Time-resolved Photoluminescence Results Research Articles

Temperature-dependent photoluminescent behavior of millimeter-scale Cs4PbBr6/CsPbBr3 bulk crystals and their application to white light-emitting diodes

The zero-dimensional perovskite composite Cs4PbBr6/CsPbBr3 has attracted significant attention for its remarkable photoluminescence (PL), which remains highly effective even in solid state. This work presents a detailed analysis of the steady-state and time-resolved PL (TRPL) behavior of millimeter-scale Cs4PbBr6/CsPbBr3 crystals over a temperature range of 80 to 360 K, which covers exciton binding energy, phonon energy, and PL peak energy shifting with increasing temperature. According to the results, Cs4PbBr6/CsPbBr3 exhibits high exciton binding energy and phonon energy, with calculated values of 358.7 and 94.8 meV, respectively. Specifically, when the temperature is below ∼235 K, thermal expansion dominates to influence the TRPL and peak energy, whereas electron-phonon interaction becomes the dominant factor as temperature rises from 235 to 325 K. It is found that Cs4PbBr6/CsPbBr3 has a PL behavior similar to CsPbBr3, and characterization and TRPL results demonstrate that nanometer-scale CsPbBr3 crystals embed in the Cs4PbBr6 bulk matrix. Meanwhile, a white light-emitting diode (WLED) device based on Cs4PbBr6/CsPbBr3 with luminous efficiency of 64.56 lm/W is fabricated, and its color coordinate is measured as (0.34, 0.31) under 20 mA, which is in close proximity to the standard white color coordinate. Moreover, the color gamut of the device is measured as 128.66% of the National Television Systems Committee (NTSC). The WLED electroluminescence (EL) spectra show high Correlated Color Temperature (CCT) stability for the working current varying from 5 to 100 mA, and after continuous operation for 12 h, the EL intensity decreases and stabilizes at ∼70% of the initial EL intensity. These findings suggest that Cs4PbBr6/CsPbBr3 crystals are a promising candidate for WLEDs.

Enhanced Hole Mobility of p-Type Materials by Molecular Engineering for Efficient Perovskite Solar Cells.

Star-shaped triazatruxene derivative hole-transporting materials (HTMs), namely, 3,8,13-tris(4-(8a,9a-dihydro-9H-carbazol-9-yl)phenyl)-5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-TY1) and 3,8,13-tris(4-(8a,9a-dihydro-9H-carbazol-9-yl)phenyl)-5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-TY2), containing electron-rich triazatruxene cores and donor carbazole moieties, were synthesized and successfully used in triple-cation perovskite solar cells. All the HTMs were obtained from relatively inexpensive precursor materials using well-known synthesis procedures and uncomplicated purification steps. All the HTMs, including the 5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-H) main core, had suitable highest occupied molecular orbitals (HOMOs) for perovskite (TAT-H: -5.15 eV, TAT-TY1: -5.17 eV, and TAT-TY2: -5.2 eV). Steady-state and time-resolved photoluminescence results revealed that hole transport from the valence band of the perovskite into the HOMO of the new triazatruxene derivatives was more efficient than with TAT-H. Furthermore, the substitution of n-hexylcarbazole and 9-phenylcarbazole in triazatruxene altered the crystalline nature of the main core, resulting in a smooth and pinhole-free thin-film morphology. As a result, the hole mobilities of TAT-TY1 and TAT-TY2 were measured to be one order of magnitude higher than that of TAT-H. Finally, TAT-TY1 and TAT-TY2 achieved power conversion efficiencies of up to 17.5 and 16.3%, respectively, compared to the reference Spiro-OMeTAD. These results demonstrate that the new star-shaped triazatruxene derivative HTMs can be synthesized without using complicated synthesis strategies by controlling the intrinsic morphology of the TAT-H main core.

