Colloidal Quantum Dot Thin Films Research Articles

(Invited) Parameter Prediction in Colloidal Quantum Dot Solar Cells Using Residual Neural Networks Trained on Experimental Data

Colloidal quantum dots (CQDs) are an attractive third-generation material for photovoltaics, especially as infrared materials for multi-junction solar cells. Characterization and materials parameter prediction both play a critical role in the rising efficiencies in this field. However, there are numerous materials properties that need to be measured in order to fully characterize a device, leading to high research costs and slow development times. Here, we leverage recent advancements in machine learning (ML), along with a recently-demonstrated multimodal spectral characterization method [1], to predict complex materials parameters in CQD solar cells from simple current density-voltage (JV) curves, using an algorithm trained on experimental data.[2]Past work ML prediction of materials properties has focused on using simulated data due to the difficulty in collecting enough experimental data to fully train an algorithm. While it is not feasible to fabricate thousands of devices in a typical research lab, it is possible to measure several thousand points on a single device. We used a micrometer-resolution 2D characterization method with millimeter-scale field of view to acquire enough training data using only a handful of devices, while learning the spatial relationships between materials parameters. Photoluminescence, trap state density, transient photovoltage, transient photocurrent, and carrier mobility were predicted (Figure 1) by five different residual neural networks, each using a ResNet block and containing over 300k learnable parameters. The design of the networks were based on the ResNet50 architecture [3]. We also implemented a novel neighborhood approach to reduce errors and account for spatial correlations in device behavior, leading to insights into transport mechanisms and correlation lengths in CQD thin films. This method could be used to study a wide range of material systems and devices, ultimately leading to a universal prediction model that greatly simplifies the characterization process for optoelectronic devices and accelerates development times.

Pixelated vertical-cavity surface-emitting laser arrays from colloidal quantum dot films

Pixelated arrays of vertical-cavity surface-emitting lasers from colloidal quantum dot thin films have been demonstrated. CdSe/ZnCdS core/shell structured colloidal quantum dots have been used as laser gain medium. 10 pairs of alternating layers between GaN and nanoporous GaN consisted highly reflective distributed Bragg reflectors, exhibiting more than 99% of reflectivity in the spectral range of 600 nm–650 nm. Close-packed colloidal quantum dot solid thin films were sandwiched between the two GaN/nanoporous GaN distributed Bragg reflectors to achieve well-defined lasing outputs under pulsed optical excitation with the lasing threshold as low as 11.0 μJ/cm2. The resulting CQD-VCSEL devices exhibited a lasing output power of 1.5 μW and a conversion efficiency of 11.4%. Finally, a two-axis scanning micro-mirror was used to complete the 25 (5 by 5) pixelated laser arrays system.

Understanding the growth mechanisms of ultrasmall silver selenide quantum dots for short-wave infrared detectors

Colloidal quantum dots (CQDs) allow for wavelength-tunable, economic, and elegant access to the infrared (IR) spectrum by utilizing inter- and intraband excitonic transitions in narrow bandgap semiconductors. With the emergence of new applications for IR detection, developing competitive non-toxic and environmentally benign alternatives to heavy metal-containing semiconductors is of increasing importance. Over the last decade, silver selenide (Ag2Se) has emerged as an alternative to lead- and mercury-based QDs, especially with large, intraband Ag2Se CQDs showing promising performance in the mid-wave IR. Herein we explored the critical nucleation and growth mechanisms of ultrasmall (2.5–3.5 nm) interband absorbing Ag2Se CQDs operating in the near to short-wave IR. Classical nucleation and growth was observed at most reaction conditions, with a growth temperature of 140 °C and a Ag:Se precursor molar ratio of 1:2 providing the most robust control over the IR absorption spectrum. We proceeded to perform an in-depth study of the impact of surface ligand chemistry of CQD thin-films on resulting optoelectronic properties. By tuning the functional groups and optimizing ligand exchange parameters, we obtained IR responsivity values of ∼25 mA/W in photoconductor devices and ∼1.8 mA/W in photodiode devices in the range 800 – 1250 nm.

