awn

Thienothiophene-Based Cation Treatment

In article number 2103130, Selcuk Yerci, Gorkem Gunbas, and co-workers fabricate perovskite solar cells (PSC) by treating 3D perovskite surfaces with a novel cation, which enables higher power conversion efficiency (PCE) and improved stability. The PCE enhancement is explained by drift diffusion modeling. Additionally, treated 3D perovskite based semitransparent PSCs are realized with increased stability and with one of the highest reported efficiencies for double cationic 3D perovskites.

Two nonhalogenated polymers, PB1 and PB2, with different side-chain orientations and deep highest occupied molecular orbital (HOMO) levels are reported. In organic photovoltaic (OPV) cells, PB1 only produces a power conversion efficiency (PCE) of 5.3%, while PB2 gives an outstanding PCE of 17.7%. More importantly, PB2 has good compatibility with various electron acceptors. PB2 achieves excellent PCEs of 18.4% and 27.1% for ternary OPV cells and indoor light photovoltaic devices, respectively.

Abstract

Nonhalogenated polymers have great potential in the commercialization of organic photovoltaic (OPV) cells due to their advantage in low-cost preparation. However, non-halogenated polymers usually have high highest occupied molecular orbital (HOMO) energy levels and inferior self-aggregation properties in solution, thus resulting in low power conversion efficiencies (PCEs). Herein, two nonhalogenated polymers, PB1 and PB2, are prepared. When the polymers are used to fabricate OPV cells with BTP-eC9, the PB1-based device only gives a PCE of 5.3%, while the PB2-based device shows an outstanding PCE of 17.7%. After the introduction of PBDB-TF as the third component, the PB2:PBDB-TF:BTP-eC9-based device with an optimal weight ratio of 0.5:0.5:1 achieves a PCE up to 18.4%. More importantly, PB2 exhibits good compatibility with various nonfullerene acceptors to achieve better PCEs than those of classical polymer (PBDB-T and PBDB-TF)-based devices. When PB2 is combined with a wide-bandgap electron acceptor (F-BTA3), this device shows excellent PCE of 27.1% and 24.6% for 1 and 10 cm² devices, respectively, under light intensity of 1000 lux light-emitting diode illumination. These results provide new insight in the rational design of novel nonhalogenated polymer donors for further development of low-cost materials and broadening the application of OPV cells.

Pure formamidinum (FA)-based 2D perovskite solar cells (PSCs) achieve a record power conversion efficiency (PCE) of 21.07% (certified over 20%), the highest efficiency for low-dimensional PSCs (n ≤ 10) reported to date, together with the improved device stability. The high-efficiency device exhibits a narrowed bandgap and unique 2D–3D intermixing phase distribution for improved light absorption and superior charge transport.

Abstract

Owing to their insufficient light absorption and charge transport, 2D Ruddlesden–Popper (RP) perovskites show relatively low efficiency. In this work, methylammonium (MA), formamidinum (FA), and FA/MA mixed 2D perovskite solar cells (PSCs) are fabricated. Incorporating FA cations extends the absorption range and enhances the light absorption. Optical spectroscopy shows that FA cations substantially increase the portion of 3D-like phase to 2D phases, and X-ray diffraction (XRD) studies reveal that FA-based 2D perovskite possesses an oblique crystal orientation. Nevertheless, the ultrafast interphase charge transfer results in an extremely long carrier-diffusion length (≈1.98 µm). Also, chloride additives effectively suppress the yellow δ-phase formation of pure FA-based 2D PSCs. As a result, both FA/MA mixed and pure FA-based 2D PSCs exhibit a greatly enhanced power conversion efficiency (PCE) over 20%. Specifically, the pure FA-based 2D PSCs achieve a record PCE of 21.07% (certified at 20%), which is the highest efficiency for low-dimensional PSCs (n ≤ 10) reported to date. Importantly, the FA-based 2D PSCs retain 97% of their initial efficiency at 85 °C persistent heating after 1500 h. The results unambiguously demonstrate that pure-FA-based 2D PSCs are promising for achieving comparable efficiency to 3D perovskites, along with a better device stability.

Herein, Sr-doped CsPbI₃ quantum dots (QDs) as an interfacial layer for CsPbIBr₂ solar cells are introduced. The simultaneous Sr²⁺ ion doping and surface Cl⁻ ion passivation for CsPbI₃ QDs results in an enhanced photoluminescence quantum yield with an increased carrier lifetime. The resulting CsPbIBr₂ solar cells achieve a highly efficient power conversion efficiency of 10.32%.

