Student: Yu-Ling Guo Advisor: Yu-Chun Wu Date: 13/06/2017

Presentation on theme: "Student: Yu-Ling Guo Advisor: Yu-Chun Wu Date: 13/06/2017"— Presentation transcript:

1 Student: Yu-Ling Guo Advisor: Yu-Chun Wu Date: 13/06/2017
Flexible synthesis of anatase TiO2 nanocrystallites for dye-sensitized solar cells applied at regular sunlight and room light conditions Student: Yu-Ling Guo Advisor: Yu-Chun Wu Date: 13/06/2017 Good afternoon everyone, thank you for your interest in my presentation. (先謝謝他們的聆聽) I am Yu-Ling Guo a master student from national cheng kung university in Taiwan.(講一下是學生跟台灣) Today, I am going to talk about “Flexible synthesis of anatase TiO2 nanocrystallites for dye-sensitized solar cells applied at regular sunlight and room light conditions ” Department of Resource Engineering National Cheng Kung University MOST M MOST E

2 Outline Introduction Experimental Section Results and Discussion
Under sun light Under room light This is my outline, the introduction, Experimental Section and Results and Discussion. Moreover, the photovoltaic performance under sun light and room light condition will be discussed. From here, we use 200 lux as the room light condition.

3 Dye-ensitized Solar Cells (DSSCs)
First of all, (加個簡單的起頭) Because of low cost and simple fabrication, DSSC are attracting academic and commercial interest. The main material for working electrode of DSSC is TiO2 nanoparticles, which was for dye loading, electron transport and light transmittance. Therefore, in our study, we want to develop a simple and time-effective process to fabricate the TiO2 photoanode for the applications ofDSSC under sun light and room light conditions. 染敏電池至今仍備受矚目的原因是因為其組裝較容易，且成本較低， 在DSSC中二氧化鈦扮演重要的角色，其中包含染料的吸附、電子傳遞以及光穿透的途徑。 因此本研究主要開發快速且簡易的製程，製備出適用於不同光強下的二氧化鈦光電極。 DSSC的working: A dye molecule self-assembled on the surface of mesoporous TiO2 photoanode absorbs a photon of the incident light, and the resulting excited electron on the lowest unoccupied molecular orbital (LUMO) is injected rapidly into the conduction band of the TiO2 and subsequently diffuses to the TCO glass. Oxidized dye molecules are then reduced by I− in the electrolyte, and I− is oxidized to form I3 electrode, the reverse reaction takes place to reduce I3−. At the Pt − back toI− to complete the entire cycle. TiO2 plays an important role in DSSCs for high power conversion efficiencies.  Dye loading  Electron transport  Light transmittance

4 TiO2 colloidal solution
Process of TiO2 paste Conventional method Our study Titanium isopropoxide (TIP) TIP in solvent Add water Solvothermal TiO2 colloidal solution Paste In conventional method, TiO2 were prepared by hydrothermal process. After that, the powder drying and grinding as well as solvent transformationwere necessary for the paste preparation. However, In our study, we can use clean and time efficient process to produce TiO2 paste. we can only use titanium isopropoxide (TIP) as precursor dissolve in solvent, and drop water into the solution without any common additive. Besides, by using microwave assisted reactor, we can reduce the process of powder drying and powder grinding for paste formation. In addition, compared with transitional method, our own process can reduce the time about six tuple for making TiO2 paste.(稍微改一下) 一般普遍的製程是利用水熱法合成出二氧化鈦，將粉末進行乾燥後，研磨，把溶劑置換成醇類才能製成漿料。 但是我們的製程僅須利用二氧化鈦前驅物與醇類均勻混和後加入微量的水進行溶熱反應便可以獲得乾淨的二氧化鈦膠體溶液，不須經過粉末乾燥和研磨即可直接用膠體溶液製作成漿料。大幅縮短漿料製成程序與時間。 Clean : Without the need of additive Time-efficient : Using microwave-assisted reactor to increase the reaction rate ( for 30 mins) Reduce the time of six tuple

