- 學位論文
以界面工程提升高分子太陽能電池之效率及穩定性
Boosting the Efficiency and Stability of Polymer Solar Cells by Interfacial Engineering
摘要
本研究係將各式界面修飾層材料，應用於高分子太陽能電池反式結構元件，以修飾主動層與電子傳導層材料-氧化鋅(ZnO)之界面，提升太陽能電池元件之效率與長期穩定性。 第一個主軸建立在主動層為全高分子之太陽能電池(all-polymer solar cells)反式結構元件，其主動層為PBDB-T:N2200系統，並引入不同官能基與表面能之PEI、PEIE以及天然材料cellulose作為界面修飾層材料，其中cellulose材料具備天然、無毒、環保以及生物可降解之特色。ZnO元件最高元件效率為6.06%，經使用PEI、PEIE以及cellulose作為界面修飾層，最高元件效率分別提升至6.99、7.06以及7.33%。使用X射線光電子能譜(XPS)和第一原理計算，證明cellulose具有高吸附能以及能與ZnO之界面間形成強氫鍵作用力，且本質具有較低之表面能，能有效鈍化ZnO表面以及提升界面相容性。在相對溼度40%、溫度25 ℃條件之濕度穩定性， ZnO/cellulose元件之T80元件壽命為2023小時，大幅超越ZnO/PEIE、ZnO/PEI和ZnO元件之T80元件壽命，它們分別為95、45以及26小時。原因來自於cellulose具有優異耐水性，其薄膜形貌助益於延長水分子滲透通道和時間，並且透過界面間形成強氫鍵作用力而延緩水分子吸附。另外，cellulose具有高玻璃轉移溫度(Tg)以及與主動層和ZnO之優良界面相容性，因此在N2環境、溫度75℃條件測試熱穩定性，相較於ZnO元件之T80元件壽命為1746小時，ZnO/cellulose提升至2712小時。ZnO/cellulose元件亦可以存放在相對濕度50-60%、溫度75 ℃，承受複雜環境並維持572小時之T80元件壽命。 第二個主軸為建立在非富勒烯高分子太陽能電池(non-fullerene based polymer solar cells)反式結構元件，其主動層為二元系統(binary system) 的PM6:Y6以及三元系統(ternary system) 的PM6:Y6:PC71BM，亦探討主動層與電子傳輸層材料ZnO之界面，使用富勒烯衍生物C60(OH)x作為界面修飾層材料。以XPS和傅立葉轉換紅外光譜儀(FTIR)證明，C60(OH)x能與ZnO (O-H···O)和Y6 (O-H···F)形成極強之氫鍵作用力，提升界面相容性、電荷傳輸以及抑制電荷再結合，進而強化界面性質。以C60(OH)x作為界面修飾層之二元與三元系統其最高元件效率，分別從14.41%提高到15.80%、由16.10%至16.81%。其熱穩定性在N2環境、65 ℃條件，二元系統之ZnO/C60(OH)x元件保持T80元件壽命長達 6187小時，並以阿瑞尼斯方程式(Arrhenius equation)預測其室溫(25 °C)之T80元件壽命可以維持34635小時 (約3.95年)，而三元系統之ZnO/C60(OH)x元件經過4200小時亦能保持 87%之元件壽命。富勒烯衍生物材料之本質具有良好的自由基捕獲能力，因此ZnO/C60(OH)x元件於N2環境、AM 1.5 G太陽光譜及100 mW/cm²光強度連續照光1200小時，在二元以及三元系統分別維持88%和91%之元件效率。 第三主軸為探討以低成本、環保永續之天然抗氧化材料Vitamin C作為主動層與電子傳輸層ZnO之界面修飾層，亦應用於非富勒烯高分子太陽能電池反式結構元件，其主動層為二元PM6:Y6以及三元PM6:Y6:PC71BM系統。以XPS與FTIR光譜證明，Viatmin C能與ZnO (O-H···O)和Y6 (O-H···F)形成氫鍵作用力，促進界面相容性、電荷傳輸效率以及抑制電荷再結合。以Viatmin C作為界面修飾層之二元與三元系統其最高元件效率，分別從14.28%提高到15.21%、由16.04%上升至16.54%。Viatmin C本質具有優異之自由基捕獲與抗氧化能力，二元系統之ZnO/Viatmin C元件以AM 1.5 G太陽光譜及100 mW/cm²光強度連續照光2100小時、N2環境，能維持85%之元件效率，而三元系統之ZnO/Viatmin C元件經連續照光1680小時，亦能保持80%之元件效率。其熱穩定性在N2環境、65 ℃條件，二元系統之ZnO/Viatmin C元件保持T80元件壽命長達4437小時，而三元系統之ZnO/Viatmin C元件經過4200小時亦能維持 85%之元件壽命。
並列摘要
This work aims to enhance the power conversion efficiency (PCE) and long-term stability of polymer solar cells by improving the interface between active layer and zinc oxide, an electron transport layer (ETL) with various organic molecules. In the frist part, three polymers (PEI, PEIE, and cellulose) were applied as the interlayer of inverted all-polymer solar cells (All-PSCs) which comprised the blend of PBDB-T and N2200 as the photoactive materials. The champion cells exhibited PCEs of 6.06, 6.99,7.06, and 7.33% for the bare ZnO, ZnO/PEI, ZnO/PEIE, and ZnO/cellulose systems, respectively. Among them, cellulose possessed the lowest surface energy, highest adsorption energy, and strongest hydrogen bonding with ZnO, effectively passivating the surface defects of ZnO and improving the interfacial compatibility of active layer. The device stability was evaluated by aging the unencapsulated cells at 25 °C and a relative humidity (RH) of 40% under N2 atmosphere and measuring the dependence of PCE on storing time. The ZnO/cellulose device exhibited an outstanding T80 of 2030 hr, which significantly surpassed the T80s of the ZnO/PEIE (95 hr), ZnO/PEI (45 hr), and pristine ZnO (26 hr) devices, because cellulose showed superior water resistance, strong hydrogen bonding with ZnO, and its dense morphology effectively prolonged the infiltration time of water molecules to reach the ZnO surface. Furthermore, the device’s thermal stability was examined at 75 ℃ in an atmosphere of N2 gas. Similarly, the ZnO/cellulose device showed the highest capability against