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J. Phys. Chem. C 2010, 114, 6822–6830
Surface-Enhanced Raman Study of the Interaction of PEDOT:PSS with Plasmonically Active Nanoparticles Marina Stavytska-Barba and Anne Myers Kelley* School of Natural Sciences, UniVersity of California, Merced, 5200 North Lake Road, Merced, California 95343 ReceiVed: January 6, 2010; ReVised Manuscript ReceiVed: March 10, 2010
Raman and surface-enhanced Raman spectra have been obtained for poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS), a polymer blend widely used as a semitransparent, hole-transporting electrode coating in organic polymer-based photovoltaic cells. Spectra of thin films are reported for both the as-received (oxidized or doped) PEDOT:PSS and samples in which the PEDOT has been chemically reduced with hydrazine. The Raman spectra of the polymer blend are dominated by lines attributable to PEDOT and the oxidized and reduced species have clearly distinct Raman spectra at excitation wavelengths of 457.9, 514.5, and 632.8 nm. The Raman spectra in the ring stretching region exhibit changes in the presence of Au or Ag which depend on the method of deposition of the nanoparticles. Ag nanoparticles appear to facilitate reoxidation of chemically reduced PEDOT. Light-induced changes in the Raman spectra in the presence of Ag nanoparticles are consistent with addition of oxygen to the sulfur atoms in the thiophene rings of PEDOT. These results are discussed in the context of the reported enhancement of organic solar cell performance when metal nanoparticles are incorporated into or are in contact with the PEDOT:PSS layer. Introduction Organic polymer blends have great potential in photovoltaic devices as low-cost materials that can be deposited readily on large-area flexible substrates. A typical solar cell device consists of a semiconducting polymer, usually a poly(thiophene) or poly(phenylenevinylene) derivative, as the primary light absorbing and hole transport material and an electron acceptor, usually a C60 derivative, as the electron transport material.1–3 Light enters through a transparent indium tin oxide (ITO) electrode and is absorbed by the conjugated polymer, producing excitons which diffuse to an interface between the polymer and the electron transporting material. Because of the relatively short exciton diffusion lengths in conjugated polymers (a few nanometers),4 most efficient device designs use the bulk heterojunction geometry in which the conjugated polymer and the electron transporting material phase separate and form interpenetrating networks.1,2,5,6 At the donor/acceptor interfaces the excited polymer transfers an electron to the fullerene. The conjugated polymer transports holes to the ITO electrode, while the C60 phase transports electrons to a top electrode that may be aluminum or another low work function metal. The best performing solar cells often contain other components as well, including optical spacer layers between the conducting polymer and the metal electrode7,8 and the polyelectrolyte poly(3,4ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) to improve hole collection at the ITO electrode.9,10 Power conversion efficiencies of about 5% have been achieved for single-cell devices based on poly(3-hexylthiophene) (P3HT) and the C60 derivative (6,6)-phenyl-C61-butyric acid methyl ester (PCBM).7,11,12 These efficiencies are still considerably lower than those achievable with silicon. Among the strategies for further increasing the efficiencies of such cells, a particularly intriguing one is the incorporation of metallic nanoparticles.13–27 At least three distinct mechanisms have been * To whom correspondence should be addressed.
proposed for the enhancement of organic solar cell efficiency by metal nanoparticles. First, the nanoparticles might act as charge carriers, improving charge mobility within the device13,17,19,24 and/or charge transfer at the electrodes.25,28 Second, metal nanoparticles absorb and scatter light strongly through excitation of surface plasmons, collective oscillations of the conduction electrons.29 The plasmon resonance frequencies depend on the size and shape of the particles but typically fall within the visible spectrum for silver and gold. Absorption of light by metallic nanoparticles can therefore increase the fraction of the solar spectrum absorbed by the cell.30 These surface plasmons also produce large local electromagnetic fields near the nanoparticle surface, which have long been exploited in surface-enhanced Raman spectroscopy (SERS).31–34 The large local fields are proposed to increase the absorption of light by the organic materials, increasing overall efficiency.14,16–21,23,27 Third, the same large local fields may increase the quantum yield for dissociation of excitons into charge carriers, thereby increasing the efficiency.15,23 A surprising aspect of these studies is the number of different configurations in which metal nanoparticles have been reported to enhance solar conversion efficiencies. Enhancement has been reported when the metal is on the opposite site of a glass substrate from the ITO electrode,22 directly in contact with the ITO electrode,15,18 spaced from the ITO by a polyelectrolyte layer,21 mixed in with the PEDOT:PSS coating the ITO electrode,23 used as a substitute for PEDOT with polystyrene,25 incorporated into the conducting polymer layer24,26 (although see ref 35 for a negative result), or deposited between the conducting polymer and the low work function metal electrode.20 The most common mechanism offered to explain the enhancement of solar conversion efficiency is plasmonic enhancement of light absorption by the conducting polymer, but it is questionable how much enhancement persists at a distance of 20-40 nm (the typical thickness of the PEDOT:PSS layer)18,21,36 from the surface of the metal. SERS studies have shown that
10.1021/jp100135x 2010 American Chemical Society Published on Web 03/19/2010
Interaction of PEDOT:PSS with Active Nanoparticles
Figure 1. Structures of PSS (left) in its fully ionized form, and of PEDOT (right) in its neutral (reduced) form.
