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.

Bilateral Chemical Linking at NiOx Buried Interface Enables Efficient and Stable Inverted Perovskite Solar Cells and Modules.

Inverted NiOx-based perovskite solar cells (PSCs) exhibit considerable potential because of their low-temperature processing and outstanding excellent stability, while is challenged by the carriers transfer at buried interface owing to the inherent low carrier mobility and abundant surface defects that directly deteriorates the overall device fill factor. Present work demonstrates a chemical linker with the capability of simultaneously grasping NiOx and perovskite crystals by forming a Ni-S-Pb bridge at buried interface to significantly boost the carriers transfer, based on a rationally selected molecule of 1,3-dimethyl-benzoimidazol-2-thione (NCS). The constructed buried interface not only reduces the pinholes and needle-like residual PbI2 at the buried interface, but also deepens the work function and valence band maximum positions of NiOx, resulting in a smaller VBM offset between NiOx and perovskite film. Consequently, the modulated PSCs achieved a high fill factor up to 86.24%, which is as far as we know the highest value in records of NiOx-based inverted PSCs. The NCS custom-tailored PSCs and minimodules (active area of 18 cm2) exhibited a champion efficiency of 25.05% and 21.16%, respectively. The unencapsulated devices remains over 90% of their initial efficiency at maximum power point under continuous illumination for 1700 hours.

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.

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.

Emerging Nonlinear Photocurrents in Lead Halide Perovskites for Spintronics.

Lead halide perovskites (LHPs) containing organic parts are emerging optoelectronic materials with a wide range of applications thanks to their high optical absorption, carrier mobility, and easy preparation methods. They possess spin-dependent properties, such as strong spin-orbit coupling (SOC), and are promising for spintronics. The Rashba effect in LHPs can be manipulated by a magnetic field and a polarized light field. Considering the surfaces and interfaces of LHPs, light polarization-dependent optoelectronics of LHPs has attracted attention, especially in terms of spin-dependent photocurrents (SDPs). Currently, there are intense efforts being made in the identification and separation of SDPs and spin-to-charge interconversion in LHP. Here, we provide a comprehensive review of second-order nonlinear photocurrents in LHP in regard to spintronics. First, a detailed background on Rashba SOC and its related effects (including the inverse Rashba-Edelstein effect) is given. Subsequently, nonlinear photo-induced effects leading to SDPs are presented. Then, SDPs due to the photo-induced inverse spin Hall effect and the circular photogalvanic effect, together with photocurrent due to the photon drag effect, are compared. This is followed by the main focus of nonlinear photocurrents in LHPs containing organic parts, starting from fundamentals related to spin-dependent optoelectronics. Finally, we conclude with a brief summary and future prospects.

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.

Honeycomb-Type TiO2 Films Toward a High Tolerance to Optical Paths for Perovskite Solar Cells.

Given the advantages of high power conversion efficiencies (PCEs), antisolvent-step free production, and suitability for device production in ambient conditions, perovskite solar cells (PSCs) based on ionic-liquid solvents have attained particular research interest. To further improve device performance, light management could be optimized to increase light harvesting in the perovskite layer. Here, ordered honeycomb-like TiO2 (Hc-TiO2 ) structures with a periodicity of around 450 nm were fabricated through a sacrificial template method. With this photonic crystal structure, the control to light flow and the confinement effect for perovskite growth were achieved simultaneously in the Hc-TiO2 , leading to improved light absorption as well as preferred crystal orientation. Furthermore, a reduced trap-state density and a well-aligned energy level induced by the perovskite/pore interlayer facilitated the charge-carrier extraction from the perovskite layer to electron transport layer. As a result, the structured devices performed better than the planar cells. And the angular dependent J-V sweeps show that the structured device reserved 76 % of its initial short circuit current density (Jsc ), whereas the planar cell showed more than a half loss under the incident light of 40°, demonstrating a reduced downward trend in Jsc with the presence of photonic crystal structures. This occurrence also suggests that the structured PSCs in this work have a high tolerance to optical path changes.

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.

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.

