Advanced near-infrared photodetectors based on lead sulfide quantum dots

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Tang, Haodong ORCID: https://orcid.org/0000-0002-5632-5096 (2021). Advanced near-infrared photodetectors based on lead sulfide quantum dots. University of Birmingham. Ph.D.

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Abstract

This thesis investigated an advanced material: lead sulfide (PbS) quantum dots (QDs), designed, optimized and performed different strategies to enhance the performance of near-infrared (NIR) photodetectors based on PbS QDs.

Previous researchers have performed some great studies suggesting that PbS QDs have great potential in NIR detection and photovoltaic area. Compared with traditional semiconductor bulk materials, the QD materials could have wide, adjustable absorption band, which enables long range detection of light through ultraviolet to infrared, covering entire visible light. Compared with another popular material in optoelectrical area, perovskite materials, PbS QDs showed its advantage in stability and NIR detection. Though, it has been 15 years since PbS QDs first been used for NIR detection, there are still a long way from the PbS QD photodetectors (PDs) to the commercial applications.

Thus, in this thesis, three different technologies are used to optimize the overall performance of PbS QD PDs. Considered of the low carrier extraction efficiency in PbS QD solids, a two-step ligand exchange process is introduced to further exchange the OA- ligands with I- ligands, which could reduce the inner separation between QDs. This two-step ligand exchange technology would cause a more compact stacking behavior of QDs and result in a higher carrier extraction efficiency, which finally reveal to an enhanced responsivity of the fabricated device. Furthermore, the deposition method for PbS QDs is also investigated. By comparing spray-coating technology with traditional spin-coating technology, spray-coated PbS QD PD showed an enhanced responsivity. The enhancement is mainly related to the more compact stacking of PbS QDs, which is confirmed by grazing-incidence small-angle X-ray scattering (GISAXS) analysis. Longer time during the film formation, the PbS QDs would have higher chance to form a self-assembled body-centered cubic (BCC) structure, rather than the distorted superlattice situation in the spin-coated QD solid. Similarly, this enhancement is also brought by reduced inner separation between PbS QDs.

On the other hand, a new structure for NIR detection and display at the same time is proposed by stacking a light-emitting diode (LED) onto the PD. The PD works as a NIR controlled light switch, only under the illumination of NIR light, the LED device could work normally. Otherwise, even there is strong bias voltage, the LED will not be turned-on. This is achieved by well designed energy bands in this device. The injected carriers are blocked by the carrier blocking layers, then the circuit is connected by the photogenerated carriers produced in PD structure when the device is illuminated by NIR light. The device could be regarded as a up-converter which absorbs NIR light and emit red light. Also, the device could be considered as a NIR photodetector which introduced excess carriers from the circuit during the illumination of NIR light.

Eventually, these methods have enhanced the performance of PbS QD PDs. However, the problems of stability, large area fabrication, cost and packaging should be solved before the commercial or industrial applications based on PbS QD PDs could be realized.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):