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Work function engineering of ZnO electrodes by using p-type and n-type doped carbon
nanotubes
View the table of contents for this issue, or go to the journal homepage for more
Work function engineering of ZnOelectrodes by using p-type and n-typedoped carbon nanotubes
Antonio Urbina1,2, Ji Sun Park3, Ju Min Lee3,4, Sang Ouk Kim3,4 andJi-Seon Kim1,3
1 Department of Physics and Centre for Plastic Electronics, Imperial College London, Prince ConsortRoad, London SW7 2AZ, UK2 Department of Electronics, Technical University of Cartagena, Plaza Hospital 1, 30202 Cartagena,Spain3 Department of Materials Science and Engineering, Korea Advanced Institute of Science andTechnology (KAIST), 305-701, Daejeon, Republic of Korea4 Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701,Republic of Korea
Received 3 June 2013, in final form 11 September 2013Published 6 November 2013Online at stacks.iop.org/Nano/24/484013
AbstractTransparent electrodes in organic electronic devices are strongly needed in order to replaceindium tin oxide (ITO). Some of the best candidates are ZnO films, which have shown bothgood electronic properties and solution processability compatible with roll-to-roll productionof the devices. We present the possibility to engineer the work function of ZnO by blending itwith carbon nanotubes (CNTs). B-doped (p-type), N-doped (n-type) and undoped CNTs aswell as their blends with ZnO have been characterized by atomic force microscopy (AFM),scanning Kelvin probe microscopy (SKPM) and Raman spectroscopy. The results of Ramanspectroscopy demonstrate the substitutional doping of carbon nanotubes, which preserves theircovalent structure although increasing the disorder within the nanotubes. The roughness andaverage shape of grains of ZnO when blended with the doped nanotubes have been measuredby AFM. Finally, SKPM shows that the work function of the blends can be engineered from4.4± 0.1 to 4.9± 0.1 eV according to the kind of nanotube that is blended even if only asmall amount of nanotubes is added to the blend (0.08 wt%).
S Online supplementary data available from stacks.iop.org/Nano/24/484013/mmedia
(Some figures may appear in colour only in the online journal)
1. Introduction
Emerging photovoltaic technologies are predicted to meet gridparity costs for electricity production in the coming years [1].One of the roadmaps for achieving this ambitious task is thesearch for a polymeric photovoltaic device (or ‘plastic solarcell’ [2]) that can be fabricated by a continuous roll-to-rollprocessing method from solution, which will significantlyreduce the economic cost and the environmental impact ofthe manufacturing process [3]. Life cycle analysis of the
manufacturing process of such devices, which also includesa detailed material inventory and the energy inputs associatedto each material, has pointed to indium tin oxide (ITO) as oneof the bottlenecks for the future production of low embeddedenergy and low cost solar cells [4]. It is therefore necessaryto look for alternative transparent conducting electrodes fororganic solar cells [5] (and other optoelectronic devices, suchas OLEDs and phototransistors).
In recent years, ZnO has seen a revival in its technologicalinterest. This semiconducting material has a wide band
gap, Eg = 3.1–3.3 eV, and a good electron mobility,µe = 150–440 cm2 V−1 s−1, with carrier concentrationsup to ne = 1.2 × 1017 cm−3, which makes it a goodcandidate for transparent electrodes (for a comprehensivereview of ZnO see [6]). ZnO also has much simplercrystal-growth technology than the most commonly usedtransparent electrode, ITO, resulting in a potentially lowercost for ZnO-based devices and compatibility with ‘printing’technologies from solution. It crystallizes in either cubiczinc-blende or hexagonal wurtzite structure, where each anionis surrounded by four cations at the corners of a tetrahedron,and vice versa [7]; this tetrahedral coordination is typical ofsp3 covalent bonding, suggesting that these materials alsohave a substantial ionic character, which makes it difficult tocreate good electrical contacts due to the built-in interfacialdipoles. The conductivity of ZnO at room temperature isdifficult to explain in terms of thermal excitation of carriersgiven the large band gap. Instead, an explanation based onn-type doping due to vacancies in the crystalline structurehas been proposed theoretically [8] and demonstratedexperimentally [9] using irradiated samples to study theconductivity for samples with different densities of nativedefects. Nevertheless, the tuning of doping and thereforeFermi level position within the band gap of ZnO controlledby irradiation of samples is not compatible with inexpensiveand fast manufacturing processes. Furthermore, the use ofradiation damage to dope the samples is not compatiblewith most of the well-controlled growth mechanisms whichnowadays yield most ZnO nanostructures: single crystalepitaxial films deposited by pulsed laser deposition [10],nanoparticle synthesis from MeOH solution [11], nanocoralfabricated on top of a Zn layer by radio-frequency magnetronsputtering followed by thermal oxidation at 500 ◦C [12],nanofibres (electrospun mats at 450 ◦C) [13], nanorodsgrown by chemical vapour deposition [15], tetrapods from‘powder’ in ethanol solution [14], and many other approachesincluding hybrid ZnO/metal nanostructures [15]. If ZnO isto be integrated in low temperature roll-to-roll processingof organic solar cells, ZnO production methods should bebased ideally on solution at processing temperatures lowerthan 140 ◦C and at the same time keep good crystallinity (ormicrocrystallinity) for good charge transport and a thin aspectratio for reliable flexibility on plastic substrates. This has beenachieved for small molecule solar cells [16], dye sensitizedsolar cells [17], hybrid solar cells (using nanoparticles mixedwith polymers in the active layer) [18, 19] and polymericsolar cells [20–24], but the performance of the devices(power conversion efficiency) is always lower than that of thebenchmark device which uses ITO, mainly due to the lowermobility and the mismatch in energy levels for standard orinverted structures.
