Hybrid Photovoltaic Devices Based on Poly (3-hexylthiophene) and

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Hybrid Photovoltaic Devices Based on Poly (3-hexylthiophene) and Ordered Electrospun ZnO Nanofibers Sujuan Wu, Qidong Tai, and Feng Yan* Department of Applied Physics, The Hong Kong Polytechnic UniVersity, Hong Kong, People’s Republic of China ReceiVed: NoVember 17, 2009; ReVised Manuscript ReceiVed: March 6, 2010

Hybrid photovoltaic devices based on poly(3-hexylthiophene) (P3HT) and an ordered electrospun ZnO nanofibrous network have been investigated. The diameters of the ZnO nanofibers have been controlled within 30-150 nm. The performance of the P3HT/ZnO hybrid solar cell is dependent on fabrication conditions, especially the thickness of the nanofibrous film. It has been found that the lifetime of carriers is lower in the device consisting of thicker ZnO nanofibrous films due to the higher density of surface traps in the ZnO nanofibers. The device with optimum fabrication conditions exhibits a power conversion efficiency of 0.51%. I. Introduction Photovoltaic devices based on composites of n-type inorganic nanomaterials and p-type conjugated polymers have attracted much attention recently because of the relatively high electron mobility and good physical and chemical stability of inorganic nanocrystals and their potential for the production of flexible and large-area solar cells at dramatically low costs.1-7 However, in such hybrid bulk heterojunction photovoltaic devices, high efficiencies are often limited by poorly formed organic-inorganic interfaces for electron transfer4,5,7 and noncontinuous electrontransport paths in the active layer.2,4 Therefore, rigid nanocrystalline structures that present continuous paths for photogenerated electrons to the collecting electrode have been popularly used, including template porous structures,8 tetrapods,9 or vertically aligned nanorods.10-15 ZnO is a good candidate for the application in hybrid photovoltaic devices because of its high electron mobility, easy control of shape and size, and environmental friendliness. Various hybrid photovoltaic devices have been developed by incorporating ZnO nanoparticles1-3 or nanorod arrays.10-15 It is worth noting that the power conversion efficiencies (PCE) of the devices based on ZnO nanorod arrays are even lower than those based on nanoparticles, which is partially due to the smaller interface area in the former devices. ZnO nanorods are normally fabricated by the hydrothermal method.9,10 However, increasing the surface area of the nanorod array depends on growing higher aspect ratio rods, which remains a significant technological challenge for hydrothermal methods. In this aspect, the electrospinning technique is a suitable method for fabricating ZnO nanofibers with extremely high aspect ratios. More importantly, this technique has provided a simple, cost-effective, and large area approach to prepare various materials with 1D nanostructures. Electrospun TiO2 nanofibers have already been used in various photovoltaic devices.16-22 Electrospun ZnO nanofibers have been used in dye-sensitized solar cells, and a PCE of 3.02% has been achieved.23,24 However, to the best of our knowledge, such ZnO nanofibers have never been used in organic-inorganic hybrid solar cells. We consider that fabrication of the hybrid solar cells by incorporating electrospun ZnO nanofibers can be a feasible way for realizing high-performance * To whom correspondence should be addressed. Tel: (852)-2766-4054. Fax: (852)-2333-7629. E-mail: [email protected].

Figure 1. Stucture of the hybrid solar cell based on electrospun ZnO nanofibers and P3HT.

devices because the density, ordering, and size of the ZnO nanofibers can be controlled by electrospinning conditions. In this paper, we will first report the hybrid solar cells based on ordered electrospun ZnO nanofibers and poly(3-hexylthiophene) (P3HT) with a tunable thickness and good adhesion to ITO substrates. The devices have been optimized by changing the thickness of the organic and ZnO nanofiber layers, and PCEs up to 0.51% have been achieved without any surface modification on the ZnO nanofibers. The lifetime of carriers at the ZnO/ P3HT interface has been characterized using electrochemical impedance spectroscopy (EIS).25 II. Materials and Methods ZnO Nanofiber Synthesis. ZnO nanofibers were electrospun on substrates from a precursor gel containing zinc acetate (Aldrich, 5%), ethanolamine (1.25%), poly(vinylpyrrolidone) (PVP) (Aldrich, MW ) 1.3 × 106 g mol-1, 5%), isopropanol (35.5%), and 2-methoxyethanol (53.25%) at a flow rate of 0.15 mL h-1. A voltage of 10 kV was applied between a metal orifice and a grounded rotating collector with a distance of 15 cm. This simple apparatus can be used to prepare aligned nanofibers, as reported before.26 The electrospinning time for each layer of ZnO nanofibers was controlled to be 30 min. The next layer was deposited with an alignment direction perpendicular to that of the previous one. Therefore, the whole thickness of the ZnO nanofibrous film was controlled by the number of cross-aligned layers. The as-spun ordered ZnO nanofibers were then calcined in oxygen at 450 °C for 5 min by rapid thermal processing to remove organic components and allow the nucleation and growth of ZnO. Solar Cell Device Fabrication. Figure 1 shows the schematic diagram of the hybrid solar cell. The devices were fabricated on patterned ITO glass substrates with a sheet resistance of 15

