A Two‐Stage Annealing Strategy for Crystallization Control of CH3NH3PbI3 Films toward Highly Reproducible Perovskite Solar Cells

The solvent‐engineering method is widely used to fabricate top‐performing perovskite solar cells, which, however, usually exhibit inferior reproducibility. Herein, a two‐stage annealing (TSA) strategy is demonstrated for processing of perovskite films, namely, annealing the intermediate phase at 60 °C for the first stage then at 100 °C for the second stage. Compared to conventional direct annealing temperature (DHA) at 100 °C, using this strategy, MAPbI₃ films become more controllable, leading to superior film uniformity and device reproducibility with the champion device efficiency reaching 19.8%. More specifically, the coefficient of variation of efficiency for 49 cells is reduced to 5.9%, compared to 9.8% for that using DHA. The TSA process is carefully studied using Fourier transform infrared spectroscopy, X‐ray diffraction, and UV–vis absorption spectroscopy. It is found that in comparison with DHA the formation of hydrogen bonding and crystallization of perovskite are much slower and can be better controlled when using TSA. The improvements in film uniformity and device reproducibility are attributed to: 1) controllable MAPbI₃ crystal growth stemming from the progressive formation of hydrogen bonding between methylammonium and halide; 2) suppression of intermediate phase film dewetting, which is believed to be due to its decreased mobility at the initial low‐temperature annealing stage.     

Extrinsic Ion Distribution Induced Field Effect in CsPbIBr2 Perovskite Solar Cells

Excellent power conversion efficiency (PCE) and stability are the primary forces that propel the all‐inorganic cesium‐based halide perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high density of trap state and internal nonradiative recombination of CsPbIBr₂ perovskite film are the barriers that limit its development. In the present study, a facile additive strategy is introduced to fabricate highly efficient CsPbIBr₂ PSCs by incorporating sulfamic acid sodium salt (SAS) into the perovskite layer. The additive can control the crystallization behaviors and optimize morphology, as well as effectively passivate defects in the bulk perovskite film, thereby resulting in a high‐quality perovskite. In addition, SAS in perovskite has possibly introduced an additional internal electric field effect that favors electron transport and injection due to inhomogeneous ion distribution. A champion PCE of 10.57% (steady‐output efficiency is 9.99%) is achieved under 1 Sun illumination, which surpasses that of the contrast sample by 16.84%. The modified perovskite film also exhibits improved moisture stability. The unencapsulated device maintains over 80% initial PCE after aging for 198 h in air. The results provide a suitable additive for inorganic perovskite and introduce a new conjecture to explain the function of additives in PSCs more rationally.     

Interface Engineering for All‐Inorganic CsPbI2Br Perovskite Solar Cells with Efficiency over 14%

In this work, a SnO₂/ZnO bilayered electron transporting layer (ETL) aimed to achieve low energy loss and large open‐circuit voltage (V_oc) for high‐efficiency all‐inorganic CsPbI₂Br perovskite solar cells (PVSCs) is introduced. The high‐quality CsPbI₂Br film with regular crystal grains and full coverage can be realized on the SnO₂/ZnO surface. The higher‐lying conduction band minimum of ZnO facilitates desirable cascade energy level alignment between the perovskite and SnO₂/ZnO bilayered ETL with superior electron extraction capability, resulting in a suppressed interfacial trap‐assisted recombination with lower charge recombination rate and greater charge extraction efficiency. The as‐optimized all‐inorganic PVSC delivers a high V_oc of 1.23 V and power conversion efficiency (PCE) of 14.6%, which is one of the best efficiencies reported for the Cs‐based all‐inorganic PVSCs to date. More importantly, decent thermal stability with only 20% PCE loss is demonstrated for the SnO₂/ZnO‐based CsPbI₂Br PVSCs after being heated at 85 °C for 300 h. These findings provide important interface design insights that will be crucial to further improve the efficiency of all‐inorganic PVSCs in the future.     

Morphology Engineering for High‐Performance and Multicolored Perovskite Light‐Emitting Diodes with Simple Device Structures

The film morphology is extremely significant for solution processed perovskite devices. Through fine morphology engineering without using any additives or further posttreatments, a full‐coverage and high quantum yield perovskite film has been achieved based on one‐step spin‐coating method. The morphologies and film characteristics of MAPbBr₃ with different MABr:PbBr₂ starting material ratios are in‐depth investigated by scanning electron microscopy, atomic force microscopy, X‐ray diffraction, photoluminescence, and time resolved photoluminescence. High performance organometal halide perovskite light‐emitting didoes (PeLEDs) based on simple device structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/perovskite/TPBi/Ca/Al are demonstrated. The green PeLED based on MAPbBr₃ shows a maximum luminance of 8794 cd m^?2 (at 7.3 V) and maximum current efficiency of 5.1 cd A^?1 (at 5.1 V). Furthermore, a class of hybrid PeLEDs by adjusting the halide ratios of methylammonium lead halide (MAPbX₃, where X is Cl, Br, or I) are also demonstrated at room temperature. These mix‐halogenated PeLEDs show bright luminance (above 100 cd m^?2) with narrow and clean emission bands over the wide color gamut.     

