Diffusion Length Of Charge Carriers Research Articles

ОСОБЛИВОСТІ ДОСЛІДЖЕНЬ ПЕРЕХІДНИХ ШАРІВ В НАПІВПРОВІДНИКОВИХ ГЕТЕРОСИСТЕМАХ

Studies of spectral characteristics in Schottky's contact specimens showed that photemf generated by mono­chromatic light, according to the formation mechanism, has a predominantly lateral nature, that is, in a hetero­system there are at least two areas separated by a potential barrier along the interface, with different conductivity levels. The feature of the photoemf spectral characteristics is a variations its appearance when changing the thermal annealing temperature of the studied heterosystems. A significant characteristic and a small amplitude of the characteristic indicates the formation of a transition layer, relatively homogeneous and with insignificant, compared with the volume of GaAs, the doping concentration. If the spectral characteristic has one maximum and amplitude, which several times exceeds the amplitude of a significant characteristic, which means the formation of a transition layer in the Schottky contact depletion area with high conductivity, compared with a quasine-neutral region of a semiconductor. The distribution of lateral photoemf along the sample also has a significant character. In order to obtain the correct results relative to the heterosystem transition layer, it is necessary to measure spectral acute characteristics at a distance from the point change sign of the emf that several times the diffusion length of non-equilibrium charge carriers in GaAs. The problem of the formation of a metal-semiconductor contact and other heterosystems accompanied by the occurrence of heterogeneous transition layers, always paid attention. The use of the proposed photovoltaic method allows to establish the degree of homogeneity of semiconductor layers, components of the structure and predict the redistribution of current density flowing through the physical scope of the device.

UV light absorbers executing synergistic effects of passivating defects and improving photostability for efficient perovskite photovoltaics

Metal halide perovskite-based solar cells (PSCs) have rapidly-increased power conversion efficiency (PCE) exceeding 25% but poor stability especially under ultraviolet (UV) light. Meanwhile, non-radiative recombination caused by diverse defects in perovskite absorbers and related interfaces is one of the major factors confining further development of PSCs. In this study, we systematically investigate the role of 2-(2-hydroxy-5-methylphenyl)benzotriazole (UVP) additive in perovskite layers. By adjusting the amount of doped UVP, the quality of perovskite absorbers is significantly improved with enlarged grains, longer lifetime and diffusion length of charge carriers. Furthermore, UVP not only reduces defects for less non-radiative recombination, but also matches energy level alignment for efficient interfacial charge extraction. X-ray photoelectron spectroscopy confirms that N-donor of UVP molecule coordinates with undercoordinated Pb2+ on the surface. Interestingly, UVP incorporated in PbI2 protects the perovskite by absorbing UV through the opening and closing of the chelating ring. Eventually, the UVP treated PSCs obtain a champion PCE of 22.46% with remarkably enhanced UV stability, retaining over 90% of initial PCE after 60 mW/cm2 strong UV irradiation for 9 h while the control maintaining only 74%. These results demonstrate a promising strategy fabricating passivated and UV-resistant perovskite materials simultaneously for efficient and stable perovskite photovoltaics.

Ionic Liquid for Perovskite Solar Cells: An Emerging Solvent Engineering Technology

