ITO-free low-cost organic solar cells with highly ...

Solar Energy Materials & Solar Cells 95 (2011) 3573–3578

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ITO-free low-cost organic solar cells with highly conductive poly(3,4 ethylenedioxythiophene): p-toluene sulfonate anodes Mohammad Arifur Rahman b,n, Abdur Rahim a, Md. Maniruzzaman a, Kiyeul Yang a, Chiyoung Lee a, Hoseok Nam a, Hoesup Soh a, Jaegab Lee a,nn a b

Center for Materials and Processes of Self-Assembly, School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Republic of Korea Department of Chemistry, University of Dhaka, Dhaka-1000, Bangladesh

a r t i c l e i n f o

abstract

Article history: Received 14 February 2011 Received in revised form 6 September 2011 Accepted 9 September 2011 Available online 25 September 2011

Indium tin oxide (ITO)-free organic solar cells were fabricated with highly conductive and transparent tosylate-doped poly(3,4-ethylenedioxythiophene: p-toluene sulfonate) (PEDOT:PTS) anodes of various thicknesses that were prepared by the vapor-phase oxidative polymerization of EDOT using Fe(PTS)3 as an oxidant. Both solution-processable layers – PEDOT:PSS and photoactive P3HT:PCBM – were spin coated. The anodes transmittance and conductivity varied with thickness. Power conversion efficiency was maximized at 1.4%. The ITO-free organic solar cells photovoltaic characteristics are qualitatively compared with those of ITO-based organic solar cells to explore the possibility of replacing costly, vacuum-deposited ITO with highly conductive, patterned polymer films fabricated by inexpensive vapor-phase polymerization. & 2011 Elsevier B.V. All rights reserved.

Keywords: Organic solar cell ITO-free Conductive polymer anodes

1. Introduction Conjugated polymer (CP)-based photovoltaic devices (PVDs), which initially suffered from low efficiencies and short lifetimes, have been engineered to show improved performance by controlling their band gaps, morphologies, and processing parameters [1–3]. Krebs et al. [4] suggested the possibility of fabricating large-area (10 10 cm2) polymer solar cells based on poly(phenylenevinylene) derivatives. Such organic optoelectronic devices generally employ indium tin oxide (ITO) anodes due to their low resistance and high optical transparency. However, ITO electrodes are brittle and have relatively high thermal expansion coefficients and poor interfacial compatibility with organic materials [5,6]. Therefore, new, low-cost, ITO-free electrodes [7–13] that are suitable for large-scale processing [14–17] are being developed. ITO-free solar cells have various anodes employed, including flexible polymers [9], silver-nanoparticles/polyethyleneternaphthalate (PEN) [10], bio degradable poly(lactic acid) [11], and PEDOT/silicate hybrid [12]. ITO requires vacuum processing, which adds to the cost of conventional anodes, increasing the desirability of low-cost, solution-processable, transparent electrodes that can be fabricated over large areas [19–23] by simple wet coating [7,8], roll-to-roll [13–18], or printing [20–23] methods.

The high cost of indium precludes the large-scale use of ITO. Organic-based electrodes are hoped to find applicability in both flexible- and rigid-substrate devices. Conductive p-conjugated polymers have potential applicability as electrodes in a variety of organic devices [24,25]. Poly(3,4ethylenedioxythiophene) (PEDOT) is a particularly appealing example due to its excellent environmental stability, high transparency in the visible wavelengths, and moderately high conductivity [26]. Patterning by the selective deposition of PEDOT films on oxidants such as iron (III) p-toluenesulfonate hexahydrate Fe(PTS)3 via vaporphase polymerization (VPP) has been reported to result in high conductivity and very smooth surfaces [26]. This work reports the fabrication of ITO-free organic solar cells, with organic-based electrodes replacing conventional ITO. A recently developed, highly conductive, transparent PEDOT:PTS formulation [26] was used as the polymer anode. It was prepared by the vaporphase, oxidative polymerization of EDOT using Fe(PTS)3 as an oxidant. Both solution-processable layers of the solar cells - the highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS layer and the photoactive poly(3-hexylthiophene):(6,6-phenyl-C61-buteric acid methyl ester), P3HT:PCBM layers - were spin coated onto the PEDOT:PTS-anodic layer.

