Effects of Oxidation State and Crystallinity of Tungsten Oxide Interlayer

Article pubs.acs.org/JPCC

Effects of Oxidation State and Crystallinity of Tungsten Oxide Interlayer on Photovoltaic Property in Bulk Hetero-Junction Solar Cell Ji-Seon Lee, In-Hyuk Jang, and Nam-Gyu Park* School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea ABSTRACT: Tungsten oxide thin films with thickness of ∼30 nm are prepared from ammonium tungstate solution to be used as a hole-selective interlayer in the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM)-based bulk heterojunction solar cell. The prepared tungsten oxide films are confirmed to be n-type semiconductor, as observed by Hall measurement. Photovoltaic performance is investigated in terms of tungsten oxide film annealing condition. The annealing temperatures at 150 and 300 °C produce amorphous phase, whereas crystalline phase is formed at 400 °C. At annealing temperature of 150 °C, the annealing in vacuum shows the conversion efficiency of 0.71%, whereas the annealing in air exhibits two times higher efficiency of 1.42%. X-ray photoelectron spectroscopic (XPS) analysis confirms that only W6+ is presented under air annealing condition, whereas W5+ appears under vacuum-annealing condition. Shunt resistance is decreased, and series resistance is increased upon annealing in vacuum. Under air annealing condition, efficiency is further improved from 1.42 to 2.01% as temperature changes from 150 to 300 °C due to a removal of the chemisorbed water and in part a complete conversion of ammonium tungstate to tungsten trioxide. Crystalline phase is formed at 400 °C, where photovoltaic performance is declined to 1.27% due to the lowered shunt resistance. Hole injection efficiency is evaluated based on the ultraviolet photoelectron spectroscopy (UPS) studies. Except for the 150 °Cvacuum-annealed WO3−x, work functions (ϕ) for the air-annealed tungsten oxide films lie in between the work function of ITO (4.66 eV) and the HOMO level (5.1 eV) of P3HT, which leads to better hole transport through the air-annealed WO3 interlayer than the vacuum-annealed one. Among the air-annealed samples, work function and conduction band minimum for the 300 °Cannealed one are most properly positioned for hole transportation, associated with highest photovoltaic performance. coating solution could corrode ITO.5,6 Recently, metal oxides have been introduced on ITO to be used as hole-selective layers because metal oxides can be deposited without degradation of ITO. As hole-selective layers, nickel oxide,7−12 tungsten oxide,13−19 vanadium oxide,20−23 and molybdenum oxide24−28 have been studied. It is interesting to see that n-type MoOx and p-type NiO are able to be used as hole-selective interlayer. According to the previous report,29 there is a distinction to be made between electron blocking and hole collection in the BHJ architecture, where electron is anticipated to be effectively blocked through the p-type interlayer; on the other hand, hole can be effectively collected through n-type buffer layer. Among the studied hole-selective interlayer materials, tungsten oxide was found to be effective in hole collection. When introducing the WO3−x layer at the positive electrode in the cell with the ZnO layer at the negative electrode, photovoltaic performance was improved compared with the device with only ZnO layer.14 Amorphous WO3−x layer deposited by thermal evaporation method was found to improve photovoltaic performance.15 In the inverted structure with TiO2 thin layer on ITO (negative electrode), the presence of WO3−x layer at metal electrode

1. INTRODUCTION Since the first discovery of organic solar cell in 1986 based on layer-by-layer junction of p-type (or donor) copper phthalocyanine and n-type (or acceptor) perylene tetracarboxylic derivative,1 a quantum jump of power conversion efficiency in organic solar cell was made by Yu et al. in 1995 using bulk heterojunction (BHJ) structure.2 Unlike the layer-by-layer pn junction structure, BHJ structure is composed of one active layer with randomly mixed p- and n-type organic materials. Short exciton diffusion length has been critical issue in layer-bylayer pn junction, which was overcome by BHJ. Although BHJ structure is beneficial to exciton diffusion length, leading to better charge separation, problem in charge collection at electrode is emerged because donor and acceptor are not distinctly separated at electrode interface.3 It has therefore been required to block effectively electron transporting at positive electrode and hole transporting at negative electrode. Indium tin oxide (ITO) has been used as a positive electrode and aluminum as a negative electrode. A polymer complex of poly(3,4-ethylendioxythiophene) and poly(styrene sulfonate) (PEDOT:PSS) was explored to prevent electron collection at the positive electrode by deposition of PEDOT:PSS on an ITO substrate.4 It has been argued, however, that the PEDOT:PSS layer coating procedure would decline the photovoltaic performance because the strong acidic nature of PEDOT:PSS © 2012 American Chemical Society