ConspectusPerovskite solar cells (PSCs) have, in recent years, become one of the most in-depth photovoltaic materials in many different disciplines due to their long-range charge carrier diffusion lengths, strong light absorption, easy tuning of the band gaps, low defect density, and solution processability. The power conversion efficiency (PCE) of single-junction PSCs has reached a certified value of 25.5%, which has caught up with or even surpassed traditional photovoltaic technologies, such as silicon (Si) solar cells, thin film solar cell, etc. In addition to the performance of PSCs comparable to traditional photovoltaic technology, its biggest feature is that it can be prepared by a solution method. The preparation process of the solution method simplify the film preparation process and greatly reduce the preparation cost. In addition, the solution method provides a favorable option for the energy supply of flexible wearable devices in the future.At present, the preparation of PSCs mainly uses organic molecular solvents, including N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and their mixed solvents. However, most of these commonly used organic solvents are unsafe and are likely to cause water and soil pollution, harm to the human body, and safety accidents. In addition, most solvents have a high coordination number, which limits the crystallization of perovskite. Furthermore, high-quality perovskite films require the introduction of a large amount of high boiling point, polar and aprotic antisolvents to assist crystallization, such as chlorobenzene, ether, and ethyl acetate. In addition to the toxicological problems of the antisolvent, the technology has a relatively narrow operating time window, relatively high operating environment requirements, and relatively low repeatability, which greatly limits the large-scale production of PSCs. Therefore, It is very necessary to develop a new solvent engineering technology for PSCs, which is green and pollution-free, is a simple process, and has high efficiency, long-term stability, and low cost. Recently, we have developed a series of ionic liquids, including MAAc, MAFa, BAAc, etc., which can be used as solvents to prepare efficient and stable PSCs. It allows the production of smooth and continuous polycrystalline perovskite thin films in ambient air through a simple spin coating methodIn this Account, we started with the discovery of ionic liquids as solvents to PSCs. We have proposed new strategies for improving and enhancing ionic liquid-based PSCs, including interface engineering, solvent engineering, and additive engineering. Furthermore, the stable and efficient mechanism of ionic liquid-based perovskite solar cells has been revealed. In view of the designable characteristics of ionic liquids, some functional ionic liquids have been designed and applied to different systems of halide perovskites, which has important guiding significance for the development of more functional ionic liquids for PSCs. Finally, the challenges and future directions of ionic liquid-based PSCs from both large-scale production and technological perspectives are proposed to provide a new method for the commercialization of PSCs.

Ultrasensitive organic-inorganic nanotube thin films of halogenated perovskites as room temperature ammonia sensors

Recently organic-inorganic halide perovskites like MAPbX3 (XBr, Cl, I), are being investigated as a substitute to commonly used metal oxide semiconductor-based sensors for the detection of various volatile organic compounds and toxic gases at room temperature. The present work provides detailed insights into chlorine halide incorporation on the ammonia sensing performance of the most popular MAPbI3 (MAPI) perovskite thin films. Here, we shown that the MAPbI3−xClx (MAPIC) sensor exhibits a high response of 591%, nearly13 times larger than the MAPI sensor for 10 ppm of ammonia exposure at room temperature. This work demonstrates excellent potential of MAPIC as an ultra-sensitive, stable and reversible NH3 sensor vis-à-vis MAPI sensor for early detection of NH3. The proposed sensing mechanism suggests that by engineering the morphology at the nanoscale and doping induced excellent charge carrier diffusion length could enhance ammonia sensor responsivity.

Numerical modelling: Design and investigation of uniformly and non-uniformly doped absorber layer based PN homojunction perovskite solar cell variants

Perovskite Solar Cell (PSC) is the fastly emerging solar technology. The unexpected optoelectronic properties of perovskite material and high performance of these cells makes them an exceptional choice for solar photovoltaics. This work aims to propose and investigate four design variants of regular homojunction lead-based PSC namely uniform absorber PSC, uniform absorber optimum PSC, graded absorber PSC and graded absorber optimum PSC with structure as TCO/WS2/MAPbI3(n)/MAPbI3(p)/MoO3/Metal. The homojunction structure explored in this work identify absorber thickness, doping density, defect density, temperature, diffusion length, and lifetime of charge carriers as strong contestants in PSC design. In this paper, all the key parameters of uniform absorber PSC are evaluated, analyzed, discussed, and further fine-tuned to obtain optimum results with Power Conversion Efficiency (PCE) of 26.81%. Further, the impact of graded absorber layer on performance of PSC is investigated. Results show that grading in absorber layer will improve the performance parameters of PSC. Graded absorber optimum PSC shows efficiency of 28.26% which is higher than the uniform absorber PSC by 7.48%.

Recombination and localization: Unfolding the pathways behind conductivity losses in Cs2AgBiBr6 thin films

Cs2AgBiBr6 (CABB) has been proposed as a promising nontoxic alternative to lead halide perovskites. However, low charge carrier collection efficiencies remain an obstacle for the incorporation of this material in optoelectronic applications. In this work, we study the optoelectronic properties of CABB thin films using steady state and transient absorption and reflectance spectroscopy. We find that optical measurements on such thin films are distorted as a consequence of multiple reflections within the film. Moreover, we discuss the pathways behind conductivity loss in these thin films, using a combination of microsecond transient absorption spectroscopy and time-resolved microwave conductivity measurements. We demonstrate that a combined effect of carrier loss and localization results in the conductivity loss in CABB thin films. Moreover, we find that the charge carrier diffusion length and grain size are of the same order of magnitude. This suggests that the material's surface is an important contributor to charge-carrier loss.

