A facile route for preparation of conjugated polymer functionalized inorganic semiconductors and direct application in hybrid photovoltaic devices

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 1293–1299

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A facile route for preparation of conjugated polymer functionalized inorganic semiconductors and direct application in hybrid photovoltaic devices Hongwei Geng a,b, Ying Guo a,b, Ruixiang Peng a,b, Shikui Han a,b, Mingtai Wang a,b,c,n a

Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, PR China Key Laboratory of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, PR China c School of Materials Science and Chemical Engineering, Anhui University of Architecture, Hefei 230022, PR China b

a r t i c l e in fo

abstract

Article history: Received 16 November 2009 Received in revised form 22 March 2010 Accepted 29 March 2010 Available online 18 April 2010

In this work, ZnO nanorods surface was functionalized with poly(1-methoxy-4-(2-ethylhexyloxy)-pphenylenevinylene) (MEH-PPV), offering a hybrid nanocomposite (MEH-PPV  ZnO) that was directly applied for the preparation of active layer in hybrids photovoltaic devices. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and photophysical properties showed that the conjugated polymer chains intimately contact with the inorganic semiconductors. The performance of the resulting photovoltaic devices was investigated by current–voltage (J–V) characteristics and intensity-modulated photovoltage spectroscopy (IMVS). The photovoltaic performance was greatly enhanced via direct application of the nanocomposites as the active layer in photovoltaic devices, giving an optimized device performance of a short-circuit current density of 0.19 mA/cm2, an open-circuit voltage of 0.59 V and a fill factor of 0.43, with a power conversion efficiency of about 0.30% under 470 nm monochromatic illumination (15.8 mW/cm2) that was 50% higher than that of the device based on the simple blend of two components. In addition, intensity-modulated photovoltage spectroscopy (IMVS) response curves revealed that a longer electron lifetime in the MEH-PPV  ZnO bulk heterojunction photovoltaic devices leads to a higher open-circuit voltage of the devices. & 2010 Elsevier B.V. All rights reserved.

Keywords: MEH-PPV ZnO nanorods Nanocomposites Photovoltaic Electron lifetime

1. Introduction During the last decades, the polymer-based photovoltaic (PV) devices are attractive for their potential to fabricate low-cost, large area and flexible solar cells [1–8]. The device stabilities of polymer-based solar cells also have been extensively investigated [9–13], and it has been demonstrated that polymer-based PV devices performed the long-time air stability under ambient condition [14–17]. Recently, significant developments in manufacture and demonstration of polymer-based PV devices have been achieved [18–22]. The hybrid polymer-based PV devices consisting of conjugated polymers and metal oxide nanocrystals have gained attention as promising alternatives for future hybrid PV devices [23–26]: the hybrid polymer-based PV devices combine the unique properties (e.g., higher electron mobility, electron affinities and better stability) of inorganic semiconductors with the good film-forming properties of the conjugated polymers. Meanwhile, the size, shape as well as the optoelectronic properties of inorganic semiconductor

n Corresponding author at: Key Laboratory of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, PR China. Tel./fax: 86 551 5593171. E-mail address: [email protected] (M. Wang).

0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.03.036

nanocrystals can be tailored individually during synthesis process before the organic phase is incorporated [27]. Generally, the active hybrids of PV devices are usually prepared by simply blending nanoparticles with conjugated polymers. Different strategies have also been applied to prepare the composites as the active layer, including so-called ‘graft onto’ method by attaching poly(3-hexylthiophene) chains onto the (DOPO-Br)-functionalized CdSe surface [28], ligand exchange process of quantum dots with the branched oligothiophene dendrons [29] and blending the nanoparticles with functionalized end-group of conjugated polymers [30,31]. Even though directly blending of the two components is widely used, such chemical attachments of the conjugated polymers to the inorganic nanoparticles surface can effectively control the morphology [29,31], and prevent the aggregation of nanoparticles. Furthermore, this could improve the compatibility between the polymer and inorganic semiconductors and influence the energetic and electronic properties of nanoparticles. For example, enhancement of charge/energy transfer between conjugated polymers and quantum dots was observed by chemical grafting of the conjugated polymer on the nanoparticle surface [29,32,33]. Some works [28–31] have been related to the photovoltaic applications of nanoparticles functionalized with conjugated polymers, but only one previous report [31] demonstrated that

