共軛型交聯添加劑對有機光伏太陽能電池之熱穩定性 探討

Abstract

本篇論文設計合成含噻吩共軛臂鏈之星狀小分子材料並添加至P3HT 與 PC61BM 為主動層之有機太陽能電池中產生物理性/化學性交聯反應，以提供穩定性。利用三苯胺(Triphenylamine)為星狀之核心，反應合成雙噻吩共軛臂鏈，末端合成溴、羥、疊氮基之烷基鏈段，並探討加入不同交聯官能基之小分子材料(TBT-Br, TBT-OH, TBT-N3)對有機太陽能電池之熱穩定性影響。由傅里葉轉換紅外光譜分析儀(FT-IR)鑑定TBT-N3 之化學性交聯反應生成，經過高溫作用下，疊氮基與PC61BM 之碳球產生共價鍵結；鑑定TBT-OH 之物理性交聯反應生成，羥基與PC61BM 之羰基產生氫鍵鍵結。再者，由光學顯微鏡(optical microscopy, OM) 證明TBT-Br 之光催化化學性交聯反應發生，主動層之相分離程度明顯較未經光催化化學性交聯反應減緩。在高溫環境下維持穩定光伏轉換效率，主動層表面形貌之調控尤為重要。利用光學顯微鏡觀察P3HT 與PC61BM 組成之主動層，添加適量之TBT-N3 與TBT-Br，於長時間150 oC 高溫環境下主動層形貌之相分離程度減緩，結晶數量減少；添加適量之TBT-OH，因物理性氫鍵耐熱溫度有限，無法應用於150 oC 高溫環境，但於120 oC 高溫環境下仍抑制主動層結晶產生，其效果較有限。由紫外光-可見光光譜與螢光光儀譜發現，添加適量之TBT-N3 與TBT-Br於長時間高溫環境下能使UV-vis 吸收光與PL 吸收光維持相同強度，提供熱穩定性。將P3HT 與PC61BM (1:0.9, w/w) 混摻不同比例小分子材料以製備成太陽能電池之主動層，元件採正式結構。在AM 1.5 照度下，以150 oC 加熱144 小時，混摻5wt % TBT-N3 之光伏參數:開路電壓為0.55 V，短路電流為6.88 mAcm-2，填充因子為60.5%，轉換效率為2.60 %；混摻5wt % TBT-Br 之開路電壓為0.58 V，短路電流為4.88 mAcm-2，填充因子為58.7 %，轉換效率為1.74 %。而添加1wt %以下之TBT-OH 系統仍能在室溫維持不錯的光伏轉換效率，但添加量提升後，因無法與主動層有好相容性，相分離嚴重，故於長時間高溫環境下無法穩定元件效率。結果顯示元件中主動層混摻適量TBT-Br 與TBT-N3 小分子材料在高溫下有好之穩定作用。

A novel type of star-shaped small crosslinkers, TBT-Br, TBT-OH and TBT-N3 were developed based on triphenylamine as core, bithiophene as conjugated arm, and crosslinkable bromide, hydroxyl and azide groups respectively bounded to bithiophene via alkyl chains. To investigate the long term thermal stability of BHJ photovoltaics, TBT-Br, TBT-OH and TBT-N3 were respectively blended into the active layer which consisted of two components: a semiconducting polymer (P3HT) and a fullerene derivative (PC61BM). According to FT-IR measurements, the chemical crosslinking reaction of TBT-N3 with PC61BM at high temperatures would form an aziridine ring. Moreover, the hydroxyl group of TBT-OH was found to interact with the carbonyl group of PC61BM via hydrogen bond, indicating the presence of physical interactions between TBT-OH and PC61BM. Based on optical microscopy (OM), the results indicate that the photo-crosslinking reaction of TBT-Br in the active layer would bring about more stable morphology at high temperatures when compared to the sample without photo-crosslinking reaction. To offer optimum photovoltaic performances, the morphology of active layer has to be carefully controlled. The OM results also indicate that upon heating the samples at 150 oC for 144 h, only few fullerene crystals were present in the active layer blended with 5% TBT-N3 or 5% TBT-Br crosslinker. However, for the active layer with TBT-OH crosslinker, the inhibition of fullerene crystal formation appeared only within 120oC owing to the gradual destruction of hydrogen bonds at higher temperatures. Long term thermal stability of active layers was determined by UV-visible spectroscopy and photoluminescence. Before and after crosslinking, there was no change in the absorption peaks for the active layers respectively blended with 5% TBT-N3 and 5% TBT-Br. Normal organic solar cells (OSCs) were fabricated by spin-coating the blends of P3HT as donor, the fullerene derivative (PC61BM) as acceptor, and different amounts of TBT-Br, TBT-OH and TBT-N3. Morphological studies indicate that the respective incorporation of TBT-N3 and TBT-Br crosslinkers into the active layers, and subsequent crosslinking reactions would effectively impede heat-promoted fullerene aggregation, thus leading to stable morphology. In terms of photovoltaic performance, upon heating the thermally crosslinked sample at 150 oC for 144 h, the OSC with 5% TBT-N3 showed a power conversion efficiency of 2.60 %. Under the same thermal treatment, a power conversion efficiency of 1.74 % was observed for the photo-crosslinked sample with 5% TBT-Br. At room temperature, stable device performance for the active layers blended with less than 1wt % TBT-OH was achieved. However, the device with more than 2wt % TBT-OH exhibited poor performance. This is owing to the presence of immiscibility between TBT-OH and the active layer. Based on the above, the crosslinking reactions would bring about stable morphology and further lead to more stable device performance.