2. Experimental n

Corresponding author. Corresponding author. E-mail addresses: [email protected] (M.A. Rahman), [email protected] (J. Lee). nn

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.09.019

Glass substrates were cleaned in piranha solution (H2SO4: H2O2 ¼4:1) for 10 min and exposed to UV illumination for 30 min to create hydroxyl (–OH) groups that could form siloxane

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M.A. Rahman et al. / Solar Energy Materials & Solar Cells 95 (2011) 3573–3578

bonds with the octadecyltrichlorosilane (OTS) monolayer. OTS monolayer was coated by immersing the substrates in silane solution (0.3 mM OTS in 50 mL hexane) for 60 min. After rinsing with hexane and subsequent drying by blowing N2, samples were exposed to UV illumination for 6 min through a shadow mask; 10–30 wt% Fe(PTS)3 with 0.5–2 wt% pyridine in ethanol was spread over the monolayer surfaces and spun at 3000 rpm with a spin coater for 60 s. Pyridine was added to suppress crystallite formation of the Fe salt; it also reduced the growth rate of the PEDOT:PTS films [27]. A Fe(PTS)3 to pyridine ratio of 20:1 was considered optimal to produce highly conductive PEDOT:PTS films at moderate growth rates. The thickness of the spin-coated Fe(PTS)3 increased linearly with Fe(PTS)3 concentration, thus determining the corresponding thickness of the PEDOT:PTS films formed at the expense of the oxidant. After Fe(PTS)3 coating, samples were heated at 80 1C to evaporate EDOT and to form PEDOT thin films on their surfaces. The PEDOT-coated samples were rinsed with methanol and dried in desiccators prior to characterization. Fabrication of organic solar cells (OSCs) shown in Fig. 1 started with the patterning of PEDOT:PTS conducting films on the glass substrate: the OTS-coated glass surface was UV-exposed using a shadow mask to activate the OTS surface by creating the surface functional groups, which allow for the selective coating of Fe(PTS)3 solution on the UV-exposed monolayer surfaces. Fe(PTS)3 solution was spin-coated over the UV-exposed OTS monolayer surfaces at 3000 rpm for 60 s. The selectively oxidant-coated substrates were placed on sample holders in the EDOT evaporation chamber with their surfaces facing downwards. EDOT was selectively polymerized on the pre-patterned oxidant surfaces at 80 1C. Samples were then rinsed with methanol and dried in a desiccator. After that, spin-coating was conducted at 2000 rpm for 60 s to produce ca. 100 nm-thick layers of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS (Clevious, P Lot NR: GNU 1061, provided by H.C. Starck), on PEDOT:PTS patterned on glass substrates. Samples were thermally annealed at 110 1C for 10 min while covered with a shallow glass Petri dish. The active layers were then deposited by spin-coating (at 1500 rpm for 60 s) equal weights of P3HT (Rieke Metals) and PCBM (Nano-C) in o-dichlorobenzene (ODCB) to produce ca. 140 nm-thick bulk-heterojunction (BHJ) film;

Fig. 1. An ITO-free PVD with a PEDOT:PTS polymer anode on glass (glass/ PEDOT:PTS/PEDOT:PSS/P3HT:PCBM/Al).

100 nm aluminum cathodes were deposited using an e-beam evaporator through a shadow mask, producing devices with active areas of 4 mm2. The devices were thermally annealed at 150 1C for 10 min while covered with a shallow glass Petri dish. Optical transmittance and absorption spectra of the PEDOT films were measured using a UV–visible spectrophotometer (UV3150, Shimadzu, Japan). Their sheet resistances were measured using a four-point probe (Chang Min Co. Ltd, Korea). Field emission scanning electron microscopy (FESEM, JEOL-7401F) was used to examine the films surface morphologies and thicknesses. Surface morphologies were also examined by atomic force microscopy (AFM, Seiko, Japan; trapping mode). To evaluate the adhesion of the PEDOT:PTS films to the glass surfaces, 3M Scotchs tape was used. The photovoltaic properties of the fabricated solar cells were measured under AM 1.5G conditions (with a solar simulator; EKO Instrument System, Japan; intensity: 100 mW/cm2) using a potentiostate (CHI 608C, CH Instruments, Austin, TX). The light intensity of the solar simulator was calibrated with a reference cell (PV measurements, USA).