High Performance and Stable All‐Inorganic Metal Halide Perovskite‐Based Photodetectors for Optical Communication Applications

Photodetectors are critical parts of an optical communication system for achieving efficient photoelectronic conversion of signals, and the response speed directly determines the bandwidth of the whole system. Metal halide perovskites, an emerging class of low‐cost solution‐processed semiconductors, exhibiting strong optical absorption, low trap states, and high carrier mobility, are widely investigated in photodetection applications. Herein, through optimizing the device engineering and film quality, high‐performance photodetectors based on all‐inorganic cesium lead halide perovskite (CsPbI_xBr_3–x), which simultaneously possess high sensitivity and fast response, are demonstrated. The optimized devices processed from CsPbIBr₂ perovskite show a practically measured detectable limit of about 21.5 pW cm^?2 and a fast response time of 20 ns, which are both among the highest reported device performance of perovskite‐based photodetectors. Moreover, the photodetectors exhibit outstanding long‐term environmental stability, with negligible degradation of the photoresponse property after 2000 h under ambient conditions. In addition, the resulting perovskite photodetector is successfully integrated into an optical communication system and its applications as an optical signal receiver on transmitting text and audio signals is demonstrated. The results suggest that all‐inorganic metal halide perovskite‐based photodetectors have great application potential for optical communication.     

Europium and Acetate Co‐doping Strategy for Developing Stable and Efficient CsPbI2Br Perovskite Solar Cells

All‐inorganic perovskite solar cells have developed rapidly in the last two years due to their excellent thermal and light stability. However, low efficiency and moisture instability limit their future commercial application. The mixed‐halide inorganic CsPbI₂Br perovskite with a suitable bandgap offers a good balance between phase stability and light harvesting. However, high defect density and low carrier lifetime in CsPbI₂Br perovskites limit the open‐circuit voltage (V_oc < 1.2 V), short‐circuit current density (J_sc < 15 mA cm^?2), and fill factor (FF < 75%) of CsPbI₂Br perovskite solar cells, resulting in an efficiency below 14%. For the first time, a CsPbI₂Br perovskite is doped by Eu(Ac)₃ to obtain a high‐quality inorganic perovskite film with a low defect density and long carrier lifetime. A high efficiency of 15.25% (average efficiency of 14.88%), a respectable V_oc of 1.25 V, a reasonable J_sc of 15.44 mA cm^?2, and a high FF of 79.00% are realized for CsPbI₂Br solar cells. Moreover, the CsPbI₂Br solar cells with Eu(Ac)₃ doping demonstrate excellent air stability and maintain more than 80% of their initial power conversion efficiency (PCE) values after aging in air (relative humidity: 35–40%) for 30 days.     

Synergistic Effect of Surface p-Doping and Passivation Improves the Efficiency,Stability, and Reduces Lead Leakage in All-Inorganic CsPbIBr2-Based Perovskite Solar Cells

Wide-bandgap inorganic cesium lead halide CsPbIBr₂ is a popular optoelectronic material that researchers are interested in because of the character that balances the power conversion efficiency and stability of solar cells. It also has great potential in semitransparent solar cells, indoor photovoltaics, and as a subcell for tandem solar cells. Although CsPbIBr₂-based devices have achieved good performance, the open-circuit voltage (V_oc) of CsPbIBr₂-based perovskite solar cells (PSCs) is still lower, and it is critical to further reduce large energy losses (E_loss). Herein, a strategy is proposed for achieving surface p-type doping for CsPbIBr₂-based perovskite for the first time, using 1,5-Diaminopentane dihydroiodide at the perovskite surface to improve hole extraction efficiency. Meanwhile, the adjusted energy levels reduce E_loss and improve V_oc of the CsPbIBr₂ PSCs. Furthermore, the Cs- and Br-vacancies at the interface are filled, reducing structural disorder and defect states and thus improving the quality of the perovskite film. As a result, the target device achieves a high efficiency of 11.02% with a V_oc of 1.33 V, which is among the best values. In addition to the improved performance, the stability of the target device under various conditions is enhanced, and the lead leakage is effectively suppressed.     