In all cases the problem of controlled doping of ZnOremains a challenge. Novel approaches have tried to overcomethis problem by using mixtures of ZnO and carbon nanotubes(CNTs) as electrodes instead of bare ZnO. This strategyavoids the need for native defect control by irradiation asmentioned above. CNTs have been shown to have a largepotential for room temperature ballistic transport due to
quantization of charge in individual nanotubes, includingboth single wall carbon nanotubes (SWNTs) [25] andmultiwall carbon nanotubes (MWNTs) [26]. However, deviceapplications have been difficult to achieve mainly due toproblems with charge transfer between nanotubes and to thesubsequent electrode connection to the network of nanotubes.A mixture of ZnO and SWNTs has been used for theelectrodes in P3HT:PCBM organic solar cells [27]. It wasshown that the short-circuit current and efficiency of thesolar cells were increased by a factor of ∼2, whereasthe open-circuit voltage was virtually unchanged by theaddition of the nanotubes. Nevertheless, the power conversionefficiencies achieved so far are lower than 2%, whichcompares poorly with the performance of the same activelayer P3HT:PCBM in a conventional ITO cell (around 5%,as reported by several laboratories [3]). Poor performance ofCNT mesh in comparison to metal or ITO electrodes was alsoreported [28]. More recently, a spray patterned film of SWNTs(from aqueous solution using SDS as surfactant) was used asthe electrode in a P3HT:PCBM solar cell and the efficiencywas improved to 3.6%, which is comparable to conventionaldevices using ITO [29].
CNTs may be n-type or p-type doped in order tomodify their semiconducting or metallic properties andimprove the charge transport of a network (or entangledmat) of nanotubes [30, 31]. Raman characterization of dopednanotubes shows features in some bands that allow the amountof disorder to be quantified, in both nitrogen-doped [32]and boron-doped [33, 34] nanotubes. In this paper we reporta detailed characterization of doped carbon nanotubes byatomic force microscopy (AFM), including scanning Kelvinprobe microscopy (SKPM), and Raman spectroscopy. When0.08 wt% concentration of CNTs, which is a small amountof nanotubes, is mixed with ZnO, it is shown that the workfunction of the mixture can be engineered according to thedoping of the nanotubes. These electrodes give improvedcharge transport and additional flexibility for level matchingin different kinds of organic optoelectronic devices, such asorganic light emitting diodes and organic solar cells.
2. Sample preparation and experimental details
The samples were prepared following procedures describedin detail in [35, 36]. They consist of thin films of threekinds: (i) ZnO, (ii) B-doped (p-type), N-doped (n-type) orundoped carbon nanotubes (CNTs) and (iii) ZnO blendedwith a small amount of doped or undoped CNTs, all ofthem deposited on fluor-doped tin oxide (FTO) substrates byspray pyrolysis in a glove box. All samples were stored ininert atmosphere at room temperature (glove box); they wereonly in contact with air during measurements. The CNTshad substitutional doping of electron-deficient boron (B) orelectron-rich nitrogen (N), which systematically altered thework function of the CNTs: the doping levels of the B-CNTsand N-CNTs were 3.0 and 3.4 atom%, respectively [35].The ZnO:CNT solutions were obtained from a precursorof zinc acetate dihydrate (Sigma-Aldrich, 16.5 mg) whichwas dissolved in 2-methoxyethanol (Sigma-Aldrich, 100 ml)
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Nanotechnology 24 (2013) 484013 A Urbina et al
with stabilizer of ethanolamine (Sigma-Aldrich, 5 ml) andthen mixed by ultrasonication for 4 h with 10.5 mg of thecorresponding CNTs followed by centrifugation, precipitationand redispersion until a stable solution of the desiredcomposition was obtained [36]. Finally, solution processing ofthin films of ZnO/CNT (∼80 nm) nanocomposite layers andCNTs via spray pyrolysis on FTO (120 nm)/glass substrateswas performed. The spray pyrolysis was carried on 400 ◦Csubstrates using a hot plate and a conventional air-sprayer.The CNT content within the ZnO layer was maintained at0.08 wt%. Previous UV–vis spectrum measurements haveconfirmed that the blended nanotubes do not significantlyinfluence the bandgap (∼3.2 eV) or the optical transmittanceof the ZnO layer [32].
