Polymer solar cells incorporating one-dimensional polyaniline nanotubes

Organic Electronics 9 (2008) 1136–1139

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Letter

Polymer solar cells incorporating one-dimensional polyaniline nanotubes Mei-Ying Chang a, Chong-Si Wu a, Yi-Fan Chen a, Bi-Zen Hsieh b, Wen-Yao Huang a, Ko-Shan Ho b, Tar-Hwa Hsieh b, Yu-Kai Han b,* a b

Institute of Electro-Optical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 27 April 2008 Received in revised form 21 July 2008 Accepted 6 August 2008 Available online 15 August 2008

PACS: 71.20.Rv 73.61.Ph 73.63.Fg 73.50.Pz 79.60.Jv

a b s t r a c t We have fabricated polymer solar cell devices based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and incorporating one-dimensional nanostructured acid-doped polyaniline nanotubes (a-PANINs) as an interfacial layer. The power conversion efficiency of an annealed device incorporating the a-PANIN layer reached 4.26% under AM 1.5 G (100 mW/cm2) illumination, an increase of ca. 26% relative to that of the annealed device lacking an a-PANIN interfacial layer. The incorporation of the a-PANINs in the solution-processed polymer solar cells was reproducible; the high conductivity, controlled tubular nanoscale morphologies, and mobility of the annealed a-PANIN layer led to efficient extraction of photogenerated holes to the buffer layer and suppression of exciton recombination, thereby improving the photovoltaic performance. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Solar cell Acid-doped polyaniline Nanotube

The properties of bulk heterojunction structures, which hold great promise for use in high-efficiency polymeric solar cells, are controlled by the nature of their interfaces. Because exciton diffusion lengths in organic solar cells range from 10 to 100 nm [1,2], only those excitons generated within a short distance of the donor–acceptor interface have the possibility of dissociating into free electrons and holes. Although ideally the interface should have as large a contact area as possible, the morphologies of the donor and acceptor should still permit the charge carriers to travel along unrestricted transport pathways to their respective electrodes. Many organic solar cells based on conjugated polymers and soluble fullerenes have been developed since their efficient photo-induced charge separation at donor–acceptor interfaces was first reported simultaneously with the finding of continuous pathways * Corresponding author. Tel.: +886 7 3814526; fax: +886 7 3830674. E-mail address: [email protected] (Y.-K. Han). 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.08.001

for free charge carriers transport to appropriate electrodes [3,4]. Several techniques related to the preparation of the materials and modification of the device architectures have also been reported [3–6]. Poly(3-hexylthiophene) (P3HT) is at present among the most suitable electron–donor conjugated polymers for use in bulk heterojunction polymer solar cells. When incorporated with double-walled carbon nanotubes [7] or multi-walled carbon nanotubes [8] as an interpenetrating hole-extracting electrode, it improves the photovoltaic performance of solar cells. Polyaniline (PANI) nanotubes have attracted almost as much interest as carbon nanotubes because of the unique chemical and physical properties of their doped and un-doped structures and for their future potential applications in polymeric conducting devices [9]. In addition to PANI nanotubes (PANINs) [10], several other PANI morphologies are known, including needle-like structures [11], hollow microspheres [12], and self-assembled nanofibers [13]. Because of their one-dimensional (1-D)

M.-Y. Chang et al. / Organic Electronics 9 (2008) 1136–1139

nanostructures and metal-like conductivities, conducting polymers, nanotubes, and nanofibers have attracted much attention for their potential applications in nanodevices [14]. In this study, we synthesized acid-doped [15] PANI nanotubes (a-PANINs) for use as the interfacial layer in P3HT:[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)based polymer bulk heterojunction solar cells to collect holes efficiently from the active layer and transport them to the buffer layer under the internal electric fields of the fabricated ITO/buffer/a-PANINs/P3HT:PCBM/Al devices. Regioregular P3HT with 98% HT–HT coupling and PCBM (99.5% purity) were obtained from Aldrich Co. A solution of n-dodecylbenzenesulfonic acid (DBSA) (3 g) and ammonium persulfate (6 g) in de-ionized water (30 mL) was mixed with a solution of aniline (1 g) in HCl (ca. pH 1.5, 5 mL). The resulting dark-green mixture was gently stirred for 5 min and then a further charge of aniline (10 g) was added. After 4 h of gentle magnetic stirring, the green/ black precipitate of the a-PANINs was suction-filtered and washed with copious amounts of de-ionized water and methanol. Drying under vacuum at 100 °C for 12 h yielded a dark-green powder [16–19]. A 60-nm-thick film of PEDOT-PSS (Baytron AI 4083) was spin-coated at 4000 rpm from an aqueous solution onto pre-cleaned indium tin oxide (ITO)-coated glass (sheet resistance: 15 X/ h; Ritek) in a clean-room atmosphere (class 10000) and then dried at 180 °C for 5 min. The a-PANIN layer (30– 50 nm) was then spin-coated from toluene solution (0.2 mg/mL, 5 mL) onto the ITO/PEDOT-PSS layer. Next, a P3HT:PCBM active layer was spin-coated [P3HT:PCBM, 1:1 (w/w) in o-xylene; 30 mg/mL] onto the a-PANIN layer