Received: December 19, 2011 Revised: May 29, 2012 Published: June 5, 2012 13480

dx.doi.org/10.1021/jp2122505 | J. Phys. Chem. C 2012, 116, 13480−13487

The Journal of Physical Chemistry C

Article

with a VG microtech ESCA 2000 using a focused monochromatized Al Kα radiation (1486.6 eV). The binding energy was calibrated with the C 1s line (284.6 eV). The XPS data were analyzed by curve fit based on linear background and combination of Gaussian (80%) and Lorentzian (20%) distributions.31,32 X-ray diffraction (XRD) analysis was performed on a Bruker D8 Discover diffractometer using Cu Kα radiation at scan rate of 4°/min under operation condition of 40 kV and 40 mA. Raman spectra were obtained with a Renishaw RM 1000 using a 514 nm Argon ion laser. Thermogravimetric (TG) analysis was performed using a Seico Inst. TG/DTA 7300 at the heating rate of 10 °C/min. Hall voltage was measured under bias current of 1 mA and applied field from 1 to 5 kG using LaKeShore 7500 series Hall Measurement System at room temperature. The properties (resistivity, doping type, sheet carrier density, and mobility of majority carrier) of tungsten oxide were calculated by van der Pauw method. Ultraviolet photoelectron spectroscopy (UPS) spectra were recorded with an AXIS-NOVA using a He I (21.2 eV) gas discharge lamp. Tungsten oxide films on ITO were used as samples for UPS measurement.

(positive electrode) improved efficiency significantly from 0.13 to 2.53%.16 It was demonstrated that WO3 interlayer worked well even in tandem structure.19 Although a significant improvement of power conversion efficiency was achieved by introduction of WO3−x interlayer, the basis for the improvement has not been addressed. In this Article, we report on dependence of photovoltaic performance on oxidation state and crystallinity of tungsten oxide thin films deposited on ITO, where tungsten oxide films prepared using ammonium tungstate solution are annealed under different condition and temperature. Physico-chemical and opto-electronic properties are characterized using XRD, XPS, and IPCE. Hole collection property is investigated in terms of energetics using UPS.

