新穎性奈米結構太陽能電池元件之製作與特性研究

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行政院國家科學委員會專題研究計畫成果報告

新穎性奈米結構太陽能電池元件之製作與特性研究研究成果報告(精簡版)

計畫類別：個別型

計畫編號： NSC 96-2221-E-011-083-

執行期間： 96 年 08 月 01 日至 97 年 07 月 31 日執行單位：國立臺灣科技大學化學工程系

計畫主持人：陳良益

計畫參與人員：碩士班研究生-兼任助理人員：吳書漢

處理方式：本計畫涉及專利或其他智慧財產權，1 年後可公開查詢

中華民國 97 年 10 月 30 日

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PREPAREING II-VI QUANTUM DOTS ATTACHED ZnO NANORODS AS PHOTOANODE FOR PHOTOVOLTIC DEVICES

Shu-Han Wu and Liang-Yih Chen

Department of Chemical Engineering, National Taiwan University of Science and Technology Taipei, TAIWAN

E-mail: sampras@mail.ntust.edu.tw

Abstract

In conventional Grätzel type solar cell, dye molecules and titanium oxides nanoparticles thin film were used as solar energy absorbed materials and photoanodes, respectively. However, nanoparticles dye sensitized solar cells (DSSCs) rely on the trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency. In addition to organic dyes, semiconductor quantum dots (QDs) which absorbed light in the visible spectrum are also good candidates to be a sensitizer of a DSSC cell. In this report, we prepared II-VI quantum dots (QDs) containing zinc oxide nanorods (ZnO NRs) as the wide band gap semiconductor photoanodes of photovoltaic devices. An array of ZnO nanorods were grown onto transparent and conducting indium-doped tin oxide (ITO) substrates by hydrothermal methods at low temperature. Before anchoring QDs, the surfaces of ZnO NRs were modified by bifunctional surfactant. The surface modification affects the QDs to link on the surfaces of ZnO NRs and the separation efficiency of electron and hole into two different phases.

Introduction

The photoanodes of dye-sensitized solar cell (DSSCs) are typically constructed using thick films of TiO₂, SnO₂ or ZnO nanoparticles that are deposited as a paste and sintered to produce electrical continuity.^[1-2] The nanoparticle film provides a large internal surface area for the anchoring of sufficient sensitizer to yield high light absorption in the range of 400-800nm, where much of the solar flux is incident. The nature of electron transport in nanoparticle film is fairly well understood. Time-resolved photocurrent and photovoltage measurement and modeling studies indicate that electron transport in nanoparticle network proceeds by a trap-limited diffusion process.^[3-4] Despite the extremely slow nature of such trap-mediated transport, electron diffusion lengths of 7-30 µm have been reported for cells operating at light intensities up to 0.1 Sun.^[3-5] One promising solution to this impasse is to increase the electron diffusion length in anode by replacing the nanoparticle film with an array of oriented single-crystalline nanorods. Electron transport in single crystalline is expected to be several orders of magnitude faster than percolation through a random polycrystalline network. Additionally, semiconductor quantum dots (QDs) can be used as solar energy absorber to replace dye molecules. QDs offer several significant advantages over dyes.^[6] QDs provide the ability to match the solar spectrum better because their absorption spectrum can be tuned with size. Recently, QDs have been shown that can generate multiple electron-hole pairs per photon, which could improve the efficiency of the solar cells^[7].

In this work, we reported the results of ZnO NRs array photoanodes sensitized with II-VI QDs.

