Vertically Oriented Perovskites with Minimized Intrinsic Boundaries for Efficient Photovoltaics.

Intrinsic boundaries formed by grain stacks of randomly oriented perovskite crystallites seriously restrict charge transport in the resultant photovoltaic devices, whereas direct passivation of these defects remains unexplored, and it is desirable to modulate perovskite growth with uniform orientation. Herein, we report a simple but effective method to regulate perovskite crystallization by employing a volatile and polymerizable monomer of hydroxyethyl methacrylate (HEMA), which can simultaneously interact with both FA+ and Pb2+ via hydrogen and coordination bonding, respectively, to seed perovskite crystallization with accelerated nucleation and retarded crystal growth. Upon thermal annealing, the gradual volatilization and partial self-condensation of the HEMA drive the perovskite growth perpendicularly to the substrate, leading to largely suppressed defect states, improved crystallinity, and a reduced Young's modulus of the perovskite film. As a result, champion efficiencies exceeding 24 and 22% with improved operational and mechanical stability of the optimized perovskite solar cells based on rigid and flexible substrates were finally achieved.

Phase-Pure α-FAPbI3 Perovskite Solar Cells via Activating Lead-Iodine Frameworks.

Narrow bandgap cubic formamidine perovskite (α-FAPbI3) is widely studied for its potential to achieve record­breaking efficiency. However, its high preparation difficulty caused by lattice instability is criticized. A popular strategy for stabilizing the α-FAPbI3 lattice is to replace intrinsic FA+ or I- with smaller ions of MA+, Cs+, Rb+, and Br-, whereas this generally leads to broadened optical bandgap and phase separation. Studies show that ions substitution-free phase-pure α-FAPbI3 can achieve intrinsic phase stability. However, the challenging preparation of high-quality films has hindered its further development. Here, a facile synthesis of high-quality MA+, Cs+, Rb+, and Br--free phase-pure α-FAPbI3 perovskite film by a new solution modification strategy is reported. This enables the activation of lead-iodine (Pb─I) frameworks by forming the coated Pb⋯O network, thus simultaneously promoting spontaneous homogeneous nucleation and rapid phase transition from δ to α phase. As a result, the efficient and stable phase-pure α-FAPbI3 PSC is obtained through a one-step method without antisolvent treatment, with a record efficiency of 23.15% and excellent long-term operating stability for 500 h under continuous light stress.

Screen-Printing Technology for Scale Manufacturing of Perovskite Solar Cells.

As a key contender in the field of photovoltaics, third-generation thin-film perovskite solar cells (PSCs) have gained significant research and investment interest due to their superior power conversion efficiency (PCE) and great potential for large-scale production. For commercialization consideration, low-cost and scalable fabrication is of primary importance for PSCs, and the development of the applicable film-forming techniques that meet the above requirements plays a key role. Currently, large-area perovskite films are mainly produced by printing techniques, such as slot-die coating, inkjet printing, blade coating, and screen-printing. Among these techniques, screen printing offers a high degree of functional layer compatibility, pattern design flexibility, and large-scale ability, showing great promise. In this work, the advanced progress on applying screen-printing technology in fabricating PSCs from technique fundamentals to practical applications is presented. The fundamentals of screen-printing technique are introduced and the state-of-the-art studies on screen-printing different functional layers in PSCs and the control strategies to realize fully screen-printed PSCs are summarized. Moreover, the current challenges and opportunities faced by screen-printed perovskite devices are discussed. This work highlights the critical significance of high throughput screen-printing technology in accelerating the commercialization course of PSCs products.

Efficient and Stable ß-CsPbI3 Solar Cells through Solvent Engineering with Methylamine Acetate Ionic Liquid.

CsPbI3, an all-inorganic perovskite material with suitable band gap and excellent thermal stability, has garnered significant attention for its potential in perovskite solar cells (PSCs). However, CsPbI3 is susceptible to phase changes from photoactive to photoinactive in humid environments. Hence, it is crucial to achieve controllable growth of CsPbI3 perovskite thin films with the desired ß-crystal phase and compact morphology for efficient and stable PSCs. Herein, MAAc was used as a solvent for the CsPbI3 precursor to fabricate ß-CsPbI3 perovskite. An intermediate compound of CsxMA1-xPbIxAc3-x was initially formed in the MAAc solution, and during annealing, the MA+ and Ac- ions were replaced by Cs+ and I- ions, respectively. Furthermore, the incorporation of strong C═O···Pb coordination stabilized the black-phase ß-CsPbI3 and facilitated the growth of crystals with a narrow vertical orientation and large grain size. As a result, the PSCs with an efficiency of 18.9% and improved stability (less than 10% decay after 2000 h of storage in N2 and less than 30% decay after 500 h of storage in humid air without any encapsulation) were achieved.

