Diffusion Length Of Charge Carriers Research Articles

CsPbBr3 Single Crystal X-ray Detector with Schottky Barrier for X-ray Imaging Application

A high Z CsPbBr3 halide perovskite with large charge carrier diffusion length was used for radiation detection applications. The high sensitivity X-ray detector is expected to be used for imaging a...

Correlation between Charge Transport Length Scales and Dielectric Relaxation Time Constant in Hybrid Halide Perovskite Semiconductors

The charge carrier diffusion length and dielectric relaxations are important parameters which decide the performance of various optoelectronic devices, in particular for photovoltaic devices. A comparative study is carried out on the charge transport length scale (L) for passivated and pristine CH3NH3PbI3 (MAPI) thin-film-based perovskite solar cells (PSCs) through scanning photocurrent microscopy (SPM). The SPM study suggested an improved L and degree of ambipolarity of photogenerated charge carriers (electron and hole) in passivated as compared to pristine MAPI-based PSCs. These results were found to be correlated with frequency-dependent photocurrent measurement, which shows that the relaxation time of the charge carrier is relatively lower in passivated MAPI-based PSCs. This mechanism could be explained by trap-assisted recombination, where trap states are induced by ion migration in halide perovskite films. Furthermore, passivation of traps showed an increased degree of ambipolarity in the perovskite semiconductor thin film.

Bulk heterojunction perovskite solar cells incorporated with solution-processed TiOx nanoparticles as the electron acceptors

In the past ten years, perovskite solar cells were rapidly developed, but the intrinsic unbalanced charge carrier diffusion lengths within perovskite materials were not fully addressed by either a planar heterojunction or meso-superstructured perovskite solar cells. In this study, we report bulk heterojunction perovskite solar cells, where perovskite materials CH3NH3PbI3 is blended with solution-processed n-type TiOx nanoparticles as the photoactive layer. Studies indicate that one-step solution-processed CH3NH3PbI3:TiOx bulk-heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap-induced charge recombination. Thus, bulk heterojunction perovskite solar cells by CH3NH3PbI3 mixed with 5 wt% of TiOx, which is processed by one-step method rather than typical two-step method, show a short-circuit current density of 20.93 mA/cm2, an open-circuit voltage of 0.90 V, a fill factor of 80% and with a corresponding power conversion efficiency of 14.91%, which is more than 30% enhancement as compared with that of perovskite solar cells with a planar heterojunction device structure. Moreover, bulk heterojunction perovskite solar cells possess enhanced device stability. All these results demonstrate that perovskite solar cells with a bulk heterojunction device structure are one of apparent approaches to boost device performance.

Call for papers on special issue “Perovskite photovoltaics and optoelectronic devices”

Aim & Scope: Metal halide perovskitehave been regarded as promising classes of materials for photovoltaics and optoelectronic devices, owing to the unique characteristics, such as long charge carrier diffusion lengths, precise tunable bandgaps, high light absorption coefficients, and high defect tolerance. Research on perovskite in the fields including photovoltaics, light-emitting diodes, lasers, X-ray imaging, and photodetectors has been gaining increasingly interest over the past years. Up to now, the efficiency of perovskite solar cells has grown from 3.8% in single-junction solar cells in 2009 to more than 25%, catching up the efficiency level of commercial silicon cells. Up to now, the key issues of perovskite photovoltaics and optoelectronic devices have become the stability, performance and large-scale production. This requires optimization of the film morphology, interface, device structure and the fabrication process. A lot work has been done on this issue and has made remarkable progress. We kindly invite you to submit a manuscript(s) for this Special Issue. Full papers, communications, and reviews are all welcome.

Optimal ranges determination of morphological parameters of nanopatterned semiconductors quality for solar cells

