J. Phys. Chem. C 2010, 114, 11273–11278
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Hybrid Bulk Heterojunction Solar Cells Based on P3HT and Porphyrin-Modified ZnO Nanorods A. J. Said,† G. Poize,† C. Martini,† D. Ferry,† W. Marine,† S. Giorgio,† F. Fages,† J. Hocq,‡ J. Boucle´,§,| J. Nelson,§ J. R. Durrant,§ and J. Ackermann*,† Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), UPR 3118, CNRS, Aix-Marseille UniVersite´, Campus de Luminy, France, Department of Physics, Imperial College London, Prince Consort Road, London SW7 2BW, United Kingdom, Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom, and XLIM UMR 6172, UniVersite´ de Limoges/CNRS, 123 AVenue Albert Thomas, 87060 Limoges Cedex, France ReceiVed: NoVember 23, 2009; ReVised Manuscript ReceiVed: March 31, 2010
This work presents hybrid bulk heterojunction solar cells based on dye-sensitized zinc oxide (ZnO) nanorods blended with poly(3-hexylthiophene) (P3HT). Tetra(4-carboxyphenyl)porphyrin (TCPP) molecules were grafted onto the surface of ZnO nanorods to enlarge the absorption spectrum of the blend. We demonstrate that additional bands in the external quantum efficiency (EQE) spectra corresponding to Soret and Q-band absorption can already been observed at very low dye concentration at the ZnO surface. Therefore, direct grafting of TCPP onto ZnO nanorods leads to very efficient electron injection process into the ZnO nanorods after the light absorption of the dye. However, the overall photocurrent of the devices decreases gradually with TCPP concentration at the ZnO nanorod surface. The recombination dynamics of the photogenerated charges at the P3HT:ZnO interface are investigated by transient absorption spectroscopy on micro- to millisecond time scales. We observe that the lifetime of the P3HT polarons is reduced by an order of magnitude by grafting TCPP of already low concentration at the ZnO surface. Furthermore, high-resolution transmission electronic microscopy analysis of the blend morphology reveals that aggregation of ZnO nanorods within the P3HT is strongly increased by TCPP grafting. Therefore, we conclude that TCPP grafting is beneficial for additional photocurrent generation in the P3HT:ZnO blend but introduces strong modification of the blend morphology and charge carrier dynamics at the P3HT/ZnO interface, which finally reduces the overall photocurrent generation. 1. Introduction Solar cells based on conjugated polymers or oligomers are promising candidates for future low-cost photovoltaics (PVs).1 The use of interpenetrating donor-acceptor networks of nanoscale phase separation, so-called bulk heterojunctions (BHJ), has led to a tremendous increase in solar cell efficiency over the past decade. The best PV devices are based on blends of phenyl-C61-butyric acid methyl ester (PCBM) and poly(3hexylthiophene) (P3HT)2,3 as well as phenyl-C71-butyric acid methyl ester (PC70BM) and PCDTBT4 and show power conversion efficiency up to 6% under simulated AM1.5 illumination. An interesting alternative to these all-organic solar cells is the replacement of the organic n-type material by inorganic nanoparticles. Such hybrid nanomaterials aim at combining the versatile solution processability of conjugated polymers with high electron mobility and the relative environmental stability of inorganic semiconductors. Various inorganic NPs such as CdSe,5,6 TiO2,7-14 ZnO,15-17 PbS,18 and silicon19 nanoparticles have been successfully applied as n-type semiconductor to solution processed BHJ solar cells. Whereas the highest power conversion efficiencies have been demonstrated for hybrid solar cells using CdSe tetrapods,6 semiconducting nanoparticles such * Corresponding author. E-mail address:
[email protected]. † Centre Interdisciplinaire de Nanoscience de Marseille (CINaM). ‡ Department of Physics, Imperial College London. § Department of Chemistry, Imperial College London. | Universite´ de Limoges/CNRS.