Interfacial recombination and nonradiative recombination in inorganic CsPbIBr₂ solar cells impede the device performance. Herein, surface passivation of CsPbIBr₂ inorganic perovskite layers with Sr-doped CsPbI₃ quantum dots (QDs), which act as an efficient interlayer to reduce interfacial recombination and enhance hole extraction as well, is reported for the first time. It is found that the simultaneous Sr²⁺ ion doping and surface Cl⁻ ion passivation for CsPbI₃ QDs results in an enhanced photoluminescence quantum yield with an increased carrier lifetime. The resulting perovskite solar cells achieve a highly efficient power conversion efficiency of 10.32% with enhanced high open circuit V _oc of 1.20 V. It is logically inferred that this approach can be a promising tool for improving device performance in inorganic perovskite solar cells.

Super flexible p–i–n-type transparent conducting oxide (TCO)-free perovskite solar cells are demonstrated on lithium bis(trifluoromethane) sulfonimide (Li-TFSI)-doped graphene electrodes on polydimethylsiloxane substrates. Li-TFSI increases the single-layered graphene conductivity while codoping the poly(triarylamine) hole-transporting layer. The resulting flexible TCO-free perovskite solar cells produce a best power conversion efficiency of 19.01% and show excellent bending and long-term photostability.

Highly efficient organic–inorganic hybrid perovskite solar cells (OIHP-SCs) are often fabricated on a transparent conducting oxide (TCO) substrate such as indium tin oxide (ITO). However, the presence of TCOs is disadvantageous to the development of flexible OIHP-SCs due to the brittle nature of ITO which is easily breakable during bending. Herein, a flexible TCO-free OIHP-SC is demonstrated by using lithium bis(trifluoromethane)sulfonimide (Li-TFSI) as a codopant for the single-layer graphene transparent conducting electrode and poly(triarylamine) hole-transporting material (HTM) on a flexible polydimethylsiloxane substrate. The optical and electrical properties of the Li-TFSI-doped graphene substrate are measured by controlling the doping amount and the best conditions for charge extraction are established at a doping concentration of 20 mm Li-TFSI, thus optimizing the device photovoltaic performance. As a result, a highest power conversion efficiency of 19.01% is demonstrated by the flexible TCO-free OIHP-SC devices with an active area of 1 cm². In addition, the flexible TCO-free OIHP-SCs exhibit good bending stability after 5000 bending cycles at radii of 6, 4, and 2 mm and excellent light soaking stability under 1 Sun light intensity over 1000 h as opposed to the poor stability when using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate as the HTM.

Herein, it is demonstrated that a two-step postfiring bias treatment is able to evidently enhance the efficiency of commercial gallium-doped passivated emitter and rear cell solar cells by up to 0.1% absolute. Furthermore, based on the proposed new model in light of hydrogen behavior under electric field, the mechanisms underlying the two-step bias treatments on improving cell efficiency are discussed.

Passivated emitter and rear cell (PERC) solar cells have dominated the photovoltaic market in recent years. Continuously improving the efficiency of PERC solar cells is of great importance to enable the goal of low electricity cost, which is cheaper than the cost of thermal power generation. Herein, it is demonstrated that a two-step postfiring bias treatment is able to evidently enhance the efficiency of commercial gallium-doped PERC solar cells by up to 0.1% absolute. In detail, the first-step bias treatment is done by forward biasing the PERC solar cells at 12 A and 200 °C for 60 min, resulting in an average efficiency enhancement at around 0.05% absolute. The second-step bias treatment is done by reverse biasing the PERC solar cells at −0.1 or −0.2 V and at the elevated temperatures for certain times, leading to another average efficiency enhancement at around 0.05% absolute. To explore the mechanism underlying the two-step bias treatments on improving cell efficiency, a new model in light of hydrogen behavior under electric field is proposed to explain this phenomenon.

Contrary to all semiconductor particles possessing photoluminescence blinking, MAPbI₃ crystals show a clear maximal characteristic time for the switching process on the order of 1–10 s. This observation may mean that MAPbI₃ does not have a memory of crystal stress/atom rearrangement longer than 10 s. This unique phenomenon for a crystal solid possibly originates from the viscoelasticity of the material.