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and then the system was dried at 150 °C for 10 min. Subsequently, an Al cathode (200 nm) was deposited on top of the active layer through thermal evaporation at ca. 2  106 torr through a mask, defining an active area of 3 mm2, and then dried at 150 °C for 30 min. The work functions of these materials in air were measured through photoelectron spectroscopy (PESA, AC-2). Using a Hall measurement system (ECOPI, HMS-3000), we determined that the electrical conductivities (rRT) of the as-spun a-PANIN and PEDOT-PSS films were in the ranges 3–4 and 2  104 –4  104 S/cm, respectively. Fig. 1a displays the energy levels of the compounds used in this study. Fig. 1b and d present the device structure and chemical structures of the materials, respectively. The field-emission scanning electron microscopy (FE-SEM) image in Fig. 1c reveals that the product was composed of >12-lm-long tubes having mean and effective diameters in the ranges 300–500 and 400–600 nm, respectively. The inset to Fig. 1c indicates that the well-extended 1-D nanostructure of each a-PANIN had the form of a 1-D nanotube, the hollow tunnel of which could aid in the transport of charges further through the tubular structure, preventing interference, recombination with electrons, or defects. The length scale of phase separation in the photoactive layer is a key factor affecting the performance of polymer solar cells. To achieve high-performance cells based on P3HT:PCBM, the PCBM component must form clusters having lateral dimensions of the order of 20–50 nm within, and continuous pathways through, the whole film [20,21]. The sub-micron scale of the features in the a-PANIN surface was appropriate for the P3HT:PCBM domain, providing a well-defined contact area for the transport of

Fig. 1. (a) Energy-level diagram, (b) device structure, (c) FE-SEM image of the a-PANINs (inset: TEM image), and (d) chemical structures of the compounds used in this study.

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Fig. 2. UV–Vis–near-IR spectra of the regular polyaniline particles and the a-PANIN nanofibers.

free charge carriers. Fig. 2 presents the UV–Vis–near-IR spectra of the regular polyaniline particles (bulk) and the a-PANINs. The UV–Vis spectra of the a-PANINs revealed that they existed in the emeraldine oxidation state, i.e., the formation of polarons after proton doping with DBSA and Cl ions, with peaks centered at ca. 440 and 800– 1600 nm. The emeraldine oxidation state of polyaniline, which contains 50% imino-type and 50% amino-type nitrogen atoms, can be represented by the formula in Fig. 1d. An interesting feature of the nano-polyaniline is the extended free carrier tail absorption in the near-IR region; it is usually found when polyaniline is secondarily doped by DBSA and Cl because of the prolonged conjugation chain length that results from rearrangement of coiled backbones into straighter conformations. The prolonged polyaniline molecules are arranged helically into nanofibers or nanotubes to prevent intra- or inter-molecular complexation and to retain the high-conjugation length (the presence of the free carrier tail), leading to the high absorbance at the near-IR region relative to that of the regular polyaniline particles. This behavior reveals that the extended conjugation length of the a-PANINs can provide longer pathway for holes to travel without exciton recombination or interference from impurities.

Fig. 3. (a) FE–SEM cross-sectional image of the ITO/PEDOT:PSS/a-PANINs/ P3HT:PCBM layers; (b) XRD patterns of the bulk, pristine, and annealed aPANINs films.