3. Results and discussion Testing with Scotchs tape characterized the adhesion of PEDOT:PTS films formed on the 6 min-UV-exposed OTS monolayer and on bare glass. PEDOT films were coated on both surfaces and perpendicularly scratched with a diamond pencil. Tape was then attached to the PEDOT surfaces and removed. PEDOT film remained intact on the UV-exposed surface but was removed from the bare SiO2 surface after peeling off the tape (Fig. 2). UVexposure has been reported to create hydrophilic functional groups, such as C–O and C¼O terminal groups, on partly photooxidized OTS monolayer surfaces, which allow the selective coating of Fe(PTS) oxidants, upon which PEDOT films could be formed. The significantly improved adhesion of the PEDOT film and the UV-exposed OTS surface was due to direct chemical bonding between the PEDOT and the oxygen containing functional groups (C–O, C ¼O, C–OH), similar to chemical bonding between PEDOT and aminosilane SAM surfaces, which is likely responsible for the strong bonding of PEDOT to APS(3-aminopropyl trimethoxysilane) surfaces [28]. The roughness of the electrodes surface can affect free charge carrier transfer at the interface of the anode, PEDOT: PSS, and the active layer. AFM images of PEDOT:PTS films of various thicknesses deposited on UV-treated OTS monolayer (Fig. 3(a)) and PEDOT:PSS films deposited on the PEDOT:PTS/UV-treated OTS and ITO (Fig. 3(b)) show that very smooth PEDOT:PTS films formed on the UV-treated monolayer via the VPP of EDOT. The films root mean square (RMS) roughness increased from 1.05 to 2.20 nm as PEDOT:PTS thickness increased from 35 nm to 120 nm. Coating PEDOT:PSS (100 nm) onto PEDOT:PTS (50 nm) resulted in negligibly increased RMS roughness (1.35 nm). This was due to the beneficial influence of the smooth underlying PEDOT:PTS surface. Consequently, PEDOT:PSS/PEDOT:PTS (50 nm) was much smoother than PEDOT:PSS/ITO (150 nm), which showed an RMS roughness of 2.52 nm. It was also smoother than other electrodes, such as ITO (RMS roughness, 4.1 nm) and PEDOT:PSS (RMS roughness, 3.5 nm). [12] The PEDOT:PTS-coated glass substrates optical transparencies were measured and compared with that of an ITO-on-glass reference (Fig. 4(a)). The films optical transmittance decreased with their increasing thickness. The 35–50 nm thick films showed excellent transparencies at 300–550 nm, comparable to that of ITO. The 75 nm and thicker films were less transparent than ITO because of their greater thickness. While increasing thickness of the PEDOT:PTS anodic films decreased transmittance


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Fig. 2. PEDOT adhesion testing: PTS deposited on OTS after UV exposure for 6 min: (a) Scotch tape and (b) PEDOT:PTS film.

Fig. 3. AFM images of (a) PEDOT:PTS films and (b) PEDOT:PSS on PEDOT:PTS and ITO substrates.

at 510 nm, the maximum absorption peak of P3HT:PCBM blend [12]increased conductivity (Fig. 4(b)). The highest conductivity (670 S/cm) was observed for the 120 nm-thick PEDOT:PTS film. The 50 nm-thick PEDOT:PTS film showed a conductivity of 650 S/cm.

The 50 nm-thick PEDOT:PTS film showed acceptable conductivity and transmittance. Electrodes work functions significantly affect devices performance; therefore the PEDOT:PTS films work functions were

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Fig. 5. PEDOT:PTS patterned film formed by the selective self-assembly of EDOT monomers on UV-exposed OTS via vapor-phase polymerization. (a) PEDOT:PTS (50 nm) and (b) PEDOT:PTS (120 nm).

Fig. 4. Changes of (a) transmittance with thickness of PEDOT:PTS film and (b) transmittance at a wavelength of 510 nm and conductivity with thickness of PEDOT film.

assessed by Kelvin force microscopy (Seiko, PA 400, Japan). They were found to be 5.0 70.1 eV, comparable to the reference ITO’s 4.8 eV, suggesting that minimal adjustment of photoactive components energy levels is required for PEDOT:PTS to replace ITO in the anodes of photovoltaic cells. Polymer film anodes require a suitable patterning method; chemical etching by photo-lithography and dry etching using laser beams have been used, but electrode lines for the conducting polymer films cannot be easily applied. The PEDOT:PTS films were easily patterned by the selective formation of conducting films on the UV exposed-OTS layers (Fig. 5). Solar cells consisting of PEDOT:PTS film (35, 42, 50, 75, and 120 nm)/PEDOT:PSS/P3HT:PCBM active layers were fabricated to evaluate the effects of the thickness of the PEDOT:PTS anode on cell performance (Fig. 6). The variation of open circuit voltage (Voc) with PEDOT:PTS film thickness was ca. 1.5% (0.67–0.68 V, Fig. 6(a)). Voc was largely invariant because it was determined by the unchanged energy gap between the P3HT donor’s highest occupied molecular orbital (HOMO) and the PCBM acceptor’s lowest unoccupied molecular orbital (LUMO). Cells fill factors (FFs) increased with increasing thickness of the PEDOT:PTS films (Fig. 6(b)), because thinner anodes lead to higher sheet resistances and increase series resistance (Rs). FF is generally closely related to Rs, which is the sum of the bulk and the contact resistances of the materials (Rs ¼RAnode þ RPEDOT:PSS þ