Abstract

Photoluminescence (PL) blinking is a common phenomenon in nanostructured semiconductors associated with charge trapping and defect dynamics. PL blinking kinetics exhibit very broadly distributed timescales. The traditionally employed analysis of probability distribution of ON and OFF events suffers from ambiguities in their determination in complex PL traces making its suitability questionable. Here, the statistically correct power spectral density (PSD) estimation method applicable for fluctuations of any complexity is employed. PSDs of the blinking traces of submicrometer MAPbI₃ crystals at high frequencies follow power law with excitation power density dependent parameters. However, at frequencies less than 0.3 Hz, the majority of the PSDs saturate revealing the presence of a maximal characteristic timescale of blinking in the range of 0.5–10 s independently of the excitation power density. Super-resolution optical microscopy shows the characteristic timescale to be an inherent material property independent of polycrystallinity. Thus, for the first time the maximum timescale of the multiscale blinking behavior of nanoparticles is observed demonstrating that the power law statistics are not universal for semiconductors. It is proposed that the viscoelasticity of metal-halide perovskites can limit the maximum timescale for the PL fluctuations by limiting the memory of preceded deformations/re-arrangements of the crystal lattice.

Virus-Inoculated Perovskites

In article number 2101221, Jin-Woo Oh, Hyung Do Kim, Il Jeon, and co-workers report genetic modification of M13 bacteriophages to amplify amino acid K, which functions as a perovskite growth template and a stronger passivator than the wild-type virus in PSCs. The modified virus-added perovskite solar cells exhibited a higher efficiency of 23.6% than the wild-type counterparts (22.8%). The observed enhancement is attributed to slightly larger perovskite grains and strong grain boundary passivation. The obtained device efficiency has been certified by the national research center.

An interdigitated bulk heterojunction structure of the active layer is developed by preforming the porous donor polymer film and then processing the acceptor on top, which yields an impressive high power conversion efficiency of 18.74% for polymer solar cells.

Abstract

The most popular approach to fabricating organic solar cells (OSCs) is solution processing a mixture of donor (D) and acceptor (A) materials into an active layer with a bulk heterojunction (BHJ) nanostructure. Herein, it is demonstrated that the interdigitated heterojunction (IHJ) is a more suitable nanostructure of the active layer for high-performance OSCs whereas it is a long standing challenge to realize well-defined IHJ structures. In this study, a facile and versatile sequential solution processing method is developed to produce an IHJ nanostructure with power conversion efficiency reaching 18.74% (18.10% for BHJ the counterpart) by fabricating a donor film with nanopores created by a wax additive, sequentially casting the acceptor on top of infiltrating the nanopores. Compared to the BHJ, the IHJ structure with an interpillar distance within the exciton diffusion length can afford a large bulk D/A interface for efficient exciton dissociation with a minimized charge recombination while free electrons and holes can transport to the respective electrodes through more straightforward pathways, thus enhance performance. Furthermore, the D or A phase in the IHJ device contacts with only one electrode, which can prevent shunting between the anode and cathode and facilitate the industrial mass production of OSCs.

Stability and toxicity of PSCs are bottleneck challenges for their commercial development. Herein, biomimetic Di-g molecules are introduced to the encapsulated FPSCs as the interface layer, which improves the efficiency of 1.01 cm² FPSCs up to 20.29%. Importantly, they demonstrate excellent mechanical stability and lead leakage suppression, the efficiency maintains 85% of initial value without ion leakage under 10 000 bending cycles.

Abstract

Although outstanding power conversion efficiency (PCE) has been achieved in flexible perovskite solar cells, unsatisfactory operational stability and toxicity caused by the moisture transmittance of polymer packaging are still the bottleneck challenges that limit their applications. Herein, inspired by the non-selective permeability of inactivated cell membrane, the diphosphatidyl-glycerol (Di-g) is tactfully introduced as a self-shield interface upon the perovskite layer. 96% of lead leakage is suppressed because the amphipathic Di-g can simultaneously bind tightly to the divalent lead ion and afford an interfacial water-resistance. More importantly, the gradient distribution of lattice residual stress perpendicular to the substrate are optimized. The resultant flexible devices achieve a PCE of 20.29% and 15.01% at effective areas of 1.01 and 21.82 cm² respectively, yielding excellent environmental and mechanical stability. This strategy exhibits the feasibility of developing interfacial encapsulation to stabilize scalable PSCs with negligible lead leakage.

Herein, by partial substitution of the A-site cation using diethylammonium iodide (DEAI), deeper energy levels are obtained. At the same time, the trap density is reduced, and the grain size is significantly improved. The fabricated solar cell shows much enhanced efficiency from 7.31% to 10.28% with the stability of 50 days maintaining 78%.

