Increased Carrier Mobility and Lifetime in CdSe Quantum Dot Thin Films through Surface Trap Passivation and Doping

Abstract

Passivating surface defects and controlling the carrier concentration and mobility in quantum dot (QD) thin films is prerequisite to designing electronic and optoelectronic devices. We investigate the effect of introducing indium in CdSe QD thin films on the dark mobility and the photogenerated carrier mobility and lifetime using field-effect transistor (FET) and time-resolved microwave conductivity (TRMC) measurements. We evaporate indium films ranging from 1 to 11 nm in thickness on top of approximately 40 nm thick thiocyanate-capped CdSe QD thin films and anneal the QD films at 300 °C to densify and drive diffusion of indium through the films. As the amount of indium increases, the FET and TRMC mobilities and the TRMC lifetime increase. The increase in mobility and lifetime is consistent with increased indium passivating midgap and band-tail trap states and doping the films, shifting the Fermi energy closer to and into the conduction band.

High-Mobility Toolkit for Quantum Dot Films

“…Nevertheless surprisingly high mobilities were reported lately by several groups, [12][13][14][15][16][17][18] in high quality films made of different materials, suggesting that band-like transport through extended states is indeed achievable in CQD arrays, provided the surface traps are effectively passivated [18][19][20][21][22][23][24][25] and the separation between dots is reduced sufficiently by the use of extremely short ligands or inorganic capping. 26 This hypothesis is supported by the observed temperature dependence of mobility and conductivity [12][13][14][15][16][17][18] whereas the spectral broadening and red shifts of the 1S exciton peak observed in these systems, 13,27 may be indicative of strong electronic coupling between QDs, as are the remarkable values of diffusion lengths and lifetimes of charge carriers measured in QD solids.…”

Engineering the surface chemistry of lead chalcogenide nanocrystal solids to enhance carrier mobility and lifetime in optoelectronic devices

“…However unlike single crystal semiconductor devices, m and t of majority and minority carriers in NC solids may be considerably different. [15][16][17][18] For example, inorganic chalcogenide, halide, and pseudohalide ligands are known to increase m, whereas short organic thiol and acid ligands are known to create NC solids with long carrier t. 15 Hybrid passivation methods using halide and organic ligands have been conducted to reduce the density of trap states and increase t. 2,3,[19][20][21] Here, we seek to design NC thin films with both a high m and a long t for majority and minority carriers, to optimize the m-t product and therefore optoelectronic device performance. As-synthesized NCs have long organic ligands at their surface, which create large interparticle separations that limit carrier transport and lead to low m, insulating thin films.…”

“…22 The lack of PCE in SCN-treated lead chalcogenide NC Schottky junctions can be understood as SCN is known to sparsely cover the surface of metal chalcogenide NCs, leaving a large percentage of the NC surface unpassivated. [15][16][17] The high density of in-gap states is more detrimental to solar cell than to FET performance. 1,3,4 Short carrier t in SCN-treated metal chalcogenide NC films have been previously reported.…”

“…4C and Table SII and Fig. 17,28 TRMC measures the product of the carrier yield and the sum of m electron and m hole and the t of those carriers. 17,28 TRMC measures the product of the carrier yield and the sum of m electron and m hole and the t of those carriers.…”

Low‐Temperature Atmospheric Pressure Plasma Processes for “Green” Third Generation Photovoltaics

“…This was, for instance, observed in the synthesis of Si QDs, whereby the introduction of even very small amount of hydrogen in the core (not at the surface) can determine the amorphous versus crystalline nature of the QD . Secondly, defects at the surface are easier to address with a range of surface treatments, which can achieve superior passivation and eliminating most defects and dangling bonds . There are also aspects that relate to the device architecture and in particular it should be noted that mobility requirements, e.g., first generation PVs are far higher than those for 3‐Gen devices, due to the absorber layer expected to be order of magnitudes thinner in the latter ones.…”