5 Experimental Section Dye-sensitized Solar Cells Fabrication
Active area : 0.16 cm2 Pretreatment and After-treatment : 40 mM TiCl4 Counter electrode : Pt DSSC for 1 sun Dye : N719 Electrolyte : Iodine-based electrolyte (0.3 M I2) DSSC for room light (200 lux) Dye : Z907+CDCA Electrolyte : Iodine-based electrolyte (0.01 M I2) This is DSSC fabrication. In this study, we use different formula in DSSC for 1 sun and room light applications. The concentration of iodine in electrolyte for 1 sun is higher, whereas the concentration of iodine for room light is lower. Because the iodine show deep brown color, which may compete the incident light with dye, therefore we need to make lower iodine content electrolyte for the application under lower light intensity. 接下來是染敏電池的組裝，而本研究所使用適用於1 sun 電池配方與室內光所適用的配方式不同的。 在太陽光下電解液濃度較高，室內光下因為碘具有顏色會和染料競爭吸光，所以濃度大幅降低 染料是N719，電解液是以碘為系統的A6141。室內光所適用的電池配方則是以染料Z907+CDCA (The coadsorption of CDCA can hinder the formation of dye aggregates and improve electron injection yield ) We need to make lower concentration of iodine based electrolyte for application under room light.

6 Results and Discussion
Let’s move on to the result and discussion. (沒人用enter啦)

7 Major exposed crystallite
TiO2 Powder Characterizations (001) [001] Ti O C Symmetrical alkoxy group The TiO2 nanocrystallites prepared in isopropanol and octanol were labeled as IPA and OCT for the following presentation. The XRD patterns shows 比較好講,用現在式 that IPA and OCT were Anatase titanium dioxide. During the solvothermal process, isopropanol will form an symmetrical alkoxy group bound on the (001) facets, which retards the growth of anatase TiO2 single crystals along [001] direction. On the other hand, the primary alcohols, Octanol, may decrease the uncoordinated Ti (titanium) site and thus restrain the capping effect on the crystal growth, leading to the formation of a spherical crystallite shape and small crystallite size. From TEM images, IPA was nm with mainly exposed at the {001} and {010} facets. OCT crystallites were with sizes of 5-7 nm with dominant {101} facets. 經由XRD鑑定可以確定合成出的二氧化鈦皆為銳鈦礦相二氧化鈦。 在溶熱反應過程中，由於異丙醇會產生具對稱性烷氧基吸附在{001}結晶面上，抑制晶粒沿[001]方向成長，而具有{001}晶面。 而OCT所產生的長鏈狀烷氧基則會吸附在各個表面，此溶劑附著現象會阻礙二氧化鈦成核成長的作用，使得合成的晶粒有變小的趨勢。 從TEM的影像圖也可以發現，M-IPA晶粒約為20-27nm的長方形，而OCT則是為粒徑大小5 nm的晶粒，沒有特殊形狀，主要具有典型的(101)晶面。 Paste Reactant solvent Major exposed crystallite Particle size (nm) IPA Isopropanol (001)/(010) 20-27 OCT Octanol Spherical 5-8 J Nanopart Res (2013) 15:1686

8 Nanocrystallites size (nm)
Mixing of IPA and OCT Particles Paste Nanocrystallites size (nm) Dye loading (mmol/cm3) BET (m2/g) Pore size (nm) OCT 5-8 0.218 107.59 8±3 IPA 20-27 0.130 83.25 17±5 In this study, we attempted to mix IPA and OCT to form heterogeneous TiO2 photoanode, aiming to make good interconnection among the TiO2 particles. Due to the high dye-loading capacity of OCT, we want to intergrate the the characteristics of IPA and OCT further enhance the photovoltaic performances of DSSC. Packing of fine particles in interstices among coarse particles. (可用來講級配) 為了提高二氧化鈦晶粒間的聯通性，我們嘗試將小顆粒的OCT與IPA做混和，並且因為OCT染料吸附量高，因此希望透過OCT的添加提高染料吸附量，藉此提升光電流。 Solar Energy Materials & Solar Cells 159 (2017) 336–344

9 Mixing of IPA and OCT Particles
Isopropanol (IPA) This is our process of mixing paste. Owing to the high boiling point of octanol, the solvent was replaced with isopropanol for OCT to simplify the solvent evaporation. After that, mix IPA and OCT colloidal solution together for the paste preparation. From TEM image, we observe that the IPA and OCT can mix very well. 這是我們的混合型漿料製程，OCT的部分需要做溶劑置換，置換成IPA後以體積比1:1進行混合，製成混合型膠體溶液後，就可以用來製做成漿料。 從右邊的TEM可以觀察到大小顆粒分布均勻，顯示IPA與OCT是有達到均勻混合的。 The mixture is well dispersion

