複合薄膜對電極與擬固態電解質於染料敏化太陽能電池之研究

On the Composite Counter Electrodes and Quasi-Solid-State Electrolytes for Dye-Sensitized Solar Cells

黃冠傑(Kuan-Chieh Huang)

指導教授 ： 何國川

國立臺灣大學/工學院/化學工程學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.00389

摘要

近年太陽能為綠色能源科技主要研究範疇之一，其中新型態太陽能電池：染料敏化太陽能電池具備製程簡易、可撓曲特性與豐富色彩美觀之優勢，並可提供相當潛力之光致電轉換效率，因此成為世界各地學者極力開發重點。本論文主要探討各種奈米合成材料應用於染料敏化太陽能電池中之對電極與電解質部分，並以 (1) 提升電池元件之光致電轉換效率 (第三章)、(2) 降低元件之製作成本 (第四章與第五章)，以及 (3) 增加元件之長期穩定性 (第六章與第七章) 為研究目標。
在提升電池元件之光致電轉換效率方面 (第三章)，利用白金奈米粒子搭配多壁奈米碳管製備出複合薄膜，以旋轉塗佈方式將其沉積於導電玻璃基材上，作為電池之對電極，製程中，使用合成高分子 poly(oxyethylene)-segmented imide (POEM) 作為多壁奈米碳管在水溶液中之良好穩定分散劑。對此複合薄膜進行 390 oC 退火處理後，非導電性 POEM 的裂解、白金前驅鹽完全轉換成適當尺寸的奈米白金粒子，以及薄膜表面的粗糙度，使薄膜擁有還原 tri-iodide (I3-) 離子之最佳催化能力，進而幫助電池使用此製備條件下之對電極提供 8.47 ± 0.21% 光致電轉換效率 (一個太陽光照射下)，此值遠高於傳統使用純白金作為電池之對電極薄膜產生的效率 (7.41 ± 0.24%)。
在降低電池元件之製作成本方面，第一部分 (第四章)，以電聚合方式將奈米石墨球/導電高分子聚苯胺形成複合薄膜在導電玻璃基材上，取代傳統貴金屬白金作為電池之對電極，經由冷凝回流處理過之奈米石墨球/苯胺單體，具有在電聚合鍍液中較好之分散能力，以利於其被聚合至導電玻璃基材上，相較於單純聚苯胺薄膜，奈米石墨球均勻分布在複合薄膜中增加了複合薄膜之導電性，因而給予電池搭配奈米石墨球/聚苯胺對電極相對於電池使用白金對電極之效率的 98.3%，此複合薄膜具有與白金薄膜媲美之催化能力也以掃描式電化學顯微鏡證實。第二部分 (第五章)，同樣使用冷凝回流搭配電聚合方法，將聚苯胺中空球成膜在導電玻璃基材上，作為電池之對電極，藉由旋轉盤電極分析結果證實，相較於單純聚苯胺薄膜擁有 0.126 cm2 之活性表面積，薄膜包含聚苯胺中空球提供更大之表面積值 (0.191 cm2)，增加還原 I3- 離子的機會，同時給予電池搭配聚苯胺中空球對電極相對於電池使用白金對電極之效率的 95.4%，故此薄膜也可成為替換白金當作對電極催化層之潛力候選。
在增加電池元件之長期穩定性方面，第六章首先以高分子 poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) 對有機液態電解質進行膠化作用，形成擬固態電解質應用於電池元件，再加入熱穩定性高之奈米粒子氮化鋁於此電解質中，造成 PVDF-HFP 之結晶性下降，使電解質中之 iodide (I-) 離子之擴散係數由 2.97 × 10-6 cm2 s-1 增加至 3.52 × 10-6 cm2 s-1，因而在一個太陽光照射下，使電池搭配此擬固態電解質產生 5.27 ± 0.23% 之光致電轉換效率，優於無添加氮化鋁電解質之電池效率 (4.75 ± 0.08%)，在一千小時量測下，電池僅有初效率的 5% 損失。另一方面，第七章以完全無有機液體之非揮發性離子液體 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) 為電解質主體，置入新合成之多壁奈米碳管/皇冠醚複合物於此電解質中，研究結果先指出皇冠醚 15-crown-5 之洞穴尺寸比 12-crown-4 與 18-crown-6 對電解質成分中的 lithium (Li+) 離子有較佳之捕獲能力，因此添加多壁奈米碳管/15-crown-5 複合物於 EMIBF4 中，複合物的 15-crown-5 削弱 Li+ 與 I- 之間的靜電吸引力，提高氧化還原對 I- 與 I3- 的 exchange reaction，而多壁奈米碳管則有助於電子傳輸，相較於電池使用單純 EMIBF4 電解質，電池採用 EMIBF4 電解質含有此複合物之短路電流與光致電轉換效率分別提升了 71.2% 與 38.8%，並在一千兩百小時量測下，具有穩定不遞減之電池效率表現。

