Density Of Trap States Research Articles

Photoexcited Carrier Dynamics in Iodine-Doped CH3NH3PbBr3 Single Crystals.

Iodine-doped bromide perovskite single crystals (IBPSCs) have important applications in optoelectronic fields, such as in solar cells. Currently, much research has aimed to study the phase separation phenomenon and device performance improvements in IBPSCs. However, important intrinsic photoexcited carrier dynamics are often overlooked in IBPSCs. Here, we explored the photoexcited carrier dynamics in typical iodine-doped MAPbBr3 single crystals using the excitation intensity-dependent steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) technique. We found that the trap state density changes with an increase in the amount of doped iodine. Further, we noticed that there is an influence of carrier diffusion on the photoexcited carrier dynamics, and then, we evaluated the carrier diffusion coefficients and recombination constants via numerical simulations of the PL kinetics. Consequently, we found that the electron shallow trap-related carrier behaviors substantially impacted the PL kinetics. Our results greatly facilitate a deeper understanding of the fundamental characteristics of mixed halide perovskite material.

Interface Defects Dependent on Perovskite Annealing Temperature for NiOX-Based Inverted CsPbI2Br Perovskite Solar Cells.

Nickel oxide (NiOX) is an ideal inorganic hole transport material for the fabrication of inverted perovskite solar cells owing to its excellent optical and semiconductor properties. Currently, the main research on developing the performance of NiOX-based perovskite solar cells focuses on improving the conductivity of NiOX thin films and preventing the redox reactions between metal cations (Ni3+ on the surface of NiOX) and organic cations (FA+ or MA+ in the perovskite precursors) at the NiOX/perovskite interface. In this study, a new type of interface defects in NiOX-based CsPbI2Br solar cells is reported. That is the Pb2+ from CsPbI2Br perovskites can diffuse into the lattice of NiOX surface as the annealing temperature of perovskites changes. The diffusion of Pb2+ increases the ratio of Ni3+/Ni2+ on the surface of NiOX, leading to an increase in the density of trap state at the interface between NiOX and perovskites, which eventually results in a serious decline in the photovoltaic performance of solar cells.

Luminescence and two-photon absorption assisted optical limiting properties of Ag-doped CdWO4 nanorods

The emerging demand for the fabrication of nonlinear optical materials with high limits of optical power is obvious in nonlinear optics due to their wide application in photonic devices. We report the linear and third-order nonlinear optical properties of Ag-doped CdWO4 nanorods and their dependence on Ag-doping concentration. CdWO4 nanorods with different Ag concentrations were synthesized by the chemical precipitation method and characterized by UV–Vis absorption, X-ray diffraction (XRD), Raman, Photoluminescence and its lifetime studies, X-ray Photoelectron spectroscopy and Transmission Electron Microscopy (TEM). The introduction of Ag ions has led to an increased redshift in the CdWO4 absorption spectra extended into longer wavelength bands, thus enhancing optical absorption within the visible range. The XRD pattern showed that the nanorods crystallized in a monoclinic wolframite structure. The incorporation of Ag in CdWO4 decreases the luminescence emission intensity due to surface quenching effect. Luminescence lifetime measurement reveals the longer decay lifetime for Ag-doped CdWO4 nanorods due to the reduction of density of defects and deep trap state. The nonlinear optical behaviour of the nanorods was investigated by an Open aperture Z-scan technique at 532 nm using pulsed wave Nd: YAG laser. Experimental results showed that reverse saturable absorption is responsible for the observed nonlinearity in the nanorods. The addition of Ag can significantly improve the nonlinear absorption coefficient and optical limiting effects of CdWO4 which can be used as a potential material in nonlinear optical devices.

