JIALINGBO

The modification of charge alignment between hole transporting layer and perovskite film is done by addition of potassium acetate interfacial layer. This study demonstrates the importance of band alignment modification where the device short-circuit current of the devices is improved regardless of the device configuration or perovskite bandgap, thus proving its application as universal interfacial passivation material.

A passivation strategy for the perovskite/HTL interface is presented based on potassium acetate (K-Ac). Since K-Ac is soluble in both polar and nonpolar solvent, deposition of K-Ac on top and bottom of perovskite is possible. With this advantage, the universality of potassium interfacial passivation at the HTL/perovskite interface applied to various configurations with various ranges of perovskite bandgap is reported. Regarding the p–i–n configuration, various materials characterizations reveal that a potassium passivation layer underneath perovskite modifies perovskite orientations, resulting in better charge transport and film properties. Furthermore, the potassium passivation layer shifts the valence band position of the HTL upward, which results in a better extraction of charges (holes) across the HTL/perovskite interface, thus improving the short-circuit current density (J _sc). The modification of the band alignment at the HTL/perovskite by the potassium interfacial passivation layer is confirmed in n–i–p devices with both WBG and CBG perovskites. Compared to reference solar cells without a passivation layer, an increase in J _sc of approximately 1 mA cm⁻² is observed in all cases, resulting in power conversion efficiencies of 19.42%, 20.06%, and 21.57% for WBG p–i–n, CBG p–i–n and n–i–p solar cells, respectively, demonstrating the wide applicability of the passivation strategy.

It is discovered that the morphology of the as-prepared perovskite film is greatly affected by the surrounding temperature during spin-coating. The optimal surrounding temperature of 29 °C is observed to dramatically enlarge the perovskite crystal size and increase the power conversion efficiency of perovskite solar cell to 20.91%, which is the highest reported value among inverted MAPbI₃ solar cells.

Controlling the crystallization of perovskite film is critical for high-performance perovskite solar cells (PVSCs), and temperature is the key factor dominating the nucleation and crystal growth. Herein, it is demonstrated that the inconspicuous ambient temperature plays an important role in reaching high-quality perovskite film and high-performance PVSCs. It is observed that the ambient temperature greatly affects the composition of as-prepared perovskite film, which dramatically influences the perovskite film morphology during the subsequent annealing process. Remarkably, the device prepared at the optimal ambient temperature of 29 °C exhibits the best power conversion efficiency of 20.91% with little hysteresis. This represents one of the best results for inverted PVSCs based on pristine MAPbI₃ system. In addition, the PVSC prepared at 29 °C shows good stability with 80% of its initial efficiency after 30d in air with a humidity of 45%. Overall, the importance of ambient temperature for crystallization of the perovskite film is emphasized, and this work should have implications for controlling the morphology for high-performance PVSCs.

The structure–properties relationships of hole transport materials (HTMs) with fused and unfused core units are investigated. The HTM with a fused core exhibits a high hole mobility and an efficient charge extraction capability, leading to a remarkable efficiency of 19.63% when it is used as an HTM without any dopant, much higher than unfused HTM-based devices (10.03%).

Replacing the dominating and dopant-needing 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spiro-bifluorene (spiro-OMeTAD) with dopant-free organic hole transport materials (HTMs) in n-i-p structured perovskite solar cells (PSCs) is a big challenge. Herein, a class of conjugated organic semiconductor materials, namely, ZT-H1 and ZT-H2, with unfused and fused core units, respectively, are successfully designed and synthesized for dopant-free HTMs. It is found that the HTM ZT-H1 exhibits a hole mobility of 7.08 × 10⁻⁵ cm² V⁻¹ s⁻¹, which is improved to 5.16 × 10⁻⁴ cm² V⁻¹ s⁻¹ for HTM ZT-H2 due to the enlarged molecular planarity of ZT-H2, leading to efficient intermolecular π–π interaction. Further investigation indicates that ZT-H2 is more fit to facilitating hole extraction, restraining charge recombination, and guaranteeing long-term stability of the devices. Consequently, a planar n-i-p structured device using ZT-H2 as HTM without any dopants exhibits a remarkable efficiency of 19.63%, which is much higher than that of ZT-H1-based devices (10.64%). Importantly, ZT-H2-based devices are much more stable than the control devices using ZT-H1 or spiro-OMeTAD as the HTM. The findings reveal that the fused central core unit with extended π-conjugation is an efficient strategy for rationally designing dopant-free HTMs toward stable and efficient PSCs.

