低能隙之共軛高分子應用於有機光伏電池及苝二酰亞胺物衍生物做為電子傳輸材料應用於鈣鈦礦太陽能電池之研究

Study of low band-gap π-conjugated polymers for organic photovoltaics and perylene diimide derivatives as electron transporting materials for perovskite solar cells

吳昭霖(Jhao-Lin Wu)

指導教授 ： 陳昭岑

共同指導教授 ： 陳錦地(Chin-Ti Chen)

國立臺灣大學/理學院/化學研究所/博士(2018年)

https://doi.org/10.6342/NTU201801216

摘要

本論文分為兩大部份，為兩大部份，第一部分是開發有機共軛高分子並將其應用於有機光伏電池的主動層材料，第二部分是開發有機材料作為鈣鈦礦太陽能電池的電子傳輸層。
於第一部分，本論文設計二酮吡咯並吡咯(diketopyrrolopyrrole, DPP)做為拉電子基團並與六種不同之寡聚-3-己基噻吩 (oligo-3-hexylthiophene) 聚合得到一系列之低能隙共聚高分子。根據寡聚-3-己基噻吩的分子結構，可分為具有區域選擇性但重複單元數目不同之2T、3T、4T分子；另一系列則是以不同碳鏈方向的四聚-3-己基噻吩異構物，分別為4T0, 4T1及4T2分子，其中0、1、2表示立體障礙的數目。根據吸收光譜、電化學分析及理論計算可知當3-己基噻吩構成的單體具有較佳的平面性時，其吸收波長紅移，而導致能隙下降並有高的電荷傳遞速率，因此在電池的表現上有高的短路電流值，但也導致HOMO能階的上升，而有小的開路電壓。相反的，當3-己基噻吩構成的單體有立體效應存在時，則有較深的HOMO能階，因此可以有較大的開路電壓，但相對使短路電流下降。其中，分子結構中有一個立體效應的HDDPP4T1高分子，恰好取得理想的電子能階及能隙，並且於製程中加入添加劑，將使高分子與PC71BM混摻形成的薄膜構成優異的微結構，因此其光電轉換效率達7.51%，根據這樣的結果將有助於開發高效率之有機高分子光伏電池。
於第二部分，由於富勒烯衍生物由於具有優異電荷傳遞速率，故廣泛運用於相關的光電領域的研究中，然而其低加工性、弱的吸光能力及高成本等因素，也限制了太陽能電池製程的開發及元件效率的表現。因此本論文選擇以具有分子穩定性及高電子傳輸率的苝二酰亞胺 (Perylene bisiimide, PDI) 衍生物中做為鈣鈦礦太陽能電池中電子傳輸材料。在PDI的結構設計上，將鹵素分子 (氟, 溴等) 導入PDI分子的灣位上，分別得到三種不同的X-PDI分子。在光學、電化學及薄膜表面形貌的實驗中，X-PDI分子對於有機溶劑的溶解度及本身的堆疊特性，將影響其導電度、電子遷移率及鈣鈦礦太陽能的電池表現。其中，由於Br-PDI分子具有優異溶解度且與主動層三鹵化甲胺鉛介面具有極佳的作用力，將有助於成膜性、電子遷移率並使的元件中的漏電效應，因此其能量轉換效率可達到3.2% (PC61BM為4.1%)。F-PDI分子由於有強的分子間作用力，如氟原子間、π軌域與氟原子間及π軌域間的作用力，將使得F-PDI有最低的電子遷移率、明顯的漏電效應及弱的介面間作用力，因此幾乎沒有轉換效率。而為提升整體的元件表現，另外選用氧化鋅奈米粒子導入電子傳輸層及電極間的介面，由於氧化鋅奈米粒子的導電能力及修飾了電子傳輸層的表面形貌，使其更為平緩，將可使X-PDI及PC61BM的轉換效率明顯提升，其中Br-PDI可提升到10.5% (PC61BM為11.6%)。藉由此結果，預測Br-PDI分子將具有潛力可以取代PC61BM作為新興的電子傳輸材料。