關鍵字

染料敏化太陽能電池 ； 對電極 ； 掃描式電化學顯微鏡 ； 旋轉盤電極 ； 擬固態電解質 ； 離子液體

並列摘要

The exploration of solar energy is one of most intensive studies on the technologies of green energy in recent years. A new generation of solar cell, dye-sensitized solar cell (DSSC), has been investigated worldwide due to the advantages, including facile development of cell, flexibility, and colorful appearance. In terms of (1) improving the power-conversion efficiency (η) of cell (Chapter 3), (2) reducing the consumption of fabrication of cell (Chapter 4 and Chapter 5), and (3) enhancing the long-term durability of cell (Chapter 6 and Chapter 7), various nanomaterials have been synthesized and prepared for the counter electrodes (CEs) and the electrolytes of the DSSCs in this dissertation.
For improving the η of DSSC, a conducting glass substrate spin-coated with a composite thin film, consisting of platinum nanoparticles (PtNPs) and multi-wall carbon nanotubes (MWCNTs), is prepared for a CE of a DSSC (Chapter 3). A homemade polymer, poly(oxyethylene)-segmented imide (POEM), is used and served as stabilizer and dispersant for MWCNTs in the aqueous solution. The best electro-catalytic ability of the film for the reduction of tri-iodide (I3-) ions is obtained after the film is annealed at 390 oC. This is attributed to the complete decomposition of non-conducting POEM, to the formation of PtNP with a moderate crystalline size, and to the surface roughness of film. Thus, an η of 8.47 ± 0.21% of the DSSC with the CE based on the composite film shows much higher than that of a DSSC applying a Pt-coated CE (η = 7.41 ± 0.24%) under illumination of 100 mW cm-2.
For reducing the consumption of fabrication of DSSC, an electro-polymerized composite thin film consisting of nanographites (NGs) and polyaniline (PANI) on the conducting glass substrate is prepared for the replacement of costly Pt as the CE of a DSSC (Chapter 4). A well dispersing ability of NG/aniline particles in the depositing solution is obtained after the particles are treated by reflux-condensation; thereby improving the depositing quality of NG/PANI composite film on the substrate by electro-polymerization. The fine distribution of NGs in the film results in an enhanced conductivity of it, with reference to the conductivity of pristine PANI film. Thus, a percentage of 98.3%, corresponding to the η of a DSSC made of Pt-coated CE, for the DSSC with the NG/PANI CE is acquired. The competitive electro-catalytic ability of the film in comparison to that of film of Pt is confirmed by scanning electrochemical microscopy. In addition, hollow spherical PANI (hsPANI) particles are also deposited on the conducting glass substrate by means of reflux-condensation and electro-polymerization for the CE of a DSSC (Chapter 5). A larger active surface area (A) of thin film consisting of hsPANI particles is estimated to be 0.191 cm2, with reference to that of pristine PANI film (A = 0.126 cm2) by rotating disk electrode. The increased A is beneficial for the reduction of I3-. Thus, a percentage of 95.4%, corresponding to the η of a DSSC with a Pt-coated CE, for the DSSC made of hsPANI CE is obtained. The film consisting of hsPANI particles can serve as a potential alternative for the replacement of Pt catalyst on the CE.
For enhancing the long-term durability of DSSC, a liquid electrolyte based on organic solvent is converted into a quasi-solid-state electrolyte by the gelation using a polymer, poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP), for a DSSC (Chapter 6). The crystallinity of PVDF-HFP decreases when high thermal stable nanoparticles of aluminum nitride (AlN) are incorporated in the quasi-solid-state electrolyte. Thus, the diffusion coefficient of iodide (I-) is increased from 2.97 × 10-6 to 3.52 × 10-6 cm2 s-1. Under 1 sun illumination, the η of a DSSC with this quasi-solid-state electrolyte gives a higher value of 5.27 ± 0.23%, compared to that of a DSSC without adding AlN in its electrolyte (η = 4.75 ± 0.08%). Merely a loss of 5% in η of the DSSC with reference to its initial η is observed for the at-rest durability of the quasi-solid-state DSSC in a period of 1,000 h. In addition, a solvent-free ionic liquid (IL)-based electrolyte containing a synthesized composite of MWCNT/crown ether is prepared for a quasi-solid-state DSSC (Chapter 7). An IL, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), is used. Prior to the characterization of performance for the DSSC with this IL electrolyte, crown ether, 15-crown-5, is preferably chosen owning to its appropriate size of cavity for capturing the lithium (Li+) in the electrolyte in comparison to the capturing abilities of 12-crown-4 and 18-crown-6. Thus, the decrease in electrostatic force between Li+ and I- leads to an improvement of the exchange reaction of I- and I3- by adding the MWCNT/15-crown-5 composite in the EMIBF4 electrolyte. The transport of electrons is facilitated by MWCNTs. Consequently, the values of short-circuit current density and η of the DSSC with both MWCNT/15-crown-5 and EMIBF4 in its electrolyte exhibits increases by 71.2 and 38.8%, respectively, with reference to these values of a DSSC with a bare EMIBF4. The at-rest durability of this quasi-solid-state DSSC is found to be unfailing for a period of 1,200 h.

並列關鍵字

Counter electrode ； Dye-sensitized solar cell ； Ionic liquid ； Quasi-solid-state electrolyte ； Rotating disk electrode ； Scanning electrochemical microscopy

參考文獻

[1] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338-344.

[2] W. G. Adams, R. E. Day, The action of light on selenium, Philosophical Transactions of the Royal Society of London 167 (1877) 313-349.

[3] C. E. Fritts, On a new form of selenium photocell, Procedures American Association for the Advancement of Science 33 (1883) 97.

[4] D. M. Chapin, C. S. Fuller, G. L. Pearson, A new silicon p-n junction photocell for converting solar radiation into electrical power, Journal of Applied Physics 25 (1954) 676-677.

[5] A. W. Blakers, M. A. Green, 20% efficiency silicon solar cells, Applied Physics Letters 48 (1986) 215-217.