In this paper, the origin of main defects in perovskite solar cells (PSCs), their effect on photovoltaic performance, and operational stability are focused upon. Then, the corresponding passivation strategies are introduced. Finally, a brief summary is made and the thorny issues that need to be solved in the future development of PSCs are looked forward to.

Abstract

Defects are considered to be one of the most significant factors that compromise the power conversion efficiencies and long-term stability of perovskite solar cells. Therefore, it is urgent to have a profound understanding of their formation and influence mechanism, so as to take corresponding measures to suppress or even completely eliminate their adverse effects on device performance. Herein, the possible origins of the defects in metal halide perovskite films and their impacts on the device performance are analyzed, and then various methods to reduce defect density are introduced in detail. Starting from the internal and interfacial aspects of the metal halide perovskite films, several ways to improve device performance and long-term stability including additive engineering, surface passivation, and other physical treatments (annealing engineering), etc., are further elaborated. Finally, the further understanding of defects and the development trend of passivation strategies are prospected.

Two novel triarylamine-based donor-acceptor copolymers featuring an imide-functionalized backbone are developed. Benefiting from the good energy level alignment, appropriate film morphology, and most importantly, improved hole mobility, the pristine PTTI-TPA based inverted perovskite solar cells achieve a high power conversion efficiency of up to 21% with negligible hysteresis and substantial stability.

Abstract

Dopant-free hole-transporting layers (HTLs) are highly desired for realizing efficient and stable perovskite solar cells (PVSCs), but only very few of them can enable power conversion efficiencies (PCEs) over 20%. Herein, two imide-functionalized triarylamine-based donor-acceptor (D-A) type copolymers, PBTI-TPA and PTTI-TPA, are developed and applied as dopant-free HTLs in inverted PVSCs. The combination of a classic redox-active triphenylamine donor unit and an electron-withdrawing oligothiophene imide co-unit with rigid and planar backbone furnishes the two polymers with quasi-planar backbone, suitable frontier molecular orbital (FMO) energy levels, favorable thermal stability, appropriate film morphology, and passivation effect. More importantly, the greatly improved hole mobility renders them as promising HTLs for PVSCs. As a result, the undoped PTTI-TPA-based inverted PVSCs deliver a remarkable PCE up to 21% as well as negligible hysteresis and substantial long-term stability, outperforming the devices based on PBTI-TPA and PTAA. The performance also represents one of the highest PCEs reported to date for PVSCs based on dopant-free polymeric HTLs. The results highlight the great potentials of oligothiophene imides for constructing donor-acceptor polymeric HTLs for enabling high-performance dopant-free PVSCs.

Inorganic CsPbI₃ perovskite is well-known for its resistance to organic cation substitution. Here, a specific organic cation of tetrabutylammonium (TBA⁺) with strong ionic binding to the Pb-I octahedral framework can effectively intercalate into CsPbI₃ and substitute the Cs⁺ cation. A post-synthesis TBAI passivation then leads to in situ formation of TBAPbI₃ layer to heal CsPbI₃ perovskite with lower defect density and enhanced stability.

Abstract

The in situ formation of reduced dimensional perovskite layer via post-synthesis ion exchange has been an effective way of passivating organic-inorganic hybrid perovskites. In contrast, cesium ions in Cs-based inorganic perovskite with strong ionic binding energy cannot exchange with those well-known organic cations to form reduced dimensional perovskite. Herein, we demonstrate that tetrabutylammonium (TBA⁺) cation can intercalate into CsPbI₃ to effectively substitute the Cs cation and to form one-dimensional (1D) TBAPbI₃ layer in the post-synthesis TBAI treatment. Such TBA cation intercalation leads to in situ formation of TBAPbI₃ protective layer to heal defects at the surface of inorganic CsPbI₃ perovskite. The TBAPbI₃-CsPbI₃ perovskite exhibited enhanced stability and lower defect density, and the corresponding perovskite solar cell devices achieved an improved efficiency up to 18.32 % compared to 15.85 % of the control one.

