Effects of Ligands on Charge Generation and Recombination in Hybrid Polymer/Quantum Dot Solar Cells

Control of quantum dot surface chemistry offers a direct approach to tune the molecular interface between donor and acceptor constituents in hybrid bulk heterojunction photovoltaics incorporating organic semiconductors and colloidal quantum dots. We investigate the effects of altering the quantum dot surface chemistry via ligand exchange in blends of PbS quantum dots with the conjugated polymer poly­((4,8-bis­(octyloxy)­benzo­(1,2-<i>b</i>:4,5-<i>b</i>′)­dithiophene-2,6-diyl)­(2-((dodecyloxy)­carbonyl)­thieno­(3,4-<i>b</i>)­thio­phenediyl)) (PTB1). We study organic ligands with both thiol and carboxylic acid functional groups including 1,2-ethanedithiol (EDT), 3-mercapto­propionic acid (MPA), and malonic acid (MA), in addition to inorganic halide ions such as tetrabutylammonium iodide (TBAI). We show that the different ligand treatments influence hybrid solar cell efficiency primarily through changes in open-circuit voltage (<i>V</i>_OC) and fill factor (FF). We use photoinduced absorption (PIA) spectroscopy to probe the generation of long-lived polarons resulting from charge transfer between the donor and acceptor constituents. We further characterize the recombination dynamics in the hybrid devices using transient photovoltage (TPV) and charge extraction (CE) techniques. Both methods show that ligand exchange with MPA yields superior device performance by promoting longer carrier recombination lifetimes under open-circuit conditions

Facile, Economic and Size-Tunable Synthesis of Metal Arsenide Nanocrystals

Synthesis of colloidal nanocrystals (NC) of important arsenide nanomaterials (e.g., InAs, Cd₃As₂) has been limited by the lack of convenient arsenic precursors. Here we address this constraint by identifying a convenient and commercially available As precursor, tris-dimethylaminoarsine (As­(NMe₂)₃), which can be used to prepare high quality InAs NCs with controlled size distributions. Our approach employs a reaction between InCl₃ and As­(NMe₂)₃ using diisobutylaluminum hydride (DIBAL-H) to convert As­(NMe₂)₃ in situ into reactive intermediates AsH_<i>x</i>(NMe₂)_3–<i>x</i>, where <i>x</i> = 1,2,3. NC size can be varied by changing DIBAL-H concentration and growth temperature, with colloidal solutions of InAs showing size dependent absorption and emission features tunable across wavelengths of 750 to 1450 nm. We also show that this approach works well for the colloidal synthesis of Cd₃As₂ NCs. By circumventing the preparation of notoriously unstable and dangerous arsenic precursors (e.g., AsH₃ and As­(SiMe₃)₃), this work improves the synthetic accessibility of arsenide-based NCs and, by extension, the potential of such NCs for use in infrared (IR) applications such as communications, fluorescent labeling and photon detection

Size-Dependent Charge Transfer Yields in Conjugated Polymer/Quantum Dot Blends

We investigate the effect of quantum dot size on photocurrent and photoinduced charge transfer yields in blends of the conjugated polymer, poly­((4,8-bis­(octyloxy)­benzo­(1,2-<i>b</i>:4,5-<i>b</i>′)­dithiophene-2,6-diyl)­(2-((dodecyloxy)­carbonyl)­thieno­(3,4-<i>b</i>)­thiophenediyl)) (PTB1), with PbS nanocrystal quantum dots (QDs). These hybrid solar cells exhibit external quantum efficiencies of over 70% and power conversion efficiencies of up to 2.8%. We use photoinduced absorption (PIA) spectroscopy and device EQE measurements to probe long-lived charge transfer at the polymer/QD interface as a function of QD size. We observe that both the PIA signal associated with charge formation on the polymer, as well as the external quantum efficiency of the hybrid photovoltaic devices decrease in magnitude with increasing quantum dot size, despite the broader absorption spectrum of the larger dots. We interpret these results as evidence that PTB1/PbS blends behave at least partially as bulk heterojunction (BHJ) solar cells, and conclude that the long-lived charge transfer yield is diminished at larger dot sizes as the energy level offset at the polymer/quantum dot interface is changed through decreasing quantum confinement

New Forms of CdSe: Molecular Wires, Gels, and Ordered Mesoporous Assemblies

This work investigates the structure and properties of soluble chalcogenidocadmates, a molecular form of cadmium chalcogenides with unprecedented one-dimensional bonding motifs. The single crystal X-ray structure reveals that sodium selenocadmate consists of infinite one-dimensional wires of (Cd₂Se₃)_<i>n</i>^2<i>n</i>– charge balanced by Na⁺ and stabilized by coordinating solvent molecules. Exchanging the sodium cation with tetraethylammonium or didodecyldimethylammonium expands the versatility of selenocadmate by improving its solubility in a variety of polar and nonpolar solvents without changing the anion structure and properties. The introduction of a micelle-forming cationic surfactant allows for the templating of selenocadmate, or the analogous telluride species, to create ordered organic–inorganic hybrid CdSe or CdTe mesostructures. Finally, the interaction of selenocadmate “wires” with Cd²⁺ ions creates an unprecedented gel-like form of stoichiometric CdSe. We also demonstrate that these low-dimensional CdSe species show characteristic semiconductor behavior, and can be used in photodetectors and field-effect transistors

Understanding and Curing Structural Defects in Colloidal GaAs Nanocrystals

GaAs is one of the most important semiconductors. However, colloidal GaAs nanocrystals remain largely unexplored because of the difficulties with their synthesis. Traditional synthetic routes either fail to produce pure GaAs phase or result in materials whose optical properties are very different from the behavior expected for quantum dots of direct-gap semiconductors. In this work, we demonstrate a variety of synthetic routes toward crystalline GaAs NCs. By using a combination of Raman, EXAFS, transient absorption, and EPR spectroscopies, we conclude that unusual optical properties of colloidal GaAs NCs can be related to the presence of Ga vacancies and lattice disorder. These defects do not manifest themselves in TEM images and powder X-ray diffraction patterns but are responsible for the lack of absorption features even in apparently crystalline GaAs nanoparticles. We introduce a novel molten salt based annealing approach to alleviate these structural defects and show the emergence of size-dependent excitonic transitions in colloidal GaAs quantum dots