Diffusion Length Of Charge Carriers Research Articles

Interface and Defect Engineering for Metal Halide Perovskite Optoelectronic Devices.

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge-carrier diffusion lengths, and high defect tolerance. Perovskite-based solar cells and light-emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect-free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Influence of Urbach Energy, Temperature, and Longitudinal Position in the Active Layer on Carrier Diffusion Length in Perovskite Solar Cells.

The diffusion length of charge carriers in the active layer of a perovskite solar cell (PSC) of the structure Glass/PEDOT: PSS/CH3 NH3 PbI3 /PC60BM/Al is modelled. It is found that the diffusion length depends on the position x in the active layer measured from the PEDOT: PSS interface, Urbach energy and temperature. By varying the voltage in the range from zero to , it is shown that the dependence of diffusion length on the position x in the active layer reduces at higher voltage. The combined influence of applied voltage and temperature on the diffusion length of charge carriers is investigated and it is found that in the low voltage range the diffusion length is temperature independent, but it becomes significantly temperature dependent at higher voltages. Also, it is found that the diffusion length decreases as the applied voltage increases and this reduction becomes much more significant at higher voltage and temperatures. The combined influence of applied voltage and Urbach energy on diffusion length of charge carriers reveals that the diffusion length decreases when both the applied voltage and Urbach energy increase. However, the reduction in the diffusion length due to the increase in Urbach energy becomes less significant at higher voltage.

Adjusting the Reduction Potential of Electrons by Quantum Confinement for Selective Photoreduction of CO2 to Methanol

The production of CH3 OH from the photocatalytic CO2 reduction reaction (PCRR) presents a promising route for the clean utilization of renewable resources, but charge recombination, an unsatisfying stability and a poor selectivity limit its practical application. In this paper, we present the design and fabrication of 0D/2D materials with polymeric C3 N4 nanosheets and CdSe quantum dots (QDs) to enhance the separation and reduce the diffusion length of charge carriers. The rapid outflow of carriers also restrains self-corrosion and consequently enhances the stability. Furthermore, based on quantum confinement effects of the QDs, the energy of the electrons could be adjusted to a level that inhibits the hydrogen evolution reaction (HER, the main competitive reaction to PCRR) and improves the selectivity and activity for CH3 OH production from the PCRR. The band structures of photocatalysts with various CdSe particle sizes were also investigated quantitatively to establish the relationship between the band energy and the photocatalytic performance.

Adjusting the Reduction Potential of Electrons by Quantum Confinement for Selective Photoreduction of CO2 to Methanol

AbstractThe production of CH3OH from the photocatalytic CO2 reduction reaction (PCRR) presents a promising route for the clean utilization of renewable resources, but charge recombination, an unsatisfying stability and a poor selectivity limit its practical application. In this paper, we present the design and fabrication of 0D/2D materials with polymeric C3N4 nanosheets and CdSe quantum dots (QDs) to enhance the separation and reduce the diffusion length of charge carriers. The rapid outflow of carriers also restrains self‐corrosion and consequently enhances the stability. Furthermore, based on quantum confinement effects of the QDs, the energy of the electrons could be adjusted to a level that inhibits the hydrogen evolution reaction (HER, the main competitive reaction to PCRR) and improves the selectivity and activity for CH3OH production from the PCRR. The band structures of photocatalysts with various CdSe particle sizes were also investigated quantitatively to establish the relationship between the band energy and the photocatalytic performance.

Utilizing MVAD method to optimize crystallization and nanostructured surface of the perovskite film: Toward electroluminescent and ultraviolet photodetective bifunctional optoelectronics

Metal-halide perovskite of MAPbBr3, by exhibiting several remarkable optoproperties such as tunable energy bandgap, high absorption coefficients, high color purity, and long charge carrier diffusion lengths, has made great contributions to photovoltaic and electroluminescent (EL) technologies, i.e., solar cell, photodetector, light-emitting diode, and newly developed bifunctional optoelectronics. To realize high performance bifunctional optoelectronics, it is imperative to promote the balance of the trade-off between effective radiative recombination during EL process and fast exciton dissociation in photodetective process. Herein, we optimize the crystallization and nanostructured surface of MAPbBr3 film through applying multistep vapour assisted deposition (MVAD) method. Then, we enhance the optical field distribution and charge transfer feature of MAPbBr3 film by establishing an optical simulation model. Based on our modifed MAPbBr3 active layer, finally, we demonstrate a high performance device with EL and ultraviolet photodetective bifunctions, which exhibits a 6.27-fold improvement of brightness and a 3 orders of magnitude increase of detectivity. To the best of our knowledge, this work not only reports high bifunctional perovskite optoelectronics performance, but also illustrates the optical field redistribution process in horizontal orientation surface of MAPbBr3 films for relevant researchers.

