High Defect Tolerance Research Articles

Combining Perovskites and Quantum Dots: Synthesis, Characterization, and Applications in Solar Cells, LEDs, and Photodetectors

AbstractMetal halide perovskites having high defect tolerance, high absorption characteristics, and high carrier mobility demonstrate great promise as potential light harvesters in photovoltaics and optoelectronics and have experienced an unprecedented development since their occurrence in 2009. Semiconductor quantum dots (QDs), on the other hand, have also been proved to be very flexible toward shape, dimension, bandgap, and optical properties for constructing optoelectronic devices. Of late, a strategic combination of both materials has demonstrated extraordinary promise in photovoltaic applications and optoelectronic devices. Combining QDs and perovskites has proved to be quite an effective strategy toward the formation of pinhole‐free and more stable perovskite crystals along with tunability of other properties. To boost this exciting research field, it is imperative to summarize the work done so far in recent years to provide an intriguing insight. This review is a critical account of the advanced strategy toward combining these two fascinating materials, including their different synthetic approaches regarding heteroepitaxial growth of perovskite crystals on QDs, carrier dynamics at the interface and potential application in the field of solar cells, light emitting diodes, and photodetectors.

Solution chemistry quasi-epitaxial growth of atomic CaTiO3 perovskite layers to stabilize and passivate TiO2 photoelectrodes for efficient water splitting

Perovskite oxides with unique crystal structures and high defect tolerance are promising as atomic surface passivation layers for photoelectrodes for efficient and stable water splitting. However, controllably depositing and crystalizing perovskite-type metal oxides at the atomic level remains challenging, as they usually crystalize at higher temperatures than regular metal oxides. Here, we report a mild solution chemistry approach for the quasi-epitaxial growth of an atomic CaTiO3 perovskite layer on rutile TiO2 nanorod arrays. The high-temperature crystallization of CaTiO3 perovskite is overcome by a sequential hydrothermal conversion of the atomic amorphous TiOx layer to CaTiO3 perovskite. The atomic quasi-epitaxial CaTiO3 layer passivated TiO2 nanorod arrays exhibit more efficient interface charge transfer and high photoelectrochemical performance for water splitting. Such a mild solution-based approach for the quasi-epitaxial growth of atomic metal oxide perovskite layers could be a promising strategy for both fabricating atomic perovskite layers and improving their photoelectrochemical properties.

Searching for Band-Dispersive and Defect-Tolerant Semiconductors from Element Substitution in Topological Materials.

Topological insulators and semimetal materials composed of heavy elements usually have inverted and dispersive band structures. It is interesting to notice that if lighter elements with reduced spin-orbit coupling are substituted for the heavy elements, the topological materials can be mutated into semiconductors with variable band gaps; for example, topological HgTe and Bi2Se3 can be mutated into CdTe and Sb2Se3, which are excellent optoelectronic semiconductors because the element substitution opens the band gap and meanwhile inherits the large band dispersion and high carrier mobility. Recently, many topological materials have been reported, and their databases have been built. Here, we demonstrate that these new topological materials can be used as the starting points to search for semiconductors with high carrier mobility and defect tolerance through element substitution. We take three recently discovered topological materials, Na3Bi, Pb2Bi2Te5, and EuCd2Sb2, as the benchmark systems to show the general validity of this strategy and find that the derived Na3P, Na3As, Sn2Sb2S5, and CaZn2N2 are all band-dispersive and defect-tolerant semiconductors with potential optoelectronic applications. For Na3P, Na3As, and Na3Sb, the new P3̅c1 structure derived from the topological Na3Bi is found unexpectedly to be their ground-state structure, more stable than their well-known structures reported in the literature. This study not only gains new insights into the physical properties of these semiconductors but also proposes an effective strategy for the search of band-dispersive and defect-tolerant semiconductors that can be generalized to other topological materials.