10 Thick Film Structure IPA/OCT OCT IPA Paste dye loading (mmol/cm3) BET
(m2/g) Porosity (%) IPA 0.130 83.25 54.14 IPA/OCT 0.187 67.87 48.91 OCT 0.218 107.59 54.38 Next, let’s move on to the BAT analysis. The BET and porosity of IPA/OCT (mixed one) was the lowest, which was attributed to the powder grading and the necking after sintering process. From the pore size distribution of different TiO2 films, the main pore size of OCT was below 10 nm, however, the aggregation caused a wide pore size distribution. In addition, the dye loading, from the result, we speculated that the dye uptake capacity of {001}/{010} may be lower, which made IPA with higher BET, but the lowest dye loading. 從BET的分析中可以得知，IPA/OCT比表面積最低，是因為大小顆粒混和後，晶粒堆積產生級配的效果，因此在燒節過後使得比表面積為最低者，孔隙率也是最低的。 從孔徑分布圖中可以發現OCT主要孔徑落在10nm以下，且因為有凝聚現象，仍有大孔隙的存在。IPA/OCT的小孔隙數量明顯少於OCT，顯示晶粒確實有燒結的現象。 染料吸附量的部分，從結果來看，{001}/{010}晶面的染料吸附能力可能較{101}低，使IPA即使具有較高的比表面積，染料吸附量仍為最低。 From the result, we speculated that the dye uptake capacity of {001}/{010} may be lower, which made IPA with higher BET, but the lowest dye loading.

11 Optical Property IPA/OCT OCT IPA
IPA → high transparency, large pore , {001} facet. IPA/OCT → good interconnection, but lower porosity and surface area. OCT → high surface area, but lower transparency. The IPA films were highly homogeneous, and the crystallites showed good dispersion. OCT films were aggregated largely. The mixed one combined IPA and OCT which made crystallites aggregate partially. Also, from the UV visible spectrum, we observe that TiO2 particles with largely aggregation may decreased the total transmittance. We assume that the film for light is crucial for light harvesting efficiency in the application of DSSC. Here, we summarize the properties of these three kinds of photoanodes. IPA with high transparency, large pore and {001} facet which is helpful for electron inject. The mixed one has good particles interconnection, but lower porosity and surface area. OCT with high surface area, but low transmittance. {先講IPA →IPA/OCT→OCT} 從SEM中可以觀察晶粒分散的情況，IPA/OCT為混合型，有部分的凝聚。推測晶粒分散情況會影響穿透度，從下圖的全穿透光譜中可以得知，IPA因晶粒分散均勻，光穿透度及高可以達到80%，OCT與IPA/OCT因為有凝聚現象使光穿透度明顯下降。

12 Photovoltaic Performances
The Jsc variation was in correlation with the dye loading ability. The Voc were with obvious difference. Dye loading (mmol/cm3) 0.130 0.187 0.218 Thickness (µm) Jsc (mA/cm2) Voc (mV) FF (%) PCE IPA 15.0 14.88 805 70.1 8.40 IPA/OCT 13.3 16.90 816 65.6 9.05 OCT 10.0 17.25 785 71.8 9.58 Dyesol 18NR 14.0 13.14 810 70.6 7.52 Let’s move on the optimal photovoltaic performance. From the table, we can observe that our TiO2 is better than commercial TiO2 paste Dyesol. The Jsc was in correlation with the dye loading. The highest Jsc was observed by the pure OCT with only 10 um due to its higher dye loading . Tthe Voc were with obvious different. We will discuss later, 以下為各漿料在最佳厚度下製成DSSC的電性比較，Jsc與染料吸附量的結果一致。OCT僅需要達到10um就可以達到最高的電流。IPA在最高的厚度下電流仍為最低。 Voc的部分卻有明顯不同，後續將作深入討論。 在這邊值得一提的是，與商用漿料dyesol比較，本研究所開發出的二氧化鈦應用於DSSC，其光電轉換效率優於市售漿料。

13 Band Structure Voc Binding energy (eV) Fermi level (eV) IPA 16.16
-5.04 IPA/OCT 15.94 -5.26 OCT 16.29 -4.91 The theoretical voltage of OCT is the highest; however, the Voc was 785 mV. The theoretical voltage of IPA/OCT is the lowest ,whereas the Voc was 816 mV. We also use UPS to analyze the fermi level of different TiO2. The theoretica voltage of OCT is the highest; however, the voltage of IPA/OCT is the lowest. This means there has something to affect that the voltage of OCT is the lowest. 將UPS的結果，畫成能帶圖可以得知三種樣品的理論電壓應為OCT最大，IPA/OCT最低。 顯示OCT樣品應該具有最高的電壓，但可能因為其他因素影響使他的電壓大幅降低。