the plasmonic enhancement falls off by an order of magnitude within 2-3 nm from the metal surface,34 but fluorescence intensities are typically maximized at distances of 5-20 nm as a result of the balance between electromagnetic field enhancement and excited-state quenching.37 Metal surfaces can interact with organic polymers in a variety of other ways including changing the conformation and/or charge distribution of the polymer, participating in charge-transfer interactions, and forming new chemical species, and it seemed worth exploring other mechanisms that may contribute to the reported effects. Raman spectroscopy is a well-established technique that can provide detailed information about molecular structure and bonding. When the excitation wavelength is on or near resonance with an allowed electronic transition, the resonance Raman intensities additionally provide a window into excitedstate molecular structure and photophysics.38,39 The resonance effect also affords enhanced sensitivity and selectivity for those molecular species that absorb near the exciting laser wavelength. When systems containing gold or silver nanoparticles are excited near the plasmon resonances in SERS, the Raman scattering from molecules closest to the metal surface can be further enhanced by large factors (103-106 or more).31–34 This makes SERS a sensitive and surface selective probe of molecular properties. As the plasmonically active metals that are reported to enhance solar cell efficiencies are the same metals that lead to surface enhancement of Raman scattering, SERS is an obvious technique for probing the nature of the metal-conjugated polymer interaction. Prior SERS studies on organic polymer solar cell components40–43 and other conjugated polymers42,44–47 have not addressed the enhancement of solar cell efficiencies by SERS-active metals. In most of the studies that report nanostructured metal enhancement of solar cell efficiency, the metal is in direct contact with the PEDOT:PSS layer typically used to improve hole transfer to the ITO electrode. We have therefore chosen to focus first on Raman and SERS spectroscopy of PEDOT: PSS. Figure 1 shows the chemical structures of PSS and PEDOT. PEDOT in its neutral form exhibits a strong, broad absorption that peaks near 600 nm and covers much of the visible spectrum. It is readily oxidized (e.g. in air) to a form that is only weakly absorbing in the visible and is characterized by a very broad absorption band centered around 1200 nm. At intermediate levels of oxidation there are two absorption bands near 850 and 1800 nm.48 Figure 2 shows structures of PEDOT in neutral, “singly oxidized”, and “doubly oxidized” forms. The actual number of charges and unpaired spins per monomeric unit in PEDOT are continuously variable.49 Most synthetic routes to PEDOT result in a material that is insoluble and not processable. However, an aqueous dispersion of colloidal PEDOT stabilized by poly(styrene sulfonate) (PSS) can be spin-cast and dried to form a thin film that is highly conducting, relatively transparent, and insoluble.50 Spin-coated films consist of 10-50 nm conductive PEDOT particles separated by insulating PSS layers.9,10,51,52 While PEDOT:PSS films exhibit remarkable stability over time in the ambient
J. Phys. Chem. C, Vol. 114, No. 14, 2010 6823 atmosphere, the conductivity is degraded by UV light through photooxidation processes.53 A few prior Raman studies of PEDOT have been reported. Garreau et al. used Raman spectroelectrochemistry, coupled with normal mode calculations based on an empirical force field, to characterize the neutral and oxidized forms of PEDOT.48 Interestingly, the strong vibration attributed mainly to stretching of the thiophene ring of the neutral form appears at 1423 cm-1 with 1064 nm excitation but at 1434 cm-1 with 514 nm excitation,48 probably as a result of distributions of effective conjugation lengths in the polymer.54 This group subsequently utilized Raman spectroscopy to examine the structure and doping level of PEDOT in nanofibers.55 Kim et al. used micro-Raman spectroscopy with 633 nm excitation to characterize the nonemissive “black spots” in polyfluorene-based light-emitting diodes (LEDs) that employ PEDOT:PSS on ITO as the anode.56 Furukawa and co-workers also used 633 nm excited Raman spectroscopy to examine polyfluorene/PEDOT:PSS LEDs. They found that the relative intensity of the PEDOT Raman bands increases after operation of the LED, and used this to argue that the PEDOT becomes dedoped during operation; however, their arguments were based on intensities and assumed resonance effects, while the frequencies of the Raman bands were hardly affected.57 This group also used the Stokes and anti-Stokes scattering of the 440 cm-1 band of PEDOT to make in situ temperature measurements of the PEDOT:PSS layer during LED operation.58 Bowmaker et al. used Raman spectroscopy at 785 and 1064 nm as an analytical tool to follow the reversible chemical and electrochemical doping and dedoping of PEDOT.59 Helmy’s group studied the effect of thermal annealing on the 633 nm excited Raman spectra of PEDOT:PSS films.60 There are also a few reports on the Raman spectroscopy of PEDOT associated with metal nanoparticles. Seeber’s group prepared composite materials consisting of PEDOT with gold nanoparticles encapsulated by bulky