Purpose: The paper aims to determine the values of the main morphological characteristics of nanopatterns, which can be considered as the reference for use as surfaces of solar cells. Design/methodology/approach: The article uses an approach based on the definition of reference indicators of nanopatterns for solar cells by analysing the main parameters of solar cells and comparing them with the possible values of morphological parameters. Correlations of pore radius and visible wavelength, porosity and visible range, wavelength of de Broglie, nanopatterned layer thickness and charge carriers diffusion length, etc., are analysed. Compliance verification of morphological characteristics of nanopatterns with the specified criteria was performed on the example of porous silicon layers. Findings: The conducted research allowed to define the basic values of morphological parameters of porous nanopatterns, namely porousness, pore size (effective diameter), the thickness of the porous layer, and form factor. Reference ranges of morphological parameters of nanopatterns formed on the surface of semiconductors for applications in solar cells are established. Research limitations/implications: The article is devoted to the choice of optimal morphological characteristics of porous nanopatterns on the surface of semiconductors for solar cells. However, for solar cells, other types of nanopatterns can also be applied, for which it is also necessary to develop methods for selecting optimal parameters. Moreover, the prospect of research on this topic is to check the intrusion into a certain range of values of real nanopatterns formed on the surface of semiconductors. Practical implications: In the article the methodology allowing to choose optimal values of morphological parameters of nanopatterns for its application for solar cells is considered. Such studies are of great practical importance for the production of high-quality solar cells based on nanopatterned semiconductors. Originality/value: The article for the first time considers the choice of the nanopattern type and the ranges of morphological parameters in terms of quality assurance of the final product – the solar cell. It is determined that it is necessary to take into account such factors as porousness, pore size, thickness of the porous layer and roundness. A range of optimal values is selected for each of the indicators.

ORGANIC-INORGANIC PEROVSKITE CH3NH3PbI3: MORPHOLOGICAL, STRUCTURAL AND PHOTOELECTROPHYSICAL PROPERTIES

The organic-inorganic perovskite films CH3N-H3PbI3 were synthesized from solutions with different ratios of initial reagents (PbI2 to CH3NH3I were taken in the ratio 1:1, 1:2 and 1:3). To deposit films of organic-inorganic perovskite, solutions with different ratio of initial reagents were applied to the substrates by the spin-coating method. The organic-inorganic perovskites synthesized were of one chemical composition in spite of the fact that different chemical reactions proceeded during the synthesis. It was found that the formation of perovskite occurs according to different schemes depending on the ra-tio of PbI2 and CH3NH3I: without the formation of intermediate compounds (at ratio 1:1) and with the formation of one (CH3NH3)2PbI4 (1:2) and two intermediate compounds (CH3NH3)3PbI5, (CH3NH3)2PbI4 (1:3).It was established that regardless of the ratio of the initial reagents, organic-inorganic perovskites with different morphology are formed. At the ratio of the initial reagents 1:1, needle particles formed, and at the ratio of 1:2 and 1:3, particles have the form of a maple leaf and round shape, respectively.To improve the film stability, polyvinyl butyral polymer was used. It is an amorphous colorless pol-ymer which is characterized by high optical properties, environmental (in particular, H2O, O2 and O3) and light resistance. The stability of films of organic-inorganic perovskite without and with a polymer were investigated by XRD, fluorescence spectroscopy and non-contact optical methods. The stability of the films was evaluated by the content of the additional phase of PbI2, which is formed due to the degradation of the organic-inorganic perovskite film CH3NH3PbI3. It was established that the presence of a polymer layer results in improved stability of samples and decrease the rate of surface recombination velocity compared to samples without a polymer layer.The diffusion length of minority charge carriers of the organic-inorganic perovskite films with the polymeric layer was estimated by the method of spectral dependences of the surface photovoltage. The spectra of surface photovoltage and the diffusion length of minority charge carriers of organic-inorganic perovskites with a polymer layer were compared with the literature data for samples without a polymer layer. This comparison shown that the characteristics of the samples with polymer layer are somewhat worse. It is determined that the organic-inorganic perovskite with the polymer layer is characterized by a smaller diffusion length (by 10%) of the minority charge carriers. The prepared perovskite films CH3NH3PbI3 are promising for the development of effective solar cells.

Monolithic all-perovskite tandem solar cells with 24.8% efficiency exploiting comproportionation to suppress Sn(ii) oxidation in precursor ink

Combining wide-bandgap and narrow-bandgap perovskites to construct monolithic all-perovskite tandem solar cells offers avenues for continued increases in photovoltaic (PV) power conversion efficiencies (PCEs). However, actual efficiencies today are diminished by the subpar performance of narrow-bandgap subcells. Here we report a strategy to reduce Sn vacancies in mixed Pb–Sn narrow-bandgap perovskites that use metallic tin to reduce the Sn4+ (an oxidation product of Sn2+) to Sn2+ via a comproportionation reaction. We increase, thereby, the charge-carrier diffusion length in narrow-bandgap perovskites to 3 μm for the best materials. We obtain a PCE of 21.1% for 1.22-eV narrow-bandgap solar cells. We fabricate monolithic all-perovskite tandem cells with certified PCEs of 24.8% for small-area devices (0.049 cm2) and of 22.1% for large-area devices (1.05 cm2). The tandem cells retain 90% of their performance following 463 h of operation at the maximum power point under full 1-sun illumination. Improvements in the efficiency and stability of low-bandgap perovskite solar cells are key to enabling all-perovskite solar cells. Here, Lin et al. use metallic tin to prevent oxidation in such low-gap perovskite and demonstrate 24.8%-efficient tandems that are stable for over 400 h under operating conditions.

