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This article discusses the current status and future prospects of multi-bandgap colloidal quantum dot-based optoelectronic devices.

Two major challenges exist before colloidal nanocrystal solar cells can take their place in the market: So far, these devices are based on Pb/Cd-containing nanocrystals, and second, the synthesis of these nanocrystals takes place in an inert atmosphere at elevated temperatures due to the use of air-sensitive chemicals. In this report, a room-temperature, ambient-air synthesis for nontoxic AgBiS2 nanocrystals is presented. As this method utilizes stable precursors, the need for the use of a protective environment is eliminated, enabling the large-scale production of AgBiS2 nanocrystals. The production cost of AgBiS2 NCs at room temperature and under ambient conditions reduces by ∼60% compared to prior reports based on hot injection, and the solar cells made of these nanocrystals yield a promising power conversion efficiency (PCE) of 5.5%, the highest reported to date for a colloidal nanocrystal material free of Pb or Cd synthesized at room temperature and under ambient conditions.

Solid-state broadband light emitters in the visible have revolutionized today's lighting technology achieving compact footprints, flexible form factors, long lifetimes, and high energy saving, although their counterparts in the infrared are still in the development phase. To date, broadband emitters in the infrared have relied on phosphor-downconverted light emitters based on atomic optical transitions in transition metal or rare earth elements in the phosphor layer resulting in limited spectral bandwidths in the near-infrared and preventing their integration into electrically driven light-emitting diodes (LEDs). Herein, phosphor-converted LEDs based on engineered stacks of multi-bandgap colloidal quantum dots (CQDs) are reported as a novel class of broadband emitters covering a broad short-wave infrared (SWIR) spectrum from 1050–1650 nm with a full-width-half-maximum of 400 nm, delivering 14 mW of optical power with a quantum efficiency of 5.4% and power conversion efficiency of 13%. Leveraging the electrical conductivity of the CQD stacks, further, the first broadband SWIR-active LED is demonstrated, paving the way toward complementary metal–oxide–semiconductor integrated broadband emitters for on-chip spectrometers and low-cost volume manufacturing. SWIR spectroscopy is employed to illustrate the practical relevance of the emitters in food and material identification case studies.

Pb-chalcogenide colloidal quantum dots (CQDs) are attractive materials to be used as tuneable laser media across the infrared spectrum. However, excessive nonradiative Auger recombination due to the presence of trap states outcompetes light amplification by rapidly annihilating the exciton population, leading to high gain thresholds. Here, a binary blend is employed of CQDs and ZnO nanocrystals in order to passivate the in-gap trap states of PbS-CQD gain medium. Using transient absorption, a fivefold increase is measured in Auger lifetime demonstrating the suppression of trap-assisted Auger recombination. By doing so, a twofold reduction is achieved in amplified spontaneous emission (ASE) threshold. Finally, by integrating the proposed binary blend to a distributed feedback (DFB) resonator, single-mode lasing emission is demonstrated at 1650 nm with a linewidth of 1.23 nm (0.62 meV), operating at a low lasing threshold of ≈385 μJ cm-2 . The Auger suppression in this system has allowed to achieve unprecedented lasing emission stability for a CQD laser with recorded continuous operation of 5 h at room temperature and ambient conditions.

The turn-on voltage of a light-emitting diode (LED) is an important parameter as it determines the power consumption of the LED and influences the effective power conversion efficiency. LEDs based on nanoscale engineering of the blended PbS [mixture of quantum dots (QDs) with two different bandgaps] colloidal QDs have recently shown record performance in the infrared region. One of the most intriguing findings for these blended devices is the substantially lower-than-bandgap turn-on voltage and the achievement of an open circuit voltage ( V

Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution-processed semiconductors because they possess high photoluminescence quantum yield (PLQY) and color tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed-halide (CsPbBrI2) NCs obtained via anion exchange reactions under ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I–, thus inducing a PL red shift up to 676 nm and obtaining a high PLQY in film (65%). Solar cell devices operating in the wavelength range 350–660 nm were fabricated in air with two different deposition methods. The solar cells display a photoconversion efficiency of 5.3% and open circuit voltage (Voc) up to 1.31 V, among the highest reported for perovskite-based solar cells with band gap below 2 eV, clearly demonstrating the potential of this material.

