高分子太陽能電池之光作用層的形態控制及其對於元件效率之影響探討

Morphology Control of Photoactive Layer in Polymer Solar Cells and Its Impact on Device Performance

梁晉瑋(Chin-Wei Liang)

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(2011年)

https://doi.org/10.6342/NTU.2011.10247

摘要

本論文分成五個研究主題，第一個研究主題是藉由控制poly(3-hexylthiophene)(P3HT)的立體規則度和分子量製備出不需退火處理之高效率的P3HT:PCBM高分子太陽能電池。研究結果發現P3HT的%RR值對於太陽能電池的性能有強烈的影響，這可以歸咎於高%RR值的P3HT有較強的分子堆疊特性，因而提升光學吸收和載子移動能力的性質所致。除此以外，P3HT的分子量亦是一影響元件性能的重要參數，P3HT擁有適度的分子量不但有更好的載子移動能力，同時與PCBM的不相容性達到較佳的平衡，在未經退火處理就已形成最佳化 之異質接面形態，由於此特性使該元件在沒有任何退火處理下，效率已能接近4%。
第二個研究主題是探究碳六十上的化學結構對於元件性能的影響。我們合成了一系列新型具有不同烷基鏈段長度的碳六十衍生物(3a-e)，烷基鏈段的長度分別為乙基、丁基、己基、辛基和十二烷基。研究結果顯示碳六十衍生物上之烷基鏈段的長度些微的變化對於碳六十衍生物的溶解度、自身聚集行為以及與P3HT的相容性都有很大的影響。從TEM的結果發現異質接面形態的相分離程度會隨著碳六十衍生物上之烷基鏈段的長度增加而增加，這證實烷基鏈度的長度增加會降低碳六十衍生物與P3HT之間的相容性。從載子遷移率的結果顯示載子傳遞的能力會隨著烷基鏈段的長度增加而下降，我們推論載子遷移率的下降是因碳六十衍生物分子間的堆疊受到烷基鏈段的長度增加而變得越差所導致，這亦證實烷基鏈段的長度會影響碳六十衍生物自身聚集的行為。3a-e與P3HT以:1:1的重量比例混攙後製備成元件，其光電轉換效率分別為0.88、2.37、2.94、2.30以及0.09%，此一結果清楚說明3a-e與P3HT之相容性和其載子遷移率對於元件性能皆扮演重要的角色。本實驗最佳的光電轉換效率為碳六十衍生物3c與P3HT以重量比為1:0.8混攙時，可達3.60%。
第三個研究主題目：為了提高了高分子塊狀異質太陽能電池的開路電壓，因此我們應用一個帶有強推電子基團-triphenylamin(TPA)之單加成物的 (TPA-C60 mono)和雙加成物(TPA-C60 di)的碳六十衍生物作為元件中之電子受體。研究結果顯示TPA-C60 mono的LUMO能階相較於PCBM足足提升了0.3 eV，這說明了在C60上導入推電子基團是可以有效提升碳六十衍生物的電子能階。進一步與P3HT製作成元件後，其開路電壓比P3HT/PCBM元件提升了0.17 V，可達0.77 V。而光電轉換效率亦比P3HT/PCBM元件的3.05%些為增加至3.26%。而TPA-C60 di亦觀察到相似的結果，元件開路電壓也比Bis-PCBM提高了0.21 V，可高達0.91 V。
第四個研究主題：主要針對異質接面層在垂直成分不理想的形貌進行改質，利用P3HT作為緩衝層導入至PEDOT:PSS和P3HT:PCBM光作元層之間，有效地利用沉積於底部的PCBM與P3HT緩衝層形成更多的donor/accpetor界面，並且有效增加載子分離的效率。研究結果顯示P3HT緩衝層所使用之P3HT的分子量對增進元件的光電轉換效率影響甚鉅。當P3HT的分子量太小時，由於溶解度太好導致易於被P3HT:PCBM光作用層完全溶解而失去作為緩衝層之功效。反之，當分子量太大時，會受到其載子移動率不佳影響，而無法達到增進元件光電轉換效率之功效。而作為此緩衝層的最適分子量介於40~60 KDa，它能提升20%的光電流，光電轉換效率從原先的3.98%提升至5.05%。另外，此一P3HT緩衝層還能進一步增進PEDOT:PSS阻隔電子的能力。
第五個研究主題：我們提出一個有效提升P3HT/PCBM高分子太陽能電池的熱穩定性的方法，其係利用添加一個非晶的碳六十衍生物-TPA-C60至P3HT/PCBM系統中。如是製備之元件在160℃加熱900分鐘後，元件效率依舊與初始效率相當，相對於未添加TPA-C60之P3HT/PCBM元件，經相同程序加熱後，其效率則從原先的3.5%降至0.7%。而元件熱穩定性提升的主要原因為TPA-C60有效抑制了PCBM聚集，進而穩定異質接面形態，使之不易形成巨觀之相分離。除此之外， TPA-C60還能進一步降低P3HT/PCBM異質接面形態的相分離程度，增加donor/acceptor界面面積，讓元件的光電轉換效率從原先的3.51%提升至3.87%。