Surface-functionalized boron nanosheets and their assembled suprastructures with unprecedented proton-transport properties.

Two-dimensional (2D) boron nanomaterials have received considerable attention due to their distinct physicochemical properties in contrast to bulk boron. However, the susceptibility to oxidation in air has limited their practical applications. In this study, we synthesize an environmentally stable bifunctionalized boron nanosheet via a wet chemical route. By lyophilization, we have hierarchically assembled the boron nanosheets into various well-ordered macroscopic forms, which exhibit unique structural features, such as stacking-induced nanochannels for proton transport. The resulting suprastructures show exceptionally high proton conductivity (∼90 mS cm-1 at 85 °C) and humidity sensitivity (response >40 000% at 97% RH). These findings demonstrate the immense potential of boron nanomaterials in electrochemical applications.

Phase-Pure α-FAPbI3 for Perovskite Solar Cells.

Because of the narrow bandgap and superior thermal stability, FAPbI3 is considered the most promising perovskite material for high-performance single-junction PSCs. Nevertheless, the metastable properties of the photoactive α-FAPbI3 becomes a primary obstacle for the development of FA-based PSCs. The main reasons for the instability of α-FAPbI3 are the rotation disorder of the FA cation and large anisotropic lattice strain, which lead to the high formation energy of α-FAPbI3. In this Perspective, we review various strategies for preparing phase-pure α-FAPbI3, such as engineering, intermediate phase engineering, and dimensionality engineering. These strategies can stabilize α-FAPbI3 by reducing the system energy, regulating the phase transition process and energy barrier, reinforcing the lattice structure, and passivating film defects. In addition, we investigate fundamental challenges of α-FAPbI3 PSCs and propose our perspective on preparing high-quality and high-purity α-FAPbI3.

SnO2 Passivation and Enhanced Perovskite Charge Extraction with a Benzylamine Hydrochloric Interlayer.

Perovskite solar cells (PSCs) have gained much attention because of their expressive power conversion efficiency (PCE) of up to 25.5%. A good contact and a well-aligned energy level at the buried interfaces between electron transport layers (ETLs) and perovskite films play an essential role in promoting charge-carrier collection and suppressing nonradiative recombination. Currently, low-temperature-processed SnO2 thin films are widely used as the ETLs to achieve efficient and stable planar PSCs. However, fabricating proper SnO2/perovskite interfaces with a good contact and a well-aligned energy level is necessary but implies a great challenge. Herein, we modify the SnO2 ETL using benzylamine hydrochloride (BH), which is expected to facilitate the energy level alignment and to enhance perovskite crystallization. Moreover, the BH interlayer is found to effectively reduce the trap-state density and thereby improve the charge-carrier extraction between the ETL and the perovskite layer. Consequently, the PSC with BH modification yields a higher PCE, a lower hysteresis, and better stability than the device without a BH interlayer. This study highlights the key role of molecule modification of ETLs in designing efficient and stable PSCs.

Improving the Air Resistance of the Precursor Solution for Ambient-Air Coating of an Sn-Pb Perovskite Film with Superior Photovoltaic Performance.

Owing to narrow band gap and low toxicity, tin-lead (Sn-Pb) hybrid perovskites have shown great potential in photovoltaic applications, and the highest power conversion efficiency (PCE) of Sn-Pb perovskite solar cells (PSCs) has recently reached 23.6%. However, it is still challenging to prepare Sn-Pb films in open-air condition due to the Sn2+ oxidation of the precursor solution under this condition. In this work, we report the stabilizing of the Sn-Pb perovskite precursor solution by using ionic liquid methylammonium acetate (MAAc) as the solvent, which enables the fabrication of Sn-Pb films in air. MAAc is found to coordinate with the Sn-Pb precursor via abundant hydrogen bonding, which stabilizes the colloids and protects the Sn2+ stability in the precursor solution in air. Therefore, the durability of the Sn-Pb precursor solution based on the MAAc solvent is greatly improved, which enables the fabrication of efficient PSCs and achieves a champion PCE of ∼16% with robust device stability. Moreover, due to the chemical interactions of MAAc with Sn-Pb perovskites, the Pb leakage is also suppressed in the MAAc-based Sn-Pb PSCs. This work demonstrates a feasible strategy for reliable fabrication of Sn-Pb PSCs, which could also be applied in many other optoelectronic devices.