Elucidating the role of two-dimensional cations in green perovskite light emitting diodes

Perovskite light emitting diodes (PeLEDs) have emerged as promising candidates for applications requiring visible and near-infrared emission. Lead-based quasi-2D perovskite materials are commonly employed as an emissive layer for PeLEDs as they promote radiative emission via spatially confining excitons and funnelling of charge carriers. Designing efficient emissive layers based on these quasi-2D perovskites requires that the ratio between inorganic [PbX 6 ] 4- component and the large organic cations (spacers) is carefully optimized. In this work, we demonstrate the importance of such compositional tuning using three different 2D cations namely phenylethylammonium (PEA), its monofluorinated analogue FPEA and a custom-made bulkier cation BPEA containing an extra phenyl ring. Our results show that the tuning of the ratio between 2D cation and the [PbX 6 ] 4- provides a trade-off between electrical transport in the device and the emission properties of the emissive layer. Generally, a large excess of cations is required to enhance the external quantum efficiency (EQE) of PeLEDs. Among the various cations, FPEA leads to PeLEDs with the highest EQE up to 7.7%, while BPEA resulted in the smallest EQE. Atomic force microscopy, space-charge-limited current, steady-state and time-resolved photoluminescence results revealed that FPEA-based perovskite have lower number of pinholes and traps compared to PEA, while it is also likely to have better charge transport properties due to its smaller size in contrast to BPEA. Our results provide guidelines regarding the multiple functions an organic spacer needs to fulfil for fabricating efficient PeLEDs. • Three quasi-2D cations are employed in perovskite emissive layer. • A change in the molar ratio of cation impact optoelectronic properties. • A trade-off between electrical transport and emission properties is found. • Peak EQE of 7.7% is obtained for optical FPEABr cation. • The 2D cations improved stability of green PeLEDs.

Improved Modulation Bandwidth of Blue Mini-LEDs by Atomic-Layer Deposition Sidewall Passivation

By adopting atomic layer deposition (ALD) sidewall passivation, the electroluminescence (EL) and communication performances of mini-light emitting diodes (mini-LEDs) of different sizes were improved. In particular, the most significant improvement in the electroluminescence properties of the external quantum efficiency (EQE) (a 7.2% increase), leakage current, and the communication properties of the modulation bandwidth (a 20% increase) transmission rate and bit error rate (BER) was found in the smallest mini-LEDs (80 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}\,\,\times $ </tex-math></inline-formula> 120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> ). According to the results of time-resolved photoluminescence (TRPL) measurements, the carrier lifetime of the samples can be affected by both the size and ALD sidewall passivation. In addition, the effects of ALD sidewall passivation for visible-light communication (VLC) were demonstrated.

Unveiling the role of carbonate in nickel-based plasmonic core@shell hybrid nanostructure for photocatalytic water splitting

Though carbonates are known for several decades, their role in sun-light driven photocatalysis is still hidden. Herein, carbonate boosted solar water splitting in nickel-based plasmonic hybrid nanostructures is disclosed for the first time via in-situ experiments and density-functional theory (DFT)-based calculations. Ni@NiO/NiCO3 core@shell (shell consisting of crystalline NiO and amorphous NiCO3) nanostructure with varying size and compositions are studied for hydrogen production. The visible light absorption at ∼470 nm excludes the possibility of NiO as an active photocatalyst, emphasizing plasmon driven H2 evolution. Under white light irradiation, higher hydrogen yield of ∼80 µmol/g/h for vacuum annealed sample over pristine (∼50 µmol/g/h) complements the spectroscopic data and DFT results, uncovering amorphous NiCO3 as an active site for H2 absorption due to its unique electronic structure. This conclusion also supports the time-resolved photoluminescence results, indicating that the plasmonic electrons originating from Ni are transferred to NiCO3 via NiO. The H2 evolution rate can further be enhanced and tuned by the incorporation of NiO between Ni and NiCO3.