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the device efficiency could be improved by bringing the conjugated polymers and nanoparticles into an intimate contact. Poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) is commonly used for hybrid solar cells [34–36]. In this work, we report a facile synthesis route for the preparation of MEH-PPV functionalized ZnO nanorods (MEH-PPV ZnO), by adding the preformed ZnO nanorods into the polymerization reaction system of MEH-PPV. Before feeding ZnO nanorods, the polymerization reaction of monomers is completed and the carbanions on the end of polymer chains are still ‘living’. Therefore, the molecular weight of the conjugated polymer attached on the nanoparticles can be easily controlled during the polymerization reaction of the monomers. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and photophysical properties testified the formation of strong interaction between MEH-PPV and ZnO nanorods. The performance of the resulting photovoltaic devices was investigated by J–V characteristics and intensity-modulated photovoltage spectroscopy (IMVS). For comparison, the composites of MEH-PPV and ZnO nanorods (MEH-PPV/ZnO) were prepared using a conventional blending approach. The device performance, in particular, the open-circuit voltage of MEH-PPV ZnO PV devices was significantly improved as compared to that of MEHPPV/ZnO PV devices. IMVS response curves revealed that a longer electron lifetime in the MEH-PPV ZnO device leads to a higher open-circuit voltage of the device, which is due to the greatly reduced recombination of photogenerated electrons.

2. Experimental 2.1. Materials and chemicals 4-Methoxyphenol, 2-ethylhexyl bromide, paraformaldehyde and potassium tert-butoxide were purchased from Acros. All other reagents were obtained from the Sinopharm Chemical Reagent Co., Ltd. and used as received, unless otherwise specified. Tetrahydrofuran (THF) was distilled from sodium benzophenone prior to use. Chlorobenzene was distilled under low pressure before use. MEH-PPV was synthesized following the method described by Neef and Ferraris [37] (detailed experimental procedure is listed in Appendix A). The average molecular mass (Mn) of the as-synthesized polymer was 27,000 and the polydispersity index was 1.2, as determined with a WATERS 150-C gel permeation chromatography (GPC) with THF as solvent and monodispersed polystyrene as standard. ZnO nanorods were synthesized according to the literature [38]. The as-synthesized nanorods were dispersed into the anhydrate THF. The content of nanoparticle in the THF was calculated by the weighting method. The average dimension of ZnO nanorods is  7 nm  20 nm, the microstructure of nanorods was measured by a transmission electron microscope. 2.2. Synthesis of MEH-PPV  ZnO nanocomposites In a typical procedure, to a round bottom flask, well flushed with N2 and equipped with a magnetic stirrer bar and a reflux condenser, a solution of potassium tert-butoxide in THF (100 ml, 1.0 M), 4-methoxyphenol (0.048 mmol) was added. A solution of a,a0 -dibromo-2-methoxy-5-(2-ethylhexyloxy) xylene (2.0 g, 4.74 mmol) in dry THF (20 ml) was then added in 60 min. After complete addition of the monomer, the reaction was stirred further for 16 h. Then the dispersion (40 ml, 50 mg/ml solution in dry THF) of ZnO nanorods was added into the three-neck flask under N2 atmosphere, and stirred for 24 h. The reaction solution

