High Infrared Photoconductivity in Films of Arsenic-Sulfide-Encapsulated Lead-Sulfide Nanocrystals

Abstract

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2 × 1013 Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.

PbS Colloidal Quantum Dot Photodetectors operating in the near infrared

“…Best devices exhibit a detectivity as high as 2·10 10 cmHz 1/2 W −1 at 1.3 μm and 1 V bias, for optical power <5 nW. Such value compares well with those reported in literature at low voltage bias182526.…”

“…For example, by using silver nanocrystals, a dramatic decrease of the dark current has been obtained (nA range) at the cost of a low responsivity (<0.1 A/W)23. Sometimes, a more favorable balance between dark current and responsivity was achieved, leading to device with D* in the 10 12 –10 13  cmHz 1/2 W −1 range (although operated at relatively high voltage bias)1831. Such approaches make the device fabrication more complex and may reflect on reduced repeatability and reliability.…”

“…However, their sensitivity is limited by the fundamental limitation of <1 quantum efficiency17. Photoconductors exhibit very large responsivity thanks to their photoconductive gain but, on the other hand, show extremely low cutoff frequency and large dark current18. An alternative approach is based on phototransistors, a three terminal device where the gate potential is optically controlled, modulating the channel conductivity.…”

“…Several studies have been conducted aiming to the control of material doping, carrier mobility and lifetime often exploiting different ligands and engineering oxidation processes of the QDs1821222324. Such research works lead to significant advance in the field and relevant device performance were demonstrated.…”

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.…”