A low-dimensional perovskite layer is constructed between the perovskite absorber and carbon electrode in printable triple-mesoscopic perovskite solar cells by posttreatment. By forming graded type-II band alignment, the device performance is significantly enhanced, delivering a power conversion efficiency of 17.47% with an open-circuit voltage of 1.02 V.

Abstract

Printable hole-conductor-free perovskite solar cells (PSCs) have attracted intensive research attention due to their high stability and simple manufacturing process. However, the cells have suffered severe potential loss in the absence of the hole transporting layer. The dimensionality of the perovskite absorber in the mesoporous carbon electrodes by conducting post-treatments is reduced. The low-dimensional perovskites possess wide-bandgaps and form type-II band alignment, favoring directional charge transportation and thus enhancing the device performance. For the cells using MAPbI₃ (MA = methylammonium) as the light absorber, the open-circuit voltage (V _OC) is significantly enhanced from 0.92 to 0.98 V after posttreatment, delivering an overall efficiency of 16.24%. For the cells based on FAPbI₃ (FA = formamadinium), a high efficiency of 17.47% is achieved with V _OC of 1.02 V, which are both the highest reported values for printable hole-conductor-free PSCs. This strategy provides a facile method for tuning the energy level alignment for mesoscopic perovskite-based optoelectronics.

The stabilization of black-phase formamidinium lead iodide (α-FAPbI₃) perovskite under various environmental conditions is considered necessary for solar cells. However, challenges remain regarding the temperature sensitivity of α-FAPbI₃ and the requirements for strict humidity control in its processing. Here we report the synthesis of stable α-FAPbI₃, regardless of humidity and temperature, based on a vertically aligned lead iodide thin film grown from an ionic liquid, methylamine formate. The vertically grown structure has numerous nanometer-scale ion channels that facilitate the permeation of formamidinium iodide into the lead iodide thin films for fast and robust transformation to α-FAPbI₃. A solar cell with a power-conversion efficiency of 24.1% was achieved. The unencapsulated cells retain 80 and 90% of their initial efficiencies for 500 hours at 85°C and continuous light stress, respectively.

The charge transport behavior of molecular perovskite under X-ray excitation based on centimeter-scale TMCM-CdCl₃ (TMCM⁺, trimethylchloromethyl ammonium) single crystal is explored. The X-ray detector shows an efficient photoresponse with good sensitivity and a low detection limit. This work may open up an effective way to discover a new generation of X-ray detection.

Abstract

Molecular perovskites have demonstrated great potential for ferroelectrics and nonlinear optics; however, their charge transport properties for optoelectronics have rarely been explored. Here, understanding of charge transport behavior of molecular perovskite under X-ray excitation based on centimeter-scale TMCM-CdCl₃ (TMCM⁺, trimethylchloromethyl ammonium) single crystal is demonstrated. The crystal is fabricated from an aqueous solution and exhibits a large bandgap of 5.51 eV, with the valence band maximum mainly dominated by the Cl-p/Cd-d states and the conduction band minimum primarily by Cd-s/Cl-p states. Charge mobility exceeding 40 cm² V⁻¹ s⁻¹ and mobility–lifetime (µτ) product on the order of 10⁻⁴ cm² V⁻¹ for the crystal are observed. These excellent optoelectronic properties translate to an efficient photoresponse under X-ray excitation, with the sensitivity reaching 128.9 ± 4.64 µC Gy_air ⁻¹ cm⁻² [fivefold higher than that of the commercialized amorphous selenium (α-Se)] and a low detection limit of 1.06 μC Gy_air ⁻¹ s⁻¹ (10 V bias). This work pioneers a superior metal-based molecular perovskite single-crystal based paradigm for optoelectronic investigation, which may lead to the discovery of a new generation of X-ray detection and imaging materials.

