摘要
本研究以室溫化學浴法將複合層氧化鋅奈米結構建構於ITO-PET導電塑膠基板，應用於可撓式染料敏化太陽能電池之光陽極材料。以滴鍍法將粒徑約20 nm之氧化鋅顆粒沉積於可撓式導電基板上為模板，利用室溫化學浴法於其表面成長厚度約5 nm之氧化鋅殼層結構。此全室溫條件建構之氧化鋅奈米殼層/顆粒結構，以D149敏化後之光陽極製作之可撓式染料敏化太陽能電池，在未經高溫燒結與機械高壓處理下，可達到4.11%之效率。藉由研究太陽能電池電子傳輸與再結合之特性，確認此氧化鋅殼層結構提供電子快速傳導之通道，故可大幅提升氧化鋅結構之電子收集效率。為了進一步提高光陽極結構於長波長光之光電流表現，本研究塗佈氧化鋅光散射顆粒於奈米殼層/顆粒結構上，並再次進行室溫化學浴處理，使氧化鋅光陽極結構之染料吸附量上升。同時發揮散射顆粒之特點，促進長波長光之收集，使電流提升。以此氧化鋅光散射顆粒/奈米殼層/顆粒複合層結構為光陽極，製作之可撓式染料敏化太陽能電池，其光電流值與氧化鋅奈米殼層/顆粒結構電池相比，提高近25%，效率值也提升30%，達5.16%之高效率表現。
此外，為了提升可撓式染料敏化太陽能電池於近紅外光之光收集效率，本研究亦使用紫質染料YD2-oC8敏化氧化鋅奈米結構之光陽極。觀察太陽能電池之光電轉換效率量測，可發現YD2-oC8染料能將650 ~ 700 nm波長之光電轉換效率提升。但由於具有高分子量之YD2-oC8容易於氧化鋅表面產生堆疊，導致染料分子於其上之吸附量下降。因此由氧化鋅奈米殼層/顆粒結構光陽極與氧化鋅光散射顆粒/奈米殼層/顆粒結構所建構之可撓式染料敏化太陽能電池，轉換效率僅分別達到1.89%與2.04%。故於紫質染料溶液之成份調控，仍有很大的改善空間。

Room Temperature Construction of Hierarchical ZnO Nanostructured Anodes
for Flexible Dye-sensitized Solar Cells

Shuo-Yen Lin
Jih-Jen Wu

Department of Chemical Engineering, National Cheng Kung University

SUMMARY

A notable efficiency of 5.16% is achieved in the flexible dye-sensitized solar cell (DSSC) using the ZnO particle-templated nanostructured anode facilely fabricated on the indium tin oxide (ITO) coated-polyethylene terephthalate (PET) substrate. Two ZnO particle layers are simply drop-cast on the ITO-PET substrate to respectively serve as the templates for the room-temperature (RT) chemical bath depositions of main-matrix and light-scattering portions of the flexible ZnO nanostructured anode. Dynamics of electron transport and recombination measurements indicate that an efficient electron collection is performed in the ZnO nanostructured anode fabricated free of high-temperature treatment and mechanical compression.

Key words: Room-Temperature Chemical Bath Deposition, ZnO Nanostructures Template, Plastic Substrates, Flexible Dye-Sensitized Solar Cells

INTRODUCTION

Dye-sensitized solar cells (DSSCs) fabricated on flexible and light-weight conducting plastic substrates have attracted considerable interests because they are one of the promising renewable and mobile power sources for portable electronic devices. The high-temperature annealed TiO2 nanoparticles (NPs) films on the transparent conducting oxide (TCO) coated glasses are typically employed to be anodes in the rigid DSSCs. An efficient electron transport pathway through the sintered NP film to TCO is curcial for achieving an efficient DSSC. Therefore, a cirtical challenge of the low-temperature preparation of TiO2 anodes on the conducting plastic substrates restricts the plastic-substrate DSSCs to the relative low conversion efficiencies.

With a similar energy band gap, ZnO has been employed to be an alternative of TiO2 for DSSC anodes. However, the efficienies of the ZnO DSSCs are always inferior to those of TiO2 DSSCs, which is mainly attributed to no efficient dye specially designed for ZnO anodes. Nevertheless, with the superior properties of high electron mobility, anisotropic growth behavior, and low crystallization temperature, ZnO is a very promising material to be the flexible DSSC anode.

In this work, a facile route is developed to fabricate high-performance flexible ZnO DSSC anodes. A ZnO particle template is drop-cast on the ITO-PET substrate, instead of the time-consuming grown NW-array template, followed by the RT growth of ZnO nanostructures to construct the flexible ZnO nanostructred anode. A notable efficiency of 5.16% is attained in the flexible DSSC with the RT-grown ZnO particle-templated nanostructured anode on the ITO-PET substrate.

