PbS Colloidal Quantum Dot Photodetectors:Enhanced Device Performance Based On Nanocomposites

Since the large Borh radium (18nm) and narrow band gap (0.4eV) of lead sulfide (PbS), strong quantum confinement experienced by electrons and holes in PbS colloidal quantum dots (CQDs) enables their band gap tunable from0.4eV to2.0eV through CQD diameter engineering, resulting in effectively light absorption from600nm to3000nm. Recently, the photoconductive photodetectors based on PbS CQDs have attracted great research interests because of low-cost fabrication, high device performance, easy integration onto any substrate and low temperature processing for flexible devices fabrication. So far, most of reports focusd on the responsivity enhancement of PbS CQDs photodetectors, little attention has been paid to the suppression of dark current and construction of photodetector using hybrid CQD nanocomposites. Guided by the theory of applied physics, this thesis incorporated metal and semiconductor nanocrystals (NCs) into PbS CQDs film in the hope of performance enhancement by their synergetic effects. These binary NC nanocomposites exhibited not only the properties of the individual building blocks but also novel collective properties through coupling between CQD components which have successfully reduced the dark current and increased the photocurrent simultaneously. Furthermore, we also fabricated flexible and sensitive PbS CQDs photodetectors by utilizing stone paper as the substrates and pencil-drawn graphite as the electrodes.We first concerned on the problem of sluggish photoresponse in traditional ethanedithiol (EDT) and benzenedithiol (BDT) treated PbS CQD photoconductivity photodetectors. In this work we chose3-mecaptoisopropionic acid (MPA), cetyltrimethylammounium bromide (CTAB) as the chemicals to passivate PbS CQDs and compared device performance with EDT treated PbS CQDs. Device results indicated that CTAB treatment lead to defects with shallowest depth, followed by MPA and then EDT. As a results, CTAB treated device showed the best device performance, with the normalized detectivity being8.9x109Jones.In order to further improve device performance based on CTAB passivation, the Au (NCs) were utilized to blend with PbS CQDs, the influence of Au NCs on the photoresponse performance of PbS CQDs photodetectors was identified as the formation of a low resistance conduction channel for holes in the PbS CQD (work function is4.8eV)/Au NC (work function is5eV) nanocomposites, increasing the carrier mobility and hence the responsivity. The effect of Au NCs on the photoresponse of PbS CQD photodetectors was systematically investigated. The results showed that the highest detectivity is1.9x1010Jones when the weight ratio of PbS CQDs to Au NCs was100:2, and the photoresponse speed had not obvious change.Since incorporation of AuNCs increased the dark current of PbS CQD photodetectors, for the hope of decreasing dark current, we explored the mixing of n type CdS NCs with PbS CQDs in the hope to form a bulk nano-heterojunction based on the theory of pn junction. In order to understand the function of CdS NCs inthe nanocomposites, the effect of MPA, CTAB and EDT passivation on the photoresponse performance of CdS NC photodetectors thin film were first investigated. Results showed that CTAB treated CdS NCs device again had best performance. The rough surface caused by CTAB treatment was very useful for oxygen absorption and desabsorption. Unfortunately, incorporating CdS NCs into PbS CQD film lead to device with increased dark current, no matter which chemicals (MPA, CTAB and EDT) were employed for the passivation. We proposed that CdS NCs incorporation probably increased carrier lifetime, hence increase dark current.Based on the finding that n-type CdS NCs increased device photocurrent, we introduced Ag NCs into PbS CQDs and studied device performance in this chapter. When treated by MPA, the reaction between Ag NCs and MPA produced Ag2S on Ag NC surface, forming Ag core/Ag2S shell NCs. Furthermore, formation of a Schottky junction betwee low work-function Ag NCs and p-type PbS CQDs accounted for dark current suppression. Hence, the synergetic effect of Ag core/Ag2S shell NCs simultaneously increased device photocurrent and reduced dark current, yielding photodetectors with1.2x1010Jones detectivity. Photoluminescence (PL) study revealed that facile charge transfer between PbS CQDs and Ag NCs, thus reducing the intensity of PL. X-ray photoemission spectroscopy (XPS) studied confirmed the in-situ formation of Ag2S, and its effect in photocurrent enhancement was supported by the experimental observation that AgNO3treatment of PbS CQD film also resulted in device with increased photocurrent. In addition to Ag NC incorporation, we further introduced Schottky contacts instead of traditional ohmic metal electrodes to reduce dark current of PbS CQD photodetectors. Latest developed n-type PbS CQD film was used to form the Schottky contact with Au electrodes, avoiding the reproducibility problem of Schottky contacts between p-type PbS CQD film and Al electrodes. As expected, Device performance showed that Schottky contact design was indeed better than ohmic contact in dark current reduction. As prepared PbS CQD photodetector acheived detectivity of2.8×1010Jones.Last, we explored the advantage of low temperature solution processing by building flexible PbS CQD photodetectors. PbS CQD/AuNC and PbS CQD/AgNC nanocomposites were spun onto stone paper and pencil drawn graphite was utilized as electrodes. Devices performance showed less than10%decay when worked at different bending angles (0-80°) and after multiple bending cycles (0-400), suggesting that PbS CQD films are very competitive for next-generation flexible photodetector applications.