High performance NIR photodetector based on Cd(1-x)CuxS colloidal quantum dots thin films

Thin films of Cd(1-x)CuxS (x = 0.0 to 0.05 with interval of 0.01) colloidal quantum dots (CQDs) are employed to develop NIR photodetectors (PDs) with significantly high performance parameters and selectivity. Two very cost effective and easy chemical routes are used to synthesize Cd(1-x)CuxS CQDs and to fabricate the PD devices. The Cd(1-x)CuxS CQDs thin films are primarily characterized by various techniques in order to study the influence of Cu doping on different properties of the films. Further, the impact of doping on device performance under the exposure of laser light of different wavelengths is investigated. The tunability in sensing range from visible (635 nm) to NIR (782 nm) of the PDs with a huge improvement in selectivity and device performance due to the incorporation of the dopant is noticed. The maximum selectivity is observed for device S2 (2% Cu in CdS) with maximum responsivity and external quantum efficiency of ∼39 A/W and ∼6.2 × 103% respectively. Whereas the device S4 (4% Cu in CdS) is found to be the fastest with response times of 102 ms/109 ms, sensitivity of ∼3 × 105%, and detectivity of ∼1 × 1014 cmHz1/2W−1. In present study, device S4shows best optimal performance parameters.

Tutorial: Lead sulfide colloidal quantum dot infrared photodetector

Lead sulfide (PbS) colloidal quantum dots (CQDs) is emerging as a promising material for infrared photodetectors because of its strong absorbance in the near infrared region, the tunable energy gap, facile fabrication, and excellent electrical properties. This Tutorial gives a comprehensive introduction of materials and devices of PbS CQD based infrared photodetectors. First, we introduce the concept of CQDs and the typical characters of CQDs. The synthesis of PbS CQDs and a solid film fabrication method are then summarized. Subsequently, the study for defect passivation and carrier transport in a PbS CQD thin film is presented. In the end, the device structure, the electrical diagram, and strategies for enhancing device performance of three kinds of infrared photodetector are concluded.

(Keynote) New Materials and Spectroscopies for Colloidal Quantum Dot Solar Cells

Colloidal quantum dots (CQDs) are an attractive third-generation material for photovoltaics due to their solution-processability, lightweight and flexible nature, and bandgap tunability, allowing them to be used as infrared materials for multi-junction solar cells. Here, we describe several methods for building new lead sulfide-based CQD materials and thin films for improving efficiencies in both single-junction and multi-junction solar cells.First, we demonstrate that the power conversion efficiency in single-junction PbS CQD solar cells is limited in part by the performance of the hole transport layer (HTL), traditionally made from ethanedithiol-passivated lead sulfide CQDs, due to the sub-optimal carrier mobility and doping density in this material. We use sulfur doping of the HTL, as well as incorporation of 2D transition metal dichalcogenide nanoflakes to address these issues and demonstrate absolute power conversion efficiency improvements of greater than 1% in single-junction devices.Next, we demonstrate a micrometer-resolution 2D characterization method with millimeter-scale field of view for assessing CQD solar cell film quality and uniformity. Our instrument simultaneously collects photoluminescence spectra, photocurrent transients, and photovoltage transients. We use this high-resolution morphology mapping to quantify the distribution and strength of the local optoelectronic property variations in CQD solar cells due to film defects, physical damage, and contaminants across nearly the entire test device area, and the extent to which these variations account for overall performance losses. We also use the massive data sets produced by this method to train machine learning models that take as input simple illuminated current-voltage measurements and output complex underlying materials parameters, greatly simplifying the characterization process for optoelectronic devices.Finally, we use artificial photonic band engineering as a method for achieving spectral selectivity in absorbing PbS CQD thin films for applications in multi-junction photovoltaics. We show that a structured periodic CQD thin film is able to maintain a photonic band structure, including the existence of a reduced photonic density of states, in the presence of weak material absorption, enabling modification of the absorption, transmission, and reflection spectra. We use a machine learning-based inverse design process to generate CQD thin film photonic structures with targeted absorption, transmission, and reflection spectra for multi-junction photovoltaics and narrow bandwidth photodetectors.

