Fabrication and Characterization of Front-illuminated Titanium Oxide Nanotube-based Dye Sensitized Solar Cells (DSSC)
|關鍵字:||二氧化鈦奈米管;染料敏化太陽能電池;陽極處理;直流濺鍍;剝除轉移法;TiO2 nanotube;Dye-sensitized solar cell;Anodization;DC sputter;Detachment-transfer method|
|摘要:|| 自組裝二氧化鈦奈米管薄膜可應用在光催化、自我清潔、氣體感測器、鋰電池與染料敏化太陽能電池 ( Dye-sensitized solar cells, DSSC )。奈米管薄膜因為其具一維傳導路徑可提升電子傳遞速度，使電荷分離速率較高，電子電洞再結合機率下降。二氧化鈦奈米管薄膜直接在鈦板上進行陽極處理而得，其元件需以背面照光的方式運作，入射光會先經過塗佈觸媒層Pt的陰極而造成光線散射，再經過電解液的吸收，才能到達TiO2-Dye的工作區域。本實驗為了改善傳統背面照光二氧化鈦奈米管元件之缺點，將陽極的Ti板置換成FTO ( Fuorine-doped tin oxide ) 導電基板，成為正面照光式的二氧化鈦奈米管DSSC元件。
首先，我們以薄膜轉移法製備正面照光DSSC元件，透過定電壓60 V陽極處理12 h成長27 □m的二氧化鈦奈米管薄膜，經460 ℃熱處理1 h後，再以20 V低電壓作二次陽極處理4 h，將剝除下的薄膜黏著於TCO導電基板上，奈米管薄膜以底部朝上方式黏著(Bottom-up式)，並利用BCl3/Cl2進行高密度離子電漿蝕刻90秒開孔後，將薄膜浸泡在N719染料中 ( c = 3□10-4 M, acetonitrile : tert-butanol=1:1 ) 18 h，組成TNT-DSSC元件 ( TiO2 nanotube, TNT )，其與背面照光元件相比光電流從9.3 mA/cm2提高到12.9 mA/cm2，效率由4.5 %提升到了6.2 %；而與一般的Face-up式元件相比，效率可由4.7 %提升到6.2 %，由此可知薄膜底部之barrier layer會阻礙電子在此界面的傳遞。
此外，我們亦以濺鍍後陽極處理法製備正面照光元件，為維持濺鍍鈦薄膜之結晶形貌與高附著力，本實驗採分段式沉積法，使沉積原子有足夠時間進行表面擴散與重組，爾後進行40 V定電壓陽極處理，3.5 □m厚之鈦膜成長為4.65 □m透明二氧化鈦奈米管薄膜，與3.5 □m背照式元件相比，JSC由4.6 mA/cm2提升到9.2 mA/cm2，元件整體效能提升了73 %。|
Self-organized anodic titania nanotube (TNT) arrays have a great potential as materials for photocatalysis, self-cleaning, gas sensing, lithium batteries and dye-sensitized solar cells (DSSC). Although 1D nanotube arrays feature excellent charge collection efficiency and slow charge recombination due to their rapid electron transport, the TNT-DSSC made by anodization on Ti foil requires back-side illumination which significantly attenuated the incident light through passing the Pt-coated counter electrode and the electrolyte. To improve the cell performance from a traditional back-illuminated structure, we replaced Ti foil by ﬂuorine-doped tin oxide (FTO) conductive glass as a working electrode for TNT-DSSC. First, the detachment-transfer method was employed to fabricate transparent front-illuminated TNT-DSSC. The ordered TNT arrays with length of 27 □m were produced by first anodization at 60 V for 12 h. After annealed at 460 oC for 1 h, the as-prepared TNT arrays were detached by the second anodization at 20 V for 4 h. The detached TNT arrays were then transferred and attached onto a TCO substrate upside down. The bottom-side closed-end TNT arrays were opened by dry etching in a high-density plasma reactor under BCl3/Cl2 for 90 s. After immersion of the TNT film in a solution of N719 dye (c = 3□10-4 M) in acetonitrile/tert-butanol (1:1 v/v) for 18 h, the TNT-DSSC device was fabricated according to a standard procedure. The front-illuminated TNT-DSSC show a significantly improved cell performance compared to that of the back-illuminated counterpart (□ = 6.2 % vs. 4.5 %) owing to the much higher Jsc value of the former than the latter (12.9 vs. 9.3 mA cm-2). Furthermore, we found that the devices fabricated based on a normal face-up transfer procedure exhibit much poorer performance (□ = 4.7% vs. 6.2%), indicating that the barrier layer of the TNT arrays might hinder the electron transport across the interface. Second, anodization after sputtering a thin Ti film on FTO was also employed to fabricate transparent TNT-DSSC. To keep the grain shape in transition region (zone T) consisting of densely packed fibrous grains and enough adhesion with FTO. We use stepwise deposition to sputter the Ti layer on FTO substrare, because the deposited atoms on the substrate would have enough time to reorganize and diffuse. Then we used conventional anodization method (constant voltage at 40 V) to transform Ti film (3.5 □m) into transparent TiO2 nanotube arrays (4.65 □m). We found that JSC / mA cm-2 of the device increase from 4.6 mA cm-2 to 9.244 mA cm-2, giving the overall performance increasing by 73%.