2. EXPERIMENTAL SECTION To form tungsten oxide layer on ITO glass substrate, ITO was cleaned with 2-propanol and acetone. A 0.3 M ammonium tungstate (NH4)2WO4 solution was prepared by dissolving 0.3 g of WO3 powder (Sigma Aldrich) in 5 mL of ca. 5.8 wt % ammonium hydroxide solution (1.05 mL of 30% ammonium hydroxide solution was diluted by the addition of 3.95 mL of deionized water) according to the method reported elsewhere.30 To improve coating quality, we mixed aqueous ammonium tungstate coating solution with anhydrous 2propanol (1:1 v/v). Prior to coating the solution, ITO was treated with UV/ozone for 20 min. The ammonium tungstate solution was then coated on the ITO substrate by using spincoating method at spinning rate of 2500 rpm, which was followed by drying for 1 h at 120 °C in vacuum oven. The dried film was treated with 1 N HCl for 30 s to convert ammonium tungstate to tungsten oxide, rinsed with ethanol, and dried under nitrogen flow, which was annealed in air or vacuum for 30 min at temperature ranging from 150 to 400 °C. The annealed film in air was colorless, whereas the one in vacuum was dark blue. For a single active layer, poly(3-hexylthiophene) (P3HT) (Flexink, U.K., 99%) and fullerene derivative6,6-phenyl-C61butyric acid methyl ester (PCBM) (Nano-C, USA, 99.5%) were blended with weight ratio of 1:0.8 in chlorobenzene (Sigma Aldrich, 97.5%). The concentration of mixture was 21.6 mg/ mL. The P3HT:PCBM admixed solution was coated on the tungsten-oxide-coated ITO substrate by spin-coating method at 1000 rpm for 15 s. Thickness of the P3HT:PCBM layer was adjusted to be 90 nm. 100 nm thick Al electrode layer was deposited on the P3HT:PCBM layer using thermal evaporator under 3 × 10−6 Torr. Photocurrent and voltage were measured from a solar simulator equipped with 450 W xenon lamp (Newport 6279NS) and a Keithley 2400 source meter in an ambient atmosphere. Light intensity was adjusted with the NRELcalibrated Si solar cell having KG-2 filter for approximating one sun light intensity (100 mW/cm2). Incident photon-to-current conversion efficiency (IPCE) was measured using an IPCE system (PV Measurement). A 75 W xenon lamp was used as a light source for generating monochromatic beam. Calibration was accomplished using a silicon photodiode, which was calibrated using the NIST-calibrated photodiode G425 as a standard. IPCE data were collected at DC mode. Surface morphology of tungsten oxide films was obtained by a fieldemission scanning electron microscope (FE-SEM, Jeol JSM 7500F) under accelerating voltage of 15 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed

3. RESULTS AND DISCUSSION Figure 1 shows scanning electron micrographs of tungstenoxide-coated ITO substrates as a function of annealing

Figure 1. Plan view of SEM for (a) ITO, (b) WO3−x layer annealed at 150 °C for 30 min in vacuum, (c) WO3−x layer annealed at 150 °C for 30 min in air, (d) WO3−x layer annealed at 300 °C for 30 min in air, and (e) WO3−x layer annealed at 400 °C for 30 min in air, along with (f) α-step profile of WO3−x layer (150 °C for 30 min in air).

condition. Surface profiler measurement confirms that thicknesses of the annealed WO3−x layers are ∼28 nm, as can be seen in Figure 1f. At 150 °C annealing temperature, small islands of WO3−x with cracks are formed under vacuum condition, whereas cracks are reduced under air atmosphere. Small grains become bigger when increasing the annealing temperature from 150 to 300 °C and 400 °C, which is 13481



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indicative of possibly forming crystals at elevated temperature. Color of tungsten oxide film is changed by annealing condition (air or vacuum). Heat treatment in air leads to colorless film, whereas bluish film is formed after heat treatment under vacuum. The blue-colored film is likely to be due to oxygen deficiency32 or reduction of W6+.33 Figure 2 shows photocurrent density−voltage curves and IPCE spectra for the OPV devices with tungsten oxide

Figure 3. Dark current density versus bias voltage for the BHJ solar cells with the 150 °C-air-annealed and the 150 °C-vacuum-annealed WO 3−x interlayers. Empty circles represent the measured data, and the solid lines represent fit data.

resistances are obtained from the inverse slope at vicinity of zero bias voltage and at vicinity of VOC, respectively.34 The data listed in Table 2 show that shunt resistance of the 150 °C-air-

Figure 2. (a) Short-circuit photocurrent density (JSC) versus opencircuit voltage (VOC) curves measured at AM 1.5G 1 sun light intensity (100 mW/cm2) and (b) IPCE spectra for the bulk heterojunction solar cells with the WO3−x interlayer annealed at 150 °C in air and under vacuum. (c) Transmittance (T) as a function of wavelength for the 150 °C-air-annealed and 150 °C-vacuum-annealed WO3−x interlayers.

Table 2. Dark Current Fitting Results for the BHJ Solar Cells with the 150 °C-air- and 150 °C-vacuum-annealed WO3−x Interlayersa

interlayer formed at 150 °C in air or under vacuum. The photovoltaic parameters are listed in Table 1. The tungsten Table 1. Short-Circuit Photocurrent Density (JSC), OpenCircuit Voltage (VOC), Fill Factor (FF), and Efficiency (η) for the Bulk Heterojunction Solar Cells with the WO3−x Interlayer annealed at 150 °C in Air and under Vacuuma

a

annealing condition

JSC (mA/cm2)

VOC (V)

FF

η (%)

150 °C/air 150 °C/vac.

