具低能隙之導電高分子其在溶液態製程之層疊式高分子太陽能電池上的應用

Abstract

本研究藉由材料上的合成與應用及元件製程上的調控，以得到高開路電壓 (Voc)，高光電轉換效率 (power conversion efficiency, PCE) 的全溶液態製程之層疊式高分子太陽能電池為目標。在層疊式元件結構中，其以有機薄膜與無機薄膜疊加而成，因此首先進行載子傳輸層材料的選擇與吸光材料的合成。我們開發一低溫Sol-gel製程之氧化鋅 (Zinc oxide, ZnO) 作為電子傳輸層材料，以及將Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) 加入一定比例的異丙醇與介面活性劑Zonyl FSN作改質，使其能夠在旋轉塗佈後完整附著於有機材料上當作優秀的電洞傳輸層。而改質之PEDOT:PSS (modified PEDOT:PSS, m-EPDOT:PSS) 與氧化鋅的導電度和透明度都相當良好，故在層疊式太陽能電池內，其兩者組成之互連層 (interconnecting layer, ICL) 也能夠有效地連接兩個子電池。 而一個成功的層疊式有機太陽能電池，除了優秀的互連層以外，前層電池 (front cell) 與後層電池 (rear cell) 所使用的吸光材料也是關鍵因素之一，這兩種吸光材料必須在單電池 (single cell) 中有良好的光電性質表現，且其吸光範圍必須沒有顯著的重覆性。為此，我們選用Poly(3-hexylthiophene) (P3HT): indene C60 bisadduct (IC60BA) 當作前層電池的主動層，另以一新合成的單體bisbenzodicyclopentadithiophene (BBCPDT) 與5,6-difluorobenzo [c][1,2,5]thiadiazole (FBT) 做共聚合，得到一具低能隙的導電高分子PBBCPDT-FBT，並利用之和 [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) 混摻當作後層電池的主動層。P3HT:IC60BA應用在反式單電池中其Voc為0.88 V、短路電流 (Jsc) 為8.84 mA/cm2、填充係數 (FF) 為59.4 %、PCE為4.62 %。而PBBCPDT-FBT:PC71BM應用在反式單電池中其Voc為0.81 V、Jsc為12.82 mA/cm2、FF為55.5 %、PCE為5.76 %。由於兩者吸光材料在元件上的效能表現突出，且其吸光波段的重疊性並不高，故最後我們製作的層疊式有機太陽能電池其Voc可疊加至1.62 V、Jsc為8.03 mA/cm2、FF為54.4 %、PCE能提昇至7.08 %。 藉由開發電荷傳輸材料與主動層材料，及精準設計元件的結構與製程條件，成功做出一極高Voc與PCE的全溶液態製程之層疊式有機太陽能電池。在我們的認知中，此為目前雙層主動層的層疊式太陽能電池研究報導中最高的Voc值。再者，由於此元件構造相當簡約，在未來將能替換不同的吸光材料，以達更突出的光電轉換效率。

This study aims to develop high power conversion efficiency (PCE) with high open circuit-voltage (Voc), all-solution-processed tandem polymer solar cells through the combinations of rational development of new materials and optimization of device fabrications. Because the tandem solar cells are stacked by organic and inorganic thin films tightly, we need to synthesize and choose comportable absorption materials and solution-processed charge transporting layers (CTLs). First, we developed a low-temperatured, sol-gel-processed Zinc oxide (ZnO) for electron transporting layer materials, and modified Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with isopropanol and Zonyl FSN for hole transporting layer materials. The conductivity and transmittance of ZnO and modified-PEDOT:PSS are impressive, so we combine them into a interconnecting layer (ICL) for connecting two subcells in the tandem device. Except ICL, another keypoint for a successful organic tandem solar cell is the two absorption materials in the front cell and rear cell. These two absorption materials must have good performances in the single cell and the overlap of their absorption ranges need to be reduced. For this reason, we choose Poly(3-hexylthiophene) (P3HT): indene C60 bisadduct (IC60BA) to be the active layer in front cell, and a novel low-bandgap polymer PBBCPDT-FBT, which is synthesized by coplymerization of bisbenzodicyclopentadithiophene (BBCPDT) and 5,6-difluorobenzo [c][1,2,5]thiadiazole (FBT) blend with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) to be the active layer in rear cell. The PCE of P3HT:IC60BA in the inverted bulk-heterojunction device is 4.62 %, Voc ~ 0.88 V, short circuit-current (Jsc) ~ 8.84 mA/cm2, fill factor (FF) ~ 59.4 %. The PCE of PBBCPDT-FBT:PC71BM in the inverted bulk-heterojunction device is 5.76 %, Voc ~ 0.81 V, Jsc ~ 12.82 mA/cm2, FF ~ 54.4 %. Due to their impressive performance in the inverted device and little overlap of their absorption ranges, the PCE of the organic tandem solar cell is increased to 7.08 %, and total Voc is summed to 1.62 V, which is nearly a summation of individual cells. We succefully demonstrated a high PCE with high Voc, solution-processed, series-connected organic tandem solar cell by way of combinating the development of new materials and optimization of device fabrications. To the best of our knowledge, this is the highest Voc for a tandem solar cell with double active layers. Furthermore, owing to the simple architecture of this tandem device, we can replace the polymers with other absorption materials for better performance in the future.