有機小分子異質混摻太陽能電池主動層薄膜結構 相行為暨形貌探討

Abstract

有機薄膜太陽能電池的形貌 (morphology)表現是決定光轉電效率的重要因素，但是由於有機小分子成膜性較差，因此以濕式製成的有機小分子太陽能電池的的效率往往比較低。為了改善這個問題，我們從熱動力學的角度探討了有機小分子在主動層的相行為機制，希望藉此調控主動層薄膜形貌成為一具有雙連續通道網路的相結構，如此會有利於提升有機太陽能的轉換效率表現。 本次研究首先藉由熱分析方法分析2,1,3-Benzothiadiazole,4,7-bis(5'-hexyl[2,2'-bithiophen]-5-yl (BT(DTC6)2)和[6,6]-phenyl C61 butyric acid methyl ester (PC61BM)混摻形成的系統，並搭配Flory-Huggins理論計算兩材料之間的作用力參數()。經計算得到結果，我們發現兩材料間的作用力參數小於零，且理論的混合熵(Smix)大於零，因此在熱力學上，此一系統將會傾向於互溶。並且當溫度及系統混摻比例條件的改變，系統的自由能也會有所改變，導致系統的形貌表現有所變化。因此我們首先藉由熱示差掃瞄分析儀 (Differential scanning calorimetry，DSC)量測不同混摻比例的樣品，求得系統的共晶相圖後，進一步以偏光顯微鏡(Polarized Optical Microscope，POM)和電子穿透顯微鏡(Transmission Electron Microscope，TEM)觀察混摻比例對於BT(DTC6)2/PC61BM相行為的影響。結果發現BT(DTC6)2分子的晶體會隨著PC61BM比例的增加而有縮小的情況，並在BT(DTC6)2/PC61BM混摻比例達到4:6時，晶體表現最不明顯，顯示此時有最均勻的混摻。接著我們分別以熱退火以及溶劑退火兩種後處理修飾混摻薄膜的表面形貌以及結晶度表現。在原子力顯微鏡(Atomic force microscope，AFM)的結果中，我們可以發現越高溫的處理可以使得薄膜的粗糙度(Root-mean-square，RMS)有更顯著的下降，同時進一步以溶劑處理，則除了薄膜有更好的平整度 (smoothness)以外，相位差(Phase contrast)也會同時提昇。這樣的結果可以低掠角薄膜X光(Graze incidence small angle X-ray scattering，GI-SAXS)試驗中證實。經由後處理，混摻材料的結晶度確實也會有顯著的提升。因此，根據以上結果，我們認為藉由調控不同混摻重量比例以及退火的條件，我們可以成功的改善BT(DTC6)2:PC61BM的薄膜形貌，因此可以預期有機太陽能電池會有更好的效率輸出。

Optimized interpenetrating bicontinuous network made from a physcial blend of a p-type conjugated polymer and an n-type fullerene derivative is a prerequisite to achieve high power conversion efficiencies in bulk heterojunction (BHJ) organic solar cells (OSCs). According to Flory-Huggins theory, the physical state of a binary blend at constant pressure is governed by the free energy of mixing (Gmix). In this work, the phase behavior of a binary blend of two conjugated molecules BT(DTC6)2 and PC61BM was investigated through thermal analysis. The influence of processing temperature and blend ratio on Gmix was first determined. Interaction parameter (), entropy of mixing (Smix), and enthalpy of mixing (Hmix) were deduced based on the differential scanning calorimetry results. The binary blend of BT(DTC6)2/PC61BM has an interaction parameter ()= - 8*10-3 , which implies a negative Hmix and its Smix is positive. Thus the Gmix of the binary blend BT(DTC6)2/PC61BM is negative and the mixing phases of the two components should be spontaneous. In DSC study, we also find that crystalline temperature (Tc) of the components in the blend system relys on the blend ratio and resulted in a eutectic phase diagram. POM and TEM studies indicate the crystalline domain of BT(DTC6)2 decreases as the PC61BM ratio increases. Thermal and solvent annealing were further used to control the morphology of BT(DTC6)2/PC61BM. From atomic force microscope (AFM) and grazing-incidence small-angle X-ray scattering (GI-SAXS), we prove that we successful induce the crystallinity of BT(DTC6)2:PC61BM. It may indicate that while applying these methods to active layers of solar cells, we can accordingly optimize the electron donor and electron acceptor interface and sustain its crystallinity.