Novel Bilayer SnO2 Electron Transport Layers with Atomic Layer Deposition for High-Performance α-FAPbI3 Perovskite Solar Cells.

Perovskite solar cells (PSCs) based on the SnO2 electron transport layer (ETL) have achieved remarkable photovoltaic efficiency. However, the commercial SnO2 ETLs show various shortcomings. The SnO2 precursor is prone to agglomeration, resulting in poor morphology with numerous interface defects. Additionally, the open circuit voltage (Voc ) would be constrained by the energy level mismatch between the SnO2 and the perovskite. And, few studies designed SnO2 -based ETLs to promote crystal growth of PbI2 , a crucial prerequisite for obtaining high-quality perovskite films via the two-step method. Herein, we proposed a novel bilayer SnO2 structure that combined the atomic layer deposition (ALD) and sol-gel solution to well address the aforementioned issues. Due to the unique conformal effect of ALD-SnO2 , it can effectively modulate the roughness of FTO substrate, enhance the quality of ETL, and induce the growth of PbI2 crystal phase to develop the crystallinity of perovskite layer. Furthermore, a created built-in field of the bilayer SnO2 can help to overcome the electron accumulation at the ETL/perovskite interface, leading to a higher Voc and fill factor. Consequently, the efficiency of PSCs with ionic liquid solvent increases from 22.09% to 23.86%, maintaining 85% initial efficiency in a 20% humidity N2 environment for 1300 h.

Stabilization of Component-Pure α-FAPbI3 via Volatile Additives for Stable Photovoltaics.

State-of-the-art high-performance perovskite solar cells are mainly based on formamidinium (FA)-dominated perovskites because of their narrow band gap and remarkable thermal resistance. However, photoactive α-FAPbI3 is prone to transit to the photoinactive phase, and pioneering phase stabilization strategies can induce undesirable band gap broadening or phase segregation, seriously restricting the efficiency and long-term stability of the resultant photovoltaics. Herein, a small molecule of ammonium acetate (NH4Ac) was introduced as an additive in a modified ripening method to fabricate component-pure α-FAPbI3. Owing to the strong interaction between NH4Ac and PbI2, FAI via Pb-O coordination, and N-H···N hydrogen bonding, vertically oriented perovskites with relaxed crystal strain were first generated, which were fully converted to α-FAPbI3 in a further ripening process. The NH4Ac was fully volatized after the perovskite formation, resulting in component-pure α-FAPbI3 with a band gap of 1.48 eV and remarkable stability under light illumination. Ultimately, a champion device efficiency of above 21% was obtained based on the component-pure α-FAPbI3 and over 95% of the initial efficiency can be maintained after 1000 h of aging.

Perovskite solar cells based on screen-printed thin films.

One potential advantage of perovskite solar cells (PSCs) is the ability to solution process the precursors and deposit films from solution1,2. At present, spin coating, blade coating, spray coating, inkjet printing and slot-die printing have been investigated to deposit hybrid perovskite thin films3-6. Here we expand the range of deposition methods to include screen-printing, enabled by a stable and viscosity-adjustable (40-44,000 cP) perovskite ink made from a methylammonium acetate ionic liquid solvent. We demonstrate control over perovskite thin-film thickness (from about 120 nm to about 1,200 nm), area (from 0.5 × 0.5 cm2 to 5 × 5 cm2) and patterning on different substrates. Printing rates in excess of 20 cm s-1 and close to 100% ink use were achieved. Using this deposition method in ambient air and regardless of humidity, we obtained the best efficiencies of 20.52% (0.05 cm2) and 18.12% (1 cm2) compared with 20.13% and 12.52%, respectively, for the spin-coated thin films in normal devices with thermally evaporated metal electrodes. Most notably, fully screen-printing devices with a single machine in ambient air have been successfully explored. The corresponding photovoltaic cells exhibit high efficiencies of 14.98%, 13.53% and 11.80% on 0.05-cm2, 1.00-cm2 and 16.37-cm2 (small-module) areas, respectively, along with 96.75% of the initial efficiency retained over 300 h of operation at maximum power point.

Unraveling the Role of Chloride in Vertical Growth of Low-Dimensional Ruddlesden-Popper Perovskites for Efficient Perovskite Solar Cells.

