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

Abstract

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.