Ultrafast photoexcitation dynamics of low dimensional PbSe nanostructures
Date of Issue2017-01-16
School of Physical and Mathematical Sciences
Multiple exciton generation (MEG) in PbSe quantum dots (QDs) opens up new opportunities to improve solar cell efficiency beyond the Shockley-Queisser limit. Despite the demonstration of efficient MEG efficiency in PbSe QDs, power conversion efficiency of PbSe QDs solar cells remains low. One of the limitations to the concept of MEG solar cells is that after MEG, excitons recombine very fast via Auger recombination and unavailable to extract at the electrodes. So far charge extraction is limiting the advantages of MEG in solar cells. Auger recombination time is proportional to the volume of the nanostructures. Therefore, Auger recombination rate should be low in elongated nanostructures such as nanowires (NWs, 1D) and nanosheets (NSs, 2D). Efficient MEG can be achieved along the confined directions of 1D and 2D nanostructures and charges can move along the elongated direction. To optimize the quantization, Auger recombination, charge extraction and transport properties of nanodevices, a detailed understanding of photoexcitation dynamics of carriers on the femtosecond scale is needed. In this work, photoexcitation dynamics of low dimensional PbSe nanostructures are explored by studying the transient absorption (TA) spectra and dynamics. We investigated MEG QY and the origin of the photon energy threshold for MEG in PbSe QDs. The comparison of visible TA spectra and dynamics for different pump wavelengths, below, close and above MEG threshold provides unambiguous evidence of the role of the Σ transition in slowing down the exciton cooling process, which allows MEG taking over the phonon relaxation process. Moreover, we infer that the MEG QY could be measured directly from the visible TA spectra. Quantum confined PbSe NWs of bandgap 0.72 eV was synthesized. In addition, two more optical transitions are observed in the absorption spectra. These three optical transitions are also predicted by the density function theory (DFT) calculations. TA spectra and dynamics were measured with excitation wavelength of 800 nm. Dispersive TA spectra of PbSe NWs were observed at short pump-probe delays (within 1 ps) due to the extended length of the NWs. After 3 ps, dispersive TA spectra changes to absorptive TA spectra and shows the optical transitions. Anisotropy was also observed in the photoexcitation dynamics of PbSe NWs, which is explained classically by the dielectric mismatch and long aspect ratio of NWs. To our best knowledge, this is the first report for the synthesis of quantum confined PbSe NSs. Quantum confined PbSe NSs of bandgap 0.64 eV were synthesized using the microwave-assisted technique. In addition one more optical transition is observed in the absorption spectra. These two transitions are also predicted by DFT calculations. TA spectra of PbSe NSs were measured with a pump wavelength of 800 nm and probe wavelength range from 950 nm to 1500 nm. At low pump fluence, TA spectra were dominated by the photo-bleaching of the second transition. However, at high pump fluence, TA spectra were dominated by the nonlinear absorption of the excitons. Anisotropy is also observed in PbSe NSs which is the combined effect of intrinsic and extrinsic anisotropies. In conclusions, we propose that ordered PbSe NWs and NSs architectures can be employed to design MEG solar cells for effective carrier extraction along preferential axis to improve the solar cell efficiency. PbSe NWs and NSs can also be used as large-scale nanodevices like single NW and NS FET. The nonlinear properties of PbSe NSs can be used in pulse shaping of the laser beam, ultrafast switch, high power IR sensor and IR eye protector etc.