Remove the water-induced traps toward improved performance in organic solar cells
Mumin ShiTao WangRui SunQiang WuDandan PeiHui WangWenyan YangWei WangYao WuGuohua XieTao WangLong YeJie Min
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Organic Electronics
Realizing cheap-end cost-effective electronics has been the principle motivation behind the research in organic photovoltaic (OPV) field. The promise of low cost relies on using semiconducting polymers which can be cheaply synthesized with electronic properties tailored according to need. These cheaper organic semiconductors can be wet-chemically processed at room temperature on flexible substrates. This entire process can be scaled up to large scale roll-to-roll process which opens the door to a new field of “plastic electronics”. OPV has been considered to be the next generation of photovoltaics. However, among other challenges, the understanding of efficient charge carrier extraction with the use of optimized buffer layer has been one of the major drawbacks in these OPV systems.
In this PhD work, PEDOT:PSS has been used as a model anode buffer layer for a P3HT-PCBM based bulk heterojunction solar cells, used here as a standard organic solar cells. Although PEDOT:PSS is the most common buffer layer used in P3HT-PCBM based organic solar cells, it is not the most optimized anode buffer layer available. This thesis creates a path toward the understanding of buffer layers in organic solar cells based on P3HT-PCBM systems with a primary focus on the anode buffer layer. PEDOT:PSS has been used in the majority of this work as the standard buffer layer system for P3HT-PCBM active layer. It has been modified and treated with various additives and solvents in order to understand
the mechanisms for device performance enhancement. A novel electropolymerization method for depositing the polythiophene anode buffer layer, namely the timedependent cyclic-potential-sweep method, was developed and successfully applied for enhanced performance of the organic solar cells. Similarly, the area dependence seen in OSCs while treating PEDOT:PSS with some additives and solvents has been systematically studied and explained with simple models.
Organic Electronics
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Buffer (optical fiber)
Organic semiconductor
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The original vision of organic electronics comprises the use of organic conductors and semiconductors specifically designed to accommodate large strains to enable highly deformable and mechanically robust devices for organic photovoltaics, biosensors, and electronic skins. However, mechanical properties of organic materials are often overlooked; as a result, many of these materials are unable to accommodate the mechanical stresses required for their intended applications. Thus, it is important to understand the parameters that govern mechanical properties of these materials. Chapter 1 provides an introduction to the characteristics, applications, and fabrications of stretchable electronics. The idea of intrinsically stretchable electronics comprising molecularly designs of semiconducting polymers is outlined. Chapter 2 focuses on the mechanical degradation and stability of organic solar cells. The key highlights are the importance of mechanical properties and mechanical effects on the viability of organic solar cells during manufacture and in operational environment. Chapter 3 and Appendix A investigate the effects of the length of the alkyl side chains in poly(3-alkylthiophenes) on the deformability of the pure polymer films and their blends with fullerenes. Chapter 4, 5, and Appendix B provide studies on the inherent competition between good photovoltaic performance and mechanical compliance; a critical length of the alkyl side chains on the poly(3-alkylthiophene) allows for co-optimization of both photovoltaic and mechanical properties. In Chapter 6 and Appendix C, the effect of incompletely separated grades of electron acceptors on the mechanical deformability of organic solar cells is investigated in an effort to simultaneously improve the mechanical robustness of the organic solar cells and reduce the energy of production. Chapter 7 describes the plasticization of the common transparent electrode using common processing additives. Chapter 8, 9, and 10 investigate the mechanical properties of low-bandgap polymers as the function of the molecular structure and solid-state packing. Chapter 11 introduces a novel experimental method, photovoltaic mapping (PVMAP), which combines the use of non-damaging electrode and gradients in processing parameter to spatially map the photovoltaic properties of organic solar cells.
Stretchable electronics
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Organic semiconductor
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A practical parameter, the volume density of organic thin films, found to affect the electronic properties and in turn the performance of organic photovoltaics (OPVs), is investigated in order to benefit the polymer synthesis and thin film preparation in OPVs. To establish the correlation between film density and device performance, the density of organic thin films with various treatments was obtained, by two-dimensional X-ray diffraction measurement using the density mapping with respect to the crystallinity of thin films. Our results suggest that the OPV of higher performance has a denser photoactive layer, which may hopefully provide a solution to the question of whether the film density matters in organic electronics, and help to benefit the OPV industry in terms of better polymer design, standardized production, and quality control with less expenditure.
Organic Electronics
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Organic electronics hold the promise of enabling the field of flexible electronics. Several novel organic transistor concepts based on the technology of molecular doping are presented that open new directions to improve the performance of OFETs. The realization of doped organic transistors as well as organic junction field-effect transistors is demonstrated. Furthermore, vertical transistor concepts with channel lengths in the sub-micrometer regime are discussed.