10.1021/jp910921a  2010 American Chemical Society Published on Web 03/16/2010

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Figure 2. (a, b) SEM images of the calcined ZnO nanofibrous network. (c) Cross-sectional view of the stacked ZnO nanofiber layers. (d) TEM image of an electrospun single ZnO nanofiber. Inset: the selected area electron diffraction (SAED) pattern of the ZnO fiber. (e) Cross-sectional view of a device under SEM.

Ω/sq. First, a thin layer of ZnO film with a thickness of 40 nm was fabricated on the ITO glass substrates by the sol-gel method, as reported before.11 A solution of zinc acetate dissolved in 2-aminoethanol was spin-coated on the ITO substrates and then subsequently annealed at 300 °C for 5 min in air by rapid thermal processing. The ZnO layer was used to improve the adhesion of electrospun ZnO nanofibers on the substrate and to work as a hole-blocking layer in the device. Sham et al18 reported that hybrid solar cells with ordered electrospun TiO2 nanofibers show higher PCEs than those with randomly collected nanofibers. Therefore, many layers of cross-aligned electrospun ZnO nanofibers were fabricated on the ZnO/ITO/glass substrates for the hybrid solar cells. P3HT was then infiltrated into the ZnO nanofibrous films by spin-coating a solution of regioregular P3HT (Reike, regioregularity is ∼93%) dissolved in toluene (30 mg/mL), followed by thermal annealing at 120 °C for 20 min in a glovebox filled with high-purity nitrogen. We have found that the contact angle for toluene on flat ZnO film is about 6°, which is lower than those for chloroform (∼7°) and chlorobenzene (∼22°). Therefore, toluene is an optimum solvent for the fabrication of such devices, which enables the infiltration of P3HT in the spaces among the ZnO nanofibers. The thickness of the P3HT layer has been optimized by controlling the spin-coating speed. An ∼100 nm Au top contact was then deposited by thermal evaporation through a shadow mask under a vacuum of 10-6 mbar. Solar Cell Characterization. The cell performance was measured by using a Keithley 2400 source meter in the glovebox under an illumination of 80 mW/cm2 (Newport 91160, 150 W, solar simulator equipped with an AM1.5 filter). Scanning electron microscopy (SEM, Quanta 200 FEG System, FEI Co., U.S.A.) and transmission electron microscopy (TEM; JEOL 2010as, Japan) were used to study fiber dimensions, crystallinity, and grain size of the ZnO nanofibers. The EIS was recorded by an impedance analyzer (HP 4294) in a frequency range from 40 to 20 MHz with an oscillating voltage of 50 mV in the dark. III. Results and Discussion Figure 2 shows the SEM images of calcined ZnO nanofibers with three cross-aligned layers (electrospinning time is 1.5 h). As shown in Figure 2a,b, the calcined ZnO nanofibers are not uniform and exhibit diameters within 30-150 nm. Figure 2c shows the cross-sectional view of the cross-aligned ZnO nanofiber layers. The thickness of the nanofibrous film is about 200 nm. Figure 2d shows a high-resolution TEM picture of one

Figure 3. (a) Current-voltage (J-V) curves and (b, c) photovatic characteristics of P3HT/ZnO nanofiber hybrid solar cells with different numbers of cross-aligned ZnO layers.

TABLE 1: Photovoltaic Performance of P3HT/ZnO Solar Cells with Different Numbers of ZnO Layers number of cross-aligned layers

Jsc (mA/cm2)

Voc (V)

FF

η (%)

only ZnO film 1 2 3 4

0.287 0.508 1.46 2.15 1.14

0.377 0.422 0.418 0.491 0.363

0.46 0.40 0.38 0.387 0.338

0.062 ( 0.003 0.11 ( 0.01 0.30 ( 0.02 0.51 ( 0.03 0.18 ( 0.01

ZnO nanofiber. The average size of ZnO grains in the polycrystalline fiber is about 5 nm. The diffraction pattern indicates that the structure of the ZnO fiber is hexagonal wurtzite. Figure 2e shows the cross-sectional view of a hybrid P3HT/ZnO nanofiber solar cell. Each layer of the device is indicated by an arrow in the figure. It is worth noting that the space among ZnO fibers and the substrate was completely filled with P3HT. The thickness of the active layer is about 250 nm. Figure 3a shows the current-voltage (J-V) characterizations of P3HT/ZnO nanofiber devices with different numbers of crossaligned ZnO layers. Figure 3b,c and Table 1 show the detailed photovoltaic parameters, including short-current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE (η), of the devices as functions of the number of ZnO layers. The optimum number of layers is three, which results in a PCE of 0.51%. The increase of PCE with the increase of the number of layers when it is less than three can be attributed to the increase of the P3HT/ZnO interface area. However, for the device with four layers of ZnO nanofibers, the thickness of the active layer (>300 nm) is too big for such organic solar cells, which normally leads to a big serial resistance, a low short-circuit current density, and a low PCE of the device.13,15

Photovoltaic Devices Based on P3HT and ZnO Nanofibers

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Figure 5. Lifetime of carriers in the devices with different numbers of cross-aligned ZnO layers characterized by EIS in the dark with a forward bias of 0.5 V.