Extraordinarily High Minority Charge Carrier Lifetime Observed in Crystalline Silicon

Recent progress in surface passivation technology and wafer pretreatment already resulted in significant improvements in the achievable minority charge carrier lifetime of crystalline silicon. Herein, this is further exemplified by studying the lifetime on lowly doped crystalline silicon wafers passivated by poly‐Si. To ensure credible lifetime measurements multiple measurement techniques are compared and good agreement between the investigated approaches is found. The resulting lifetime curves are analyzed in detail and the main limitation is very likely caused by silicon bulk recombination—most likely due to impurities. This analysis indicates that even very low impurity concentrations can be a limiting factor at the extraordinary high level of charge carrier lifetime observed in this study. Despite these limitations, lifetimes of 0.18 s on p‐type and 0.5 s on n‐type crystalline silicon wafers are measured, which to our knowledge exceed previously reported lifetimes. In both cases, these measured lifetimes correspond to an effective minority charge carrier diffusion length of ≈2.5 cm.

Charge Carrier Diffusion Dynamics in Multisized Quaternary Alkylammonium-Capped CsPbBr3 Perovskite Nanocrystal Solids.

CsPbBr3 quantum dots (QDs) are promising candidates for optoelectronic devices. The substitution of oleic acid (OA) and oleylamine (OLA) capping agents with a quaternary alkylammonium such as di-dodecyl dimethyl ammonium bromide (DDAB) has shown an increase in external quantum efficiency (EQE) from 0.19% (OA/OLA) to 13.4% (DDAB) in LED devices. The device performance significantly depends on both the diffusion length and the mobility of photoexcited charge carriers in QD solids. Therefore, we investigated the charge carrier transport dynamics in DDAB-capped CsPbBr3 QD solids by constructing a bi-sized QD mixture film. Charge carrier diffusion can be monitored by quantitatively varying the ratio between two sizes of QDs, which varies the mean free path of the carriers in each QD cluster. Excited-state dynamics of the QD solids obtained from ultrafast transient absorption spectroscopy reveals that the photogenerated electrons and holes are difficult to diffuse among small-sized QDs (4 nm) due to the strong quantum confinement. On the other hand, both photoinduced electrons and holes in large-sized QDs (10 nm) would diffuse toward the interface with the small-sized QDs, followed by a recombination process. Combining the carrier diffusion study with a Monte Carlo simulation on the QD assembly in the mixture films, we can calculate the diffusion lengths of charge carriers to be ∼239 ± 16 nm in 10 nm CsPbBr3 QDs and the mobility values of electrons and holes to be 2.1 (± 0.1) and 0.69 (± 0.03) cm2/V s, respectively. Both parameters indicate an efficient charge carrier transport in DDAB-capped QD films, which rationalized the perfect performance of their LED device application.

All‐Inorganic CsPbBr3 Perovskite Nanocrystals/2D Non‐Layered Cadmium Sulfide Selenide for High‐Performance Photodetectors by Energy Band Alignment Engineering

AbstractPerovskites have attracted intensive attention as promising materials for the application in various optoelectronic devices due to their large light absorption coefficient, high carrier mobility, and long charge carrier diffusion length. However, the performance of the pure perovskite nanocrystals‐based device is extremely restricted by the limited charge transport capability due to the existence of a large number of the grain boundary between perovskite nanocrystals. To address these issues, a high‐performance photodetector based on all‐inorganic CsPbBr3 perovskite nanocrystals/2D non‐layered cadmium sulfide selenide heterostructure has been demonstrated through energy band engineering with designed typed‐II heterostructure. The photodetector exhibits an ultra‐high light‐to‐dark current ratio of 1.36 × 105, a high responsivity of 2.89 × 102 A W−1, a large detectivity of 1.28 × 1014 Jones, and the response/recovery time of 0.53s/0.62 s. The enhancement of the optoelectronic performance of the heterostructure photodetector is mainly attributed to the efficient charge carrier transfer ability between the all‐inorganic CsPbBr3 perovskites and 2D cadmium sulfide selenide resulting from energy band alignment engineering. The charge carriers’ transfer dynamics and the mechanism of the CsPbBr3 perovskites/2D non‐layered nanosheets interfaces have also been studied by state‐state PL spectra, fluorescence lifetime imaging microscopy, time‐resolved photoluminescence spectroscopy, and Kelvin probe force microscopy measurements.

