Low Trap-state Density Research Articles

MAPbI3 Single Crystals Free from Hole-Trapping Centers for Enhanced Photodetectivity

Perovskite single crystals (PSCs) are considered the next breakthrough in optoelectronics research due to their free-grain boundary and much lower density of trap states compared to those of their ...

P‐6.6: Effects of lightly Doped Drain Structure in n‐channel ELA Bridged‐Grain Poly‐Si TFTs

We lead the lightly doped drain (LDD) structure into the bridged‐grain (BG) poly‐Si thin film transistor (TFT). This combined structure can restrain the gate induced drain leakage current (GIDL) and the electrical characteristics shows not much sacrifice. A high Ion/Ioff current ratio 3.79×108 is achieved. Besides, the device reliability is improved significantly. Because the lateral electrical field decreases by LDD, lower trap state density is generated in the SiO2/poly‐Si interface and the grain boundaries of poly‐Si near the surface conduction channel, resulting in the stable on‐state drive current (Ion) and drain saturation voltage (Vdsat). All test results indicate that the BG poly‐Si TFT with a LDD structure will work well and has a great potential for system‐on‐panel application.

Investigation of annealed, thin(∼2.6 nm)-Al2O3/AlGaN/GaN metal-insulator-semiconductor heterostructures on Si(111) via capacitance-voltage and current-voltage studies

Annealed, thin(∼2.6 nm)-Al2O3/AlGaN/GaN metal-insulator-semiconductor (MIS) heterostructures on Si(111) are fabricated and studied via capacitance–voltage (C–V) measurements to quantify densities of fast and slow interface trap states and via current–voltage (I–V) measurements to investigate dominant gate current leakage mechanisms. Dual-sweep C–V measurements reveal small voltage hysteresis (∼1 mV) around threshold voltage, indicating a low density of slow interface trap states of ∼109 cm−2. Frequency-dependent conductance measurements show fast interface trap state density ranging from 8 × 1012 to 5 × 1011 eV−1 cm−2 at energies from 0.275 to 0.408 eV below the GaN conduction band edge. Temperature-dependent I–V characterizations reveal that trap-assistant tunneling (TAT) dominates the reverse-bias carrier transport while the electric field across the Al2O3 ranges from 3.69 to 4.34 MV cm−1, and the dominant Al2O3 trap state energy responsible for such carrier transport is identified as 2.13 ± 0.02 eV below the Al2O3 conduction band edge. X-ray photoelectron spectroscopy measurements on Al2O3 before and after annealing suggest an annealing-enabled reaction between Al-O bonds and inherent H atoms. Overall, we report that annealed, thin-Al2O3 dielectric is an effective (Al)GaN surface passivation alternative when minimizing passivation-associated parasitic capacitance is required, yet non-ideal for significantly suppressing gate leakage current in MIS structures due to the governing TAT carrier transport mechanism.

Increasing responsivity and air stability of PbS colloidal quantum dot photoconductors with iodine surface ligands

PbS colloidal quantum dots (QDs) are a promising material for the realization of low-cost, high-responsivity near-infrared photodetectors. Previously reported attempts showed high responsivity but a fast performance decay in air-exposed devices, demanding encapsulation of the photodetectors. Conversely, devices with very high air stability have been demonstrated but the low trap-state density hinders photoconductive gain and reduces overall responsivity. In this paper, photoconductive devices prepared with partially tetrabutylammonium iodide exchanged PbS QDs are presented with enhanced air stability and high responsivity at low voltage, low optical power.