Cation influence on carrier dynamics in perovskite solar cells

Rubidium and Cesium cations (Rb+ and Cs+) incorporation recently emerged as a viable strategy to enhance perovskite solar cells (PSCs) efficiency. However, a clear understanding of the impact of these cations on the structure-function relationship in relation to the device performance is severely lacking. Here, we systematically investigate the influence of Rb+ and Cs+ on the carrier dynamics using transient optical spectroscopy and correlate with solar cell performance. Unlike Rb+, Cs+ integrates well with methylammonium (MA+) and formamidinium (FA+) yielding increased perovskite grain size, longer charge carrier lifetimes and improved power conversion efficiency (PCE). Concomitant incorporation of Cs+/Rb+ cooperatively retards radiative recombination by ~60% in the quaternary-cation based perovskite system (RbCsMAFA) compared to the dual-cation MAFA samples. By suppressing the defect density, PCEs around 20% are obtained along with more balanced charge carrier diffusion length and comparable photoluminescence quantum yield in quaternary-cation perovskites. While the synergistic addition of Rb+ and Cs+ is attractive for controlling defects and recombination losses in efficient solar cells development, sole incorporation of Rb+ is still an engineering challenge. Importantly, our study explicates the underlying mechanisms behind the synergistic combination of cations to minimize the charge carrier losses and achieve high efficiency perovskite solar cells.

CH3NH3PbBr3 quantum dots for visible wavelength photodetector applications

Perovskite halide materials with their salient optoelectronic features such as high absorption coefficient, large charge carrier diffusion lengths and low carrier recombination, have revolutionised the field of photodetectors. In this paper we report the synthesis of CH3NH3PbBr3 quantum dots for visible wavelength photodetector applications. Quantum dots of size 2–4.5 nm were obtained. UV–Vis and photoluminescence spectroscopy revealed an optical band gap of 2.3 eV. An electrochemical band gap of 2.09 eV was obtained using cyclic voltammetry. CH3NH3PbBr3 quantum dots based visible wavelength photodetector device was fabricated. The time dependent photoresponse was found to be stable over several ON–OFF cycles of the lamp. We have achieved a responsivity, external quantum efficiency and specific detectivity of 6.11 A W−1, 1380.2% and 3.81 × 1012 Jones under AM 1.5 G, 1 sun illumination.

Inkjet printed perovskite solar cells: progress and prospects

In the field of photovoltaic materials, perovskite has attracted extensive attention during the past years, owing to its excellent photovoltaic properties, including high charge carrier mobility, low exciton binding energy, long charge carrier diffusion length, broad light absorption spectrum, large absorption coefficient, and low-cost solution processability. However, due to the limitations of film preparation methods (typical spin coating), industrial large-scale production of perovskite solar cells is still in infancy. The inkjet printing technology is a significant manufacturing technology developed from home and office printing and widely used in various printing electronics industries. Compared with other deposition methods, it possesses many advantages, including low cost, high material utilization, high patterning precision, etc. As a direct writing technology, the inkjet printing has shown great industrial potential and is expected to be employed in the industrialization of perovskite solar cells. In this paper, we review the research progress of perovskite solar cells fabricated via the inkjet printing and the application of inkjet printing technology to various functional layers (electrode, hole transport layer, electron transport layer, perovskite active layer). Finally, the challenges of inkjet printed perovskite solar cells at this stage are discussed, and the commercialization direction of inkjet printed perovskite solar cells is also pointed out.

Organic-inorganic hybrid Sn-based perovskite photodetectors with high external quantum efficiencies and wide spectral responses from 300 to 1000 nm

Organic-inorganic hybrid perovskites are ideal materials for photodetection owing to their high charge carrier mobility, long charge carrier diffusion length, low dark current density and sharp absorption edge. However, a relatively small band gap (1.6 eV) limits their photon-harvesting efficiency in the near-infrared region. In the present work, we demonstrate a hybrid methylamine iodide and Pb-Sn binary perovskite as the light absorption layer in photodetectors. Experimentally, the wavelength of photoresponse onset for the photodetectors can be extended to as great as 1,000 nm when the Sn content of the hybrid perovskite is increased to 30 mol%. In addition, the photodetectors exhibit a photoresponsivity of 0.39 A W−1, a specific detectivity of 7×1012 Jones, a fast photoresponse with rise and decay time constants and an external quantum efficiency greater than 50% in the wavelength range of 350–900 nm, with a maximum value of about 80% at 550 nm.

