Passivation Of Trap States Research Articles

Unique photoluminescence response of MoS2 quantum dots over a wide range of As (III) in aqueous media

We report the solvothermal synthesis of MoS2 based quantum dots (QDs) and the performance evaluation of bare QDs for the detection of aqueous As (III) oxidative state at room temperature and neutral pH over a vast range (0.1−1000 ppb). Concentration-dependent photoluminescence (PL) of the QDs enhances up to 50 ppb and then suppresses till 1000 ppb. It shows two distinctive slopes for enhancement and suppression. The enhancement is possibly due to the passivation of trap states or defects. The formation of tiny glassy As2S3 particles on the QD surface may be the possible reason for suppression. The pattern of optical absorption of QDs follows the similar patterns of PL. Still, it shows an enhanced absorbance in the near UV range below ≤300 nm, which increases with As (III) concentration up to 50 ppb and then decreases following the PL pattern. The MoS2 QDs were characterized by using transmission electron microscopy, x-ray diffraction, UV−Vis, and PL spectroscopy. The enhancement and suppression results were excellently fitted with the modified Stern−Volmer equation. The detection of arsenic is possible using these linear fit equations as calibration curves.

Steric effect of amino-acids as additives for perovskite solar cells

In perovskite solar cells, the molecule configuration of Lewis adducts closely related with its coordination ability, which needs careful design. In this work, two amino acids, circular proline and linear glycine are applied to reveal the steric effect of the Lewis-acid base additives. The proline with configuration and analogous size with Pb-Pb distance in perovskite crystals realizes better coordination effect, greatly promotes the perovskite crystal growth and trap states passivation. The PSCs with proline as additives in perovskite (PVK) exhibit the highest efficiency of 19.6% under 1 sun AM1.5, compared with 18.5% of PSC with glycine additive and 17.8% of the pristine PSCs. This work highlights steric effect should be considered when choosing and designing the Lewis adducts.

Trap State Passivation by Rational Ligand Molecule Engineering toward Efficient and Stable Perovskite Solar Cells Exceeding 23% Efficiency

AbstractThe nonradiative recombination losses resulting from the trap states at the surface and grain boundaries directly hinder the further enhancement of power conversion efficiency (PCE) and stability of perovskite solar cells. Consequently, it is highly desirable to suppress nonradiative recombination through modulating perovskite crystallization and passivating the defects of perovskite films. Here, a simple and effective multifunctional additive engineering strategy is reported where 11 Maleimidoundecanoic acid (11MA) units with carbonyls (carboxyl and amide) and long hydrophobic alkyl chain are incorporated into a perovskite precursor solution. It is revealed that improved crystallinity, reduced trap state density, and inhibited ion migration are achieved, which is ascribed to the strong coordination interaction between the carbonyl groups at both sides of 11MA molecules and Pb2+. As a result, improved efficiency and stability are achieved simultaneously after introducing 11MA additive. The device with 11MA additive delivers a champion PCE of 23.34% with negligible hysteresis, which is significantly higher than the 18.24% of the control device. The modified device maintains around 91% of its initial PCE after aging under ambient conditions for 3000 h. This work provides a guide for developing multifunctional additive molecules for the purpose of simultaneous improvement of efficiency and stability.

Enhanced performance through trap states passivation in quantum dot light emitting diode

Device performance enhancement in quantum dot light-emitting diodes (QLEDs) is realized by adding a small amount of insulating polymer polymethyl methacrylate (PMMA) into the emitting quantum dot layer. Pool-Frenkel effect is observed through temperature-dependent current density-voltage experiments, indicating the important role of trap states, and the addition of the insulating PMMA helps to reduce the Poole-Frenkel barrier hence the trap depth. Reduced density and depth of trap states with PMMA are indeed observed through further capacitance measurements. This work contributes to a better understanding on the effects of traps in QLEDs.

