Plasmonic light trapping leads to responsivity increase in colloidal quantum dot photodetectors

We report broadband responsivity enhancement in PbS colloidal quantum dot (CQDs) photoconductive photodetectors due to absorption increase offered by a plasmonic scattering layer of Ag metal nanoparticles. Responsivity enhancements are observed in the near infrared with a maximum 2.4-fold increase near the absorption band edge of 1 lm for 400 nm thick devices. Additionally, we study the effect of the mode structure on the efficiency of light trapping provided by random nanoparticle scattering in CQD films and provide insights for plasmonic scattering enhancement in CQD thin films.This research has been partially supported by Fundacio´ Privada Cellex Barcelona. We also acknowledge support from European Commission’s Seventh Framework Programme for Research under contract PIRG06-GA-2009-256355

Quantum-Tuned Cascade Multijunction Infrared Photodetector

Emerging applications such as augmented reality, self-driving vehicles, and quantum information technology require optoelectronic devices capable of sensing a low number of photons with high sensitivity (including gain) and high speed and that could operate in the infrared at telecom windows beyond silicon’s bandgap. State-of-the-art semiconductors achieve some of these functions through costly and not easily scalable doping and epitaxial growing methods. Colloidal quantum dots (QDs), on the other hand, could be easily tuned and are compatible with consumer electronics manufacturing. However, the development of a QD infrared photodetector with high gain and high response speed remains a challenge. Herein, we present a QD monolithic multijunction cascade photodetector that advances in the speed-sensitivity-gain space through precise control over doping and bandgap. We achieved this by implementing a QD stack in which each layer is tailored via bandgap tuning and electrostatic surface manipulation. The resulting junctions sustain enhanced local electric fields, which, upon illumination, facilitate charge tunneling, recirculation, and gain, but retain low dark currents in the absence of light. Using this platform, we demonstrate an infrared photodetector sensitive up to 1500 nm, with a specific detectivity of ∼3.7 × 1012 Jones, a 3 dB bandwidth of 300 kHz (0.05 cm2 device), and a gain of ∼70× at 1300 nm, leading to an overall gain-bandwidth product over 20 MHz, in comparison with 3 kHz of standard photodiode devices of similar areas

Optical Resonance Engineering for Infrared Colloidal Quantum Dot Photovoltaics

We report optically enhanced infrared-harvesting colloidal quantum dot solar cells based on integrated Fabry–Perot cavities. By integrating the active layer of the photovoltaic device between two reflective interfaces, we tune its sensitivity in the spectral region at 1100–1350 nm. The top and bottom electrodes also serve as mirrors, converting the device into an optical resonator. The front conductive mirror consists of a dielectric stack of SiN_<i>x</i> and SiO₂ with a terminal layer of ITO and ZnO in which current can flow, while the back mirror consists of a highly reflective gold layer. Adjusting the reflectivity and central wavelength of the front mirror as well as the thickness of the active layer allowed increases in absorption by a total of 56% in the infrared, leading to a record external quantum efficiency of 60% at 1300 nm. This work opens new avenues toward low-cost, high-efficiency rear-junction photovoltaic harvesters that add to the overall performance of silicon solar cells

Near-Unity Broadband Quantum Efficiency Enabled by Colloidal Quantum Dot/Mixed-Organic Heterojunction

Solution-processed semiconducting materials are promising for realizing high-performance, low-cost, and flexible energy conversion devices. In particular, hybrid structures comprising colloidal quantum dots (CQDs) and organic molecules have been proposed to achieve broadband absorption across the visible-to-infrared solar spectrum. However, the photophysical mismatch present at CQD/organic interfaces limits charge extraction, resulting in low power conversion efficiency (PCE). In this study, we sought to overcome this photophysical mismatch, addressing the CQD/organic interface using a library of surface ligands with different functions. We established, using both experiments and theoretical calculations, that thiol termination of the CQD surface reduced the interfacial barrier, resulting in a 4-fold higher charge transfer efficiency at the maximum power point bias. The CQD/mixed-organic heterojunction solar cells exhibit a record photocurrent density of 33.3 mA/cm2 and near-unity broadband quantum efficiency up to 1100 nm, demonstrating the potential of these devices to harvest infrared solar photons in all-solution-processed tandem devices

Halide Re-Shelled Quantum Dot Inks for Infrared Photovoltaics

Colloidal quantum dots are promising materials for tandem solar cells that complement silicon and perovskites. These devices are fabricated from solution phase; however, existing methods for making infrared-bandgap CQD inks suffer agglomeration and fusion during solution exchange. Here we develop a ligand exchange that provides robust surface protection and thereby avoids aggregation. First, we exchanged long oleic acid ligands to a mixed system comprising medium-chain ammonium and anionic chloride ligands; we then reshelled the surface using short halides and pseudohalide ligands that enabled transfer to a polar solvent. Absorbance and photoluminescence measurements reveal the retention of exciton sharpness, whereas X-ray photoelectron spectroscopy indicates halide capping. The best power conversion efficiency of these devices is 0.76 power points after filtering through silicon, which is 1.9× higher than previous single-step solution-processed IR-CQD solar cells