Environment-friendly tin perovskite solar cells (T-PKSCs) are the most suitable alternative candidate for lead-free PKSCs. However, the photovoltaic performance of such T-PKSCs is far below those of lead-based perovskite solar cells due to an energetic mismatch between the perovskite layer and charge transport layers. Herein, it is shown that, by partial substitution of the A-site cation using diethylammonium iodide (DEAI) substitution, deeper energy levels are obtained. At the same time, the trap density is reduced and the grain size is significantly improved. The fabricated solar cell shows much enhanced efficiency from 7.31% to 10.28%, short-circuit current density from 18.68 to 21.69 mA cm⁻², open-circuit voltage from 0.59 to 0.67 V, and fill factor from 0.67 to 0.71 after DEAI substitution. Such an efficiency improvement can be explained by matching energy levels at the interfaces between perovskite layer and the charge transport layers. In addition, after 50 days of storage, the modified T-PKSCs demonstrate high stability maintaining 78% of its initial efficiency, whereas the reference device degrades to 68% during 28 days storage.

Publication date: 17 November 2021

Source: Joule, Volume 5, Issue 11

Author(s): Wenxiao Zhang, Xiaodong Li, Sheng Fu, Xiaoyan Zhao, Xiuxiu Feng, Junfeng Fang

This is the first example where the fused aromatic ring unit is utilized as a conjugated side chain to extend the conjugation system of small-molecular donors. This strategy effectively achieves improved short-circuit current density and fill factor in organic solar cells (OSCs). Together with Y6 as acceptor, the all-small-molecule OSC achieves an improved power conversion efficiency of 13.6%.

Side-chain engineering influences the organic solar cell (OSC) devices active layer morphology fundamentally and it is an efficient strategy to improve device performances. A novel small molecular donor with conjugated fused-aromatic-ring side chain on a benzo[1,2-b:4,5-b′]dithiophene core is developed. To the best of our knowledge, this is the first example where fused-aromatic-ring units are being utilized as conjugated side chains to extend the conjugation system of small molecular donor materials. The material property, OSC device performance, active layer morphology, and photovoltaic mechanism are investigated. This molecular design strategy has proven to be effective in fine-tuning and enhancing the molecular stacking/orientation and phase separation, which led to the enhanced charge separation and transportation, suppressed charge recombination, and as a result, devices with an improved short-circuit current density of 24.9 mA cm⁻², fill factor of 69.9%, and power conversion efficiency of 13.6% are achieved. The results reveal that a fused-aromatic-ring utilized as conjugated side chain strategy can potentially be an effective way for molecular rational design, aiming to improve the OSC performances.

An embedding 2D/3D heterostructure is in situ formed by incorporating a small amount of 4-guanidinobutanoic acid, which markedly suppresses nonradiative recombination, leading to high efficiencies of 21.45% and 20.16% for the rigid and flexible perovskite devices, respectively.

Abstract

Flexible perovskite solar cells (f-PSCs) have attracted increasing attention because of their enormous potential for use in consumer electronic devices. The key to achieve high device performance is to deposit pinhole-free, uniform and defect-less perovskite films on the rough surface of polymeric substrates. Here, a solvent engineering to tailor the crystal morphology of FA-alloyed perovskite films prepared by one-step blade coating is first deployed. It is found that the use of binary solvents DMF:NMP, rather than the conventional DMF:DMSO, enables to deposit dense and uniform FA-alloyed perovskite films on both the rigid and flexible substrates. As a decisive step, an embedding 2D/3D perovskite heterostructure is in situ formed by incorporating a small amount of 4-guanidinobutanoic acid (GBA). Accordingly, photovoltage increases up to 100 mV are realized due to the markedly suppressed nonradiative recombination, leading to high efficiencies of 21.45% and 20.16% on the rigid and flexible substrates, respectively. In parallel, improved mechanical robustness of the flexible devices is achieved due to the presence of the embedded 2D phases. The results underpin the importance of morphology control and defect passivation in delivering high-performance flexible FA-alloyed flexible perovskite devices.

Methylammonium chloride (MACl) or formamidine chloride (FACl) of perovskite precursor is added into SnO₂ electron transport layer (ETL) and reacts with PbI₂ to form perovskite crystal nucleus, which improves the contact between SnO₂ ETL and perovskite, as well as the morphology and crystallinity of perovskite layer, and greatly improves the fill factor (FF) of perovskite solar cells (PSCs).

