Reduced Trap Density Research Articles

Effects of Microwave Annealing on High-k Dielectric HfO₂ Thin Films.

In this study, the effect of microwave annealing (MWA), which has been attracting considerable attention as a low-thermal-budget method, on the high-k materials required for next-generation ultrahigh-integration semiconductor devices is investigated. Metal-oxide-semiconductor (MOS) capacitor-structure devices are fabricated using HfO₂ films, which are typical high-k materials; post deposition annealing (PDA) is then performed by MWA and compared to conventional thermal annealing (CTA). The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the fabricated HfO₂ MOS capacitors are measured to evaluate the leakage current, time-zero dielectric breakdown (TZDB), time-dependent dielectric breakdown (TDDB), and frequency dispersion. Compared to the as-deposited state, the PDA-treated HfO₂ dielectric film is found to have reduced trap density, improved dielectric breakdown, frequency dispersion, and lifetime. In particular, the high-k HfO₂ thin films treated with MWA exhibit excellent surface properties, including a low root-mean square roughness (Rq) of 0.141 nm, thickness of 49.3 nm, and low inter face trap density (Dit) of 7.82 × 1012 cm-2eV-1 and electrical properties such as a high breakdown electric field (EBD) of 10 MV/cm, and high breakdown to charge (QBD) of 1,000 C/cm². As a result, the MWA-treated high-k HfO₂ thin films are smoother and denser than the CTA-treated thin films, and show better electrical properties and reliability, suggesting that the defects at Si the interface as well as in the high-k films are effectively removed.

Morphology control of perovskite in green antisolvent system for MAPbI3-based solar cells with over 20% efficiency

Solvent engineering has been considered a reliable process for the fabrication of pinhole-free and highly crystalline perovskite thin films. Recently, green solvents have received immense attention, as the toxic antisolvents used in the conventional fabrication process cause environmental and health hazards. In this regard, ethyl acetate (EA) is a promising environmentally friendly antisolvent. Here, we present the fabrication of perovskites with controlled morphologies by changing the composition of the perovskite and the volume of EA in ambient humidity. The incorporation of [HC(NH2)2]PbIBr2 into a CH3NH3PbI3 matrix results in a grain size up to ~1.5 μm. This induces a considerable reduction of trap density, leading to the suppression of charge recombination and, consequently, improvement in the photoluminescence characteristics. The resulting power conversion efficiencies (PCEs) of the optimized devices are 20.93% and 19.51% for active areas of 0.12 cm2 and 0.7 cm2, respectively. The reduced diffusion of moisture along grain boundaries improves device stability. Further, by virtue of the excellent humidity resistance of EA, the film morphologies obtained at high relative humidity (50%) are similar to those obtained under dry conditions, exhibiting an impressive PCE of 20.11%. We believe that our optimized fabrication process using EA can be extended to other green antisolvent systems.

Well-grown low-defect MAPbI3–xClx films for perovskite solar cells with over 20% efficiency fabricated under controlled ambient humidity conditions

The conventional [(CH3NH3I (MAI)):PbI2:PbCl2 = 3 : 0: 1 [abbreviated as (3:0:1)]] precursor solution is known to result in CH3NH3PbI3–xClx films with large grain sizes when processed in an inert atmosphere, but it gives non-uniform perovskite films containing lots of voids and cracks when processed in ambient air. Furthermore, a dramatically longer annealing time (usually 100 min) is required for these films (3:0:1) due to the slow formation of the MAPbI3 phase via MACl loss, which is not conducive to perovskite film formation under ambient conditions due to perovskite degradation upon long exposure to moisture. Pure MAPbI3 films can be formed very rapidly from (1:1:0) (MAI:PbI2:PbCl2 = 1 : 1: 0) solution within a short annealing time, but they show small grain sizes and poor film quality. This work demonstrated that a fractional substitution of PbI2 with PbCl2 in the ([MAI]:[PbI2] = 1 : 1) precursor solution has a significant influence on film morphology and quality in terms of crystallization rate, grain size, crystallinity, and trap density of the formed perovskite film. Perovskite films can be formed with 5-min annealing at 100 °C from the precursor (MAI: PbI2:PbCl2 = 1: 0.8 : 0.2) processed in ambient air (humidity, 20% RH), exhibiting more uniform, increased grain size and higher film quality with reduced trap densities compared to film (1:1:0), thus leading to significantly improved power conversion efficiency (PCE), from 16.7% for perovskite solar cells (PrSCs) based on film (1:1:0) to 20.04% for the cell based on film (1:0.8:0.2). Further, the effects of R (R = [MAI]/[PbI2+PbCl2]) on morphology, hole mobility, carrier lifetime and efficiency of PrSCs were systematically and thoroughly investigated. This study found that MAPbI3–xClx at R = 1 can enable the highest hole mobility and longest carrier lifetime, thus giving the best performance at R = 1.