Bifunctional passivation by lewis-base molecules for efficient printable mesoscopic perovskite solar cells

Printable mesoscopic perovskite solar cells (PM-PSCs) possess notable merits in terms of cost-effectiveness, easy manufacturing, and large scale applications. Nevertheless, the absence of a hole transport layer contributes to the exacerbation of carrier recombination, and the defects between the perovskite and electron transport layer (ETL) interfaces significantly decrease the efficiency of the devices. In this study, a bifunctional surface passivation approach is proposed by applying a thioacetamide (TAA) surfactant on the mesoporous TiO2 interface. The results demonstrate that TAA molecules could interact with TiO2, thereby diminishing the oxygen vacancy defects. Additionally, the amino group and sulfur atoms in TAA molecules act as Lewis base to effectively passivate the uncoordinated Pb2+ in perovskite and improve the morphology of perovskite, and decrease the trap-state density of perovskite. The TAA passivation mechanism improves the alignment of energy levels between TiO2 and perovskite, facilitating electron transport and reducing carrier recombination. Consequently, the TAA-passivated device achieved a champion power conversion efficiency (PCE) of 17.86% with a high fill factor (FF) of 79.16% and an open-circuit voltage (VOC) of 0.971 V. This investigation presents a feasible strategy for interfacial passivation of the ETL to further improve the efficiency of PM-PSCs.

Stable and efficient perovskite solar cells based on tert-butyl peroxy-2-ethylhexanoate modified hole transport layer

Nowadays, perovskite solar cells (PSCs) have gained significant attention as one of the most promising new energy technologies. However, the application of spiro-OMeTAD, a highly efficient organic hole transport material (HTM) in PSCs, is limited due to its inherent weaknesses in conductivity and hole mobility. Here, a free radical initiator, tert-butyl peroxy-2-ethylhexanoate (TBPEH) is introduced to induce the rapid and controllable oxidation of Spiro. TBPEH-modified hole transport layer (HTL) produces more free radical cations, increasing carrier mobility and electrical conductivity. A smoother surface of the modified HTL improves the charge transport efficiency. Additionally, trap-state density and defect-induced nonradiative recombination of TBPEH-modified device are decreased. In addition, better energy level matching between the HTL and perovskite layer enables more efficient hole transport. Consequently, the PSC doped with TBPEH achieves a champion power conversion efficiency (PCE) of 24.33 %, surpassing the PCE of 21.72 % obtained from the pristine device. Furthermore, the TBPEH-modified devices show better stability. Storing in the atmosphere at 30 % relative humidity for 42 days, the TBPEH-modified device maintains 90.8 % of its highest efficiency, compared to the 73.8 % of the pristine one.

Energy Transfer-Dominated Quasi-2D Blue Perovskite Light-Emitting Diodes.

The bromide-chloride mixed quasi-two-dimensional (2D) perovskite, with a natural quantum well structure and tunable exciton binding energy, has gained significant attention for high-performance blue perovskite light-emitting diodes (PeLEDs). However, the relative importance of having a low trap state density or efficient exciton transfer for high-efficiency electroluminescence (EL) performance remains elusive. Here, two molecules with the benzoic acid group, sodium 4-fluorobenzoate (SFB) and 3,5-dibromobenzoic acid (DBA), are used to modulate the phase distribution and trap state to explore the effect between energy transfer and defect passivation. As a result, when the n = 1 phase is inhibited in both films, the DBA@SFB-modified perovskite films achieve a higher photoluminescence quantum yield (PLQY) than the SFB-modified perovskite films due to effective defect passivation. However, DBA@SFB-modified PeLEDs exhibit lower external quantum efficiency (EQE) compared to SFB-modified PeLEDs due to the poor exciton transfer between the low-dimensional phase. This demonstrates that passivation strategies may enhance photoluminescence through reducing nonradiative recombination, but the effect of phase distribution is pivotal for EL performance by efficient energy transfer in quasi-2D perovskites. Femtosecond time-resolved transient absorption measurements confirm the fastest carrier dynamics in SFB-modified perovskite films, further corroborating the above result. This work provides useful information about phase modulation and defect passivation for high-efficiency blue quasi-2D PeLEDs.

Eliminating the Burn-in Loss of Efficiency in Organic Solar Cells by Applying Dimer Acceptors as Supramolecular Stabilizers.