Effect of laser scanning speed on the microstructure, phase transformation and mechanical property of NiTi alloys fabricated by LPBF

This study investigated the effects of laser scanning speed on the microstructure, phase transformation and properties of NiTi alloys fabricated by laser powder bed fusion (LPBF). In this study, the contributions of metallurgical factors under different scanning speeds, such as Ni evaporation, Ni4Ti3 precipitation, dislocations and internal stress, to the transformation temperature and transformation latent heat were clarified through specially designed experiments. Ni evaporation is found to have the most profound effect, followed by precipitation. Increasing scanning speed is found to reduce Ni loss, thus cause less increase in the transformation temperature and transformation heat of the LPBF-NiTi alloys. Increasing scanning speed also increases the microstructure non-uniformity and thus widens the transformation temperature interval. The orientations of residual stress exhibit strong crystallographic stiffness dependence. The LPBF-NiTi alloys with different scanning speeds all exhibited high strains (>13.4%) and excellent shape memory effect. A LPBF-NiTi honeycomb structure exhibited 96% shape recovery rate after a 60% pre-compressive deformation. Besides, there is an optimum scanning speed for minimum porosity and smallest average pore size. However, the pore structure is found to have weak influence on the tensile behaviour of the LPBF-NiTi alloys, possibly due to the high defect tolerance of the martensitic transformation.

Surface Defect Formation and Passivation in Formamidinium Lead Triiodide (FAPbI3) Perovskite Solar Cell Absorbers

Formamidinium lead iodide based hybrid perovskite materials with improved efficiency and stability still lack well-understood surface defect formation mechanisms. Controlling the surface termination and defects has the potential to improve the performance of both conventional 3D and latterly reduced-dimensional perovskites photovoltaics. Here, we characterized the termination and all possible defect formations in FAPbI3 surface by the first-principles calculations. We found that, among the surfaces we considered, FAI-termination exhibits the most stable surface with a high defect tolerance. The PbI2-terminated surface is also found to be relatively stable; however, certain defects, such as electron-donating FA-interstitial and Pb-interstitial defects, can create deep-level stable charge-traps, potentially limiting the optoelectronic performance. We further investigate the surface treatment on these deep defects by model small molecule additives. We found that benzene additive with delocalized electron distribution can effectively passivate the deep FA-interstitial and Pb-interstitial defects by electron donating to the surface defect through charge-transfer.

Influence assessment of artificial defects on the fatigue behavior of additively manufactured stainless steel 316LVM

The laser powder bed fusion of metals (PBF-LB/M) is one of the most promising techniques to realize lightweight optimized parts and structures. Possible design elements are internal cooling channels or topology optimized geometries. However, not only does the process suffer of instabilities causing pores and lack-of-fusion defects but also a low surface quality. A further knowledge about these defects and their influence on the mechanical behavior are needed to use additively manufactured parts in structural relevant applications. In this work, the austenitic stainless steel 316LVM (X2CrNiMo18-15-3) has been processed by PBF-LB/M. In total, four different batches were manufactured with either no intended porosity or specific cubic defects ranging between 0.3 and 1.5 mm edge length. The fatigue behavior was evaluated at stress ratio R = -1 up to 1E7 cycles. The fracture surface was analyzed by scanning electron microscopy and the relationship between artificial defect size and fatigue strength was investigated by Kitagawa-Takahashi (KT) diagram and its modification by El Haddad’s intrinsic crack length. The results show that the KT-diagram underestimates the fatigue strength of the investigated steel indicating a high defect tolerance and possible hardening mechanisms during cyclic loading such as possible nano-twinning. An influence of the entrapped process gas could also play a role. As long as this is unclear, the models can only be used conservatively as the full potential of the PBF-LB/M steel cannot be fully exploited.

Research progress of ABX3-type lead-free perovskites for optoelectronic applications: materials and devices.

In recent years, lead halide perovskite materials have attracted great interest and are widely used in solar cells and light-emitting devices due to their high photoelectronic quantum yield, high color purity, high defect tolerance, long diffusion length, high carrier mobility, and bandgap tunability. However, the application of lead halide perovskites is limited due to the presence of Pb, making lead-free perovskites an important substitute due to their same crystal structure and similar properties. Although some reports have been made on lead-free perovskite materials, there are still great challenges to realize their application due to their poor stability, easy phase transition, and low photoelectric conversion efficiency. Here, we mainly summarize the development and application of ABX3-type lead-free halide perovskite materials, especially in optoelectronic devices. The article first introduces the lattice and energy band structure, the optoelectronic properties of lead-free perovskites, including the research method of lead-free perovskites, and then analyzes the reasons for the low luminous efficiency and poor stability of lead-free perovskite materials. Second, the development history and current situation of lead-free perovskites in different optoelectronic device applications are summarized. Finally, we present the challenges and prospects for the future development of lead-free perovskites.

