Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
J Am Chem Soc. 2012 Sep 12;134(36):14877-89. doi: 10.1021/ja304198e. Epub 2012 Aug 30.
We present an optical spectroscopy study on the role of oxygen and water in electron trapping and storage/bias-stress degradation of n-type polymer field-effect transistors based on one of the most widely studied electron transporting conjugated polymers, poly{[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bisthiophene)} (P(NDI2OD-T2)). We combine results obtained from charge accumulation spectroscopy, which allow optical quantification of the concentration of mobile and trapped charges in the polymer film, with electrical characterization of P(NDI2OD-T2) organic field-effect transistors to study the mechanism for storage and bias-stress degradation upon exposure to dry air/oxygen and humid nitrogen/water environments, thus separating the effect of the two molecules and determining the nature of their interaction with the polymer. We find that the stability upon oxygen exposure is limited by an interaction between the neutral polymer and molecular oxygen leading to a reduction in electron mobility in the bulk of the semiconductor. We use density functional theory quantum chemical calculations to ascribe the drop in mobility to the formation of a shallow, localized, oxygen-induced trap level, 0.34 eV below the delocalized lowest unoccupied molecular orbital of P(NDI2OD-T2). In contrast, the stability of the polymer anion against water is limited by two competing reactions, one involving the electrochemical oxidation of the polymer anion by water without degradation of the polymer and the other involving a radical anion-catalyzed chemical reaction of the polymer with water, in which the electron can be recycled and lead to further degradation reactions, such that a significant portion of the film is degraded after prolonged bias stressing. Using Raman spectroscopy, we have been able to ascribe this to a chemical interaction of water with the naphthalene diimide unit of the polymer. The degradation mechanisms identified here should be considered to explain electron trapping in other rylene diimides and possibly in other classes of conjugated polymers as well.
我们进行了一项关于氧和水在基于研究最广泛的电子传输共轭聚合物之一的 n 型聚合物场效应晶体管中电子俘获和存储/偏压应力退化中作用的光谱研究，该聚合物为聚{[N,N9-双（2-辛基十二烷基）-萘 1,4,5,8-二羧酸二酰亚胺-2,6-二基]-交替-5,59-（2,29-噻吩）}（P（NDI2OD-T2））。我们结合电荷积累光谱学的结果，该结果允许对聚合物薄膜中可移动和俘获电荷的浓度进行光学量化，以及对 P（NDI2OD-T2）有机场效应晶体管的电学特性进行研究，以研究在暴露于干燥空气/氧气和潮湿氮气/水环境下存储和偏压应力退化的机制，从而分离这两种分子的作用并确定它们与聚合物的相互作用的性质。我们发现，在氧气暴露下的稳定性受到中性聚合物与分子氧之间相互作用的限制，这导致半导体体中的电子迁移率降低。我们使用密度泛函理论量子化学计算将迁移率的下降归因于浅的局部化氧诱导陷阱能级的形成，该能级比 P（NDI2OD-T2）的离域最低未占据分子轨道低 0.34 eV。相比之下，聚合物阴离子对水的稳定性受到两种竞争反应的限制，一种涉及水对聚合物阴离子的电化学氧化而不导致聚合物降解，另一种涉及聚合物与水的自由基阴离子催化化学反应，其中电子可以被回收并导致进一步的降解反应，因此在长时间偏压处理后，很大一部分薄膜会降解。使用拉曼光谱，我们能够将其归因于水与聚合物中萘二酰亚胺单元的化学相互作用。这里确定的降解机制应被认为是解释其他芘二酰亚胺以及可能的其他类别的共轭聚合物中的电子俘获的原因。