7.72 6.75

0.445 0.288

0.413 0.365

1.42 0.71

annealing condition

shunt resistance (kΩ·cm2)

series resistance (Ω·cm2)

ideality factor

saturation current (mA/cm2)

150 °C/air 150 °C/ vac.

257.1 8.44

18.24 64.73

1.68 2.87

1.85 × 10−5 1.37 × 10−3

a

Active area of the P3HT:PCBM layer was 0.2 cm2.

annealed WO3−x sample (257.1 kΩcm2) is much higher than that of the 150 °C-vacuum-annealed one (8.44 kΩcm2). This suggests that the 150 °C-air-annealed WO3−x layer prevents shunting current more effectively than the 150 °C-vacuumannealed one, which is indicative of better hole collection property. In addition, series resistance of 18.24 Ωcm2 for the 150 °C-air-annealed WO3−x layer is lower than that of 64.73 Ωcm2 for the 150 °C-vacuum-annealed WO3−x layer, which is related to higher FF for the 150 °C-air-annealed WO3−x layer. It is also noted that VOC is 55% higher for the air-annealed WO3−x than the vacuum-annealed one. Because VOC is reported to be dependent on ideality factor (n) and saturation current density (J0),35 n and J0 are also estimated by fit the data using eq 135,36


oxide interlayer treated under vacuum shows short-circuit photocurrent density (JSC) of 6.75 mA/cm2, open-circuit voltage (VOC) of 0.288 V, fill factor (FF) of 0.365, and conversion efficiency (η) of 0.71% at AM 1.5G 1 sun illumination (100 mW/cm2). The film heat-treated in air exhibits much better photovoltaic performance: JSC of 7.72 mA/cm2, VOC of 0.445 V, FF of 0.413, and η of 1.42%. Compared with the vacuum-annealed tungsten oxide, the absolute IPCE values for the air-annealed one are improved in the measured wavelength range, which is well-consistent with the increased JSC. It is noted that both JSC and VOC are substantially improved when the WO3−x interlayer is treated in air, resulting in two times higher efficiency than the case treated under vacuum. The higher IPCE may be associated with optical gain or hole collection efficiency. Optical spectra for the films under different annealing condition are obtained in Figure 2c to investigate the relation between optical transmittance and the improved IPCE. Little difference in transmittance is found, which indicates that the improved IPCE is probably related to better hole collection through the air-annealed tungsten oxide interlayer. Figure 3 compares dark current of the 150 °C-air-annealed WO3−x interlayer with the 150 °C-vacuum-annealed one. From the data fitting, shunt resistance, series resistance, saturation current, and ideality factor are estimated. Shunt and series

ln(JD) = ln(Jo ) +

⎛1⎞ q ⎜ ⎟ V ⎝ n ⎠ kBT b

(1)

where J, Vb, q, kB, and T represent current density, bias voltage, electron charge, Boltzmann constant, and temperature, respectively. As can be seen in Figure 3, n and J0 are determined from linear fit in the range between ∼0.3 and ∼0.5 V and listed in Table 2. It was reported that VOC is expected to increase with decreasing J0.37 Therefore, higher voltage for the 150 °C-air-annealed WO3−x interlayer is associated with two orders of magnitude lower J0. Abnormally high n of 2.87 for the 150 °C-vacuum-annealed WO3−x interlayer suggests a deviation from ideal diode and a deteriorated electrical property.37,38 Moreover, compared with the vacuum-annealed tungsten oxide, higher shunt resistance for the air-annealed tungsten oxide implies a better hole transport behavior. To understand the better performance observed from the WO3−x layer annealed in air, we analyzed the oxidation state of 13482



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reduced carrier concentration. The reduced shunt resistance as observed in Figure 3 is related to low carrier concentration. To investigate the effect of crystallinity of tungsten oxide layer on photovoltaic performance, we changed annealing temperature from 150 to 300 °C and 400 °C in air atmosphere. Figure 5 shows XRD patterns and Raman spectra of the WO3

tungsten using W 4f XPS spectra, together with oxygen 1s XPS spectra, as can be seen in Figure 4.