Interfacial Engineering for Improved Stability of Flexible Perovskite Solar Cells

Because of unique optoelectronic properties, such as extended carrier lifetime, large absorption coefficient, high defect tolerance factor, low exciton binding energy, and ambipolar diffusion, metal halide perovskites display enormous potential for the next generation of photovoltaics. Flexible perovskite solar cells with low weight, high flexibility, and conformability have attracted attention for portable electronic products. The interface is crucial in perovskite solar cells for both photovoltaic efficiency and operational stability. A high-quality interface could be obtained through interface engineering. Here, we summarize the degradation mechanisms and review interfacial engineering with the emphasis on their effects in flexible perovskite solar cells. On the basis of recent research progress in flexible devices, current challenges, possible directions, and perspectives are discussed. This would be helpful to promote the commercialization of flexible perovskite solar cells.

Highly efficient, ultrathin, Cd-free kesterite solar cells in superstrate configuration enabled by band level tuning via Ag incorporation

Kesterite, or Cu2ZnSn(S,Se)4 (CZTS) is promising in developing sustainable PV technology due to its earth-abundant, non-toxic composition. However, issues including instability of interface, high density of defects that fails to allow the short charge-collection length to meet its light absorption needs, use of Cd that fails to comply with the restriction of hazardous substances (RoSH), are promoting the development of alternative, eco-friendly device structure. Here, this study reports an important progress on this subject by adopting the superstrate configuration to kesterite, thus to realize advantageous light management and high defect tolerance in an ultrathin device. By incorporating Ag in kesterite to overcome the detrimental alignment at the pristine interface, a solar cell with PCE of 8.1% has been fabricated with ~ 200 nm absorber and ~ 15 nm TiO2 buffer, representing a PCE improvement of nearly three-fold from the baseline Cu2ZnSn(S,Se)4 device and breaking the 5% PCE limit for superstrate kesterite cells to date. Moreover, this enables the sole use of TiO2 as novel buffer material free of toxic Cd. Further analysis reveals the critical role of Ag in synergistically tailoring band offset and bandgap, along with largely reduced density of defects, leading to this substantial performance improvement.

Enhanced field emission properties of CsPbBr3 films by thermal annealing and surface functionalization with boron nitride

The relatively low work function, high defect tolerance, high electron mobility, and ease of fabrication nanostructures with high aspect ratio make all inorganic cesium lead halide perovskites promising emitter materials, but little research efforts have been devoted to them for potential applications in field emission devices. Herein, we report the enhanced field emission properties of CsPbBr3 films after thermal annealing and boron nitride modification. For as-grown CsPbBr3 films, the turn-on field at a current density of 10 μA cm−2 is 4.0 V μm−1, it decreases to 3.375 V μm−1 after thermal annealing at 150 °C, which is attributed to better crystallinity and formation of nanoscale emitter tips with sharp edges and optimized tips density. The field emission performance is further enhanced by surface functionalization with boron nitride deriving from the surface transfer induced n-type doping of CsPbBr3, and hence reduced work function. As a result, a lowest turn-on field of 2.875 V μm−1 and highest enhancement factor of 2518 are achieved. Finally, a good emission current stability of CsPbBr3 films after boron nitride modification have been demonstrated. The emission current fluctuations are 2.36% and 0.56% at initial current density of 20 and 100 μA/cm2, respectively.

Homojunction Perovskite Quantum Dot Solar Cells with over 1µm-Thick Photoactive Layer.

The solution-processed solar cells based on colloidal quantum dots (QDs) reported so far generally suffer from poor thickness tolerance and it is difficult for them to be compatible with large-scale solution printing technology. However, the recently emerged perovskite QDs, with unique high defect tolerance, are particularly well-suited for efficient photovoltaics. Herein, efficient CsPbI3 perovskite QD solar cells are demonstrated first with over 1µm-thick active layer by developing an internal P/N homojunction. Specifically, an organic dopant 2,2'-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6TCNNQ) is introduced into CsPbI3 QD arrays to prepare different carrier-type QD arrays. The detailed characterizations reveal successful charge-transfer doping of QDs and carrier-type transformation from n-type to p-type. Subsequently, the P/N homojunction perovskite QD solar cell is assembled using different carrier-type QDs, delivering an enhanced power conversion efficiency of 15.29%. Most importantly, this P/N homojunction strategy realizes remarkable thickness tolerance of QD solar cells, showing a record high efficiency of 12.28% for a 1.2µm-thick QD active-layer and demonstrating great potential for the future printing manufacturing of QDs solar cells.

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode.

