二氧化鈦奈米結構染料敏化太陽能電池之研製

Characterization of TiO2 Nanostructured Dye-sensitized Solar Cells

蘇亞洛(Alagesan Subramanian)

指導教授 ： 王宏文

中原大學/理學院/化學研究所/博士(2013年)

https://doi.org/10.6840/CYCU.2013.00372

摘要

本論文首先研究氫氧基附著在二氧化鈦光電極上對染料敏化太陽能電池效能的影響。
二氧化鈦光電極是由商用的二氧化鈦奈米粉末所製作 (Degussa P25)。氫氧基是利用雙氧水(H2O2)在熱處理時附著上去的。利用FTIR 光譜，可以確定氫氧基確實附著到二氧化鈦光電極。利用熱分析可以檢驗氫氧基在二氧化鈦光電極上的百分比。利用太陽光模擬器AM1.5，100mW/cm2 可測得經氫氧基處理的染敏電池效率。氫氧基處理過的染敏電池顯示較佳的光電轉換效率。可能原因是較佳的染料吸附率及較長的電子壽命。利用UV-vis 及EIS (電化學阻抗儀)，我們確定了這樣的變化。
第二，將摻雜硼的二氧化鈦奈米管陣列(B-TNA)做為染料敏化太陽能電池的光陽極，並與未摻雜硼之二氧化鈦奈米管陣列(TNA) 做比較。利用SEM 、XRD 以及UV-vis spectroscopy 進行材料的檢測以及推估其光轉換效率。而TNA 以及B-TNA 的化學組成則是利用energy dispersive X-ray spectroscopy 進行鑑定。XRD 的分析結果證實了TNA以及B-TNA 的主要相結構為銳鈦鑛，同時也證明了在B-TNA 試片中，硼確實有置入到TiO2的晶格結構中。藉由UV-vis spectra，可推算出二氧化鈦奈米管在摻雜硼之後其能隙變窄。光伏打特性則是利用電壓-電流計在AM 1.5 模擬光源的照射下進行量測。B-TNA 在光電流密度的表現上有明顯提升，其值為7.85 ± 0.20 mA/cm2 ，整體的轉換效率(η)為3.44 ± 0.10%。效率的改善可歸因於TiO2 導帶以及染料分子的LUMO 有完好的匹配，加速了電子的注入速率以及抑制電荷的再結合。定且由電化學阻抗光譜圖(EIS)得知，相較於未摻雜硼的TNA，B-TNA 具有較長的電子生命週期，以及較低的TiO2/dye/electrolyte介面阻抗。
另外，本篇也研究針對不同的粒徑的TiO2 奈米粒子，進行Li+離子的置入，並探討其對DSSC 光電轉換效率的影響。將已摻雜了Li+離子之不同粒徑的TiO2 奈米粒子，製做成薄膜電極，並以XRD 以及電化學阻抗光譜進行量測工作。XRD 的結果顯示其主要相結構為銳鈦鑛。在XRD 的分析結果中，觀察到Li+離子會同時置入到TiO2 奈米粒子的表面以及內部。當奈米粒子的粒徑較大時，其Li+離子主要是置入到粒子的內部；當粒徑較小時時，其Li+離子則會被侷限在粒子的表面。光伏打特性是利用電壓-電流計在AM 1.5 模擬光源的照射下進行量測。量測結果顯示，當Li+離子於較小粒徑的奈米粒子進行置入，其光電轉換效率會得到提升；而當Li+離子於較大粒徑的奈米粒子進行置入，其光電轉換效率則是會降低。當Li+離子摻入進22 nm 的奈米粒子(P25)時，其光電轉換效率減少了將近40%；摻入進19 nm 的奈米粒子(P90)時，轉換效率只有些微的降低；而當Li+離子摻入進6 nm的奈米粒子時，其轉換效率則得到了提升。Li+離子摻雜後所造成的這個現象，可以以電子陷獲以及電荷再結合來解釋。
更進一步的，本篇論文還比較了三種不同結構的光電極。第一種為單層的TiO2 結構，其TiO2 為自製的奈米粉體；第二種為雙層TiO2 結構，第一層為自製TiO2粉體，第二層則是由TNA 所構成； 第三種為具有階層性的多層結構光電極， 其組成為自製TiO2/TNA/P25。利用SEM 以及XRD 分析這些光電極的表面形貌以及相組成比例。在這些光電極中，以多層結構具有較好的轉換效率。其原因為是具有較高的染料吸附量、部分光散射所造成的高入射光子轉換電流率，以及電極結構優化後導致良好的電荷傳輸。利用UVvis進行檢測，證實了多層結構確實有較高的染料吸附量。EIS 則是分析這些電極中電荷的傳輸動力。在EIS 的分析中，優化後的多層結構電極具有大的電子注入速率、抑制再結合速率以及較長的電子生命週期。IPCE 的量測也證明了多層結構的光電極具有較高的光補獲率。
最後，在奈米粒子 (NP)及奈米管柱 (NT)這兩種不同電極結構的基礎上，改變其銳鈦鑛及金紅石相的組成比例，並比較其光伏打特性的不同。利用SEM 分析其結構形貌，並以XRD 繞射圖譜計算其相組成比例。在NP 結構的光電極中，以銳鈦鑛相樣品具有較好的表現，其短路電流密度為10.60 mA/cm2，整體轉換效率 (η)為 5.09 %。而金紅石相樣品的轉換效率則為3.98 %，降低了將近20%。在EIS 的分析中可發現，當電極結構為奈米粒子且組成相為銳鈦鑛時，由於高的染料吸附量以及長的電子生命週期，所以有高的電子注入速率。而當電池結構為奈米管柱時，其金紅石相樣品的轉換效率為 3.50 %，相較於銳鈦礦相樣品( 3.35% )，其效率反而有些微的提升。EIS 中的分析顯示，金紅石相電極的電子注入速率有些微的增加，以及低的電子生命週期。且在NP 及NT 結構中，金紅石相電極皆具有較高的Voc 以及以及電子擴散係數。這樣的結果主要是由於鍛燒溫度的關係，產生較少的再結合中心及補獲位置。這個捕捉態現象可由光致螢光光譜進行驗證。IPCE光譜則是用來確認DSSC 的Jsc 值。