anionic species. The PEDOT Raman spectra were nearly unchanged by the presence of the metal,61,62 suggesting that the encapsulating agent both prevents aggregation of the gold nanoparticles and provides an effective barrier between the PEDOT and the metal. Kumar et al. used EDOT as a reductant for a gold salt to produce aqueous dispersions of Au nanoparticles protected by PEDOT:PSS.63 They interpreted the 1064 nm-excited Raman spectra as indicating a high degree of doping and cited a paper on poly(pphenylene) and poly(phenylene vinylene),64 but the spectra they present more closely resemble the spectra of neutral PEDOT from the spectroelectrochemical studies of Garreau.48 In the present study, we use Raman spectroscopy to examine the physical and/or chemical changes that occur when PEDOT: PSS interacts with metal nanoparticles. Our main focus is on the as-received material (oxidized PEDOT) because this is the form incorporated into organic solar cells, but films containing chemically reduced PEDOT are also examined for comparison. We expect that metal nanoparticles will enhance the Raman scattering through the SERS effect, thereby focusing our observations on the material closest to the metal surfaces. Experimental and Computational Methods PEDOT:PSS was obtained from Clevios (type P VP AI 4083, OLED grade) and was deposited on either glass coverslips (Fisherbrand) or ITO-coated glass (Delta Technologies). All samples containing metal nanoparticles, and the metal-free samples used for determination of SERS enhancement factors, were prepared by spin-coating, 5 s at 500 rpm followed by 30 s at 1500 rpm. Two layers of PEDOT:PSS, each expected to have
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Figure 2. Representative resonance structures of PEDOT in its neutral and successively oxidized forms.
Figure 3. Illustration of the “metal on bottom” (left) and “metal on top” (right) samples containing metal nanoparticles. Drawings are not to scale.
a thickness of 50-80 nm based on the conditions used,65,66 were spin-coated on top of each other to provide a thicker sample. PEDOT:PSS samples without metal nanoparticles were also prepared by drop-coating in order to produce thicker samples giving stronger Raman signals. All samples were then heated for 15 min at ∼110 °C. In place of the light-absorbing conducting polymer blend used in solar cells, the PEDOT:PSS was covered with a layer of polystyrene (PS; Sigma-Aldrich, average MW 35 000) spin-coated from toluene. For experiments on PSS alone, poly(sodium 4-styrene sulfonate) was purchased from Sigma-Aldrich (MW 70 000, 30 wt % solution in water) and was either spin-coated or drop-coated onto glass slides, with or without a covering layer of PS. Gold and silver nanoparticles were synthesized in aqueous solution through the standard citrate reduction method.67 The gold particles had diameters of 15 ( 2 nm based on transmission electron microscopy (TEM) images, and an absorption maximum of 519 nm in water. The silver particles had diameters of 46 ( 11 nm by TEM and an absorption maximum of 416 nm. Note that aggregation shifts the plasmon resonances, and the optical properties of the dried nanoparticle films used in our experiments may be considerably different from those of the particles in aqueous suspension. Gold nanoparticles were centrifuged to concentrate them; silver nanoparticles were gravity concentrated. For the “metal on bottom” configuration the nanoparticles were applied to the substrate with a dropper,
allowed to dry in air, and heated for 10 min at ∼100 °C. Then the PEDOT:PSS and PS layers were spin-coated as above. In a second preparation method, “metal on top”, the PEDOT:PSS was spin-coated and annealed first, then the nanoparticles were applied to the surface of the PEDOT:PSS, dried and heated at ∼100 °C, and finally the PS layer was applied. Figure 3 illustrates the two sample configurations prepared. Neutral PEDOT was generated by reduction of the as-received PEDOT:PSS with hydrazine as described in ref 68, followed by spin-coating onto glass with or without nanoparticles in the same manner as for the as-received PEDOT:PSS. Excitation for Raman spectroscopy was provided by either a Coherent Innova 90C-5 argon ion laser (457.9 and 514.5 nm) or helium-neon lasers (543 and 632.8 nm). Excitation power at the sample was 0.2-0.3 mW. Two different Raman detection setups were employed. The majority of the data presented were obtained on a macrosampling system,69 which uses a 20 mm focal length lens to focus the excitation light onto the sample and collects the scattering in a ∼135° backscattering geometry with a condenser lens. The Raman scattering was passed through a long-pass filter, dispersed with a Spex 500M 0.5 m single spectrograph, and detected with a Roper Scientific Spec 10: 100B liquid nitrogen cooled, back-illuminated CCD providing >50% quantum efficiency from 400-850 nm. At the slit width used (100 µm), spectral resolution was 3.4 cm-1 at 632.8 nm, 5.5 cm-1 at 514.5 nm, and 7.0 cm-1 at 457.9 nm. Some data
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Figure 4. Left: Optical absorption spectra of a thin film of PEDOT:PSS on glass after preparation in ambient room light (black) and after sitting in the dark for 2 h (gray). Right: Spectrum of PEDOT:PSS reduced with hydrazine prior to spin-coating on glass.