All measurements were performed using an AFMmanufactured by NT-MDT. The technique used was non-contact amplitude modulation AFM, in which tip oscillationis kept at a minimum to ensure a true non-contact regime.Topography and phase contrast images were acquired forscans of different sizes (ranging typically from 20 µm ×20 µm to 500 nm × 500 nm images).
All AFM tips were manufactured by NanoSensors(model: PPP-NCHPt50). They were conductive tips with aPt–Ir coating (25 nm). The tips’ resonant frequencies werearound 270 kHz and their force constants were around40 N m−1; the resonant frequency was determined for eachtip used. Even if not necessary for AFM measurements,we used conducting tips because they were required for theSKPM measurements. This set of tips provided an X–Y planeresolution of 1x,1y: ±25 nm and a height resolution of1z :≤ 1 nm (tested against a calibration of a highly orientedpyrolytic graphite (HOPG) sample). The HOPG measurementalso delivered a value of around 160 mV for the work functionof the HOPG (4.6 eV), which provided a link to an absolutescale for the SKPM measurements, with a resolution (viaLorentzian fit) of 1Vcp: ±25 mV (see figures S-E1 to S-E3in the supplementary information available at stacks.iop.org/Nano/24/484013/mmedia).
The SKPM measurements were performed using thesecond pass technique and applying an oscillating electricalfield to the tip (sample grounded, second pass always at7 nm additional height) and using a second feedback loopand a lock-in amplifier which allowed us to extract the firstharmonic of the signal and nullify its value by applying aVdc component which was equal to the surface potential ofthe sample for each point of the X–Y scan. This techniqueprovides an image of the surface potential of the sample.To summarize, we used conductive tips which were biasedby Vtip (dc + ac voltage); the voltage applied to thesmall piezoelectric was Vpiezo = 0 (working in a ‘voltagemodulation mode’); the lock-in technique was applied forthe separation of harmonics, dc, 1ω, 2ω; the second feedbackwas used to nullify the first harmonic, and at this point thevalue of the applied Vdc = Vsurf was recorded (for a givenX–Y position) and a surface potential image was constructed.The measurements were performed using a retrace mode inwhich the voltage applied to the sample piezo scanner wasrecorded in a first scan (with Vtip = 0), and this allowed us to
perform a second scan at a given height on top of the recordedtopography (the second scan did not use the first feedback;KPM was also performed without retrace mode, but in allcases we applied a second pass distance of 7 nm as explainedabove, working with the two feedbacks simultaneously).
Micro-Raman measurements were performed with aRenishaw In-Via spectrometer coupled to an opticalmicroscope and using different excitation wavelengths: 785,633, 514, 488 and 457 nm; the laser power was kept at10 mW for all measurements (laser spot ∼1 µm2) unlessindicated otherwise. Only excitation with 785 nm wavelengthwas performed at higher power: 80 mW. All spectra arepresented as an accumulation of five acquisitions of 5 s each.Intensity is normalized to different peaks (as indicated in therespective figures).
3. Results and discussion
In this section, the experimental results and discussion arepresented. First, we focus on the Raman spectra, both of pureCNTs and of blends of ZnO:CNTs; then the AFM and SKPMdata are presented. Both sets of data enable the discussion ofwork function engineering and the demonstration that a tinyamount of CNTs can indeed modify the ZnO work function.
3.1. Raman spectroscopy
All Raman spectroscopy measurements show strong CNTsignals for all the excitation wavelengths. This CNT signalsare used to characterize the pure CNT samples and todemonstrate the combined signals of both ZnO and CNT whenthe spectra are measured on the blends, even for a tiny amountof CNT within the blend (0.08 wt%).