Abstract

The electron transport layer (ETL) is a key component of regular perovskite solar cells to promote the overall charge extraction efficiency and tune the crystallinity of the perovskite layer for better device performance. The authors present a novel protocol of ETL engineering by incorporating a composition of the perovskite precursor, methylammonium chloride (MACl), or formamidine chloride (FACl), into SnO₂ layers, which are then converted into the crystal nuclei of perovskites by reaction with PbI₂. The SnO₂-embedded nuclei remarkably improve the morphology and crystallinity of the optically active perovskite layers. The improved ETL-to-perovskite electrical contact and dense packing of large-grained perovskites enhance the carrier mobility and suppress charge recombination. The power conversion efficiency increases from 20.12% (blank device) to 21.87% (21.72%) for devices with MACl (FACl) as an ETL dopant. Moreover, all the precursor-engineered cells exhibit a record-high fill factor (82%).

The strategy of introducing low-cost commercial thermoplastic elastomer in the active layer is put forward to address the long-standing challenge of mechanical properties improvement of organic solar cells. Meanwhile, the morphology and mechanical performance of ternary blend films are effectively modulated and characterized with advanced scattering and microscopic techniques. This approach is effective to many kinds of high-efficiency polymer:nonfullerene systems.

Abstract

Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here, a simple and effective strategy is reported to improve the stretchability and reduce the stiffness of high-efficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents and the molecular weight dependence of SEBS on microstructure and mechanical properties are thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl and PBDB-T:ITIC. More crucially, the elastic modulus of such complex ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.

Herein, lead acetate and lead thiocyanate are explored as dual lead sources to prepare high-quality methylammonium lead iodide perovskite films in ambient air through an eco-friendly way, which results in unprecedented efficiency for perovskite solar cells prepared from non-halide lead sources. In addition, the device also shows excellent air stability.

Abstract

The realization of highly efficient perovskite solar cells (PSCs) in ambient air is considered to be advantageous for low-cost commercial manufacturing. However, it is fundamentally difficult to achieve comparable device performance to that obtained in an inert atmosphere, especially when the ambient humidity is high. Here, an effective precursor engineering that simultaneously employs non-halide lead acetate and lead thiocyanate lead sources for fabricating high-quality methylammonium lead iodide perovskite films in ambient air with enhanced moisture tolerance, is reported. The presence of Ac^– and SCN^– ions not only enables the facile formation of homogeneous and highly crystalized perovskite films, but also directs the uniform growth of the crystals along the (110) direction. Accordingly, a 20.55% efficiency is demonstrated, one of the best results for air-processed MAPbI₃ PSCs, which is also the highest value achieved with non-halide lead sources. Furthermore, the unencapsulated device shows fivefold prolonged air stability (3600 h) compared to the conventional PbI₂-based PSC. Together with the use of non-toxic antisolvent, this strategy is fully compatible with ambient air operation and thus of great potential for practical applications.

Recent developments of the quinoxaline-based D–A copolymers for the applications as polymer donor in polymer solar cells and as hole transport material in perovskite solar cells are reviewed.

Abstract

Polymer solar cells (PSCs) have achieved great progress recently, benefiting from the rapid development of narrow bandgap small molecule acceptors and wide bandgap conjugated polymer donors. Among the polymer donors, the D–A copolymers with quinoxaline (Qx) as A-unit have received increasing attention since the report of the low-cost and high-performance D–A copolymer donor based on thiophene D-unit and difluoro-quinoxalline A-unit in 2018. In addition, the weak electron-deficient characteristic and the multiple substitution positions of the Qx unit make it an ideal A-unit in constructing the wide bandgap polymer donors with different functional substitutions. In this review article, recent developments of the Qx-based D–A copolymer donors, including synthetic method of the Qx unit, backbone modulation, side chain optimization, and functional substitution of the Qx-based D–A copolymers, are summarized and discussed. Furthermore, the application of the Qx-based D–A copolymers as hole transport material in perovskite solar cells (pero-SCs) is also introduced. The focus mainly on the molecular design strategies and structure–properties relationship of the Qx-based D–A copolymers, aiming to provide a guideline for developing high-performance Qx-based D–A copolymers for the applications as donor in PSCs and as hole transport material in pero-SCs.

Energy level bending at the donor-acceptor interface of a solar cell due to the acceptor–donor–acceptor molecular architecture suggests the optimal molecular quadrupole moment of ca. 75 Debye Å for the acceptor molecule.

