功能性有機共軛分子於反式鈣鈦礦太陽能電池之介面應用

Abstract

鈣鈦礦為一種半導體結晶材料，維度可以經設計而橫跨0D至3D結構，其中，3D鈣鈦礦材常作為太陽能電池之主動層，因其具備優秀的光電性質，包括可以有效吸收並轉換可見光區光子為光電流、低激子結合能、長激子擴散長度、雙極傳輸載子之能力等等，僅僅十年，光電轉換效率已由3.8%升至25.2%，搭配其低溫低成本製備的特性，鈣鈦礦太陽能電池成為新興太陽能領域中最有望商業化，能與矽太陽能電池匹敵之重要角色。在本研究中，我著重於如何利用功能性有機共軛分子提升反式鈣鈦礦太陽能電池的性能。 首先，我利用螺烯作為電洞析取層以修飾既有且粗糙的氧化鎳電洞傳輸層，該螺烯具有之雙基特性能夠提升反式鈣鈦礦太陽能電池之載子介面傳輸能力，進而提升整體光電轉換效率。此外，螺烯所製造的疏水平面可以增強鈣鈦礦內分子作用力，因而提升旋轉塗佈製備之結晶度，高結晶度不只會大大提升光電性能，更是使其具備更強的抗水氧光熱刺激能力而提升穩定性。 雖然鈣鈦礦可以有效吸收可見光，但近紅外光區之外仍然無法有效利用，縱使可以經由調控鈣鈦礦組成而紅移擴展光電響應，但往往犧牲了光電轉換效率；在本研究中，我將低能階非富勒烯有機共軛小分子混入既有的電子傳輸層，位在鈣鈦礦層之後攔截無法被鈣鈦礦利用之近紅外光光子。然而該二元體系無法轉換任何近紅外光成電流，唯有配合P型高分子之混摻以提升電洞遷移率，促進受近紅外光激發的激子於電子傳輸層內有效分離，該三元異質結電子傳輸層才能夠有效捕獲近紅外光光子並轉換為光電流，成功擴展該反式鈣鈦礦太陽能電池之光電響應至950nm。

Perovskite is an emerging class of crystalline semiconductors, and its dimension can be tailored from 3D to 0D for distinct applications. In recent years, 3D perovskite has been recognized as the promising photoactive materials because of its excellent optoelectronic properties, including the high photon-to-electron conversion efficiency, low exciton binding energy, long carrier diffusion length, and ambipolar charge transporting capability. Within only decades, the power conversion efficiency (PCE) has reached 25.2% from 3.8%. Valuing its solution-based low-temperature fabrication processes and low production cost, perovskite solar cells (PVSCs) have become the front-runner in the photovoltaic community for commercialization. In this study, we focus on employing functional conjugated molecules to improve the performance and stability of the inverted PVSCs. First, we introduced helicene-based small molecules, 1ab and 1bb, as the hole-extracting layers to modify the surface of commonly used nickel oxide hole-transporting layer (HTL). It was revealed that the unique biradical characteristics of both 1ab and 1bb can enhance the interfacial hole transfer ability at the perovskite/HTL interface. Moreover, their hydrophobic property can enhance the vertical diffusion of perovskite itself during film evolution, resulting in better crystalline. The enhanced crystalline of perovskite not only affords better optoelectronic properties, but also provide a better robustness against the invasion of external stimulus, such as heat, moisture, oxygen and radiation. Combining all these advantages, the 1ab/1bb-based PVSCs were finally demonstrated to deliver improved PCEs than the control device. Although 3D perovskite can absorb a wide range of light, the intensive absorption range is still limited within the visible region (300~800 nm), and the photons in/beyond the near-infrared (NIR, >800 nm) region will be wasted. Usually, the absorption of perovskite materials could be red-shifted or broadened through composition tuning. However, composition engineering always sacrifices the performance owing to the inferior stability and semiconducting properties. To remedy such disadvantage, we herein extend device’s absorption into NIR region by employing a rationally designed electron-transporting layer (ETL) for an inverted PVSC. A low bandgap non-fullerene acceptor, BT-CIC, was blended into fullerene-based ETL to intercept the transmitted NIR photons. However, such binary system could not convert any exciton generated by NIR. It was found that the culprit should be the poor hole-mobility of such binary system, and it could be simply solved by adding a p-type polymer to facilitate the exciton dissociation. The ternary bulk-heterojunction ETL was finally demonstrated to successfully convert the NIR photons into photocurrent, broadening device’s photo-response to 950 nm.