Aminosilane‐Modified CuGaO2 Nanoparticles Incorporated with CuSCN as a Hole‐Transport Layer for Efficient and Stable Perovskite Solar Cells

AbstractHerein, solution‐processible inorganic hole‐transport layer (HTL) of a perovskite solar cell that consists of CuGaO2 nanoparticles and CuSCN, which leads to an improved device performance as well as long‐term stability, is reported. Uniform films of CuGaO2 are prepared by first treating CuGaO2 nanoparticles with aminosilane that leads to well‐dispersed CuGaO2 solution, followed by spin‐coating of the suspension. Subsequent spin‐coating of CuSCN solution onto the CuGaO2 forms a smooth HTL with excellent coverage and electrical conductivity. Comparing to the reference device with CuSCN HTL, the CuGaO2/CuSCN device improves carrier extraction and reduces trap density by ≈40%, as measured by photoluminescence and capacitance analysis. Excellent thermal stability is also demonstrated: ≈80% of the initial efficiency of the perovskite solar cells with the CuGaO2/CuSCN HTL is retained after 400 h under 85 °C/85% relative humidity environment.

Extending the Photovoltaic Response of Perovskite Solar Cells into the Near-Infrared with a Narrow-Bandgap Organic Semiconductor.

Typical lead-based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near-infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR-chromophore that is also a Lewis-base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis-basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination-induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one-sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR-harvesting PSCs.

Triamine‐Based Aromatic Cation as a Novel Stabilizer for Efficient Perovskite Solar Cells

AbstractOperational stability of perovskite solar cells has been a challenge from the beginning of perovskite research. In general, humidity and heat are the most well‐known degradation sources for perovskites, requiring ideal design of perovskite chemistry to withstand them. Although triple‐cation perovskite (Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3) has been already introduced as the stable perovskite material, the high reactivity of methylammonium and formamidinium in the cation sites demands further modification. Herein, 1,2,4‐triazole is suggested as an effective cation solute to improve the performance and stability of perovskite solar cells. 1,2,4‐Triazole is an aromatic cation with low dipole moment that is stable under humidity and heat. It also possesses three nitrogen atoms, forming additional hydrogen bonds in the lattice, stabilizing the material. In this study, the solar cell utilizing 1,2,4‐triazole alloying achieves a power conversion efficiency of 20.9% with superior stability under extreme condition (85 °C/85% of relative humidity (RH), encapsulated) for 700 h. The 1,2,4‐triazole‐alloyed perovskite exhibits reduced trap density and film roughness and enhanced carrier lifetime with electrical conductivity, suggesting an ideal perovskite structure for efficient and stable optoelectronic applications.

Thiol and Halometallate, Mutually Passivated Quantum Dot Ink for Photovoltaic Application.

Tunable-band-gap colloidal QDs are a potential building block to harvest the wide-energy solar spectrum. The solution-phase surface passivation with lead halide-based halometallate ligands has remarkably simplified the processing of quantum dots (QDs) and enabled the proficient use of materials for the development of solar cells. It is, however, shown that the hallometalate ligand passivated QD ink allows the formation of thick crystalline shell layer, which limits the carrier transport of the QD solids. Organic thiols have long been used to develop QD solar cells using the solid-state ligand exchange approach. However, their use is limited in solution-phase passivation due to poor dispersity of thiol-treated QDs in common solvents. In this report, a joint passivation strategy using thiol and halometallate ligand is developed to prepare the QD ink. The mutually passivated QDs show a 50% reduction in shell thickness, reduced trap density, and improved monodispersity in their solid films. These improvements lead to a 4 times increase in carrier mobility and doubling of the diffusion length, which enable the carrier extraction from a much thicker absorbing layer. The photovoltaic devices show a high efficiency of 10.3% and reduced hysteresis effect. The improvement in surface passivation leads to reduced oxygen doping and improved ambient stability of the solar cells.

