退火和接面修飾對高分子光伏元件之影響研究

Abstract

隨著能源問題的日益嚴重，太陽能電池的研究更是如火如荼的發展，近年來高分子太陽能電池之光電轉換效率已達5%，主要是利用P3HT/PCBM [poly(3-hexylthiophene) / [6,6]-phenyl C61-butyric acid methyl ester]等這類有機材料所形成的p-n結構所製作而成，由於這類有機太陽能電池具有成本低、重量輕、具可撓性及容易製造大面積元件等優勢，研究提升製程速度和效率便可使太陽能電池的價格和效能更具競爭力。

由於溶劑退火對高分子太陽能電池而言是一種相當有效能提昇效率的一種方式。因此，我們首先利用光譜和形貌的鑑定來研究製程中兩種重要的參數p-n組成比例與揮發速度對元件效率的影響，當P3HT:PCBM組成比例在1:1時，可以發現有最好的光電轉換效率，當PCBM增加時，主動層的吸收和結晶性有明顯降低，但由雷射共聚焦顯微鏡可以觀察到，當在比例為1:1，確實有均勻的相分離同時也有最多的激子分解位置，利用這些分解位置可以提升電子電洞產生機率，近而提升高分子太陽能電池之光電轉換效率。主動層對溶劑揮發速度的比較，可以發現揮發速度越慢，可以提升主動層的結晶性和吸收，此外，由吸收峰的紅移證實慢揮發速度可以使共軛高分子鏈緊密排列。最後，由光學模擬，可以得知其元件吸收增加來自於溶劑退火使主動產生光學漸變結構，使更多光能被多層薄膜太陽能電池所吸收。

為了增進產率，我們利用微波加熱的方式處理高分子太陽能電池並提升其光電轉換效率，微波會選擇性加熱主動層和金屬陰極，但不會造成能量浪費在ITO基板上，此外，金屬因其厚度小於微波的穿透深度，造成金屬陰極會吸收微波能量，產生加熱。最後，在加熱只要短短1.5分鐘內,即可以使高分子太陽能電池達到光電轉換效率達3.6%，，因此，高分子太陽能電池可以利用這類微波退火在一個短暫的時間。有效提升光電轉換效率。

為了追求更高的光電轉換效率，可以利用降低在太陽能電池等效電路中的串聯電阻，當甘露醇被添加入PEDOT:PSS中時，其導電性可以提升40倍，此外，製成光伏元件後，其光電轉換效率有提升，且串聯電阻也有所降低的趨勢。其中短路電流可從12增加到14.7mA/cm2.，最後，當甘露醇添加到60mg/mL時，太陽能電池的光電轉換效率可以達到5.2%。

The power conversion efficiency of organic photovoltaic devices up to ~5% has been achieved recently by creating organic p-n bulk-heterojunction, such as that of p-type poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl C61-butyric acid methyl ester (PCBM), in the active layer. Because of the advantages, such as low-cost, light-weight, flexible and large area fabrication capability, developing high throughput and efficiency methods are thus essential to enable the photovoltaic system more competitive in price and performance.

Solvent annealing is one of the candidates to improve the efficiency of solar cells. For investigating the effect of solvent annealing on the performance of polymer solar cells, we studied two important parameters such as the composition ratio and solvent evaporation by several measurements and morphology characterizations. The optimized composition ratio of P3HT/PCBM was 1 to 1 from light J-V curves and EQE value. When the concentration of PCBM increases, the light absorption and crystallinity of P3HT were reduced. The confocal laser scanning microscopy (CLSM) shows that the uniform and large amounts of interface area were obtained while ratio is equal to one. As a result, the moderate quantity of PCBM can effectively create exciton dissociation cites, to produce free electrons and holes and improve the performance of polymer solar cells. As for the influence of solvent evaporation, the crystallinity and light absorption was improved in longer drying time. In addition, red-shift of vibration peaks indicates the conjugated polymer chain has closed packing structure. From the optical sitmulation, the higher absorption in the active layer was resulted from optimized index-matching to form grading structure in the multi-layer device architecture.

To increase the fabrication throughput, we demonstrated a microwave annealing method to treat the polymer devices and found that it will increase the performance of the photovoltaic cells. The microwave irradiation will selectively heat the active layer and cathode and reduce energy loss in the ITO substrate. The metal cathode heat by microwave is due to the thickness which is lower than the penetration depth of microwave. Finally, the optimized efficiency of polymer solar cells can be improved to 3.6% in only 1.5 min. The microwave is an efficient way to improve the performance of the photovoltaic device.

To pursue higher power conversion efficiency in polymer PV, highly conductive PEDOT:PSS was used to reduce the series resistance in equivalent circuit of the polymer solar cells When the mannitol doped in the PEDOT:PSS, the conductivity was 40 fold improvement compared to the pristine device. The power conversion efficiency was improved and series resistance was reduced as mannitol concentration increased. Comparing with the pristine PEDOT:PSS,. Thus, the short circuit current increased from 12 to 14.7 mA/cm2. Finally, the optimized power conversion of polymer solar cell was 5.2% with 60mg/mL manntiol doping.