Figure 5. (a) XRD patterns and (b) Raman spectra of WO3 films annealed at 150, 300, and 400 °C in air.

films as a function of temperature. No XRD and Raman signals are detected for the 150 °C- and 300 °C-annealed WO3 films, indicating amorphous characteristics. However, the 400 °Cannealed WO3 film exhibits XRD and Raman peaks, which suggests that crystalline WO3 is formed at 400 °C. XRD peaks are indexed to be monoclinic phase,42 and the peaks at 272 and 322 cm−1 in Raman spectra are corresponding to W−O−W bending mode, and the peaks at 719 and 808 cm−1 can be assigned to W−O−W stretching vibration.43 Differential thermal analysis (DTA) study in Figure 6 shows an exothermic peak at temperature of 337.6 °C. This

Figure 4. W 4f and O 1s XPS spectra for the (a,c) 150 °C-air- and (b,d) 150 °C-vacuum-annealed WO3−x interlayers. Details of peak deconvolution are described in the text.

As previously mentioned, the WO3−x layer has different colors depending on annealing condition, where no color is observed under air condition and blue color is observed under vacuum condition. The 150 °C-air-annealed WO 3−x layer in Figure 4a shows two distinct 4f 7/2 and 4f 5/2 peaks, corresponding to W6+, whereas the 150 °C-vacuum-annealed WO3−x layer shows rather complicated XPS structure, as can be seen in Figure 4b. We have tried to deconvolute the peaks based on the full width at half-maximum (fwhm) of 1.41 eV, the binding energy difference between W 4f7/2 and W 4f5/2 (Δ = 2.12 eV), the peak intensity ratio of W 4f7/2 to W 4f5/2 ( I(f7/2):I(f5/2) = 4:3), and the binding energies of W5+ 4f7/2 peak (34.5 eV) and W6+ 4f7/2 peak (35.7 eV). 39 Additional doublet at 36.9 and 39.0 eV is used in deconvolution, which is related to inhomogeneity as mentioned in the literature. 39 Vacuum annealing yields substantial amount of W5+ ions. The blue color is thus associated with W5+ state. 32 The presence of W5+ state in the vacuum-annealed sample is indicative of oxygen deficiency. We compare intensity ratio of O 1s to W 4f between air- and vacuum-annealed tungsten oxides. For the case of the 150 °C-air-annealed tungsten oxide in Figure 4c, the intensity ratio of O 1s to W 4f is estimated to be 0.96 from the integrated intensities of O 1s and W 4f peaks. Meanwhile, the ratio of O 1s/W 4f is found to be 0.91 for the case of the 150 °C-vacuum-annealed tungsten oxide in Figure 4d. The ratio of oxygen to tungsten is relatively smaller under vacuum annealing condition than under air annealing condition, which indicates that the vacuum-annealed tungsten oxide film has oxygen defect. Electron transport of tungsten oxide was reported to be enhanced in the presence of W5+ state and oxygen vacancy. 40 It was also reported that the electrical resistivity value of an amorphous WO3 was 20 × 10 6 Ωcm, whereas that of a colored amorphous tungsten oxide (W6+ was partially reduced to W5+) was 400 Ωcm. 41 Therefore, it can be postulated that the presence of W5+ and oxygen defect in the vacuum-annealed WO3−x lowers hole collection ability, associated with the

Figure 6. TG and DTA curves of WO3 film prepared by acid treatment of (NH4)2WO4. Inset shows the first derivative of DTA data.