Figure 5. Left: Raman spectra of as-received PEDOT:PSS films, drop-coated onto glass and covered with a spin-coated layer of polystyrene, at three excitation wavelengths. Right: Corresponding spectra of PEDOT:PSS reduced with hydrazine. Asterisks mark peaks from the polystyrene overcoat. The spectra are scaled and vertically shifted. Peak positions labeled with a hash mark but no frequency are within (3 cm-1 of the spectrum immediately above.
were also obtained with a Jobin-Yvon T64000 Raman microscope system consisting of a 0.64-m triple spectrograph coupled to a confocal Raman microprobe based on an Olympus BX-41 microscope with an Olympus MPLFL100X objective. The much smaller focal volume of this system caused light-induced changes in the SERS spectra as discussed below. Spectra were corrected for the system sensitivity with use of a broadband light source.38 Fluorescence backgrounds were removed by using GRAMS/AI 7.02 (Thermo Galactic). The Raman spectra were calibrated in frequency by using the Raman bands of cyclohexane or polystyrene. Reported peak frequencies are the apparent maxima (or, for shoulders, inflection points) of the spectra as obtained and were not determined through any peakfitting algorithm. The reported frequencies may, therefore, be slightly distorted by peak overlap. Optical absorption spectra were obtained on a Cary 50 UV-vis spectrophotometer. Density functional theory calculations were performed by using the B3LYP hybrid functional with the 6-311g(d,p) basis as implemented in Gaussian 03.70 No scaling factor has been applied to the reported frequencies. Results Figure 4 shows the optical absorption spectra of as-received and chemically reduced PEDOT:PSS films on glass. The asreceived (oxidized PEDOT) films absorb mainly in the UV and red/near-IR regions of the spectrum, and they appear only very
faintly blue. Interestingly, even exposure to ordinary room lights changes the optical properties of the films. After sitting in the dark for 2 h, the absorbance in the UV region increases and the near-IR absorbance develops a more clearly pronounced peak near 840 nm and apparently a second peak beyond 950 nm. The light-exposed sample shows only one broad band with an apparent maximum to the red of 950 nm. The changes we observe on going from “dark” to “light” forms are similar to the changes observed in the spectroelectrochemical studies of Garreau et al. on changing the potential from +600 to +1200 mV.48 Note that the oxidized forms have very little absorption in the 400-600 nm region, so Raman spectra obtained with excitation at 457.9 or 514.5 nm should be essentially nonresonant while 632.8 nm is weakly postresonant. The neutral (reduced) form of PEDOT absorbs strongly in the midvisible, and excitation at either 514.5 and 632.8 nm, and to a lesser extent at 457.9 nm, falls within its absorption band. Figure 5 shows the Raman spectra of as-received (oxidized PEDOT) and hydrazine-reduced PEDOT:PSS films on glass without nanoparticles as a function of excitation wavelength. All of the spectra in the 900-1400 cm-1 range are fairly similar, with peaks of moderate to low intensity near 991, 1266, and 1368 cm-1. The line near 1266 cm-1 is consistently found about 8 cm-1 lower in frequency in the oxidized form than in the reduced form. The oxidized material also shows a band near 1131 cm-1 that is weaker or absent in the reduced form and is
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TABLE 1: Raman Frequencies (cm-1) of Reduced PEDOT synthetic short oligomer,54 457-1064 nm
electrochemical reduction48 (-1000 mV), 514 and 1064 nm
electrochemical reduction,59 785 and 1064 nm
chemical reduction,56 633 nm
chemical reduction, this work, 458-633 nm
1516 (w, 785) 1520 (1064)
∼1515 (m)
1567 (w, 458) 1560 (w, 514) 1553 (w, 633) 1516 (m)
1414 (s, 785) 1431 (1064)
∼1422 (s)
1439 (s, 458) 1434 (s, 514) 1427 (s, 633)
1370 (m) 1270 (1064) 1252 (m, 785) 1226 (1064) 1097 (w, 785) 1111 (1064) 990 (m)
∼1370 (m)
1368 (m) 1270 (w)
1571 (w, 457) 1562 (w, 514) 1517 (m)
1514 (m, 457) 1508 (m, 514) 1506 (m, 676) 1458 (m, 457) 1440 (s, 457) 1434 (s, 514) 1423 (s, 676) 1427 (s, 1064) 1369-1364 (m)