Enhanced efficiency and stability of inverted perovskite solar cells by carbon dots cathode interlayer via solution process

Abstract Interfacial engineering has been widely recognized as among the most effective approaches to deriving high performance perovskite solar cells (PSCs). [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and silver (Ag) were extensively used as the electron transport material and cathode material in the inverted device. However, the energy level mismatch between PC61BM and Ag cathode has a possibility to severely affect the electron extraction and transportation. Hereby, only alcohol-soluble carbon dots (CDs) were synthesized and applied as a cathode interlayer (CIL) through a simple solution-process to make alteration to the interface between PC61BM and Ag in inverted PSCs, the PC61BM/CDs layer was generated with low trap-state density, high electrical conductivity, high electron mobility and appropriate energy level. Finally, the inverted device with the CDs CIL exhibited a power conversion efficiency (PCE) of 17.34% with the steady-state output PCE reaching 16.96%. The stability of devices was also enhanced due to the hydrophobicity of the CDs cathode interlayer. The efficiency of inverted PSCs with PC61BM/CDs layer remained 70% of the initial PCE under dry condition after 40 days versus only 20% of the initial PCE for the PCBM ETL without CDs.

Self‐Powered X‐Ray Detector Based on All‐Inorganic Perovskite Thick Film with High Sensitivity Under Low Dose Rate

Hybrid perovskite materials have been one of the brightest spotlights in the radiation detection research fields due to their higher carrier mobility and lower trap state density in recent years. In this paper, the CsPbBr3 microcrystal thick film is successfully prepared using a low‐cost facile solution synthesis method. The properties of samples are characterized by field‐emission scanning electron microscope (FE‐SEM), X‐ray diffraction (XRD), and UV‐vis diffuse reflectance spectrum (UV‐vis DRS). In addition, X‐ray detectors based on the CsPbBr3 microcrystal thick film are fabricated and the photoelectric properties of the detectors are further improved through the multiple‐times dissolution–recrystallization method. An improved sensitivity of 470 µC Gyair−1 cm−2 is obtained for X‐ray photodetector based on recrystallized CsPbBr3 microcrystal thick film at zero bias under a remarkably low dose rate (0.053 µGyair s−1). This sensitivity is over 20 times higher than that of α‐Se X‐ray detectors working at a much higher field of 10 V µm−1. The current investigation can provide an efficient low‐cost and facile method to fabricate self‐powered X‐ray detectors based on CsPbBr3 microcrystal thick film that work normally at low radiation dose rate.

(Invited) Perovskite Single Crystals with Free-Hole Trapping Centers for Photodetector and Solar Cell Applications

The methylammonium lead iodide (MAPbI3) single crystals show extremely low trap-state density, high charge carrier diffusion length, long radiative carrier lifetime and high charge carrier mobility, making them outstanding candidates for a variety of potential applications including solar cells and photodetectors. However, these single crystals whatever the structure compositions still severely suffer from the presence of impurity traces such as triiodide that formed during their growth at high temperature. These traces can act as electron and hole quenching centers on the surface and at the device interface, hindering the further development of the performance of the solar cells and photodetectors based on these crystals. They also suffer from ions migrauion, vacancies, interstitials, and surface resturcturing and degradation especially under environmental conditions. Here, we will first show tremendous impact of the presence of excess triiodide on the charge carrier lifetime, carrier mobility and the performance of their solar cell and photodetector devices. We will also show the significant impact of the structure of organic cations on halogen migration, vacancies, and interstitials, carrier lifetimes as well as surface degradation of these single crystal perovskites using a combination of experimental and theoretical investigations. Finally, the unprecedented photo-response and photo-conversion efficiency after the chemical treatment of these single crystals will be presented and discussed.

Unveiling Property of Hydrolysis-Derived DMAPbI3 for Perovskite Devices: Composition Engineering, Defect Mitigation, and Stability Optimization

SummaryAdditive engineering has become increasingly important for making high-quality perovskite solar cells (PSCs), with a recent example involving acid during fabrication of cesium-based perovskites. Lately, it has been suggested that this process would introduce dimethylammonium ((CH3)2NH2+, DMA+) through hydrolysis of the organic solvent. However, material composition of the hydrolyzed product and its effect on the device performance remain to be understood. Here, we present an in-depth investigation of the hydrolysis-derived material (i.e., DMAPbI3) and detailed analysis of its role in producing high-quality PSCs. By varying the ratio of CsI/DMAPbI3 in the precursor, we achieve high-quality CsxDMA1-xPbI3 perovskite films with uniform morphology, low density of trap states, and good stability, leading to optimized power conversion efficiency up to 14.3%, with over 85% of the initial efficiency retained after ∼20 days in air without encapsulation. Our findings offer new insights into producing high-quality Cs-based perovskite materials.