Organic Electronics
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The inversion field-effect transistor is the basic device of modern microelectronics and is nowadays used more than a billion times on every state-of-the-art computer chip. In the future, this rigid technology will be complemented by flexible electronics produced at extremely low cost. Organic field-effect transistors have the potential to be the basic device for flexible electronics, but still need much improvement. In particular, despite more than 20 years of research, organic inversion mode transistors have not been reported so far. Here we discuss the first realization of organic inversion transistors and the optimization of organic depletion transistors by our organic doping technology. We show that the transistor parameters—in particular, the threshold voltage and the ON/OFF ratio—can be controlled by the doping concentration and the thickness of the transistor channel. Injection of minority carriers into the doped transistor channel is achieved by doped contacts, which allows forming an inversion layer. Inversion type transistors – which are widely used in silicon-based industries – are thought to not be obtainable in organic devices. Lüssem et al.realize the first inversion organic field-effect transistor by doping at the source and drain contacts without degrading its ON/OFF ratio.
Microelectronics
Organic Electronics
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Transistor array
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Abstract Thiazolo[5,4‐ d ]thiazoles (TzTzs) are fused bicyclic heteroaromatic compounds characterized by a rigid planar backbone and an extended π‐conjugated electronic structure. Although they have been known for many decades, interest in their properties and applications has begun to increase only recently, after their incorporation into a series of active materials used in the field of organic electronics. Recently, the thiazolo[5,4‐ d ]thiazole scaffold has been inserted into new photoactive compounds (both small‐molecule compounds and polymers) used to build bulk‐heterojunction organic solar cells (OSCs), as well as in new organic π dyes for dye‐sensitized solar cells (DSSCs). This microreview focuses on the preparation of such TzTz‐containing materials and their application in the field of organic and hybrid photovoltaics.
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유기발광다이오드 (Organic light-emitting diode - OLED)는 차세대 디스플레이와 고체조명 (Solid-state lighting – SSL)의 후보로 최근 각광받고 있다. 특히 저전력, 고효율, 장수명의 백색 OLED를 제작하는 것은 조명용으로의 OLED에서 필수적인 조건이므로 많은 연구그룹들에게 있어서 해결해야 할 중요한 문제로 자리잡고 있다. 수명과 효율을 증가시키고 구동전류를 감소시켜 안정적인 백색 OLED를 구현하기 위한 한 가지 접근방법은 소자 내의 정공주입층 (Hole-injection Layer - HIL)을 최적화하는 방식이다. 본 논문에서는 정공주입층에 초점을 맞추어 연구를 진행하였다. 먼저 단층과 복층 녹색 형광 OLED 소자를 MoO3를 정공주입층으로 도입하여 적층 녹색 OLED소자의 구동과 효율향상을 IVL과 EL 측정을 통하여 확인하였다. 나아가 MoO3 대신 직접 제작한 고분자 정공주입층인 PEDOT:PSS:PFI을 적용하여 효율의 증가를 확인할 수 있었다. 이처럼 향상된 정공주입능력과 효율은 이 고분자 블렌드를 스핀코팅할 때, PFI 층이 표면 쪽으로 자기정렬되어 표면의 일함수를 대폭 증가시키며 전자의 정공주입층으로의 주입을 막아주기 때문으로 해석할 수 있다. 고분자블렌드 정공주입층을 사용하여 제작한 단층과 복층 OLED소자를 비교하였을 때 효율은 약 1.2 ~ 1.4배 증가하는 경향을 보였다. C-V 측정을 통하여 분석한 결과 위쪽 발광층과 아랫쪽 발광층 사이의 전하 불균형에 의한 것으로 나타났다. 이 결과는 고분자 정공주입층의 변화를 통하여 전하균형을 맞춘다면 용액공정을 적층 OLED에 적용할 수 있는 가능성을 보여주었다.
또한, 본 논문에서는 MoO3를 NPB에 도핑하거나 MoO3와 NPB를 반복적으로 증착하는 방식으로 정공주입/정공전달 혼합층을 두꺼운 두께로 적용하여 녹색 형광 OLED를 구현해보았다. MoO3 도핑된 NPB는 균일하게 분산된 MoO3에 의한 층 내부의 결함과 불순물 등으로 인하여 성능의 향상을 확인할 수 없었다. 그러나 MoO3/ NPB의 반복증착을 구현한 소자에서는 두꺼운 층으로 인한 구동전류감소와 MoO3/NPB의 면적 증가에 따른 정공주입향상 등으로 인해 약 1.2배의 효율과 1.3배의 수명증가를 이루어냈다. 이 결과는 정공주입/정공전달 혼합층으로 이루어진 소자구조에서의 장수명 OLED 구현에 대한 가능성을 나타낸다는 점에서 의미 있는 결과이다.
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Silsesquioxane
Organic semiconductor
Nanomaterials
Organic Electronics
Hybrid solar cell
Hybrid material
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Organic transistors for flexible electronics applications are usually fabricated on polymeric substrates, but considering the negative impact of plastic waste on the global environment and taking into account the desirable properties of paper, there are more and more efforts to use paper as a substrate for organic transistors.
Organic Electronics
Flexible Electronics
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