Solution-processed Colloidal Quantum Dots Research Articles

Temperature- and ligand-dependent carrier transport dynamics in photovoltaic PbS colloidal quantum dot thin films using diffusion-wave methods

Solution-processed colloidal quantum dots (CQDs) are promising candidates for large-scale, low-cost, and lightweight photovoltaic and electronic devices. Carrier transport dynamics has a substantial impact on device efficiency optimization. Coupled with photocarrier radiometry (PCR) — a dynamic spectrally integrated frequency-domain photoluminescence (PL) modality, we report the derivation of a trap-state-mediated carrier hopping transport model for the extraction of multiple carrier transport parameters in PbS CQD thin films. These parameters, including effective carrier lifetime τE, hopping diffusivity Dh, trap-state-dependent carrier trapping rate RT, diffusion length Lh, and carrier thermal emission rate ei, were obtained for CQD thin films with different dot size and capping ligands: tetrabutylammonium iodide (TBAI), 1,2-ethanedithiol (EDT), and methylammonium lead triiodide perovskite (MAPbI3). Consistent with the framework of phonon-assisted carrier hopping mechanism, τE, Dh, and Lh have demonstrated a monotonic dependence on temperature in the range from 100 K to 300 K. Perovskite-passivated PbS CQD thin films, especially those with larger dot sizes which are free of apparent defect induced PL emission and have higher τE and Dh at room temperature (ca. 0.51μs and 1.80×10−2cm2/s, respectively) than their counterparts, demonstrate better photovoltaic material properties. Dot-size-dependent exciton binding energies (35.21–53.20meV) were characterized using a dynamic PCR photo-thermal spectroscopy that also characterized the trap-state-mediated carrier hopping activation energies in the range from 100meV to 280meV. To test the reliability of the best-fitted results, computational fitting uniqueness was examined using a parametric theory.

Study of Exciton Hopping Transport in PbS Colloidal Quantum Dot Thin Films Using Frequency- and Temperature-Scanned Photocarrier Radiometry

Solution-processed colloidal quantum dots (CQDs) are promising materials for realizing low-cost, large-area, and flexible photovoltaic devices. The study of charge carrier transport in quantum dot solids is essential for understanding energy conversion mechanisms. Recently, solution-processed two-layer oleic-acid-capped PbS CQD solar cells with one layer treated with tetrabutylammonium iodide (TBAI) serving as the main light-absorbing layer and the other treated with 1,2-ethanedithiol (EDT) acting as an electron-blocking/hole-extraction layer were reported. These solar cells demonstrated a significant improvement in power conversion efficiency of 8.55% and long-term air stability. Coupled with photocarrier radiometry measurements, this work used a new trap-state mediated exciton hopping transport model, specifically for CQD thin films, to unveil and quantify exciton transport mechanisms through the extraction of hopping transport parameters including exciton lifetimes, hopping diffusivity, exciton detrapping time, and trap-state density. It is shown that PbS-TBAI has higher trap-state density than PbS-EDT that results in higher PbS-EDT exciton lifetimes. Hopping diffusivities of both CQD thin film types show similar temperature dependence, particularly higher temperatures yield higher hopping diffusivity. The higher diffusivity of PbS-TBAI compared with PbS-EDT indicates that PbS-TBAI is a much better photovoltaic material than PbS-EDT. Furthermore, PCR temperature spectra and deep-level photothermal spectroscopy provided additional insights to CQD surface trap states: PbS-TBAI thin films exhibit a single dominant trap level, while PbS-EDT films with lower trap-state densities show multiple trap levels.

Surface Passivation of Lead Sulfide Nanocrystalswith Low Electron Affinity Metals:Photoluminescence and Photovoltaicperformance

During the last decade, solution-processed colloidal quantum dots (CQDs) have attracted significantattention for low-cost fabrication of optoelectronic devices. In this study, lead sulfide (PbS) CQDs weresynthesized via the hot injection method and the effect of doping elements with low electron affinity,including cadmium, calcium and zinc, on the passivation of trap states was investigated. A red-shiftin the luminescence emission was observed by doping through passivation of lead dangling bonds.Time-resolved photoluminescence measurements showed that the lifetime of charged carriers wassignificantly enhanced by cadmium doping (B80%) which is quite noticeable compared with calciumandzinc-doped nanocrystals. External quantum efficiency measurements on thin solid films (B300 nm)prepared by spin coating supported improved lifetime of carriers through passivation of mid-gap trapstates. In order to show the potential application of the doping process, bulk heterojunction CQD solarcells were fabricated. It was found that the power conversion efficiency (PCE) was improved up toB40%; the highest improvement was observed with the Cd treatment. Finally, density functional theory(DFT) and electrochemical impedance spectroscopy (EIS) were employed to study the effect of dopingon the density of states. The results showed that doping with low electron affinity metals effectivelyreduced the deep trap states of PbS QDs. Figure 1

Colloidal quantum dot solids for solution-processed solar cells

Solar cells based on solution-processed colloidal quantum dots are promising alternatives to conventional devices. This Review discusses recent advances and outstanding challenges for the field of quantum dot solar cells towards their commercialization.

Suppression of dark current through barrier engineer for solution-processed colloidal quantum-dots infrared photodetectors

In an attempt to suppress the dark current, the barrier layer engineer for solution-processed PbSe colloidal quantum-dot (CQD) photodetectors has been investigated in the present study. It was found that the dark current can be significantly suppressed by implementing two types of carrier blocking layers, namely, hole blocking layer and electron blocking layer, sandwiched in between two active PbSe CQD layers. Meanwhile no adverse impact has been observed for the photo current. Our study suggests that this improvement resides on the transport pathway created via carrier recombination at intermediate layer, which provides wide implications for the suppression of dark current for infrared photodetectors.