關鍵字

硼 ； 鋰摻雜 ； 烃基 ； 二氧化鈦 ； 染料敏化太陽能電池 ； 氧化鈦奈米管柱 ； 氧化鈦奈米粒子 ； 階層結構 ； 多層結構

並列摘要

At first, the effect of hydroxyl group attachment on the nanocrystalline TiO2 photoelectrodes for the performance of dye-sensitized solar cells (DSSCs) was investigated. The photoelectrodes were prepared using commercial TiO2 nanoparticles (Degussa P25). Hydroxyl groups were attached to the TiO2 film using hydrogen peroxide by thermal treatment method and the OH-attached sample was compared with the untreated one. The FTIR spectra evidenced the presence of hydroxyl groups attached to the TiO2 nanoparticles. Thermogravimetric analysis showed that the sample treated with hydrogen peroxide was attached with higher weight percentage of hydroxyl groups. The photovoltaic characteristics of the as-prepared DSSCs were measured by an electrochemical analyzer under the standard AM 1.5 illumination of 100 mW/cm2 light source. The hydroxyl groups attached sample showed an enhanced performance of DSSC than that of the blank P25 film. It shows that higher amount of dye was adsorbed due to the surface hydroxyl groups on the H2O2-treated samples. Electrochemical impedance spectroscopy (EIS) measurement indicated that the electron lifetime for the H2O2-treated sample was longer than that of the untreated sample. The higher dye loading due to the attached hydroxyl groups on the sample was confirmed using UV-vis measurement.
Secondly, titanium nanotubes doped with boron (B-TNA) used as the photoelectrode for dye-sensitized solar cells was investigated and compared with the undoped (TNA) one. The materials were characterized by SEM, XRD, and UV-vis spectroscopy and their photoconversion efficiencies were evaluated. The chemical compositions of TNA and B-TNA were identified by the energy dispersive X-ray spectroscopy. XRD evidenced the presence of anatase as the main phase and presented the existence of boron elements at interstitial sites between the TiO2 lattices.
The UV-vis spectra indicated the narrowing of band gap upon doping boron into titanium nanotubes. The photovoltaic properties were measured by a current-voltage meter under AM1.5 simulated light radiation. The boron-doped TiO2 nanotube arrays showed an enhanced performance with a photocurrent density of 7.85 ± 0.20 mA/cm2 and an overall conversion efficienciey(η) of 3.44 ± 0.10%. The enhanced performance was attributed to the enhanced electron injection rate and retardation of the charge recombination, which could be due to perfect matching between the LUMO of dye molecules and the conduction band of TiO2.
Electrochemical impedance spectroscopy (EIS) measurement indicated the longer electron lifetime and reduced TiO2/dye/electrolyte interface resistance for boron doped TiO2 nanotubes than that of undoped TiO2 nanotubes.
Also, the effect of Li+ insertion into different sized TiO2 nanoparticles and their influences on the photoconversion efficiency of dye-sensitized solar cells (DSSC) were investigated. TiO2 nanoparticles with different particle sizes (22nm, 14nm and 6nm) doped with Li+ were employed to form thin film electrodes and their properties were characterized by X-ray diffraction (XRD) and electrochemical impedance spectroscopy analysis. XRD evidenced the presence of anatase as the main phase. From the XRD analysis, it was observed that the Li+ ions could be inserted into both the surface and bulk of the TiO2 nanoparticles. In the larger particle size, the Li+ ions are inserted into the bulk anatase where as Li+ ions bounded on the TiO2 surface for the