Charge carrier dynamics study and morphology optimization in solvent annealed CH3NH3PbI3 perovskite for air processed stable solar cell application

We report room temperature excitation intensity dependent charge carrier dynamics in solvent-annealed organo-metal halide perovskite semiconductor CH3NH3PbI3 thin film using steady state and time resolved photoluminescence studies. The average lifetime of these solvent annealed films does not vary significantly with excitation fluence confirming the formation of almost defect free perovskite crystallites. The photocarrier dynamics are well described by a simple rate equation. The local reduction of shallow trap densities as observed in PL dynamics was correlated with scanning electron microscopy (SEM) and X-ray spectroscopy. The observed photophysical investigation and morphology study suggests that anti-solvent method as a potential approach for CH3NH3PbI3 perovskite solar cell fabrication under ambient condition. A prototype all air processed anti-solvent method based stable perovskite device yielded the efficiency of 7.54% with JSC value of 14.21 mA/cm2 and VOC of 810 mV.

Enhanced n-type β-Ga2O3 (2¯01) gate stack performance using Al2O3/SiO2 bi-layer dielectric

High performance gate stacks are critically needed for the development of β-Ga2O3 power transistors. Significant improvement in the performance of β-Ga2O3 gate stacks is demonstrated in this work by using SiO2 as an interlayer dielectric between Al2O3 and β-Ga2O3. The presence of SiO2 results in an 800× reduction in gate leakage current along with a 1.7× increase in the electric breakdown field. Additionally, the capacitance-voltage characteristics show an increase in flat band voltage (from 0.74 V to 3.25 V) that can enable normally off power transistors. The lowest interface trap density (Dit) of 5.1 × 1010 cm−2 eV−1 for the SiO2/β-Ga2O3 interface has been demonstrated through the use of a piranha clean before SiO2 deposition on β-Ga2O3 for the first time. Reduction (8×) in Dit, hysteresis (from 0.17 V to 0.05 V), and border trap density indicate substantial improvement in the quality of the β-Ga2O3/gate dielectric interface for the Al2O3/SiO2 bilayer stack as compared to only Al2O3.

Impact of metal gate electrodes on electrical properties of Y2O3/Si0.78Ge0.22 gate stacks

Electrical properties of Y2O3/SiGe metal-oxide-semiconductor (MOS) capacitors with Al, Au, W and TiN gate electrodes have been evaluated in order to study the impact of the metal gate electrodes on Y2O3/SiGe interface properties. It is found that MOS capacitors with TiN gate electrodes can provide better Y2O3/SiGe MOS interfaces than those with Al, Au or and W gate stacks after post metallization annealing (PMA) at each optimized temperature. The physical origins of interface trap density (Dit) reduction are examined from the viewpoint of the composition and the quality of interfacial layers (ILs).

Origins of Efficient Perovskite Solar Cells with Low-Temperature Processed SnO2 Electron Transport Layer

Recently, SnO2 has been noticed as a promising material for electron-transport layer of planar perovskite solar cells. SnO2 layer presents advantages of low-temperature processability and high power-conversion efficiency, and understanding the correlations between the SnO2 properties and device performance will provide a key to realize more efficient perovskite solar cells. Herein, uniform electron-transport layer using SnO2 nanoparticles is fabricated, and the effect of annealing on the solar-cell performance is discussed. Solar cells with low-temperature processed SnO2-nanoparticle layer (below 120 °C or even at room temperature) exhibit desirable short-circuit current, open-circuit voltage, and fill factor with the highest efficiency of 19.0%. Using atomic force microscopy and ultraviolet photoelectron spectroscopy, both great surface uniformity and favorable band alignment of low-temperature processed SnO2 layer have been observed, which are responsible for the device performance. Furthermore, deep electronic-trap states at the SnO2/perovskite interface are investigated via impedance analysis. Compared to the cells processed over 160 °C, low-temperature processed cells exhibit trap states shifting toward the bandedge and reduced trap density, verifying that controlling the interfacial trap states holds a dominance on the open-circuit voltage and is a critical requisite to enable efficient perovskite solar cells. These less-defective solar cells fabricated below 120 °C show high thermal stability, suggesting further commercial applications.

