具有共軛基團及烷氧基之富勒烯吡咯烷作為電子受體對於高分子太陽能電池之光伏特性的影響探討

Abstract

本論文分為兩個研究主題，探討使用不同官能化修飾的富勒烯衍生物作為電子受體，對於高分子太陽能電池之光伏特性的影響。第一個研究主題中，我們為了改善碳六十在可見光區吸收能力較弱的缺點，選用四種具有可見光區吸收的共軛基團做為碳六十上的取代基，合成出一系列具有可見光吸收的碳六十衍生物。 實驗中我們發現，將四種共軛分子鍵結在碳六十上時，它們各自的吸光範圍皆不相同，在可見光區也可達到比PCBM 更高的吸收係數。並且，碳六十上的共軛基團經可見光激發後，產生之激子可快速的轉移至C60 上，形成自由的電子與電洞。 作為電子受體並與聚(3-己基噻吩) (P3HT)混摻後所製備之太陽能電池，其元件效率最高可達到2.54%。雖然，效率不及於P3HT:PCBM 所製備之元件，但其中以 2,5-bis(4-hexylthiophen-2-yl)thiophene-fulleropyrrolidine (BTTC)作為電子受體的電池元件，在IPCE 圖譜350~400 nm 的光致電子的轉換仍可高出PCBM 達7%。這顯示，我們在碳六十上導入一吸光基團確實可對高分子太陽能電池的Jsc 產生貢獻。 此外，我們也成功利用BTTC、BTBTC 和BTBSeC 加熱後不易聚集的非晶特性，製備出具有優異熱穩定性的高分子太陽能電池。主動層在110 ℃下，連續加熱300分鐘後，其元件效率依舊維持不變。 第二部分的研究主題，是探討推電子的烷氧基團接在苯環鄰、對位置的碳六十衍生物對於元件效能的影響。導入推電子基團(2-ethylhexyloxy)的碳六十衍生物， 其LUMO 能階可高出PCBM 0.04~0.05 eV，並提升元件的Voc 大約30~50 mV。推電子基團接在苯環不同位置的碳六十衍生物，並不影響其光學特性，但會造成溶 解度上的差異。我們發現受到結構分子空間和材料本質聚集性的影響，對位取代的C60 衍生物其溶解度會低於對位取代的C60 達兩倍之多。並且，材料溶解度上的 差異性，亦造成載子遷移率以及元件主動層的異質接面形態有所不同。推電子基團在苯環鄰、對位置的碳六十衍生物作為電子受體，應用在P3HT 為予體材料之反式高分子太陽能電池中，最佳化的光電轉換效率可達3.01%和3.37%。

This thesis mainly studies the photovoltaic behaviors of polymer solar cells in which new C60 derivatives that bear various functional groups were applied as electron acceptors. It essentially consists of two parts. Firstly, to overcome the drawback of low light˗absorptivity of C60 in the visible region, four different conjugated groups were chemically bounded onto the N-methyl fulleropyrrolidine as a substituent. These molecules display distinct absorption bands and have a much higher molar absortivity than that of PCBM in the visible range. The photoluminescence (PL)experiments indicate a smooth transfer of photoexcited electrons from the conjugated substituent to the C60 cage, thereby creating free electron/hole pairs. Solar devices fabricated from the blends of poly(3-hexylthiophene) (P3HT) and these fullerene derivatives exhibit the best power conversion efficiency of 2.54%. More importantly, the comparison of the IPCE spectra of the P3HT/PCBM and P3HT/2,5-bis(4-hexylthiophen-2-yl)thiophenefulleropyrrolidine (BTTC) devices reveals the IPCE of the latter cell is higher than that of the former cell by about 7% in the wavelength range of 350~400 nm, which corresponds to the absorption band of terthiophene. This finding clearly verifies the excitons generated in the conjugated substituent can make contribution to the photocurrent. In addition, the thermal stability of the cells based on the blend of P3HT and these four C60 derivatives was examined by aging the film of photoavtive blend at 110 °C for various intervals before the evaporation of the metal cathode. Both OM and TEM images of the thermally aged blend films show the amorphous nature of BTTC,BTBTC and BTBSeC effectively suppresses the thermal-driven aggregation of C60 adducts during aging process, leading to a extremely stable blend morphology. Consequently, these devices almost retain their initial PCE even after storing at 110 °C for 300 minutes. In the second part, the influence of introducing an electron-donating, alkoxy, group into the C60 derivative and the anchoring position of such substituent on the photovoltaics of polymer solar cells, was investigated. As expected, the LUMO of these alkoxy-bearing fulleropyrrolidines is higher than that of PCBM by 30~50 mV. Although the binding position of the alkoxy moiety on N-methyl-2-phenyl fulleropyrrolidine has no obvious effect on the optical properties of the molecule, the para-substituted compound (4EHOBC) has a good solubility, which is about twice higher than that of the ortho-substituted compound (2EHOBC). This difference subsequently affects the carrier mobilities and the film morphology of their blends with P3HT. As a result, the P3HT/4EHOBC and P3HT/2EHOBC devices exhibit an optimal power conversion efficiency of 3.37% and 3.01%, respectively.