Fluorescence “turn-on” of silicon-containing nanoparticles for the determination of resorcinol

A fluorescent nanosensor based on silicon-containing nanoparticles (Si CNPs) with green fluorescence (FL) was prepared by one-step method. The prepared Si CNPs emitted green FL at 470nm under the excitation at 350nm. The FL signal of Si CNPs reveals an obvious enhancement in the presence of resorcinol (RC), due to the passivation of surface trap states of Si CNPs via the binding of OH group of RC with the NH group of Si CNPs, which allowed the formation of new radiative electron-hole recombination centers. This was confirmed by some analytical experiments performed on zeta potential, FL lifetime steady state, and the FTIR spectra. Most importantly, this nanosensor could selectively determine RC with high sensitivity and without interference from hydroquinone (HQ) and catechol (CT) as RC isomers. RC was detected in the linear range 0.05-40μM, with a detection limit of 0.012μM. The synthesized nanosensor was applied to the determination of RC in fresh fruit juice and water samples. The collected results confirmed the feasibility of our approach with high accuracy.

Scalable synthesis of highly luminescent and stable thiocyanate based CsPbX3 perovskite nanocrystals for efficient white light-emitting diodes

All-inorganic lead halide perovskites have attracted considerable attention as promising materials for optoelectronic devices because of their excellent photophysical properties. However, the stability issue of halide perovskites precludes their practical applications. For the first time, this work exhibits that the optical properties and stability of CsPbX3 perovskite nanocrystals (NCs), prepared by the saponification precipitation method, can be improved using thiocyanate (SCN−). Besides, this work offers a simple and economical synthesis method operable at room temperature and ambient atmosphere. Specifically, an increasing concentration of SCN− induces an increase in quantum yield almost by 10% and stability against air and moisture over several days due to the better passivation of trap states, irrespective of size and shape variation of NCs. The demonstration of tunable emission (454–645 nm) across the visible spectrum and the design of 3-D printed thin layers facilitate the preparation of green, yellow, and red-emitting color conversion layers. Benefitted from the decreased photon loss in as-synthesized conversion layers, the application of these color conversion layers results in a bright neutral white light (5184 K) with remarkable optical properties. Hence, this experimental study offers a low-temperature synthetic path for the scalable preparation of high-quality CsPbX3 NCs processed with SCN− and reveals the practical applications of such NCs.

Enhancing efficiency and decreasing photocatalytic degradation of perovskite solar cells using a hydrophobic copper-modified titania electron transport layer

The electron transport layer (ETL) plays a pivotal role in obtaining perovskite solar cells (PSCs) with high power conversion efficiency (PCE). Titanium dioxide is a widely used ETL, however it suffers from low electron mobility, poor conductivity and may act as a photocatalyst of chemical reactions leading to degradation of the perovskite. Herein, copper cations employed to modify the titania ETL of PSCs, mitigate the photocatalytic action of the compact layer, increase its conductivity and electron mobility, adjust favorably the energy levels, improve the ETL/perovskite interface, thus enhance perovskite’s light absorption and provoke passivation of the perovskite surface trap states. The investigation of structural and surface chemistry properties revealed the uniform distribution of Cu1+ in TiO2 and confirmed the determining role of copper in the fast extraction of the photogenerated charge carriers to the adjacent electrode. As a result, the champion planar PSCs based on the hydrophobic Cu-TiO2 ETL showed a 18.15 % PCE outperforming the reference devices (based on pristine TiO2 ETL) which showed a PCE equal to 15.78 %. The results followed the same trend also in the case of mesoporous PSCs, proving the universality of our approach. Finally, aging tests confirmed that the copper-modified devices showed higher stability in comparison with the non-modified ones retaining the 53 % of the initial PCE value after 51 days of storage in relative humidity (RH = 25 %) and dark conditions. Photostability experiments proved that the Cu-TiO2 based devices showed remarkable robustness and retained approximately 91 % of their initial PCE, even after 5 h under continuous UV stress. These results are associated with the suppression of the photocatalytic activity of the ETL and open new perspectives for improving the performance of TiO2-based PSCs.