The meta-stable active layer morphology of organic solar cells (OSCs) is identified as the main cause of the rapid burn-in loss of power conversion efficiency (PCE) during long-term device operation. However, effective strategies to eliminate the associated loss mechanisms from the initial stage of device operation are still lacking, especially for high-efficiency material systems. Herein, the introduction of molecularly engineered dimer acceptors with adjustable thermal transition properties into the active layer of OSCs to serve as supramolecular stabilizers for regulating the thermal transitions and optimizing the crystallization of the absorber composites is reported. By establishing intimate π-π interactions with small-molecule acceptors, these stabilizers can effectively reduce the trap-state density (Nt) in the devices to achieve excellent PCEs over 19%. More importantly, the low Nt associated with an initially optimized morphology can be maintained under external stresses to significantly reduce the PCE burn-in loss in devices. This research reveals a judicious approach to improving OPV stability by establishing a comprehensive correlation between material properties, active-layer morphology, and device performance, for developing burn-in-free OSCs.

Influence of highly optimized charge carrier mobility and diverse physical features toward efficient organic solar cells

Organic solar cells (OSCs) exhibit potential in low-emissive photovoltaic (PV) technology by enhancing excitonic absorption, higher trap-assist recombination, lower excitons diffusion length (Ln,p), and carrier lifetime (τ n,p). The main challenge remains the asymmetric carrier mobility (μ n,p) of the organic absorbing layer (OAL) and various physical factors affecting efficiency (η). This effort has been explored through the attributes of different fullerene derivatives based on binary blends of OAL thickness that suggest new physical insights into the roles of several contributions in the PV performances under intense light illumination. The relationship between optimum mobility ratio (β) and lower trap-state density (Nt) of OAL in OSC structures for inclusive η has been collectively investigated. With a very thin OAL and pioneering transparent hole transport layers (HTLs) can significantly reduce recombination loss and enhance transparency, focusing on near-infrared band absorption and thin hetero-interface design for η and stability. The improved thin OALs, tunable absorption bands, and carrier selectivity address efficiency–transparency trade-offs and reproducibility concerns. The outcome revealed a stable η of 6.27% with a 250 nm thinnest OAL at a temperature of 300 K, which may be interpreted as a coupled framework for effective optimization strategies to accomplish balance between photogeneration and charge carrier recombination. Thus, the observed hypothetically analyzed results have verified the further optimization of OAL thickness for fabrication perspectives with a typical interpretation of ohmic contact.

Boosting MA-based two-dimensional Ruddlesden-Popper perovskite solar cells by incorporating a binary spacer

Two-dimensional Ruddlesden-Popper (2DRP) perovskite exhibits excellent stability in perovskite solar cells (PSCs) due to introducing hydrophobic long-chain organic spacers. However, the poor charge transporting property of bulky organic cation spacers limits the performance of 2DRP PSCs. Inspired by the A-site cation alloying strategy in 3D perovskites, 2DRP perovskites with a binary spacer can promote charge transporting compared to the unary spacer counterparts. Herein, the superior MA-based 2DRP perovskite films with a binary spacer, including 3-guanidinopropanoic acid (GPA) and 4-fluorophenethylamine (FPEA) are realized. These films (GPA0.85FPEA0.15)2MA4Pb5I16 show good morphology, large grain size, decreased trap state density, and preferential orientation of the as-prepared film. Accordingly, the present 2DRP-based PSC with the binary spacer achieves a remarkable efficiency of 18.37% with a VOC of 1.15 V, a JSC of 20.13 mA cm−2, and an FF of 79.23%. To our knowledge, the PCE value should be the highest for binary spacer MA-based 2DRP (n ≤ 5) PSCs to date. Importantly, owing to the hydrophobic fluorine group of FPEA and the enhanced interlayer interaction by FPEA, the unencapsulated 2DRP PSCs based on binary spacers exhibit much excellent humidity stability and thermal stability than the unary spacer counterparts.