was poured into the rapidly stirred methanol, and then the resulting nanocomposites were collected by centrifugation. Finally, the hybrids were moved into a vacuum oven at room temperature for a couple of days to remove the solvent. The content of ZnO nanorods in the nanocomposites was determined by thermogravimetric analysis (see Appendix A). 2.3. Preparation of the blends of MEH-PPV/ZnO nanorods Blends of MEH-PPV and ZnO were prepared by mixing the MEH-PPV solution in chlorobenzene with the ZnO nanorods dispersion in THF. The exact amount of the ZnO nanorods stock solution needed to reach the desired amounts of ZnO in the blends, and the ZnO concentration in the solution was determined from the solid residue after solvent evaporation. On average, a ZnO concentration of 35 mg ml had been obtained. In a typical procedure, MEH-PPV (10 mg) was dissolved in chlorobenzene (2.193 ml). For a 70 wt% MEH-PPV/ZnO blend, 0.667 ml of the ZnO solution was added, and the blends were magnetic stirred for more than 24 h at room temperature before being used. 2.4. Device fabrication Indium–tin oxide (ITO) glass sheet (1.1 mm thick, r15 O/&, Wuhu Token Sci. Co., Ltd., China) was cut into square to yield a substrate area of 30 mm  30 mm, and ITO layer was patterned into a rectangle (10 mm  30 mm) by etching with HCl solution (36%). The substrate was first manually washed in aqueous detergent and then sequentially sonicated for 10 min twice in acetone, isopropanol and deionized water. The freshly cleaned substrate was dried with N2 stream and then spin coated (60 s, 3000 rpm) with a poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) suspension (Celvios P AI 4083) that had been passed through a 0.45 mm filter, and the coated substrate was dried for 10 min at 200 1C. The active layer was deposited over the PEDOT:PSS layer by spin coating (60 s, 1500 rpm) 100 ml composites solution under ambient condition. After the formation of the composite active layer, the substrate was stored under vacuum for 24 h and subsequently transferred to the evaporation chamber. A 1.0 nm thick layer of LiF was evaporated on top of the active layer, followed by the deposition of approximately 100 nm Al at a base pressure of less than 1  10  3 Pa over 10 min with a rate of  0.3 nm/s for the first 5 nm. The evaporation mask defined a cathode area of  4 mm2 for each of the ten solar cells on a substrate, and the four substrates loaded simultaneously into the vacuum chamber defined a batch. Finally, the solar cells were subsequently transferred to a dry N2 glove box environment (H2O less than 1 ppm and O2 less than 1 ppm) where the solar cells were sealed. 2.5. Characterization XPS was performed with a Thermo VG ESCALAB 250 instrument with a monochromatic Al Ka (hn ¼1486.6 eV) source 200 W X-ray source. The X-ray spot size was 500 mm. For all samples, the Zn 2p3/2 peak was normalized to 1021.9 eV as the reference. The pass energy was set at 70 and 20 eV for the survey and detail scans, respectively. Data acquisition and processing were achieved with the Avantage Software, version 4.20. Atomic percentage was determined using this software. FT-IR spectra were obtained using a BRUKER ALPHA FT-IR spectrometer in the range 4000–500 cm  1 from samples that were dispersed in KBr pellets. Thermogravimetric analysis (TGA) was performed on a Shimadzu DTG-60H analyzer with a heating rate of 10 1C/min at range from room temperature to 700 1C under a nitrogen

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atmosphere. Differential scanning calorimetry (DSC) measurements were made using a Shimadzu DSC-60 calorimeter. DSC scans were made from 30 to 350 1C using a scan rate of 10 1C/min under nitrogen atmosphere. TEM studies were performed using a JEOL 2010 instrument under an acceleration voltage of 200 kV. The samples were prepared by dispersing into chlorobenzene, and then dropping the dispersion onto the TEM copper grids. UV–vis absorption spectra were recorded with a Shimadzu UV-2550 spectrophotometer (wavelength range 280–800 nm). Photoluminescence spectral measurements were performed on the HITACHI F-7000 fluorescence spectrophotometer by photoexcitation at 480 nm and recording in the range of 500–850 nm. Steady-state current–voltage characteristics (J–V) and IMVS were measured on a controlled intensity-modulated photo-spectroscopy (CIMPS) (Zahner Co., Germany) in ambient conditions under an illumination through an ITO glass side. A blue light-emitting diode (LED) as light source (BLL01, lmax ¼470 nm, spectral half-width¼25 nm, Zahner Co.) driven by a frequency response analyzer and the background light intensity (I0) of 15.85 mW cm  2 was controlled by the bias voltage at the LED with a proportionality factor FP¼315.7 W m  2 V  1 that was calibrated using an IL1400A photometer with an SEL 033/W detector (International Light, Inc., USA). In the IMVS measurements, the small sinusoidal perturbation Iac ¼I0deiot (1.54 mW/cm2) was used, about a depth d ¼10% of the I0 intensity. During the measurements, the aluminum and ITO contacts were taken as negative and positive electrodes, respectively.