Bulk Heterojunction Perovskite Solar Cells Incorporated with Zn2SnO4 Nanoparticles as the Electron Acceptors.

Perovskite solar cells were developed very fast in the past decade, but hybrid perovskite materials with unbalanced charge carrier diffusion lengths were not fully addressed by either conventional or planar heterojunction device structures. In this study, high-performance perovskite solar cells with bulk heterojunction device structures where CH3NH3PbI2.55Br0.45 is blended with an n-type high-electron-mobility Zn2SnO4 nanoparticle as the photoactive layer are reported. Systematic studies indicate that the CH3NH3PbI2.55Br0.45:Zn2SnO4 bulk heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap density. Photoluminescence and time-resolved photoluminescence studies further demonstrate that there is an efficient photoinduced charge carrier transfer between CH3NH3PbI2.55Br0.45 and Zn2SnO4 nanoparticles. Thus, bulk heterojunction perovskite solar cells by the CH3NH3PbI2.55Br0.45:Zn2SnO4 thin film exhibit over 21.07% power conversion efficiency, which is more than 12% enhancement as compared to that (18.74%) observed from planar heterojunction perovskite solar cells by the pristine CH3NH3PbI2.55Br0.45 thin film. Moreover, bulk heterojunction perovskite solar cells possess significantly suppressed photocurrent hysteresis, dramatically enhanced device stability, and reproducibility. All these results demonstrate that high-performance perovskite solar cells can be realized through bulk heterojunction device structures.

Highly stable lead-free Cs3Bi2I9 perovskite nanoplates for photodetection applications

Lead halide perovskites have received tremendous attentions recently for their excellent properties such as high light absorption coefficient and long charge carrier diffusion length. However, the stability issues and the existence of toxic lead cations have largely limited their applications in optoelectronic area. Herein, we report the synthesis and investigation of highly stable and lead-free Cs3Bi2I9 perovskite nanoplates for visible light photodetection applications. The Cs3Bi2I9 nanoplates were synthesized through a facile solution-processed method, which is also applicable to various substrates. The achieved nanoplates present very good crystal quality and exhibit excellent long-term stability even exposed in moist air for several months. Photodetectors were constructed based on these high-quality perovskite nanoplates for the first time, and display a maximum photoresponsivity of 33.1 mA/W under the illumination of 450 nm laser, which is six times higher than the solution-synthesized CH3NH3Pbl3 nanowire photodetectors. The specific detectivity of these devices can reach up to 1010 Jones. Additionally, the devices exhibit fast rise and decay time of 10.2 and 37.2 ms, respectively, and highly stable photoswitching behavior with their photoresponse well retaining under alternating light and darkness. This work opens up a new opportunity for stable and low-toxic perovskite-based optoelectronic applications.

Sequentially Assembled Graphene Layers on Silicon, the Role of Uncertainty Principles in Graphene–Silicon Schottky Junctions

AbstractOwing to the atomic thickness of graphene, the out‐of‐plane velocity of carriers is entangled with the thickness of graphene layer as well as the quasiparticle lifetime by means of the position–momentum and energy–time uncertainty principles. The out‐of‐plane velocity is vital for thermionic emission of carriers across the Schottky junction at graphene–silicon interface. In this report, the effect of graphene layer thickening on the electronic and optoelectronic processes is studied in hybrid graphene–silicon Schottky junctions. The thickness of graphene layer is systematically increased via sequentially assembling of chemical vapor deposited (CVD‐grown) graphene layers onto silicon. The assembled graphene layers are found to behave as structurally uncoupled layers leading to a universal scaling of in‐plane current with the number of assembled layers. The reverse saturation current of the graphene–silicon junctions strongly correlates with the number of assembled graphene layers indicating the critical influence of graphene layer thickness on the electronic and optoelectronic properties of graphene–silicon junctions. The experimental observations are quantitatively analyzed based on the uncertainty principles. The effect of graphene layer thickening on the concentration and diffusion length of excess photogenerated charge carriers in graphene is investigated through lateral photovoltage spectroscopy. This work sheds light on the fundamental role of uncertainty principles in the hybrid 2D/3D junctions.