1434 (s, 514) 1423 (s, 1064) 1369 (m) 1270 (w)
991 (w)
992 (w)
-1
TABLE 2: Raman Frequencies (cm ) of Oxidized (doped) PEDOT PEDOT, +1000 mV,48 1064 nm 1546 (m)
PEDOT:PSS,56 633 nm ∼1570 (w) ∼1530 (m) ∼1490 (w)
∼1477 (s) ∼1454 (s) ∼1430 (s) ∼1424 (m) ∼1369 (m) 1295 (w) 1268 (m) 1239 (w) ∼1144 (m)
∼1450 (s) ∼1438 (s)
PEDOT:PSS, this work, 458-633 nm 1564-1578 (w) 1541 (w, 458) 1541 (w, 514) 1534 (m, 633) 1507 (w, 458) 1504 (w, 514) 1497 (w, 633) 1454 (m-s) 1441 (sh, 458) 1442 (s, 514) 1439 (s, 633)
PEDOT:PSS,60 on Si, 633 nm 1563 (w) 1532 (w) 1495 (m)
1421-1431 (s)
∼1422 (s) ∼1365 (m)
1365-1369 (w-m)
1366 (m)
∼1260 (w)
1258-1262 (w)
1255 (m)
∼991 (m)
probably assignable to the PSS component (see below). This line is not evident in the reduced form because of the resonance enhancement of reduced PEDOT at these excitation wavelengths. The spectra in the ring stretching region from 1400 to 1650 cm-1 vary considerably with both excitation wavelength and oxidation state. The spectra of reduced PEDOT are quite simple. There is a strong band that shifts from 1439 to 1427 cm-1 and narrows as the excitation is tuned from 457.9 to 632.8 nm. The narrowing and shifting to lower frequency is consistent with longer wavelength excitation resonantly enhancing a smaller subset of molecules that have longer effective conjugation lengths, as discussed by others.54 There is also a moderate intensity band near 1516 cm-1 that does not shift with excitation, and a weak, broad band near 1565 cm-1 that resolves into two bands with red excitation. The spectra of the as-received, oxidized material are much more complex in this frequency region. The most intense feature consists of several overlapping lines, but the apparent maximum is always at a higher frequency for the oxidized than for the reduced material at any excitation wavelength. In addition, the single 1516 cm-1 band of the reduced form is replaced in the oxidized form by one line that shifts from 1507 to 1497 cm-1 as the excitation is tuned from 458 to 633 nm, and a second line at 1541-1534 cm-1. The
1127-1135 (w) 990-994 (w-m)
1093 (w) 989 (m)
excitation wavelengths employed lie between two absorption bands of oxidized PEDOT (Figure 4), so the excitation dependence of the spectrum arises from a combination of preresonant and postresonant effects and is not simple to interpret. The spectrum of PSS alone (not shown) has strong lines at 1135, 1600, and 998 cm-1, the latter two nearly coincident with PS lines. In general the scattering from PSS is very weak compared with that from PEDOT at all three excitation wavelengths. Tables 1 and 2 compare our results with previously reported Raman frequencies of PEDOT in its reduced and oxidized forms. The previous studies used both chemical and electrochemical reduction to prepare the reduced form of PEDOT and employed a range of excitation wavelengths. Our 632.8 nm excited spectra of both the oxidized and reduced (doped and dedoped) forms agree well with those published by Kim et al. at the same excitation wavelength,56 although those workers did not report peak frequencies. However, our spectra are rather different from those of Schaarschmidt et al., which were spin-coated onto Si wafers rather than glass or ITO-coated glass.60 The spectra of both dedoped and variably doped forms reported by Chiu et al.59 are also rather different from ours, owing perhaps to their use of longer excitation wavelengths which are resonant with
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Figure 6. Left: The 632.8 nm excited Raman spectra of as-received PEDOT:PSS films on glass with or without Ag or Au nanoparticles, deposited either on the glass prior to spin-coating the polymer (“bot”) or on top of the PEDOT:PSS layer prior to overcoating with PS (“top”). Backgrounds have been subtracted and spectra scaled and vertically shifted by arbitrary factors. Asterisks mark features from the PS overcoat. Peak positions labeled with a hash mark but no frequency are within (3 cm-1 of the spectrum immediately above. Right: Same for chemically reduced PEDOT: PSS. Metal-free films were drop-coated, while metal-containing films were spin-coated.