First, we analyse the CNT radial breathing modes[37, 38], which are related to the CNT diameter by
ωRBM =A
dt+ B
where A = 227.0 ± 0.3 cm−1 nm and B = 0.3 ± 0.2 cm−1.When the influence of the environment is strong, Van derWaals forces are included in the model [39],
ωRBM =227dt
√1+ Ced2
t
with Ce ≈ 0.05–0.07.Figure 1 illustrates the lower wavenumber spectra in
which radial breathing modes (RBMs) are detected. It can beseen that both undoped and doped CNTs show RBMs, thusindicating that the substitutional incorporation of the dopingatoms preserves the covalent structure of the nanotubes. Thepeak position does not show any dependence on the differentexcitation wavelengths, as can be observed in the intensitiesof the different CNT peaks: as expected for these firstorder peaks, neither the position nor the width of the peaksdepends on the excitation wavelength. A detail of the peak at∼240 cm−1 is shown for an excitation wavelength of 514 nmin figure 1. A peak at higher wavenumber (∼310 cm−1)is attributed to the signal from an inner nanotube (with
Figure 1. The low wavenumber parts of the Raman spectra ofundoped, B-doped and N-doped CNTs for λ = 514 nm excitationwavelength. The spectra are normalized to the first RBM, whichcorresponds to external nanotubes of diameter ∼1 nm; the secondbroader peak corresponds to inner nanotubes.
approximately half diameter). A summary of the diametersdeduced from the measured RBM positions is presented intable S1 in the supplementary information (available at stacks.iop.org/Nano/24/484013/mmedia); the position of the peakand the full-width-half-maximum (FWHM) is obtained byfitting Lorentzian peaks to each individual measurement.
The G band, which corresponds to the tangential C–Cstretching mode, can be easily identified and it is slightlymodulated by the nanotube diameter, as indicated by
ωG = 1591−A
d2t,
where A can be AG+ = 0,AG-S = 47.7 cm−1 nm2 or AG-M =
79.5 cm−1 nm2; these values represent the small modulationof the main G peak (G+) for semiconducting (G-S) andmetallic nanotubes (G-M), which creates a broadening ofthe G peak related to the orientation of the C–C stretchingmode relative to the chirality of the nanotube [37, 41]. TheD band (one phonon double resonance, DR1) is an LO oriTO mode, corresponding to an intervalley scattering (q= 2k).This D band is associated with disorder; it is a dispersiveband and therefore ω depends on the laser energy [40, 41],as can be seen in figure 4 (detailed values for the Ramanshifts of the peak positions are included in table S2 inthe supplementary information available at stacks.iop.org/Nano/24/484013/mmedia). The intensity ratio between theG and D bands allows us to define a parameter δ = ID/IGwhich quantifies the amount of disorder within the nanotube.In order to illustrate this intermediate wavenumber shiftregion of the Raman spectra we plot in figure 2 (top), foran excitation wavelength of λ = 633 nm, a superpositionof spectra for undoped, B-doped and N-doped nanotubes(normalized to the G-band peak); this normalization illustratesthe doping dependence of the disorder D and D′ peaks (seemore detail in the inset of figure 2 for the D′ peak). Theshoulder at 1100 cm−1 is attributed to the FTO substrate
(detailed spectra for a pure FTO substrate are shown in thesupplementary information available at stacks.iop.org/Nano/24/484013/mmediaand used to identify the FTO peaks whichappear in the nanotube samples).
Finally, in the higher wavenumber shift regions, wehave the following peaks: G′ which is a two phonon doubleresonance process DR2, 2LO which is an overtone of LO(D + D′) modes, and 2G which is an overtone of the Gmode. First, we show in figure 2 (top right) a comparison ofdoped and undoped nanotubes for the λ = 633 nm excitationwavelength; then, an excitation wavelength dependence studywas carried out which is also illustrated in figure 2 (middle andbottom graphs) for the doped CNTs whose results are shownin detail (the results for undoped CNTs are shown in thesupplementary information available at stacks.iop.org/Nano/24/484013/mmedia); as expected, the 2G peak is not affectedby the excitation energy, but all other peaks are shifteddepending on the excitation wavelength. A summary of the D,D′ and G′ peak position dependence on excitation wavelengthis presented in table S2 of the supplementary information(available at stacks.iop.org/Nano/24/484013/mmedia) andrepresented graphically in figure 4. The values obtained for theparameters from all Raman peak fittings of CNT samples foran excitation wavelength of λ = 633 nm can be found in tableS1 of the supplementary information (available at stacks.iop.org/Nano/24/484013/mmedia); in all cases, the values arisefrom Lorentzian fitting to individual peaks. Moreover, theparameter δ = ID/IG calculated from normalized intensitiesallows us to compare the degree of disorder between theundoped and doped nanotubes. The parameter δ has beenobtained for all excitation wavelengths (figure 3) and it showsthat in the doped samples, the disorder is ∼10% larger thanin the undoped sample; this arises from the random inclusionof substituting dopant atoms (N or B, 3–4%), but also fromadditional C–C disorder induced by the chemical dopingprocess. Only for higher energy excitation does the undopednanotube show a higher degree of disorder according to thisdefinition. This higher degree of disorder is still not fullyunderstood but it could be attributed to a more susceptibledegradation of the undoped nanotube to the high energy laser.