as ZnO and TiO2 offer a promising, nontoxic, and environmental friendly alternative, but the corresponding solar cells performance remains low. Recently, surface modification was applied to n-type TiO2 nanorods by attaching dye molecules to improve the interface between the nanoparticles and the surrounding polymer.12,13 The presence of the dye has been found to improve charge carrier injection in the TiO2 nanorods, and power conversion efficiencies of 2.2% could be demonstrated.13 Such beneficial effect of ligand exchange was also demonstrated at the polymer:ZnO nanorod interfaces, for which ZnO nanorods were grown on ITO substrates and filled with P3HT to form a nanostructured hybrid solar cell or a planar structure between ZnO/P3HT.14 However, in all cases, no additional photocurrent generation due to light harvesting by the ligand could be observed, although such dyes absorb far beyond the absorption edge of P3HT. The purpose of this article is to investigate whether dye grafting can increase the spectral response of solution processed hybrid BHJ solar cells based on ZnO nanorods and P3HT blends. Porphyrins bearing carboxylic acid anchoring groups have strong absorption coefficients and show high solar energy conversion efficiency in dye sensitized solar cells.20-23 They have also been grafted successfully to ZnO nanospheres,24 nanorods,25 and tetrapods26 recently. Here we report on the effect of porphyrin grafting on the external quantum efficiency (EQE), charge carrier dynamics, and blend morphology of the hybrid ZnO/P3HT system.
10.1021/jp911125w 2010 American Chemical Society Published on Web 06/09/2010
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J. Phys. Chem. C, Vol. 114, No. 25, 2010
2. Results and Discussion Materials and Methods. ZnO nanorods were prepared following Pacholski27 and Sirringhaus.28 In brief, we added dropwise a solution of 0.4859 g of potassium hydroxide (Aldrich Chemicals 99.99%) in 23 mL of methanol to a solution of 0.8182 g of zinc acetate (Aldrich Chemicals 99.99%) in 42 mL of methanol (Acros 99,99%) and 0.25 mL of distilled water at 60 °C under magnetic stirring. After 2 h 15 min, the mixture was concentrated by a rotary pump to 10 mL and then heated to 60 °C for 48 h. Then, the solution was left at rest to decant and the white solid precipitate was washed with methanol. After the 48 h of reaction time, ZnO rods with 9 nm as diameter and 40 nm as length are formed. Dye Modification of ZnO Nanorods. Dye sensitization of ZnO nanorods was performed overnight in tetrahydrofuran (THF) solution. Tetra(4-carboxyphenyl)porphyrin (TCPP) was purchased from Sigma-Aldrich and used without further purification. Dye concentration during grafting was 1.5 × 10-8, 6 × 10-8, 1.2 × 10-7, and 2 × 10-7 mol/mg ZnO nanorods. By taking an average surface per TCPP molecule of 1.2 nm2,11 which corresponds to an average surface coverage of 12, 48, 96, and 100% respectively. Modified ZnO nanorods were centrifuged twice in THF solution to eliminate nongrafted dye molecules and then redispersed in THF. Solar Cell Fabrication. The modified ZnO nanorods were mixed with P3HT (Sigma-Aldrich) in THF (ratio 70:30 wt %). Solar cells were fabricated on ITO-coated (indium-doped tin oxide) glass substrates (Solaronix). After the ITO was cleaned with acetone and deionized H2O, a layer of 80 nm thick PEDOT/ PSS (H-Starck) was spin coated on ITO substrates, followed by annealing under argon atmosphere for 30 min at 80 °C. Then, the hybrid blend was spin coated on top of the PEDOT/ PSS-ITO substrates, followed by thermal evaporation of aluminum contacts through shadow masks under high vacuum (1 × 10-6 mbar). A final annealing step was performed at 80 °C under an argon atmosphere for 30 min. Characterization Techniques. Fourier transform infrared spectroscopy (FTIR) spectra were obtained with a Bruker Equinox 55 in the transmission mode (100 scans, resolution 2 cm-1). UV-vis absorption and fluorescence investigations of the nanohybrids in solution and thin films were recorded using a Varian CARY 50 spectrophotometer and a CARY Eclipse spectrometer, respectively. Morphological studies of dyesensitized ZnO nanorods were carried out by the high-resolution transmission electron microscope (HRTEM) JEOL 3010 (operating at an acceleration voltage of 300 kV), where samples are prepared by drop casting of diluted solution on a meshcoated carbon film. In the case of HRTEM analysis of thin films, the hybrid layers were transferred from the solar cells to meshcoated carbon film after dissolution of the PEDOT/PSS layer in deionized water. All PV characterizations were performed in air. I-V measurements were done in the dark and under 75 mW/cm2 of AM1.5 illumination using a 150 W xenon lamp. EQE measurements were measured under white-light halogen tungsten illumination combined with a Cornerstone monochromator using a calibrated Silicon solar cell as reference. The charge transfer properties of the blend films were investigated by microsecond-millisecond transient absorption spectroscopy using an experimental configuration described elsewhere.29 For this system, the excitation (nitrogen-pumped dye laser, repetition rate ∼4 Hz, pulse duration
