單層異質接面有機光伏電池使用含氟取代半導體共聚高分子

Bulk-Heterojunction Organic Photovoltaic Devices Based on Fluorine-Substituted Semiconducting Copolymers

方達(Adane Desta Fenta)

指導教授 ： 陳錦地 ； 劉瑞雄

國立清華大學/理學院/化學系所/博士(2021年)

摘要

在各種光伏電池中，高分子太陽能電池由於其溶液加工性使其具有成本效益、機械可撓曲性以及較小的環境衝擊性，而備受矚目。目前，溶液製程的體異質接面元件在性能方面已經有了非凡的進展。目前高分子太陽能電池領域最高的功率轉換效率已超過17％。這一顯著進步不僅是藉由對元件結構且同時對高分子或小分子半導體材料結構之設計優化之結果。而化學結構優化之過程包含了使用各種化學方法來調整高分子之光電特性和形貌特徵。最近的幾種高性能有機半導體材料皆使用了氟作為取代基，而將其導入在共軛高分子或小分子骨架中；在許多例子中，此方法也已成為開發有潛力的半導體有機材料的實用方法。 有鑑於此，本論文研究了含氟之噻吩並[3,4-c]吡咯-4,6-二酮（TPD）作為供體高分子，而將該一系列高分子與富勒烯或非富勒烯小分子作為受體材料共混摻而製成高分子太陽能電池元件。本研究中深入探討了體異質接面混摻形貌與元件性能之間的相依性，且觀察到氟化對高分子的光電和形貌特徵的影響以及這些性質如何反映在元件性能表現。以下兩個部分闡述了本研究中完成的主要任務和獲得的結果。
在本文的第一部分中，我們系統地設計並合成了五種噻吩並[3,4-c]吡咯-4,6-二酮（TPD）-噻吩共聚物，它們是PHT，P1F3HT，P1F1HT，P3F1HT和PFT；其化學結構上的差別在其高分子合成過程中，分別導入了0％，25％，50％的75％和100％的雙氟化噻吩單體用來取代未氟化的普通噻吩單體而與噻吩並[3,4-c]吡咯-4,6-二酮（TPD）單體共聚合。這些高分子與PC61BM混摻可製成厚的（〜300 nm）反式體異質接面(bulk-heterojunction（JSC）隨著導入的雙氟化噻吩單元逐漸增加，最高功率轉換效率（PCE）高達9.68 %；這是目前TPD高分子與PC61BM富勒烯衍生物為電子受體的太陽能電池中最高的功率轉換效率。從AFM和TEM研究中可以看出，隨著高分子中氟取代基的增加，在太陽能電池主動層中形成的高分子奈米纖維結構的纖維寬度逐漸減小。這可以歸因於疏氟效應使其溶解度下降，導致具有更多氟取代基的高分子更容易急遽聚集形成奈米纖維結構。GIWAXS研究表明，具有更多氟取代基的高分子的層狀堆積和π-π堆積結構的結晶度得到相當程度的提高。DFT計算表明，對噻吩單元與氟取代基實際上處於共平面構形。增加的VOC也與增加的氟取代基有關，這通過電化學和光電子能譜測量可證實增加氟取代基能有效降低高分子的HOMO能階，進而提升VOC。
本論文的第二部分討論了將TPD高分子製成單層三成分體異質接面主動層，即所謂的三元體系，來與二元體系有機太陽能電池作比較。三元體系已被證明是一種提高功率轉換效率的潛力策略。
為了保持主動層具較厚的厚度（100-400 nm）的同時，進一步改善PCE，我們使用寬能隙高分子作為電子供體（PFT）；結晶性稠環小分子（IT4F）以及典型的富勒烯衍生物（PC71BM）作為電子受體而製作三元系高分子太陽能電池元件。結果，最佳化之後的PFT：IT4F：PC71BM的三元系主動層元件具有12.85％的功率轉換效率，明顯優於基於PFT：IT4F的二元系元件（10.94％）。通過對元件製程的精細調整，厚主動層（≥300 nm）三元系元件的PCE達到了12.36％，相較於已知的ITIC或其衍生的NFA有機太陽能電池元件而言，具有如此厚主動層的元件是一個了不起的結果。我們已經進行了全面的研究，包括吸收和放射光譜的能量轉移、薄膜光電子能譜和電化學循環伏安法對分子軌道的能級檢測、AFM和TEM顯微鏡BHJ形貌的拍攝、GIWAXS對高分子鏈微晶結構和2D取向的繞射測量、空間電荷限制電流（SCLC）進行的載子遷移率推算、動態光致發光猝熄用於激子拆解的實驗以及通過短路電流密度（JSC）和開路電壓（VOC）對光強度的相依性進行載子再結合的描述。據此，我們闡明了為什麼三元體系的混合重量比為1：1：0.25最好，以及為什麼PFT：IT4F：PC71BM三元系OPV在如此厚（300 nm）的BHJ主動層下能獲得如此可觀的PCE。這項工作表明，同時使用具氟取代基的供體和受體以及加入適當的第三種成分（例如PC71BM）是製造高效率厚主動層OSC的有效方法，而厚的主動層乃有利於實現未來大規模生產。