Impact of PC71BM layer on the performance of perovskite solar cells prepared at high moisture conditions using a low temperature annealed ZnO thin film as the electron transport layer

ZnO is a promising electron transport material with high electron mobility compared to TiO2 and SnO2. However, its high basicity and the presence of hydroxyl groups at the ZnO surface induces thermochemical decomposition of hybrid perovskites though proton transfer reactions. In perovskite solar cells (PSCs), these deprotonation reactions produce chemical products at the interface between ZnO and perovskite, which obstacle charge carrier extraction process and lead to low efficiency of the solar cells. In this work, PC71BM thin films of three different thickness, 19, 11 and 6 nm, were deposited on top of ZnO layers, prepared by sol–gel spin coating and annealed at 150 °C. It is found that low temperature prepared ZnO films contain deep trap states, and the effective optical band gap of ZnO/PC71BM double layers is slightly reduced with the thickness of the fullerene derivative. The presence of an interfacial PC71BM layer on top of ZnO enhances the stability of the upcoming perovskite coatings and promotes the passivation of trap states at the ZnO surface. Interestingly, the best PC71BM-passivated PSC, fabricated under relative humidity (RH) of 60–65%, achieves a maximum power conversion efficiency (PCE) of 13.3%, whereas those PSCs with only ZnO as the electron transport layer show an average PCE of 5.5%. However, the stability under continuous illumination of PC71BM based PSCs is significantly lower than expected, probably due to the PC71BM degradation under high RH conditions.

Influence of end groups variation of self assembled monolayers on performance of planar perovskite solar cells by interface regulation

We present the synthesis and application of new class of self-assembled monolayer molecules (SAMs) for acquiring feasible interfacial engineering in inverted type perovskite solar cells (PSCs). The proposed SAMs bearing different electron donating terminal groups have been utilized to tune the work function of indium tin oxide (ITO) electrodes. Fine-tuning of terminal groups of the SAMs allows us to compare relationship between molecular structures and device parameters. Moreover, ionic and hybrid nature of perovskite enables forming various chemical interactions with terminal groups of SAMs. Employed SAMs has resulted in permanent increase in work function of ITO, increase power conversion efficiency (PCE) of the cells and passivation of trap states at the interface between electrode and perovskite layer. The present study provides new insights into correlation between molecular engineering and solar cell performance through treating holistic comparison of synthesized molecules at the interface of ITO and perovskite layer.

Chemical passivation of the perovskite layer and its real-time effect on the device performance in back-contact perovskite solar cells

The back-contact architecture for perovskite solar cells (PSCs) offers the possibility of positioning both electrodes on one side of the absorber layer and shining light directly on the perovskite photoactive layer. This helps us to avoid the occurrence of transmission losses caused by the charge collecting transparent conductive oxide electrode in the conventional sandwich structure for PSCs. The back-contact device architecture is also useful for conducting fundamental studies as it has an exposed photoactive area, permitting in situ measurements on the effects of chemical treatment, passivation, and annealing. A successful application of back-contact PSCs in studying the effect of chemical passivation of the perovskite photo-absorber layer trap states with pyridine and its influence on the device performance have been studied. The real-time effect of pyridine vapor treatment on the device performance is visualized by monitoring the maximum power output of the devices under operation conditions. The device performance enhancement by ∼12% owing to the surface passivating effect of pyridine is demonstrated.

Impacts of Ammonia Gas Plasma Surface Treatment on Polycrystalline-Silicon Junctionless Thin-Film Transistor

The impacts of ammonia gas (NH3) plasma treatment on the performance and positive gate bias stress (PGBS) of polycrystalline-silicon (poly-Si) junctionless thin-film transistor (JL-TFT) are studied. A −0.785-V threshold voltage ( $V_{\mathrm {TH}}$ ) shift of JL-TFT due to the NH3 plasma treatment is observed, which is attributed to the fixed oxide charge effect of the plasma-induced interfacial layer (PIL). In addition, the transconductance of JL-TFT with the NH3 plasma treatment is enhanced by ~2.73 times due to the trap state passivation of grain boundaries in the poly-Si, and the ON-state current ( $I_{\mathrm {ON}}$ ) is enhanced by ~2.91 times. In addition to the performance enhancement of JL-TFT by the plasma process, the $V_{\mathrm {TH}}$ shift of poly-Si JL-TFT under PGBS is suppressed from −0.240 to −0.063 V after the plasma treatment. It is attributed to the less degradation of insulator/channel interface due to the growth of PIL by the plasma process. Moreover, the $I_{\mathrm {ON}}$ degradation of JL-TFT after PGBS is also improved from −19% to −16% after the plasma treatment. As the carrier transport of JL-TFT is the bulk conduction rather than the surface conduction of conventional inversion-mode TFT, the degradation of insulator/channel interface would exhibit less impacts on the $I_{\mathrm {ON}}$ . The improvement of performance and PGBS of JL-TFT by the NH3 plasma treatment would be beneficial to the application of 3-D integrated circuits.