Figure 4. (a) Impedance spectra, (b) charge-transfer resistance R2 and capacitance C2, and (c) lifetime of carriers in a P3HT/ZnO nanofiber hybrid solar cell with three cross-aligned layers of ZnO nanofibers (electrospinning time ) 1.5 h) measured at different biases in the dark.

The devices with ZnO nanofibers normally show a higher Voc than that based on planar sol-gel ZnO film, which can be attributed to different Fermi level positions in ZnO nanofibers and films, as reported by Olson et al.15 A higher density of nanofibers may lead to a higher Voc. The device with three crossaligned layers of ZnO nanofibers shows the maximum opencircuit voltage. However, as shown in Table 1, the device with four ZnO layers shows a lower Voc than the other four devices, which is probably due to a higher density of interface traps and lower lifetime of carriers in the device with thicker composite film. Therefore, the open-circuit voltage can be a trade-off between the high Fermi level and the low carrier lifetime at the surface of ZnO nanofibers. To confirm this assumption, the lifetimes of carriers in the composite films have been characterized by EIS. Figure 4a shows the impedance of the device with three layers of cross-aligned ZnO nanofibers measured at different bias voltages. Each curve is composed of two semicircles and can be fitted with the equivalent circuit, as shown in the figure. The series resistance, Rs ≈ 0.2 Ω cm2, is relatively low. The left arc in the high-frequency region is rather independent of the bias voltage, whereas the right one at lower frequencies exhibits a big dependence. The resistance R1 corresponding to the left arc at different bias voltages is about 70 Ω cm2. The capacitance C1 is about 1 × 10-8 F/cm2, which is very close to the capacitance of the composite film, assuming that the dielectric constant of the film is 3 and the thickness is 250 nm. Therefore, the left arc can be attributed to the impedance of the P3HT/ ZnO composite film. The right arc corresponds to the charge-transfer process at the P3HT/ZnO interface because it changes with the forward

bias voltage. C2 and R2 extracted by least-squares fitting of the impedance spectra with the equivalent circuit at different bias voltages are shown in Figure 4b. The lifetime of carriers at the interface can be calculated according to the diffusion-recombination model.27 Figure 4c shows the lifetime of carriers in the same device decreasing with the increase of forward bias voltage, which is a reasonable result because the recombination of carriers at the p-n junction is easier at higher forward bias voltage.25 Figure 5 shows the lifetime of carriers in devices with different numbers of layers of ZnO nanofibers measured at the same bias voltage close to the Voc. The lifetime monotonously decreases with the increase of the number of layers. The Voc is strongly influenced by the recombination rate at the P3HT/ZnO interface.28 A high recombination rate of carriers will lead to a low open-circuit voltage. Therefore, the low Voc in the device with four layers of ZnO nanofibers can be explained. In addition, a lower lifetime of carriers will also result in a lower PCE in the hybrid device. The optimum thickness of the nanofibrous film is a compromise among many factors, including interface area, lifetime of carriers, serial resistance, etc. It is worth noting that the hybrid solar cell shows a relatively low PCE, which is a common problem for this type of solar cell.9,10 The average diameter of the electrospun ZnO nanofibers is about 100 nm, which is much bigger than the size of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) nanocrystallites in whole organic P3HT/PCBM solar cells.29 Therefore, the interface area of the bulk heterojunction in the ZnO/P3HT film is expected to be much smaller than that in P3HT/PCBM composite films. To increase the efficiency of the hybrid device, it is necessary to decrease the size and increase the density of the ZnO nanofibers in the composite film, which will be carefully studied in the future. In addition, surface modification on the ZnO nanofibers may help exciton dissociation and improve carrier lifetime at the interface,7 which is under investigation. IV. Conclusion In summary, the performance of hybrid solar cells based on P3HT and electrospun ZnO nanofibers is dependent on the thickness of the nanofibrous film. The device with optimum fabrication conditions exhibits a power conversion efficiency of 0.51%. The lifetime of carriers at the P3HT/ZnO interface characterized by EIS monotonously decreases with the increase of film thickness. Further improvement on the device performance by decreasing the size and doing surface modification of the ZnO nanofibers is ongoing. Acknowledgment. The authors would like to acknowledge Mr. Zhang Liang for the help with experiments. This work is

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