Perovskite Single Crystals by Vacuum Evaporation Crystallization.

Perovskite single crystals have attracted tremendous attention owing to their excellent optoelectronic properties and stability compared to typical multicrystal structures. However, the growth of high-quality perovskite single crystals (PSCs) generally relies on temperature gradients or the introduction of additives to promote crystal growth. In this study, a vacuum evaporation crystallization technique is developed that allows PSCs to be grown under extremely stable conditions at constant temperature and without requiring additives to promote crystal growth. The new method enables the growth of PSCs of unprecedented quality, that is, MAPbBr3 single crystals that exhibit an ultranarrow full width at half maximum of 0.00701°, which surpasses that of all previously reported values. In addition, the MAPbBr3 single crystals deliver exceptional optoelectronic performance, including a long carrier lifetime of 1006ns, an ultralow trap-state density of 3.67 × 109 cm-3, and an ultrahigh carrier mobility of 185.86 cm2 V-1 s-1. This method is applicable to various types of PSCs, including organic-inorganic hybrids, fully inorganic structures, and low-dimensional structures.

The physical meaning of time-delayed collection field transients from disordered devices

Charge carrier mobility and recombination determine the performance of many opto-electronic devices such as solar cells, sensors, and light-emitting diodes. Understanding how these parameters change as a function of material choice, charge carrier density, and device geometry is essential for developing the next generation of devices. The time-delayed collection field (TDCF) technique is becoming a widely used method to measure both recombination and carrier transport with values derived from this method being widely reported for many material systems. However, most novel materials are highly disordered with a high density of trap states and standard TDCF theory neglects the influence of these states. In this work, we examine how reliable TDCF can be as a measurement technique when the device contains significant energetic disorder. We identify regimes where the results can be relied upon and where the results should be taken with more caution. Finally, we provide simple and easy to use experimental tests to help the experimentalist decide whether the physical processes are dominated by trap states.

Effect of Fe Doping Profile on Current Collapse in GaN-based RF HEMTs.

Using computer-aided design (TCAD) simulation, the impact of the Fe doping profile, including concentration, decay rate, and depth of the doping region on current-collapse magnitude (▵CC) in 0.5-μm gated GaN-based high electron mobility transistors (HEMTs) is systematically investigated. Accurate simulation models are established and developed to facilitate the fabrication of electronics. It is elucidated that the intricate interplay between trapping and de-trapping of Fe-related traps at the gate-drain edge is responsible for current collapse. The concentration and decay rate of the doping region have a more significant impact on current collapse than the depth. Increased trap state density near two-dimensional electron gas (2DEG) channel caused by deep-level acceptors would boost ▵CC. However, a minor dynamic reduction in 2DEG density (nT) induces a relatively small ▵CC. By adjusting the concentration, decay rate, and depth of the doping region, ▵CC of GaN-based Radio Frequency (RF) HEMTs can be reduced by approximately 50.3 %. The optimized distribution of Fe doping discussed in this work helps to prepare GaN-based RF HEMTs with a limited current collapse effect.

Enhanced performance and stability of quasi two-dimensional perovskite solar cells by conductive phosphine oxide

Recently, quasi two-dimensional (quasi-2D) perovskite solar cells (PSCs) have been paid more attentions due to their good stability. While the efficiency of 2D PSCs is lower than that of 3D ones due to their poor charge transport property. Here we introduced a conductive material of 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13) into the precursor solution of quasi-2D perovskite GAMA5Pb5I16 (GA = guanidinium, MA = methylammonium) to improve the charge transfer property. The efficiency of the PSCs with SPPO13 is increased to 18.58 % from 16.97 % for the control, which is resulting from the enlarged grain size, increased electrical conductivity, and reduced trap state density induced by the addition of SPPO13. Moreover, the additon of SPPO13 improves the stability of the devices. The normalized PCE of the devices with SPPO13 keeps 90 % of the original value after aging for 30 days in atmosphere. This study provides a novel way to enhance the charge transfer property of 2D perovskite by conductive material for enhanced efficiency and stability of quasi-2D PSCs.