Recently, low-dimensional Ruddlesden-Popper (LDRP) perovskite-based solar cells (PSCs) have been extensively studied because of their robust stability. However, because of the poor conductivity of the organic spacer, the charge transport across the spacers in the LDRP perovskite is considerably poor, and thus regulation of the growth orientation of LDRP cells is of primary importance. So far, the key role of organic cations in controlling the growth orientation of LDRP films has been widely studied, but the impact of halogens has not been sufficiently investigated. Herein, we demonstrate the important role of halogens in determining the characteristics of benzylamine (BZA)-based LDRP perovskite films, where different BZAX salts (X = Cl, Br, I) are adopted. Compared to Br and I, Cl is shown to prominently enlarge the grain size, promote the vertical orientation, reduce the trap state density, and prolong the carrier lifetime of LDRP film, and all these merits effectively accelerate the carrier transport within the film. As a result, a PSC device based on BZACl delivers a champion PCE of 17.25% with much improved device stability. This work unravels the vital role of Cl in regulating the crystallization process of LDRP films, which provides a facile approach for boosting the performance of LDRP-based PSCs.

In Situ Interfacial Passivation of Sn-Based Perovskite Films with a Bi-functional Ionic Salt for Enhanced Photovoltaic Performance.

Environment-friendly Tin (Sn)-based perovskite solar cells (PSCs) have lately made significant development, showing tremendous promise in addressing the hazardous problems associated with Pb-based PSCs. However, even in N2 atmospheres, the thermodynamic stability of Sn-based perovskite films and long-term stability of Sn-based PSCs are demonstrated to be poor due to the presence of interfacial defect trap states. Here, we demonstrate the post-treatment of Sn-based perovskite films with ethylenediamine formate (EDAFa2) ion salt, serving as a bi-functional interface layer to in situ passivate the interfacial defect and improve the stability of Sn2+ by creating a thermodynamic chemical environment pathway. Moreover, the presence of EDAFa2 is shown to promote the interfacial energy level alignment, which is beneficial for the charge extraction at the interface. As a result, PSC devices with a bi-functional interface achieve a champion power conversion efficiency (PCE) as high as 9.40% and enhanced stability, retaining ∼95% of the original PCE stored in a N2 environment after ∼1960 h without encapsulation. This work highlights the significant role of an interfacial design in efficient and stable Sn-based PSCs.

Stabilizing black-phase formamidinium perovskite formation at room temperature and high humidity.

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.

Origin of High Efficiency and Long-Term Stability in Ionic Liquid Perovskite Photovoltaic.

Environment-friendly protic amine carboxylic acid ionic liquids (ILs) as solvents is a significant breakthrough with respect to traditional highly coordinating and toxic solvents in achieving efficient and stable perovskite solar cells (PSCs) with a simple one-step air processing and without an antisolvent treatment approach. However, it remains mysterious for the improved efficiency and stability of PSCs without any passivation strategy. Here, we unambiguously demonstrate that the three functions of solvents, additive, and passivation are present for protic amine carboxylic acid ILs. We found that the ILs have the capability to dissolve a series of perovskite precursors, induce oriented crystallization, and chemically passivate the grain boundaries. This is attributed to the unique molecular structure of ILs with carbonyl and amine groups, allowing for strong interaction with perovskite precursors by forming C=O…Pb chelate bonds and N-H…I hydrogen bonds in both solution and film. This finding is generic in nature with extension to a wide range of IL-based perovskite optoelectronics.

All-inorganic Sn-based Perovskite Solar Cells: Status, Challenges, and Perspectives.

Recently, the power conversion efficiency (PCE) of perovskite solar cells (PSC) based on organic-inorganic hybrid Pb halide perovskites has reached 25.2 %. However, the toxicity of Pb has still been a main concern for the large-scale commercialization of Pb-based PSCs. Efforts have been made during the past few years to seek eco-friendly Pb-free perovskites, and it is a growing consensus that Sn is the best choice as Pb alternative over any other Pb-free metal elements. Among Sn-based perovskites, all-inorganic cells are promising candidates for PSCs owing to their more suitable bandgap, better stability, and higher charge mobility compared to the organic-inorganic hybrid counterparts. However, the poor phase stability of all-inorganic Sn-based perovskites (AISPs) and low PCE of their PSCs are most challenging in the field at present. Herein, recent developments on PSCs based on AISPs, including CsSnX3 and Cs2 SnX6 (X=Br, I), are comprehensively reviewed. Primarily, the intrinsic characteristics of the two AISPs are overviewed, including crystallographic property, band structure, charge carrier property, and defect property. Sequentially, state-of-the-art progress, regarding the photovoltaic application of AISPs as light absorber, is summarized. At last, current challenges and future opportunities of AISP-based PSCs are also discussed.