the doped form but preresonant with the undoped form. Our spectra of both oxidized and reduced forms generally agree well with those of Garreau et al.,48 although their spectrum of electrochemically oxidized PEDOT at 1064 nm shows a strong line near 1477 cm-1 seen only as a weak shoulder in our spectra and only at 632.8 nm, again owing perhaps to the different excitation wavelengths. Table S1 (see the Supporting Information) summarizes literature values and our observed values of the vibrational frequencies of PSS and PS. The main Raman lines of PSS and PS are nearly identical except for the S-O stretching vibration of PSS near 1135 cm-1. The strong line near 1000 cm-1 from the PS coating tends to interfere with observation of the PEDOT line near 990 cm-1, but both PS and PSS have only weak Raman lines in the 1400-1580 cm-1 region where the main ring stretching vibrations of PEDOT occur. PEDOT:PSS samples prepared with gold or silver nanoparticles exhibit changes in the relative intensities of some of the Raman lines and, particularly with silver, enhancement of the Raman scattering. Both the extent of enhancement and the qualitative appearance of the spectra vary considerably from one region to another of the nanoparticle-treated slides, which is not surprising given that the microscopic appearance of the samples shows considerable variability in nanoparticle density. For this reason we have not attempted to quantitate the enhancement. Figure 6 presents representative spectra at 632.8 nm excitation. Interestingly, the spectra of the as-received material are different when the nanoparticles are deposited on the glass prior to spin-coating the PEDOT:PSS than when they are deposited onto the PEDOT:PSS after it is spin-coated (“bottom” versus “top”, see Figure 3). When either Ag or Au nanoparticles are deposited on the bottom, the spectra are quite similar to those in the absence of nanoparticles. When the nanoparticles are deposited on top, the main thiophene stretching band shows an overall shift of intensity to lower frequencies. In addition, the bands in the 1530-1580 cm-1 region are deenhanced and the 1497 cm-1 band shifts up to 1502-1506 cm-1. Raman spectra of reduced PEDOT also show changes in the presence of nanoparticles. Gold has little effect on the reduced spectrum regardless of how it is deposited, but silver nanoparticles change the spectra considerably. In particular, the spectra of chemically reduced and as-received PEDOT:PSS with Ag on the bottom are rather similar, suggesting that the reduced
PEDOT may have become reoxidized. The most suggestive evidence for reoxidation is the shift of the 1268 cm-1 line to 1262 cm-1 in the Ag bottom spectrum. Figure 7 shows Raman spectra of PEDOT:PSS films, each spin-coated to the same nominal thickness, with no rescaling of the intensity axes. Fluorescence backgrounds have been subtracted and the baselines have been adjusted for ease of viewing, but the baseline-to-peak intensities are proportional to the measured signal strengths in counts per second. The Raman signals are significantly enhanced by Ag nanoparticles and to a much lesser extent by Au. As mentioned above, the nanoparticle density is rather heterogeneous and the apparent enhancement varies considerably from one part of the sample to another. The effects of metal nanoparticles on the Raman spectra are somewhat less pronounced at 457.9 and 514.5 nm excitation and the signal-to-noise ratio of the spectra is compromised by larger fluorescence backgrounds. In addition, the SERS enhancement from Au is considerably weaker at these wavelengths, particularly at 457.9 nm. However, the presence of Ag nanoparticles also causes the “reduced” and “oxidized” spectra to appear similar, with a line near 1262 cm-1 characteristic of the oxidized form. Spectra obtained on ITO-coated slides were similar to those with bare glass as the substrate (see Figure S1 in the Supporting Information). The spectrum of reduced PEDOT:PSS with silver on the bottom (top plot on the right-hand side of Figure 6) also shows a new, weak line at 969 cm-1, and lines of moderate intensity at 1031 and 1131 cm-1. The 1131 cm-1 line may be from PSS. PS has a line near 1030 cm-1, but the 1031 cm-1 line in this spectrum cannot be attributed to PS based on the low intensity of the strong PS line at 999 cm-1. Spectra taken with the Raman microscope system, where the excitation laser is focused much more tightly, show several new lines in the 950-1100 cm-1 region. These new lines appear in both the oxidized and reduced forms of PEDOT:PSS, although more often and more strongly in the reduced form. Representative spectra showing these new features are presented in Figure 8. The new lines at 960-980, 1052, and 1075 cm-1 appear only in the presence of Ag nanoparticles, not Au. They are not found in all spectra. In some spectra none of these lines are present at the shortest observation times but the 960-980 cm-1 line grows in with time, accompanied by a loss of intensity in the main
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Figure 7. The 633 nm excited Raman spectra of as-received (“oxidized”) and chemically reduced (“reduced”) PEDOT:PSS films in the absence of metal or with Au or Ag nanoparticles. All three spectra in each frame are plotted on the same scale in counts per second, and all samples consist of two spin-coated layers on glass.
Figure 8. Raman spectra of as-received (“ox”) and chemically reduced (“red”) PEDOT:PSS films spin-coated on glass with Ag nanoparticles, obtained with the Raman microscope. New, presumably light-induced features are labeled.