Efficient Perovskite Solar Cells Fabricated Through CsCl‐Enhanced PbI2 Precursor via Sequential Deposition

The fabrication of high‐quality perovskite film highly relies on chemical composition and the synthesis method of perovskite. So far, sequentially deposited MA_0.03FA_0.97Pb(I_0.97Br_0.03)₃ polycrystalline film is adopted to produce high‐performance perovskite solar cells with record power conversion efficiency (PCE). Fewer grain boundaries and incorporation of inorganic cation (e.g., cesium) would further increase device performance via sequential deposition. Here, cesium chloride (CsCl) is introduced into lead iodide (PbI₂) precursor solution that beneficially modulates the property of PbI₂ film, leading to larger grains with cesium incorporation in the resulting perovskite film. The enlarged crystal grains originate from a slower nucleation process for CsCl‐containing PbI₂ film when reacting with formamidine iodide, confirmed by in situ confocal photoluminescence imaging. Photovoltaic devices based on CsCl‐containing PbI₂ film demonstrate a higher averaging efficiency of 21.3% than 20.3% of the devices without CsCl additives for reverse scan. More importantly, the device stability is improved by CsCl additives that retain over 90% of their initial PCE value after 4000 min tracking at maximum power point under 1‐sun illumination. This work paves a way to further improve the photovoltaic performance of mixed‐cation‐halide perovskite solar cells via a sequential deposition method.     

Non‐Preheating Processed Quasi‐2D Perovskites for Efficient and Stable Solar Cells

Although the hot‐casting (HC) technique is prevalent in developing preferred crystal orientation of quasi‐2D perovskite films, the difficulty of accurately controlling the thermal homogeneity of substrate is unfavorable for the reproducibility of device fabrication. Herein, a facile and effective non‐preheating (NP) film‐casting method is proposed to realize highly oriented quasi‐2D perovskite films by replacing the butylammonium (BA⁺) spacer partially with methylammonium (MA⁺) cation as (BA)_2?x(MA)_3+xPb₄I₁₃ (x = 0, 0.2, 0.4, and 0.6). At the optimal x‐value of 0.4, the resultant quasi‐2D perovskite film possesses highly orientated crystals, associated with a dense morphology and uniform grain‐size distribution. Consequently, the (BA)_1.6(MA)_3.4Pb₄I₁₃‐based solar cells yield champion efficiencies of 15.44% with NP processing and 16.29% with HC processing, respectively. As expected, the HC‐processed device shows a poor performance reproducibility compared with that of the NP film‐casting method. Moreover, the unsealed device (x = 0.4) displays a better moisture stability with respect to the x = 0 stored in a 65% ± 5% relative humility chamber.     

Compositional Engineering for Thermally Stable,Highly Efficient Perovskite Solar Cells Exceeding 20% Power Conversion Efficiency with 85 °C/85% 1000 h Stability

Perovskite solar cells have received great attention because of their rapid progress in efficiency, with a present certified highest efficiency of 23.3%. Achieving both high efficiency and high thermal stability is one of the biggest challenges currently limiting perovskite solar cells because devices displaying stability at high temperature frequently suffer from a marked decrease of efficiency. In this report, the relationship between perovskite composition and device thermal stability is examined. It is revealed that Rb can suppress the growth of PbI₂ even under PbI₂‐rich conditions and decreasing the Br ratio in the perovskite absorber layer can prevent the generation of unwanted RbBr‐based aggregations. The optimized device achieved by engineering perovskite composition exhibits 92% power conversion efficiency retention in a stress test conducted at 85 °C/85% relative humidity (RH) according to an international standard (IEC 61215) while exceeding 20% power conversion efficiency (certified efficiency of 20.8% at 1 cm²). These results reveal the great potential for the practical use of perovskite solar cells in the near future.     

Origins of High Performance and Degradation in the Mixed Perovskite Solar Cells

The origins of the high device performance and degradation in the air are the greatest issues for commercialization of perovskite solar cells. Here this study investigates the possible origins of the mixed perovskite cells by monitoring defect states and compositional changes of the perovskite layer over the time. The results of deep‐level transient spectroscopy analysis reveal that a newly identified defect formed by Br atoms exists at deep levels of the mixed perovskite film, and its defect state shifts when the film is aged in the air. The change of the defect state is originated from loss of the methylammonium molecules of the perovskite layer, which results in decreased J_SC, deterioration of the power conversion efficiency and long‐term stability of perovskite solar cells. The results provide a powerful strategy to diagnose and manage the efficiency and stability of perovskite solar cells.     

