Transient Optical Studies of Interfacial Charge Transfer at Nanostructured Metal Oxide/PbS Quantum Dot/Organic Hole Conductor Heterojunctions

Abstract

We report a transient absorption and luminescence study addressing the charge separation, recombination, and regeneration reactions at nanostructured metal oxide/PbS quantum dot/organic hole conductor heterojunctions. We show that yields of charge separation are significantly higher at PbS/SnO(2) interfaces relative to PbS/TiO(2) interfaces, and conclude that this behavior is a result of the ca. 300-500 meV lower conduction band edge in SnO(2) as compared to TiO(2). We also report a correlation between the PbS particle size and the yield of charge separation at PbS/SnO(2) interfaces, with a smaller PbS particle radius resulting a higher yield of charge separation. Finally we investigated the regeneration of the photooxidized PbS by an organic hole transporting material, namely, spiro-OMeTAD. The overall spiro-OMeTAD(+) yield is found to be 27% at a SnO(2)/PbS (approximately 3 nm diameter)/spiro-OMeTAD heterojunction. The addition of a lithium salt was found to raise the overall spiro-OMeTAD(+) yield from its initial value of 27% (where no Li(+) is present) to 50%. We believe this to be a result of an increase in the primary charge injection yield to near unity as the SnO(2) conduction band is lowered (with increasing [Li(+)]), increasing the driving force for electron injection. The present findings are discussed with relevance to the design of PbS-sensitized metal oxide layers for DSSCs.

Ultrasensitive PbS quantum-dot-sensitized InGaZnO hybrid photoinverter for near-infrared detection and imaging with high photogain

“…Whereas the TRPL in the PbS QD single-layer film exhibited monoexponential decay dynamics with a few hundred nanosecond time scale, the decay time observed in the PbS QD/IGZO sample was much faster and within laser-pulse repetition (12.5 ns for our Ti: sapphire laser oscillator), indicating that the photogenerated charge carriers from PbS QDs can be transferred to IGZO films. [40][41][42] The PbS QD layer processed with the AN solvent was also applied to make the photosensing layer, but we observed less effective photosensitivity (less negative V th shifts: see Supplementary Figure S5); this can be directly correlated with the degree of the EDT ligand exchange as previously mentioned in the discussion regarding absorption spectra. The photoinduced V th shifts provide additional information regarding the extraction of optical energy bandgap of the PbS QD film.…”

Solution Processed Polymer–Inorganic Semiconductor Solar Cells Employing Sb2S3 as a Light Harvesting and Electron Transporting Material

“…Transient absorption spectroscopy is a pumpprobe technique that can be used to identify photogenerated charge separated states and monitor their decay dynamics. [22][23][24][25] Full experimental details of our transient absorption spectrometer have been described previously and are provided in the Supplementary Information to this manuscript. [ 26 ] Briefl y, transient absorption data were obtained using an optical pumpprobe set-up employing nitrogen-pumped dye-laser (600 ps pulse duration) as an excitation source and a tungsten lamp as a probe source.…”

Spectroscopic Studies of Electron Injection in Quantum Dot Sensitized Mesoporous Oxide Films

“…A widely studied approach to extract carriers from QDs is to attach the QDs to a mesoporous oxide matrix that is immersed in an electrolyte solution. 2,[11][12][13][14] In this concept, light is selectively absorbed by the QDs and electrons are injected from the QDs into the oxide matrix, which serves as photoanode, and the remaining positively charged QD is reduced by a suitable redox couple in the electrolyte. The oxidized redox species migrates toward the photocathode, where it is reduced by the electrons that re-enter the system via the external circuit.…”