關鍵字

鈣鈦礦太陽能電池 ； 有機光伏電池 ； 溶液製程 ； 3-己基噻吩 ； 苝二酰亞胺

並列摘要

This thesis can be divided into 2 parts. The first was concentrated on designing and synthesizing donor materials for the organic photovoltaics (OPVs). The second part was focused on developing of electron transporting materials for the perovskite solar cells (PVSCs).
In the first part, we have synthesized and characterized a series of diketopyrrolopyrrole (DPP)-oligothiophene copolymers, of which the number of regioregular oligothiophene ring (2T, 3T and 4T) and the arrangement of the alkyl side-chain on regioirregular quarterthiophene (4T0, 4T1 and 4T2) are variable. The side chains with regioregular lead to more planar copolymer backbones and higher short circuit current (JSC), but backbone torsion (due to regioirregular side chains) generates greater open-circuit voltages (VOC) for DPP-oligothiophene copolymers. The increasing thiophene ring progressively raises HOMO energy level of copolymers but marginally affects their band gaps. Additionally, the HOMO energy level was found declined significantly with side-chain regioirregularity, because of reducing length of π-conjugation. The HDDPP4T0 exhibits the strongest absorption, extensive network structure, and high hole mobility (µh = 6.04 × 10-4 cm2 V-1 s-1). These characteristics contribute to the exceptional high JSC of 18.96 mA cm-2 for OPV with PCE = 6.12%. However, the HDDPP4T1 having an optimal combination of π-conjugation and energy level affords the second highest VOC (0.73 V) and the third highest JSC (16.89 mA cm-2), resulting the best PCE of 7.51 % among all. X-ray scattering, transmission electron microscopy, atomic force microscopy, and space-charge-limited-current (SCLC) easurements reveal that the solvent additive of diphenylether (DPE) enables PC71BM-blended copolymers thin film in crystallinic fibril with enhanced hole mobility.
In the second part, we have developed and demonstrated three solution processable perylene diimides, i.e., X-PDI, X = H, F, or Br, as nonfullerene electron accepting and electron transporting materials in inverted PVSCs. Whereas H-PDI or F-PDI performs
unsatisfactorily, our best PVSC is based on Br-PDI exhibiting PCE of 3.23%, which is just a bit shy of 4.13% of fullerene (PC61BM) PVSCs. Through a series of physical, spectroscopic, and microscopic studies, we have understood that the low solubility of F-PDI is a major factor causing the poor quality of the thin film, rendering virtually no photovoltaic effect for F-PDI. Although the solubility of H-PDI is better than F-PDI, the inferior electron mobility and conductivity make H-PDI PVSCs have relatively worse performance. Having the highest solubility, electron mobility, and conductivity among X-PDI, Br-PDI based PVSCs are almost as efficient as PC61BM based PVSCs. Within the ZnO NP as a CBL in the PVSCs, the PCEs of PVSCs based on X-PDI or PC61BM are significantly improved to 7.78%, 10.50%, and 11.07%, respectively.We infer that the CBL of ZnO NP has the function of interspacelling on the defect of X-PDI or PC61BM thin film, reducing the direct contact of Ag cathode to the perovskite material. Due to the strong molecular interaction, F-PDI aggregates significantly in thin film, creating too many and too large defects to be remedied or improved with or without CBL of ZnO NP. Very poor electron mobility and conductivity of F-PDI are two other factors devastating its PVSCs. Through this study, we have demonstrated that the simple mono-bromine substituted perylene diimide (Br-PDI), is solution processable and has potential for use as a non-fullerene electron accepting and electron transporting material in inverted PVSCs.

並列關鍵字

organic photovoltaics ； perovskites solar cells ； diketopyrrolopyrrole ； oligothiophene ； regioregularity ； nonfullerene acceptors ； perylene bisiimide

參考文獻

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