Colloidal quantum dot (CQD) light-emitting diodes (LEDs) deliver a compelling performance in the visible, yet infrared CQD LEDs underperform their visible-emitting counterparts, largely due to their low photoluminescence quantum efficiency. Here we employ a ternary blend of CQD thin film that comprises a binary host matrix that serves to electronically passivate as well as to cater for an efficient and balanced carrier supply to the emitting quantum dot species. In doing so, we report infrared PbS CQD LEDs with an external quantum efficiency of ~7.9% and a power conversion efficiency of ~9.3%, thanks to their very low density of trap states, on the order of 1014 cm−3, and very high photoluminescence quantum efficiency in electrically conductive quantum dot solids of more than 60%. When these blend devices operate as solar cells they deliver an open circuit voltage that approaches their radiative limit thanks to the synergistic effect of the reduced trap-state density and the density of state modification in the nanocomposite. PbS quantum dot ternary blends enable the realization of high-efficiency colloidal quantum dot infrared light-emitting diodes

Colloidal copper indium sulfide (CIS) nanocrystals (NCs) are Pb- and Cd-free alternatives for use as absorbers in quantum dot solar cells. In a heterojunction with TiO2, non-annealed ligand-exchanged CIS NCs form solar cells yielding a meager power conversion efficiency (PCE) of 0.15%, with photocurrents plummeting far below predicted values from absorption. Decreasing the amount of zinc during post-treatment leads to improved mobility but marginal improvement in device performance (PCE = 0.30%). By incorporating CIS into a porous TiO2 network, we saw an overall drastic improvement in device performance, reaching a PCE of 1.16%, mainly from an increase in short circuit current density (Jsc) and fill factor (FF) and a 10-fold increase in internal quantum efficiency (IQE). We have determined that by moving from a bilayer to a bulk heterojunction architecture, we have reduced the trap-assisted recombination as seen in changes in the ideality factor, the intensity dependence of the photocurrent and transient photocurrent (TPC) and photovoltage (TPV) characteristics.

Harnessing low energy photons is of paramount importance for multi-junction high efficiency solar cells as well as for thermo-photovoltaic applications. However, semiconductor absorbers with the bandgap lower than 0.8 eV have been limited to III-V (InGaAs) or IV (Ge) semiconductors that are characterized by high manufacturing costs and complicated lattice matching requirements in their growth and integration with higher bandgap cells. Here, we have developed solution processed low bandgap photovoltaic devices based on PbS colloidal quantum dots (CQDs) with a bandgap of 0.7 eV suited for both thermo-photovoltaics and low energy solar photon harvesting. By matching the spectral response of those cells to that of the infrared solar spectrum, we report a record high short circuit current (JSC) of 37 mA cm-2 under the full solar spectrum and 5.5 mA cm-2 when placed at the back of a silicon wafer resulting in power conversion efficiencies (PCEs) of 6.4% and 0.7%, respectively. Moreover, the device reached an above bandgap PCE of ∼6% as a thermo-photovoltaic cell recorded under a 1000 °C blackbody radiator.

Developing low-cost photovoltaic absorbers that can harvest the short-wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si-based and perovskite photovoltaic technologies, is a prerequisite for making high-efficiency, low-cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic-organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short-circuit current density of 34 mA cm-2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI3 perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.

Two-terminal tandem cell architectures are believed to be an effective way to further improve the power conversion efficiency in solution processed photovoltaics. To design an efficient tandem solar cell, two key issues need to be considered. First, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Second, identifying the appropriate intermediate layer (IML) to connect the subcells is necessary to minimize the optical and electronic losses. PbS colloidal quantum dots (CQDs) are a notable choice for the subcells due to their low cost, solution processability, and remarkable wide range band gap tunability. Single-layer graphene (Gr) has been proposed to be a promising IML due to its high transparency and conductivity. Here, as a proof of concept, we demonstrate a solution-processed, two-terminal PbS CQDs tandem solar cell employing chemical vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising subcells with band...