thermal stress and had an impressive T80 of 2712 hr due to the high glass transition temperature (Tg) of cellulose and improved interfacial compatibility between the active layer and ZnO. ZnO/cellulose device also can withstand complex environments by storing them at 75 °C in a relative humidity (RH) of 50-60%, the T80 lifetime of the ZnO/cellulose device is >572 hr. In the second part, fullerenol, C60(OH)x, was employed as the surface modifier of ZnO in the inverted non-fullerene acceptor based polymer solar cells (NFA-based PSCs) which comprised the physical blends of PM6:Y6 or PM6:Y6:PC71BM as the active layer. The incorporation of C60(OH)x markedly improved the PCE of the binary (PM6:Y6) and ternary (PM6:Y6:PC71BM) solar devices from 14.41% to 15.80% and from 16.10% to 16.81%, respectively because it enhanced the charge extraction ability across the active layer/ZnO interface and inhibited the trap-assisted recombination by passivating the defect on the ZnO surface. Both results from X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy indicated that C60(OH)x can form hydrogen bonds with the oxygen atoms of ZnO and the fluorine atoms of Y6, significantly enhancing the compatibility between the ETL and the organic photoactive film and thus stabilizing the interface against environmental stresses. As a result, the ZnO/C60(OH)x device for the binary system maintained 80% of its original PCE after storing at 65 °C for over 6187 hr in N2 atmosphere, and the T80 lifetime of the device at room temperature (25 °C) was predicted by Arrhenius equation to be greater than 34635 hr, ≈ 3.95 years. Besides, the ZnO/C60(OH)x device for the ternary system also reserved 87% of the initial PCE at 65 ℃ for >4200 hr in N2 atmosphere. It is well known that C60 is an excellent free radical scavenger. Therefore, the ZnO/C60(OH)x device retained 88% and 91% of initial PCE for the binary and ternary systems, respectively, after continuous light soaking with a solar simulator at 100 mW/cm² intensity for >1200 hr in N2 atmosphere. In the final part, natural-antioxidant Vitamin C was used as the surface modifier of ZnO in the inverted NFA-based binary and ternary PSCs and it effectively raised the highest PCE from 14.28% to 15.21% and from 16.04% to 16.54% for the PM6:Y6 (binary) and PM6:Y6:PC71BM (ternary) devices, respectively. This is primarily because of the restrain of the trap-assisted recombination by passivating the defect on the ZnO surface and the increment of the charge extraction ability across the active layer/ZnO interface. Furthermore, experimental results indicated that a thin layer of Vitamin C effectively suppressed the photocatalytic degradation of NFA due to its strong free-radical scavenging ability. As a result, the ZnO/Vitamin C device for the binary system retained 85% of its original PCE for >2100 hr, and the ternary system reserved 80% of the initial PCE for >1680 hr under continuous light illumination with a solar simulator at one-sun intensity in N2 condition. Additionally, the X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy measurements demonstrated that the hydroxyl moieties of Vitamin C can form hydrogen bonds with the O atoms of ZnO and the fluorine atoms of Y6, the NFA used herein, significantly stabilizing the interface between ZnO and the organic photoactive films against thermal treatment. Hence, the ZnO/Vitamin C device for the binary system maintained 80% and ternary system reserved 85% of their initial PCE after being thermally aged at 65 ℃ in N2 condition for over 4437 hr and 4200 hr, respectively.