Defects at the surface of β-Ga2O3 produced by Ar plasma exposure

Films of β-Ga2O3 grown by halide vapor phase epitaxy on native substrates were subjected to Ar inductively coupled plasma treatment. As a result, the built-in voltage of Ni Schottky diodes deposited on the plasma treated surfaces decreased from 1 V to −0.02 V due to the buildup of deep trap concentration in the near surface region. Deep level spectra measurements indicate a strong increase in the top ∼200 nm of the plasma treated layer of the concentration of E2* (Ec − 0.8 eV) and especially E3 (Ec − 1.05 eV) deep electron traps. Capacitance-voltage profiling with monochromatic illumination also indicated a large increase in the upper ∼100 nm of the film in the concentration of deep acceptors with optical threshold for an ionization of ∼2.3 eV and 3.1 eV. Such defects at the surface led to a significant increase in reverse current, an increase in the ideality factor in forward current, and a dramatic decrease in the diffusion length of nonequilibrium charge carriers from 450 nm to 150 nm.

Recent advances on organic‐inorganic hybrid perovskite photodetectors with fast response

AbstractIn the last decade, optoelectronic devices based on organic‐inorganic hybrid perovskite (OIHP) materials, which have unique advantages of direct bandgap, large absorption coefficient, low density of defects, long charge carrier lifetime, diffusion length, and solution processability, have traveled with traditional inorganic semiconductor devices. The state‐of‐the‐art OIHP photodetectors have contributed a comparable performance with Si and III‐V compound semiconductor based photodetectors. Large amount of efforts have been focused on improving sensitivity, broadening detection spectra, enlarging linear dynamic range. However, few reports emphasized the important parameter of response speed. In this review, we summarize the progress and applications of OIHP photodetectors with fast response. Based on photovoltaic and photoconductive‐type OIHP photodetectors, the working principle and key factors on determining response speed are systematically mentioned. Then, the research progress of response speed, which is composed of resistance‐capacitance (RC) time constant and charge carrier transit time is discussed in detail. Subsequently, considering the intrinsic flexibility of perovskite materials, we briefly discuss the flexible photodetectors. Finally, an outlook and potential rules for designing fast‐response OIHP photodetectors are further proposed.

All-Solution-Processed WO3/BiVO4 Core-Shell Nanorod Arrays for Highly Stable Photoanodes.

Tungsten oxide (WO3) and bismuth vanadate (BiVO4) are oneof the most attractive combinations to construct an efficient heterojunction for photoelectrochemical (PEC) applications. Here, we report an all-solution-processed WO3/BiVO4 heteronanostructure photoanode with highly enhanced photoactivity and stability for sustainable energy production. The vertically aligned WO3 nanorods were synthesized on a fluorine-doped tin oxide/glass substrate by the hydrothermal method without a seed layer and BiVO4 was deposited by pulsed electrodeposition for conformal coating. Owing to the long diffusion lengths of charge carriers in the WO3 nanorods, the ability to absorb the wider range of wavelengths, and appropriate band-edge positions of the WO3/BiVO4 heterojunction for spontaneous PEC reaction, the optimum WO3/BiVO4 photoanode has a photocurrent density of 4.15 mA/cm2 at 1.23 V versus RHE and an incident-photon-to-current efficiency of 75.9% at 430 nm under front illumination, which are a double and quadruple those of pristine WO3 nanorod arrays, respectively. Our work suggests an environment-friendly and low-cost all-solution process route to synthesize high-quality photoelectrodes.

Deep trap spectra of Sn-doped α-Ga2O3 grown by halide vapor phase epitaxy on sapphire

Epitaxial layers of α-Ga2O3 with different Sn doping levels were grown by halide vapor phase epitaxy on sapphire. The films had shallow donor concentrations ranging from 1017 to 4.8 × 1019 cm−3. Deep level transient spectroscopy of the lowest doped samples revealed dominant A traps with level Ec − 0.6 eV and B traps near Ec − 1.1 eV. With increasing shallow donor concentration, the density of the A traps increased, and new traps C (Ec − 0.85 eV) and D (Ec − 0.23 eV) emerged. Photocapacitance spectra showed the presence of deep traps with optical ionization energy of ∼2 and 2.7 eV and prominent persistent photocapacitance at low temperature, surviving heating to temperatures above room temperature. The diffusion length of nonequilibrium charge carriers was 0.15 µm, and microcathodoluminescence spectra showed peaks in the range 339–540 nm, but no band-edge emission.

(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.

A Novel Phototransistor Device with Dual Active Layers Composited of CsPbBr3 and ZnO Quantum Dots.