Fig. 3a presents a cross-sectional SEM image of the layers in the device. Because it was difficult to redissolve the a-PANIN layer into o-xylene after it had been dried at 150 °C, this layer survived the spin-coating of the active layer from o-xylene solution. Fig. 3b presents the X-ray diffraction (XRD) patterns of the bulk, pristine, and annealed a-PANIN films. We assign the sharp peak centered at a value of 2h of 3° to the periodic distance between the dopant and the nitrogen atoms on adjacent main chains. We attribute the broad bands centered at values of 2h of 20 and 27° to the periodicities parallel and perpendicular to the polymer chains [22], respectively. These results indicate that the a-PANIN layer was partly crystalline as a result of its special tubular morphology, which was further enhanced upon thermal treatment. We expected that this high crystallinity would result in higher charge mobility in the aPANIN layer and, correspondingly, improved device performance. Yang and co-workers [23] found that higher crystallinity of the interface layer results in higher solar-cell performance because of the higher charge mobility that the interface layer provides. We also prepared a p-type-only device having the structure ITO/PEDOT:PSS (80 nm)/a-PANINs (100 nm)/MoO3(10 nm)/Al (150 nm) to investigate the hole mobility behavior in the PEDOT:PSS/a-PANINs layer. We calculated the hole mobilities from the current–voltage plots (not shown here) of this p-type-only cell structure, using the Mott–Gurney space charge limited current (SCLC) model (J ¼ 9er e0 lV 2 =8L3 ; where J is the current density, er is the dielectric constant, e0 is the vacuum permittivity, l is the mobility, V is the applied voltage, and L is the gap of the electrodes) and the Poole–Frenkel law [l ¼ lo expðE=Eo Þ1=2 ; where lo is the zero-field mobility, E is the electric field (equal to V/L), and Eo is the field coefficient] [23,24]. The zero-field hole mobility (lo) increased from 7.38  104 to 1.13  103 cm2/Vs after the device had been heat-treated at 150 °C for 10 min, presumably as a result of ordering in the structure of the a-PANIN film. This finding reveals that the highly crystalline 1-D tubular morphology of the a-PANIN layer provided both a well-defined contact surface between the interfaces and efficient pathways for the transportation of free charge carriers toward their respective electrodes, thereby reducing the degree of exciton recombination within the photoactive cell. Fig. 4 displays the I–V characteristics of cells incorporating and lacking the a-PANIN interfacial layer; Table 1 summarizes the performance parameters. The solvent effect resulted in a lower power conversion efficiency (PCE) relative to that described in a previous report [25] where different organic casting solvents were used. We attribute this lower photovoltaic action to the film morphology obtained from o-xylene, which appears to be unsuitable for the formation of an interpenetrated network [26] and disrupts the charge transport in the polymer matrix. The increase in the values of Jsc, Voc, and PCE after thermal treatment might indicate that the molecules comprising each layer were sufficiently thermally agitated to interdiffuse, thereby increasing the heterojunction area [21,27,28]. A comparison of the cells incorporating the a-PANIN interfacial layers before (device B) and after (device D) annealing at 150 °C for 30 min reveals that thermal annealing of

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using single-wall carbon nanotubes (SWNTs) as the interlayer [30]. The greater order in the structure of the a-PANINs following thermal treatment improved the degree of contact between the buffer layer and the photoactive film, decreasing the series resistance of the cell while increasing both the current and the FF [24,27,31]. (iii) The separated free charge carriers proceed toward their respective electrodes to generate electric current. In conclusion, we have prepared polymer photovoltaic devices based on the structure ITO/PEDOT-PSS/a-PANINs/ P3HT:PCBM/Al. The performance of the solar cell can be improved significantly through incorporating an a-PANIN layer into the device structure without complicating the process of device fabrication. Acknowledgement

Fig. 4. Current–voltage characteristics (AM 1.5 G, 100 mW/cm2) of the devices discussed in this report.

This study was supported by the National Science Council of Taiwan (Grant Nos. NSC-95-2113-M-151-001-MY3 and NSC-95-2113-M-110-013). References

Table 1 Characteristics of the photovoltaic cells measured under AM 1.5 G, 100 mW/cm2 solar illumination Devicea

Voc (V)b

Jsc (mA/cm2)c

FF (a.u.)d

PCE (%)e

Rs (X cm2)f

A B C D

0.58 0.60 0.64 0.64

6.33 7.09 8.83 10.96

0.44 0.45 0.55 0.60

1.67 1.91 3.39 4.26

45 30 17 13

a Devices: (A) as-cast ITO/PEDOT-PSS/P3HT:PCBM/Al; (B) as-cast ITO/ PEDOT-PSS/a-PANINs/P3HT:PCBM/Al; (C) ITO/PEDOT-PSS/P3HT:PCBM/Al, annealed at 150 °C for 30 min; (D) ITO/PEDOT-PSS/a-PANINs/ P3HT:PCBM/Al, annealed at 150 °C for 30 min. b Open circuit voltage. c Short circuit current. d Fill factor. e Power conversion efficiency. f Series resistance.

the three-layer organic solar cell increased the crystallinity percentage of the a-PANIN layer, thereby resulting in improved photovoltaic performance. After annealing (150 °C, 30 min) the devices lacking (device C) and incorporating (device D) the a-PANIN interfacial layer, the PCE increased from 3.39% to 4.26%. These findings suggest that the 1-D nature of hole transport in the a-PANINs reduced the number of cul-de-sacs for holes [29]. We propose the following photovoltaic mechanisms for the functioning of the solar cells incorporating the a-PANIN interfacial layers: (i) The bulk heterojunction P3HT:PCBM photoactive layer accepts photons from white light and generates excitons. (ii) The partial excitons are swept to the wellcontacted, highly ordered a-PANIN interfacial layer under the influence of the built-in chemical and electric potentials. The high conductivity and mobility of the annealed a-PANIN layer efficiently extracts the photo-induced holes, providing conducting pathways to the buffer layer while reducing the degree of exciton recombination and resulting in a more efficient charge separation. The enhanced hole collection can be ascribed in part to geometrical field enhancement at the aPANIN layer, similar to the observation of a previous study

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