Temperature-dependent transition of charge transport in core/shell structured colloidal quantum dot thin films: From Poole–Frenkel emission to variable-range hopping

Solution-processed core/shell quantum dot films are of great significance for light-emitting diodes. It is well known that the operation of core/shell quantum dot-based light-emitting diodes largely relies on charge transport. However, the charge transport mechanism in quantum dot films is still under debate and inconclusive. Herein, the temperature-dependent charge transport properties of light-emitting core/shell colloidal quantum dot thin films are characterized and analyzed across a wide temperature range, and the charge transport mechanism is studied. The results reveal that Poole–Frenkel emission conduction is applicable in the high-temperature range. With the decrease in the temperature, the measured current can be described by the Efros–Shklovskii variable-range hopping model. It is worth noting that, in both cases, trap states and disorders in the quantum dot film play a very important role in charge transport. These findings are of great importance for optimizing quantum dot light-emitting diodes and understanding the effects of charge transport on the device performance.

Mid-IR Intraband Photodetectors with Colloidal Quantum Dots

In this paper, we investigate an intraband mid-infrared photodetector based on HgSe colloidal quantum dots (CQDs). We study the size, absorption spectra, and carrier mobility of HgSe CQDs films. By regulating the time and temperature of the reaction during synthesis, we have achieved the regulation of CQDs size, and the number of electrons doped in conduction band. It is experimentally verified by the field effect transistor measurement that dark current is effectively reduced by a factor of 10 when the 1Se state is doped with two electrons compared with other doping densities. The HgSe CQDs film mobility is also measured as a function of temperature the HgSe CQDs thin film detector, which could be well fitted by Marcus Theory with a maximum of 0.046 ± 0.002 cm2/Vs at room temperature. Finally, we experimentally discuss the device performance such as photocurrent and responsivity. The responsivity reaches a maximum of 0.135 ± 0.012 A/W at liquid nitrogen temperature with a narrow band photocurrent spectrum.

Polarization-based colour tuning of mixed colloidal quantum-dot thin films using direct patterning

Colloidal quantum-dots (cQDs) are finding increasingly widespread application in photonics and optoelectronics, providing high brightness and record-wide colour gamuts. However, the external quantum efficiencies in thin-film device architectures are still limited due to losses into waveguide modes and different strategies are being explored to promote the outcoupling of emission. Here we use a template-stripping-based direct-patterning strategy to fabricate linear gratings at the surface of cQD thin films. The linear gratings enhance optical outcoupling through Bragg scattering, yielding bright emission with a strong degree of linear polarization. By patterning linear gratings with different periodicities and orientations onto a film of mixed-colour cQDs, we demonstrate polarization-based active colour tuning of the thin-film emission.

Advanced characterization methods of carrier transport in quantum dot photovoltaic solar cells

Solution-processed colloidal quantum dot (CQD) solar cells are lightweight, flexible, inexpensive, and can be spray-coated on various substrates. However, their power conversion efficiency is still insufficient for commercial applications. To further boost CQD solar cell efficiency, researchers need to better understand and control how charge carriers and excitons transport in CQD thin films, i.e., the CQD solar cell electrical parameters including carrier lifetime, diffusion length, diffusivity, mobility, drift length, trap state density, and doping density. These parameters play key roles in determining CQD thin film thickness and surface passivation ligands in CQD solar cell fabrication processes. To characterize these CQD solar cell parameters, researchers have mostly used transient techniques, such as short-circuit current/open-circuit voltage decay, photoconductance decay, and time-resolved photoluminescence. These transient techniques based on the time-dependent excess carrier density decay generally exhibit an exponential profile, but they differ in the signal collection physics and can only be used in some particular scenarios. Furthermore, photovoltaic characterization techniques are moving from contact to non-contact, from steady-state to dynamic, and from small-spot testing to large-area imaging; what are the challenges, limitations, and prospects? To answer these questions, this Tutorial, in the context of CQD thin film and solar cell characterization, looks at trends in characterization technique development by comparing various conventional techniques in meeting research and/or industrial demands. For a good physical understanding of material properties, the basic physics of CQD materials and devices are reviewed first, followed by a detailed discussion of various characterization techniques and their suitability for CQD photovoltaic devices.