temperature is likely to be related to crystallization temperature because the X-ray amorphous phase at 300 °C is changed to crystalline phase at 400 °C. In TG curve, gradual weight loss is observed from 30 to 337.6 °C. The 6.6% weight loss from 25 to 100 °C is due to removal of water,44 and the 3.7% loss from 100 to 300 °C is attributed to removal of chemisorbed water and ammonia.45 Very small amount of ∼0.7% weight loss is observed from 300 to 337.6 °C, which is ascribed to release of water and ammonia produced by conversion of a trace of (NH4)2WO4 to WO3.46 Figure 7 compares JSC versus VOC curves for the solar cells with the WO3 interlayer annealed at 150, 300, and 400 °C in air. The photovoltaic parameters are listed in Table 3. As the temperature increases from 150 to 300 °C, JSC, VOC, and FF increase from 7.72 to 8.28 mA/cm2, 0.445 to 0.489 V, and 0.413 to 0.497, respectively. As a result, overall conversion 13483



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Table 4. Dark Current Fitting Results for the Bulk HeteroJunction Solar Cells with the WO3 Interlayers Annealed at 150, 300, and 400 °C in Aira

a

VOC (V)

FF

η (%)

150 °C/air 300 °C/air 400 °C/air

7.72 8.28 7.76

0.445 0.489 0.380

0.413 0.497 0.431

1.42 2.01 1.27

series resistance (Ω·cm2)

ideality factor

saturation current (mA/cm2)

150 °C/air 300 °C/air 400 °C/air

257.1 568.2 27.3

18.24 18.65 49.51

1.68 1.49 2.26

1.85 × 10−5 6.04 × 10−5 6.98 × 10−4


for the 150 and 300 °C-annealed samples, ideality factor of 2.86 is estimated for the 400 °C-annealed sample. Therefore, crystallization of tungsten oxide at 400 °C produces relatively large fraction of shunting current along with nonideal diode behavior. It was reported that the dark conductivity of a crystalline tungsten oxide was one order of magnitude higher than that of an amorphous one.47 Therefore, in a similar manner as observed for the 150 °C-vacuum-annealed tungsten oxide, the diminished photovoltaic performance for the 400 °Cannealed WO3 interlayer is attributed to a decrease in the hole collection efficiency that is well-correlated with the reduced shunt resistance. This could be a result of either improved bulk properties of the oxide layer due to changes in the interfacial recombination from changes in local interfacial structure or most likely a combination of these effects.48 Because the presence of W5+ is found to diminish hole collection ability according to our previous investigation, we measure XPS to investigate change of tungsten oxidation state with annealing temperature in air. XPS analysis confirms that only the peaks corresponding to W6+ are detected regardless of annealing temperature in air (Figure 9).

Table 3. JSC, VOC, FF, and η for the Bulk-Heterojunction Solar Cells with the WO3 Interlayers Annealed at 150, 300, and 400 °C in Aira JSC (mA/cm2)

shunt resistance (kΩ·cm2)

a

Figure 7. JSC versus VOC curves measured at AM 1.5G 1 sun light intensity (100 mW/cm2) for the bulk-heterojunction solar cells with the WO3 interlayers annealed at 150, 300, and 400 °C in air.

annealing condition

annealing condition


efficiency increases from 1.42 to 2.01%, corresponding to ∼42% increment. However, when the annealing temperature increases further to 400 °C, JSC, VOC, and FF are slightly decreased, and as a result of that, the conversion efficiency is deteriorated to 1.27%. The increased efficiency by increasing temperature from 150 to 300 °C is probably related to the removal of both the chemisorbed water and the trace of (NH4)2WO4, as confirmed by TG analysis. It is found that the device does not work when using (NH4)2WO4 layer (no acid treatment). The decreased performance by the further increase in temperature from 300 to 400 °C is expected to be associated with the amorphous− crystalline phase change. According to the dark current−voltage analysis from Figure 8, shunt resistance increases from 257.1 to 568.2 kΩcm2 as the

Figure 9. W 4f and O 1s XPS data for the WO3 interlayers annealed at 150, 300, and 400 °C in air.

