced screening of the built-in-field in organic solar cells: effect on charge transport and recombination, Adv. Energy Mater. 3 (2013) 321–327. C.-Y. Li, T.-C. Wen, T.-H. Lee, T.-F. Guo, J.-C.-A. Huang, Y.-C. Lin, Y.-J. Hsu, An inverted polymer photovoltaic cell with increased air stability obtained by employing novel hole/electron collecting layers, J. Mater. Chem. 19 (2009) 1643. S.K. Hau, H.-L. Yip, N.S. Baek, J. Zou, K. O’Malley, A.K.-Y. Jen, Air-stable inverted flexible polymer solar cells using zinc oxide nanoparticles as an electron selective layer, Appl. Phys. Lett. 92 (2008) 253301. I. Campbell, S. Rubin, T. Zawodzinski, J. Kress, R. Martin, D. Smith, N. Barashkov, J. Ferraris, Controlling Schottky energy barriers in organic electronic devices using self-assembled monolayers, Phys. Rev. B 54 (1996) 14321–14324. I.H. Campbell., J.D. Kress, R.L. Martin, D.L. Smith, N.N. Barashkov, J.P. Ferraris, Controlling charge injection in organic electronic devices using self-assembled monolayers, Appl. Phys. Lett. 71 (1997) 3528–3530. M. Stössel, J. Staudigel, F. Steuber, J. Simmerer1, A. Winnacker, Impact of the cathode metal work function on the performance of vacuum-deposited organic light emitting-devices, Appl. Phys. A 68 (1999) 387–390. V. Shrotriya, G. Li, Y. Yao, C.-W. Chu, Y. Yang, Transition metal oxides as the buffer layer for polymer photovoltaic cells, Appl. Phys. Lett. 88 (2006) 073508. H.-L. Yip, S.K. Hau, N.S. Baek, A.K.-Y. Jen, Self-assembled monolayer modified ZnO/metal bilayer cathodes for polymer/fullerene bulk-heterojunction solar cells, Appl. Phys. Lett. 92 (2008) 193313. J.H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, U. Scherf, A.J. Heeger, G.C. Bazan, Improved high-efficiency organic solar cells via incorporation of a conjugated polyelectrolyte interlayer, J. Am. Chem. Soc. 133 (2011) 8416–8419. C.J. Brabec, S.E. Shaheen, C. Winder, N.S. Sariciftci, P. Denk, Effect of LiF/metal electrodes on the performance of plastic solar cells, Appl. Phys. Lett. 80 (2002) 1288–1290.

5

[20] S.-H. Oh, S.-I. Na, J. Jo, B. Lim, D. Vak, D.-Y. Kim, Water-soluble polyfluorenes as an interfacial layer leading to cathode-independent high performance of organic solar cells, Adv. Funct. Mater. 20 (2010) 1977–1983. [21] S.H. Park, Y. Jin, J.Y. Kim, S.H. Kim, J. Kim, H. Suh, K. Lee, A blue-light-emitting polymer with a rigid backbone for enhanced color stability, Adv. Funct. Mater. 17 (2007) 3063–3068. [22] M.O. Reese, A.J. Morfa, M.S. White, N. Kopidakis, S.E. Shaheen, G. Rumbles, D.S. Ginley, Pathways for the degradation of organic photovoltaic P3HT:PCBM based devices, Sol. Energy Mater. Sol. Cells 92 (2008) 746–752. [23] M.O. Reese, M.S. White, G. Rumbles, D.S. Ginley, S.E. Shaheen, Optimal negative electrodes for poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester bulk heterojunction photovoltaic devices, Appl. Phys. Lett. 92 (2008) 053307. [24] S.-H. Oh, D. Vak, S.-I. Na, T.-W. Lee, D.-Y. Kim, Water-soluble polyfluorenes as an electron injecting layer in PLEDs for extremely high quantum efficiency, Adv. Mater. 20 (2008) 1624–1629. [25] W.-Y. Wong, X.-Z. Wang, Z. He, A.B. Djuriˇsiˇc, C.-T. Yip, K.-Y. Cheung, H. Wang, C.S.K. Mak, W.-K. Chan, Metallated conjugated polymers as a new avenue towards high-efficiency polymer solar cells, Nat. Mater. 6 (2007) 521–527. [26] M.J. Currie, J.K. Mapel, T.D. Heidel, S. Goffri, M.A. Baldo, High-efficiency organic solar concentrators for photovoltaics, Science 321 (2008) 226–228. [27] F. Vollmer, W. Rettig, E. Birckner, Photochemical mechanisms producing large fluorescence stokes shifts, J. Fluoresc. 4 (1994) 65–69. [28] H. Hoppe, N.S. Sariciftci, D. Meissner, Optical constants of conjugated polymer:fullerene based bulk-heterojunction organic solar cells, Mol. Cryst. Liq. Cryst. 385 (2002) [233]/113–[239]/119. [29] M. Helgesen, R. Søndergaard, F.C. Krebs, Advanced materials and processes for polymer solar cell devices, J. Mater. Chem. 20 (2010) 36–60. [30] W.J. Potscavage Jr., S. Yoo, B. Kippelen, Origin of the open-circuit voltage in multilayer heterojunction organic solar cells, Appl. Phys. Lett. 93 (2008) 193308. [31] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Energy level alignment and interfacial electronic structures at organic/metal and organic/organic interfaces, Adv. Mater. 11 (1999) 605–625. [32] N. Blouin, A. Michaud, M. Leclerc, A low-bandgap poly(2,7-carbazole) derivative for use in high-performance solar cells, Adv. Mater. 19 (2007) 2295–2300. [33] J.H. Seo, T.-Q. Nguyen, Electronic properties of conjugated polyelectrolyte thin films, J. Am. Chem. Soc. 130 (2008) 10042–10043.