A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3−x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm−2 up to >0.5 mA cm−2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm−2) at an applied bias of −0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.

Synthesizability and Properties of Metastable Semiconductors for Solar Energy Conversion

Metastable semiconductors in many chemical systems have been discovered that have desirable semiconductor properties for driving fuel-producing redox reactions from water and sunlight, such as strong visible light absorption, optimal band-edge energies, extreme defect tolerance, and enhanced carrier mobilities and charge separation. These properties are intimately related to their metastable nature, i.e., their crystalline structures and compositions lead to thermodynamic instabilities. New results will be presented that demonstrate flux-mediated synthetic routes to new metastable n-type Sn(II)-titanates and p-type Cu(I)-niobates. These semiconductors possess some the smallest known visible-light band gaps that also maintain suitable conduction and valence band energies for the photocatalytic reduction and oxidation of water to molecular hydrogen and oxygen, respectively. Their electronic structures also show advantageous features for solar energy conversion, such as small bandgaps, high band dispersion and defect tolerance. These characteristics are intimately related to their metastability and kinetic stabilization, which can be achieved via solid solutions that inhibit phase segregation. Thus, the solid-solution semiconductors Cu5(Ta1-x Nb x )11O30 and (Ba1-x Sn x )(Zr1-yTi y )O3 have been prepared using flux-mediated synthesis methods. Their band gaps, band edge energies and photocatalytic activities for the production of molecular hydrogen and molecular oxygen are found to be a sensitive function of their chemical compositions.

Near-Ergodic CsPbBr3 Perovskite Nanocrystal with Minimal Statistical Aging.

Optical robustness, uniformity, ergodicity, statistical aging, etc. dictate the applicability of nanocrystals. Based on a series of multimodal statistical analyses such as the Kolmogorov-Smirnov test, Lévy statistics, etc., we demonstrate that for CsPbBr3 perovskite nanocrystals (PNCs): (a) the extent of heterogeneity in the quality and associated physical processes is minimal; (b) the optical robustness is very high, and (c) indeed, a single PNC can depict optical behavior of its ensemble. In addition, toward prospective applications, an optically robust CsPbBr3 PNC exhibits (i) near-ergodicity and (ii) minimal statistical aging, which are extremely vital and complementary to its high defect tolerance.

Limitations and solutions for achieving high-performance perovskite tandem photovoltaics

The large light absorption coefficient, long carrier diffusion length, and high defect tolerance enable organic-inorganic lead halide perovskites for excellent photovoltaic performance. The highest certified power conversion efficiency of single-junction perovskite solar cells (PSCs) has reported 25.5%. Besides, the bandgap of perovskites can be tuned by engineering their composition. These merits have made perovskites promising candidates for tandem photovoltaics, which can cross over the Shockley-Queisser limit of single-junction PSCs with economic costs. However, there are yet some hurdles in the wide-bandgap and narrow-bandgap subcells as well as interconnected layers (ICLs), which limit the commercial applications of perovskite tandem solar cells (PTSCs). In this review, we summarize the major scientific and technical limitations of PTSCs. We firstly demonstrate the configurations and working principles of PTSCs. Then, the developments of front subcells and rear subcells are discussed. Their main drawbacks, implemented technologies, and underlying mechanisms are analyzed in detail. Subsequently, the progress of ICLs which are responsible for guaranteeing continuous current in 2 T PTSCs are discussed. In addition, the stability of PTSCs is also summarized. The purpose of this review is to map and thrive the future development of PTSCs.

A Green Lead Recycling Strategy from Used Lead Acid Batteries for Efficient Inverted Perovskite Solar Cells.

Lead is widely used as a crucial elemental for lead acid batteries (LABs) and emerging halide perovskite solar cells (PSCs). However, the use of soluble lead will raise environmental concerns. For the purpose of Pb recycling, herein, we report a reactant-recycling strategy to extract Pb from used LABs and synthesize high-purity PbI2. The recycled PbI2 shows smaller grain size, higher crystallinity, and higher thermal stability compared to the commercial sources. Perovskite films deposited with the high-quality PbI2 show larger grain size and fewer defects than the commercial ones. Consequently, the synthesized PbI2 enables a power conversation efficiency of 20.45% for the inverted MAPbI3 (MA= methylammonium) PSCs with excellent air stability. This work offers a novel strategy for lead recovery from LABs and a green path for the realization of high-performance PSCs with high defect tolerance.

Halide Perovskites for Memristive Data Storage and Artificial Synapses.