Partially replacing Pb2+ by Mn2+ in hybrid metal halide perovskites: Structural and electronic properties

Tailoring the physical properties of hybrid lead metal halide APbX3 perovskites by means of compositional engineering is one of the key factors contributing to the development of highly efficient and stable perovskite solar cells. While the beneficial effects of partial ionic replacement at the A- and X-sites are largely demonstrated, partial replacement of Pb2+ is less explored. Here, we developed a solution-based procedure to prepare thin films of mixed-metal MAPb1-aMnaI3 perovskites. Although Mn2+ ions have a size that can potentially fit in the B-sites of MAPbI3, using a combination of structural and chemical analysis, we show that only less than 10% of Pb2+ can be replaced by Mn2+. A 3% replacement of Pb2+ by Mn2+ leads to an elongation of the charge carrier lifetimes as concluded from time-resolved PL measurements. However, by analysis of the time-resolved microwave conductance data, we show that the charge carrier mobilities are largely unbalanced, which is in accordance with density functional theory (DFT) calculations indicating that the effective mass of the hole is much higher than that of the electron. Increasing the concentration of Mn2+ in the precursor solution above 10% results in formation of amorphous Mn-rich domains in the film, while the perovskite lattice becomes depleted of Mn2+. These domains negatively affect the charge carrier mobilities and shorten the lifetime of photogenerated carriers. The resulting reduction in charge carrier diffusion lengths will severely limit the photovoltaic properties of solar cells prepared from these mixed metal halide perovskites.

Research Progress of Compositional Controlling Strategy to Perovskite for High Performance Solar Cells

Over the past few years, the power conversion efficiency of perovskite solar cells have shown a tremendous progress from 3.8% in 2009 to 23.3% in 2018. Perovskites have exhibited excellent advantages in photovoltaic devices and other promising optoelectronic devices owing to their exceptional material properties, including direct and tunable bandgaps, strong light absorption, high electron/hole mobilities, long charge carrier lifetimes and diffusion lengths. The outstanding performance of perovskite solar cells is closely related with the deposition techniques and material composition of perovskite films. The preparation process of perovskite film is crucial for obtaining high efficiency devices, and it usually requires to fabricate a high coverage, compact and uniform perovskite layer. At present, the preparation technology of perovskite absorption layer mainly includes one-step processing, two-step processing, dual-source thermal evaporation processing, vapor-assisted solution processing and some scalable processing methods, and there are many reports and summaries about this work. However, perovskites still have some shortcomings such as insufficient light absorption range, poor long-term stability, the lead toxicity, which need to be overcome to realize higher power conversion efficiency and further product application. Compositional control engineering of perovskite materials becomes one of the effective ways to solve the above problems, but the summary of the research in this area is still lacking. In this review, we summarize the recent progress on the perovskite materials with different component systems, including organic-inorganic lead halide perovskite, all-inorganic lead halide perovskite, low-lead perovskite and lead-free perovskite. We also discuss some representative material compositions and the research on their corresponding preparation methods, the optimization of device structure and the effects on the device performance. Moreover, we compare and summarize the advantages and disadvantages of perovskite materials with different component systems. The purpose is to provide ideas on how to improve the efficiency and stability of perovskite solar cells through compositional controlling, and finally realize commercial application.

Simulation of the Spatial Distribution of the Local Quantum Efficiency and Photoelectric Characteristics of Photodiode-Based Infrared Focal Plane Arrays

The results of calculation of the spatial distribution of the local quantum efficiency over the area of photodiode-based IR focal plane arrays (IR FPA) are presented. The diffusion of photogenerated charge carriers in the absorber layer of the array was calculated by Monte-Carlo simulation. Methods of reducing the amount of necessary calculations based on using the symmetry properties of the array are discussed. Requirements for the photoelectric and design parameters (absorber-layer thickness, chargecarrier diffusion length and optical absorption length in this layer, the ratio of the size of n–p junctions to the geometrical dimensions of the detector pixels) are formulated that ensure the threshold sensitivity and spatial resolution of IR FPAs.

