倒置高分子太陽能電池之薄膜形態對元件特性的影響

Abstract

人類在近年來將面對到的最大問題為能源短缺，由於石油藏量與日漸減，價格卻是不停上升，為了填補能源缺口，有多種可行的替代性能源成為各國研究的重點，太陽能幾乎取之不盡的特性，使得太陽能電池被視為能夠填補能源缺口的可行方案之一，而新發展的有機高分太陽能電池有別於以往的無機太陽能電池，能夠以低成本的方式製造，且質輕可撓曲、可大量製造的特性，能使太陽能電池應用的範圍更加廣泛。 然而目前有機太陽能電池的主要問題是效率偏低，而且元件在空氣下的壽命太短，本論文研究倒置結構的有機太陽能電池，由於使用氧化鋅和銀電極取代一般結構中易水解的PEDOT:PSS和易氧化的鋁電極，這種結構具有良好的空氣穩定性，有機太陽能電池的穩定性問題可經此結構解決。 論文中首先應用Plextcore® PV2000材料於空氣穩定的倒置結構，由於PV2000能夠比被廣泛研究的P3HT:PCBM系統提供更高的開路電壓，搭配溴萘添加的慢乾法，能促進高分子的結晶性，薄膜的吸收頻譜提升，能使短路電流由9.6mA/cm2提升到11.3 mA/cm2，效率也因此從3.96%提升到4.63%。實驗中證實適當添加溴萘慢乾法可以有效增加元件效率，並推測過多的添加使薄膜乾燥時間過長反而會產生反效果。 為了使有機層更能有效利用太陽光能，因此將有機材料更換為新型低能隙的PCDTBT材料，預估能夠多利用18%的太陽光。然而在實驗中發現，原先在P3HT:PCBM系統中能夠對元件有幫助的氧化鋅薄膜表面紋路，在搭配低能隙材料時，由於低能隙材料的理想薄厚小於80奈米，然而氧化鋅薄膜表面卻有90奈米高的山丘狀結構，因此氧化鋅表面紋路會穿透有機薄膜，造成元件中有大量的漏電流。若是更換溶劑及加入TiO2中介層可以有效改善這個問題，將開路電壓由0.36V恢復到0.66V，使倒置結構可以匹配低能隙材料。這個成果能夠使將來應用其它新型有機低能隙材料時，可以成功讓薄膜不會受到氧化鋅表面皺紋狀紋路的破壞。 由於氧化鋅表面形貌會影響低能隙材料的薄膜成長，因此在本論文也研究了改變氧化鋅薄膜退火環境對表面形貌的影響，發現在封閉無空氣流動的環境退火可以成長出較平坦的氧化鋅薄膜，這種平坦無皺紋狀紋路的氧化鋅薄膜，水接觸角量測為41.94°，比起具有皺紋狀紋路的接觸角為64.02°，較具有親水性，若經由UV光持續照射後會暫時性的提升親水性。然而已知基板具親水性不利於P3HT:PCBM元件的效率，因此在此建議若要搭配親油性的氧化鋅基板，可保留表面的紋路較佳。

The humanity’s most important problem is the energy shortage. Due to the increasing oil price, several renewable energy sources are investigated to fill the energy requirement gap between demand and supply. Because solar energy is almost inexhaustible, solar cells become one of the most promising solutions for the energy issue. Organic solar cells (OSCs) are different from the previous inorganic solar cells. Since OSCs are low-cost, flexible and mass producible, it can promote the application of solar cells to more general usage. The most important problem of organic solar cells is the lower power conversion efficiency and the air-stable life time. By replacing the hydrolysis PEDOT:PSS and easily oxidized Al with ZnO and Ag, the inverted structure has great air stability. Hence, the life time issue of organic solar cells could be eliminated by the air stable inverted structure. At first, the Plextcore® PV2000 was applied to the inverted structure to providing higher Voc than P3HT:PCBM system. Cooperating with the Br-naph doping in solvent, the polymer layer achieved better crystallization and higher absorption. As a result, the Jsc increased from 9.6mA/cm2 to 11.3 mA/cm2, and the PCE(power conversion efficiency) was improved from 3.96% to 4.63%. In this experiment, the doping Br-naph to length the drying-time of active layer is effective to enhance the device efficiency, yet the excess doping may show some counter effect. To efficiently take advantage of long wavelength region photons, the low band gap polymer, PCDTBT, was used to further absorb 18% sun light. However, the ridge ZnO surface, which was helpful in the previous P3HT:PCBM system, lead to some negative effect. The wrinkled ZnO ridge was 90nm height while the polymer film was just less than 80nm. The ZnO ridge consequently penetrated the polymer thin film and cause the leakage current in device. Applying different solvent and TiO2 interlayer can reduce the ZnO penetration, and the Voc recovered from 0.36V to 0.66V, so the inverted structure can match the thin film low band gap material. This result can provide a route to match the air stable inverted structure to new developed low band gap polymer, getting rid of the wrinkled ZnO surface. Since the surface morphology strongly affects the developing process of organic thin film on ZnO surface, the different annealing environment was investigate to find out the surface morphology. The closed oven without air flow produce plane ZnO thin film and the contact angle of the plane ZnO is 41.94°, lower than the rough one 64.02°. After continuous UV illumination, the contact angle was further decreasing, indicating the more hydrophilic surface. However, the hydrophilic surface is not preferred for P3HT:PCBM. The result suggests that the rough wrinkled ZnO surface can provide for hydrophobic polymer layer.