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surface [40]. For MEH-PPVZnO (Fig. 1a), the three characteristic peaks of acetate groups eventually decreased. It indicates that most of the acetate groups on ZnO nanorods surface were removed in the MEH-PPV ZnO nanocomposites during this synthesis route. Survey and high-resolution spectra of the C1s spectra for (a) MEH-PPV ZnO and (b) MEH-PPV/ZnO are shown in Fig. 2. The survey spectrum of MEH-PPV  ZnO shows the features similar to that of MEH-PPV/ZnO. However, the increased C1s peak indicates that carbon was enriched on the ZnO nanorods surface in MEH-PPV ZnO nanocomposites. The C1s spectrum of the MEH-PPV/ZnO [Fig. 2B (b)] shows peaks at 284.6, 285.9, 287.2 and 289.5 corresponding to –C–C, –C–O, CQC and –O–CQO, respectively [41]. The –O–CQO content in MEH-PPV/ZnO estimated from XPS is 1.52 wt%. The spectrum from MEHPPV  ZnO [Fig. 2B (a)] shows a significantly smaller –O–CQO peak, corresponding to  0.94 wt% acetate groups. The acetate groups on ZnO nanorods surface of MEH-PPV  ZnO nanocomposites were removed to a great extent, which is consistent with IR results. Obviously, the C–C peak for MEH-PPV  ZnO at 285.0 eV is upshifted by 0.4 eV compared to that of MEH-PPV/ZnO due to the effect of the electronic donor, MEH-PPV. This upshift of C–C peak in C1s spectrum agrees with the observation in C1s XPS

3. Results and discussion 3.1. FT-IR, XPS and TEM characterizations The FT-IR spectra of MEH-PPV  ZnO and MEH-PPV/ZnO in Fig. 1 have a broad band between 3500 and 3000 cm  1, which indicates the presence of –OH groups and physically absorbed water on the surface of ZnO nanorods. MEH-PPV chain includes the following main characteristic absorption bands: 3058 cm  1 band of vinylene C–H stretching, 2954 and 2926 cm  1 bands of CH3 and CH2 of asymmetrical C–H stretching, 1503 cm  1 band of C–C ring semicircular stretching, 1461 cm  1 band of CH3 asymmetrical C–H bending, 1412 cm  1 band of C–C semicircular stretching, 1351, 1252, 1203 and 1040 cm  1 bands of phenylene alkoxy stretching [39]. For the spectrum of MEHPPV/ZnO (Fig. 1b), it is simply the sum of the absorption spectra of constituent parts (MEH-PPV and ZnO nanorods). The peak at 1590 cm  1 and the shoulder peaks at 1410 and 1340 cm  1 are attributed to ‘‘bridging’’ type of acetate groups on ZnO nanorods

Fig. 1. FT-IR spectra of (a) MEH-PPV  ZnO and (b) MEH-PPV/ZnO.

Fig. 2. (A) Survey and (B) C1s XPS spectra collected from (a) MEH-PPV ZnO and (b) MEH-PPV/ZnO.