A Highly Emissive Surface Layer in Mixed‐Halide Multication Perovskites

Mixed‐halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution‐processed triple‐cation mixed‐halide (Cs_0.06MA_0.15FA_0.79)Pb(Br_0.4I_0.6)₃ perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar‐equivalent illumination. It is found that the illumination leads to localized surface sites of iodide‐rich perovskite intermixed with passivating PbI₂ material. Time‐ and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide‐rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed‐halide mixed‐cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.     

Efficient Flexible Solar Cell based on Composition‐Tailored Hybrid Perovskite

Organic–inorganic hybrid perovskites (OIHPs) are new photoactive layer candidates for lightweight and flexible solar cells due to their low‐temperature process capability; however, the reported efficiency of flexible OIHP devices is far behind those achieved on rigid glass substrates. Here, it is revealed that the limiting factor is the different perovskite film deposition conditions required to form the same film morphology on flexible substrates. An optimized perovskite film composition needs a different precursor ratio, which is found to be essential for the formation of high‐quality perovskite films with longer radiative carrier recombination lifetime, smaller density of trap states, reduced precursor residue, and uniform and pin‐hole free films. A record efficiency of 18.1% is achieved for the flexible perovskite solar‐cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low temperature (≤100 °C) solution process.     

A Self‐Powered and Flexible Organometallic Halide Perovskite Photodetector with Very High Detectivity

Flexible and self‐powered photodetectors (PDs) are highly desirable for applications in image sensing, smart building, and optical communications. In this paper, a self‐powered and flexible PD based on the methylammonium lead iodide (CH₃NH₃PBI₃) perovskite is demonstrated. Such a self‐powered PD can operate even with irregular motion such as human finger tapping, which enables it to work without a bulky external power source. In addition, with high‐quality CH₃NH₃PBI₃ perovskite thin film fabricated with solvent engineering, the PD exhibits an impressive detectivity of 1.22 × 10¹³ Jones. In the self‐powered voltage detection mode, it achieves a large responsivity of up to 79.4 V mW^?1 cm^?2 and a voltage response of up to ≈90%. Moreover, as the PD is made of flexible and transparent polymer films, it can operate under bending and functions at 360 ° of illumination. As a result, the self‐powered, flexible, 360 ° omnidirectional perovskite PD, featuring high detectivity and responsivity along with real‐world sensing capability, suggests a new direction for next‐generation optical communications, sensing, and imaging applications.     

Polymer‐Passivated Inorganic Cesium Lead Mixed‐Halide Perovskites for Stable and Efficient Solar Cells with High Open‐Circuit Voltage over 1.3 V

Cesium‐based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (E_loss) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI₂Br absorber and polythiophene hole‐acceptor to minimize the E_loss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI₂Br, can significantly reduce electron‐hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole‐injection into the hole‐acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed‐halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open‐circuit voltage (V_OC) of up to 1.32 V and E_loss of down to 0.5 eV, which both are the optimal values reported among cesium‐lead mixed‐halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the E_loss.     

Atomic‐Layer Deposition‐Assisted Double‐Side Interfacial Engineering for High‐Performance Flexible and Stable CsPbBr3 Perovskite Photodetectors toward Visible Light Communication Applications

Self‐powered photodetectors (PDs) based on inorganic metal halide perovskites are regarded as promising alternatives for the next generation of photodetectors. However, uncontrollable film growth and sluggish charge extraction at interfaces directly limit the sensitivity and response speed of perovskite‐based photodetectors. Herein, by assistance of an atomic layer deposition (ALD) technique, CsPbBr₃ perovskite thin films with preferred orientation and enlarged grain size are obtained on predeposited interfacial modification layers. Thanks to improved film quality and double side interfacial engineering, the optimized CsPbBr₃ (Al₂O₃/CsPbBr₃/TiO₂, ACT) perovskite PDs exhibit outstanding performance with ultralow dark current of 10^?11 A, high detectivity of 1.88 × 10¹³ Jones and broad linear dynamic range (LDR) of 172.7 dB. Significantly, excellent long‐term environmental stability (ambient conditions >100 d) and flexibility stability (>3000 cycles) are also achieved. The remarkable performance is credited to the synergistic effects of high carrier conductivity and collection efficiency, which is assisted by ALD modification layers. Finally, the ACT PDs are successfully integrated into a visible light communication system as a light receiver on transmitting texts, showing a bit rate as high as 100 kbps. These results open the window of high performance all‐inorganic halide perovskite photodetectors and extends to rational applications for optical communication.     

FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

Mixed‐halide wide‐bandgap perovskites are key components for the development of high‐efficiency tandem structured devices. However, mixed‐halide perovskites usually suffer from phase‐impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA⁺)‐based mixed‐halide perovskites, MAPb(I_0.6Br_0.4)₃, the halide composition of the spin‐coated perovskite films is preferentially dominated by the bromide ions (Br^?). Additional thermal energy is required to initiate the insertion of iodide ions (I^?) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA⁺) in the precursor solution, it can effectively facilitate the I^? coordination in the perovskite framework during the spin‐coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high‐crystallinity perovskite film with high Br^? content. As a result, high‐quality MA_0.9FA_0.1Pb(I_0.6Br_0.4)₃ perovskite film with a bandgap (E_g) of 1.81 eV is achieved, along with an encouraging power‐conversion‐efficiency of 17.1% and open‐circuit voltage (V_oc) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.     

Effective Carrier‐Concentration Tuning of SnO2 Quantum Dot Electron‐Selective Layers for High‐Performance Planar Perovskite Solar Cells

The carrier concentration of the electron‐selective layer (ESL) and hole‐selective layer can significantly affect the performance of organic–inorganic lead halide perovskite solar cells (PSCs). Herein, a facile yet effective two‐step method, i.e., room‐temperature colloidal synthesis and low‐temperature removal of additive (thiourea), to control the carrier concentration of SnO₂ quantum dot (QD) ESLs to achieve high‐performance PSCs is developed. By optimizing the electron density of SnO₂ QD ESLs, a champion stabilized power output of 20.32% for the planar PSCs using triple cation perovskite absorber and 19.73% for those using CH₃NH₃PbI₃ absorber is achieved. The superior uniformity of low‐temperature processed SnO₂ QD ESLs also enables the fabrication of ≈19% efficiency PSCs with an aperture area of 1.0 cm² and 16.97% efficiency flexible device. The results demonstrate the promise of carrier‐concentration‐controlled SnO₂ QD ESLs for fabricating stable, efficient, reproducible, large‐scale, and flexible planar PSCs.     

Ultrathin,Core–Shell Structured SiO2 Coated Mn2+‐Doped Perovskite Quantum Dots for Bright White Light‐Emitting Diodes

All‐inorganic semiconductor perovskite quantum dots (QDs) with outstanding optoelectronic properties have already been extensively investigated and implemented in various applications. However, great challenges exist for the fabrication of nanodevices including toxicity, fast anion‐exchange reactions, and unsatisfactory stability. Here, the ultrathin, core–shell structured SiO₂ coated Mn²⁺ doped CsPbX₃ (X = Br, Cl) QDs are prepared via one facile reverse microemulsion method at room temperature. By incorporation of a multibranched capping ligand of trioctylphosphine oxide, it is found that the breakage of the CsPbMnX₃ core QDs contributed from the hydrolysis of silane could be effectively blocked. The thickness of silica shell can be well‐controlled within 2 nm, which gives the CsPbMnX₃@SiO₂ QDs a high quantum yield of 50.5% and improves thermostability and water resistance. Moreover, the mixture of CsPbBr₃ QDs with green emission and CsPbMnX₃@SiO₂ QDs with yellow emission presents no ion exchange effect and provides white light emission. As a result, a white light‐emitting diode (LED) is successfully prepared by the combination of a blue on‐chip LED device and the above perovskite mixture. The as‐prepared white LED displays a high luminous efficiency of 68.4 lm W^?1 and a high color‐rendering index of Ra = 91, demonstrating their broad future applications in solid‐state lighting fields.     

Defect‐Engineering‐Enabled High‐Efficiency All‐Inorganic Perovskite Solar Cells

The emergence of cesium lead iodide (CsPbI₃) perovskite solar cells (PSCs) has generated enormous interest in the photovoltaic research community. However, in general they exhibit low power conversion efficiencies (PCEs) because of the existence of defects. A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared by defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, while the intrinsic defect concentration is largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere and thus a glovebox is not required. By completely eliminating the labile and expensive components in traditional PSCs, the all‐inorganic PSCs based on CsPbI₃:Br:InI₃ and carbon electrode exhibit PCE and open‐circuit voltage as high as 12.04% and 1.20 V, respectively. More importantly, they demonstrate excellent stability in air for more than two months, while those based on CsPbI₃ can survive only a few days in air. The progress reported represents a major leap for all‐inorganic PSCs and paves the way for their further exploration in order to achieve higher performance.