Evaluation of colloidal CdSe quantum dots with metal chalcogenide ligands for optoelectronic applications
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Evaluation of colloidal CdSe quantum dots with metal chalcogenide ligands for optoelectronic applications Yiqiang Zhang, R. Acharya, and X. A. Cao Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, USA ABSTRACT Exchanging the original organic ligands of colloidal CdSe quantum dots (QDs) with metal chalcogenide SnS4 ligands resulted in absorption peak redshifts and complete photoluminescence quenching in QD solids. The ITO/QDs/Al structure with SnS4-capped QDs showed much higher electrical conductivity and reduced space-charge limited current. These results are indicative of carrier delocalization as well as enhanced inter-QD electronic coupling caused by the inorganic ligands. The SnS4-capped QDs were able to retain strong excitonic absorption. The photocurrent spectral response of the all-inorganic QD film resembled its absorption spectra, and was three orders of magnitude stronger than that of QDs with organic ligands. It was found that mild annealing at ~ 200 oC transformed the SnS4-capped QD film into to a more conductive assembly, degrading its absorption and photocurrent generation. These findings suggest that colloidal QDs with metal chalcogenide ligands are better suited for solar energy conversion and photodetection than use in light-emitting devices as luminophores. INTRODUCTION Colloidal quantum dots (QDs) synthesized by low-cost solution methods have many attractive properties and offer a new class of materials for optoelectronic applications [1-4], such as light-emitting diodes, photodetectors, and solar cells. However, the performance of QD-based optoelectronic devices is lagging significantly behind that of devices based on conventional bulk materials [1-3]. The device performance is largely determined by the surface properties of QDs. Surface defects trap carriers and enhance nonradiative recombination. Long-chain organic ligands introduced for dispersion during colloidal synthesis may passivate most of the surface defects, but increase the interparticle spacing and energy barriers for electronic coupling [1]. As a result, charge transport and exciton energy transfer between QDs and surrounding materials, two critical processes in optoelectronic device operation, are hampered. Complete removal of the organic ligands by different treatments has proven to be difficult and inevitably generates surface defects, deteriorating the optical properties of QDs [5,6]. Simple exchange of the ligands with small thoil molecules is another commonly used approach, but the small organic molecules are not stable against oxidation and volatility [6-8]. In an effort to seek ideal ligands which provide colloidal stabilization and electronic passivation of colloidal QDs, as well as enable facile inter-particle electronic communication, several groups recently exploited the exchange of bulky organic ligands with small inorganic ligands such as molecular metal chalcogenide complexes [9,10], chalcogenide, hydrochalcogenide, and halide cations [11,12]. The sizes of these
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