MATERIALS AND METHODS

The ZnO nanostructured anodes were grown on the particle-templated ITO-PET substrates using RT CBD. A blocking layer composed of ZnO NPs with a diameter of ~5 nm was spin-coated on the ITO-PET substrate. The particle template composed of two ZnO particle layers was then drop cast on the blocking layer. The first ZnO NP layer with the particle size of 20 nm (Alfa Aesar, 99%) was prepared by the drop casting of a butanolic solution of ZnO NPs (0.015g/3ml) on the blocking layer. Another ZnO particle film was further drop cast on the ZnO NP layer using an ethanolic solution of ZnO particles (0.1g/10ml) with sizes of 200-500 nm. The ZnO particles were hydrothermally synthesized using an aqueous solution of zinc acetate and NaOH. The ZnO particle templates are heated at 50 oC for 2hr for solvent removal. ZnO nanostructures were constructed on the template by immersing the templated ITO-PET substrate into the stirred aqueous solution of 0.062 M zinc acetate and 0.5 M NaOH at RT for 3.5 min.

Dye adsorption was carried out by immersing the anode in a 0.5 mM acetonitrile/t-butanol (1:1) solution of D149 at RT for 1 h. The sensitized electrode and platinized ITO/PET counter electrode were sandwiched together with 25-m-thick hot-melt spacers (SX 1170-25, Solaronix SA). Liquid electrolyte solutions composed of 0.5 M tetrapropylammonium iodide (TPAI) and 50 mM I2 in a 1:4 volume ratio of ethylene carbonate and acetonitrile was employed for the D149-sensitized DSSCs. The cells are fully sealed with cyanoacrylate glue. A mask on the PET substrate side of photoanode was used to create an exposed area of 0.16 cm2 for all cells.

RESULTS AND DISCUSSION

Flexible DSSCs are fabricated using the ZnO NP, ZnO RT-M, and ZnO RT-MS anodes constructed on ITO-PET substrates. Photovoltaic performances of these flexible D149-sensitized DSSCs are shown in Figure 1a. For comparison, the photocurrent density (J)-voltage (V) characteristic of the DSSC with ZnO RT-M anode on ITO-glass (named ZnO RT-M-g DSSC hereafter) is also demonstrated in the figure. The photovoltaic properties of these DSSCs are listed in Table 1. It shows that the ZnO NP DSSC, in which the NPs in the anode are not well-connected, possesses an efficiency of 1.84%. With the RT-grown ZnO nanostructures on the NP template, the performance of the flexible ZnO RT-M DSSC is significantly superior to that of the ZnO NP DSSC. More than 2-fold enhancements of the short-circuit photocurrent density (Jsc) and therefore the efficiency are attained. An efficiency of 4.11% is measured in the flexible DSSC with a 5-m-thick D149-sensitized ZnO RT-M anode. Moreover, as shown in Figure 1a and Table 1, the efficiency of the flexible ZnO RT-M DSSC is comparable to that of the ZnO RT-M-g DSSC. As the ZnO NP templates for the RT growth of the ZnO nanostructured anodes are fabricated on ITO substrates at a temperature of 50oC, similar photovoltaic performances can therefore be attained in both flexible and rigid DSSCs without the high-temperature post-treatment and mechanical compression.

With the additional light-scattering portion in the anode, the flexible ZnO RT-MS DSSC shows a 25% enrichment of Jsc compared to the ZnO RT-M DSSC. A remarkable efficiency of 5.16% is achieved in the flexible ZnO RT-MS DSSC with the anode fabricated on plastic substrate without mechanical compression, as shown in Figure 1a and Table 1. Without the time-consuming work, the high-performance RT-grown ZnO DSSC anodes have been realized on the particle templates which are facilely constructed on ITO-PET substrates by the drop casting method. The results demonstrate the outperformed characteristics of the RT-grown ZnO nanostructured anodes on the templated ITO-PET substrates for use in flexible DSSCs.

Figure 1b shows the IPCE spectra of the flexible ZnO NP, ZnO RT-M, and ZnO RT-MS DSSCs. Compared to the ZnO NP DSSC, the IPCE values are significantly increased in the ZnO RT-M DSSC. It is ascribed to the formation of the RT-grown nanostructured anode with the suitable surface for dye adsorption and superior electron transport properties (the detail of which will be discussed later). The increase of the light scattering ability due to the development of aggregate morphology after RTCBD also contributes to the enhancement of the IPCE values of the ZnO RT-M DSSC. Further increases of the IPCE values in the wavelength range of 480-700 nm are measured in the ZnO RT-MS DSSC although the light harvesting of the flexible DSSC is restricted by the ITO-PET substrate with transmittances of 70-85%, as shown in Figure 1b. Moreover, significant enrichments of the IPCE values in the long wavelength range are observed in the ZnO RT-MS DSSC, which is attributed to the addition of the light scattering portion in the flexible ZnO DSSC anode.

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