The Progress of Additive Engineering for CH3NH3PbI3 Photo-Active Layer in the Context of Perovskite Solar Cells

Methylammonium lead triiodide (CH3NH3PbI3/MAPbI3) is the most intensively explored perovskite light-absorbing material for hybrid organic–inorganic perovskite photovoltaics due to its unique optoelectronic properties and advantages. This includes tunable bandgap, a higher absorption coefficient than conventional materials used in photovoltaics, ease of manufacturing due to solution processability, and low fabrication costs. In addition, the MAPbI3 absorber layer provides one of the highest open-circuit voltages (Voc), low Voc loss/deficit, and low exciton binding energy, resulting in better charge transport with decent charge carrier mobilities and long diffusion lengths of charge carriers, making it a suitable candidate for photovoltaic applications. Unfortunately, MAPbI3 suffers from poor photochemical stability, which is the main problem to commercialize MAPbI3-based perovskite solar cells (PSCs). However, researchers frequently adopt additive engineering to overcome the issue of poor stability. Therefore, in this review, we have classified additives as organic and inorganic additives. Organic additives are subclassified based on functional groups associated with N/O/S donor atoms; whereas, inorganic additives are subcategorized as metals and non-metal halide salts. Further, we discussed their role and mechanism in terms of improving the performance and stability of MAPbI3-based PSCs. In addition, we scrutinized the additive influence on the morphology and optoelectronic properties to gain a deeper understanding of the crosslinking mechanism into the MAPbI3 framework. Our review aims to help the research community, by providing a glance of the advancement in additive engineering for the MAPbI3 light-absorbing layer, so that new additives can be designed and experimented with to overcome stability challenges. This, in turn, might pave the way for wide scale commercial use.

Explaining the Fill‐Factor and Photocurrent Losses of Nonfullerene Acceptor‐Based Solar Cells by Probing the Long‐Range Charge Carrier Diffusion and Drift Lengths

AbstractOrganic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their open‐circuit voltage loss and fill‐factor, with state‐of‐the‐art OSCs rarely displaying fill‐factor of 80% and above. The fill‐factor of transport‐limited solar cells, including organic photovoltaic devices, is affected by material and device‐specific parameters, whose combination is represented in terms of the established figures of merit, such as θ and α. Herein, it is demonstrated that these figures of merit are closely related to the long‐range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady‐state photoconductance measurements. This yields a straightforward way of determining θ and α in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices—including the high efficiency nonfullerene acceptor blends—and show that the diffusion length of the free carriers provides a good correlation with the fill‐factor. It is, finally, concluded that most state‐of‐the‐art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor.

Effects of phthalocyanine nanostructure on photovoltaic performance of its polymer composite thin films

In this work, nanocomposite polymer solar cells with PEDOT:PSS/PTB7-Th:ITIC:MPc/Al were developed. A PTB7-Th:ITIC bulk heterojunction films doped with metal phthalocyanine nanoribbons were prepared by simple spin-coating methods from chlorobenzene solution. Phthalocyanine nanoribbons were synthesized by a temperature gradient physical vapor deposition method at temperature of 440–470 °C and an argon flow rate in the reaction zone of 150 cm3/min. The loading concentration of phthalocyanine nanoribbons in the PTB7-Th:ITIC mixture was 0.5%. The surface morphology of samples were probed by an atomic force microcopy (AFM). It was found that the surface roughness (Rq) of the PTB7-TH:ITIC photoactive layer decreases approximately two times after doping with phthalocyanine nanoribbons. The absorption spectra of composite films are measured and it was found that the incorporation of phthalocyanine nanoribbons into the PTB7-Th:ITIC photoactive layer leads to the broadening of the optical spectrum and to an increase in the optical density at the absorption maximum by a factor of 1.6. I–V characteristic of PEDOT:PSS/PTB7-Th:ITIC:MPc/Al composite organic solar cells were measured. According to the I–V characteristic, doping of the PTB7-Th:ITIC photoactive layer with phthalocyanine nanoribbons increased the short-circuit current density by 12% and boosted power conversion efficiency by to 4.82% versus an undoped device. In order to understand the effect of phthalocyanine doping on charge transport dynamics, the impedance spectra of composite organic solar cells were measured. An analysis of impedance spectra showed that doping of the photoactive layer with phthalocyanine nanoribbons led to an increase in the effective diffusion length (Deff) of charge carriers by a factor of 3.2 and an increase in the mobility of charge carriers (μ) by a factor of 1.4. The quantum efficiency of composite organic cells was measured and it was revealed that the doping of the photoactive layer with phthalocyanine nanoribbons boosted the quantum efficiency by 4%.