RP3HT þRPCBM þRAl) [29]. Here, all materials and processes used to fabricate the OSCs were identical except for the thickness of the PEDOT:PTS anodes, therefore differences in the OSCs Rs and FF were dependent on the Rsheet of the PEDOT:PTS anodes. FF increased from 0.34 to 0.42 with increasing anode thickness because of the decreasing Rsheet. This result is consistent with the observed increase of conductivity with increasing thickness of PEDOT:PTS film in Fig. 4. Jsc increased with increasing anode thickness from 35 to 50 nm because of the thicker films’ lower resistances (Fig. 6(c)). However, increasing the thickness further to 120 nm, while further decreasing resistance, lowered optical transmittance, leading to poor absorption and photocurrent generation. The photocurrent of the 120 nm-thick PEDOT:PTS anode-based device was lower than those of the devices with 75 nm and 50 nm-thick PEDOT:PTS anodes. In order to know the distribution of the optical electric field in the active layer due to interference of the incident light with reflected light from the highly reflecting interface of the Al cathode [30], we have simulated the optical field distribution for a single wavelength illumination (510 nm, the maximum absorption peak of P3HT:PCBM blend) in these normal cells with different PEDOT thicknesses in Fig. 7. The simulation result (Fig. 7) revealed that the relative optical intensity decreased with the increase of PEDOT thickness, showing a similar dependence of the PEDOT:PTS-coated glass substrates optical transparencies on the PEDOT:PTS thickness as shown in Fig. 4, and implying that PEDOT:PTS thickness is the main contribution to the optical intensity in the active layer. Accordingly, the optimal PEDOT:PTS layer thickness could be 50 nm. Jsc was the greatest at this thickness, indicating that the PEDOT:PTS anode had the optimal balance of optical and electrical properties. Fig. 6(d) shows calculated power conversion efficiencies (PCEs) of the cells with various PEDOT:PTS anodes. PCE trended similar to Voc, FF, and Jsc. Since the anodes resistances limited their photocurrent output, the 50 nm anode gave the highest PCE, 1.4%, with an FF of 42% under 100 mW/cm2 AM 1.5G irradiation. To assess the feasibility of these ITO-free organic solar cells (IFOSCs), their performances were compared against an ITO-based OSC reference (Fig. 8). The reference cell, containing a 150 nmthick ITO anode with ca. 90% transmittance, was characterized by FF¼0.48; Voc ¼0.67 V, Jsc ¼7.8 mA/cm2, and PCE¼2.5%. Its better performance was attributed to the higher conductivity of ITO over PEDOT:PTS, which reduced the devices’ better series resistance [29,31]. Compared with the ITO-based cell, the decreased FF and Jsc were likely due to the higher series resistance and lower transmittance of the PEDOT:PTS (50 nm) anode. The qualitative similarity of the cells’ photovoltaic characteristics (Fig. 8) indicates that less expensive and more easily fabricated PEDOT:PTSdeposited OSCs are a possible replacement of cells containing costly vacuum-deposited indium tin oxide.


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Fig. 6. Influence of the thickness of PEDOT:PTS anodes on (a) open circuit voltage (Voc), (b) fill factor (FF), (c) short-circuit current (Jsc), and (d) power conversion efficiency (PCE).

Fig. 8. Current–voltage curves of PVDs with various anodes under AM 1.5G (100 mW/cm2) illumination.

4. Conclusion

Fig. 7. The simulated optical field distribution (for 510 nm illumination) as a function of the distance from PEDOT:PTS/P3HT:PCBM interface in these solar cells. These cells have the structures of glass/PEDOT:PTS (x nm)/PEDOT:PSS (100 nm)/ P3HT:PCBM (180 nm)/Al (100 nm) with x¼ 35, 42, 50, 75, and 120 nm.

ITO-free polymer solar cells with conductive PEDOT:PTS anodes selectively deposited on pre-patterned Fe(PTS)3 oxidants were fabricated. They showed enhanced adhesion and bonding between the PEDOT:PTS and the glass substrate. The thickness of

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the PEDOT:PTS layer was shown to be an important determinant of device performance, with thinner PEDOT:PTS electrodes performing better due to their higher transmittance, which allowed more sunlight to be incident on the cells active layers, despite the thinner electrodes showing increased series resistance and reduced photocurrent. A 50 nm PEDOT:PTS anode resulted in optimized cell performance, with a PCE of 1.4% obtained under AM 1.5, 100 mW/cm2 irradiation. Despite the unexceptional PCE, these results suggest the possibility of replacing ITO in solar cells with simply fabricated, low-cost polymers.

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