Metal halide perovskites are widely investigated in photodetection applications due to their remarkable photoelectric properties. Herein, an overview of the recent advances in perovskite photodetectors for image sensing is provided. The device structures, preparation methods, and photoelectric properties of image sensors based on different dimensional perovskites are highlighted. The single-pixel imaging and narrowband detection of perovskite photodetectors are also discussed.

Abstract

In recent years, metal halide perovskites have been widely investigated to fabricate photodetectors for image sensing due to the excellent photoelectric performance, tunable bandgap, and low-cost solution preparation process. In this review, a comprehensive overview of the recent advances in perovskite photodetectors for image sensing is provided. First, the key performance parameters and the basic device types of photodetectors are briefly introduced. Then, the recent developments of image sensors on the basis of different dimensional perovskite materials, including 0D, 1D, 2D, and 3D perovskite materials, are highlighted. Besides the device structures and photoelectric properties of perovskite image sensors, the preparation methods of perovskite photodetector arrays are also analyzed. Subsequently, the single-pixel imaging of perovskite photodetectors and the strategies to fabricate narrowband perovskite photodetectors for color discrimination are discussed. Finally, the potential challenges and possible solutions for the future development of perovskite image sensors are presented.

Despite the defect-tolerance of lead-halide perovskites, defects at the surface of colloidal nanocrystals and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the photoluminescence quantum yields, device efficiency, and stability. This Review summarizes the defects, their influence on the optical and charge-carrier transport properties, and passivation strategies to mitigate the effects of defects.

Abstract

Lead-halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge carriers into light and vice versa. However, despite the defect-tolerance of LHPs, defects at the surface of colloidal NCs and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the PLQYs, device efficiency, and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes are critical for achieving advances in performance. This Review presents the current understanding of defects in halide perovskites and their influence on the optical and charge-carrier transport properties. Passivation strategies toward improving the efficiencies of perovskite-based LEDs and solar cells are also discussed.

The self-doping effect on the light stability of perovskite solar cells (PSCs) is systemically investigated through various opto-electrical characterizations. Although both PSCs with Pb-rich and Pb-deficient conditions exhibit similar initial performance, the Pb-rich PSC degrades relatively quickly under light illumination even without H₂O and O₂, resulting in the shift of the defect state associated with the formation of deep-level defects.

Abstract

Although there have been significant advances in the stability of perovskite solar cells through encapsulation techniques to remove extrinsic degradation factors, such as moisture and oxygen, irreversible photo-degradation originating from intrinsic defects is still challenging and remains elusive. Herein, the photo-aging mechanism due to intrinsic defects is investigated in nitrogen-filled conditions, excluding extrinsic degradation factors. Devices with similar power conversion efficiencies (PCE) of 21%, but with different Fermi levels in the perovskite films, via controlling the self-doping effect, have been investigated. Opto-electronic investigations and depth profiles of the elemental constituents show that after photo-aging, strain relaxation in the perovskite lattice and a Fermi level shift towards conduction band edge are observed, implying the formation of new defect states in Pb-rich devices. Furthermore, thermal admittance spectroscopy measurement of the devices suggests that the formation of the deep-traps in the perovskite leads to irreversible degradation. Thin-film solar cells that are relatively Pb-deficient (FA-rich) exhibit improved long-term stability, retaining over 90% of their initial PCE during 500 h of continuous 1-Sun illumination. This study suggests passivation of the Pb-I related antisite defects near the grain boundaries and the interface is crucial for the fabrication of solar cells with enhanced long-term stability.

A moisture‐triggered self‐healing flexible perovskite photodetector is presented. The lateral photodetector shows a high responsivity of 11.3 A W⁻¹ and a good stability to both moisture and mechanical damage. Meanwhile, damaged perovskite film can repair cracks with the assistance of a poly(vinyl alcohol) microscaffold under a humid environment and recover to 90% of its initial performance for several cycles.

