| Summary: | 碩士 === 國立臺灣大學 === 高分子科學與工程學研究所 === 103 === 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.
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