The Sign of Exciton-Photon Coupling in GaN-Based Triangular-like Ridge Cavity

In this paper, the behavior of exciton radiative recombination in a GaN-based triangular-like ridge cavity is studied at room-temperature. The triangular-like ridge cavity is fabricated on a standard-blue-LED epitaxial wafer grown on a sapphire substrate. Through the photoluminescence (PL) and time-resolved photoluminescence (TR-PL) measurements, a clear modulation of the original spontaneous emission is found in the microcavity, a new transition channel is observed, and the effect is angle-dependent. Furthermore, by changing the tilt angle during angle-resolution photoluminescence (AR-PL), it is found that the coupling between excitons and photons in the cavity is the strongest when tilted at 10°. By simulation, the strong localization of photons in the top of the cavity can be confirmed. The PL, TR-PL, and AR-PL results showed the sign of the exciton-photon coupling in the triangular-like ridge cavity.

Modulating optical properties and interfacial electron transfer of CsPbBr3 perovskite nanocrystals via indium ion and chlorine ion co-doping.

In this work, we demonstrated an in situ approach for doping CsPbBr3 nanocrystals (NCs) with In3+ and Cl- with a ligand-assisted precipitation method at room temperature. The In3+ and Cl- co-doped NCs are characterized by the powder x-ray diffraction patterns, ultraviolet-visible, photoluminescence (PL) spectroscopy, time-resolved PL (TRPL), ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, and transmission electron microscopy. Based on PL and TRPL results, the non-radiative nature of In3+-doping induced localized impurity states is revealed. Furthermore, the impact of In3+ and Cl- doping on charge transfer (CT) from the NCs to molecular acceptors was investigated and the results indicate that the CT at the interface of NCs can be tuned and promoted by In3+ and Cl- co-doping. This enhanced CT is attributed to the enlarged energy difference between relevant states of the molecular acceptor and the NCs by In3+ and Cl- upon co-doping. This work provides insight into how to control interfacial CT in perovskite NCs, which is important for optoelectronic applications.

Rapid thermal annealing induced engineering of surface and photoluminescence properties of (K,Na)NbO3 thin films for optoelectronic applications

The present study is focused on understanding the mechanism of luminescence behavior and how it gets affected with post annealing temperature via rapid thermal annealing of (K,Na)NbO3 thin films. The crystalline KNN perovskite structure with preferred (001)-orientation is observed upon annealing at 750 and 800 °C. After annealing, the evolution kinetics of surface features is resulted from the simultaneous evaporation-recondensation and volume diffusion of particles. The volatilization of alkali species after annealing and, in contrast, the lattice oxygen and Nb5+ chemical state of niobium related to KNN is enhanced. Narrowing of optical band gap after annealing is attributed to the generation of shallow defects deep below the conduction band owing to order–disorder and/or distortion in NbO6 octahedra. Photoluminescence (PL) quenching behavior is observed after RTA with 290 nm excitation wavelength. A strong blue emission is observed at 800 °C after exciting with 266 nm wavelength. The kinetics of decay lifetime from time-resolved PL results revealed the enhancement of transitions more likely via radiative states. The RTA induced results specify that KNN can be one of the promising candidates for use in LEDs, smart glass windows, optical switches/sensors, and optoelectronic devices.

Improved minority carrier lifetime in p-type GaN segments prepared by vacancy-guided redistribution of Mg