Broad-Band Photodetectors Based on Copper Indium Diselenide Quantum Dots in a Methylammonium Lead Iodide Perovskite Matrix.

Low-temperature solution-processed methylammonium lead iodide (MAPbI3) crystalline films have shown outstanding performance in optoelectronic devices. However, their high dark current and high noise equivalent power prevent their application in broad-band photodetectors. Here, we applied a facile solution-based antisolvent strategy to fabricate a hybrid structure of CuInSe2 quantum dots (CISe QDs) embedded into a MAPbI3 matrix, which not only enhances the photodetector responsivity, showing a large on/off ratio of 104 at 2 V bias compared with the bare perovskite films, but also significantly (for over 7 days) improves the device stability, with hydrophobic ligands on the CuInSe2 QDs acting as a barrier against the uptake of environmental moisture. MAPbI3/CISe QD-based lateral photodetectors exhibit high responsivities of >0.5 A/W and 10.4 mA/W in the visible and near-infrared regions, respectively, partly because of the formation of a type II interface between the respective semiconductors but most significantly because of the efficient trap-state passivation of the perovskite grain surfaces, and the reduction in the twinning-induced trap density, which stems from both CISe QDs and their organic ligands. A large specific detectivity of 2.2 × 1012 Jones at 525 nm illumination (1 μW/cm2), a fast fall time of 236 μs, and an extremely low noise equivalent power of 45 fW/Hz1/2 have been achieved.

Defect/Interface Recombination Limited Quasi-Fermi Level Splitting and Open-Circuit Voltage in Mono- and Triple-Cation Perovskite Solar Cells.

Multication metal-halide perovskites exhibit desirable performance and stability, compared to their monocation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanisms in solution-processed MAPbI3 (MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (triple cation) perovskite solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi-Fermi level splitting than for the triple cation perovskite absorbers, which defines the open-circuit voltage of the MAPI cells. PL measurements at low temperatures (∼20 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the nonradiative recombination loss at room temperature. We conclude that while triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states within the MAPI films is the primary consideration for device optimization.

Self-Repairing Tin-Based Perovskite Solar Cells with a Breakthrough Efficiency Over 11.

The development of tin (Sn)-based perovskite solar cells (PSCs) is hindered by their lower power conversion efficiency and poorer stability compared to the lead-based ones, which arise from the easy oxidation of Sn2+ to Sn4+ . Herein, phenylhydrazine hydrochloride (PHCl) is introduced into FASnI3 (FA = NH2 CH  NH2 + ) perovskite films to reduce the existing Sn4+ and prevent the further degradation of FASnI3 , since PHCl has a reductive hydrazino group and a hydrophobic phenyl group. Consequently, the device achieves a record power conversion efficiency of 11.4% for lead-free PSCs. Besides, the unencapsulated device displays almost no efficiency reduction in a glove box over 110 days and shows efficiency recovery after being exposed to air, due to a proposed self-repairing trap state passivation process.

Heavy Water Additive in Formamidinium: A Novel Approach to Enhance Perovskite Solar Cell Efficiency.

Heavy water or deuterium oxide (D2 O) comprises deuterium, a hydrogen isotope twice the mass of hydrogen. Contrary to the disadvantages of deuterated perovskites, such as shorter recombination lifetimes and lower/invariant efficiencies, the serendipitous effect of D2 O as a beneficial solvent additive for enhancing the power conversion efficiency (PCE) of triple-A cation (cesium (Cs)/methylammonium (MA)/formaminidium (FA)) perovskite solar cells from ≈19.2% (reference) to 20.8% (using 1 vol% D2 O) with higher stability is reported. Ultrafast optical spectroscopy confirms passivation of trap states, increased carrier recombination lifetimes, and enhanced charge carrier diffusion lengths in the deuterated samples. Fourier transform infrared spectroscopy and solid-state NMR spectroscopy validate the N-H2 group as the preferential isotope exchange site. Furthermore, the NMR results reveal the induced alteration of the FA to MA ratio due to deuteration causes a widespread alteration to several dynamic processes that influence the photophysical properties. First-principles density functional theory calculations reveal a decrease in PbI6 phonon frequencies in the deuterated perovskite lattice. This stabilizes the PbI6 structures and weakens the electron-LO phonon (Fröhlich) coupling, yielding higher electron mobility. Importantly, these findings demonstrate that selective isotope exchange potentially opens new opportunities for tuning perovskite optoelectronic properties.