Interfacial charge transport in MAPbI3 perovskite on ZnO

Abstract Interfacial charge transport of methylammonium lead iodide (MAPbI3) perovskite close to the ZnO nanocrystal (NC) interface was probed using a field effect transistor (FET) device in which a perovskite/ZnO bilayer was formed between the source and drain electrodes. Through electric field gating the role of the underlying ZnO NC layer on the charge transport in the MAPbI3 perovskite was elucidated. In the presence of the ZnO NC layer, the FET was turned on earlier indicating that the carrier concentration at zero gate voltage is higher than that without a ZnO NC layer. This is attributed to reduced trap density by passivating the interfacial traps at the SiO2/perovskite interface. No significant change was observed for the FET mobility depending on the presence of the ZnO NC layer.

Enhanced Charge Transport in 2D Perovskites via Fluorination of Organic Cation.

Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky spacer cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluorophenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with better aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained a power conversion efficiency of >13% for (F-PEA)2MA4Pb5I16-based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.

Limiting factor of performance for solution-phase ligand-exchanged PbS quantum dot solar cell

Despite great increases in photovoltaic performance due to the insertion of ligand-exchanged PbS quantum dot layer via 1,2-ethanedithiol (EDT), detailed carrier transport characteristics of solar cells with solution-phase ligand-exchanged PbS quantum dot (QD) still remains unknown. In this paper, relaxation time, recombination resistance, chemical capacitance, carrier lifetime, carrier diffusion length, and defect density corresponding to the depletion width were measured through electrochemical impedance spectroscopy using the Schottky diode model to understand limiting factors for photovoltaic performance. The activation energy, which is estimated from the temperature-dependence of resistance, was reduced from 0.06 eV to 0.02 eV by inserting the PbS-EDT layer, leading to the improved photovoltaic performance. We also evaluated QD solar cells with varying thicknesses of PbS QD layers to investigate the impedance analysis and photovoltaic measurements. The depletion width as well as the carrier diffusion length was larger than the physical thickness of the layer, and this indicates that an increased short-circuit current density can be realized at the thick PbS QD layer due to the resulting large absorbance. Furthermore, the reduced trap density of the QD layer improves the open circuit voltage and fill factor. As a result, a high photoconversion efficiency of 9.41% was achieved.

Modulation of recombination zone position for quasi-two-dimensional blue perovskite light-emitting diodes with efficiency exceeding 5%

In recent years, substantial progress has been made in developing perovskite light-emitting diodes with near-infrared, red and green emissions and over 20% external quantum efficiency. However, the development of perovskite light-emitting diodes with blue emission remains a great challenge, which retards further development of full-color displays and white-light illumination based on perovskite emissive materials. Here, firstly, through composition and dimensional engineering, we prepare quasi-two-dimensional perovskite thin films with improved blue emission, taking advantages of reduced trap density and enhanced photoluminescence quantum yield. Secondly, we find a vertically non-uniform distribution of perovskite crystals in the PEDOT:PSS/perovskite hybrid film. Through modulating the position of the recombination zone, we activate the majority of quasi-two-dimensional perovskite crystals, and thus demonstrate the most efficient blue perovskite light-emitting diode to date with emission peak at 480 nm, record luminance of 3780 cd m−2 and record external quantum efficiency of 5.7%.