To accelerate the development of GaN power devices, reproducible fabrication of p-type GaN (p-GaN) segments by ion-implantation (I/I) that enables selective-area doping is preferred. In this Letter, the results of time-resolved photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements on p-GaN fabricated by I/I are presented. The p-GaN samples of a 220-nm-deep box-shaped Mg concentration profile ([Mg] = 3×1018 cm−3) were fabricated by sequential I/I of Mg and N followed by atmospheric-pressure post-implantation annealing (PIA). During PIA, Mg redistribution was guided by N-I/I-induced vacancies. The intensities of PL peaks originating from Mg acceptors were increased by increasing the PIA temperature (Ta), indicating progressive activation of Mg and simultaneous decrease in the concentration of nonradiative recombination centers (NRCs). As a result, a record-long room-temperature PL lifetime for the near-band edge emission of Ga-polar Mg-implanted p-GaN (1.4 ps) was obtained when Ta = 1300 °C. Because the species of major vacancy-type defects in the present sample was determined as vacancy clusters comprised of few Ga-vacancies (VGa) and N-vacancies (VN) such as (VGaVN)2–3, the NRC concentration is roughly estimated at around 1017 cm−3 provided that (VGaVN)2–3 are major NRCs with the electron-capture-coefficient of 7×10−6 cm3 s−1. Cross-sectional CL linescan images revealed certain activation of Mg from the surface to the leading edge of the box-shaped [Mg] profile, while NRCs remained at the depths right below the box-shaped profile. The results support advantages of using the vacancy-guided Mg redistribution technique for controlling the activated [Mg] profile and realizing p-type Ga-polar GaN:Mg segments by I/I.

The red light emission in 2D (C4SH3CH2NH3)2SnI4 and (C4OH7CH2NH3)2SnI4 perovskites.

Two-dimensional (2D) perovskites have a large exciton binding energy due to the structure of the quantum confinement, which produces a faster radiative recombination, and so are promising potential materials for light-emitting diodes. However, most of the highly efficient hybrid halide perovskites are based on the toxic Pb-based materials, so the replacement of Pb with less toxic and suitable substitute elements has been investigated for environmental efficient materials. Herein, we report the Sn-based 2D perovskites, which include (TPM)2SnI4 (TPM = C4SH3CH2NH3) and (TFF)2SnI4 (TFF = C4OH7CH2NH3), as red emission materials. Structural characterization by single crystal X-ray diffraction reveals that (TPM)2SnI4 undergoes a structural evolution from the orthorhombic space group Cmc21 (100 K) to Pbca (298 K), while the (TFF)2SnI4 perovskite exhibits the monoclinic space group P21/c at 100 K and 298 K. The inorganic framework of (TFF)2SnI4 was separated by the bilayer TFF chains with an empty space, which is an effective structure to increase the quantum confinement effect. The band gaps of the (TPM)2SnI4 (1.80 eV) and (TFF)2SnI4 (1.73 eV) compounds indicate the direct band gap semiconductor materials. From the time-resolved photoluminescence results, it can be seen that (TPM)2SnI4 produces uniform short emission (0.73 ns) throughout the entire powder crystals, whereas (TFF)2SnI4 has a uniform and long emission life time (47 ns). Temperature-dependent photoluminescence (PL) studies indicate that the (TPM)2SnI4 and (TFF)2SnI4 perovskites have a strong split red emission at low temperature due to the vibration of the inorganic framework. As the temperature increases, the PL spectra shift to the high energy region and the emission intensity decreases. The PL spectra of (TPM)2SnI4 and (TFF)2SnI4 perovskites have maximum peak wavelengths at 622 nm and 640 nm, and show the photoluminescence quantum yields of 0.30% and 1.71%, respectively.

A facile surface passivation method for efficient inorganic CsPbI2Br perovskite solar cells with efficiencies over 15%

Recently, perovskite solar cells (PVSCs) based on CsPbI2Br have attracted increasing attention owing to their good balance between efficiency and stability. Solution-processed CsPbI2Br perovskites usually contain various defects which need passivation for PVSCs with high-performance as well as good stability. Since the degradation of perovskite films usually begins at the grain boundaries, here we report a facile defect passivation strategy by spin-coating a KF solution on the CsPbI2Br perovskite surface. The deposited KF salt mostly locates at the grain boundaries of the perovskite surface, resulting in PVSCs with improved stability. Both steady-state and time-resolved photoluminescence results suggest that the defects of perovskite were significantly passivated by KF treatment. Consequently, the best-performance PVSC based on CsPbI2Br with KF treatment shows an enhanced power conversion efficiency (PCE) of 15.01% with a larger open circuit voltage (VOC) of 1.26 V in comparison with the pristine CsPbI2Br-based counterpart which exhibits an inferior PCE of 14.14% with a VOC of 1.18 V.