PEDOT features. In other spectra a band at 960-980 cm-1 is present at the earliest observable times, while in still other cases no new lines in this region appear despite prolonged irradiation. The lines near 1052 and 1075 cm-1 always have approximately the same frequency and are reasonably sharp, while the lower frequency feature is usually broader and its maximum varies over a range of nearly 20 cm-1. Spectra obtained on the Raman microscope with 457.9 and 514.5 nm excitation show these as well as other time-dependent changes including loss of intensity in the thiophene ring stretching region and increased intensity around 1585 cm-1, both of which are apparent in the top two spectra of Figure 8. Discussion The excitation wavelengths used in this study all fall within the absorption spectrum of the neutral form of PEDOT, while they are near a minimum in the absorption of the oxidized forms. Therefore, we expect that any neutral PEDOT present in our samples will be preferentially enhanced in the Raman spectra. Our spectra of as-received PEDOT:PSS, excited at 632.8 nm, are similar to those reported by Garreau at 1064 nm for the fully oxidized form of PEDOT (+1000 mV potential)48 and are nearly identical with the 633 nm-excited fully doped PEDOT spectra presented by Kim et al.56 In addition, the spectra of chemically reduced PEDOT:PSS are quite different from those
of the as-received material at all excitation wavelengths as shown in Figure 5. We therefore conclude that the spectra shown in Figure 5 (left panel) do originate almost entirely from PEDOT in a highly doped (oxidized) form. The spectra in the main thiophene ring stretching region (1425-1455 cm-1) vary considerably with excitation wavelength. This has been observed previously in PEDOT54 and attributed to wavelength-selective resonance enhancement of segments with different effective conjugation lengths. The spectra of Figure 5 suggest two empirical marker bands for oxidation state: 1268-1271 cm-1 in the reduced form versus 1258-1262 cm-1 in the oxidized form (this may be a different normal mode that shifts up in frequency rather than the 1270 cm-1 mode shifting down),48 and 1516 cm-1 in the reduced form versus 1497-1507 cm-1 in the oxidized form. Published calculations on neutral PEDOT assign the 1270 cm-1 mode as the inter-ring CsC stretch48,71 and the 1516 cm-1 mode as asymmetric CdC stretching of the sides of the thiophene ring.48 How the frequencies of these vibrations should change upon oxidation depends on the relative contributions of the resonance forms in Figure 2 to each of the oxidation states and is difficult to predict. However, the empirical criteria suggest that chemically reduced PEDOT:PSS deposited over Ag nanoparticles on the bottom undergo a considerable degree of reoxidation (Figure 6, top right). There is also some indication of reoxidation when the Ag is deposited on top of the PEDOT:PSS. The formation of nonemissive “black spots” in polyfluorene-based lightemitting diodes (LEDs) has been attributed to dedoping of the PEDOT.56,57 Our results suggest that metal nanoparticles might protect PEDOT against this type of damage and thereby enhance solar cell performance, although we observe this effect only with silver, not with gold. The oxidant is most likely atmospheric oxygen; while our samples are somewhat protected by the PS coating, no effort was made to exclude oxygen during sample preparation. Aqueous emulsions of PEDOT:PSS consist of regions of relatively hydrophobic PEDOT surrounded and solubilized by PSS.72 A variety of studies suggest that this morphology persists in the spin-coated films.9,10,72,73 As plasmonic Raman enhancement falls off rapidly with increasing distance from the metal surface, a morphology in which PEDOT grains are coated by PSS should result in the PSS being more strongly enhanced by metal nanoparticles than the PEDOT component. Figure 6 shows that the strongest Raman mode of PSS, the SO3 stretch at ∼1133 cm-1, is indeed slightly stronger (relative to the PEDOT
Interaction of PEDOT:PSS with Active Nanoparticles vibrations) in the “metal bottom” configuration than in the absence of metal nanoparticles. However, the PSS component does not appear to be preferentially enhanced when the metal is deposited on top. In addition, when the metal is on the bottom the thiophene ring stretching region shows a general shift of intensity to higher frequencies, suggesting a shorter effective conjugation length that might imply less extended chains. When the metal is on the top, the PEDOT chains appear to be more extended and also less protected from the metal by the PSS layer. Light-induced degradation of PEDOT has been observed by other workers and attributed to a variety of processes. As noted above, Furukawa and co-workers used Raman intensities to argue that the PEDOT in polyfluorene/PEDOT:PSS LEDs becomes dedoped during operation,57 but their conclusion was not supported by Raman frequency changes. Kim et al., in contrast, did observe changes in the Raman spectrum characteristic of reduction of the PEDOT in LEDs.56 Marciniak and co-workers used X-ray photoelectron spectroscopy to characterize UV light-induced changes in doped PEDOT as well as both neutral and doped forms of an alkylated PEDOT.53,72 They observed addition of oxygen to the thiophene sulfur to form sulfone groups (R-SO2-R) as well as chain scission accompanied by the addition of carbonyl/carboxyl groups, both disrupting π-conjugation