Abstract

Flexible devices are urgently required to meet the demands of next‐generation optoelectronic devices and metal halide perovskites are proven to be suitable materials for realizing flexible photovoltaic devices. However, the tolerance to moisture corrosion and repeated mechanical bending remains a critical challenge for flexible perovskite devices. Herein, a self‐healing formamidinium lead iodide (FAPbI₃) film is fabricated to cure mechanical damage by absorbing moisture from the surrounding environment. A poly(vinyl alcohol) microscaffold is designed not only to stabilize the black phase of the FAPbI₃ film but also to endow it with self‐healing ability in a humid environment. The photodetector based on a self‐healing film exhibits a high responsivity of 11.3 A W⁻¹ and recovers to over 90% of the initial responsivity after the self‐healing process. This work provides an effective self‐healing strategy to stabilize the operation of flexible perovskite devices under normal high‐humidity environmental conditions.

A one-source strategy using the same polymer donor material to simultaneously dope CsPbI₂Br perovskite films by antisolvent engineering and fabricating the hole transport layer is proposed. The perovskite solar cell (pero-SC) based on this one-source strategy exhibits a remarkable power conversion efficiency of 16.40% and possesses excellent thermal stability and operational stability at the same time.

Abstract

All-inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. However, the inferior film quality and doped hole transport layer (HTL) have a strong tendency to degrade the perovskite under high temperatures or harsh operating conditions. To solve these problems, a one-source strategy using the same polymer donor material (PDM) to simultaneously dope CsPbI₂Br perovskite films via antisolvent engineering and fabricating the HTL is proposed. The doping assists perovskite film growth and forms a top–down gradient distribution, generating CsPbI₂Br with enlarged grain size and reduced defect density. The PDM as the HTL suppresses the energy barrier and forms favorable electrical contacts for hole extraction, and assemble into a fingerprint-like morphology that improves the conductivity, facilitating the creation of a dopant-free HTL. Based on this one-source strategy using PBDB-T as PDM, the CsPbI₂Br perovskite solar cell with a dopant-free HTL achieves a power conversion efficiency (PCE) of 16.40%, which is one of the highest PCEs reported among all-inorganic CsPbI₂Br pero-SCs with a dopant-free HTL. Importantly, the devices exhibit the highest thermal stability at 85 °C and operational stability under continuous illumination even with Ag as the top electrode and present good universality.

Elastic properties of two isostructural molecular perovskite ferroelectrics are systematically investigated. Strikingly, MDABCO-KI₃ shows much higher moduli than those of MDABCO-NH₄I₃ due to the marked different strengths between KI coordination bonds and NH₄…I hydrogen interactions. This work demonstrates that it is possible to manage elastic properties of molecular ferroelectrics via facile chemical substitution.

Abstract

Managing elastic properties of ABX₃ type molecular perovskite ferroelectrics is critical to their future applications since these parameters determine their service durability and reliability in devices. The abundant structural and chemical viability of these compounds offer a convenient way to manipulate their elastic properties through a facile chemical approach. Here, the elastic properties and high-pressure behaviors of two isostructural perovskite ferroelectrics, MDABCO-NH₄I₃ and MDABCO-KI₃ (MDABCO = N-methyl-N′-diazabicyclo[2.2.2]octonium) is systematically investigated, via the first principles calculations and high-pressure synchrotron X-ray diffraction experiments. It is show that the simple replacement of NH₄ ⁺ by K⁺ on the B-site respectively results in up to 48.1%, 52.4%, and 56.3% higher Young's moduli, shear moduli and bulk moduli, which is attributed to the much stronger KI coordination bonding than NH₄…I hydrogen bonding. These findings demonstrate that it is possible to tune elastic properties of molecular perovskite ferroelectrics via simply varying the framework assembling interactions.

Thin-film, cover, and hybrid encapsulation technologies, that function as a moisture and oxygen permeation barrier and mechanical protection to prevent leakage of toxic by-product, and limit decomposition of reactants in a confined space, can be applied in organic light emitting diodes, organic and perovskite solar cells, leading to robust stability and long lifetime in three types of devices.

