Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces

We investigate the impact of grain boundaries and interfaces on dynamics of photoexcited charge carriers in polycrystalline lead sulfide (PbS) films and at interfaces between polycrystalline PbS and ZnO by studying transient photoconductivity over sub-picoseconds to microseconds timescales using time-resolved terahertz spectroscopy and time-resolved microwave conductivity measurements. Narrow band gap bulk-like polycrystalline PbS with high absorption in the infrared paired with wide band gap metal oxide current collectors holds promise for infrared photodetectors and photovoltaics for converting infrared radiation to electricity. We find that grain boundaries in polycrystalline PbS suppress long-range conductivity and confine photoexcited carriers within individual crystallites. The mobility of photoexcited holes inside the ∼150 nm crystallites reaches 750 cm²/V s, and their lifetime exceeds hundreds of microseconds, while electrons get rapidly trapped at grain boundary states. The presence of PbS/ZnO interfaces dramatically reduces the lifetime of the photoexcited free holes in the PbS crystallites. Moreover, we detect no injection of free electrons from PbS to ZnO. Optimal transfer of photoexcited electrons, as is needed for optoelectronic devices with PbS/ZnO heterojunctions, may require engineering PbS/ZnO heterojunctions with buffer layers or organic ligands to passivate deleterious interface states

Quantum Dot Color-Converting Solids Operating Efficiently in the kW/cm² Regime

With rapid progress in the use of colloidal quantum dots (QDs) as light emitters, the next challenge for this field is to achieve high brightness. Unfortunately, Auger recombination militates against high emission efficiency at multiexciton excitation levels. Here, we suppress the Auger-recombination-induced photoluminescence (PL) quantum yield (QY) loss in CdSe/CdS core–shell QDs by reducing the absorption cross section at excitation wavelengths via a thin-shell design. Studies of PL vs shell thickness reveal that thin-shell QDs better retain their QY at high excitation intensities, in stark contrast to thicker-shell QDs. Ultrafast transient absorption spectroscopy confirms increased Auger recombination in thicker-shell QDs under equivalent external excitation intensities. We then further grow a thin ZnS layer on thin-shell QDs to serve as a higher conduction band barrier; this allows for better passivation and exciton confinement, while providing transparency at the excitation wavelength. Finally, we develop an isolating silica matrix that acts as a spacer between dots, greatly reducing interdot energy transfer that is otherwise responsible for PL reduction in QD films. This results in the increase of film PL QY from 20% to 65% at low excitation intensity. The combination of Auger reduction and elimination of energy transfer leads to QD film PL QY in excess of 50% and absolute power conversion efficiency of 28% at excitation powers of 1 kW/cm², the highest ever reported for QDs under intense illumination