有機太陽能電池之小分子材料設計、合成與應用

Abstract

有機太陽能電池相較於其他再生能源，具有低成本、輕量化、材料多樣性及可撓曲等優點，近年來許多研究團隊致力於開發新穎的有機材料和元件架構以提升光電轉換效率。為了提升元件效率，本論文針對已開發的 D-A-A (Donor-Acceptor-Acceptor) 型有機小分子進行改良。

第一部分使用噻二唑并 [3,4-f] 二酮異吲哚作為中央的拉電子基，以苯環作為推拉電子基團中間的 π 架橋，末端拉電子基團分別為氰基和拉電子性更強的二氰乙烯基，合成出 DTCPTID 和 DTDCPTID 兩個分子。和苯并噻二唑作為中央拉電子基團的分子比較，發現兩者的最高佔有軌域 (HOMO) 差距不大，但最低未佔有軌域 (LUMO) 則差了0.5 eV 以上，有效的減少分子材料的能隙，使分子的吸光波段落在太陽光光子密度較為密集處，其中以 DTCPTID 作為電子予體搭配 C70 為電子受體，利用 BHJ 結構進行蒸鍍型的元件製作，其光電轉換效率 (power conversion efficiency, PCE) 為 5.53%，短路電流 (short circuit current, Jsc) 為 11.61 mA/cm2、開路電壓 (open circuit voltage, Voc) 為 0.85 V、填充因子 (fill factor, FF) 為 56.04%。在室內弱光元件部分，其 PCE 為 10.82%，Jsc 為 87.01 μA / cm2、Voc 為 0.60 V、FF 為 60.14%。

第二部分則是改變受體和予體間的 π 架橋，將苯環和苯并噻二唑置換成拉電子能力更強的氟取代苯并噻二唑 (FBT) 以及吡啶并噻二唑 (PT) 基團，設計並且合成出 DTDCDBT-F 和 DTDCPTBT 兩個分子。透過循環伏安法的測定，可以觀察到在置換成拉電子能力更強的基團上，對於降低 HOMO 能階有顯著的效果。希望此分子設計能有助於提升有機太陽能電池元件中的開路電壓，使其能有更好的元件表現。

Organic photovoltaics (OPVs) have attracted tremendous attentions because of their potential advantages such as light-weight, material diversity and mechanical flexibility. The device performance of an OPV devices is determined by three parameters inclusive of short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (FF). In order to improve device performances, we have developed series of Donor-Acceptor-Acceptor-configured small-molecule organic materials.

First, we have designed and synthesized DTCPTID and DTDCPTID by replacing benzo[2,1,3]thiadiazoe (BT) with thiadiazolo[3,4-f]isoindole-5,7-dione (TID). The introduction of more electron-deficient TID could drastically lower the LUMO level while slightly lowering the HOMO level, thus decreasing the bandgap.The device employing DTDCPTID as donor and C70 as acceptor exhibited a PCE of 5.53% for vacuum-deposited BHJ solar cells, along with a Jsc of 11.61 mA/cm2, a Voc of 0.85 V and a FF of 56.04%. This device could also display a PCE close to 11% under the illumination of TLD-840 fluorescent lamp.

Second, we have designed and synthesized DTDCDBT-F and DTDCPTBT by utilizing difluorobenzo[1,2,5]thiadiazole (FBT) and pyridal[2,1,3]thiadiazole (PT) as π-bridge, respectively. The energy levels calculated from cyclic voltammetry indicated that the stronger electron-withdrawing group in π-bridge, the deeper HOMO levels are. Hence, higher Voc values could be obtained because Voc values are positively related to the difference between the HOMO levels of donors and the LUMO levels of acceptors, and higher device performance could be expected in OPVs.