Ligand exchange on CdSe nanoplatelets for the solar light sensitization of TiO2 and ZnO nanorod arrays

Abstract

In quantum dot (QD) solar cells, the ex situ sensitization of wide band gap semiconductors (WBSCs) makes it possible to control the shape and the passivation of the nanosized sensitizer. Hence, ex situ techniques can be used to investigate how the band gap of the sensitizers affects the performance of quantum dot solar cells. The latter can be precisely controlled in 1D confined structures such as quasi-2D nanoplatelets (NPLs), the thickness of which is defined with an atomic precision. In this work, we tested and thoroughly characterized the attachment of 7, 9 and 11 monolayers thick CdSe NPLs (as well as QDs for the sake of comparison) to ZnO and to TiO2 nanorods. A crucial point of the ex situ techniques is the choice of bifunctional ligands that link the nanosized sensitizers to the WBSCs. Besides the well-known mercaptopropionic acid, we also studied two 'atomic linkers' (OHand SH-) to minimize the distance between the sensitizer and the oxide. The asprepared systems have been analyzed by UV/VIS absorption and Raman spectroscopy.

Large Band Edge Tunability in Colloidal Nanoplatelets

“…We use periodic 2D structural approximants, as shown in Figure a, with thicknesses ranging from 2.5 to 8.5 monolayers (MLs) following the notation of refs and that accounts for the Cd termination of both facets. These thicknesses of 2.5–8.5 MLs correspond to the experimental values that are precisely controlled, ,,,,,,,,− while the 2D approximants follow negligible changes in the absorption and emission spectra for lateral dimensions >5 nm, as observed in experiments. , We study ligand binding to Cd atoms via thiolate (R′–S – ) and carboxylate (R′–Ac – ) groups, with distinct R′– functional groups: methyl groups (Me–S – and Me–Ac – for CH 3 –S – and CH 3 –Ac – , respectively) and phenyl groups (Ph–S – and Ph–Ac – , where Ph represents C 6 H 5 ) where the phenyl ring is functionalized by substituting −H with −F, −SH, −CH 3 , −CF 3 , etc. We also study H 3 PO 3 ligands for comparison with experimental results of CdSe NPLs with phosphonic acid ligands. , Those ligands are found in as-synthesized colloidal zinc blende CdSe NPLs , or accessed via ligand exchange techniques. ,,,, All Cd atoms at the surfaces are passivated with ligands in order to saturate all of the dangling bonds …”

“…Owing to their outstanding optical properties, − quasi-two-dimensional (2D) colloidal semiconductor nanoplatelets (NPLs) are being actively explored for deployment in a variety of optoelectronic devices, such as light-emitting diodes, , solar cells, and lasers. ,, Over the past decade, efficient synthetic strategies have been proposed to control the magnitude of their optical band gaps from the near-IR to the UV range . Specifically, the one-dimensionally quantum-confined electronic structure of NPLs is tunable through their chemical composition, ,,,,, core/shell heterostructuring in the out-of-plane direction, ,, and the interaction of their surface states with organic ligands .…”

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS–PIETA NMR Spectroscopy

“…Next to the formation of heterostructures, diverse postsynthetic chemical modifications are pursued to impart desired optical and electronic characteristics of NPLs. These include the exchange of insulating long-chain ligands with mercaptopropionic acid, OH – and SH – for solar light sensitization of metal-oxide nanorods, various alkyl thiols, , or halide ligands . The latter shifts the optical signals to the red, releases lattice strain in cadmium chalcogenide NPLs, and passivates the surface, which maintains or even enhances the high PL QY of NPLs .…”

Manipulation of the Optical Properties of Colloidal 2D CdSe Nanoplatelets

“…[ 80 ] The effect of ligands also reflects on the self‐assembly process when the freestanding NPLs are stacked into columnar structures [ 23,43 ] upon the addition of different surfactant agents. Moreover, ligands help to tune the optical features [ 12 ] as well as the charge transport properties [ 81 ] of the engineered NPLs. Among all these ligand‐related effects, we will focus on the adjustment of electronic band structures of CdSe NPLs via ligand treatment.…”