Improved perovskite solar cell efficiency by tuning the colloidal size and free ion concentration in precursor solution using formic acid additive

Abstract Improving the quality of the perovskite active layer is crucial to obtaining high performance perovskite solar cells (PSCs). In this work, by introducing formic acid into the formamidinium lead iodide (FAPbI3) precursor solution, we managed to achieve reduced colloidal size in the solution, leading to more uniform deposition of FAPbI3 film with lower trap state density and higher carrier mobility. The solar cells based on the FAPbI3 absorber layer modified with formic acid show significantly better photovoltaic performance than that on the reference FAPbI3 film without formic acid. The device performance shows a close correlation with the colloidal size. Within the range studied from 6.7 to 1.0 nm, the smaller the colloidal size is, the higher the solar cell efficiency. More specifically, the cell efficiency is improved from 17.82% for the control cell without formic acid to 19.81% when 0.764 M formic acid was used. Formic acid has also been added into a CH3NH3PbI3 (MAPbI3) precursor solution, which exhibits a similar effect on the resulting MAPbI3 films and solar cells, with efficiency improved from 16.07% to 17.00%.

Synergistic Surface Passivation of CH3 NH3 PbBr3 Perovskite Quantum Dots with Phosphonic Acid and (3-Aminopropyl)triethoxysilane.

CH3 NH3 PbBr3 perovskite quantum dots (PQDs) are synthesized by using four different linear alkyl phosphonic acids (PAs) in conjunction with (3-aminopropyl)triethoxysilane (APTES) as capping ligands. The resultant PQDs are characterized by means of XRD, TEM, Raman spectroscopy, FTIR spectroscopy, UV/Vis, photoluminescence (PL), time-resolved PL, and X-ray photoelectron spectroscopy (XPS). PA chain length is shown to control the PQD size (ca. 2.9-4.2 nm) and excitonic absorption band positions (λ=488-525 nm), with shorter chain lengths corresponding to smaller sizes and bluer absorptions. All samples show a high PL quantum yield (ca. 46-83 %) and high PL stability; this is indicative of a low density of band gap trap states and effective surface passivation. Stability is higher for smaller PQDs; this is attributed to better passivation due to better solubility and less steric hindrance of the shorter PA ligands. Based on the FTIR, Raman, and XPS results, it is proposed that Pb2+ and CH3 NH3 + surface defects are passivated by R-PO3 2- or R-PO2 (OH)- , whereas Br- surface defects are passivated by R-NH3 + moieties. This study establishes the combination of PA and APTES ligands as a highly effective dual passivation system for the synergistic passivation of multiple surface defects of PQDs through primarily ionic bonding.

Interfacial evolution of AgBiS2 absorber layer obtained by SILAR method in hybrid solar cells

AgBiS2 is a promising and environmentally friendly absorber material for use in hybrid solar cells (HSC). Here, we report a study on the evolution of interfacial phenomena observed during deposition of AgBiS2 onto mesoporous TiO2 by the two-stage successive ionic layer adsorption-reaction method. With this approach, inorganic–organic HSC were assembled using Co2+ doped P3HT as hole transport layer. Surface photovoltage spectroscopy and contact potential difference measurements corroborated a low density of trap states in the ternary chalcogenide and lack of majority carrier barriers, compared to the binary absorbers used as reference. The best HSC exhibits a power conversion efficiency of 2.87% under irradiation of 100 mW cm−2, which is attractive for an easily scalable, no capping, no passivating synthesis of AgBiS2.

