Optoelectronic Applications Of Lead Sulfide Quantum Dot:Studies On Photodetectors And Light Emitting Diodes

Quantum Dot?QD?optoelectronics have been used to fabricate low-cost,large-area and spectrally-tunable optoelectronic devices,such as photodetectors,phototransistors,solar cells,and light emitting diodes?LEDs?.According to the material types,QD can be mainly divided into III-V,II-VI,IV-VI group compound and IV group substance.Lead sulfide?PbS?QD has mature preparation processes and good optical properties,attracting a great deal of interest.In particular,the bandgap of PbS bulk is 0.4 eV and the bandgap of PbS QD is adjustable due to quantum confinement effect.The absorption edge of PbS QD is tunable in the range of 600 to 3000 nm by controlling the QD size during synthesis.Thus,PbS QD can be used as light absorbing layers for infrared photodetectors and photovoltaic devices,as well as light emitting layers for infrared LEDs.Earlier studies have found that carrier mobility of PbS QD thin film is too low and surface defects of PbS QD are too high,which restrict the performance of PbS QD optoelectronic devices.This topic is mainly on these two aspects to study and enhance PbS QD photodetectors and LEDs.PbS QD photodetectors mainly use short-chain ligands to exchange OA on PbS QD surface for improving the mobility of PbS QD films.We pioneered to utilize high-mobility multi-walled carbon nanotube?MWCNT?to form a composite structure with PbS QD,in which photo-generated carriers in PbS QD were transferred to MWCNT,thereby improving the transmission of photo-induced carriers.Transmission electron microscopy?TEM?and Fourier transform infrared spectroscopy?FTIR?results showed that MWCNT and PbS QD self-assembled to form PbS QD/MWCNT composite structure.Photoluminescence?PL?quenching proved that the photo-generated carriers in PbS QD were efficiently transferred to MWCNT.PbS QD/MWCNT photodetectors showed significant improvements in sensitivity and flexibility over pure PbS QD photodetectors.Responsivity and detectivity of PbS QD/MWCNT photodetectors reached up to 583mA/W and 3.25?10¹² Jones,respectively.The PbS QD/MWCNT device could withstand more than 10,000 folds and up to 80°fold.Finally,the PbS QD/MWCNT device also successfully realized wearable heart rate detection,which was expected to be integrated into the electronic skin.Based on the principle of photo-generated carriers in PbS QD transferred by high-mobility materials,two-dimensional?2D?graphene and PbS QD based photodetectors have shown ultra-high sensitivity.Here,new-type 2D tin disulfide?SnS₂?nanosheet?NS?field effect tube?FET?was used as a high-mobility conductive channel to transmit photo-generated carriers from PbS QD.PbS QD/SnS₂ NS photodetectors took advantage of strong wide-spectrum absorption of PbS QD and high mobility of SnS₂ NS to successfully realize wide-spectrum and ultra-sensitive photodetection.Compared with other PbS QD/2D material based photodetectors,the PbS QD/SnS₂ NS photodetectors showed more balanced performance,with a responsivity of up to 10⁶ A/W and a response time of up to 20 ms.Due to different working mechanism in long-wave and short-wave bands,the PbS QD/SnS₂ NS photodetectors showed positive response in short-wave band?photocurrent greater than dark current?and negative response in long-wave band?photocurrent lower than dark current?.By establishing a physical model,we qualitatively analyzed that the wavelength-dependent positive and negative photoresponse was caused by a competition between increase of photo-generated charge concentration in SnS₂ NS channel and rise of potential barrier between SnS₂ NS and gold electrodes.By using the high sensitivity and spectral discriminability of PbS QD/SnS₂ NS photodetectors,we realized color temperature discrimination of commercial white LED with different emission spectra,demonstrating a unique application that distinguished them from other broadband or narrowband photodetectors.Besides being used as light absorption layers of photodetectors,PbS QD is considered as one of the most promising material choices for light emitting layers of next-generation near infrared?NIR?LEDs.PbS QD has a very high photoluminescence quantum yield?PLQY?in NIR range that is unattainable by organic luminescent materials or other QDs.Compared to high external quantum efficiency?EQE?achieved by QD LED in visible band,the EQE of PbS QD NIR LED needs to be further improved.The improvement of PbS QD LED EQE relies on improvement of carrier-to-exciton dynamic processes and PLQY enhancement of PbS QD light-emitting layer,which are related to PbS QD surface passivation.However,the two main passivation strategies reported so far are organic passivation and wide-bandgap-material capping,which can't simultaneously achieve efficient carrier-to-exciton dynamics and high PLQY.Therefore,we proposed a new strategy of using PbS QD coated by quasi two-dimensional perovskite?Q2DPVK?as light-emitting layers of NIR LEDs,which combined the advantages of both reported strategies.By solving compatibility of PbS QD and perovskite precursor solution and adjusting concentration of PbS QD,we prepared a high-quality PbS QD/Q2DPVK thin film with uniform PbS QD distribution and homogeneous Q2DPVK orientation.The PbS QD/Q2DPVK film showed a roughness of only 0.3 nm,a PLQY of up to 45%and an 80%charge transfer efficiency in the way of exciton from Q2DPVK to PbS QD.When used as a light-emitting layer of NIR LEDs,this thin film ensured uniform and balanced charge injection and efficient light emission.The NIR LEDs based on PbS QD/Q2DPVK reached up to 8.08%EQE and 7.38 W Sr^-1 m^-2 radiance with emission peaks continuously adjustable from 986 nm to 1564 nm.