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peak of graphene sheets, which was functionalized with pyrene1-sulfonic acid sodium salt as an electronic donor [42]. Fig. 3 shows the TEM images of MEH-PPV/ZnO and MEH-PPV  ZnO prepared by physically mixing MEH-PPV and as-synthesized ZnO nanorods and by chemically functionalizing ZnO nanorods with MEH-PPV, respectively. The blends of MEHPPV/ZnO exhibited a significant aggregation of ZnO nanorods as evidenced in Fig. 3b. In contrast, the ZnO nanorods were easily dispersed in MEH-PPV ZnO nanocomposites (Fig. 3a). It is concluded that the MEH-PPV functionalized ZnO nanorods can prevent the aggregation of ZnO nanorods. The results of AFM height images of MEH-PPV  ZnO and MEH-PPV/ZnO also agree with the observation in TEM images (see Appendix A). It is noteworthy that no clear MEH-PPV coating at the periphery of the ZnO nanorods can be imaged, which may be due to that the

Fig. 3. TEM images of (a) MEH-PPV ZnO and (b) MEH-PPV/ZnO.

electron density of ZnO nanorods is relatively low and the rigid MEH-PPV chains make it difficult for MEH-PPV chains to collapse at the surface of ZnO nanorods core into a dense shell layer [33]. According to the above analysis, there is reliable evidence for the successfully functionalizing ZnO nanorods with MEH-PPV chains and the formation of strong interaction between MEH-PPV and ZnO nanorods, but further investigations are required to clarify the mechanism of grafting. 3.2. Photophysical properties To explore the differences in photophysical properties between MEH-PPV ZnO and MEH-PPV/ZnO (ZnO nanorods content is 80 wt% in the blends) was prepared. The absorption spectra of MEH-PPV, ZnO nanorods, MEH-PPV/ZnO and nanocomposites of MEH-PPV ZnO are displayed in Fig. 4. Firstly, for the spectrum of MEH-PPV/ZnO composites, two peaks at 362 and 507 nm are seen, corresponding to MEH-PPV and ZnO nanorods absorption, respectively. Secondly, a slight red-shift of the absorption band of MEH-PPV can be observed, which indicates that the polymer chains in the two composite films are more extended and the p electrons in the polymer backbone have a longer conjugation length [43]. Thirdly, the absorption peak of ZnO nanorods of MEH-PPV  ZnO nanocomposites was red-shifted about 8 nm (Fig. 4). This can be attributed to the changes in the dielectric environment (i.e., placing ZnO nanorods in intimate contact with MEH-PPV) [33,44]. The PL intensity of the nanocomposites of conjugated polymers and inorganic semiconductors can depend on various reasons, such as, annealing [45,46], morphological disorder [47] and charge transfer [33] between the polymers and the nanoparticles. The PL spectrum (Fig. 5) of MEH-PPV/ZnO is quenched compared to pristine MEH-PPV, which agrees with the results of other researchers [38,48]. However, the PL intensity of MEH-PPV ZnO nanocomposites increases evidently, which should be attributed to several factors. Firstly, the configuration of the p-conjugated polymer plays an important role in the luminescent properties of polymers [49]. The higher glass transition temperature (Tg) of MEH-PPV confirms the more rigid polymer backbone in MEH-PPV ZnO nanocomposites (see Appendix A Fig. S4). Meanwhile, a stronger interaction between conjugated polymer and inorganic nanoparticles will result in lowering the efficiency of charge transfer, as was observed by

Fig. 4. Absorption spectra of pristine MEH-PPV, ZnO nanorods, MEH-PPV/ZnO and MEH-PPV ZnO. The MEH-PPV, MEH-PPV  ZnO and MEH-PPV/ZnO films were prepared by spin-coating on quartz substrates. ZnO nanorods were dispersed into chlorobenzene.

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Table 1 Photovoltaic performance of MEH-PPV/ZnO nanorods hybrid photovoltaic devices with different nanoparticle contents in blends (under 470 nm blue light illumination).

Fig. 5. Photoluminescence spectra of MEH-PPV/ZnO, MEH-PPV  ZnO and pristine MEH-PPV on quartz substrates. The MEH-PPV film was prepared by spin-casting 3.0 mg/ml solution of MEH-PPV in chlorobenzene.

