高分子半導體材料側鏈結構設計於有機光電子元件應用

Side Chain Engineering on Polymeric Semiconductors for Optoelectronic Device Applications

吳泓錦(Hung-Chin Wu)

指導教授 ： 陳文章

國立臺灣大學/工學院/化學工程學研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.00647

摘要

半導體高分子材料因其於有機場效電晶體、光伏打電池、與記憶體元件上之廣泛應用而備受囑目。近期新穎光電高分子的發展中，發現經由高分子側鏈結構設計，可有效調控材料於有機溶劑中之溶解度、高分子鏈間排列結構、甚至其薄膜形態。本論文目標在於開發具備不同共軛或是烷基側鏈結構之高分子材料，並探討側鏈化學結構設計對於高分子薄膜形態與光電特性之影響，系統性的探討側鏈化學結構對於載子遷移率、太陽能電池效率、以及記憶體資訊儲存性質之影響。側鏈結構開發策略主要可分為三大類：

1. 合成二維延伸分枝側鏈八噻吩共軛高分子於於場效電晶體與光伏打電池應用：本文第二章開發一系列具二維延伸分枝側鏈八噻吩結構(8T)之半導體高分子(P8TSe、P8TT、和P8TTT)，並探討其化學結構與光電性質的關係。8T基團本身側鏈分枝的結構可以有效降低最高填滿電子軌域能階，將有利於提升高分子光伏打電池之開路電壓。P8TTT具有最佳之載子遷移率5.0×10-5 cm2V-1s-1；同時，P8TTT/PC71BM光伏打電池可以達到2.81 %之光電轉換效率。為了進一步探討共軛側鏈構形對光電性質的影響，本文第三章延續開發二維線性延伸四噻吩和分枝延伸八噻吩與雙鍵官能基共聚合成之半導體高分子P4TV和P8TV。P4TV與P8TV最適化之電洞遷移率分別為0.12與0.0018 cm2V-1s-1，同時其最佳高分子/PC71BM光伏打電池光電轉換效率分別為4.04與2.69 %，P4TV因線性側鏈結構具有較優異之電荷傳輸能力，故擁有較佳之光伏打電池光電轉換能力；另一方面，P8TV光伏打電池則由於高分子材料本身具分枝共軛側鏈而具有非常優異之環境穩定性。

2. 開發新型主鏈電子施體-側鏈電子受體共軛高分子於軟性高分子記憶體元件應用：本文第四章合成新穎主鏈噻吩-芴系電子施體與側鏈咪唑電子受體共軛高分子，PFT-PI，利用主側鏈分別傾向捕捉電洞與電子的特性，成功在單一共軛高分子的使用條件下製備出軟性阻抗式快取記憶體元件，並具有低操作電壓(正負2V)、高電流開關比(104)、在高導電態或是低導電態皆可以維持104秒以上等特點，同時此軟性記憶體於撓曲環境下可維持穩定的操作；研究發現元件主要因為側鏈對於電子的捕獲，才穩定形成記憶體之特性。基於第四章之研究結果，本文第五章延續側鏈電子咪唑受體之高分子構形，開發一系列主鏈具有不同芳香環-雙鍵之高分子材料，並對此系列高分子之光物理特性以及記憶體元件性質做深入探討。由於不同高分子主鏈結構設計，使得材料之電子能隙與能階有顯著的不同，此差異同時也代表三個高分子側鏈電子受體對於電荷捕捉能力有所不同，進而成功調控記憶體元件電荷儲存特性。

3. 烷基側鏈結構對於高分子半導體薄膜電荷傳遞特性之影響與其於拉伸式電晶體元件應用潛力探討：本文的第六章開發一軟性接觸層壓法，可快速且有效率地鑑定適合用於拉伸式電晶體元件之半導體高分子材料，同時藉由實驗結果探討高分子烷基側鏈化學結構對於拉伸式高分子電晶體應用之影響。實驗中深入探討半導體高分子於不同拉伸程度之下的薄膜形態與載子傳導特性；根據高分子側鏈烷基化學結構組成改變，尋找適合應用於拉伸式場效電晶體元件之潛力材料。同時為進一步研發高效能共軛高分子材料，本文第七章合成一系列新穎異靛藍(isoindigo)為基礎之電子施體-受體高分子，同時高分子烷基側鏈具有不同碳長之矽氧(siloxane)末端基結構設計，其可調控高分子間堆疊排列之性質，並進一步影響場效電晶體之載子遷移率。其高分子之最適化電洞遷移率可以達到4.8 cm2V-1s-1，並具有緊密的分子間π-π堆疊(3.379 Å)，極具應用於下一世代光電元件之潛力。