The effect of MoO3 anode buffer layer on power conversion efficiency and stability of small molecular photovoltaic devices based on CuPc/PTCDI-C8 heterojunction was investigated. Substantial improvement in device efficiency and stability was achieved with the incorporation of a thin MoO3 anode buffer layer. The improvement in efficiency is attributed to the decrease in contact resistance due to the reduction of barrier height at the CuPc/ITO interface. For the optimized MoO3 thickness the device exhibited a fill factor of 50.32% and efficiency of 1.6%. Stability enhancement is due to the prevention of oxygen and humidity infiltration by MoO3 anode buffer layer.

The power conversion efficiency of colloidal PbS-quantum-dot (QD)-based solar cells is significantly hampered by lower-than-expected open circuit voltage (VOC). The VOC deficit is considerably higher in QD-based solar cells compared to other types of existing solar cells due to in-gap trap-induced bulk recombination of photogenerated carriers. Here, this study reports a ligand exchange procedure based on a mixture of zinc iodide and 3-mercaptopropyonic acid to reduce the VOC deficit without compromising the high current density. This layer-by-layer solid state ligand exchange treatment enhances the photovoltaic performance from 6.62 to 9.92% with a significant improvement in VOC from 0.58 to 0.66 V. This study further employs optoelectronic characterization, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy to understand the origin of VOC improvement. The mixed-ligand treatment reduces the sub-bandgap traps and significantly reduces bulk recombination in the devices.

In this article on sequential adaptive testing, we have studied the optimal allocation between two populations for testing a composite hypothesis involving the parameters, with the goal of decreasing allocation of one of the treatments to the order of the logarithm of the sample size while decreasing the probability of incorrect selection to zero. We have proved the result for large sample sizes both mathematically and by simulation studies.

In our present work we have incorporated PTCDI-C8 nanoribbons into the bulk donor polymer in nanostructured zinc oxide based hybrid inverted solar cell which resulted in a 100% increment in the device efficiency.

We report a systematic study on the dielectric properties of epoxy based positive tone photoresist and its use as a gate dielectric for n-channel organic field effect transistors (OFETs) made with N,N'-Dioctyl-3,4,9,10-perylenedicarboximinde (PTCDI-C8) as the active semiconducting layer. We find that the photoresist has high dielectric constant (k = 12 at 10 kHz) and thus can be used in fabricating low operating voltage OFET devices. Highly smooth gate dielectric surface was obtained using the photoresist with the highest root mean square (rms) roughness of 0.239 nm for the films annealed at 200 °C. Consequently, the semiconducting layer (on photoresist dielectric annealed at 100 °C) also exhibited highly uniform surface with rms roughness of 0.382 nm. The turn-on voltage (VT), inverse subthreshold slope (S) and saturation mobility of electrons (μsat) of the transistor device were estimated to be 4.3 V, 13 V decade−1 and 6 × 10−5 cm2 Vs−1, respectively, when the device was operated in ambient, which is better than some of the earlier reported works under similar experimental conditions.

We report here the formation of vertical N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanostructure with simple chemical treatment over thermally evaporated thin film of PTCDI-C8. Here, we have used PTCDI-C8 nanostructures as an electron accepting material and a conjugated polymer as an electron donor material in the solar cell device. The study shows that the utilization of nanostructure over bilayer architecture gives better performance. The higher donor/acceptor effective interface and its effect on the device performance are realized with the nanostructured PTCDI-C8 layer.

The performance of ZnO/PbS colloidal quantum dot (CQD)-based heterojunction solar cells is hindered by charge carrier recombination at the heterojunction interface. Reducing interfacial recombination can improve charge collection and the photocurrent of the device. Here we report the use of a mixed nanocrystal (MNC) buffer layer comprising zinc oxide nanocrystals and lead sulfide quantum dots at the respective heterojunction interface. Remote trap passivation of the PbS CQDs taking place within this MNC layer reduces interfacial recombination and electron back transfer which in turn improves charge collection efficiency. Upon the addition of the MNC layer, the overall power conversion efficiency increases from 9.11 to 10.16% and Short-circuit current density (JSC) increases from 23.54 to 25.23 mA/cm2. Optoelectronic characterization of the solar cells confirms that the effects underlying device improvement are reduced trap density and improved charge collection efficiency due to the presence of the MNC buffer layer.