Palaniappan et al. [30]. The two factors may explain why the remarkable PL quenching [29,32,33] was not observed in MEHPPV  ZnO nanocomposites and the short-circuit current density of PV devices fabricated from MEH-PPV  ZnO nanocomposites slightly decreased as compared to that of MEH-PPV/ZnO PV devices (see Section 3.3). In a word, the unique photophysical properties of MEH-PPV  ZnO nanocomposites also testify the mutual effects between MEH-PPV chains and ZnO nanorods as a result of the conjugated polymer intimately contacting with nanoparticles. The strong interaction between conjugated polymers and inorganic semiconductors could significantly influence the photovoltaic performance of the hybrid PV devices. 3.3. Photovoltaic performance To investigate the influence of the MEH-PPV functionalized ZnO nanorods on photovoltaic performance, two series of PV devices were fabricated in the same manner except for the preparation of composites of MEH-PPV and ZnO nanorods, i.e., the hybrid MEH-PPV/ZnO devices with different amounts of ZnO nanorods and the hybrid MEH-PPV ZnO devices with different concentrations of nanocomposites. The photovoltaic properties were studied by J–V characteristics and IMVS spectra, respectively. IMVS has been widely applied in the study of dye-sensitized solar cells (DSCs) [50–52]. IMVS measures the periodic photovoltage response to a small sinusoidal perturbation of light, Iac ¼I0deiot, superimposed on a lager steady background I0 and provides information about the recombination kinetics under open-circuit condition. In the case of typical DSCs, the electron lifetime tn in the TiO2 nanoparticles can be calculated from the frequency fmin value while the lowest imaginary component on a complex plane IMVS plot according to tn ¼(2pfmin)  1. The polymer/nanoparticles bulk heterojunction PV devices show some similarities to the typical DSCs with respect to their working principles and the device architecture, i.e., the conjugated polymer replaces the dye and electrolyte, and plays the role of both sensitizer and hole transporter [53–55]. The performance of MEH-PPV/ZnO devices is strongly dependent on the content of ZnO in the blends. Table 1 summarizes some of the observed effects. The best performance has been obtained at 70 wt% content of ZnO nanorods, reaching an efficiency of 0.20% under 470 nm blue light illumination. Note that power conversion efficiency for our MEH-PPV/ZnO devices is less than the highest values previously reported (0.26%) [56],

Content (%)

