共軛高分子/富勒烯衍生物之高分子太陽能電池熱穩定性質探討

Investigation on The Thermal Stability of Polymer/Fullerene Derivatives Bulk Heterojunction Solar Cells

劉岳欣(Wei-Shin Liu)

指導教授 ： 邱文英

共同指導教授 ： 王立義(Leeyih Wang)

國立臺灣大學/工學院/化學工程學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.00288

摘要

近幾年來，高分子太陽能電池之光電轉換效率的發展極為迅速，而元件之使用壽命與穩定性，儼然成為高分子太陽能電池商業化所需面臨的最大課題。本論文即分為三個主題，分別探討影響主動層異質接面形態之熱穩定性的因素與機制。
第一個研究主題中，為了改善P3HT:PCBM主動層熱穩定性不佳與碳六十在可見光區吸收能力較弱的缺點，我們設計了四種與P3HT具有相似結構且具有可見光區吸收能力的共軛基團做為碳六十上的取代基，合成出一系列具有可見光吸收能力的富勒烯衍生物。作為電子受體並與P3HT混摻後製備高分子太陽能電池，其中4,6-bis(4-(2-ethylhexyl)thiophen-2-yl)thieno[3,4-b]thiophene-fulleropyrrolidine (2TTT-EH-C60)作為電子受體的太陽能電池元件效率最高，可達3.14%。雖然效率不及於P3HT:PCBM所製備的太陽能電池元件，但我們成功利用此系列富勒烯衍生物加熱後不易聚集的非晶特性，製備出優異熱穩定性的高分子太陽能元件，主動層在130℃下熱處理480分鐘，其元件效率幾乎維持不變。除此之外，我們導入碳六十的共軛基團與P3HT具有相似的結構，因此，將其添加至P3HT:PCBM系統作為P3HT與PCBM間之增容劑，可以避免P3HT與PCBM在長時間熱處理後發生急劇的相分離現象，並有效地穩定主動層之異質接面形態，以達到優異熱穩定性的高分子太陽能元件。
第二部分的研究主題則是著重於主動層異質接面形態在長時間熱處理後的相分離機制與影響因素。實驗中，我們分別利用含有機不純物、除去有機不純物、高RR值與低RR值之P3HT作為電子予體，並與PCBM混摻製備主動層。經過長時間熱處理後，四個系統的主動層在巨觀下皆會產生大規模且大尺度的PCBM聚集，但其熱穩定性卻有極大的差異。由GISAXS的實驗結果中，我們發現大規模的PCBM聚集並非影響太陽能元件性能之穩定性的最主要原因，主動層於微觀下之相分離變化才是影響熱穩定性的關鍵因素。含低分子量且低規則度這類有機不純物的P3HT與低RR值之P3HT的結晶性較差，在主動層成膜的過程形成晶種數量少但粒徑較大的PCBM團簇，熱處理後，團簇間的距離過遠，使電子傳遞路徑的阻力變大，因而降低其穩定性；相反的，除去不純物的P3HT與高RR值的P3HT系統，將形成晶種數量多且單一團簇粒徑較小的PCBM團簇，於熱處理後，因為團簇的數量多，所以仍可以串成良好的電子傳遞通道，維持優異的穩定性。
第三部分的研究主題，是探討低能隙共軛高分子PTB7-Th與PCBM混摻之異質接面形態的熱穩定性，PTB7-Th:PCBM主動層於100℃熱處理900分鐘後，主動層薄膜會出現大規模的PCBM聚集，使D/A接觸面積減少、降低電洞遷移率，因而使元件效率由6.41%大幅下降至2.85%，降幅超過五成，顯示其熱穩定性不佳。當我們於主動層中添加非晶性的材料bis-PCBM作為相分離抑制劑，可大幅降低PCBM在長時間熱處理後產生的聚集，使主動層之異質接面形態具有良好的熱穩定性，主動層於100℃熱處理900分鐘後，其元件之光電轉換效率仍可維持於原本效率的75%，其電流值更降低不到一成。明確地顯示，以bis-PCBM作為相分離抑制劑可以有效的穩定主動層之異質接面形態，提升主動層之熱穩定性，進而穩定太陽能元件之光電性質。

關鍵字

高分子太陽能電池 ； 形態熱穩定性 ； 聚(3-己基噻吩) ； 富勒烯衍生物 ； bis-PCBM

並列摘要

In recent years, there is a tremendous growth in the performance of polymer solar cells. The lifetime and durable under prolonged exposure to sunlight and weather of polymer solar cells are much more important for commercialization. In this research, it will be discussed with the factors and mechanisms of thermal stabilities in the morphology of bulk heterojunction polymer solar cells.
In the first aspect, we have designed and synthesized a series of fullerene derivatives with four visible-region absorption compounds which are having a similar structure with P3HT and conjugated groups with absorption in visible region, to improve the light-absorptivity of C60 and the thermal stability of the active layer. The best power conversion efficiency (PCE) is 3.14% in the polymer solar cells which based on 4,6-bis(4-(2-ethylhexyl)thiophen-2-yl)thieno[3,4-b]thiophene-fulleropyrrolidine (2TTT-EH-C60) as electron accepter blending with P3HT. Although the PCE is not better than the devices fabricated with P3HT:PCBM, we have successfully constructed a devices with high thermal stability. Both OM and TEM images of the thermally aged blend film show the amorphous nature of 3T-EH-C60 and 2TTT-EH-C60 which effectively suppresses the thermal-driven aggregation of C60 adducts during aging process, leading to a extremely stable morphology. Consequently, the PCE is stable on annealing at 130℃ for 480 minutes. Besides, the conjugated groups have similar structure with P3HT, it may act as the compatibilizer of P3HT and PCBM to avoid the phase separation during long time thermal annealing inorder to achieve the superior thermal stability of polymer solar cells.
Second aspect, is mainly focused on the mechanism and factors in phase separation with long time thermal annealing in the active layer. We have carried out, using P3HT with organic impurities, without organic impurities, with higher RR value or lower RR value, as electron donor to blend with PCBM. After long time thermal annealing, the active layers may form the large-scale PCBM aggregations, but the stabilities have the large difference. GISAXS study revealed that the large-scale PCBM aggregations is not the major reason for thermal instability. The critical factor on thermal stabilities is the nanoscale phase separation in the active layer. Materials with low molecular weight and low RR value may have the poor crystallinity, it may form the PCBM clusters with a small amounts of grains with large size. After annealing, the stability may decrease because of the huge distance between the clusters with increasing resistance and pathways. Otherwise, the P3HT without organic impurities or high RR value may shape a large amounts of grains with smaller size. It observed that the photovoltaic performance is stable because of the large amount of clusters forming the effective electron transporting pathways.
Third aspect, it is observed that the PTB7-Th:PCBM may shape the large-scale PCBM aggregations after annealing at 100℃ for 900 minutes. The aggregation may decrease the interface area with donor and accepter, hole mobility and photovoltaic performances from 6.41% to 2.85%. Furthermore, while adding bis-PCBM as phase separation inhibitors, it is observed that it may decrease the PCBM aggregation with long time annealing, in order to get better thermal stability. Besides, the device performance i.e., the PCE can still remain at 75% after annealing at 100℃ for 900 minutes.

並列關鍵字

polymer solar cells ； morphological stability ； poly(3-hexylthiophene) ； fullerene derivatives ； bis-PCBM

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