Department of Graphic Arts Information Engineering, Pukyong National University , Sinseon-ro 365, Nam-gu, Busan 608-739, Republic of Korea.
Organic Materials Lab, Samsung Advanced Institute of Technology, Samsung Electronics Company, Samsung-ro, Suwon, Gyeonggi 443-370, Republic of Korea.
ACS Appl Mater Interfaces. 2016 Nov 9;8(44):30344-30350. doi: 10.1021/acsami.6b09993. Epub 2016 Oct 27.
Field-effect mobility of a polymer semiconductor film is known to be enhanced when the gate dielectric interfacing with the film is weakly polarizable. Accordingly, gate dielectrics with lower dielectric constant (k) are preferred for attaining polymer field-effect transistors (PFETs) with larger mobilities. At the same time, it is also known that inducing more charge carriers into the polymer semiconductor films helps in enhancing their field-effect mobility, because the large number of traps presented in such a disorder system can be compensated substantially. In this sense, it may seem that employing higher k dielectrics is rather beneficial because capacitance is proportional to the dielectric constant. This, however, contradicts with the statement above. In this study, we compare the impact of the two, i.e., the polarizability and the capacitance of the gate dielectric, on the transport properties of poly[(diketopyrrolopyrrole)-alt-(2,2'-(1,4-phenylene)bisthiophene)] (PDPPTPT) semiconductor layers in an FET architecture. For the study, three different dielectric layers were employed: fluorinated organic CYTOP (k = ∼2), poly(methyl methacrylate) (k = ∼4), and relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (k = ∼60). The beneficial influence of attaining more carriers in the PDPPTPT films on their charge transport properties was consistently observed from all three systems. However, the more dominant factor determining the large carrier mobility was the low polarizability of the gate dielectric rather than its large capacitance; field-effect mobilities of PDPPTPT films were always larger when lower k dielectric was employed than when higher k dielectric was used. The higher mobilities obtained when using lower k dielectrics could be attributed to the suppressed distribution of the density of localized states (DOS) near the transport level and to the resulting enhanced electronic coupling between the macromolecules.
众所周知，当与薄膜相互作用的栅介质的极化率较弱时，聚合物半导体薄膜的场效应迁移率会提高。因此，为了获得具有较大迁移率的聚合物场效应晶体管（PFET），通常更喜欢介电常数（k）较低的栅介质。同时，人们也知道，将更多的载流子引入聚合物半导体薄膜中有助于提高其场效应迁移率，因为在这种无序体系中存在大量的陷阱可以得到很大程度的补偿。从这个意义上说，使用更高的 k 介电常数似乎更为有利，因为电容与介电常数成正比。然而，这与上述说法相矛盾。在这项研究中，我们比较了两种介电常数（即栅介质的极化率和电容）对聚[二酮吡咯吡咯烷--alt-（2,2'-(1,4-亚苯基)双噻吩）]（PDPPTPT）半导体层在 FET 结构中的输运性质的影响。在这项研究中，我们使用了三种不同的介电层：氟化有机 CYTOP（k = ∼2）、聚甲基丙烯酸甲酯（k = ∼4）和弛豫铁电聚（偏二氟乙烯-三氟乙烯-氯三氟乙烯）（k = ∼60）。从这三个体系中都观察到了 PDPPTPT 薄膜中获得更多载流子对其电荷输运性质的有益影响。然而，决定大载流子迁移率的更主要因素是栅介质的低极化率，而不是其大电容；当使用低 k 介电常数时，PDPPTPT 薄膜的场效应迁移率总是大于使用高 k 介电常数时的迁移率。使用低 k 介电常数可以获得更高的迁移率，这归因于抑制了输运能级附近局域态密度（DOS）的分布，以及高分子之间增强的电子耦合。
