全固態染敏電池之電洞傳導層/敏化劑介面的改良與性質探討

Improvement of the Interface at Sensitizer/Hole Transport Material in Solid-State Dye-sensitized Solar Cells

陳韋志(Wei-Chih Chen)

指導教授 ： 王立義

國立臺灣大學/工學院/高分子科學與工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.02458

摘要

本論文分為三個研究主題，第一個主題是調查染料分子與電洞傳導材料(hole transport material，HTM)之間結構相容性對元件效率的影響。在實驗方面，我們分別利用Z907以及在ancillary group的位置有噻吩單元取代之衍生物，CYC-B11，當一敏化劑並結合Poly(3-hexylphene) (P3HT)或(2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9')-spirobifluorene (OMeTAD)作一HTM來進行全固態染敏化太陽能電池(solid-state dye-sensitized solar cells，ss-DSCs)的製備。研究發現CYC-B11/P3HT元件相較於Z907/P3HT與CYC-B11/OMeTAD兩元件有較高的短路電流密度。根據光致電子轉換效率(incident photo-to-electron conversion efficiency，IPCE)以及交流阻抗儀(impedance)的結果顯示，P3HT取代OMeTAD充當一HTM，在光電流上有著顯著的提升並與CYC-B11的吸收光譜是一致的，且在TiO2/dye/HTM之異質介面間的電荷轉移電阻(charge-transfer resistance)明顯下降。因此，CYC-B11/P3HT之ss-DSC在一較薄厚度(0.5 μm)之中孔徑材料TiO2下，其光電轉換效率可達3.66 %。
第二個主題，我們合成不同分子量之P3HT分別為65000(P3HT65)、47000(P3HT47)與25000(P3HT25) g/mol，並搭配CYC-B11為一染料製備ss-DSCs。根據空間電荷限制電流(space-charge-limited-current，SCLC)、impedance與IPCE的量測結果指出，P3HT之非結晶相可增加P3HT的電洞遷移率，且可有效降低元件之TiO2/dye/HTM之異質介面間的電荷轉移電阻(charge-transfer resistance)，因而有效提升元件之光電流。此外，從暫態光電壓的實驗結果發現CYC-B11/P3HT47元件有較長的electron lifetime。此結果暗示P3HT47可有效地均勻覆蓋於吸附染料之TiO2表面，而導致電荷複合的機率降低，其將促使元件有較高的開路電壓。因此，CYC-B11/ P3HT47元件之光電轉換效率可達4.72 %，相較於CYC-B11/ P3HT25元件，其效率提升了接近50 %。為了進一步優越化元件效率，我們導入一染料CYC-B19，其相較於CYC-B11有更高的吸收係數與寬廣的吸收光譜，因此元件效率可進一步提升至4.85 %，且呈現不錯的再現性。
第三個主題是利用Grignard metathesis (GRIM)之聚合方式合成一共軛嵌段共聚高分子，poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-hexylthiophene) (PPP-b-P3HT)，並將其與P3HT分別當一HTM應用於ss-DSCs系統中。此共聚高分子以spin-drying方式的製膜過程中發現，P3HT鏈段會自主裝形成一fibrous-like之結晶結構，且長度約在數個微米的尺度，因此賦與此共聚高分子擁有較快的電洞遷移率。根據暫態光電壓的量測指出，CYC-B11/PPP-b-P3HT元件 相較於參考元件(CYC-B11/P3HT)有較長的electron lifetime。此外，比較兩元件之交流阻抗分析圖譜，其結果顯示CYC-B11/PPP-b-P3HT元件在TiO2/dye/HTM之異質介面間較容易進行電荷轉移。根據上述所觀察的現象暗示著PPP鏈段促進共聚高分子與染料之間有更緊密的接觸，進而大幅提升元件的光電轉換效率。因此，CYC-B11/PPP-b-P3HT元件其光電轉換效率可達4.65 %，此外，此研究也證實共聚高分子對於發展高效能之ss-DSCs而言是一良好的電洞傳輸材料。
第四個主題是導入一具吸光能力之compatibilizers於ss-DSCs系統中，並搭配在Red/NIR有高莫耳吸收係數的SQ2作一染料來探討其對元件光伏特性的影響。從實驗結果顯示此compatibilizer在此系統中，除了扮演共敏化劑的角色來提供可見光區額外的吸收之外，並同時充當dye/HTM之異質介面的修飾劑，進而有效降低異質介面間電荷轉移之電阻並減少charge recombination的機率，因此其光電轉換效率在AM 1.5G的條件下最高可達2.68 % (DS3)，相較於參考元件足足提升了50 %。此外，此研究也有別於目前文獻所發表之共敏化劑或relay dye的功用，這對於發展高效能之ss-DSCs提供了一有效的研究方向。