14 Resistance Analysis The impedence of different Interface:
TiO2 / dye / electrolyte Smaller Charge transfer resistance Rt → It would make the electron transport efficiently. Lower Recombination resistance Rct → It would occur charge recombination easily. Before we discuss the result from the EIS to know the impedence on different interfaces between TiO2, dye and electrolyte. When the interrconnection of TiO2 is better, the electron can transfer smoothly, which will have small charge transfer resistance Rt. The amount of grain boundaries resulted in a drowback which increased the interfacial charge recombination, and this will make higher recombination resistance Rct. Therefore, the optimal DSSC need to have higher Rct and lower Rt. 接下來在進行交流阻抗的分析的比較前，先介紹一下Rt和Rct這兩個阻抗所代表的意義。 當二氧化鈦間的連接性比較好的時候，電子就可以傳的比較順暢，這時候電子傳輸阻抗Rt就會比較低；那如果二氧化鈦晶界比較多的時候，則可能增加電子電洞再結合的發生，Rct會比較低。 因此在這邊我們會希望理想的電池其再結合阻抗越大越好，但電子傳輸阻抗越小越好。 Rct and Rt are the electron recombination and chrarge transfer resistance The optimal DSSCs need to have higher Rct and lower Rt.

15 Resistance Analysis At 0.55 V bias potential
IPA with {001} and {010} facets exhibitd a high surface energy can facilitate the electron inject efficiently, which is with bigger Rct; whereas the big pore size may cause the electron transport hard. The recombination resistance of OCT was the lowest, and this result indicated that it would occur electron recombination on the interface easily, which was interrelated with low Voc. The Rt of the mixed one is similar to OCT, which means that the particle aggregation can help electron transport easily. 接下來我們從阻抗分析的結果可以推知，IPA擁有較多利於電子注入的{001}晶面，最不易發生電子電洞再結合。IPA的電子傳遞阻抗最大，顯示電子傳遞可能因為二氧化鈦晶粒較分散，膜的孔洞相對較大，造成電子傳遞的過程有阻礙。 OCT剛好相反，因為晶粒間連通性較好，電子傳遞阻抗是三者中最低，電子傳遞較順暢。但因具有較多的grain boundary，使電子在傳遞時介面上容易發生再結合，在高厚度下再結合的影響就更明顯。 混合型IPA/OCT的阻抗性質都介於兩者之間，但電子傳遞的部分與OCT差不多，表示凝聚確實讓電子傳遞更順暢，結合了兩者的優點，正面大於負面影響使得混合型的電壓較高。 OCT with much grain boundaries would occur electron recombination on the interface easily, which was interrelated with low Voc.

16 Photovoltaic Performance under Indoor Light Irradiation
Film thickness : 4+4 µm OCT with low transmittance made low Jsc. Next, let’s talk about the photovoltaic performance under room light irradiation. The indoor light (200 lux) due to its low photon flux, the DSSC devices do not require too much dye molecules. Also, we control the active layer at 4 um to help voltage stay high. With the limit input photon, a little loss of photon would obviously affect the cell performance. The OCT based cells exhibited the lowest photocurerrent, which is totally different with the result from 1 sun condition. OCT with low transmittance loses more incident light than others do and hence loses photocurrent as well. Therefore, to improve the light harvetimg , we try to decrease the thickness of active layer at 2 um. 在室內光下入射光子數量會減少，不需要太多的染料作用，同時為了維持高Voc，因此二氧化鈦主動層控制在 4 um疊加4 um散射層組成DSSC於200 lux下量測其電性。 值得注意的是，OCT樣品的光電流最低，與在1 sun下的結果完全不同,顯示高染料吸附量在弱光條件下沒有明顯增益的效果。且已知OCT的光穿透度低，在入射光子數量少的情況下可能有更大的損失，導致Jsc最低。因此因此嘗試降低主動層厚度至２um來提升光捕獲率。 Jsc (µA/cm2) Voc (mV) FF (%) PCE Pout (µW/cm2) IPA 24.3 540 57.3 11.6 7.5 IPA/OCT 20.3 501 59.8 9.4 6.1 OCT 17.6 459 67.3 8.37 5.4 Dyesol 18NR 21.8 516 65.2 11.3 7.3