In Situ Interface Engineering for Highly Efficient Electron-Transport-Layer-Free Perovskite Solar Cells.

Electron-transport-layer free perovskite solar cells (ETL-free PSCs) have attracted great attention due to their low cost and simple manufacturing process. However, an additional interface layer has to be introduced, and the currently achieved efficiency remains far from full-structure PSCs. Here, we report an in situ interface engineering strategy by the methylammonium acetate (MAAc) ionic liquid perovskite precursor. We found that a dipole layer was in situ constructed through the physical adsorption of the residual MAAc polar molecules on the indium tin oxide electrode, which is significantly different from the treatment by the interface layer in previous reports. This allows a decrease of the effective work function and enables in situ band bending in the perovskite semiconductor. The in situ band bending facilitates charge collection and hinders interfacial charge recombination, leading to ETL-free PSCs with a maximum power conversion efficiency of 21.08%, which is the highest report to date.

Tailoring Component Interaction for Air-Processed Efficient and Stable All-Inorganic Perovskite Photovoltaic.

All-inorganic lead halide perovskites are promising candidates for optoelectronic applications. However, fundamental questions remain over the component interaction in the perovskite precursor solution due to the limitation of the most commonly used solvents of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Here, we report an interaction tailoring strategy for all-inorganic CsPbI3-x Brx perovskites by involving the ionic liquid solvent methylammonium acetate (MAAc). C=O shows strong interaction with lead (Pb2+ ) and N-H⋅⋅⋅I hydrogen bond formation is observed. The interactions stabilize the perovskite precursor solution and allow production of the high-quality perovskite films by retarding the crystallization. Without the necessity for antisolvent treatment, the one-step air-processing approach delivers photovoltaic cells regardless of humidity, with a high efficiency of 17.10 % along with long operation stability over 1500 h under continuous light illumination.

Centimeter-Sized Single Crystal of Two-Dimensional Halide Perovskites Incorporating Straight-Chain Symmetric Diammonium Ion for X-Ray Detection.

Two-dimensional (2D) AA'n-1 Mn X3n+1 type halide perovskites incorporating straight-chain symmetric diammonium cations define a new type of structure, but their optoelectronic properties are largely unexplored. Reported here is the synthesis of a centimeter-sized AA'n-1 Mn X3n+1 type perovskite, BDAPbI4 (BDA=NH3 C4 H8 NH3 ), single crystal and its charge-transport properties under X-ray excitation. The crystal shows a staggered configuration of the [PbI6 ]4- layers, a band gap of 2.37 eV, and a low trap density of 3.1×109  cm-3 . The single-crystal X-ray detector exhibits an excellent sensitivity of 242 µC Gyair -1 cm-2 under the 10 V bias (0.31 V µm-1 ), a detection limit as low as 430 nGyair s-1 , ultrastable response current, a stable baseline with the lowest dark current drift of 6.06×10-9  nA cm-1 s-1 V-1 , and rapid response time of τrise =7.3 ms and τfall =22.5 ms. These crystals are promising candidates for the next generation of optoelectronic devices.

Improved Performance of CH3NH3PbI3-xClx Resistive Switching Memory by Assembling 2D/3D Perovskite Heterostructures.

The rapidly growing demand for fast information storage and processing has driven the development of resistive random access memories (RRAMs). Recently, RRAMs based on organometal halide perovskite materials have been reported to have promising memory properties, which are essential for next-generation memory devices. In this study, a hybrid two-dimensional/three-dimensional (2D/3D) perovskite heterostructure has been created by depositing n-butylammonium iodide on top of the CH3NH3PbI3-xClx (MAPbI3-xClx) surface. The perovskite film is fabricated by a facile one-step spin-coating method with room-temperature molten salt methylammonium acetate in the air. Resistive switching memory devices with a 2D/3D perovskite heterostructure exhibit a significantly improved switching window (ON/OFF ratio over 103) with a lower operation voltage compared with their 3D counterparts. The 2D/3D perovskite heterostructure is advantageous for fabricating uniform-crystalline-grain, highly compact structures and can passivate defect states for the MAPbI3-xClx film and the interface, which results in improved memory properties. These results provide a new perspective for developing high-performance perovskite-based memory devices.