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Experimental

ZnO nanorods have been grown on indium doped tin oxide (ITO) glass substrates. Before ZnO NRs synthesis, ZnO seed layer was deposited on ITO using physical deposition for 20 hours and annealed at 500°C for 30 minutes in air. The substrates covered with the seed layer were then placed in a 200mL aqueous growth solution containing 0.5 mM polyethyleneimine, 10 mM zinc acetate dehydrate and 10 mM hexamethylenetetramine and heated to 90°C to initiate nanorod growth. Because nanorods growth slowed after 3Hr, substrates were repeatedly introduced to fresh solution baths for long rods array (Total reaction times of up to 24 Hr). In order to link II-VI QDs onto the surfaces of ZnO NRs, 3-mercaptoproponic acid (3-MPA) and 11-mercaptoundecanoic acid (11- MUA) were used as linker to modify the surfaces of ZnO NRs. The ZnO NRs array substrates were immersed into a solution containing 3-MPA or 11-MUA tetrahydrofuran (THF) solution of 34 mM for 24 hours, then washing with THF and toluene. After linker molecules modification, the substrates were immersed in a CdSeS or CdS QDs toluene solution for another 24 hours. The morphology analysis of the nanorods was characterized by scanning electron microscopy (SEM, JEOL JSM 6500F) and transmission electron microscopy (Philips Tecnai F20, 200kV). The optical properties were analyzed by photoluminescence (PL) spectrophotometer (Hitachi F7000 spectroscopy).

Results and Discussion

Figure 1 (a)-(c) shows the SEM image of ZnO NRs. From Figure 1 (a)-(b), the diameter distribution of ZnO NRs was very uniform and the perfect hexagonal facets of ZnO NRs can be observed. From cross-sectional SEM analysis shown in Figure 1 (c), we know the length of ZnO NRs around 6µm grown vertically on ITO substrate.

Figure 2 (a) and (b) shows the PL spectra of 3-MPA capped ZnO NRs before and after sensitizing with CdSeS and CdS QDs, respectively. When the QDs were anchored onto the surfaces of ZnO nanorods, emission at 593 nm and 471nm can be observed due to the emission of CdSeS and CdS QDs, respectively. The emission peak positions are different to the ZnO emission peak around 387 nm. In addition, the PL spectra of CdSeS or CdSe QDs anchored onto the ZnO NRs are identical to that of QDs CdSeS and CdS QDs dispersed in toluene.

The detail carrier transport mechanism need to study more detail in the future.

Conclusion

In this work, QDs have successfully been anchored onto the surface of ZnO nanorods by using bifunctional surfactant, 3-MPA. However, the performance of QDs sensitized ZnO NRs solar cells still have poor performance compared with conventional dye-sensitized solar cell based on TiO2 nanoparticle thin film. How to increase the anchoring amount of QDs and prevent carrier recombination in the interfaces is our target in the future..

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Figures and captions

Figure 1 SEM image of ZnO nanorods (a) tilt 45^o view ;(b) high magnification of (a); (c) cross-sectional Image of a cleaved nanorods array on ITO substrate.

Figure 2 (a) (From Top to down) PL spectra of CdSeS QDs attached ZnO nanorods, as-grown ZnO nanorods, and CdSeS QDs dispersed on toluene solution, respectively; (b) (From Top to down) PL spectra of CdSe QDs attached ZnO nanorods, as-grown ZnO nanorods, and CdSe QDs dispersed on toluene solution, respectively.

REFERENCES

[1] Nick Vlachopoulo, Paul Liska, Jan Augustynski and Michael Grätzel, J, Am. Chem. Soc. 110, 1216 (1988);

M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos and M.

Grätzel, J. Am. Chem. Soc. 113, 6382 (1993).

[2] Qifeng Zhang, Tammy P. Chou, Bryan Russo, Samson A. Jenekhe, and Guozhong Cao, Adv. Func. Mater. 18, 1654 (2008); T. P. Chou, Q. F. Zhang, G. E. Fryxell, G. Z. Cao, Adv. Mater. 19, 2588 (2007).

[3] A. C. Fisher, L. M. Peter, E. A. Phonomarev, A. B. Walker, K. G. U.Wijayantha, J. Phys. Chem. B, 104, 949 (2000).

[4] T. Oekermann, D. Zhang, T. Yoshida, H. Minoura, J. Phem. Chem. B, 108, 2227 (2004).

[5] S. Nakade et al. J. Phys. Chem. B, 106, 10004 (2002).

[6]A. J. Nozik, Phys. E, 14, 115 (2002).

[7] R. D. Schaller, V. I. Klimov, Phy. Rev. Lett. 92, 186601 (2004).