Revealing the stability origins of 596 days-humidity-stable semitransparent perovskite solar cells

Hybrid organic–inorganic perovskites (HOIPs) hold promise in the field of optoelectronics due to their excellent photoelectric conversion efficiency. However, the ion migration and hygroscopicity of these perovskite solar cells need to be addressed. Here, we presented semitransparent perovskite solar cells (ST-PSCs) using hole transport layer (HTL) combined with polyaniline (PANI) to stabilize HTL/perovskite interface, achieving a humidity durability (RH, 50%–90%) for 596 days (14304 h) without encapsulation. Moreover, the decrease in hydrolysis products (LiF) showed the interaction between PANI with the additives in HTL dramatically inhibited the water uptake and corrosion on MAPbI3 layer. The PANI modified samples had a higher I/Pb ratio and lower trap state density, which indicated the passivation effect of PANI on the uncoordinated Pb2+ and iodine vacancies. Subsequently, PANI successfully stabilized the interface and perovskite by inhibiting the formation of Pb0 and Au migration as long period storage. This work presented an interfacial design to develop HOIP in air with high humidity stability.

Reduced trap-density and boosted performance of CH3NH3PbI3 solar cells by 1-Pentanethiol enhanced anti-solvent washing route

Performance and the stability of the perovskite-based photovoltaic devices are directly linked to existing trap-states or defect profiles at the surface and/or in the bulk of perovskite layers. Hence identification of stemming the defects during perovskite formation is crucial for achieving superior and long-lasting performances. Here, we present the effect of 1-Pentanethiol incorporation into the one-step deposition of perovskite layers. A feasible glove box-free route results in high-quality CH3NH3PbI3 layers under highly humid conditions (RH > 50%) but at low temperatures (T < 18 °C). 1-Pentanethiol addition into the washing solvent leads to the refinement of I/Pb stoichiometry, elimination of the iodide deficiencies, and reduction of the trap-state densities. Consequently, a precise amount 1-Pentanethiol addition enhances photovoltaic performances, resulting in a 54% PCE improvement for CH3NH3PbI3-based inverted solar cells.

Ni-doped ZnO nanoparticles derived by the sol–gel method: structural, optical, and magnetic characteristics

A simple sol–gel route was used for preparation of Ni doped ZnO nanoparticles (Ni:ZnO) with 0, 3, 5, 7 and 9 wt% of Ni to Zn precursor salts. Structural, optical and room temperature magnetic properties were studied. The prevailing structural arrangement that observed in all the examined samples is the wurtzite structure of ZnO. Nevertheless. The orderly development of NiO rock-salt phase on at the surfaces of ZnO nanoparticles upon Ni doping is confirmed by Rietveld refinements. The ZnO wurtzite structure is dominant in all the samples as confirmed by both XRD and Raman results. The average crystallite size showed almost no change upon Ni doping. RTFM behavior is being exhibited for all Ni:ZnO nanoparticles that is demonstrating using of ZnO as the host structure allowed for the efficient formation of DMS. The Near-infrared solar reflectance (RNIR) and the visible solar reflectance (RVis) values reduced with increasing of the Ni content as a result of the rising in density of trap states upon Ni doping of the samples. Kubelka-Munk method was used to determine the energy band gap (Eg) and band Urbach energy (EU) tails from diffused reflectance (DRS) results. The obtained energy gap values were very close in all studied samples with an average value of energy gap is 3.164 eV. The EU values raise from 123 to 173 meV upon increasing of Ni content in the samples in agreement with lower values of RNIR and RVis and their effect on optical performance of the investigated samples in applications.

Improved Performance and Stability of Quasi-Two-Dimensional Perovskite Solar Cells by Post-treatment with Acetaminophen.

