The effects of inorganic surface treatments on photogenerated carrier mobility and lifetime in PbSe quantum dot thin films

Abstract

We used flash-photolysis, time-resolved microwave conductivity (TRMC) to probe the carrier mobility and lifetime in PbSe quantum dot (QD) thin films treated with solutions of the metal salts of Na 2 Se and PbCl 2 . The metal salt treatments tuned the Pb:Se stoichiometry and swept the Fermi energy throughout the QD thin film bandgap. A stoichiometric imbalance favoring excess Se heavily p-doped the QD thin film, shifted the Fermi energy toward the valence band, and yielded the highest TRMC mobility and lifetime. Introducing Pb first compensated the p-doping and shifted the Fermi level through mid-gap, decreasing the TRMC mobility. Further Pb addition created an excess of Pb, n-doped the QD thin film, moved the Fermi level to near the conduction band, and again increased the TRMC mobility. The increase in TRMC mobility as the Fermi energy was shifted toward the band edges by non-stoichiometry is consistent with the QD thin film density of states. 2 Graphical abstract Keywords Time-resolved microwave conductivity; quantum dot; doping; spectroscopy; optoelectronic; lead selenide Highlights  Na 2 Se and PbCl 2 treatments modified the surface chemistry of PbSe quantum dots  Excess Se (Pb) p-doped (n-doped) PbSe quantum dot thin films  Carrier mobility and lifetime were studied using time-resolved microwave conductivity  Mobility increased as the Fermi level approached the band edges

Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals

“…The experimental setup for TRMC has been described previously. 55 In brief, a group of methylammonium lead triiodide crystals were fixed to a glass slide using double-sided Scotch tape and the sample was loaded into a home-built resonant cavity fabricated from a WR90 waveguide. Microwaves around 9 GHz were generated in a voltage-controlled oscillator (SiversIMA).…”

“…TRMC provides a contact-free probe of photogenerated carriers regardless of their recombination mechanism. 54,55 When the absorption of microwave radiation following a visible pump pulse is measured, the temporal evolution in conductance (ΔG) upon photoexcitation can be used to extract the free carrier lifetime and mobility according to eq 3…”

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.…”

“…S4, ESI †). However, given the substantially larger m and t of majority than minority carriers in NC solids, 18 the TRMC measurements are dominated by the majority carrier characteristics. However, given the substantially larger m and t of majority than minority carriers in NC solids, 18 the TRMC measurements are dominated by the majority carrier characteristics.…”

Increased efficiency in pn-junction PbS QD solar cells via NaHS treatment of the p-type layer

“…Chalcogenide salts such as Na 2 S, Na 2 Se, and K 2 S are soluble in polar solvent and have been demonstrated to effectively alter the PbS, PbSe, or CdSe stoichiometry and increased p-type doping in transistors, leading to higher p-type currents and charge carrier mobilities in field effect transistors. [26][27][28][29][30] However, the success of these salts in solar cell structures has been limited, likely due to the extremely high reactivity of these salts, which can introduce trap states into the film and cause a pronounced fusion of the QDs, leading to a lower V OC .…”