unite solution processing with desirable optoelectronic properties such as tunable light emission and long carrier lifetimes and diffusion lengths. [9][10][11] The rapid development of mixed-halide CsPbBr x /I 3−x nanocrystals via compositional tuning has enabled an EQE of 20.3% in the red with a full-width at half maximum (FWHM) of 40 nm. [12] Unfortunately, they have yet to rise to match the operating stability of organic [13,14] and inorganic quantum dot [15,16] LEDs: device operating stability (T 50 ) to date, at an initial luminance of 140 cd m -2 , has been limited to hours. [12] The organic ligands used in synthesis, and the subsequent exchange, provide colloidal stability in solution. However, these surface ligands are labile, exhibit limited surface binding affinity, and provide high surface coverage only when present in excess in solution. [18,19] They are readily desorbed upon dilution and washing, inducing incomplete passivation of surface sites.Introducing inorganic ligands in order to better passivate surfaces has shown promising progress in single halide composition in our previous work in both blue and red LEDs. [10,27] However, these strategies rely on a highly polar solvent (DMF) to carry out the inorganic salts. The introduction of the highly polar solvent compromises perovskite nanocrystal structural stability and harms the operating stability of LEDs.Here, we report a strategy wherein we introduce inorganic ligands in the antisolvent used in nanocrystal purification. We show that working with a mildly polar antisolvent, such as ethyl acetate used in this work, allows a gentler processing of the vulnerable perovskite quantum dots. Only by introducing ultrasonication during the introduction of the antisolvent were we able to develop an exchange that was successful, and substantially complete. The inorganic ligands replace, in situ, the organic ligands that are detached from the dot surface (Figure 1a): the small inorganic cations provide a rich surface coverage superior to that offered by long-chain organic ligands, and thus prevent trap formation. The inorganic ligands fill surface defects and improve material conductivity and charge-carrier injection in LEDs. The strategy improves bandgap stability, resulting in MHP solids with a storage stability of 1 year in ambient conditions (25 °C and 40% humidity), in comparison to controls that show phase changes after 7 days under the same conditions.Instability in mixed-halide perovskites (MHPs) is a key issue limiting perovskite solar cells and light-emitting diodes (LEDs). One form of instability arises during the processing of MHP quantum dots using an antisolvent to precipitate and purify the dots forming surface traps that lead to decreased luminescence, compromised colloidal stability, and emission broadening. Here, the introduction of inorganic ligands in the antisolvents used in dot purification is reported in order to overcome this problem. MHPs that are colloidally stable for over 1 year at 25 °C and 40% humidity are demonstrated and films ...
We studied the elastic and piezoelectric properties of buckled honeycomb group III–V monolayers (GaP, GaAs, GaSb, InP, InAs and InSb) by DFT calculations. Those buckled monolayers are ferroelectric and have nonzero e11, e31, d11 and d31 piezoelectric coefficients. Our calculations show that those monolayers are good piezoelectric materials and a pronounced periodic trend of the piezoelectric coefficients e11, e31, d11 and d31 was found. Group III–V monolayers are promising candidates for future atomically thin piezoelectric applications such as transducers, sensors, and energy harvesting devices.
CQD solutions processability and tunability of physical properties, but also leads to an increased surface area/volume ratio. [4] Consequently, CQDs suffer from high surface trap density, resulting in significantly reduced photovoltaic performance. [5,6] The surface passivation then becomes a critical step to reduce energy loss (e.g., open-circuit voltage [V oc ] loss) and realize high device performance. [7][8][9] Exploring efficient passivation strategies has always been the overarching theme throughout the history of CQD solar cells. [10] However, this issue has far from been solved. Up to now, the V oc loss in high-efficiency PbS CQD solar cells is mostly around 0.45 V, [11][12][13][14][15] which is significantly larger compared to the V oc loss of ≈0.35 V for silicon, gallium arsenide, and perovskite solar cells. [16][17][18] Among all the photovoltaic parameters, the low V oc is evidently the one limiting the device performance of current CQD solar cells. Therefore, it's most urgent to explore an innovative passivation strategy to further reduce V oc loss and realize the breakthrough of CQD photovoltaic performance.After synthesis, most CQD surface trap states are generated during the ligand-exchange process, in which the insulating oleate ligands are replaced with short ones to facilitateThe high open-circuit voltage (V oc ) loss arising from insufficient surface passivation is the main factor that limits the efficiency of current lead sulfide colloidal quantum dots (PbS CQDs) solar cell. Here, synergistic passivation is performed in the direct synthesis of conductive PbS CQD inks by introducing multifunctional ligands to well coordinate the complicated CQDs surface with the thermodynamically optimal configuration. The improved passivation effect is intactly delivered to the final photovoltaic device, leading to an order lower surface trap density and beneficial doping behavior compared to the control sample. The obtained CQD inks show the highest photoluminescence quantum yield (PLQY) of 24% for all photovoltaic PbS CQD inks, which is more than twice the reported average PLQY value of ≈10%. As a result, a high V oc of 0.71 V and power conversion efficiency (PCE) of 13.3% is achieved, which results in the lowest V oc loss (0.35 eV) for the reported PbS CQD solar cells with PCE >10%, comparable to that of perovskite solar cells. This work provides valuable insights into the future CQDs passivation strategies and also demonstrates the great potential for the direct-synthesis protocol of PbS CQDs.
The zinc oxide (ZnO) nanoparticles (NPs) are well‐documented as an excellent electron transport layer (ETL) in optoelectronic devices. However, the intrinsic surface flaw of the ZnO NPs can easily result in serious surface recombination of carriers. Exploring effective passivation methods of ZnO NPs is essential to maximize the device's performance. Herein, a hybrid strategy is explored for the first time to improve the quality of ZnO ETL by incorporating stable organic open‐shell donor‐acceptor type diradicaloids. The high electron‐donating feature of the diradical molecules can efficiently passivate the deep‐level trap states and improve the conductivity of ZnO NP film. The unique advantage of the radical strategy is that its passivation effectiveness is highly correlated with the electron‐donating ability of radical molecules, which can be precisely controlled by the rational design of molecular chemical structures. The well‐passivated ZnO ETL is applied in lead sulfide (PbS) colloidal quantum dot solar cells, delivering a power conversion efficiency of 13.54%. More importantly, as a proof‐of‐concept study, this work will inspire the exploration of general strategies using radical molecules to construct high‐efficiency solution‐processed optoelectronic devices.
PbS quantum dots (QDs) are promising building blocks for solution-processed short-wavelength infrared (SWIR) devices. The recently developed direct synthesis of semi-conductive PbS QD inks has substantially simplified the preparation processing and reduced the material cost, while facing the challenge to synthesize large-size QDs with absorption covering the SWIR region. Herein, we for the first time realize a low-cost, scalable synthesis of SWIR PbS QD inks after an extensive investigation of the reaction kinetics. Finally, based on these PbS SWIR QD inks, the solar cell demonstrates a record-high power conversion efficiency (PCE) of 1.44 % through an 1100 nm cutoff silicon filter and the photodetector device shows a low dark current density of 2 × 10 À 6 A cm À 2 at À 0.8 V reverse bias with a high external quantum efficiency (EQE) of 70 % at � 1300 nm. Our results realize the direct synthesis of low-cost and scalable SWIR QD inks and may accelerate the industrialization of consumer SWIR technologies.
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