Enhanced power conversion efficiency of perovskite solar cells based on mesoscopic Ag-doped TiO2 electron transport layer

Organic-inorganic hybrid perovskite solar cells (PSCs), a superstar of photovoltaic devices, have attracted people’s attention owing to their adjustable energy band gap value, extremely high absorption coefficient, long charge carrier diffusion length, and dramatically high power conversion efficiency. In recent years, researchers have tried hard to explore new strategies in order to obtain better optical characteristics. Metal ion-doped electron transport layer (ETL) is a convenient way to improve the optical properties of PSCs. By this method, the conduction band and valence band position can be changed. This can lead to the carriers can effectively transport, and the charge recombination between ETL and absorber layer be reduced. In our study, we have enhanced the photovoltaic performance and the hysteresis behavior of the device by using various concentrations of mesoscopic Ag-doped TiO2 (meso-Ag:TiO2) as the ETL. We also systematically discussed the surface morphology, the charge carrier dynamic, the electron mobility, and the electrical conductivity for our devices. Finally, by optimizing the parameters of PSCs and the energy band alignment between perovskite absorber layer and ETL, the power conversion efficiency (PCE) of the champion device with 1.00 mol% meso-Ag:TiO2 ETL reached as high as 17.7%.

Revealing the Performance-Limiting Factors in α-SnWO4 Photoanodes for Solar Water Splitting

α-SnWO4 is an n-type metal oxide semiconductor that has recently attracted attention as a top absorber material in a D4-tandem device for highly efficient solar water splitting due to the combination of an ideal bandgap (∼1.9 eV) and a relatively negative photocurrent onset potential (∼0 V vs RHE). However, up to now, α-SnWO4 photoanodes have not shown high photoconversion efficiencies for reasons that have not yet been fully elucidated. In this work, phase-pure α-SnWO4 films are successfully prepared by pulsed laser deposition. The favorable band alignment is confirmed, and key carrier transport properties, such as charge carrier mobility, lifetime, and diffusion length are reported for the first time. In addition, a hole-conducting NiOx layer is introduced to protect the surface of the α-SnWO4 films from oxidation. The NiOx layer is found to increase the photocurrent for sulfite oxidation by a factor of ∼100, setting a new benchmark for the photocurrent and quantum efficiency of α-SnWO4. These results p...

All-Inorganic Perovskite Quantum Dot-Monolayer MoS2 Mixed-Dimensional van der Waals Heterostructure for Ultrasensitive Photodetector.

2D transition metal dichalcogenide (2D‐TMD) materials and their van der Waals heterostructures (vdWHs) have inspired worldwide efforts in the fields of electronics and optoelectronics. However, photodetectors based on 2D/2D vdWHs suffer from performance limitations due to the weak optical absorption of their atomically thin nature. In this work, taking advantage of an excellent light absorption coefficient, low‐temperature solution‐processability, and long charge carrier diffusion length, all‐inorganic halides perovskite CsPbI3− xBrx quantum dots are integrated with monolayer MoS2 for high‐performance and low‐cost photodetectors. A favorable energy band alignment facilitating interfacial photocarrier separation and efficient carrier injection into the MoS2 layer inside the 0D–2D mixed‐dimensional vdWHs are confirmed by a series of optical characterizations. Owing to the synergistic effect of the photogating mechanism and the modulation of Schottky barriers, the corresponding phototransistor exhibits a high photoresponsivity of 7.7 × 104 A W−1, a specific detectivity of ≈5.6 × 1011 Jones, and an external quantum efficiency exceeding 107%. The demonstration of such 0D–2D mixed‐dimensional heterostructures proposed here would open up a wide realm of opportunities for designing low‐cost, flexible transparent, and high‐performance optoelectronics.

Optimization of Charge Carrier Extraction in Colloidal Quantum Dots Short‐Wave Infrared Photodiodes through Optical Engineering

AbstractColloidal quantum dots (QDs) have attracted scientific interest for infrared (IR) optoelectronic devices due to their bandgap tunability and the ease of fabrication on arbitrary substrates. In this work, short‐wave IR photodetectors based on lead sulfide (PbS) QDs with high detectivity and low dark current is demonstrated. Using a combination of time‐resolved photoluminescence, carrier transport, and capacitance–voltage measurements, it is proved that the charge carrier diffusion length in the QD layer is negligible such that only photogenerated charges in the space charge region can be collected. To maximize the carrier extraction, an optical model for PbS QD‐based photodiodes is developed, and through optical engineering, the cavity at the wavelength of choice is optimized. This universal optimization recipe is applied to detectors sensitive to wavelengths above 1.4 µm, leading to external quantum efficiency of 30% and specific detectivity (D*) in the range of 1012 Jones.