The linear dependence of the Raman wavenumbers of theD and G′ peaks with excitation energy can be fitted with thefollowing simple equation as shown in figure 4:
ωD,G′ = AElaser + B.
The parameters A and B obtained from the fits are summarizedin table 1. The dependence of the G′ peak is particularlyinteresting since it comes from a resonant second order,symmetry allowed, Raman scattering process involving anin-plane transverse optical (iTO) phonon, typical of all sp2
carbon materials near the K point of the hexagonal Brillouinzone. A phonon with wavevector −2k will be selected by anelectron with wavevector k in a double resonance process (iflinear and angular momentum are conserved in the 1D carbonnanotube structure). If we consider that the energy of thelaser is exciting electrons from the valence to the conductionband in the nanotube, Elaser = Ec − Ev = 2hvFk, where vFis the Fermi velocity of the electron, then the modulus of
Figure 2. Top: Raman spectra of doped and undoped CNTs for an excitation wavelength of λ = 633 nm normalized to the intensities of theG and G′ peaks respectively. The inset shows a detail of the G and D′ peaks. The shoulders at 1100 and 2500 cm−1 are attributed to the FTOsubstrate. Middle: Raman spectra for B-doped CNTs (left) and N-doped CNTs (right) for four different excitation energies. The G peak isused for normalization. The strong intensity and Raman wavenumber variation of the D peaks can be appreciated. Bottom: Raman spectrafor B-doped CNTs (left) and N-doped CNTs (right) for different excitation energies. The G′ peak is used for normalization. The strongintensity and Raman wavenumber variation of the G′ and 2LO peaks are easily appreciated, while 2G remains stable.
Table 1. Summary of the parameters obtained by fitting the plots shown in figure 4 to the linear equation which relates Raman shiftwavenumber to the laser excitation energy.
Figure 3. The parameter defined to quantify disorder from theintensity ratio of peaks D and G (δ = ID/IG) for undoped, B-dopedand N-doped carbon nanotubes is plotted as a function of excitationenergy.
the electron momentum k can be changed by changing Elaser,and therefore Raman spectroscopy (phonon measurements)can be used to probe the electronic structure of the nanotube.According to the values shown in table 1, the doped nanotubeshave a renormalization in electron and phonon energy whencompared to the undoped ones: the A parameter is reduced,indicating an increase in the electronic energy gap, while the B(which is the G′ frequency at the K point, where the dispersionrelation of the iTO phonon has a minimum) is increased,indicating an upwards shift in the dispersion relation of thephonons in the more disordered (doped) nanotubes.
Finally, in figure 5 a Raman spectrum of the ZnO blendedwith undoped carbon nanotubes is presented in order todemonstrate that a simultaneous signal both from the ZnO andfrom the nanotubes can be achieved; it is shown for a Ramanexcitation wavelength of 633 nm. When the samples includea small amount of carbon nanotubes (0.08 wt%), the signal isdominated by the G, D and D′ bands of the nanotubes which
Figure 5. Raman spectrum of a ZnO + undoped CNT sample on anFTO substrate for λ = 633 nm excitation wavelength. The spectrumshows the simultaneous measurement of peaks attributed to FTO, toZnO and to CNTs, all normalized to the G-CNT peak.
might be due to a stronger Raman scattering cross section ofCNTs compared to ZnO; nevertheless, it is possible to getRaman spectra which simultaneously contain features fromthe ZnO, the FTO and the carbon nanotubes. (The features ofpure FTO and ZnO/FTO samples are identified in the spectrashown in the supplementary information, sections C andF respectively available at stacks.iop.org/Nano/24/484013/mmedia.)
3.2. AFM measurements: topography and phase contrast
We took several images of topography and phase contrast,of different scan sizes, for all samples; two examples areshown in figures 6 and 7 for ZnO:N-doped and ZnO:undopednanotubes. The analysis of the images focused on theextraction of the following parameters.
Figure 4. Peak positions for the D and G′ bands as a function of excitation energy for all kinds of nanotubes. Linear fits are superimposedon the plots and the resulting parameters are shown in table 1.
Figure 7. FTO/ZnO:undoped CNT topography (top left and right, 1z = 35 nm) and contact potential (top centre, 1V = 100 mV) images.The height profiles at the lines indicated in the images are shown in the bottom graphs.
(i) Peak-to-peak maximum height (Z): this is an automaticprocedure which delivers a single number for each imageand it has been applied to the larger scans. It allows us todetect anomalous protruding peaks. It is also a test for therelation of Z and X–Y resolution since the largest peak-to-peakmaximum height value should not exceed the largest observedX–Y feature for each sample and should be typically ten timesthe rms value of surface roughness; this is observed for all thesamples.
