Chalcogenide nanocrystals (NCs) hold substantial promise for applications in cost-effective and energy-efficient light capture (solar cells), detection, and generation (LEDs). A good understanding of the processes of charge injection and transport in NCs and NC arrays is essential for development of these applications. The efficiency of injection of charges into semiconducting NCs depends on the specifics of the electronic NC band structure and surface properties. To understand the variables affecting the charge injection, a wide array of experimental techniques has been applied, such as ultraviolet photoelectron spectroscopy (UPS) or photoelectron spectroscopy in air (PESA). An alternative approach is based on electrochemical methods, which can yield a relevant information in a cost-effective manner. Nevertheless, when dealing with semiconducting NCs, distinguishing between charge injection into the conduction band minima (CBM) or valence band maxima (VBM) and charge injection into surface defect states can be challenging. The combination of electrochemistry and spectroscopy emerged as a powerful approach not only for identifying the electronic band structure but also for investigating the optical properties of charged semiconducting NCs.

One of the essential parts for any particle device application is thin film fabrication. Investigation and understanding of the charge transport in these NC thin films is very important from the prospective of basic science and device application. In this work, we prepared and investigated charge (electron/hole) injection mechanism in chalcogenide NCs in thin film configuration. The goal was to use spectro-electrochemistry (SEC), a combination of electrochemistry and spectroscopy as a tool, to understand the charge injection and transport mechanism in NCs and its effect in the optical properties of NCs. In previous studies, SEC was used as a method for acquiring energy level diagrams [1] by injecting electrochemical charges into quantum confined band. Several SEC studies of SC materials yielded information about the electronic band structure of CdSe, CdS, PbSe etc [2-6]. Injection of electron into the conduction band of chalcogenide QDs bleaches the first excitonic absorption (1S_h→1S_e) and initiate new absorption due to interaction of intra band (1S_e→1P_e) electronic excitation. Boehme et. al. [7] showed that beyond a certain size (i.e., 3.7 nm of CdSe QDs) charge injection is not possible in thin film configuration, whereas, Arun et. al. [8] showed charge injection in 3.2 nm QDs in liquid phase. As the QD size is reduced, the film becomes increasingly compact, which imposes constraints on electrochemical charge injection. These studies showed that size and shape of the NC is an important parameter determining the efficiency of charge injection into NC thin films. QDs are different form nanorods (NRs), not only in terms of energy band aliment [9], but also in terms the resulting thin film morphology, with NRs yielding less ordered films. However, so far the information about the effect of morphology on the charge injection and transport in NC films has been limited. This work focuses on investigation of the mechanism of the charge injection and transport in NC films assembled from NCs of various shape (quantum dot vs nanorod) and sizes of CdSe NC thin films. This investigation is complemented by in-situ spectroscopic characterization to gain a deeper understanding of the kinetics and dynamics of charge transport under an external applied potential. We show that in addition to effects of shape, surfactant or ligands also significantly affect the charge injection and charge transport processes.