smaller crystallite size. The photovoltaic properties were measured by a currentvoltage meter under AM1.5 simulated light radiation. It exhibited that the overall photoconversion efficiency of DSSC was decreased in the larger particles while it was enhanced in the smaller nanoparticles when Li+ was doped into the TiO2 nanoparticles. A nearly 40% decrease in the efficiency (η) of DSSC was observed upon intercalation of Li+ ions into 22 nm sized TiO2 nanoparticles (P25). The 14 nm sized TiO2 nanoparticles (P90) showed slightly less efficiency (η) upon Li+ doping than that of the undoped sample. However, the smallest sized TiO2 nanoparticles (6 nm) showed higher efficiency than that of the undoped one. This phenomenon is explained based on electron trapping and charge recombination due to lithium doping.
Furthermore, a single layer nanocrystalline film electrode and bi-layered structure of TiO2 electrode were prepared and compared with the hierarchal multi-layer structured photoelectrode. SEM and XRD were used to analyze the morphology and crystal phases of these electrodes, respectively. The multilayer-structured photoelectrode showed a superior performance compare to that of the other two electrodes. The enhancement was attributed to higher amount of dye adsorption, large incident photon to current conversion yield due to greater fraction of light scattering and the good charge transportation due to the optimization of the TiO2 electrode structure. The UV-vis measurement confirmed the higher dye loading capacity of the multilayered structure. EIS was used to analyze the charge transport kinetic in these electrodes.
The EIS analysis showed large electron injection rate, the suppression of recombination rate and longer electron lifetime in this electrode due to the optimized structure. The IPCE measurement confirmed the greater light harvesting efficiency of the multilayer structured photoelectrode.
To the end, the photovoltaic properties of the anatase and rutile phases using nanoparticles (NP) and nanotube (NT) based photoelectrodes were compared. The surface morphology was analyzed by the SEM. The quantity of phases present in the NP and NT based electrode was calculated from the XRD patterns. The NP based anatase sample presents a highest performance, compare to NP based rutile electrode, with a short-circuit current density (Jsc) of
10.60 mA/cm2 and an overall conversion efficiency (η) of 5.09 %. A nearly 20 % decrease in the DSSC efficiency (3.98 %) was observed for the NP based rutile electrode. From the EIS analysis, it was found that NP based anatase electrode has high electron injection rate due to high dye loading and large electron lifetime. In the case of the NT based cells, rutile sample showed a slight enhancement (3.50 %) in the conversion efficiency compare to that of the anatse cell (3.35%). EIS analysis showed that a slight increase electron injection rate and low electron lifetime for the rutile phase electrode. The Voc value and Deff value were higher for both the rutile NP and NT based electrodes. This was attributed to less number of recombination center/trap sites due to the annealing temperature. The trap state phenomena was confirmed by photoluminescence study.
The IPCE spectra were used to confirm the obtained Jsc value in the DSSC.

並列關鍵字

Multilayer structure ； Dye-sensitized solar cell ； Titania nanotubes ； Hierarchical structure ； TiO2 ； Titania nanoparticles ； Hydroxyl ； Lithium doping ； Boron

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