Halide perovskites have been noted for their exotic properties such as fast ion migration, tunable composition, facile synthetic routes, and flexibility in addition to large light absorption coefficients, long carrier diffusion lengths, and high defect tolerance. These properties have made halide perovskites promising materials for memristors. Applications in the field of resistive switching memory devices and artificial synapses for neuromorphic computing are especially noteworthy. This Perspective covers state-of-the-art perovskite-based memristive devices. Moreover, the fundamental mechanisms and characteristics of perovskite-based memristors are elucidated. Interesting opportunities to improve the performance of perovskite-based memristors for commercialization are provided, including improving film uniformity and air stability, controlling the stoichiometry, finding new all-inorganic and lead-free halide perovskites, and making perovskites into single crystals or quantum dots. We expect our Perspective to be the foundation of realizing next-generation halide perovskite-based memristors.

Perovskite Passivation with a Bifunctional Molecule 1,2‐Benzisothiazolin‐3‐One for Efficient and Stable Planar Solar Cells

Even though the perovskite material itself has a high defect tolerance compared with other semiconductors, the defects existing at grain boundaries (GBs) or the surface still cause high densities of the trap state which impair the efficiency and stability of perovskite solar cells (PSCs). Herein, the small molecule 1,2‐benzisothiazolin‐3‐one (BIT) is investigated to passivate the defects at the GBs and surface of perovskite films. The results reveal that the BIT molecule belongs to a type of bifunctional species, which functions as cations and anions to passivate the perovskite surface. The BIT passivation generates lower formation energy and a more stable perovskite structure, resulting in improved performance and stability of the PSCs. The power conversion efficiency (PCE) of the optimized inverted photovoltaic device reaches 21.83%. Meanwhile, high photostability is achieved for PSCs with an efficiency drop of less than 10% under continuous light illumination over 500 h.

Enhancing Defect Tolerance with Ligands at the Surface of Lead Halide Perovskites.

High defect tolerance has been considered a primary reason for the long charge carrier lifetime and high photoluminescence quantum yield in bulk lead halide perovskites (LHPs). On the other hand, surface defects play a critical role in determining charge carrier dynamics and optical properties, especially for LHP nanocrystals and quantum dots. Understanding the nature of surface defects and developing strategy for their effective passivation are thus of strong interest. Focusing on a prototypical LHP, CsPbBr3, our work uses first-principles calculations to reveal that interstitial sites and antisites can have lower formation energies when they form at the surface while simultaneously creating deep trap states within the bandgap. Meanwhile, the formation of halide vacancies is energetically less favorable. On the basis of a new surface defect model, we demonstrate the explicit role of molecular ligands in passivating these defects, which eliminate trap states in favor of shallow states and enhance photoluminescence.

Suppressing ion migration of CsPbBr x I3-x nanocrystals by Nickel doping and the application in high-efficiency WLEDs

All inorganic perovskite nanocrystals CsPbX3(X = Cl, Br, I) are the great potential candidates for the application of high-performance light emitting diodes (LED) due to their high Photoluminescence Quantum Yield (PLQY), high defect tolerance, narrow full-width half-maximum and tunable wavelength of 410–700 nm. However, the application of red-emitting (630–650 nm) CsPbBr x I3-x nanocrystals are perplexed by phase segregation due to the composition of mixed halides and the difference in halide ion mobility. Herein, we provide an effective strategy to suppressing the migration of Br/I ions through Ni2+ doping via a facile Hot-Injection method and the PLQY was improved as well. DFT calculations show that the introduction of Ni2+ causes a slight contraction of the host crystal structure, which improves the bond energy between Pb and halides and reduces the level of surface defects. Therefore, the phase stability is improved by Ni2+ doping because the phase segregation caused by ion migration in the mixed phase is effectively inhibited. Meanwhile, the non-radiative recombination in the exciton transition process is reduced and the PLQY is improved. What’s more, benefiting from the suppressed ion migration and enhanced PLQY, we combine the Ni2+-doped CsPbBr x I3-x nanocrystals with different Br/I ratios and YAG: Ce3+ phosphors as color conversion layers to fabricate high efficiency WLED. When the ratio of Br/I is 9:11, WLED has a color coordinate of (0.3621, 0.3458), the color temperature of 4336 K and presents a high luminous efficiency of 113.20 lm W−1, color rendering index of 94.9 under the driving current of 20 mA and exhibits excellent stability, which shows great potential in the application of LED.