運用次微米尺度操控分子相分離技術製作有機太陽能電池

Abstract

相分離之控制是決定高分子太陽能電池效率的重要關鍵，特別對於總體異質接面(bulk heterojunction)太陽能電池。在理想的異質接面結構中，主動層兩材料藉由均勻地垂直相分離形成互相貫穿的通道，以確保電荷傳輸至電極並減少再結合現象發生。為了實現理想的高分子太陽能電池，在本論文中我們提出一個新穎的方法促使太陽能電池主動層材料P3HT/PCBM達成自主性排列(self-organized)的次微米結構。有別於傳統分子量相近之兩高分子間相分離控制，本論文首先提出利用微接觸列印技術(microcontact printing)結合自組裝薄膜(self-assembled monolayers)特性讓基板表面能產生差異性而導致高分子太陽能電池p-型共軛高分子(P3HT)與n-型小分子材料(PCBM)間相分離現象。由實驗結果所示，我們證實利用所提出自主性排列的方法確實能夠控制次微米(sub-micron)等級的相分離現象。藉由相分離控制，太陽能電池元件有較佳的特性，此外，在不改變開路電壓值情況下，其短路電流隨著donor-acceptor界面面積增加而提升。因為較大的界面面積在光電轉換過程能提升激子產生速率與激子分離的機率，而光電轉換效率也可藉由grating size 縮小來改善。當高分子太陽能元件中主動層材料相分離間距縮小至0.5μ，最佳化光電轉換效率可達到2.59％。此外，本文也將針對光電流及效率提升現象的機制加以探討。

The processes of phase separation determine the efficiency of polymer solar cell dramatically, especially in bulk hetrojunction (BHJ) structure with donor and acceptor. In an ideal BHJ solar cell, this two phases uniform interpenetrated through vertical phase separation to ensure charge carrier transport to the electrode without recombination. To achieve this idea polymer solar cell, herein, we proposed a new method to fabricate a self-organized structure in P3HT/PCBM blending on ITO substrates by self-assembled monolayers (SAMs) confinement. Different from conventional phase separation based on two polymers with similar molecular weight, we firstly manipulate the phase separation in conjugated polymer and fullerene blending by soft lithography. As a result, we have demonstrated that this self-alignment method can control the phase separation between the donor and acceptor down to sub-micron scale. Through the phase separtion controlling, the solar cell shows better performance compared to the device without phase separation. In addition, the short circuit current also increases with the interfacial area in BHJ structure instead of reducing of open circuit voltage. Because the exciton generation rate and dissociation probability were both enhanced by larger interfacial area, the power conversion efficiency of polymer solar cell is improved by grating size reducing. With reducing the gating size to 0.5μm, the optimized power conversion efficiency of 2.59% was obtained in the BHJ polymer-based photovoltaic devices.