Encapsulation of colloid perovskite nanocrystals into solid polymer matrices: Impact on electronic transition and photoluminescence

To improve the stability and enable luminescent devices, colloidal perovskite nanocrystals (NCs) are usually encapsulated by polymer host matrices. Here, colloidal CsPbX3 (X = Cl, Br or I, or mixture) NCs with bright emissions covering wide color gamut (~410–700 nm) are incorporated into the off-stoichiometry thiol-ene (OSTE) for solid state luminescence. The impact of polymer encapsulation on electronic transitions is investigated by comparison of absorption, photoluminescence (PL), and time resolved PL (TRPL) of the CsPbX3 NCs before and after the encapsulation by the OSTE. The PL bandwidth increase pronouncedly, while the PL peak position apparently blue shifts after encapsulation. The spectral changes suggest the energy transfer between the CsPbX3 NCs and OSTE matrix and decreased exciton binding energy due to the suppressed dielectric screen effect, which are further validated by the TRPL results. This work sheds light on the fluorescence of CsPbX3 NCs incorporated in the polymer host matrices, which will facilitate their future applications in light-emitting and other optoelectronic devices.

Defect passivation of CsPbI2Br perovskites through Zn(II) doping: toward efficient and stable solar cells

Defect passivation is an important strategy to achieve perovskite solar cells (PVSCs) with enhanced power conversion efficiencies (PCEs) and improved stability because the trap states induced by defects in the interfaces and grain boundaries of perovskites are harmful to both large open circuit voltage and high photocurrent of devices. Here, zinc cations (Zn2+) were used as a dopant to passivate defects of the CsPbI2Br perovskite leading to Zn2+-doped CsPbI2Br film with fewer trap states, improved charge transportation, and enhanced light-harvesting ability. Thus, the best-performance PVSC based on CsPbI2Br with the optimal Zn2+ doping shows a higher PCE of 12.16% with a larger open-circuit voltage ( V OC) of 1.236 V, an improved short-circuit current ( J SC) of 15.61 mA cm−2 in comparison with the control device based on the pure CsPbI2Br which exhibits a PCE of 10.21% with a V OC of 1.123 V, a J SC of 13.27 mA cm−2. Time-resolved photoluminescence results show that the Zn2+ doping leads to perovskite film with extended photoluminescence lifetime which means a longer diffusion length and subsequently enhanced photocurrent and open circuit voltage. This work provides a simple strategy to boost the performance of PVSCs through Zn2+ doping.

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Summary Electron-transport-layer-free (ETL-free) device architectures are promising designs for perovskite photovoltaics because they offer simpler configurations, low cost, and convenience for versatile optoelectronics. However, the development of ETL-free photovoltaics is hindered by their low performance. Herein, we reveal that a low electron-injection rate at the ETL-free interface is responsible for the performance loss. Moreover, we demonstrate that improving carrier lifetimes in the perovskite films can remedy the poor carrier extraction at interfaces, enabling carrier collection efficiency in ETL-free photovoltaics to approach that in ETL-containing devices. Using perovskite films with microsecond carrier lifetimes, we obtained ETL-free devices with a power conversion efficiency (PCE) of 19.5%, nearly eliminated hysteresis, and good stability. Such a PCE value is comparable to that (20.7%) of the analogous ETL-containing photovoltaics. These results offer opportunities for ETL-free architecture designs in the perovskite photovoltaics family. More importantly, this research provides a general approach to improving the performance of photovoltaics with low-injection-rate interfaces.