Abstract

Efficiencies of organic solar cells have practically doubled since the development of non-fullerene acceptors (NFAs). However, generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor–acceptor interface. It is shown that an acceptor–donor–acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is suggested that the molecular quadrupole moment of ≈75 Debye Å balances the losses in the open circuit voltage and gains in charge generation efficiency.

A thorough overview is given on the fundamental properties of tin halide perovskites tailored to their application in solar cells. A detailed discussion of the crystal structure, electronic properties, photophysics, and thin-film crystallization of tin halide perovskites provides a foundation to highlight its exceptional properties and reveals research strategies to improve the performance of tin halide solar cells.

Abstract

Metal halide perovskites have unique optical and electrical properties, which make them an excellent class of materials for a broad spectrum of optoelectronic applications. However, it is with photovoltaic devices that this class of materials has reached the apotheosis of popularity. High power conversion efficiencies are achieved with lead-based compounds, which are toxic to the environment. Tin-based perovskites are the most promising alternative because of their bandgap close to the optimal value for photovoltaic applications, the strong optical absorption, and good charge carrier mobilities. Nevertheless, the low defect tolerance, the fast crystallization, and the oxidative instability of tin halide perovskites currently limit their efficiency. The aim of this review is to give a detailed overview of the crystallographic, photophysical, and optoelectronic properties of tin-based perovskite compounds in their multiple forms from 3D to low-dimensional structures. At the end, recent progress in tin-based perovskite solar cells are reviewed, mainly focusing on the detail of the strategies adopted to improve the device performances. For each subtopic, the current challenges and the outlook are discussed, with the aim to stimulate the community to address the most important issues in a concerted manner.

Lanthanum ions doped 3D all-inorganic perovskite possess a band-edge PL from the α-phase and a self-trapped exciton emission from the δ-phase. They present a color tuning capability spanning from green to red by extension of the soaking time, and the emission properties can recover by removing the light-soaking and undergoing a certain recovery time.

Abstract

Understanding and manipulating the photoluminescence (PL) of all-inorganic perovskites is significant in applications toward light-emitting diodes. Doping has proved to be a very promising approach for tuning the luminescence properties of perovskites. Herein, rare earth (Er and Yb) doped 3D all-inorganic perovskite flakes (CsPb(Cl/Br)₃) are synthesized. At room temperature, they possess a narrow emission peak at 506 nm with 10 nm linewidth and a quite broad peak at 700 nm with 170 nm linewidth, which are from band-edge PL of α-CsPb(Cl/Br)₃ and self-trapped excitons of δ-CsPb(Cl/Br)₃, respectively. When exposing the flakes under a light-soaking, the samples present a color tuning capability spanning from red to green by extension of the soaking time, and the emission properties can recover by removing the light-soaking and undergoing a certain recovery time. Time-resolved photoluminescence indicates a photoinduced reduction of defects density in the doped perovskite flakes. Based on further density functional theory calculations, a photoinduced O₂-diffused defect-passivation mechanism is proposed. The discovery is expected to promote the optical tuning capability of the all-inorganic perovskites and expand their potential application in optoelectronics devices.

2D mixed halide perovskites with bandgap tunability and increased stability are employed in photovoltaics, light-emitting diodes, and optoelectronics. Inherent halide ion mobility (instability) of mixed halides drives photoinduced halide ion segregation. The important role of dimensionality across 3D to 2D perovskites governs the excited-state dynamics and the kinetics of halide segregation and dark recovery.

Abstract

2D lead halide perovskites, which exhibit bandgap tunability and increased chemical stability, have been found to be useful for designing optoelectronic devices. Reducing dimensionality with decreasing number of layers (n = 10–1) also imparts resistance to light-induced ion migration as seen from the halide ion segregation and dark recovery in mixed halide (Br:I = 50:50) perovskite films. The light-induced halide ion segregation efficiency, as determined from difference absorbance spectra, decreases from 20% to <1% as the dimensionality is decreased for 2D perovskite film from n = 10 to 1. The segregation rate constant (k _segregation), which decreases from 5.9 × 10⁻³ s⁻¹ (n = 10) to 3.6 × 10⁻⁴ s⁻¹ (n = 1), correlates well with nearly an order of magnitude decrease observed in charge-carrier lifetime (τ_average = 233 ps for n = 10 vs τ_avg = 27 ps for n = 1). The tightly bound excitons in 2D perovskites make charge separation less probable, which in turn decreases the halide mobility and resulting phase segregation. The importance of controlling the dimensionality of the 2D architecture in suppressing halide ion mobility is discussed.