Limits to Electrical Mobility in Lead-Halide Perovskite Semiconductors.

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH3NH3PbI3. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators.

Influence of Ga back grading on voltage loss in low‐temperature co‐evaporated Cu(In,Ga)Se2 thin film solar cells

AbstractThe performance of Cu(In,Ga)Se2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (VOC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (ΔGGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the ΔGGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both VOC deficit and time‐resolved photoluminescence lifetime are observed with increasing ΔGGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a ΔGGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high‐performance CIGS solar cells and devices.

In-Situ Nano-Auger Probe of Chloride-Ions during CH3NH3PbI3-xClx Perovskite Formation.

Organo-halide perovskite solar cells (PSCs) have emerged as next-generation photovoltaics, owing to their high power-conversion efficiency (PCE), lower production cost, and high flexibility. ABX3-structured methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) perovskite is a widely studied light-absorbing material in PSCs. Interestingly, a small amount of chlorine incorporation into MAPbI3 increases charge carrier diffusion lengths (from 129 nm to 1069 nm), which enables planar structured PSCs with high PCEs. However, existence of chloride ions in the final perovskite film is still under debate. Contrastingly, few studies reported a negligible amount or absence of chloride ions in the final film, while others reported detection of chloride ions in the final film. Herein, we observed the microstructure and chlorine content of MAPbI3−xClx thin films with increasing temperature via an in-situ nano-Auger spectroscopy and in-situ scanning electron microscopic analysis. The relative precipitation of MAPbI3−xClx films occur at lower temperature and MAPbI3−xClx grains grow faster than those of MAPbI3 grains. Local concentrations of chlorine at intragrain and the vicinity of grain boundary were analyzed to understand the behavior and role of the chloride ions during the microstructural evolution of the MAPbI3−xClx films.

Advanced Series Resistance Imaging for Silicon Solar Cells via Electroluminescence

Herein, a method for advanced series resistance imaging via electroluminescence (EL) for silicon solar cells is presented. The well‐known method by Haunschild et al. is revisited. The Fuyuki assumption of a linear relation between diffusion length and EL signal is shown to be not applicable to silicon devices nowadays due to larger minority charge carrier diffusion lengths and thinner solar cells. A new relation derived by Breitenstein is used here instead. The updated method for series resistance and saturation current imaging based on two EL images, which show a far superior separation of contrast and almost 60% shorter data acquisition times in comparison with the original method by Haunschild, is presented. The Weber contrast of the unwanted signal caused by recombination in resistance image is reduced from 0.89 to 0.44, using the advanced method for a prominent feature on the sample cell. The contrast due to resistance stays at the same level. The dark saturation current density images show 20% higher peaks at recombination active areas and also a 5% low.

Photonic crystal based photoelectrochemical cell for solar fuels

AbstractThe solar fuel production efficiency of photoelectrochemical cells based on TiO2 is primarily limited by the low optical absorption coefficient and concomitant long penetration depth of visible light compared with the minority charge carrier diffusion length. A TiO2 photoelectrode consisting of a slanted conical‐pore photonic crystal immersed in water and integrated on a reflective back‐contact is proposed to reduce the light penetration depth by more than 10 times and branch the electron and hole transports into dimensions matching their diffusion lengths. This brings the photoelectrolytic process into the high‐efficiency operating regime. The maximum achievable photocurrent density for a lightly doped TiO2 with the imaginary part of the dielectric constant 0.01 in the visible range reaches 7.65 mA cm‐2.