17 Photovoltaic Performance under Indoor Light Irradiation
At 0.6 V bias potential Film thickness : 2+4 µm IPAOCT with the lowest specific surface area decreased the charge recombination. After decreasing the thickness of photoanode, the Jsc of IPA/OCT has obvious increased. It actually improve the light harvesting. Morever, the Voc is getting higher because it decreased the charge recombination probability. In room light condition, Rct of OCT is the lowest. Also, the low concentration of iodide ion would decrease the regeneration rate of dye molecules. On the other hand, the mixed one improves the interconnection among the TiO2 particles and with the lowest surface area, also combine the advantage of IPA, decrease the recombination on the interface. As a result, the PCE reached 12.46%. 主動層厚度降低後可以發現，混合型與OCT的Jsc部分均有明顯上升，確實改善了光捕獲以及光電流。除此之外，也降低再結合發生機率，提升了Voc。 OCT的再結合阻抗仍然是最低的，最容易發生再結合，且可能是因為在室內光下光子數減少，受激發的染料也減少時，電子傳遞的過程中因為有較多的晶界使得再結合發生的機率提升，電壓是三者中最低的。 混合型混合型透過大小顆粒混合產生的級配效應，使比表面積最低外，也保留IPA特點，使發生在界面上的損失都大幅降低，使光電轉換效率達12.46%。 適用於室內光的電池其電解液中碘濃度較低，可能使染料再生的能力減緩，就算主動曾厚度降低，仍然使OCT在室內光照下的效果不如預期。 4+4 Voc (mV) IPA 540 IPA/OCT 501 OCT 459 4+4 Jsc (µA/cm2) IPA 24.3 IPA/OCT 20.3 OCT 17.6 2+4 Jsc (µA/cm2) Voc (mV) FF (%) PCE Pout (µW/cm2) IPA 22.6 575 59.5 11.92 7.8 IPA/OCT 23.3 556 62.6 12.46 8.1 OCT 20.0 517 65.5 10.43 6.8 Dyesol 18NR 20.4 531 67.0 11.18 7.3 459 517

18 Conclusion We obtained TiO2 anatase crystallites in size of 5 nm and nm with {101} or {001}/{010}facets through the micro-wave assisted solvothermal process. The dye loading capacity was the crucial factor for the 1 sun illumination. OCT based cell is optimal for DSSC applied under 1 sun condition because of its high specific surface area and light scattering effect. Its PCE reaches 9.58% with only 10 µm without the need of scattering layer. The film transparency and charge recombination were the most important points for the room light illumination. IPA mixed with OCT (IPA/OCT) with good interconnection among the TiO2 particles decreased the charge recombination and thus the PCE reached 12.46%. This is my conclusion, we proposed simple and time-efficient process to form anatase with different shape and size, in our process we do not need any additive. Under 1 sun condition, dye loading is the crucial factor. OCT with high surface area and light scattering effect, the PCE of it reaches 9.58%. The film transparency and electron recombination rate were the key points for room light condition. IPA mixed with OCT with good interconnection among the TiO2 particles decreased the charge recombination and thus the PCE reached 12.46%. 本研究確實透過了簡單快速的製程，可利用不同溶劑合成出不同形狀及大小的銳鈦礦相二氧化鈦，且不需要任何添加劑就可以使二氧化鈦顆粒大小均一。 18

19 Thank you for your attention !!
Thank you for your attention, and I will be happy for answer any question.

20 Q&A

21 Under 200lux illumination Illumination power density :
200 lux condition Fluorescent lamp T5 SOMA S-2440 Spectrometer We measure the J-V curve of DSCs under the 200 lux condition by using fluorescent T5 lamp in the black box. The 200 lux spectrum is calibrated with a lux spectrometer. Under 200lux illumination Illumination power density : 65 uW/cm2 Active area : 0.16 cm2

22 Equivalent circuit model for DSSC under middle bias potential
Charge transfer resistance (Rt) Recombination resistance (Rct) Rt 凝聚 所以經歷連接性較好 Rct 晶界多的話容易發生再結合

23 Equivalent circuit model for DSSC under high bias potential
Rt 凝聚 所以經歷連接性較好 Rct 晶界多的話容易發生再結合

24 Sol-gel method The sol-gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides. Typical precursors are metal alkoxides.