The chemistry and structure of the WO3−x interlayer are examined and found to be related to photovoltaic performance. However, those may not be enough to make clear correlation with the device performance. Therefore, we conduct UPS measurement to be used to clarify the connection. First, the prepared WO3−x interlayers on ITO are confirmed to be n-type semiconductor according to Hall measurement, where all samples exhibit negative Hall coefficients ranging between 6 × 10−4 and 12 × 10−4 cm3/C. Because n-type semiconductors are inserted in between ITO and donor−acceptor mixture, work function and conduction band minimum (CBM) are anticipated to be crucial role in device performance. In Figure 10, UPS of the 150 °C-air-annealed WO3 is compared with that of the 150 °C-vacuum-annealed WO3−x. Work function (ϕ), associated with Fermi level (EF), is determined by ϕ = photon

Figure 8. Dark current density versus bias voltage for the BHJ solar cells with the WO3 interlayers annealed at 150, 300, and 400 °C in air.

annealing temperature increases from 150 to 300 °C, whereas it is drastically declined to 27.33 kΩcm2 upon heat treatment at 400 °C, as listed in Table 4. The increased shunt resistance at 300 °C suggests the improvement of hole collection characteristics, whereas the significant drop in shunt resistance after heat treatment at 400 °C is indicative of deterioration in hole collection property. In addition, the reverse saturation current increases an order of magnitude from 6.04 × 10−5 to 6.98 × 10−4 mA/cm2 as annealing temperature increases from 300 to 400 °C, which is associated with the decreased voltage. Furthermore, compared with the ideality factor of 1.68 and 1.49 13484



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states are empty. This result is also well-consistent with the XPS data. The CBM, associated with electron affinity, is determined based on the optical band gap. Figure 12 shows Tauc plot50

Figure 10. UPS spectra of the 150 °C-air-annealed WO3 and the 150 °C-vacuum-annealed WO3−x, showing (a) the secondary edge at high binding energy region, related to work function, (b) low-energy region, related to valence band maximum, and (c) top of valence band, related to gap states.

energy (21.2 eV) − the binding energy of the secondary edge (high binding energy cutoff) in the UPS spectra (Figure 10a). The 150 °C-air-annealed WO3 shows ϕ = 4.73 eV, whereas ϕ of the 150 °C-vacuum-annealed WO3−x is found to be 4.59 eV. From the low binding energy region of the UPS spectra in Figure 10b, the energies that appear at 2.82 and 3.06 eV are valence band maximum (VBM), associated with ionization energy, with respect to EF for the 150 °C-air-annealed and the 150 °C-vacuum-annealed tungsten oxides, respectively. Therefore the VBMs are estimated to be 7.55 and 7.65 eV with respect to vacuum level for the 150 °C-air-annealed and the 150 °C-vacuum-annealed tungsten oxide, respectively. In Figure 10c, as compared with the 150 °C-air-annealed tungsten oxide, small peaks located at ∼0.6 eV appear for the 150 °C-vacuumannealed tungsten oxide, which indicates that the W 5d states, located just below the conduction band edge inside the gap, are occupied,49 which is presumably due to reduction of W6+ or oxygen defect. This result is well-consistent with the presence of W5+ observed in XPS. Similarly, work functions of the WO3 films annealed in air at 300 and 400 °C are determined to be 4.87 and 4.97 eV, respectively, based on the UPS spectra in Figure 11a. From the

Figure 12. Tauc plot ((αhν)1/2 versus hν plot) of the tungsten oxide films annealed in (a) vacuum at 150 °C, (b) air at 150 °C, (c) air at 300 °C, and (d) air at 400 °C. Open circles and solid lines represent the observed data and the fit data, respectively.