An Air Knife–Assisted Recrystallization Method for Ambient‐Process Planar Perovskite Solar Cells and Its Dim‐Light Harvesting

The photovoltaic performance of perovskite solar cells is highly dependent on the control of morphology and crystallization of perovskite film, which usually requires a controlled atmosphere. Therefore, fully ambient fabrication is a desired technology for the development of perovskite solar cells toward real production. Here, an air-knife assisted recrystallization method is reported, based on a simple bath-immersion to prepare high-quality perovskite absorbers. The resulted film shows a strong crystallinity with pure domains and low trap-state density, which contribute to the device performance and stability. The proposed method can operate in a wide process window, such as variable relative humidity and bath-immersion conditions, demonstrating a power conversion efficiency over 19% and 27% under 1 sun and 500-2000 lux dim-light illumination respectively, which is among the highest performance of ambient-process perovskite solar cells.

High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level.

Colloidal quantum dot (CQD) light-emitting diodes (LEDs) deliver a compelling performance in the visible, yet infrared CQD LEDs underperform their visible-emitting counterparts, largely due to their low photoluminescence quantum efficiency. Here we employ a ternary blend of CQD thin film that comprises a binary host matrix that serves to electronically passivate as well as to cater for an efficient and balanced carrier supply to the emitting quantum dot species. In doing so, we report infrared PbS CQD LEDs with an external quantum efficiency of ~7.9% and a power conversion efficiency of ~9.3%, thanks to their very low density of trap states, on the order of 1014 cm-3, and very high photoluminescence quantum efficiency in electrically conductive quantum dot solids of more than 60%. When these blend devices operate as solar cells they deliver an open circuit voltage that approaches their radiative limit thanks to the synergistic effect of the reduced trap-state density and the density of state modification in the nanocomposite.

Pressure-Assisted Annealing Strategy for High-Performance Self-Powered All-Inorganic Perovskite Microcrystal Photodetectors.

Owing to their low trap-state density, high carrier mobility, and high thermal stability, CsPbBr3 perovskite microcrystals (MCs) have attracted significant attention for applications as photodetectors (PDs). However, solution synthesis processes lead to MC films with high void density, seriously limiting the performance of the PDs. Here, a pressure-assisted annealing strategy is introduced to significantly reduce the void density and decrease the surface roughness. The resulting self-powered all-inorganic CsPbBr3 perovskite MC thick-film PDs show improved performance characteristics, with responsivities and detectivities of up to 0.206 A W-1 and 7.23 × 1012 Jones, respectively. Moreover, the on/off ratios of the devices are up to 106, and the highest linear dynamic range reaches 123.5 dB. These improved results indicate that the pressure-assisted annealing method is an effective strategy to enhance the performance of solution-synthesizedperovskite MC PDs.

CH3NH3PbBr3 Quantum Dot-Induced Nucleation for High Performance Perovskite Light-Emitting Solar Cells.

Solution-processed organometallic halide perovskites have obtained rapid development for light-emitting diodes (LEDs) and solar cells (SCs). These devices are fabricated with similar materials and architectures, leading to the emergence of perovskite-based light-emitting solar cells (LESCs). The high quality perovskite layer with reduced nonradiative recombination is crucial for achieving a high performance device, even though the carrier behaviors are fundamentally different in both functions. Here CH3NH3PbBr3 quantum dots (QDs) are first introduced into the antisolvent in solution phase, serving as nucleation centers and inducing the growth of CH3NH3PbI3 films. The heterogeneous nucleation based on high lattice matching and a low free-energy barrier significantly improves the crystallinity of CH3NH3PbI3 films with decreased grain sizes, resulting in longer carrier lifetime and lower trap-state density in the films. Therefore, the LESCs based on the CH3NH3PbI3 films with reduced recombination exhibit improved electroluminescence and external quantum efficiency. The current efficiency is enhanced by 1 order of magnitude as LEDs, and meanwhile the power conversion efficiency increases from 14.49% to 17.10% as SCs, compared to the reference device without QDs. Our study provides a feasible method to grow high quality perovskite films for high performance optoelectronic devices.