(ii) Shape and size of the X–Y features: manualmeasurement on the X–Y plane of the image of the textureand grain’s aspect ratio for all the samples has been carriedout. For the FTO substrate and unmodified ZnO, the shapeof grains is rounded, but for the composite ZnO:CNTsamples the shape is elongated and slightly larger. No clearcorrelation of the shape with the CNT doping is observed,with the exception of the ZnO:N-doped CNT sample forwhich systematically a small protruding tip always appearsat one end of the elongated grain. This protruding tip createsa larger contrast in the phase images (see figure 6), indicatinga different interaction between the protrusion and the rest of
the sample. This different interaction could be provided bya segregation of one of the materials (improbable) or by theappearance of surface dipoles of the ZnO:CNT compositewhich change the tip–sample interaction for this particularlocation (these surface dipoles have been reported widely inthe literature for pure ZnO samples [6]).
Finally, (iii) roughness (both average and rms) isautomatically calculated for all scans, delivering a coupleof values for each sample. For the rms calculations there isalways a small dependence on the relative size of the scanand the X–Y features of the sample, and therefore the valuesshown in table 2 are always taken for scans of 10 µm×10 µmfor all samples to allow a better comparison between samples.All FTO/ZnO:CNT samples show a larger rms roughnesscompared to FTO, FTO/ZnO or FTO/CNT samples.
The AFM results are summarized in table 2. The analysisof the data shows that the grain size for ZnO is increased uponaddition of nanotubes from around 150 nm (round shape) tolarger grains of around 150 nm × 300 nm (elongated shape).This result is remarkable given the small amount (0.08 wt%)of nanotubes present in the blend. Also, the roughness of
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Nanotechnology 24 (2013) 484013 A Urbina et al
Figure 8. Surface contact potential histograms for (A) FTO/ZnO, (B) FTO/ZnO:undoped CNTs, (C) FTO/ZnO:B-doped CNTs and(D) FTO/ZnO:N-doped CNTs, and the corresponding Gaussian fits showing a bimodal distribution of contact potential values.
Table 2. Summary of AFM (topography and phase contrast measurements) of all samples. The values shown in the corresponding columnsare averages of several measurements at different locations on the same sample for different samples of each kind.
Sample
AFM
Max. peak-to-peak (nm) (±0.5 nm) X–Y features (nm) (±25 nm)
the sample surface is increased upon addition of nanotubes(almost a twofold increase), and more protruding areas aresticking out of the surface.
3.3. SKPM measurements
For the SKPM measurements, a histogram of all imageswas systematically obtained; the statistical approach includedseveral scans at different locations on the same sample (10),and also measurements of different samples of the samebatch (4). The histogram of counts of the measured contactpotential for each sample was then fitted using Gaussian peaks(see figure 8). In most of the samples, a single Gaussianpeak was enough to get a good fit to the experimentaldata; a ‘noisy’ background (very broad Gaussian) was also
included in some fits to account for intrinsic noise inour measurements. The experimental value, which measurescontact potential differences within each image, can then beconverted into an absolute work function value using theHOPG calibration performed for all tips (see details in thesupplementary information available at stacks.iop.org/Nano/24/484013/mmedia). For each sample, both the measuredsurface contact potential and the calculated work function arepresented in table 3. For some of the images, the appearanceof a bimodal distribution of values was evident by eye, but thehistogram showed it without doubt via the use of two Gaussianpeaks of similar width to fit the experimental histogram. Thisis the clearly the case for the two composite samples whichcombine ZnO and doped CNTs; for the ZnO:undoped CNTsample two Gaussians are also needed although both are
Figure 9. FTO/undoped CNT topographic (left, 1z = 100 nm) and surface contact potential (KPM, centre, 1V = 250 mV) images. Thehistograms of surface contact potential measurements and the corresponding Gaussian fits are shown for all the CNT samples:FTO/undoped CNTs, FTO/B-doped CNTs and FTO/N-doped CNTs.
Table 3. Summary of the SKPM results on all samples. The measured surface contact potential and the obtained work function aftercalibration are shown.