從以上三類材料開發策略中，可成功利用側鏈結構設計調控有機高分子半導體元件之光物理特性、載子傳遞能力、太陽能光電轉換效率、以及數位資訊存取性質；另一方面，實驗結果亦證明半導體材料結構設計對於下一世代軟性可撓式以及可拉伸式電子元件開發具有深遠的影響。

關鍵字

側鏈結構 ； 高分子半導體 ； 形態學 ； 軟性電子 ； 光電子元件

並列摘要

Polymeric semiconductors have received great attentions for organic electronic and optoelectronic devices, such as field-effect transistors (FETs), photovoltaic cells (PVs), and memory devices. In the recent progress of polymer community, side chains are act as a crucial component in the design of novel conjugated polymers. They not only directly relate to the solubility but also affect the molecular packing motifs and thin film morphologies. The goal of this thesis is to address the effect of conjugated or alkyl side chain structures on the polymer thin film morphologies and the optoelectronic properties. In addition, the field-effect mobilities, photovoltaic, or memory characteristics are also probed to investigate the side chain engineering design on polymeric semiconductors for optoelectronic devices systematically. Three different strategies are explored in this thesis, as shown in followings:

1. Syntheses of two-dimensional branched thiophene extended octithiophene‐based conjugated polymers for field-effect transistors and photovoltaic cells:

In Chapter 2, three octithiophene (8T)-based conjugated copolymers, including P8TSe, P8TT, and P8TTT, have been synthesized. The larger atomic radius selenium (Se) atom possesses higher polarizability than sulfur (T), inducing stronger intermolecular interactions in solid state. Also, 8T moiety could significantly lower the HOMO level and lead to the enhanced open circuit voltage because of its branched conformation. The hole mobilities of these 8T-based copolymers were in the range of 1.32×10-5 to 5.00×10-5 cm2V-1s-1 with on/off ratio of 104. Among them, P8TTT showed better characteristics than the other polymers due to the fused-ring TT can promote self-organization and minimize the steric interactions. The power conversion efficiencies (PCE) of the copolymers/PC71BM based photovoltaic cells were in the range of 1.28 - 2.30% under the illumination of AM 1.5G (100 mW cm-2). In particular, P8TTT showed the best PCE of 2.81%, as the blend films are prepared from the mixed solvent of o-dichlorobenzene (DCB) and 1,8-diiodoctane (DIO) (DCB/DIO = 97%:3% by volume).

In Chapter 3, the synthesis, morphology and optoelectronic device applications of 2D extended quaterthiophene (4T)- and octithiophene (8T)-vinylene conjugated polymers, P4TV and P8TV, were explored. P4TV and P8TV exhibited smaller energy band gaps of 1.69 and 1.78 eV than that of parent polythiophenes, respectively, due to the reduced conformation distortion by the vinylene linkage. The highest field-effect hole mobilities of P4TV and P8TV were 0.12 and 0.0018 cm2V-1s-1, respectively, with on/off ratios around 104-105. In addition, the power conversion efficiency (PCE) of the P4TV/PC71BM based photovoltaic cells under the illumination of AM 1.5G (100 mW cm-2) was 4.04 %, which was significantly higher than that of P8TV/PC71BM with 2.69 %, due to its superior charge transport ability. However, P8TV had a better environmental stability attributed to its low-lying HOMO energy level.