Single-Exciton Gain and Stimulated Emission Across the Infrared Telecom Band from Robust Heavily Doped PbS Colloidal Quantum Dots

Materials with optical gain in the infrared are of paramount importance for optical communications, medical diagnostics and silicon photonics. The current technology is based either on costly III-V semiconductors that are not monolithic to silicon CMOS technology or Er-doped fiber technology that does not make use of the full fiber transparency window. Colloidal quantum dots (CQD) offer a unique opportunity as an optical gain medium in view of their tunable bandgap, solution processability and CMOS compatibility. The 8-fold degeneracy of infrared CQDs based on Pb-chalcogenides has hindered the demonstration of low-threshold optical gain and lasing, at room temperature. We demonstrate room-temperature, infrared, size-tunable, band-edge stimulated emission with linewidth of ~14 meV. Leveraging robust electronic doping and charge-exciton interactions in PbS CQD thin films, we reach gain threshold at the single exciton regime representing a four-fold reduction from the theoretical limit of an eight-fold degenerate system, with a net modal gain in excess of 100 cm-1.

Local Defects in Colloidal Quantum Dot Thin Films Measured via Spatially Resolved Multi-Modal Optoelectronic Spectroscopy.

The morphology, chemical composition, and electronic uniformity of thin-film solution-processed optoelectronics are believed to greatly affect device performance. Although scanning probe microscopies can address variations on the micrometer scale, the field of view is still limited to well under the typical device area, as well as the size of extrinsic defects introduced during fabrication. Herein, a micrometer-resolution 2D characterization method with millimeter-scale field of view is demonstrated, which simultaneously collects photoluminescence spectra, photocurrent transients, and photovoltage transients. This high-resolution morphology mapping is used to quantify the distribution and strength of the local optoelectronic property variations in colloidal quantum dot solar cells due to film defects, physical damage, and contaminants across nearly the entire test device area, and the extent to which these variations account for overall performance losses. It is found that macroscopic defects have effects that are confined to their localized areas, rarely prove fatal for device performance, and are largely not responsible for device shunting. Moreover, quantitative analysis based on statistical partitioning methods of such data is used to show how defect identification can be automated while identifying variations in underlying properties such as mobilities and recombination strengths and the mechanisms by which they govern device behavior.

Temperature- and Size-Dependent Exciton Dynamics in PbS Colloidal Quantum Dot Thin Films Using Combined Photoluminescence Spectroscopy and Photocarrier Radiometry

Lead salt colloidal quantum dots (CQDs) are promising active components for applications in electronic and optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells. Detailed understanding of exciton dynamics in these nanocrystalline films is crucial for their practical applications. Photocarrier radiometry (PCR) is a dynamic spectrally integrated frequency-domain photoluminescence (PL) method, the spectral bandwidth of which is gated to eliminate thermal infrared photons due to nonradiative recombination. On the other hand, PL spectroscopy is a steady-state method that offers complementary spectrally resolved information. Combined PL temperature spectra and PCR temperature and frequency measurements were applied to PbS CQD thin films capped with oleic acid. Enhanced PL intensities at high temperatures originating in thermally activated exciton transfer from trap to exciton states were found in two samples consisting of two different sizes of quantum dots. A theoretical exciton...

