Structural, Optical, and Electrical Properties of PbSe Nanocrystal Solids Treated Thermally or with Simple Amines

Abstract

We describe the structural, optical, and electrical properties of films of spin-cast, oleate-capped PbSe nanocrystals that are treated thermally or chemically in solutions of hydrazine, methylamine, or pyridine to produce electronically coupled nanocrystal solids. Postdeposition heat treatments trigger nanocrystal sintering at approximately 200 degrees C, before a substantial fraction of the oleate capping group evaporates or pyrolyzes. The sintered nanocrystal films have a large hole density and are highly conductive. Most of the amine treatments preserve the size of the nanocrystals and remove much of the oleate, decreasing the separation between nanocrystals and yielding conductive films. X-ray scattering, X-ray photoelectron and optical spectroscopy, electron microscopy, and field-effect transistor electrical measurements are used to compare the impact of these chemical treatments. We find that the concentration of amines adsorbed to the NC films is very low in all cases. Treatments in hydrazine in acetonitrile remove only 2-7% of the oleate yet result in high-mobility n-type transistors. In contrast, ethanol-based hydrazine treatments remove 85-90% of the original oleate load. Treatments in pure ethanol strip 20% of the oleate and create conductive p-type transistors. Methylamine- and pyridine-treated films are also p-type. These chemically treated films oxidize rapidly in air to yield, after short air exposures, highly conductive p-type nanocrystal solids. Our results aid in the rational development of solar cells based on colloidal nanocrystal films.

Nonmonotonic Size Dependence in the Hole Mobility of Methoxide-Stabilized PbSe Quantum Dot Solids

“…Note that the 1s core level of R 3 CÀO is expected at 286.4 eV. 12 Thin films of all oleate-capped samples have the same S e r S h transition energy as the colloidal sample in tetrachloroethylene. Upon MeO À treatment, all QD samples exhibit a pronounced red shift consistent with results for hydrazine-, ethanedithiol-, or dicarboxylic acid-capped PbSe QDS.…”

Phase transformation of PbSe/CdSe nanocrystals from core-shell to Janus structure studied by photoemission spectroscopy

“…Heating due to irradiation is, thus, lower than 200 • C, a temperature that is still low enough to prevent the desorption of oleate ligands attached to Pb atoms at the surface of PbSe NCs. 63 Therefore, although Cd atoms in excess at the surface of the shell reintegrate in the CdSe lattice upon annealing, the ligands preferentially diffuse from the CdSe facets to the PbSe facets and bind to Pb adatoms. A possible alternative would be that excess Pb adatoms diffuse at the surface of the HNCs toward the CdSe facets and saturate the oleate ligands there.…”

Schottky Solar Cells Based on Colloidal Nanocrystal Films

“…We have found that using methylamine instead of EDT yields similar device performance (Supporting Information, Figure 1). 2 We have also fabricated working devices from PbS and CdSe NCs (Supporting Information, Figures 2 and 3), which indicates that the approach adopted here is not restricted to EDT-treated PbSe NCs and that it should be possible to improve cell efficiency by engineering the surface of the NCs to attain longer carrier diffusion lengths and higher photovoltages through surface state passivation and prevention of Fermi level pinning.When tested in nitrogen ambient under simulated 1-sun test conditions (100 ( 5 mW cm -2 ELH white light illumination), EDT-treated PbSe devices exhibit large shortcircuit photocurrent densities (J SC ) and modest open-circuit voltages (V OC ) and fill factors (FF), with one of the most efficient devices yielding J SC ) 24.5 mA cm -2 , V OC ) 239 mV, FF ) 0.41 and a mismatch-corrected 3 AM1.5G efficiency of 2.1% (Figure 1a; see Supporting Information regarding spectral mismatch). The mismatch-corrected J SC values of these devices are reproducibly larger than those of other nanostructured solar cells, including the best organic 4 and dye-sensitized devices, 5 which is remarkable considering the unsintered, glassy microstructure of our NC films and the fact that the NCs retain quantum confinement effects despite strong inter-NC carrier transport.…”

“…We have found that using methylamine instead of EDT yields similar device performance (Supporting Information, Figure 1). 2 We have also fabricated working devices from PbS and CdSe NCs (Supporting Information, Figures 2 and 3), which indicates that the approach adopted here is not restricted to EDT-treated PbSe NCs and that it should be possible to improve cell efficiency by engineering the surface of the NCs to attain longer carrier diffusion lengths and higher photovoltages through surface state passivation and prevention of Fermi level pinning.…”