- 學位論文
以PCBM分子表面鈍化劑誘發FAPbI3晶體薄膜內的碘遷移及其於太陽能電池之應用
PCBM-molecule surface passivators induce iodide migration in the FAPbI3 crystal thin films and the application of solar cells
摘要
善用再生能源一直是人類需要重視的議題，其中鈣鈦礦太陽能電池的發展在近十多年更是在能源領域佔有一席之地。使用α晶相的FAPbI3鈣鈦礦晶體作為吸光層的鈣鈦礦太陽能電池不僅不會像MAPbI3晶體內的MA+有機陽離子因脫氫(dehydrogenation)而對環境造成危害，同時FAPbI3中的FA+陽離子與碘離子之間有更強的氫鍵，使得α晶相的FAPbI3晶體的能隙(1.48 eV)更小，可以吸收更寬頻的太陽光，因此α晶相的FAPbI3被視為目前更適合拿來做成鈣鈦礦太陽能電池的吸光材料。然而，FAPbI3薄膜卻面臨一個最大的問題，即是容易在室溫下從立方晶系(cubic)的α晶相 FAPbI3產生相變變成六方晶系(hexagonal)的δ晶相FAPbI3，前者的吸光能力好且晶格對稱，後者的吸光能力差且晶格也較為扭曲，亦對應較大的激子束縛能(Exciton binding energy)，因此不利於形成自由電子及電洞。為了解決這個問題，本論文研究α晶相FAPbI3晶體中碘離子遷移的機制，並透過優化電子傳遞層來提升FAPbI3太陽能電池的功率轉換效率(Power Conversion Efficiency, PCE)。實驗結果顯示當使用BrB:CB(1:4, v/v)混合溶劑溶解PCBM小分子做成的FAPbI3太陽能電池的PCE較佳。透過吸收度光譜、光激發螢光光譜、拉曼散射光譜及X光繞射圖譜可知此條件下的PCBM小分子可有效鈍化PCBM/FAPbI3的介面缺陷，同時亦保有良好的堆疊特性，對α晶相FAPbI3晶體的晶格扭曲程度影響最小，使得元件於30天後依舊保有穩定的光伏表現，其超過12%且穩定的功率轉換效率亦證實了穩定α晶相FAPbI3晶體之形成。
並列摘要
Effective use of renewable energy is an issue that human must focus on. The development of perovskite solar cells has occupied a place in the energy field in past decade. The perovskite solar cells which use α-phase FAPbI3 perovskite crystal as the absorption layer is not only friendly to the environment since it won’t have the dehydrogenation process caused by MA+ cation in MAPbI3 crystal, but also stable in the crystal structure since the FA+ cation has the stronger hydrogen bond with the iodide ions, which results in the smaller absorption band gap and thereby harvesting the broader wavelength range of the sunlight. As mentioned above, α-phase FAPbI3 crystal is regarded as the appropriate light absorbing material in perovskite solar cells currently. However, the FAPbI3 thin film is facing a huge problem which is α-phase FAPbI3 can easily transform into δ-phase FAPbI3 at room temperature. The former one has an efficient light absorbing ability and a symmetric lattice structure while the latter one has a poor light absorption ability and a distorted lattice. Besides, the photo-inactive δ-phase FAPbI3 has the larger exciton binding energy, which cannot easily form free electrons and holes. To solve this problem, we explore the mechanism of iodide ions migration in the α-phase FAPbI3 thin films, and optimize the electron transport layer in order to increase the power conversion efficiency (PCE) of the resultant FAPbI3 solar cells. The experimental results show that the FAPbI3 solar cells have the higher PCE when the PCBM molecules are dissolved by a bromobenzene:chlorobenzene (1:4, v/v) mixture used to prepare the PCBM thin films. The absorbance spectra, photoluminescence spectra, Raman scattering spectra and X-ray diffraction patterns show that the optimized PCBM solution can simultaneously achieve an effective defects passivation at the PCBM/FAPbI3 interface, a high molecular packing structure in the PCBM thin film, and a low effect on the lattice distortion of α-phase FAPbI3. The formation of stable α-phase FAPbI3 thin films is also confirmed by the high PCE of about 12% after 30 days.