關鍵字

高分子太陽能電池 ； 富勒烯受體材料 ； 非富勒烯受體材料 ； 薄膜 ； 單層異質接面 ； 氟 ； 厚度 ； 溶劑加工添加劑

並列摘要

Among the varied photovoltaic devices used to convert solar energy to electricity, polymer solar cells have gained considerable attention due to their solution-processability, which allows cost effective, mechanically flexible and low environmental-impact production. Typically, solution-processed bulk heterojunction devices have experienced an extraordinary evolution in performances, achieving power conversion efficiencies over 17%. This remarkable progress is the result of a simultaneous development of more advanced device structures and optimized polymeric or small molecular semiconducting materials. Various chemical methods have been employed to modulate the optoelectronic and morphological polymeric parameters needed to attain high efficiencies. Incorporating fluorine atom into the polymer or small molecule backbone has emerged a practical approach for the development of potential semiconducting organic materials, as most recently high performing semiconducting materials are using this curious substituent in their conjugated backbone. Therefore, this thesis work investigates polymer solar cells made of a blend system of fluorine-containing Thieno[3,4-c] pyrrole-4,6-dione (TPD)-based donor copolymers and fullerene or non-fullerene acceptors. The interdependence between the bulk heterojunction morphology and the performance of the devices has been intensively examined. The influence of fluorination on optoelectronic and morphological characteristics of the bulk and hence device performances have been also observed in this study. The major tasks done and results obtained in this study are given in the following two broad portions.

In the first part of this thesis, five thieno[3,4-c]pyrrole-4,6-dione (TPD)-terthiophene copolymers, PHT, P1F3HT, P1F1HT, P3F1HT, and PFT having 0%, 25%, 50%, 75%, and 100% of fluorine substituent on the center of terthiophene were systematically designed and synthesized. Accompanying with PC61BM in a thick (~300 nm) BHJ of inverted polymer solar cells, these copolymers exhibit progressive increase of open-circuit voltage (VOC), short-circuit current density (JSC), and hence power conversion efficiency (PCE) up to 9.68% of PC61BM-blended copolymer photovoltaic, which is the highest PCE reported for TPD-based polymer solar cells with fullerene derivatives as the electron acceptor. It can be seen in AFM and TEM studies that the width of copolymer nano-fibril formed in solar cells decreases progressively with the increasing fluorine substituents of the copolymers. A declining solubility, which is attributed to the fluorophobic effect, narrows the nano-fibril of copolymers with more fluorine substituents. The GIWAXS studies reveal the improved crystallinity of lamellar stacking and π-π stacking structure of copolymers with more fluorine substituents. DFT calculation indicates a virtually coplanar conformation of terthiophene unit with fluorine substituents. The increasing VOC is also associated with the increasing fluorine substituents, which lower the HOMO energy level of the copolymers verified by the electrochemical and photoelectron spectroscopic measurements.

The second part of the thesis addresses utilizing a single three-component Bulk heterojunction layer the so called ternary system, which is practically proofed as a potential strategy for boosting the power conversion efficiencies than the binary counterpart in organic solar cells. Aimed at achieving further improved PCE while maintaining broad active layer thickness toleration (100-400 nm), ternary OSCs are fabricated by using a wide-bandgap polymer donor (PFT), a crystalline fused-ring small molecule (IT4F) as host electron acceptor, and a typical fullerene derivative (PC71BM) as a third component. As a result, the optimized PFT:IT4F:PC71BM based ternary device exhibits a decent PCE of 12.85%, which significantly outperforms the PFT: IT4F-based binary control devices (10.94%). With a fine tuning of the device fabrication process, a remarkable PCE of 12.36% was achieved in thick-film (≥ 300 nm) ternary device, which is an outstanding result for devices based on ITIC or its derivative NFAs having such thick active layers. We have conducted a comprehensive study, including energy transfer by absorption and emission spectroscopy, thin film photoelectron spectroscopy and (electrochemical) cyclic voltammetry for the energy level of molecular orbitals, BHJ morphology by AFM and TEM microscopy, polymer chain crystallite and alignment orientation by 2D GIWAXS, charge carrier mobility by space-charge limited current (SCLC), dynamic photoluminescence quenching for exciton dissociation, and charge recombination by light-intensity dependence of short-circuit current density (JSC) and open-circuit voltage (VOC). Accordingly, we have elucidated why blending weight ratio of 1:1:0.25 of the ternary system is the best and why PFT:IT4F:PC71BM ternary OPVs achieve respectful PCE with such thick (300 nm) BHJ. This work demonstrates that adoption of both fluorinated donor and acceptor pair together with appropriate third component (e.g PC71BM) is an effective approach to fabricate efficient thick film OSCs, which can facilitate the future large scale production.

並列關鍵字

Polymer solar cells ； fullerene acceptors ； non-fullerene accepters ； thin film ； bulk heterojunction ； fluorine ； thickness ； solvent processing additives

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