Ultra-sensitive tandem colloidal quantum-dot photodetectors.

The solution-processed PbSe colloidal quantum-dot (CQD) infrared photodetector with tandem architecture is proposed to address the high dark current issue. The electrical transport mechanism in tandem has been fundamentally changed in which the recombination of carriers at an intermediate layer becomes dominant rather than carriers hopping between nearest neighbors in CQD materials. As a result, the tandem photodetector exhibits ultra-high detectivities of 4.7 × 10(13) Jones and 8.1 × 10(13) Jones under 34 μW cm(-2) illumination at 1100 nm, at 275 K and 100 K, respectively.

Charge Trapping Dynamics in PbS Colloidal Quantum Dot Photovoltaic Devices

The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2-0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 0.3-0.5 eV) is observed on the ~0.1-10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development.

Field-effect transistor-based solution-processed colloidal quantum dot photodetector with broad bandwidth into near-infrared region

We demonstrate a solution-processed colloidal quantum dot (CQDs) photodetector with the configuration of a field-effect transistor (FET), in which the drain and source electrodes are fabricated by a shadow mask. By blending PbS CQDs into the hybrid blend, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM), the photosensitive spectrum of the nanocomposite blend is extended into the near-infrared region. A FET-based photodetector ITO/PMMA (180 nm)/P3HT:PCBM:PbS (110 nm)/Al, in which PMMA (polymethylmethacrylate) acts as the dielectric layer and P3HT:PCBM:PbS (in weight ratio of 1:1:1) as the active layer, shows a broad spectral bandwidth, a responsivity of 0.391 mA W−1 and a specific detectivity of 1.31 × 1011 Jones are obtained at VGS = 1 V under 600 nm illumination with an intensity of 30 μW cm−2. Therefore, it provides an easy way to fabricate such a FET-based photodetector with a channel length of some hundreds of micrometers by a shadow mask.

Colloidal quantum dot photodetectors enhanced by self-assembled plasmonic nanoparticles

Self-assembled colloidal plasmonic silver nanoparticles monolayers were integrated into solution-processed colloidal quantum dot (QD) photodetectors. We observed plasmon enhancement of photodetector responsivity over a board spectrum range (400–600 nm), with a 1.2–1.6 fold enhancement for a 440-nm-thick QD film device and a 2.4–3.3 fold enhancement for a 100-nm-thick QD device. The enhancement behavior was wavelength dependent with higher enhancement factor at longer wavelengths. The origins of responsivity enhancement were discussed.

Solution-processed colloidal quantum dot photovoltaics: A perspective

Deployed solar cells today are predominantly first-generation devices based on silicon. Single-crystal materials offer the advantage of high efficiency, but costs associated with materials, manufacturing, and installation remain high. Second generation, or thin-film, technologies, such as those based on CdTe and CuInGaSe2, offer dramatically improved costs per square meter, but at the price of lower efficiencies. Third-generation photovoltaics aspire to unite high efficiency with low cost. Strategies include nanostructured semiconductors, inorganic‐organic hybrid structures, and molecular assemblies. Solution-processed colloidal quantum dot (CQD) photovoltaic devices offer considerable promise as a third-generation photovoltaic candidate. Their solution-synthesis and -coating offer a path to low-cost roll-to-roll manufacturing atop flexible, lightweight substrates. The bandgap of CQDs is tuned via the nanoparticles’ diameter, allowing them to absorb the sun’s broad visible and infrared spectrum within a single materials-processing strategy. CQD photovoltaics have, with only a half-decade’s intense activity, reached above 5% solar power conversion efficiency, 1,2 and progress shows no signs of abating. Here we review the latest advances in the field and discuss the avenues for further progress towards commercially compelling performance. We include a discussion of the materials themselves, considering their synthesis and their photophysical properties. We review briefly the fundamentals of semiconductors and solar cells. We highlight the major progress and achievements in CQD photovoltaics, focusing on the introduction of improved device architectures.

Solar Cells, Photodetectors, and Optical Sources from Infrared Colloidal Quantum Dots

AbstractOptoelectronic devices made via spin‐coating of soft materials onto an arbitrary substrate enable ready integration, low cost, and physical flexibility. The use of solution‐processed colloidal quantum dots offers the added advantage of quantum‐size‐effect tuning of material bandgap. Tuning across the near‐ and short‐wavelength infrared (SWIR) spectral regions enables applications in fiber‐optic communications, night vision and biomedical imaging, and efficient solar energy collection. Here we review progress in infrared solar cells, light sensors, and optical sources based on solution‐processed materials. The latest solution‐processed photovoltaics now provide 4.2% power conversion efficiencies in the infrared, placing them a factor of three away from enabling a doubling in overall solar power conversion efficiency of visible‐wavelength solution‐processed photovoltaics. The best solution‐processed photodetectors now provide sensitivities of 1013 Jones D* (normalized detectivity), exceeding the sensitivity of the best epitaxially grown SWIR photodetectors. Infrared optical sources, both broadband light‐emitting diodes and, more recently, lasers, have now also been reported at 1.5 µm.

Bright and Color-Saturated Emission from Blue Light-Emitting Diodes Based on Solution-Processed Colloidal Nanocrystal Quantum Dots

We report a multilayer solution-processed blue light-emitting diode based on colloidal core/shell CdS/ZnS nanocrystal quantum dots (QDs). At a low-operating voltage of 5.5 V, the device emits spectrally pure blue radiation at 460 nm with a narrow full-width-at-half-maximum bandwidth of 20 nm and high brightness up to 1600 cd/m2. Broad-band, long-wavelength emission from the polymer components and deep traps in the QDs are minimized to less than 5% of the total emission.