Voc/V

Jsc/mA cm  2

80 70 60 50

0.125 0.400 0.325 0.350

0.453 0.210 0.182 0.133

FF

g/%

sn/ms

0.34 0.38 0.38 0.37

0.12 0.20 0.14 0.11

20.9 101 159 199

owing to the different architectures and fabrication processes used here (for example, LiF/Al was used as the material of cathode instead of Ba/Al). Increase in or decrease in the content of ZnO nanorods in the blends ultimately always leads to a lower open-circuit voltage (Voc) of the PV devices. The short-circuit current densities (Jsc) of MEH-PPV/ZnO devices are enhanced with an increase in ZnO nanorods content in the blends, which should be attributed to the formation of more effective pathways for the charge transport at high content of ZnO nanorods [38]. With further increase in ZnO nanorods content, Jsc increases whereas Voc decreases. It can be concluded that the exciton dissociation is further improved, but more significantly electron transport through the nanorods network without recombination with the holes in the polymer becomes more likely [57]. In addition, the lowest Voc is accompanied by the shortest tn, which is in agreement with the observation in dye-sensitized solar cells [58,59]. The influence of nanocomposites concentration on MEH-PPV ZnO PV device performance was investigated, and the obtained results are listed in Table 2. It can be seen that the Voc and FF are not sensitive to the concentration of nanocomposites. Jsc, however, is more significantly dependent on it. The MEH-PPV ZnO device with 15 mg/ml concentration performed the highest Jsc (0.190 mA/cm2) and the maximum power conversion efficiency (0.30%). The active layer spin-coated with 12 mg/ml solution exhibits the lowest Jsc of device, because the absorbance of light decreases and thin film likely forms pinholes leading to shunts [38], while increase in the concentration (4 15 mg/ml) leads to an inhibition of Jsc. Therefore, the active layer of the optimized hybrid device was spin-coated with 15 mg/ml solution, which should be attributed to the devicelimiting trade-off between optical absorption and electrical performance [4,60,61]. At the same time, it is found that the power conversion efficiency of MEH-PPV ZnO PV device (fabricated from the 15 mg/ml solution) is not only higher than that of MEH-PPV/ZnO PV device with 80 wt% content of ZnO nanorods, but also superior to the highest efficiency in the series of MEH-PPV/ZnO PV devices. Fig. 6 shows the J–V characteristics of MEH-PPV/ZnO (70 wt%) and MEH-PPV ZnO (15 mg/ml) devices under monochromatic illumination and in the dark, which perform the highest efficiency in the two series of bulk heterojunction PV devices, respectively. The Voc increases to as high as 0.59 V for the MEH-PPV  ZnO device, about 50% greater than its initial value of 0.40 V for the MEH-PPV/ZnO device. Meanwhile, the dark curve of MEHPPV  ZnO device shows insignificant shift toward higher voltage in the forward current. The PV device based on MEH-PPV  ZnO gives an optimized device performance of a short-circuit current density of 0.19 mA/cm2, an open-circuit voltage of 0.59 V, a fill factor of 0.43 and a power conversion efficiency of about 0.30% under simulated 470 nm illumination (15.8 mW/cm2), and exhibits an efficiency 50% higher than that of MEH-PPV/ZnO device. The higher Voc compensates the lower Jsc than that of the MEH-PPV/ZnO devices, which results in a higher efficiency. In addition, the enhancement of Voc could be attributed to the

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Table 2 Photovoltaic performance of MEH-PPV  ZnO nanorods hybrid photovoltaic devices with different concentrations of nanocomposites in solutions (under 470 nm blue light illumination). Concentration of nanocomposites (mg/ml)

Voc/V

Jsc/mA cm  2

20 17 15 12

0.520 0.625 0.590 0.490

0.102 0.109 0.190 0.089

FF

g/%

sn/ms

0.43 0.35 0.43 0.43

0.14 0.15 0.30 0.12

192 753 381 304

for the Returned Overseas Chinese Scholars, State Education Ministry, the National Natural Science Foundation of China (no. 20474066) and the National 973 project (2007CB936602) and the President Foundation of Hefei Institutes of Physical Science.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2010.03.036.

References

Fig. 6. The J–V characteristics of MEH-PPV  ZnO and MEH-PPV/ZnO bulk heterojunction photovoltaic devices under 470 nm blue light illumination with irradiation intensity of 15.8 mW/cm2. The inset shows the architecture of bulk heterojunction photovoltaic devices.

reduction in back charge recombination [62–64] due to interfacial modification. It is consistent with the increased tn of MEHPPV  ZnO PV device.

4. Conclusions In summary, we present a simple route to prepare the MEHPPV functionalized ZnO nanorod nanocomposites, which could bring the conjugated polymers into an intimate contact with inorganic nanoparticles. This synthesis route can be easily extended to other conjugated polymer–inorganic hybrid systems. The strong interaction between conjugated polymers and inorganic semiconductors could lead to the unique photophysical properties of the MEH-PPV  ZnO nanocomposites synthesized via this method. The PL intensity of MEH-PPV ZnO nanocomposites was remarkably increased due to the rigid polymer chains intimately contacting with nanoparticles in the nanocomposites. The photovoltaic performance of MEH-PPV  ZnO PV devices had been investigated and compared with those of the MEH-PPV/ZnO PV devices. It is found that the open-circuit voltage and power conversion efficiency of the MEH-PPV  ZnO PV devices were greatly enhanced, about 50% greater than those of the MEH-PPV/ ZnO PV devices. The improvement in the device performance originates from the greatly increased open-circuit voltage due to the reduced back charge recombination.

Acknowledgments This work was supported by the ‘100-Talent Program’ of Chinese Academy of Sciences, the Scientific Research Foundation

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