Numerical analysis of ultrathin Sb2Se3-based solar cells by SCAPS-1D numerical simulator device

Antimony selenide is considering as an emerging photovoltaic solar cell absorber. In this paper, Solar Cell Capacitance Simulator in 1 Dimension (SCAPS-1D) is used to investigate the possibility of realizing ultrathin Sb2Se3-based solar cells. The comparison of the current-voltage characteristic and output performances simulation results of CdS/Sb2Se3 solar cells with and without HTL are in agreement with the experimental results. In the first step, by considering the cell without HTL, the best PCE of 5.29% is obtained with WS2 buffer layer. Thereafter, we simulated the impact of the charge carriers diffusion length and the doping concentration on the output performances. By combining a high quality absorber and doping concentration in the order of 1015 cm−3, Sb2Se3 solar cell achieves high PCE above 10%. Secondly, we introduced a HTL between the absorber layer and back metal contact, which led to n-i-p configuration. This configuration with CZ-TA HTL shows a best PCE of 6.29%. For a high quality absorber, Sb2Se3-based solar cell achieves best PCE of 11.10% and better stability for a thickness of 250 nm and doping concentration of 1014 cm−3 of the Sb2Se3 absorber layer. Our numerical solar cell design provides an approach to further improve the efficiency of Sb2Se3-based solar cells.

Device simulation of all-perovskite four-terminal tandem solar cells: towards 33% efficiency

Inorganic–organic hybrid perovskites offer wide optical absorption, long charge carrier diffusion length, and high optical-to-electrical conversion, enabling more than 25% efficiency of single-junction perovskite solar cells. All-perovskite four-terminal (4T) tandem solar cells have gained great attention because of solution-processability and potentially high efficiency without a need for current-matching between subcells. To make the best use of a tandem architecture, the subcell bandgaps and thicknesses must be optimized. This study presents a drift-diffusion simulation model to find optimum device parameters for a 4T tandem cell exceeding 33% of efficiency. Optimized subcell bandgaps and thicknesses, contact workfunctions, charge transport layer doping and perovskite surface modification are investigated for all-perovskite 4T tandem solar cells. Also, using real material and device parameters, the impact of bulk and interface traps is investigated. It is observed that, despite high recombination losses, the 4T device can achieve very high efficiencies for a broad range of bandgap combinations. We obtained the best efficiency for top and bottom cell bandgaps close to 1.55 eV and 0.9 eV, respectively. The optimum thickness of the top and bottom cells are found to be about 250 nm and 450 nm, respectively. Furthermore, we investigated that doping in the hole transport layers in both the subcells can significantly improve tandem cell efficiency. The present study will provide the experimentalists an optimum device with optimized bandgaps, thicknesses, contact workfunctions, perovskite surface modification and doping in subcells, enabling high-efficiency all-perovskite 4T tandem solar cells.

Progress of lead-free perovskite and its resistance switching performance

With the rapid development of the information age, the demand for information storage capacity and miniaturization of memory units has been being increased. However, the commonly used silicon-based flash memory has nearly approached to its physical limit. The resistive switching random access memory (ReRAM) has become one of the promising candidates for the next-generation non-volatile memory due to its simple structure, fast operation speed, excellent flexibility, and long endurance. Recently, we witnessed that the lead halide perovskites, as hot star materials, have been widely used in optoelectronic fields owning to their advantages of low cost, excellent photoelectric properties, and solution process ability. Moreover, the lead halide perovskite has been successfully used as the active layer in ReRAM device because of its tunable bandgap, long charge carrier diffusion length, fast ion migration, and high charge carrier mobility. Whereas the toxicity of lead in halide perovskite is a very horrible problem in lead halide perovskite-based ReRAM devices. The lead-free halide perovskite is considered to be the most promising material for perovskite-based ReRAM devices because it does not contain lead element. Most recently, a large number of scientists from different groups have begun to study lead-free perovskite-based ReRAM devices. For example, tin, bismuth, antimony, and copper-based halide perovskite materials have been utilized in ReRAM devices and exhibited excellent resistance switching (RS) performances. Here in this paper, the recent development of lead-free perovskite and its RS performance are reviewed, including lead-free halide perovskite materials, RS performances, and RS mechanisms of lead-free perovskite-based ReRAM. Finally, the key problems and development prospects of lead-free perovskite-based ReRAM are also presented, which provides a fundamental step towards developing the RS performance based on lead-free halide perovskites.