本論文研究以聚(3-己烷基噻吩) (Poly (3-hexylthiophene), P3HT)與碳六十衍生物 ( [6,6]- phenyl - C61 – butyric acid methyl ester, PCBM)相互混摻形成單層異質接面結構(bulk-heterojunction)的有機薄膜太陽能電池。利用溶液製程搭配旋轉塗佈方式製作P3HT:PCBM主動層薄膜，進而製備太陽能電池元件，選用氯仿(高揮發性的溶劑)和三氯苯(低揮發性的溶劑)為共溶劑，探討共溶劑比例對太陽能電池元件電性參數的影響，並使用吸收光譜儀、拉曼光譜儀、x-ray 繞射儀與原子力顯微鏡進行主動層薄膜結構與形態學分析，進而探討太陽能電池元件電性參數的關連性。
研究發現當使用純氯仿為溶劑時，可獲致最佳的太陽能電池元件，能量轉換效率(power conversion efficiency，PCE) 可高於3 %，最大的短路電流(10.73 mA/cm2) 和開路電壓(0.59 V)；而加入三氯苯溶劑後，元件的PCE 皆低於2%，短路電流和開路電壓皆隨著三氯苯比例增加而下降，但當使用純三氯苯為溶劑時，則可獲得最大的填充因子（0.51）。顯然地，溶劑的選用與共溶劑的比例，確實嚴重影響P3HT : PCBM太陽能電池元件的性能。
從UV吸收光譜圖可以發現若僅剩氯仿為溶劑，所製作之主動層薄膜會有最強的吸收效果，且隨著三氯苯在共溶劑中占有的比例變多，吸收強度有逐一變小的趨勢；薄膜微結構的部分，由拉曼光譜分析可以發現，當氯仿所佔有的溶劑比例愈高，其周遭的環境均勻性愈差，表示P3HT和PCBM混摻的愈均勻，P-N接觸面積因而變得更多，電子-電洞對分離的機率也就有所提升，進而獲得較多的光電流；另外，X-ray繞射光譜分析指出，三氯苯對於主動層薄膜於垂直基板的方向上，鏈長和鏈長間的堆疊會有較好的排序，因此當三氯苯在共溶劑中占有的比例愈多將有助於載子在分子間的傳遞；搭配原子力顯微鏡觀察到薄膜表面形態的結果，也指出隨著共溶劑中三氯苯的比例增加，主動層薄膜分子間的排列愈均勻。
綜合而論，共溶劑中三氯苯溶劑比例較高，薄膜內P3HT高分子鏈的排列較為緊密，有利於載子的傳輸，但不利於電子-電洞對的分離；但對於共溶劑中氯仿溶劑比例較多的情況，薄膜所顯現的特性則正好相反。推論氯仿溶劑比例多的元件，其載子在傳輸的過程中容易因較差的微結構有序性而複合消失，但相對的此微結構型態分離出的電子、電洞也較多，故最後實際接收到的電荷仍然是最多，所顯現元件電特性自然也會是最好的。

We studied the thin-film structures and photovoltaic properties of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) organic bulk-heterojunction solar cells. The P3HT:PCBM blended active layers were prepared by solution deposition via a spin-coating technique using chloroform (CF) and 1,3,5-trichlorobenzene (TCB) cosolvents. The effect of the cosolvent concentration on the correlation between the morphology of the active layer and the photovoltaic characteristics of the solar cells were studied.
For the electrical properties, we found that the solar cells that were created using pure CF as a solvent for depositing the P3HT:PCBM active layers show the best device performance, including a power conversion efficiency (PCE) above 3 %, a short-circuit current density (JSC) as high as 10.73 mA/cm2, and an open-circuit voltage (Voc) of 0.59 V. However, when TCB:CF cosolvents were used to prepare the active layer, all the solar devices exhibited an inferior PCE of below 2%. We observed that the JSC and Voc decreased when the TCB concentration was increased for the cosolvents. The results of our experiment suggest that the cosolvent composition used when preparing P3HT:PCBM active layers played an important role in the development of the photovoltaic characteristics of organic solar cells.
The P3HT:PCBM active layers were characterized using UV-vis absorption spectroscopy, Raman spectroscopy, x-ray diffraction (XRD), and atomic force microscopy (AFM). The results revealed that an increase in the CF concentration for the cosolvents resulted in films with higher absorbance and better P3HT:PCBM blend homogeneity, thus suggesting the formation of a relatively large amount of excitons and an increased probability of exciton dissociation. This provides a reasonable basis for the higher photocurrent of the solar cells. On the contrary, we also observed an increased crystalline size in the P3HT domains with increasing TCB concentrations, thus reducing the contact between P3HT and PCBM, i.e., creating a smaller p-n junction area.
In summary, we observed a good correlation between the morphology of the P3HT:PCBM active layer and the electrical properties of the solar cells when utilizing the cosolvent approach. In spite of the decreasing order of the P3HT chains, the PCE of the device created with the CF solvent is larger than that of the device made with them CF:TCB cosolvents. Hence, we suggest that the enhanced absorbance and improved p-n junction contribute to the enhancement of the photocurrent in organic bulk-heterojunction-type solar cells despite the formation of small crystalline domains that could limit the transport of charge carriers.

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