Enhanced Charge-Carrier Mobility in High-Pressure-Crystallized Poly(3-hexylthiophene)
Enhanced Charge-Carrier Mobility in High-Pressure-Crystallized Poly(3-hexylthiophene)
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DOI：
10.1021/ma102529f
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发表时间：
2011-03-22
期刊：
影响因子：
5.5
通讯作者：
Stingelin, Natalie
中科院分区：
文献类型：
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作者：
Mueller, Christian;Zhigadlo, Nikolai D.;Stingelin, Natalie
聚合物有机半导体，像许多其他大分子系统一样，可以显示出从基本无序或无定形到高度结晶的全光谱微观结构。许多电子性质和过程都严重依赖于这种分子和超分子秩序。然而，通常仍然不清楚哪些特定的微观结构方面有助于这些材料的宏观电子属性。在商品聚合物领域，如聚乙烯（PE）、等规聚丙烯（i-PP）、尼龙和聚酯，一个结构特征已经被发现对优化机械功能至关重要，包括杨氏模量，特别是抗拉强度，是链延伸程度和相关的层状晶体厚度1（见示意图图1a）。类似地，在本研究中，我们关注这一特定特征，并探索通过不同处理方案变化的l是否影响共轭聚合物物质中的电荷输运。这种关系的初步迹象是存在的。例如，Zhang等人基于相对较低的重量-平均分子量Mw （2.4 kg mol r1< Mw< 18 kg mol r1）的聚（3-己基噻吩）s （P3HT）范围，建立了“重量-平均等高线长LW”与场效应晶体管电荷-载流子迁移率μFET之间的相关性，μFET随着LW的增加而增加。考虑到Zhang等人研究的材料相对较低的分子量，可以从Brinkmann和Rannou的工作中假设这些p3ht形成链延伸晶体，这意味着LW ~ 1。在这里，我们关注较大分子量的p3ht，以确保大分子的长度远远超过它们自然形成延伸链晶体的范围，即在链折叠开始的状态下。并在其熔体或浓缩溶液中形成链缠结（图示见图1b；另见参考文献6r11）。选择两种不同的p3ht（重量-平均分子量Mw= 60和344 kg mol r1），在常温和高压下从熔体中凝固。为了进行比较，这两种聚合物也是由溶液铸造的，因为这是该聚合物家族最常用的加工方法。此外，从低分子量p3ht为Mw= 22 kg mol r1的溶液中制备薄膜，其不具有分子连接的晶体部分，并且在环境条件下加工时也可以形成链延伸晶体。后者从它们的脆性拉伸行为中可以明显看出（支持信息表S1）。[注1]我们观察到，在Mw= 60 kg mol r1时，材料开始发生塑性变形，表明达到了弹性渗流。这意味着晶体实体通过单个大分子（“结分子”，见辅助信息图S1中的示意图）连接在一起，这导致了常见的半晶“塑料”的典型结构，即晶片和非晶片交替(很大程度上是
Polymeric organic semiconductors, like many other macro-molecular systems, can display the full spectrum of microstructures, from essentially disordered, or amorphous, to highly crystalline. Many electronic properties and processes are critically depending on this molecular and supramolecular order. 1r4 However, it often is still unclear which particular microstructural aspects contribute to the macroscopic electronic attributes of these materials. In the field of commodity polymers, such as polyethylene (PE), isotactic polypropylene (i-PP), nylons, and polyesters, one structural characteristic that has been found to be of paramount importance for optimizing mechanical functionalities, including Young’s modulus and, in particular, tensile strength, is the degree of chain extension and associated lamellar crystal thickness l (see for a schematic Figure 1a). In analogy, in the present study we focused on this specific feature and explored whether l, varied through different processing schemes, influences charge transport in conjugated polymeric matter. Initial indications for such a relation exist. On the basis of a range of poly (3-hexylthiophene) s (P3HT) of relatively low weight-average molecular weight Mw (2.4 kg mol r1< Mw< 18 kg mol r1), Zhang et al. have, for example, established a correlation between “weight-average contour length LW” and field-effect transistor charge-carrier mobilities μFET, with μFET increasing with LW. 5 Considering the relatively low molecular weight of the materials investigated by Zhang et al., one can assume from Brinkmann and Rannou’s work that these P3HTs form chain extended crystals, which would imply that LW∼ l. Here, we focus on P3HTs of larger molecular weights to ensure that the macromolecules are of a length well above the range where they naturally form extended-chain crystals, ie, in the regime where chain folding sets in, and chain entanglements form in their melt or concentrated solutions (schematically indicated in Figure 1b; see also refs 6r11). Two different P3HTs (weight-average molecular weight Mw= 60 and 344 kg mol r1) were selected and solidified from the melt at ambient and under elevated pressure. For comparison, both polymers were also cast from solution, as this is the most frequently adopted processing method for this polymer family. In addition, thin films were prepared from solution at ambient from a low-molecular-weightP3HT of Mw= 22 kg mol r1, which does not feature molecularly connected crystalline moieties and can be expected to form chain extended crystals also when processed at ambient conditions. The latter is evident from their brittle tensile behavior (Supporting Information Table S1).[NB We observe the onset of plastic deformation for the material of Mw= 60 kg mol r1, indicating that elastic percolation is reached. This implies that the crystalline entities are connected through individual macromolecules (“tie molecules”; see schematic in Supporting Information Figure S1), which results in the typical structure of common semicrystalline “plastics” of alternating crystalline lamellae and amorphous (largely