Sample SKPM (surface contact potential)
Measured contactpotential (mV) (±25 mV)
Work function (using HOPGcalibration) (eV) (±0.1 eV)
a Results for pure nanotubes on FTO; the statistical procedure for obtaining the workfunction is different from that for the rest of samples as explained in the main text.
centred at the same Vcp value (without the second Gaussianit is not possible to fit the height of the ‘apparently’ singlepeak, see figure 8(B)). For the samples of only CNTs onFTO, the appearance of the SKPM is different. It seems thataggregations of carbonaceous material provide a non-uniformvalue of the contact potential. The CNTs are not seen inthe topographic images, although if a histogram of only the‘aggregation’ areas is taken in the contact potential images(the number of counts is much lower compared with theprevious histograms), the Gaussian fit provides values thatallow us to distinguish the different dopings of the CNTs (seefigure 9, and values in table 3 marked with an ‘a’). The workfunction has been obtained for these pure nanotube on FTOsamples following the same procedure as explained above;the values obtained are (i) 4.6 ± 0.1 eV for FTO:undopedCNTs; (ii) 5.1 ± 0.1 eV for FTO:B-doped CNTs; and(iii) 4.3 ± 0.1 eV for FTO:N-doped CNTs. All these values,obtained from the SKPM measurements, are very similar tothe values obtained by ultraviolet photoelectron spectroscopy(UPS) [36] and electron energy loss spectroscopy (EELS) [35]measurements for the same material.
The most important result is the demonstration that thework function of the pure nanotubes can be transferred tothe ZnO:CNT blend even if the amount of nanotubes issmall (0.08 wt%), delivering values for the work functionof 4.6 ± 0.1, 4.9 ± 0.1 and 4.4 ± 0.1 eV respectively. TheCNT is acting as a doping agent for the ZnO without the
need for structural modification of the wurtzite structure ofZnO; furthermore, the ZnO:CNT samples are very uniformand variations of the measured parameter between differentsamples are within the error margin of the measurements(±25 mV). This is not the case for the pure CNT samples(doped or undoped), where although the samples are globallysimilar to each other, there is a higher variation of the value ofthe measured contact potential within the same sample, as canbe seen in figure 9, where the data dispersion is larger, bothfor the value of the contact potential (around ±40 mV, almosttwice the experimental error for the measurement, whichis ±25 mV) and for the number of counts for each value.Interestingly, the dispersion between different locations onthe sample disappears for the ZnO:CNT samples, indicatingthat the small amount of CNTs is uniformly dispersed andmodifies the value of the contact potential of the whole surfacewithout variations from one spot to the next (at least for the 10spots that were measured within each sample).
The experimental results demonstrate that control ofthe work function of ZnO:CNT blends has been achieved.This can be exploited in the design and fabrication of bothorganic solar cells (OSCs) and light emitting diodes (OLEDs),where an optimum match between the electrodes and theHOMO or LUMO levels of active materials is required fora good performance of the device. This is usually achievedby the inclusion of different interfacial charge transportlayers that can be considered as electron (or hole) blocking
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Nanotechnology 24 (2013) 484013 A Urbina et al
layers. In particular, for the case of OSCs, the fabricationof inverted structures (ITO acting as cathode) which havesimilar efficiencies but are more stable than the equivalentstandard structure (ITO acting as anode) requires the inclusionof a ZnO layer, which works well for the P3HT:PCBMactive layer, but which will require a slightly different valuefor the work function if a polymer with lower band gap(and different HOMO level) is used in the solar cell: thisnew required value can be achieved upon addition of thecorresponding doped CNTs to the ZnO-based interlayer.Similarly, for OLEDs, polyelectrolyte polymers have beenused as hole injecting layers with good results; ZnO:CNTinterlayers can similarly be used to modify the work functionbut without the additional complication of built-in chargesthat appear in the case of polyelectrolytes. Nevertheless, someprocessing issues need to be addressed in order to obtain asuccessful device with the new ZnO:CNT interlayer: first,the high temperature of the substrates during the pyrolysisprocess involved in the sample preparation should be reducedif flexible plastic-based substrates in an R2R processing lineare going to be used; secondly, the carbon nanotubes mayoccasionally protrude from the surface, as shown in table 2for the maximum peak-to-peak height, which considerablyexceeds the rms value of surface roughness; these protrusionsmay create shorts and therefore reduce the performance ofthe devices (lower the shunt resistance of even completelyshunted devices).
4. Conclusions
Doped and undoped carbon nanotubes (CNTs) havebeen characterized by Raman spectroscopy using differentwavelength excitations. The Raman spectra demonstratethat the CNTs keep their covalent structure after beingsubstitutionally doped by 3–4% of nitrogen or boron atoms,delivering respectively n-type and p-type semiconductingmaterial. Raman spectroscopy allowed us to characterize thenanotubes using radial breathing modes and to quantify theamount of disorder induced by the doping process, around10% when compared with undoped nanotubes. Additionally,the G′ band dependence on the excitation energy demonstratesa renormalization of electron and phonon energies in themost disordered nanotubes. FTO substrates were spraycoated with the nanotubes and characterized by AFM andSKPM techniques, showing a modification of the workfunction depending on the type of doping of the nanotubes:4.6 ± 0.1 eV for undoped, 5.1 ± 0.1 eV for B-doped and4.3± 0.1 eV for N-doped nanotubes. When the nanotubes areblended with ZnO (0.08 wt%) and an FTO substrate is spraycoated with the blend, the work function of the surface can beengineered according to the kind of nanotube that was blendedwith the ZnO, delivering the following values of the workfunction: 4.6 ± 0.1, 4.9 ± 0.1 and 4.4 ± 0.1 eV. Althoughthe surface roughness is still high (10–17 nm rms) and it isan issue that has to be addressed in the future, the controlof the work function of the electrode has a large potential toimprove the performance of organic electronic devices whichuse these ZnO:CNT blends as transparent electrodes, as has
already been shown for organic light emitting diodes whichhave benefited from ZnO:N-doped CNT electrodes.