Time resolved photoluminescence studies of degradation in GaInP/GaAs/Ge solar cells after 1MeV electron irradiation

Time resolved photoluminescence (TRPL) is a powerful method to character the behaviors of carriers as it has high time resolution that could reflect the reactions of carriers within nanoseconds. For solar cells, minority carrier lifetime is the most important parameter. TRPL has been used to measure the lifetime for uniform materials. However, for homojunction solar cells, doping distribution and carrier drift make the spectroscopy analysis much difficult. Thus one dimension numerical calculations are used to study the time-dependent photoluminescence (TRPL) decay of GaAs sub-cell in GaInP/GaAs/Ge solar cells. Calculation shows that both lifetime of minorities and light intensities could determine the line shape of TRPL. The bimolecular recombination under high injection modifies the curve from single-exponential to non-single-exponential one. For TRPL of homojunction solar cell, the photoluminescence decay process is not synchronized in all parts, and the decay is fast in emitter but slower in base regions. To get the lifetimes of minority carriers after 1MeV electron irradiation, carrier generation parameter G0 should be fitted by numerical method firstly. The damage factor Kτ=4.8×10-15 cm2/ns is fitted from TRPL results. Photoluminescence spectra (PL) are also used to get Kτ=5.5×10-15 cm2/ns which is similar with the value obtained from TRPL.

Influence of negative-U centers related carrier dynamics on donor-acceptor-pair emission in fluorescent SiC

E 1/E2 defects are the typical negative-U centers in n-type 6H silicon carbide (SiC). They are the main contributors to non-radiative recombination, which limits the carrier lifetime. In this study, two fluorescent 6H silicon carbide (f-SiC) samples and one bulk substrate were characterized via time-resolved photoluminescence (TRPL) and static photoluminescence (PL) measurements, where all the samples were nitrogen-boron co-doped 6H n-type. The existence of E1/E2 defects, which caused the diminution of the internal quantum efficiency (IQE) and luminescence intensity of each sample, was confirmed by applying a carrier dynamics model based on negative-U centers. The carrier dynamics simulation reveals that the density of the E1/E2 defects in bulk 6H SiC is two orders of magnitude higher than that of the f-SiC sample, causing much lower PL intensity in the bulk substrate compared to the two f-SiC samples. The IQE of the two f-SiC samples was extracted from the corresponding TRPL results, where the contrast between their IQE was further confirmed by the related PL measurement results. The slight difference in IQE between the two f-SiC samples was attributed to slightly different E1/E2 defect concentrations. On the other hand, by implementing a steady-state donor-acceptor-pair (DAP) recombination calculation, it was found that the f-SiC sample with lower IQE had a higher DAP transition probability due to the higher doping level. This prompted further optimizations in the f-SiC crystal growth conditions in order to decrease the E1/E2 defects while maintaining the correct doping parameters.

Energy band engineering of InGaN/GaN multi-quantum-well solar cells via AlGaN electron- and hole-blocking layers

In this paper, we perform a comprehensive study on energy band engineering of InGaN multi-quantum-well (MQW) solar cells using AlGaN electron- and hole-blocking layers. InGaN MQW solar cells with AlGaN layers were grown by metalorganic chemical vapor deposition, and high crystal quality was confirmed by high resolution X-ray diffraction measurements. Time-resolved photoluminescence results showed that the carrier lifetime on the solar cells with AlGaN layers increased by more than 40% compared to that on the reference samples, indicating greatly improved carrier collections. The illuminated current-density (J–V) measurements further confirmed that the short-circuit current density (Jsc) of the solar cells also benefited from the AlGaN layer design and increased 46%. At room temperature, the InGaN solar cells with AlGaN layers showed much higher power conversion efficiency (PCE), by up to two-fold, compared to reference devices. At high temperatures, these solar cells with AlGaN layers also delivered superior photovoltaic (PV) performance such as PCE, Jsc, and fill factor than the reference devices. These results indicate that band engineering with AlGaN layers in the InGaN MQW solar cell structures can effectively enhance the carrier collection process and is a promising design for high efficiency InGaN solar cells for both room temperature and high temperature PV applications.

