不同官能基之共軛聚電解質:合成、性質和應用於 p-i-n鈣鈦礦太陽能電池特性分析

Abstract

本文旨在設計、合成及分析帶有不同官能基的共軛聚電解質，透過鑑定高分子的官能基修飾，並探討分析高分子結構與性質，以開發高效性電洞傳導層材料，應用於p-i-n結構之鈣鈦礦太陽能電池。 本研究第一部份使用芴(fluorene, FL)與苯并噻二唑(benzothiadiazole, BT)作為高分子主鏈的共軛主結構，其中FL在九號位上兩個氫原子容易被取代成含有末端官能基之雙碳鏈結構，而另一部分則引入BT，此為強拉電子基團，可使高分子HOMO能階有效下降，而兩旁的thiophene則為π-spacer，用以舒緩FL與BT之間的扭轉角，增加主鏈平面性。本研究共合成三種不同官能基的共軛聚電解質，分別為PFNH3BT (-NH3+)、PFNM3BT (-NMe3+)及PFSO3BT (-SO3-)，並使用為p-i-n鈣鈦礦太陽能電池的電洞傳導層。元件分析上，因為這些共軛聚電解質擁有較深HOMO能階(依序為-5.38 eV、-5.44 eV及-5.40 eV)，可有效減少電動傳遞時的能量損失，增加開環電壓值。另外，它們的表面均比PEDOT:PSS疏水，使得在其表面成長的MAPbI3晶粒比在傳統常用的PEDOT:PSS表面成長者大，對短路電流有所提升。因此，使用此等共軛聚電解質來取代PEDOT：PSS作為電洞傳導層，可顯著地將元件最高效率從12.89 %分別大幅提升到17.71 % (PFNH3BT)、14.92 % (PFNM3BT)及17.14 % (PFSO3BT)。另外，三種官能基間存在微小的差異，一個表面性親疏水性導致鈣鈦礦成長的差異，另一個是對ITO表面功函數修飾的差異，在元件表現上PFNH3BT相似於PFSO3BT而略好於PFNM3BT。 第二部分置換HTL為P3HT-COOH之p-i-n鈣鈦礦太陽能電池結構，延用帶有-NMe3+之高分子PFNM3BT，用以修飾PC60BM與陰極(Ag)間的界面，有效降低Ag的表面功函數從4.60 eV到4.40 eV，同時改善兩層間的接觸面狀態、降低再結合速率，將填充因子從76.92 %增加到80.86 %，而電流則微小的提升，最後效率從16.30 %提高到17.11 %。 最後一部份則著重於設計另一種新的共軛主鏈，其係將環戊二噻吩(cyclepentadithiophene, CPDT)及二氟噻吩(difluorothiophene)進行交替式共聚合，形成帶有羧酸根之共軛聚電解質，PC3HDFT及PC8HDFT。紫外光-可見光吸收光譜顯示，各別的溶液態與薄膜態間並無明顯的紅位移，薄膜能階分析上，這兩者皆有深的HOMO能階(分別為 -5.38 eV和-5.42 eV)，最後作為p-i-n鈣鈦礦太陽能電池結構之電洞傳導層，初步嘗試元件效率約為14.44 %及14.29 %。

A series of conjugated polyelectrolyte (CPE) with different functional groups were designed and synthesized by Stille polymerization. The correlation between chemical structures and properties of such CPEs and the effect of these polymers as the hole transport layer of perovskite solar cells on photovoltaic properties were fully investigated. In the first part, three CPEs with the same conjugated backbone were synthesized using fluorene (FL) and benzothiadiazole (BT) as building blocks. FL is a useful monomer to introduce various functional groups into polymer main chain by simply grafting the 9-position of the ring with two functionalized alkyl chains. Herein, an electron-deficient BT monomer and two thiophenes were applied to lower the HOMO and reduce the twist angle between FL and BT, producing three new CPEs with ammonium (NH3+), trimethylammonium (NM3+) and sulfonate (SO3-) moieties, namely PFNH3BT、PFNM3BT and PFSO3BT, respectively. These CPEs were then utilized as a hole transport layer (HTL) to fabricate inverted perovskite solar cells (PSCs). The solar cells based on our new CPEs outperformed the one with PEDOT:PSS, which is the most common HTL. The improved open-circuit voltage (Voc) was attributed to reduced energy loss of transferring holes from perovskite to HTL because the HOMO levels (- 5.38 eV, -5.44 eV and -5.40 eV) of our CPEs are deeper than that (-5.20 eV) of PEDOT: PSS, minimizing the gap to the HOMO (-5.62 eV) of perovskite. In addition, the perovskites can grow into bigger crystals and grains on the conjugated polyelectrolytes than on the PEDOT:PSS surface, reducing charge carries recombination and then enhancing the short-circuit current density (Jsc). However, the type of functional groups greatly influenced the growth behavior of perovskites and interfacial charge recombination resistance. As a result, replacing PEDOT:PSS with PFNH3BT, PFNM3BT and PFSO3BT as HTL effectively increases the power conversion efficiency (PCE) from 12.89 % to 17.71%, 14.92 % and 17.14 %, respectively, for champion cells. In the second part, PFNM3BT was chosen as an interlayer sandwiched between PC60BM as the electron transport layer, and silver as the electron collection electrode because it is soluble in high polar alcohol solvents. This polyelectrolyte interlayer successfully reduced the WF of Ag from 4.60 eV to 4.40 eV, as measured by photoelectron spectroscopy in air (PESA), and increased charge recombination resistance, as measured by electrochemical impedance spectroscopy (EIS), increasing the fill factor from 76.62 % to 80.86 %, and slightly improving the Jsc. Consequently, the PCE was raised from 16.30 % to 17.11 %. In the final part, we utilized cyclepenta-dithiophene (CPDT) and difluorothiophene (DFT) as building blocks to synthesize new type of CPE with carboxyl acid groups. Two such PCEs with propyl and octyl chains were prepared and named as PC3HDFT and PC8HDFT, respectively. The PESA measurements indicated both CPEs had deep HOMO levels. Preliminary PCEs of the PSCs based on PC3HDFT and PC8HDFT as HTL were 14.44 % and 14.29 %, respectively.