Influence of OMMT on Surface Breakdown of Epoxy Resin in Liquid Nitrogen

Epoxy resin is employed as the main insulation materials or support components in the high-temperature superconducting (HTS) system. But surface breakdown of epoxy resin threatens the safe operation of the HTS system. Introducing organic montmorillonite (OMMT) into epoxy resin is an effective way to improve the resistance of surface breakdown. The OMMT with fraction of 0, 1, 3, 5, and 7 wt% is filled into the epoxy resin to investigate the conductivity properties, partial discharge, and trap distribution characteristics. The results shows that the conductivity of epoxy resin is reduced by incorporating OMMT into the matrix, and the smallest value is obtained with the filler content of 5 wt%. The modified partial discharge (PD) properties are related to trap distribution of epoxy resin/OMMT composites, which is with larger trap energy level and trap density than the pure epoxy resin one in liquid nitrogen. However, the PD resistance is decreased with filler content increasing from 5 wt% to 7 wt%, which is associated with the reducing trap energy level and trap density induced by the overlapping of OMMT. The research establishes the foundation for design and development of advanced polymer nanocomposites for HTS system.

One-step-spin-coating route for homogeneous perovskite/pyrrole-C60 fullerene bulk heterojunction for high performance solar cells

To overcome obstacle of unbalanced hole and electron diffusion behavior in perovskites, a novel type of rationally designed bulk-heterojunction with C60-fullerene electron extraction material coupled with perovskite is for the first time reported. A facile strategy is developed for pyrrole to modify C60 to realize co-soluble of C60 and perovskites, achieving co-deposition of perovskite/pyrrole-fullerene bulk-heterojunction film via a one-step-spin-coating process. The unique perovskite/pyrrole-fullerene bulk-heterojunction exhibits increased mobility, quenched fluorescence intensity, decreased electron lifetime, large recombination resistance and reduced trap density of states, greatly enhancing charge extraction and transfer ability, thus boosting photoelectric conversion efficiency of solar cells. Introducing pyrrole-fullerene can effectively reduce grain boundaries and electron trap density to suppress electron-hole recombination. The enlarged interfacial area between pyrrole-fullerene and perovskite is essential for facilitating extraction of more free electrons from perovskite to fullerene electron acceptor, resulting in a balance of charge extraction and transport. A maximum power conversion efficiency of 18.9% with virtually no hysteresis for the bulk-heterojunction based perovskite solar cells with an optimized concentration of bulk-heterojunction can be obtained. The present strategy for high-performance bulk-heterojunction perovskite solar cells can be potential for industrial photovoltaic applications for its facile and low-cost process route.

Bright perovskite light-emitting diodes with improved film morphology and reduced trap density via surface passivation using quaternary ammonium salts

Abstract Organic-inorganic hybrid perovskite light-emitting diodes (PeLEDs) have emerged as one of the promising candidates for display and illumination devices with low-cost and high color purity. However, PeLEDs exhibit bad film uniformity and high defect density, which lead to poor device performance. Herein, we introduce an additive engineering strategy employing quaternary ammonium salts, namely benzyltrimethylammonium bromide (BTABr), phenyltrimethylammonium bromide (PTABr) and benzyltrimethylammonium chloride (BTACl), in the precursor solution to prepare uniform and dense MAPbBr3 perovskite films with reduced grain sizes. These quaternary ammonium salts, which have cations containing a bulky benzyl unit, are more effective at reducing the grain sizes of MAPbBr3 perovskites. More importantly, the BTABr additive performs a dual-passivation effect to the MAPbBr3, effectively reducing the trap density within the perovskite layer as well as the proportion of non-radiative recombination of PeLEDs, and balancing the charge injection. Consequently, PeLEDs incorporating a BTABr-modified MAPbBr3 layer exhibit a reduced turn-on voltage from 4.2 to 2.6 V, and the maximum luminance and current efficiency are enhanced from 3490 Cd/m2 and 0.24 Cd/A of the reference device to 12026 Cd/m2 and 1.27 Cd/A, respectively.

Bi-functional additive engineering for high-performance perovskite solar cells with reduced trap density

The bi-functional additive of ammonium benzenesulfonate enables the fabrication of low-defect and high-performance perovskite solar cells.

Trap passivation and efficiency improvement of perovskite solar cells by a guanidinium additive

Incorporation of an effective additive 1,3-diaminoguanidine monohydrochloride in MAPbI3 film can increase the grain size and reduce trap density, consequently increasing the carrier lifetime and reducing charge recombination, leading to an improved efficiency from 19.1% to 20.3% in a p–i–n structured solar cell.