Abstract

Organic light emitting diodes (OLEDs) employing organic thin-film based emitters have attracted tremendous attention due to their widespread applications in lighting and as displays in mobile devices and televisions. The novel thin-film photovoltaic techniques using organic or organic–inorganic hybrid materials such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs) have become emerging competitive candidates with regard to the traditional photovoltaic techniques on account of high-efficiency, low-cost, and simple manufacturing processing properties. However, OLEDs, OPVs, and PSCs are vulnerable to the undesired degradation induced by moisture and oxygen. To afford long-term stability, a robust encapsulation technique by employing materials and structures that possess high barrier performance against oxygen and moisture must be explored and employed to protect these devices. Herein, the recent progress on specific encapsulation materials and techniques for three types of devices on the basis of fundamental understanding of device stability is reviewed. First, their degradation mechanisms, as well as, influencing factors are discussed. Then, the encapsulation technologies and materials are classified and discussed. Moreover, the advantages and disadvantages of various encapsulation technologies and materials coupled with their encapsulation applications in different devices are compared. Finally, the ongoing challenges and future perspectives of encapsulation frontier are provided.

A novel IDTT‐based small molecule (SM) additive (IDTT‐ThCz) is developed and introduced into perovskite solar cells (PSCs) through an anti‐solvent engineering method. This IDTT‐ThCz passivates defect states of perovskite layers, providing efficient charge extraction as well as preventing the decomposition of perovskite crystals. Therefore, IDTT‐ThCz treated PSCs achieve a highest efficiency of 22.5% and remarkable thermal stability.

Abstract

Although perovskite solar cells (PSCs) have attracted enormous attention owing to their fascinating optoelectronic properties and solution processability, defects in PSCs, which adversely affect efficiency and stability, are still not completely resolved. Herein, a novel indacenodithieno[3,2‐b]thiophene‐based small molecule (SM) additive (IDTT‐ThCz), capable of interacting with perovskite layers, is developed. In particular, the IDTT‐ThCz, which can perform a surface passivation, is introduced into the perovskite layer to significantly suppress perovskite defects via antisolvent treatment. Furthermore, this facile surface passivation not only significantly improves the charge extraction capability, but also prevents perovskite degradation. The IDTT‐ThCz‐treated PSCs exhibits a power conversion efficiency (PCE) of 22.5% and retains 95% of its initial PCE after 500 h storage under thermal condition (85 °C), representing the most remarkable efficiency as well as stability among the SM additives reported to date.

Bottom-surface defect passivation of perovskite film is enabled by covalently attaching –OH to a hole-transporting polymer. A solvent evaporation-induced self-assembly of the resultant amphiphilic hole-transporting polymer enriches –OH on the film surface, passivating defects of the upper perovskite layer. Inverted perovskite solar cells based on this polymer afford an efficiency of 20.12% with improved device stability compared to its poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) counterpart.

Abstract

Bottom-surface defect passivation of perovskite film, lagging far behind easily conducted bulk and top-surface passivations in perovskite solar cells (PSCs), remains rather challenging because most passivation molecules/groups can be eroded by polar solvents used for the subsequent perovskite deposition. In this work, an effective bottom-surface passivation is enabled for enhanced performance of inverted PSCs by covalently attaching a passivation group (hydroxyl) to a hole transporting polymer. A short linker (methylene) between the hydroxyl and the conjugated backbone bearing hydrophobic long alkyl chains is adopted to improve the resistance of the resultant amphiphilic polymer to polar solvents. A solvent evaporation-induced self-assembly of the amphiphilic hole transporting polymer is developed to enrich hydroxyl groups on the film surface, passivating defects of the upper perovskite layer via interactions with undercoordinated Pb²⁺ and I^– sites. Inverted PSCs based on this hole transporting film are superior in efficiency (20.12%), reproducibility, large-area fabrication, and stability to its classical poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) counterpart. This work demonstrates that rational introduction of passivation groups into the hole transporting layer combined with self-assembly-modulated component distributions is useful to realize bottom-surface passivation of the perovskite layer for improved photovoltaic performance.