Colloidal Quantum Dot Photovoltaics Enhanced by Perovskite Shelling

Solution-processed quantum dots are a promising material for large-scale, low-cost solar cell applications. New device architectures and improved passivation have been instrumental in increasing the performance of quantum dot photovoltaic devices. Here we report photovoltaic devices based on inks of quantum dot on which we grow thin perovskite shells in solid-state films. Passivation using the perovskite was achieved using a facile solution ligand exchange followed by postannealing. The resulting hybrid nanostructure created a more intrinsic CQD film, which, when incorporated into a photovoltaic device with graded bandstructure, achieved a record solar cell performance for single-step-deposited CQD films, exhibiting an AM1.5 solar power conversion efficiency of 8.95%

Nanoimprint-Transfer-Patterned Solids Enhance Light Absorption in Colloidal Quantum Dot Solar Cells

Colloidal quantum dot (CQD) materials are of interest in thin-film solar cells due to their size-tunable bandgap and low-cost solution-processing. However, CQD solar cells suffer from inefficient charge extraction over the film thicknesses required for complete absorption of solar light. Here we show a new strategy to enhance light absorption in CQD solar cells by nanostructuring the CQD film itself at the back interface. We use two-dimensional finite-difference time-domain (FDTD) simulations to study quantitatively the light absorption enhancement in nanostructured back interfaces in CQD solar cells. We implement this experimentally by demonstrating a nanoimprint-transfer-patterning (NTP) process for the fabrication of nanostructured CQD solids with highly ordered patterns. We show that this approach enables a boost in the power conversion efficiency in CQD solar cells primarily due to an increase in short-circuit current density as a result of enhanced absorption through light-trapping

10.6% Certified Colloidal Quantum Dot Solar Cells via Solvent-Polarity-Engineered Halide Passivation

Colloidal quantum dot (CQD) solar cells are solution-processed photovoltaics with broad spectral absorption tunability. Major advances in their efficiency have been made via improved CQD surface passivation and device architectures with enhanced charge carrier collection. Herein, we demonstrate a new strategy to improve further the passivation of CQDs starting from the solution phase. A cosolvent system is employed to tune the solvent polarity in order to achieve the solvation of methylammonium iodide (MAI) and the dispersion of hydrophobic PbS CQDs simultaneously in a homogeneous phase, otherwise not achieved in a single solvent. This process enables MAI to access the CQDs to confer improved passivation. This, in turn, allows for efficient charge extraction from a thicker photoactive layer device, leading to a certified solar cell power conversion efficiency of 10.6%, a new certified record in CQD photovoltaics

Molecular Doping of the Hole-Transporting Layer for Efficient, Single-Step-Deposited Colloidal Quantum Dot Photovoltaics

Employment of thin perovskite shells and metal halides as surface-passivants for colloidal quantum dots (CQDs) has been an important, recent development in CQD optoelectronics. These have opened the route to single-step-deposited high-performing CQD solar cells. These promising architectures employ a CQD hole-transporting layer (HTL) whose intrinsically shallow Fermi level (<i>E</i>_F) restricts band-bending at maximum power-point during solar cell operation limiting charge collection. Here, we demonstrate a generalized approach to effectively balance band-edge energy levels of the main CQD absorber and charge-transport layer for these high-performance solar cells. Briefly soaking the CQD HTL in a solution of the metal–organic p-dopant, molybdenum tris­(1-(trifluoroacetyl)-2-(trifluoromethyl)­ethane-1,2-dithiolene), effectively deepens its Fermi level, resulting in enhanced band bending at the HTL:absorber junction. This blocks the back-flow of photogenerated electrons, leading to enhanced photocurrent and fill factor compared to those of undoped devices. We demonstrate 9.0% perovskite-shelled and 9.5% metal-halide-passivated CQD solar cells, both achieving ca. 10% relative enhancements over undoped baselines

Efficient Photon Recycling and Radiation Trapping in Cesium Lead Halide Perovskite Waveguides

Cesium lead halide perovskite materials have attracted considerable attention for potential applications in lasers, light-emitting diodes, and photodetectors. Here, we provide the experimental and theoretical evidence for photon recycling in CsPbBr₃ perovskite microwires. Using two-photon excitation, we recorded photoluminescence (PL) lifetimes and emission spectra as a function of the lateral distance between PL excitation and collection positions along the microwire, with separations exceeding 100 μm. At longer separations, the PL spectrum develops a red-shifted emission peak accompanied by an appearance of well-resolved rise times in the PL kinetics. We developed quantitative modeling that accounts for bimolecular recombination and photon recycling within the microwire waveguide and is sufficient to account for the observed decay modifications. It relies on a high radiative efficiency in CsPbBr₃ perovskite microwires and provides crucial information about the potential impact of photon recycling and waveguide trapping on optoelectronic properties of cesium lead halide perovskite materials