Red-Carbon-Quantum-Dot-Doped SnO2 Composite with Enhanced Electron Mobility for Efficient and Stable Perovskite Solar Cells.

An efficient electron transport layer (ETL) plays a key role in promoting carrier separation and electron extraction in planar perovskite solar cells (PSCs). An effective composite ETL is fabricated using carboxylic-acid- and hydroxyl-rich red-carbon quantum dots (RCQs) to dope low-temperature solution-processed SnO2 , which dramatically increases its electron mobility by ≈20 times from 9.32 × 10-4 to 1.73 × 10-2 cm2 V-1 s-1 . The mobility achieved is one of the highest reported electron mobilities for modified SnO2 . Fabricated planar PSCs based on this novel SnO2 ETL demonstrate an outstanding improvement in efficiency from 19.15% for PSCs without RCQs up to 22.77% and have enhanced long-term stability against humidity, preserving over 95% of the initial efficiency after 1000 h under 40-60% humidity at 25 °C. These significant achievements are solely attributed to the excellent electron mobility of the novel ETL, which is also proven to help the passivation of traps/defects at the ETL/perovskite interface and to promote the formation of highly crystallized perovskite, with an enhanced phase purity and uniformity over a large area. These results demonstrate that inexpensive RCQs are simple but excellent additives for producing efficient ETLs in stable high-performance PSCs as well as other perovskite-based optoelectronics.

Enhanced Performance of Perovskite Light-Emitting Diodes via Diamine Interface Modification.

Interfacial engineering between charge transport layers and perovskite light-emitting layers has been applied as an effective strategy to enhance performance of perovskite light-emitting diodes (PeLEDs). Herein, we introduce a Lewis base diamine molecule [2,2-(ethylenedioxy)bis(ethylammonium), EDBE] to modify the interface between the ZnMgO electron transport layer (ETL) and perovskite light-emitting layer in PeLEDs. With two amino groups in EDBE, one amine can interact with ZnMgO beneath to tune the growth of perovskite films, resulting in improved electron injection and suppressed current leakage. Meanwhile, the other amine can passivate the surface trap states of the polycrystalline perovskite films, which would eliminate trap-mediated nonradiative recombination. An enhanced performance for near-infrared PeLEDs is achieved with external quantum efficiency from 9.15 to 12.35% after incorporating the EDBE interfacial layer. This work demonstrated that the introduction of Lewis base diamine molecules as the ETL/perovskite interfacial agent is a promising way for developing high-performance PeLEDs.

Stable, Efficient Near-Infrared Light-Emitting Diodes Enabled by α/δ Phase Modulation.

Although formamidinium lead iodide (FAPbI3) perovskite shows excellent prospects in the field of optoelectronics, it is limited by the conversion of the black perovskite phase (α-FAPbI3) to the yellow non-perovskite phase (δ-FAPbI3). Here, we first introduce diamine cations [EDBE, 2,2-(ethylenedioxy)bis(ethylamine)] into FAPbI3 for stable and efficient near-infrared perovskite light-emitting diodes (NIR PeLEDs). By rationally regulating the α/δ phase, we have successfully prepared a NIR PeLED with an external quantum efficiency (EQE) of ≤11.40%. In addition, the perovskite film has excellent stability and does not undergo any significant change after being in air for 270 days. These results show that the introduction of EDBE reduces the formation energy of α-FAPbI3 perovskite and the hydrogen bonds formed between adjacent EDBE molecules increase the stability of the perovskite devices. This work could help guide the phase transition mechanism of FAPbI3 and further improve the performance of LEDs.

Reduced-Dimensional Perovskite Enabled by Organic Diamine for Efficient Photovoltaics.

Reduced-dimensional (RD) perovskite solar cells (PSCs) are emerging as highly attractive alternatives to three-dimensional (3D) PSCs due to their dramatically improved environmental stability and photostability. Diamine-based RD perovskites with a single organic amine interlayer possess orderly inorganic sheets and a smaller insulation area, indicating great potential in combining high efficiency and long-term stability. Here, we report an efficient and stable RD PSC based on 1,4-butanediamine (BDA). We found that the BDA-based RD perovskite exhibits improved crystallinity, reduced trap-state densities, and enhanced charge mobility compared to those of butylamine (BA)-based RD (BA-RD) perovskite. A high power conversion efficiency of 17.91% was achieved with negligible hysteresis. Moreover, the device showed improved stability compared to those of BA-RD and 3D films and devices. The findings may inspire new developments in introducing organic diamine for efficient and stable RD PSCs.