obtained from UV−vis transmittance spectra, where the indirect band gaps of the WO3−x films are evaluated by extrapolating the linear region of the curves (αhν)1/2. The band gaps are determined to be 3.49, 3.47, 3.44, and 3.43 eV for the 150 °C-vacuum-annealed, the 150 °C-air-annealed, the 300 °Cair-annealed, and the 400 °C-air-annealed tungsten oxide films, respectively. The observed band gaps are consistent with the values reported elsewhere.51,52 A slight increase in the band gap with increasing temperature is probably due to phase transition from amorphous to crystalline. The CBMs are thus estimated to be 4.16, 4.08, 4.23, and 4.07 eV for the 150 °C-vacuumannealed, the 150 °C-air-annealed, the 300 °C-air-annealed, and the 400 °C-air-annealed tungsten oxide films, respectively. It has been proposed that optimized hole-selective interlayers in contact with organic layers should have valence band energies, EVB, and work functions, ϕ, closely matched to the EVB or HOMO of the donor such as P3HT, whereas electronselective interlayers should have conduction band energies, ECB, and ϕ close to the ECB or LUMO of the acceptor such as PCBM, so that electronic equilibrium can be easily established and charge injection barriers minimized.29 We establish energy diagram based on UPS data for the various WO3−x interlayers, which is depicted in Figure 13. It was reported from the holeonly device ITO/MoO3/hole-transport-material/Ag that hole injection mechanism through MoO3 was related to CBM and EF of MoO3, EF of ITO, and HOMO of hole-transportmaterial.53 Except for the 150 °C-vacuum-annealed WO3−x, EF lies in between the work function of ITO and the HOMO level of P3HT for the air-annealed WO3 interlayers. Therefore, hole collection is likely to be better for the air-annealed WO3 interlayers than for the vacuum-annealed one. It was also proposed that hole injection could be enhanced as the EF was pinned near CBM.53 Among the air-annealed samples, the gap between EF and CBM is shortest for the 300 °C-annealed one,

Figure 11. UPS spectra of the WO3 films annealed in air at 150, 300, and 400 °C showing (a) the secondary edge at high binding energy region, related to work function, (b) the low-energy region, related to valence band maximum, and (c) the top of valence band, related to gap states.

UPS spectra in Figure 11b, the VBMs are estimated to be 2.80 and 2.53 eV with respect to the Fermi level, which is corresponding to the VBMs of 7.67 and 7.50 eV with respect to the vacuum level for the 300 °C-air-annealed and the 400 °C-air-annealed tungsten oxides, respectively. Figure 11c reveals no evolution of gap states, which indicates that W 5d 13485



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Figure 13. Energy diagram of the P3HT:PCBM-based BHJ solar cell with hole-selective WO3−x interlayer between ITO and polymer blend film. CBM, EF, and VBM represent conduction band minimum, Fermi level, and valence band maximum, respectively.

which leads to the improvement of hole transportation and is likely to be responsible for the highest performance.

4. CONCLUSIONS Tungsten oxide thin films with thickness of ∼30 nm were prepared to be used as the hole-selective layer in BHJ architecture. Photovoltaic performance was investigated in terms of tungsten oxide film annealing condition. The annealing temperature below 300 °C produced amorphous phase, whereas crystalline phase was formed at 400 °C. At annealing temperature of 150 °C, only W6+ was presented under air annealing condition, whereas W5+ and W6+ were presented under vacuum annealing condition. The presence of W5+ in tungsten oxide film was found to deteriorate hole collection ability, leading to poor photovoltaic performance, which was related to low work function. Under air annealing condition, photovoltaic performance was improved as temperature changed from 150 to 300 °C, whereas a little decrease in the performance was observed for the 400 °C-annealed WO3. From the UPS study, the effect of air annealing temperature on photovoltaic performance was found to be related to work functions. It is thus concluded that oxidation state and amorphous nature in tungsten oxide interlayer play the crucial role in hole collection from P3HT to ITO electrode. In addition, the work function, electron affinity, and ionization energy of tungsten oxide interlayer are found to correlate with hole transport efficiency.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-31-290-7241. Fax: +82-31-290-7272. E-mail: npark@ skku.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea under contract nos. 2011-0016441, 2011-0030359, and R31-2008-10029 (WCU program) and the Korea Institute of Energy Technology Evaluation and planning (KETEP) grant funded by the Ministry of Knowledge Economy under contract no. 20103020010010.

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Effects of Oxidation State and Crystallinity of Tungsten Oxide Interlayer

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