Field-effect enhanced triboelectric colloidal quantum dot flexible sensor

Flexible electronics, which is of great importance as fundamental sensor and communication technologies for many internet-of-things applications, has established a huge market encroaching into the trillion-dollar market of solid state electronics. For the capability of being processed by printing or spraying, colloidal quantum dots (CQDs) play an increasingly important role in flexible electronics. Although the electrical properties of CQD thin-films are expected to be stable on flexible substrates, their electrical performance could be tuned for applications in flexible touch sensors. Here, we report CQD touch sensors employing polydimethylsiloxane (PDMS) triboelectric films. The electrical response of touching activity is enhanced by incorporating CQD field-effect transistors into the device architecture. Thanks to the use of the CQD thin film as a current amplifier, the field-effect CQD touch sensor shows a fast response to various touching materials, even being bent to a large curvature. It also shows a much higher output current density compared to a PDMS triboelectric touch sensor.

Solvent Engineering for High-Performance PbS Quantum Dots Solar Cells

PbS colloidal quantum dots (CQDs) solar cells have already demonstrated very impressive advances in recent years due to the development of many different techniques to tailor the interface morphology and compactness in PbS CQDs thin film. Here, n-hexane, n-octane, n-heptane, isooctane and toluene or their hybrids are for the first time introduced as solvent for comparison of the dispersion of PbS CQDs. PbS CQDs solar cells with the configuration of PbS/TiO2 heterojunction are then fabricated by using different CQDs solution under ambient conditions. The performances of the PbS CQDs solar cells are found to be tuned by changing solvent and its content in the PbS CQDs solution. The best device could show a power conversion efficiency (PCE) of 7.64% under AM 1.5 G illumination at 100 mW cm−2 in a n-octane/isooctane (95%/5% v/v) hybrid solvent scheme, which shows a ~15% improvement compared to the control devices. These results offer important insight into the solvent engineering of high-performance PbS CQDs solar cells.

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.

Direct Patterning of Colloidal Quantum-Dot Thin Films for Enhanced and Spectrally Selective Out-Coupling of Emission.

We report on a template-stripping method for the direct surface patterning of colloidal quantum-dot thin films to produce highly luminescent structures with feature sizes less than 100 nm. Through the careful design of high quality bull’s-eye gratings we can produce strong directional beaming (10° divergence) with up to 6-fold out-coupling enhancement of spontaneous emission in the surface-normal direction. A transition to narrow single-mode lasing is observed in these same structures at thresholds as low as 120 μJ/cm2. In addition, we demonstrate that these structures can be fabricated on flexible substrates. Finally, making use of the size-tunable character of colloidal quantum dots, we demonstrate spectrally selective out-coupling of light from mixed quantum-dot films. Our results provide a straightforward route toward significantly improved optical properties of colloidal quantum-dot assemblies.

Hybrid organic-inorganic inks flatten the energy landscape in colloidal quantum dotsolids.

Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (Voc) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher Voc and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.

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.

Quantitative Analysis of Trap-State-Mediated Exciton Transport in Perovskite-Shelled PbS Quantum Dot Thin Films Using Photocarrier Diffusion-Wave Nondestructive Evaluation and Imaging

Perovskite-shelled colloidal quantum dots (CQDs) with low trap-state density are promising candidates for large-scale, low-cost, and lightweight solar cell applications. However, even minimal trap states can significantly limit CQD-based solar cell efficiency. We reported trap-state-mediated exciton transports in methylammonium lead triiodide (MAPbI3) perovskite-passivated PbS CQD thin films. Excitation power-dependent photocarrier radiometry (PCR) intensity study demonstrated the free (electrons/holes)-to-bound (acceptors/donors) and trap-state-related transition-induced nonlinear response of CQD thin films to excitation. The existence of shallow trap states (activation energy: 33.8–40.7 meV) was characterized using photothermal emission spectra at different modulation frequencies. CQD thin films were imaged, for the first time, by InGaAs-camera-based nondestructive homodyne and heterodyne lock-in carrierographies (LIC; spectrally gated dynamic photoluminescence imaging), clearly showcasing photocarrier ...