Designing Ionic Liquids as the Solvent for Efficient and Stable Perovskite Solar Cells.

As a green solvent, ionic liquids (ILs) are considered as a promising alternative to conventional polar aprotic solvents for the production of efficient and stable perovskite solar cells (PSCs). Moreover, with the use of IL solvents, perovskite films can be prepared without antisolvent treatments in an ambient environment instead of in a glovebox with inert gases, which simplifies the film manufacturing process and is favorable for industrialization production. However, the type of IL solvents that have been studied is limited, and the influence of IL molecular structures on the perovskite-film crystallization and device performance is not completely understood. In this work, four different ILs, methylammonium formate (MAF), methylammonium acetate (MAAc), methylammonium propionate (MAP), and mthylammonium isobutyrate (MAIB), are synthesized as the perovskite precursor solvents. The interaction between the functional groups of the synthesized solvents and Pb2+ in the precursor solution is studied, which has a direct impact on the morphology and crystallization of the deposited perovskite film. It is found that MAP solvent gives a high-quality perovskite film, which leads to the best photovoltaic performance with a champion PCE of 20.56% compared to the devices based on the other IL solvents. Moreover, the MAP-based device maintains 88% of its original PCE after 1000 h of storage in a N2 atmosphere, demonstrating excellent device stability. Therefore, it is concluded that MAP is the most suitable solvent for MAPbI3 films with respect to photovoltaic applications as compared to the other ILs.

Solvent Engineering of the Precursor Solution toward Large-Area Production of Perovskite Solar Cells.

Solar cells based on emerging organic-inorganic hybrid perovskite materials have reached certified power conversion efficiency as high as 25.5%, showing great potential in the next generation of photovoltaics toward large-scale industrialization. The most competitive feature of perovskite solar cells (PSCs) is that the perovskite light absorber can be fabricated by a low-cost solution method. For the solution method, the characteristics of the solvent play a key role in determining the crystallization kinetics, growth orientation, and optoelectronic properties of the perovskite film. Although significant progress has been made in the field of solvent engineering in PSCs, it is still challenging for the solution method to sustainably produce industrial-scale PSCs for future commercialization applications. Herein, the advanced progress of solvent engineering of precursor solution in terms of coordination regulation and toxicity reduction is highlighted. The physical and chemical characteristics of different solvents in reducing the toxicity of the solvent system, regulating the coordination property of the precursor solution, controlling the film-forming process of the perovskite film, and adjusting the photovoltaic performance of the PSC are systematically discussed. Lastly, important perspectives on solvent engineering of the perovskite precursor solution toward future industrial production of high-performance PSCs are provided.

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.

Efficient and Stable Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells Enabled by Reducing Tunnel Barrier.

Low-dimensional Ruddlesden-Popper (LDRP) perovskite solar cells (PSCs) have attracted increasing attention due to their excellent long-term stability over three-dimensional (3D) counterparts. However, the introduction of insulated long-range bulkier organic ammonium spacers hindered the charge transport. Here, the short-range organic ammonium spacers, 1-amino-3-butene hydrochloride (BEACl) and 3-butyn-1-amine hydrochloride (BYACl), were employed to construct LDRP perovskites, instead of common butylamine hydrochloride (BACl). We found that charge transport can be significantly improved by controlling the tunneling effect. Moreover, highly oriented and flat perovskite films without pinholes were obtained. Consequently, high PCEs, exceeding 16% for BEA- and 15% for BYA-based devices, which is much higher than that of the BA-based analogous device (13.8%), were achieved. Most importantly, the BEA- and BYA-based LDRP perovskite films and devices show much improved stability. The finding is of great significance for the exploration of new organic ammonium spacers for highly efficient and stable LDRP PSCs.