in the ring. UV or visible irradiation of poly(3-alkylthiophene) films in the presence of oxygen leads to new features in the IR spectrum attributed to hydroxyl, carbonyl, ether, and a variety of sulfur-containing oxidative products.74,75 The new Raman lines we observe in the presence of Ag nanoparticles at 1075, 1052, and 960-980 cm-1 may result from structures in which oxygen has been added to the thiophene ring sulfur. The Raman spectra of two EDOT-containing sulfone model compounds did not reveal any lines clearly attributable to the SO2 group.76 However, photooxidation of P3HT has been shown to produce new IR features at 1080 and 1050 cm-1, the latter assigned to the S-O stretch of a sulfoxide (R-SO-R) group.75 Density functional theory calculations give the symmetric SO2 stretch of EDOT sulfone at 1090 cm-1 and the S-O stretch of EDOT sulfoxide at 1031 cm-1. We therefore tentatively assign the bands near 1075 and 1052 cm-1 to sulfone and sulfoxide stretching, respectively. Any assignment for the bands at 960-980 cm-1 is highly speculative, although desulfonation of PSS has been observed as a damage mechanism72 and the strongest Raman bands of aqueous SO42- and HSO4are at 980 and 1040 cm-1, respectively.77 In the surface-enhanced Raman scattering community it is well-known that organic molecules in close contact with SERSactive metal nanoparticles, particularly silver, often exhibit rapid photochemical degradation.33,41,78–81 Broad bands near 1590 cm-1 (see the 514.5 nm excited spectra of Figure 8) are observed as contaminants or photoproducts in many SERS experiments and are often attributed to amorphous carbon,79–81 although many other species could also give rise to features in this region. The new Raman lines we observe in the 900-1100 cm-1 region appear more specific to PEDOT. The incident light intensities in our Raman microscope experiments (prior to plasmonic enhancement) are hundreds of MW/m2, at least 5 orders of magnitude greater than solar fluxes. A photochemical process that occurs with a rate constant of seconds to minutes under our experimental conditions would require months to years to occur in a functioning device if the process is linear in photon flux. Nevertheless, our observation of significant photoinduced changes in PEDOT:PSS in the presence of silver nanoparticles
J. Phys. Chem. C, Vol. 114, No. 14, 2010 6829 suggests that even if solar conversion efficiency is initially enhanced, the long-term stability of such devices may be problematic. Conclusions The Raman spectra of PEDOT:PSS films in contact with gold or silver nanoparticles exhibit not only enhancement but also changes in frequency and relative intensity that suggest effects of the metal on PEDOT chain morphology. The spectra also suggest that silver nanoparticles facilitate reoxidation of chemically reduced PEDOT to its original doped form. At higher light intensities, metal nanoparticle-containing PEDOT:PSS films exhibit new Raman lines in the 950-1150 cm-1 region that are tentatively assigned as products of oxidative addition of oxygen to the PEDOT ring sulfur atoms and/or desulfonation of PSS. These morphological and/or chemical effects may contribute to the performance of organic polymer solar cells which incorporate metal nanoparticles into or in contact with PEDOT:PSS. Acknowledgment. The authors acknowledge the Donors of the American Chemical Society Petroleum Research Fund for support of this research through grant no. 48820-ND10. TEM images of the metal nanoparticles were obtained at the UC Merced Imaging and Microscopy Facility with the assistance of Mike Dunlap. Brandon Hernandez, an American Chemical Society Project SEED student, carried out the initial density functional theory calculations on EDOT dimers and trimers and their sulfones. Supporting Information Available: Raman frequencies of PSS and PS and 633 nm excited Raman spectra of PEDOT: PSS with and without metal nanoparticles on ITO-coated glass. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15–26. (2) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533– 4542. (3) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. AdV. Funct. Mater. 2006, 16, 2016–2023. (4) Markov, D. E.; Amsterdam, E.; Blom, P. W. M.; Sieval, A. B.; Hummelen, J. C. J. Phys. Chem. A 2005, 109, 5266–5274. (5) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789–1791. (6) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841–843. (7) Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.; Heeger, A. J. AdV. Mater. 2006, 18, 572–576. (8) Andersson, B. V.; Huang, D. M.; Moule´, A. J.; Ingana¨s, O. Appl. Phys. Lett. 2009, 94, 043302. (9) Nardes, A. M.; Janssen, R. A. J.; Kemerink, M. AdV. Funct. Mater. 2008, 18, 865–871. (10) Pingree, L. S. C.; MacLeod, B. A.; Ginger, D. S. J. Phys. Chem. C 2008, 112, 7922–7927. (11) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617–1622. (12) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 083506. (13) Barazzouk, S.; Hotchandani, S. J. Appl. Phys. 2004, 96, 7744– 7746. (14) Ha¨gglund, C.; Zach, M.; Kasemo, B. Appl. Phys. Lett. 2008, 92, 013113. (15) Morfa, A. J.; Rowlen, K. L.; Reilly, T. H., III; Romero, M. J.; van de Lagemaat, J. Appl. Phys. Lett. 2008, 92, 013504. (16) Nah, Y.-C.; Kim, S.-S.; Park, J.-H.; Park, H.-J.; Jo, J.; Kim, D.-Y. Electrochem. Commun. 2007, 9, 1542–1546. (17) Rand, B. P.; Peumans, P.; Forrest, S. R. J. Appl. Phys. 2004, 96, 7519–7526.
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