共軛高分子/富勒烯吡咯烷衍生物之高分子太陽能電池之光伏特性探討

Abstract

目前在異質接面高分子太陽能電池中，主動層材料通常是以共軛高分子作為電子予體，以富勒烯衍生物作為電子受體，其中又以聚(3-己基噻吩)(P3HT):PCBM 之系統最被廣泛研究。本論文即分為兩部份，分別探討與尋找適用於異質接面高分子太陽能電池中的電子予體及電子受體材料。 在第一個研究主題，我們設計了三種以噻吩(thiophene)為架構的二維共軛高分子P(2EHT-BT)、P(2EHT-TT)、P(2EHT-DTT)，以側鏈為三噻吩之共軛側鏈結構為主體，主鏈分別導入雙噻吩(bithiophene)和駢環之雙環(thieno[3,2-b]thiophene)與三環(dithieno[3,2-b:2’,3’-a]thiophene)結構，探討主鏈結構共軛長度對材料結晶行為、光學、電化學性質以及與PCBM 混摻所製備之太陽能電池之光伏特性的影響。由實驗結果我們得知，當主鏈中導入龐大地側鏈基團時，主鏈結構越長，對於材料之結晶行為及電洞傳遞速率表現越好，此外，在此系統中，主鏈若導入共平面的駢環結構，則溶解度會大幅下降，不利於元件之製備。在光學性質方面，三種共軛高分子皆具有兩個吸收峰且吸收範圍相當寬廣，而在材料之能階方面，材料之HOMO 能階皆比P3HT 來得低。由於側鏈貢獻了部份的光電流，且因為HOMO 能階較低，因而元件有較高的開環電壓，由此三種共軛高分子與PCBM 所構成的太陽能電池中，其光電轉換效率最高可達到4.62 %。 而在第二部份的研究主題中，我們發現富勒烯吡咯烷衍生物在氮上接上不同取代基時，對於與P3HT 混摻之高分子太陽能電池的光伏特性有很大的影響。首先，由碳六十衍生物N-phenyl、N-methyl、N-isopentyl、N-hexyl 分別與P3HT 混摻所製備成的順式結構元件中發現，在富勒烯吡咯烷衍生物氮原子上接上烷基鏈段的碳六十衍生物與P3HT 所製備成的元件有S-shape 的光伏特性產生，經由載子傳遞率的量測，我們發現元件之電洞傳遞速率過慢而導致了S-shape 光伏特性的產生。我們由混摻薄膜之TEM、XRD、UV及PL 光譜的量測，觀察到P3HT 在混摻薄膜中仍有自組裝排列的情形且與這些碳六十衍生物混摻之後也有足夠的混摻異質接面，與P3HT:N-phenyl 之混摻薄膜並無太大差異，而由SIMS 之縱身分佈數據可以得知接有烷基鏈段的碳六十衍生物較容易沉降在光作用層底部，使得電洞之遷移率明顯較慢，進而產生了S-shape 電壓電流特性圖。由簡單的UV 吸收光譜、模擬計算得知，接有烷基鏈段的碳六十衍生物較易沉降的原因為碳球本身的帶電量較高，使得其與P3HT 之電荷轉移作用力較弱，同時本身之聚集行為也較強，導致在成膜之過程中，聚集至一定之重量後就沉降至光作用層底部。為了驗證我們的推測，我們進一步在N-phenyl 之苯環外面分別接上了推電子與拉電子基團，N-methoxyl及N-carbonyl，由模擬計算的結果得知N-methoxyl上的碳球帶電量高於N-carbonyl，其與P3HT 混摻所製備而成的順式結構太陽能電池也產生了S-shape 的光伏特性，此一結果也如我們所預期。順著沉降的問題，我們更利用了反式太陽能電池之架構，解決了S-shape 的光伏特性，此結果除了提供我們一個方向去設計適用於反式高分子太陽能電池的新型碳六十衍生物之外，也使我們了解，除了碳六十衍生物的溶解度會影響混摻薄膜之形態外，改變碳球本身的電子雲密度也會影響到碳六十衍生物與P3HT 混摻之後的形態。

In the bulk heterojunction polymer solar cells, it usually takes conjugating polymer as electron donor and fullerene derivatives as electron acceptor in the active layer, especially in P3HT-PCBM system of intensive study but optimization of their properties are still in need for better efficiency. This thesis includes two aspects for searching for the appropriate electron donor and acceptor in the bulk heterojunction polymer solar cells. In the first aspect, a series of new two-dimensional polythiophene derivatives with terthiophene as pendant group in which bithiophene (BT) and fused thiophene families, thieno[3,2-b]thiophene (TT) and dithieno[3,2-b:2’,3’-a]thiophene (DTT), were employed as comonomer to reduce the steric hindrance of bulky side-chain and increase the extent of coplanarity of polymer main chain, yielding P(2EHT-BT), P(2EHT-TT), and P(2EHT-DTT), respectively, were applied as electron donor blending with PCBM as electron acceptor to fabricate polymer solar cells. The photoactive blends were characterized by UV-vis, SCLC carrier mobility, TEM measurements. These polymers possess not only a broad light-harvesting range with two absorption bands but also a low-lying HOMO energy level, resulting in high open-circuit-voltage photovoltaic cells. Therefore, the P(2EHT-BT) device exhibits a promising power conversion efficiency (PCE) of 4.62%. However, the use of fused thiophene as backbone spacer significantly lowers the solubility of polymer that in turn decreases the short-circuit current and the PCE. In the second part, a series of N-substituted fulleropyrrolidines were employed as electron acceptor blending with P3HT as electron donor to fabricate polymer solar cells. It was found that the type of substituent significantly influences the photovoltaic behavior of the solar devices. As the substituent is an alkyl group, such as N-methyl, N-isopentyl, N-hexyl, abnormal S-shape current-voltage (I-V) curves are resulted. Conversely, the cell based on N-phenyl fulleropyrrolidine exhibits a normal I-V behavior. The analysis of the polymer/fullerene photoactive blend by TEM, XRD, UV-vis, and PL shows no obvious difference between the P3HT/N-alkyl fulleropyrrolidine and P3HT/N-phenyl fulleropyrrolidine films. In the result of SIMS, it can be ascribed that the N-alkyl fulleropyrrolidine forms an interlayer at the bottom of the photoactive film in the course of spin-dying, blocking the transport of holes to the anode and thus inducing S-shape I-V curves. Moreover, the results from UV-vis absorption experiment and molecular simulation of fulleropyrrolidine derivatives suggest the N-substituent may induce changes on the electron density of C60 core that reinforces or deteriorates the interaction between the fullerene and the thiophene units of P3HT. A weak association of fullerene on P3HT leads to the settle of the fulleropyrrolidine acceptor down to the bottom forming a fullerene-rich layer because of its relative high surface energy compared with that of P3HT. To verify this speculation, the para position of N-phenyl substituent in fulleropyrrolidine was functionalized with an electron-donating group, methoxyl, or an electron-withdrawing group, carbonyl, to manipulate the electron density on fullerene cage. As expected, the cell using N-methoxylphenyl fulleropyrrolidine as acceptor has an S-shape I-V curve but the one based on N-carbonylphenyl fulleropyrrolidine group behaves a normal photovoltaic performance. Interestingly, as an inverted cell structure is adopted to reverse the transport routes of carriers inside the photoactive blend, the problem of S-shape kink associated with all N-alkyl fulleropyrrolidine are totally solved, suggesting these new fullerene acceptor can be applied as effective acceptor to develop high-efficiency polymer solar cells.