關鍵字

全固態染敏化太陽能電池 ； 結構相容性 ； 聚(3-己基噻吩) ； 電洞傳導材料 ； 共軛嵌段共聚高分子 ； 自主裝 ； 相容劑

並列摘要

This thesis consists of three topics concerning solid-state dye-sensitized solar cells (ss-DSCs). In the first part, ss-DSCs are fabricated using Z907 or its thiophene derivative, CYC-B11, as a dye, and poly(3-hexylthiophene) (P3HT) or (2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9')-spirobifluorene (OMeTAD) as a hole transport material (HTM). The effect of the structural compatibility of dye molecules with HTM on device performance is investigated. The CYC-B11/P3HT device has a much higher short-circuit current density than those for Z907/P3HT and CYC-B11/OMeTAD devices. Results from the incident photo-to-electron conversion efficiency and impedance measurements support the use of P3HT, in place of OMeTAD, as HTM markedly increases the photocurrent throughout the absorption spectrum of CYC-B11 and significantly reduces the charge-transfer resistance at the TiO2/dye/HTM interface. As a result, the CYC-B11/P3HT ss-DSC that is fabricated from a thin (0.5 μm) mesoporous TiO2 layer exhibits an outstanding power conversion efficiency (PCE) of 3.66%.
In the second part, we applied P3HT with three different molecular weights, 65000 (P3HT65), 47000 (P3HT47) and 25000 (P3HT25) g/mol, as HTM to fabricate ss-DSCs in which CYC-B11 was used as sensitizer. Results from the space-charge-limited-current, impedance and incident photo-to-electron conversion efficiency measurements indicate that the amorphous phase of P3HT increases its hole mobility and efficiently reduces the charge-transfer resistance at the TiO2/dye/HTM interface, thereby increasing the photocurrent. Moreover, the transient photovoltage experiments show that the CYC-B11/ P3HT47 device has the longest electron lifetime. This result suggests a complete coverage of the dyed-TiO2 surface with P3HT47, resulting in the suppression of charge recombination that causes the increase of open-circuit voltage. Accordingly, the CYC-B11/ P3HT47 device exhibits a striking PCE of 4.72 %, which is ~50 % better than that of the ss-DSCs using P3HT25 as HTM. The replacement of CYC-B11 with CYC-B19 as the sensitizer to improve the light-harvesting capability further increases the PCE up to 4.85 %.
In the third part, An all-conjugated diblock copolymer, poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-hexylthiophene) (PPP-b-P3HT), was synthesized and applied as HTM for the fabrication of ss-DSCs. This copolymer is characterized by an enhanced crystallinity, enabling its P3HT component to self-organize into interpenetrated and long-range ordered crystalline fibrils upon spin-drying and ultimately endowing itself to have a faster hole mobility than that of the parent P3HT homopolymer. Transient photovoltage measurements indicate that the photovoltaic cell based on PPP-b-P3HT as the HTM has a longer electron lifetime than that of the reference device based on P3HT homopolymer. Moreover, comparing the two ss-DSCs in terms of the electrochemical impedance spectra reveals that the transfer of charge carriers across the TiO2/dye/HTM interface is substantially easier in the PPP-b-P3HT device than in the P3HT cell. Above observations suggest that the PPP block facilitates an intimate contact between the copolymer and dye molecules absorbed on the nanoporous TiO2 layer, which significantly enhances the performance of the resulting device. Consequently, the PPP-b-P3HT ss-DSC exhibits a promising power conversion efficiency of 4.65%. This study demonstrates that conjugated block copolymers can function as superior HTMs of highly efficient ss-DSCs.
In the last part, we introduced colored comptibilizers (namely DS1 and DS3) into the ss-DSCs with a squaraine dye (SQ2). The results imply that colored comptibilizers can act not only as interface modifiers between dye and hole transporting material (HTM) but also as co-sensitizers by forster energy transfer. Results from incident photo-to-electron conversion efficiency, surface energy and impedance measurements indicate that the introduction of colored comptibilier increase extra light-harvesting and efficiently reduce the charge-transfer resistance at the TiO2/dye/HTM interface, thereby increasing the photocurrent. Moreover, the transient photovoltage experiments show that the DS modified devices have the longer electron lifetime, leading to the increment of open-circuit voltage compared to the reference cell. This result suggests a complete coverage of the TiO2-SQ2 with OMeTAD through DSs treatment, resulting in the suppression of charge recombination between electrons from the TiO2 conduction band and dye cations. Consequently, the ss-DSC using SQ2 and OMeTAD as dye and HTM with the treatment of DS3 compabilizer, exhibits a promising PCE of 2.68 %, which is ~50 % better than that of the reference cell.

並列關鍵字

solid-state dye-sensitized solar cells ； structural compatibility ； poly(3-hexylthiophene) ； hole conductor ； all-conjugated block copolymer ； self-assembly ； compatibilizer

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