To realize the full potential of colloidal quantum dot (CQD) based solar cells, it is important to address the issue of large open-circuit voltage (VOC) deficit which is a major roadblock in reaching higher efficiencies. The origin of the VOC deficit in these solar cells lies primarily in the presence of sub-bandgap trap states of the QDs. Here, we present a synergistic engineering framework to passivate these sub-bandgap states in PbS QDs through chemical surface passivation and remote passivation exploiting ligand and architecture engineering. In particular, we form bulk nanoheterojunctions (BNH) by mixing PbS QDs with ZnO nanocrystals in conjunction with mixed ligand treatments to passivate surface traps. We employ the mixed ligand system of zinc iodide and 3-mercatopropyonic acid to leverage the benefits of both organic and inorganic ligands for surface passivation and improved charge transport. This mixed ligand treatment in BNH architectures leads to record low VOC deficit for PbS QDs of 0.4–0.55 V compared to previously reported 0.6–0.8 V for the range of 1.1–1.35 eV bandgap PbS QDs.

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 (CQDs) 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 a line width of ~14 meV. Leveraging robust electronic doping and charge-exciton interactions in PbS CQD thin films, we reach a gain threshold at the single exciton regime representing a 4-fold reduction from the theoretical limit of an 8-fold degenerate system, with a net modal gain in excess of 100 cm-1. The authors acknowledge financial support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 725165), the Spanish Ministry of Economy and Competitiveness (MINECO), and the ‘Fondo Europeo de Desarrollo Regional’ (FEDER) through grant TEC2017- 88655-R. The authors also acknowledge financial support from Fundacio Privada Cellex, the program CERCA, and from the Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0522). S.C. acknowledges support from a Marie Curie Standard European Fellowship (NAROBAND, H2020-MSCA-IF-2016-750600). I.R. acknowledges support from the Ministerio de Economiá , Industria y Competitividad of Spain via a Juan de la Cierva fellowship. We are also thankful to Dr. Kaifen Wu for fruitful discussions.

Surface passivation of PbS colloidal quantum dots (QDs) with iodide has been used in highly efficient solar cells. Iodide passivation is typically achieved by ligand exchange processes on QD films. Complementary to this approach, herein we present a non-intrusive solution-based strategy for doping QDs with iodide to further optimize solar cell performance. The doping step is applied in-situ at the end of the synthesis of the QDs. The optimum precursor I/Pb ratio is found to be in the 1.5-3% range at which iodide substitutes S without excessively altering the dots´ surface chemistry. This allows for band engineering and decreasing the density of deep trap states of the QDs which taken together lead to PbS QD solar cells with efficiency in excess of 10%.

Unbalanced charge injection is deleterious for the performance of colloidal quantum dot (CQD) light emitting diodes (LEDs) as it deteriorates the quantum efficiency (QE), brightness and operational lifetime. CQD LEDs emitting in the infrared have previously achieved high quantum efficiencies but only when driven to emit in the low radiance regime. At higher radiance levels, required for practical applications, the efficiency decreased dramatically in view of the notorious efficiency droop. Here we report a novel methodology to regulate charge supply in multinary bandgap CQD composites that facilitates improved charge balance. Our approach is based on engineering the energetic potential landscape at the supra-nanocrystalline level that has allowed us to report short-wave infrared (SWIR) PbS CQD LEDs with record-high external QE in excess of 8%, most importantly, at a radiance level of ~ 5 WSr-1m2, an order of magnitude higher than prior reports. Furthermore, the balanced charge injection and Auger recombination reduction has led to unprecedentedly high operational stability with radiance half-life of 26068 hours at a radiance of 1Wsr-1m-2.