Interface Engineering of Colloidal CdSe Quantum Dot Thin Films as Acid-Stable Photocathodes for Solar-Driven Hydrogen Evolution.

Colloidal semiconductor quantum dot (CQD)-based photocathodes for solar-driven hydrogen evolution have attracted significant attention because of their tunable size, nanostructured morphology, crystalline orientation, and band gap. Here, we report a thin film heterojunction photocathode composed of organic PEDOT:PSS as a hole transport layer, CdSe CQDs as a semiconductor light absorber, and conformal Pt layer deposited by atomic layer deposition (ALD) serving as both a passivation layer and cocatalyst for hydrogen evolution. In neutral aqueous solution, a PEDOT:PSS/CdSe/Pt heterogeneous photocathode with 200 cycles of ALD Pt produces a photocurrent density of -1.08 mA/cm2 (AM-1.5G, 100 mW/cm2) at a potential of 0 V versus reversible hydrogen electrode (RHE) ( j0) in neutral aqueous solution, which is nearly 12 times that of the pristine CdSe photocathode. This composite photocathode shows an onset potential for water reduction at +0.46 V versus RHE and long-term stability with negligible degradation. In the acidic electrolyte (pH = 1), where the hydrogen evolution reaction is more favorable but stability is limited because of photocorrosion, a thicker Pt film (300 cycles) is shown to greatly improve the device stability and a j0 of -2.14 mA/cm2 is obtained with only 8.3% activity degradation after 6 h, compared with 80% degradation under the same conditions when the less conformal electrodeposition method is used to deposit the Pt layer. Electrochemical impedance spectroscopy and time-resolved photoluminescence results indicate that these enhancements stem from a lower bulk charge recombination rate, higher interfacial charge-transfer rate, and faster reaction kinetics. We believe that these interface engineering strategies can be extended to other colloidal semiconductors to construct more efficient and stable heterogeneous photoelectrodes for solar fuel production.

Surface passivation with nitrogen-doped carbon dots for improved perovskite solar cell performance

Undercoordinated lead cations and halide anions on the surface of perovskite layer can form surface trap states and cause electronic disorders which reduce the performance of perovskite solar cells. Nitrogen-doped carbon dots (NCDs) that have rich nitrogen- and oxygen-containing functional groups can effectively interact with the unsaturated metal sites and halide anions on the surface and boundaries of perovskite grains. Herein, low-cost NCDs are utilized as efficient additives to passivate the surface of a solution-processed CH3NH3PbI3 perovskite film, which remarkably reduce charge carrier recombination, as evidenced by the results of time-resolved photoluminescence and electrochemical impedance spectrum measurements. FTIR spectra indicate the formation of hydrogen bonds between the undercoordinated iodine ions on perovskite grains and hydroxyl as well as nitrogenous groups of NCDs. In addition, NCDs additives also help increase interfacial charge transfer from perovskite to electron-transporting layer, leading to an improvement in power conversion efficiency for the solar cell device from 12.12 ± 0.28% (standard cell fabricated in same conditions) to 15.93 ± 0.15%.

Photoluminescence study of carbon dots from ginger and galangal herbs using microwave technique

Carbon dots are new type of fluorescent nanoparticle that can be synthesis easily from natural sources. We have synthesized carbon dots from ginger and galangal herbs using microwave technique and studied their optical properties. We synthesized colloidal carbon dots in water solvent by varying microwave processing time. UV-Vis absorbance, photoluminescence, time-resolved photoluminescence, and transmission electron microscope were utilized to study properties of carbon dots. We found that microwave processing time significantly affect optical properties of synthesized carbon dots. UV-Vis absorbance spectra and time-resolved photoluminescence results show that luminescent of carbon dots is dominated by recombination process from n-π* surface energy level. With further development, these carbon dots are potential for several applications.