Diammonium Cations in the FASnI3 Perovskite Structure Lead to Lower Dark Currents and More Efficient Solar Cells

Hybrid halide perovskite solar cells with mixed cations demonstrate superior optical and electrical properties, especially for lead-based perovskite devices. Here, we report lead-free tin-based perovskite solar cells with diammonium cations, which can significantly improve the device performance. Formamidinium tin iodide (FASnI3) perovskite can incorporate propylenediammonium (PN) and trimethylenediammonium (TN) and retain its three-dimensional structure while at the same time providing better film morphology and optoelectronic properties. As a result, solar cell devices using FASnI3 absorbers mixed with 10% PN and 10% TN achieve higher power conversion efficiencies of 5.85% and 5.53%, respectively, compared to 2.53% of the pristine FASnI3 solar cell. This difference in device performance can be mainly attributed to the reduced leakage current, lower trap-state density, and reduced recombination, as evidenced by our dark current–voltage, space-charge-limited current, and impedance measurements. The result...

Constructing Sensitive and Fast Lead-Free Single-Crystalline Perovskite Photodetectors.

We developed a high-performance photodetector based on (CH3NH3)3Sb2I9 (MA3Sb2I9) microsingle crystals (MSCs). The MA3Sb2I9 single crystals exhibit a low-trap state density of ∼1010 cm-3 and a long carrier diffusion length reaching 3.0 μm, suggesting its great potential for optoelectronic applications. However, the centimeter single crystal (CSC)-based photodetector exhibits low responsivity (10-6 A/W under 1 sun illumination) due to low charge-carrier collection efficiency. By constructing the MSC photodetector with efficient charge-carrier collection, the responsivity can be improved by three orders of magnitude (under 1 sun illumination) and reach 40 A/W with monochromatic light (460 nm). Furthermore, the MSC photodetectors exhibit fast response speed of <1 ms, resulting in a high gain of 108 and a gain-bandwidth product of 105 Hz. These numbers are comparable to the lead-perovskite single-crystal-based photodetectors.

Metal Halide Perovskite Single Crystals: From Growth Process to Application

As a strong competitor in the field of optoelectronic applications, organic-inorganic metal hybrid perovskites have been paid much attention because of their superior characteristics, which include broad absorption from visible to near-infrared region, tunable optical and electronic properties, high charge mobility, long exciton diffusion length and carrier recombination lifetime, etc. It is noted that perovskite single crystals show remarkably low trap-state densities and long carrier diffusion lengths, which are even comparable with the best photovoltaic-quality silicon, and thus are expected to provide better optoelectronic performance. This paper reviews the recent development of crystal growth in single-, mixed-organic-cation and fully inorganic halide perovskite single crystals, in particular the solution approach. Furthermore, the application of metal hybrid perovskite single crystals and future perspectives are also highlighted.

Perovskites: Between the Grains

Defect reduction and device stabilization will continue to be two important thrusts toward efficient long-lasting polycrystalline perovskite solar cells. Recently in Chem, Zong et al. show that a complementary hydrophobic/hydrophilic triblock copolymer continuously incorporates into methylammonium lead tri-iodide grain boundaries. This improves solar cell stability and performance, resulting in a stabilized 19.4% power conversion efficiency, 92% of which is retained after 480 hr of continuous 1-sun illumination.

Self-Powered All-Inorganic Perovskite Microcrystal Photodetectors with High Detectivity.

Organic-inorganic lead halide perovskite microcrystal (MC) films are attractive candidates for fabricating high-performance large-area self-powered photodetectors (PDs) because of their lower trap state density and higher carrier mobility than their polycrystalline counterparts and more suitability of synthesizing large lateral area films than their single-crystal counterparts. Here, we report on the fabrication of self-powered all-inorganic CsPbBr3 perovskite MC PDs with high detectivity, using a modified solution synthesis method. The MCs are up to about 10 μm in size, and the MC layer is also about 11 μm in thickness. Under 473 nm laser (100 mW) illumination, the CsPbBr3 MC PDs show responsivity values of up to 0.172 A W-1, detectivity values of up to 4.8 × 1012 Jones, on/off ratios of up to 1.3 × 105, and linear dynamic ranges of up to 113 dB. These performances are significantly better than those of PDs based on polycrystalline perovskite thin films and comparable with those of PDs based on perovskite single crystals.