2. Syntheses of main chain donor tethered side chain phenanthro[9,10-d]imidazole acceptor conjugated polymers for high performance flexible resistive memory devices:

In Chapter 4, a bipolar-recorded resistive memory device consisting of a single-layer donor-acceptor conjugated polymer fabricated on plastic polyethylene naphthalate (PEN) have been developed. The newly designed conjugated polymer with a main-chain donor of fluorene and thiophene and a side-chain acceptor of phenanthro[9,10-d]-imidazole (PFT-PI) was synthesized as an active memory material. The reproducible, nonvolatile flash switching characteristics of each sandwiched PEN/Al/PFT-PI/Al memory device was demonstrated under bending. The flexible nonvolatile resistor memory devices with low threshold voltages (±2 V), low switching powers ( 100 μW cm−2), large ON/OFF memory windows (104), good retention (>104 s) and excellent endurance against electric and mechanical stimulus. The simple and facile device fabrication was obtained from a single PFT-PI memory material, without using charge injection layers or a complex multilayer structure.

In Chapter 5, the synthesis and resistive memory device characteristics of new donor-acceptor conjugated poly(arylene vinylene), PVC-PI, PVT-PI, and PVTPA-PI, have been explored. The studied polymers possess similar HOMO energy levels (-5.08 ~ -5.18 eV), but with different LUMO energy levels (-2.24, -3.40, and -2.60 eV for PVC-PI, PVT-PI, and PVTPA-PI, respectively). The PVC-PI flexible memory with the sandwich configuration of PEN/Al/polymer/Al reveals the volatile static random access memory (SRAM) characteristic while the PVTPA-PI device exhibits the nonvolatile write-once-read-many-times (WORM) switching behavior. The above two devices could operate at low voltages (less than 2.5 V) with high ON/OFF current ratios (over 104) and exhibit excellent durability upon repeated bending tests. The PVT-PI device, however, only shows a diode-like electrical behavior. The polymer conformation affects the strength of D-A electrical polarization and charge trapping ability, leading to the variation on the volatility of the memory devices.

3. Effects of alkyl side chain design on charge transport: Synthesis, morphology, and stretchable transistor applications:

In Chapter 6, three polymers with variant alkyl side chain structures (i.e. short linear, long linear, and branched alkyl side chains), namely P3HT, PTDPPTFT4, and PII2T, are evaluated for stretchable field-effect transistors. In addition, a facile method to efficiently identify suitable semiconducting polymers for organic stretchable transistors using soft contact lamination is described. In this method, the various polymers investigated are first transferred on elastomeric poly(dimethylsiloxane) (PDMS) slab, and subsequently stretched (up to 100 %) along with the PDMS. The polymer/PDMS matrix is then laminated on source/drain electrode-deposited Si substrates equipped with a PDMS dielectric layer. The polymer semiconductors can be repeatedly interrogated with laminate/delaminate cycles under different amounts of tensile strain, and the strain limitation of semiconductors enable different side chain structures can be derived.

In Chapter 7, a series of isoindigo-based conjugated polymers (PII2F-CmSi, m=3-11) with alkyl siloxane-terminated side chains have been prepared, in which the branching point is systematically “moved away” from the conjugated backbone by one carbon atom. All soluble PII2F-CmSi (m=5-11) polymers exhibited hole charge carrier mobilities over 1 cm2V-1s-1, while the reference polymer with the same polymer backbone showed a much lower mobility of 0.13 cm2V-1s-1. PII2F-C9Si showed the highest mobility of 4.76 cm2V-1s-1, even though PII2F-C11Si exhibited the smallest π-π stacking distance at 3.379 Å. We concluded that it is beneficial that the branching site was further away from conjugated backbones to improve charge transport characteristics.

The above studies demonstrate that the optoelectronic properties, charge carrier transport ability, solar cell efficiency, and memory behaviors can be manipulated using side chain engineering design. The device performances were tuned by controlling the chemical structures of conjugated side chains. Moreover, with variant alkyl side chain structures, the charge carrier mobility in stretched polymer thin films were changed, indicating the design of side chain on polymeric semiconductors plays a crucial role for next-generation electronic device application.

並列關鍵字

Side chain ； polymeric semiconductors ； morphology ； flexible electronics ； optoelectronic devices

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