Estimation of the Maximum Nonequilibrium Charge-Carrier Concentration in GaN Under Electron-Beam Irradiation

An empirical expression describing the lateral distribution of the nonequilibrium charge-carrier generation rate in GaN by a focused electron beam is obtained. The maximum nonequilibrium charge-carrier concentration as a function of the beam energy and diffusion length is calculated using this expression. Comparison of the results with values obtained using previously proposed approximate expressions provides an opportunity to evaluate the applicability of these approximations. Within the study of GaN by the cathodoluminescence and electron-beam-induced-current methods, the possibility to achieve a low injection level is analyzed considering the fact that the diffusion lengths of nonequilibrium charge carriers can be measured correctly only at low injection levels. It is shown that, in spite of the submicron values of the diffusion lengths in GaN, rather low beam currents are required to achieve a low injection level.

Defects responsible for charge carrier removal and correlation with deep level introduction in irradiated β-Ga2O3

Carrier removal rates and electron and hole trap densities in β-Ga2O3 films grown by hydride vapor phase epitaxy (HVPE) and irradiated with 18 MeV α-particles and 20 MeV protons were measured and compared to the results of modeling. The electron removal rates for proton and α-radiation were found to be close to the theoretical production rates of vacancies, whereas the concentrations of major electron and hole traps were much lower, suggesting that the main process responsible for carrier removal is the formation of neutral complexes between vacancies and shallow donors. There is a concurrent decrease in the diffusion length of nonequilibrium charge carriers after irradiation, which correlates with the increase in density of the main electron traps E2* at Ec − (0.75–0.78) eV, E3 at Ec − (0.95–1.05) eV, and E4 at Ec − 1.2 eV. The introduction rates of these traps are similar for the 18 MeV α-particles and 20 MeV protons and are much lower than the carrier removal rates.

Visualization and Investigation of Charge Transport in Mixed‐Halide Perovskite via Lateral‐Structured Photovoltaic Devices

AbstractMixed‐halide perovskites, CH3NH3PbI3−xClx, can be used to fabricate highly efficient perovskite solar cells; the presence of chlorine ions affects the perovskite morphology and enhances optoelectronic properties. However, the reported device performances of mixed‐halide perovskites are comparable to those of triiodide perovskites, CH3NH3PbI3. Thus, the benefits of the presence of Cl− ions in mixed‐halide perovskites are uncertain. To clarify the effects of Cl− on the optoelectronic properties of perovskite devices, a suitable device structure is required. In this study, lateral‐structured perovskite devices are fabricated and their electrical properties are investigated. A clear contrast is found between the optoelectronic properties of the mixed‐halide and triiodide perovskite; a suitable platform is also constructed for the spectroscopic analysis. Scanning photocurrent spectroscopy is used to visualize the charge carrier diffusion in the perovskite films and it is found that the charge carrier diffusion length of the mixed‐halide perovskite is longer. It is also demonstrated by using conducting atomic force microscopy that the charge carrier pathways in the mixed‐halide perovskite are more efficient, which means that it has longer charge carrier recombination life‐times and higher photocurrents. This lateral‐structured device has an architecture that makes it suitable for the observation of charge carrier diffusion and spectroscopic analysis.

Investigation of the stability for self-powered CsPbBr3 perovskite photodetector with an all-inorganic structure

All-inorganic cesium lead halide perovskites have attracted intensive interest as light absorbing materials due to their large absorption coefficient, long charge carrier lifetime and diffusion length, especially they show better stability than their organic and inorganic hybrid counterparts. Herein, we demonstrate a high-performance self-powered CsPbBr3 perovskite photodetector based on an all-inorganic structure. Our self-powered devices show high performance with the responsivity of 0.3 A/W, detectivity of 1.15 × 1013 cm Hz1/2/W and on/off ratio of 4.5 × 102. Notably, ZnO nanorods as a scaffold, which functions as a protection layer, are used to effectively enhance the stability and durability of perovskite photodetectors. Our all-inorganic device exhibits improvement of the stability in the air, has a certain high temperature endurance and shows the anti-corrosion ability to water and alcohol. These good performances and high stability present a potential application in harsh environments without external power sources.