Charge Transport Characteristics of Surface‐Complexed Quantum Dot in a Thin Film Transistor (original) (raw)

2020, Advanced Materials Interfaces

high photoluminescence quantum yield as well as excellent carrier mobility and should also facilitate a balanced ambipolar conduction (both electron and hole conduction), which is crucial for efficient exciton recombination through radiative pathways. [5,10] However, there is always a trade-off between carrier mobility and luminescence in case of organic semiconductors. [11] Colloidal semiconductor nanocrystal or quantum dots (Qdots) are reported to be excellent light emitters and have exhibited high charge transport capability. [12-15] Importantly, Qdot-integrated LETFT are efficient in terms of both emissive and switching properties. [9,13] However, most of the devices are based on toxic metal chalcogenides such as PbS, PbSe, and CdSe Qdots. [14,16-18] These heavy metals cause adverse oxidative stress in human body. [19,20] The toxic nature of these Qdots puts limitations to their practical usability and thus supports the development of environmentally benign ones. The principal bottleneck for the fabrication of devices using Qdots is single step deposition of high quality and defect free layer. Generally, Qdots capped with insulating long organic chains induce a potential barrier to proficient charge transport and electronic coupling between Qdots in thin films and thus restrict their application in the field of electronics. [21] To reduce the interdot spacing and enhance the electronic coupling between the Qdots, a large number of surface modification strategies have been already employed and among them, ligand exchange with halides, [22,23] pseudohalides, [24,25] chalogenides, [26] oxoanion, [27] chalcogenideometallate complex, and halometallates is popular. It has been established that the nature of the capping ligand controls the mobility of carriers and transport characteristics. Typically, the type of ligand might influence the type of channel-whether n-type, p-type or ambipolar-in field effect transistors. [23,28] The ambipolar nature of the transistor is due to near-intrinsic doping of the Qdot. Recently, tetrabutylammonium iodide-treated PbS Qdots, [13] metal chalcogenide complex (Na 4 Sn 2 S 6 •14H 2 O, Na 4 Sn 2 Se 6 , and KInSe 2)-capped CdSe Qdots, [14] and 3-mercaptopropionic acid ligand-exchanged PbS Qdots [9] have been used in thin film transistors to achieve efficient charge transport and high carrier mobilities. However, ligand exchange with compact organic or inorganic ligands in Qdots do not always assure high carrier mobility and better charge transport in Qdot thin films. On the other hand, the ligand exchange may introduce additional Ambipolar transport characteristics of thin film transistors fabricated from nontoxic white light emitting quantum dot complexes (QDCs) are herein reported to exhibit efficient carrier mobilities. The QDCs are synthesized by forming bluish green emitting zinc quinolate complex on the surface of orange emitting Mn 2+-doped ZnS quantum dots (Qdots) using 8-hydroxyquinoline 5-sulfonic acid as the chelating ligand. The device exhibits efficient ambipolar transport characteristics with high I ON /I OFF ratio of 10 4 and electron mobility and hole mobility of 2.95 × 10 −02 and 1.06 × 10 −02 cm 2 V −1 s −1 , respectively. The subthreshold slope of Qdot complex-integrated thin film transistor increases from that of Mn 2+-doped ZnS Qdot-integrated thin film transistor from 0.35 to 0.79 V dec −1 in p-field effect transistor (FET) and from 0.59 to 0.97 V dec −1 in n-FET operations, which annotates an increase in trap state density due to surface complexation of the Qdot. These results suggest that white light emitting QDC can be used as an efficient transport as well as an emissive material, which open up new paradigm for advanced optoelectronic applications.