Acknowledgments
This work was supported by the ESPRC SUPERGENExcitonic Solar Cell Consortium (EP/G031088/1) andWorld Class University (WCU) Program in Korea (GrantNo. R32-10051). SOK and JML were supported by theInstitute for Basic Science (IBS) in Korea. AU wants toacknowledge financial support from the Spanish MICINN(Grants Consolider-HOPE CSD2007-00007 and MAT2010-21267-C02 including FEDER funds).
Appendix. Work function measurements byscanning Kelvin probe microscopy
Reliable measurements of the work function with submicronresolution in air can be achieved by means of scanning Kelvinprobe microscopy (SKPM). Two general modes for AFMmeasurement can be used. The first is a mechanically drivenmode in which a voltage Vpiezo is applied to the small piezoof the cantilever while the tip is kept at ground or fixed bias(Vtip = 0 or Vtip = Vdc constant),
Vpiezo = Vdc + Vac sin(ωpt)
z = z0 + A(ωp) sin(ωpt + ϕc
)where z is the tip–sample distance. Then, the detection ofresonant frequency shift is performed while feedback adjustsωp (the ‘driving’ frequency) in order to keep maximal A(ωp)
(frequency dependent oscillation amplitude). An alternative isdetection of amplitude change or phase shift, both at constantωp, which is the mode that we used in the AFM used for ourtopography and phase measurements.
The second mode can be called the voltage modulationmode, in which the driving voltage of the piezo is set to zero(Vpiezo = 0) and a conductive tip is biased by a dc+ ac voltagegiven by
Vtip = Vdc + Vac sin (ωt)
z = z0 + A0 + A1 sin (ωt + ϕ1)+ A2 sin (2ωt + ϕ2)
where z0 is the tip–sample separation when Vtip = 0 and A0is the static response, and two sets of amplitude and phaseresponse (first and second harmonic) can be separated bylock-in techniques and used for Kelvin probe microscopytechniques.
When there exists electrostatic interaction, the force isdescribed by
Fel =12(1V)2
∂C(z)
∂z+
QsQt
4πε0z2
where 1V is the difference in potential between the sampleand the tip, Vtip − Vsurf, C(z) is the tip–sample capacitanceand Qs, Qt are the electrostatic charges of the sample and thetip. If there is no surface charge or built-in surface dipoles,the second term of the right hand side is much smaller thanthe first one and it is a good approximation in Kelvin probe
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Nanotechnology 24 (2013) 484013 A Urbina et al
microscopy (KPM) to consider only the capacitance variationwith tip–sample distance. In this case, substituting
1V = Vtip − Vsurf
and considering Vtip as a superposition of dc and ac voltagesas described in the equation above, we obtain
F(z) =12∂C(z)
∂z
[(Vdc − Vsurf)
2+
12
V2ac [1− cos (2ωt)]
+ 2 (Vdc − Vsurf)Vac sin (ωt)
]which can be separated into three terms
F = Fdc + F1ω + F2ω
which are, respectively,
Fdc(z) =12∂C(z)
∂z
[(Vdc − Vsurf)
2+
12
V2ac
]F1ω(z) =
∂C(z)
∂z(Vdc − Vsurf)Vac sin (ωt)
F2ω(z) = −14∂C(z)
∂zV2
ac cos (2ωt) .
Lock-in techniques enable us to extract F1ω, then a feedbackloop to Vdc(tip) is employed to keep it equal to zero, so F1ω =
0 and then Vdc = Vsurf, therefore mapping the surface potential(Kelvin probe microscopy). The second harmonic signalprovides information about the z-derivative of the capacitance.The information achieved by this technique can deliverdirectly the differences in surface potential between differentparts of the same sample (ideally within the same image);this information can be obtained by statistical treatment ofthe pixelated image. In order to provide absolute values ofthe work function of the measured surface, it is necessaryto calibrate the tip using a well known sample, usually afresh surface of highly oriented pyrolytic graphite (HOPG),and then convert the relative surface potential measurementsto absolute work function values. Ideally the tips should becalibrated before and after each experiment.
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