Recently, perovskite solar cells (PSCs) based on quasi-two-dimensional (quasi-2D) perovskites have drawn more attention due to their excellent stability, although their efficiencies are still lower than those of 3D ones. Here we applied post-treatment of 2D perovskite GAMA5Pb5I16 (GA = guanidinium, MA = methylammonium) films with acetaminophen (AMP) to improve their performance. The efficiency of the solar cells with 2 mg/mL AMP post-treatment increased to 18.01% from 16.72% for those without post-treatment. The efficiency improvement results from the enlarged grain size, reduced trap state density, and better energy level matching after AMP post-treatment. In addition, the stability of the solar cells is improved. The solar cells with AMP post-treatment maintain 91% of the original power conversion efficiency value after aging for 30 days in the atmosphere. This work opens a new approach for the efficiency and stability enhancement of quasi-2D PSCs.

Reduced Surface Trap States of PbS Quantum Dots by Acetonitrile Treatment for Efficient SnO2-Based PbS Quantum Dot Solar Cells.

The solution-phase ligand-exchange strategy offers a simple pathway to prepare PbS quantum dots (QDs) and their corresponding solar cells. However, the production of high-quality PbS QDs with reduced surface trap state density for efficient PbS QD solar cells (QDSCs) still faces challenges. As the hydroxyl group (-OH) has been demonstrated to be the primary source of the surface trap states on PbS QDs in the general oleic acid method, here, we present an effective and facile strategy for reducing the surface -OH content of PbS QDs by using acetonitrile (ACN) as precipitant to wash the surface of QDs, which significantly decreases the trap state density and enables the preparation of superior PbS QDs. The resulting solar cell with an ITO/SnO2/n-PbS/p-PbS/Au structure obtained an improved photoelectric conversion efficiency (PCE) from 8.53 to 10.49% with an enhanced air storage stability, realizing a high PCE for SnO2-based PbS QDSCs.

Investigation of atomic layer deposition methods of Al2O3 on n-GaN

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH3 or H2 plasma pre-treatment is studied. Planar MIS capacitors are used to investigate the electrical properties and robustness of the gate insulators. In vacuo x-ray photoelectron spectroscopy (XPS) is used to study the changes in chemical composition after every surface treatment. XPS shows that all plasma pre-treatments efficiently remove all carbon contamination from the surface, but only NH3 plasma is observed to additionally remove the native oxide from the n-GaN surface. The water precursor step in the ThALD process does not completely remove the CH3 ligands of the trimethylaluminum precursor step, which might electrically be associated with a reduced forward bias robustness. The O2 plasma step in the PEALD process is associated with the removal of carbon and a tremendous increase of the O content in the GaN surface region. Electrically, this strongly correlates to an enhanced forward bias robustness and an increased forward bias hysteresis, respectively. The ThALD/PEALD stack method mitigates the shortcomings of both ALD processes while maintaining its advantages. Electrical measurements indicate that the stack method alongside NH3 plasma pretreatment provides the best characteristics in terms of hysteresis, threshold voltage, forward bias robustness, and interface trap density of states.

Enhancing energy transfer via inhibition defect density of coupled quasi-2D perovskite layers for efficient sky-blue light-emitting diodes

Metal halide perovskite materials have emerged as unique and promising candidates for full-color display and solid-state lighting applications. While substantial progress has been made in developing green, red and near-infrared perovskite light-emitting diodes (PeLEDs) with external quantum efficiency (EQE) approaching their theoretical upper limit, the performance of blue PeLEDs is still lagging far behind. Here, synergistic long and short hydrobromide chains are strategically introduced into the perovskite to regulate composition and dimensional engineering for promoting coupled quasi-two-dimensional perovskite thin films with enhanced photoluminescence quantum yield (PLQY) and reduced trap state density. The inclusion of the hydrobromide can also simultaneously passivate the perovskite defects by reducing nonradiative recombination loss and increasing the formation of n >1 phases for efficient exciton energy transfer. Encouraged by these findings, we finally demonstrate the most stable sky-blue PeLEDs to date with a half-lifetime of 515 min at high external quantum efficiency (EQE＞10 %), emitting at 486 nm with a peak EQE of up to 15.6 %.