Taking advantage of a large light absorption coefficient, long charge carrier diffusion length and low-cost solution processing, all-inorganic halides perovskite CsPbBr3 quantum dots (QDs) are combined with a ZnO QD film to construct a high-performance photodetector. In this work, a novel photodetector device based on transistor structure with dual active layers composed of CsPbBr3 and ZnO film is proposed. In this structure, CsPbBr3 film functions as the light-absorbing layer and ZnO film acts as the conducting layer. Owing to the high electron mobility and hole-blocking nature of the ZnO QDs film, the photo-induced electron-hole pairs can be separated efficiently. As a result, the device exhibits high performance with response of 43.5 A/W, high detection up to 5.02 × 1011 Jones and on/off ratio of 5.6 × 104 under 365 nm light illumination. Compared with the ZnO-only phototransistor (the photodetector with the structure of transistor) the performance of the CsPbBr3 phototransistor showed significant improvement, which is superior to the majority of photodetectors prepared by perovskite. This work demonstrates that the ZnO QDs film can be applied in the photodetector device as a functional conducting layer, and we believe that the hybrid CsPbBr3/ZnO phototransistor would promote the development of low-cost and high-performance photodetectors.

Contactless measurements of photocarrier transport properties in perovskite single crystals

The remarkable properties of metal halide perovskites arising from their impressive charge carrier diffusion lengths have led to rapid advances in solution-processed optoelectronics. Unfortunately, diffusion lengths reported in perovskite single crystals have ranged widely – from 3 μm to 3 mm – for ostensibly similar materials. Here we report a contactless method to measure the carrier mobility and further extract the diffusion length: our approach avoids both the effects of contact resistance and those of high electric field. We vary the density of quenchers – epitaxially included within perovskite single crystals – and report the dependence of excited state lifetime in the perovskite on inter-quencher spacing. Our results are repeatable and self-consistent (i.e. they agree on diffusion length for many different quencher concentrations) to within ± 6%. Using this method, we obtain a diffusion length in metal-halide perovskites of 2.6 μm ± 0.1 μm.

Theoretical Prediction of Chiral 3D Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskites (HOIPs), in particular 3D HOIPs, have demonstrated remarkable properties, including ultralong charge-carrier diffusion lengths, high dielectric constants, low trap densities, tunable absorption and emission wavelengths, strong spin-orbit coupling, and large Rashba splitting. These superior properties have generated intensive research interest in HOIPs for high-performance optoelectronics and spintronics. Here, 3D hybrid organic-inorganic perovskites that implant chirality through introducing the chiral methylammonium cation are demonstrated. Based on structural optimization, phonon spectra, formation energy, and ab initio molecular dynamics simulations, it is found that the chirality of the chiral cations can be successfully transferred to the framework of 3D HOIPs, and the resulting 3D chiral HOIPs are both kinetically and thermodynamically stable. Combining chirality with the impressive optical, electrical, and spintronic properties of 3D perovskites, 3D chiral perovskites is of great interest in the fields of piezoelectricity, pyroelectricity, ferroelectricity, topological quantum engineering, circularly polarized optoelectronics, and spintronics.

Combustion Synthesized Zinc Oxide Electron‐Transport Layers for Efficient and Stable Perovskite Solar Cells

AbstractPerovskite solar cells (PSCs) have advanced rapidly with power conversion efficiencies (PCEs) now exceeding 22%. Due to the long diffusion lengths of charge carriers in the photoactive layer, a PSC device architecture comprising an electron‐ transporting layer (ETL) is essential to optimize charge flow and collection for maximum performance. Here, a novel approach is reported to low temperature, solution‐processed ZnO ETLs for PSCs using combustion synthesis. Due to the intrinsic passivation effects, high crystallinity, matched energy levels, ideal surface topography, and good chemical compatibility with the perovskite layer, this combustion‐derived ZnO enables PCEs approaching 17–20% for three types of perovskite materials systems with no need for ETL doping or surface functionalization.

Strategies for Modifying TiO2 Based Electron Transport Layers to Boost Perovskite Solar Cells

The research on solution processed metal halide perovskite solar cells (PSCs) as a new type of solar cells has experienced explosive growth since the first report in 2009. It is impressive that solar energy conversion efficiency has increased to over 23%. Outstanding optoelectronic properties including high absorption coefficient, high mobility, and long diffusion length of charge carriers have been revealed in the family of hybrid organic inorganic halide perovskite materials that are considered the heart of solar cells. A long-anticipated feature for solar cells that the diffusion lengths of charge carriers outstrip the active layer thickness of a device has been demonstrated in PSCs so that the efficiency of extracting photocarriers, particularly electrons at the interfaces becomes a key parameter controlling global device performance. The n-type semiconductor TiO2 with the merits of thermal and chemical stability, low cost, and suitable band edge positions has been regarded an ideal electron transport...