關鍵字

立體規則度 ； 分子量 ； 聚(3-己基噻吩) ； 碳六十衍生物 ； 異質接面形態 ； 熱穩定性 ； 高分子太陽能電池

並列摘要

This thesis consists of five topics concerning polymer photovoltaics. In the first part, an annealing-free method to prepare high-efficiency P3HT:PCBM solar cells was developed based on the use of highly regioregular P3HT with suitable molecular weight. Our results showed the regioregularity of P3HT has strong influence on the performance of solar cell. Highly regioregular polymer chains can easily form ordered packing during spin drying, leading to an enhanced optical absorption and improved carrier mobility. In addition, the molecular weight (MW) of P3HT plays an important role in determining the cell performance. Suitable MW of P3HT possesses faster carrier mobility and it also promotes the formation of an optimum morphology upon blending with PCBM to maximize the area of heterojunction and produce bi-continuous carrier transport routes. The thus-prepared devices exhibit the best power conversion efficiency of around 4% under AM1.5 illumination at 1 sun intensity without any pre- or post-treatments.
In the second part, we explored the effects of the substitution group on C60 core on the device performance. Herein, a series of methanofullerenes with various alkyl chain lengths (C2~C12), (benzyl-n-alkyl)(1,2-methanofullereneC60)-61,61- dicarboxylates (3a-e), was synthesized via the Bingel cyclopropanation of malonic ester-bearing molecules with C60. Our results showed that slight variation in the length of alkyl substitutions of 3a-e causes significant changes in their solubility, aggregation behavior, and compatibility with P3HT. As expected, the solubility of methanofullerenes is getting better as the alkyl chain is getting longer. However, the TEM images revealed that the domain size of two constitutes increase with increasing alkyl chain lengths, indicating the compatibility of 3a-e and P3HT decreases as the alkyl chain lengthens. The electron mobility of pristine 3a-e was determined by space-charge-limit current method. It exhibited a monotonous decay with increasing alkyl chain length. This finding suggested that the presence of long insulating alkyl groups may block C60 cores to form closed packing, thus raising the hindrance of electron hopping among C60s. As a hexyl group was employed in methanofullerenes, the device has the best energy conversion efficiency of approximate 3.6% under AM1.5 illumination at 100 mW/cm2.
In the third part, new fullerene derivatives with one electron-donating bistriphenylamine (TPA-C60-mono) or two substituents (TPA-C60-di) were utilized as acceptor to blend with P3HT for making solar devices. Very interestingly, the cyclic voltamograms showed TPA-C60-mono and PCBM have similar reduction potentials, but the combination of AC-2 measurements and UV-vis absorption spectra demonstrated that the LUMO of TPA-C60-mono film is 0.3 eV higher than that of PCBM film. This is probably because the triphenylamine moieties and neighbor C60 cores form charge transfer complexes in the solid film. Consequently, the device based on TPA-C60-mono and P3HT exhibites a high open circuit voltage of 0.77 V, which is 30% greater than that of the device based on PCBM and P3HT. Similarly, the TPA-C60-di’s LUMO is 0.3 eV higher than bis-PCBM’s and the TPA-C60-di-based cell’s Voc (0.91 V) is 0.21 V higher than bis-PCBM-based cell’s.
In the forth part, we presented an approach for improving the unfavorable vertical composition gradients of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in the photoactive layer of bulk heterojunction solar cells. The proposed method involves simply depositing a thin layer of P3HT on top of poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) prior to the P3HT:PCBM blend is spin-coated. The results from photoluminescence and photovoltaic measurements indicated that incorporating this P3HT layer significantly enhances the electron blocking ability of PEDOT:PSS, the efficiency of photo-induced electron transfer and the photocurrent of the device, resulting in an improvement of the power conversion efficiency from 3.98% to 5.05%.
In the last part, we demonstrated an effective method for improving thermal stability of the morphology of P3HT/PCBM blend by applying amorphous TPA-C60-mono as anticoagulant of PCBM. Both TEM and OM images indicated that replacing PCBM with only 9 wt% of TPA-C60-mono in the P3HT/PCBM blend effectively suppresses the formation of micro-sized PCBM aggregates and large-scaled phase separation after long-term heat treatment. Photovoltaic measurements also revealed that no apparent degradation in power conversion efficiency was observed for the device with TPA-C60-mono after heating the photoactive layer at 160 °C for 15 hours prior to depositing metal cathode, but it degrades dramatically from 3.5% to 0.7% for the device without TPA-C60-mono. Besides, the replacement of PCBM with a small amount of TPA-C60-mono resultes in the reduction of the domain size in P3HT/PCBM blend and then the enhancement of donor/acceptor interfacial area, improving the power conversion efficiency from 3.51% to 3.87%.

並列關鍵字

polymer solar cells ； morphology ； bulk heterojunction ； regioregularity ； moclecular weight ； poly(3-hexylthiophene) ； C60 derivatives ； thermal stability

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