Surface Passivation of Perovskite Film by Sodium Toluenesulfonate for Highly Efficient Solar Cells

Ionic defects at the surfaces of organolead halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells (PSCs). Herein, sodium p‐toluenesulfonate (STS) is applied during the surface modification of perovskite layer for the first time, leading to the efficient surface passivation of the perovskite film and consequently significant enhancements in both efficiency and stability of mixed‐cation PSC devices. Upon incorporating STS atop the perovskite layer, the power conversion efficiency of the Cs0.05MA0.12FA0.83PbI2.55Br0.45 (abbreviated as CsMAFA) mesoporous‐structure mixed‐cation PSC devices improves from 18.70% to 20.05% with reduced hysteresis. The sulfonate (–SO3−) anion of STS coordinates with the Pb2+ of CsMAFA perovskite, and the Na+ cation of STS electrostatically interacts with the anions (I−/Br−) of CsMAFA perovskite, resulting in the surface passivation of the CsMAFA perovskite film with reduced electron and hole trap state densities. In addition, STS modification induces an upshift of the valence band of perovskite, facilitating hole extraction from the perovskite layer to the hole transport layer with suppressed interfacial charge recombination. Moreover, such a trap state passivation of perovskite film leads to improvement of the ambient stability of PSC devices.

Shallow and Deep Trap State Passivation for Low-Temperature Processed Perovskite Solar Cells

While perovskite solar cells (PSCs) have emerged as promising low-cost solar power generators, most reported high-performance PSCs employ electron transport layers (ETLs, mainly TiO2) treated at hi...

Alleviation of abnormal NBTI phenomenon in LTPS TFTs on polyimide substrate for flexible AMOLED

AbstractThis letter investigates the negative‐bias temperature instability (NBTI) behavior of p‐channel low‐temperature polycrystalline silicon thin‐film transistors (LTPS TFTs) on plastic substrate. The measurements reveal that the threshold‐voltage positive shift is highly correlated to the passivation of grain boundary trap states. By applying the established phenomenon such as NBTI recovery and H diffusion from PI substrate, a new model is introduced to explain the mechanism and verified by the experiment. With the thick buffer and bottom metal layer or newly processed PI substrate, we succeeded in adjusting the NBTI behavior of LTPS TFTs on plastic substrate.

The enhancement of excitonic emission crossing Saha equilibrium in trap passivated CH3NH3PbBr3 perovskite

Metal-halide semiconductor perovskites have received great attention for the development of stable and efficient light emitting diodes and lasers, since they combine high charge carrier mobility and light emission spectral-purity with low-cost fabrication methods. Nevertheless, the role of excitons, free carries and trap states in perovskite light emission properties is still unclear due to their interdependence. In this paper we selectively manage trapping and light emission mechanisms by a reversible laser-assisted trap-passivation process performed on a CH3NH3PbBr3 perovskite layer, coupled to the inner modes of a high-quality micro-cavity, which only affects the radiative recombination. We show that photoluminescence is dominated by exciton radiative decay process and that trap states passivation increases the exciton gemination rate by reducing coulombic scattering of free electrons due to the ionized impurities. This picture provides a more general description than the model based on trap states-free Saha thermodynamic equilibrium between photo-generated species.

Engineering of the Electron Transport Layer/Perovskite Interface in Solar Cells Designed on TiO2 Rutile Nanorods

AbstractThe engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single‐crystalline TiO2 nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl4 treatment combined with a subsequent PC61BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show JSC, VOC, and fill factor as high as 23.2 mA cm−2, 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO2 NR‐based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements.