Additive Engineering for Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have reached a certified 25.2% efficiency in 2019 due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap. However, due to the nature of solution processing and rapid crystal growth of perovskite thin films, a variety of defects can form as a result of the precursor compositions and processing conditions. The use of additives can affect perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, as well as influence the interface tuning of structure and energetics. Here, recent progress in additive engineering during perovskite film formation is discussed according to the following common categories: Lewis acid (e.g., metal cations, fullerene derivatives), Lewis base based on the donor type (e.g., O‐donor, S‐donor, and N‐donor), ammonium salts, low‐dimensional perovskites, and ionic liquid. Various additive‐assisted strategies for interface optimization are then summarized; additives include modifiers to improve electron‐ and hole‐transport layers as well as those to modify perovskite surface properties. Finally, an outlook is provided on research trends with respect to additive engineering in PSC development.     

Chlorinated Fullerene Dimers for Interfacial Engineering Toward Stable Planar Perovskite Solar Cells with 22.3% Efficiency

A major limit for planar perovskite solar cells is the trap‐mediated hysteresis and instability, due to the defective metal oxide interface with the perovskite layer. Passivation engineering with fullerenes has been identified as an effective approach to modify this interface. The rational design of fullerene molecules with exceptional electrical properties and versatile chemical moieties for targeted defect passivation is therefore highly demanded. In this work, novel fulleropyrrolidine (NMBF‐X, X?H or Cl) monomers and dimers are synthesized and incorporated between metal oxides (i.e. TiO₂, SnO₂) and perovskites (i.e. MAPbI₃ and (FAPbI₃)_x(MAPbBr₃)_1‐x). The fullerene dimers provide superior stability and efficiency improvements compared to the corresponding monomers, with chlorinated fullerene dimers being most effective at coordinating with both metal oxides and perovskite via the chlorine terminals. The non‐encapsulated planar device delivers a maximum power conversion efficiency of 22.3% without any hysteresis, while maintaining over 98% of initial efficiency after ambient storage for 1000 h, and exhibiting an order of magnitude improvement of the T₈₀ lifetime.     

Composition Engineering in Doctor‐Blading of Perovskite Solar Cells

Organic–inorganic halide perovskite (OIHP) solar cells with efficiency over 18% power conversion efficiency (PCE) have been widely achieved with lab scale spin‐coating method which is however not scalable for the fabrication of large area solar panels. The PCEs of OIHP solar cells made by scalable deposition methods, such as doctor‐blading or slot‐die coating, have been lagging far behind than spin‐coated devices. In this study the authors report composition engineering in doctor‐bladed OIHP solar cells with p–i–n planar heterojunction structure to enhance their efficiency. Phase purer OIHP thin films are obtained by incorporating a small amount of cesium (Cs⁺) and bromine (Br^?) ions into perovskite precursor solution, which also reduces the required film formation temperature. Pinhole free OIHP thin films with micrometer‐sized grains have been obtained assisted by a secondary grain growth with added methylammonium chloride into the precursor solution. The OIHP solar cells using these bladed thin films achieved PCEs over 19.0%, with the best stabilized PCE reaching 19.3%. This represents a significant step toward scalable manufacture of OIHP solar cells.     

Universal Strategy with Structural and Chemical Crosslinking Interface for Efficient and Stable Perovskite Solar Cells

Due to the limited interface contact and weak interfacial interaction, planar heterojunction perovskite solar cells (PSCs) have space for further improvement. Herein, a structural and chemical crosslinking interface is proposed and constructed by introducing an extra layer, which blends tin dioxide (SnO₂) nanoparticles with chloride salts. Since the incorporated materials can be dissolved during the fabrication of perovskite, the quality of perovskite films is improved, leading to larger grain size and reduced trap-state density. Also, more chloride ions at the SnO₂/perovskite interface are observed and the interaction between Cl⁻ and Sn⁴⁺ is confirmed. It results in more pronounced n-type SnO₂ with better conductivity and deeper conduction bands, leading to preferable energy level alignment between SnO₂ and perovskite. Consequently, the open-circuit voltage and fill factor of the devices increase, and target cells present better stability, retaining 98% of initial efficiencies after >10 000 h storage in dry air (≈5% relative humidity) and maintaining 85.50% of the initial efficiency after 1000 h of operation under light. This strategy enables the achievement of 25.28% efficiency with a low bandgap (1.53 eV) perovskite composition, and it is confirmed to be universal when other related materials are utilized.     

Interface Engineering for Highly Efficient and Stable Planar p‐i‐n Perovskite Solar Cells

Organic‐inorganic halide perovskite materials have become a shining star in the photovoltaic field due to their unique properties, such as high absorption coefficient, optimal bandgap, and high defect tolerance, which also lead to the breathtaking increase in power conversion efficiency from 3.8% to over 22% in just seven years. Although the highest efficiency was obtained from the TiO₂ mesoporous structure, there are increasing studies focusing on the planar structure device due to its processibility for large‐scale production. In particular, the planar p‐i‐n structure has attracted increasing attention on account of its tremendous advantages in, among other things, eliminating hysteresis alongside a competitive certified efficiency of over 20%. Crucial for the device performance enhancement has been the interface engineering for the past few years, especially for such planar p‐i‐n devices. The interface engineering aims to optimize device properties, such as charge transfer, defect passivation, band alignment, etc. Herein, recent progress on the interface engineering of planar p‐i‐n structure devices is reviewed. This review is mainly focused on the interface design between each layer in p‐i‐n structure devices, as well as grain boundaries, which are the interfaces between polycrystalline perovskite domains. Promising research directions are also suggested for further improvements.     

New Strategy for Two‐Step Sequential Deposition: Incorporation of Hydrophilic Fullerene in Second Precursor for High‐Performance p‐i‐n Planar Perovskite Solar Cells

In p‐i‐n planar perovskite solar cells (pero‐SCs) based on methylammonium lead iodide (MAPbI₃) perovskite, high‐quality MAPbI₃ film, perfect interfacial band alignment and efficient charge extracting ability are critical for high photovoltaic performance. In this work, a hydrophilic fullerene derivative [6,6]‐phenyl‐C₆₁‐butyric acid‐(3,4,5‐tris(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)phenyl)methanol ester (PCBB‐OEG) is introduced as additive in the methylammonium iodide precursor solution in the preparation of MAPbI₃ perovskite film by two‐step sequential deposition method, and obtained a top‐down gradient distribution with an ultrathin top layer of PCBB‐OEG. Meanwhile, a high‐quality perovskite film with high crystallinity, less trap‐states, and dense‐grained uniform morphology can well grow on both hydrophilic (poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonic acid)) and hydrophobic (polytriarylamine, PTAA) hole transport layers. When the PCBB‐OEG‐containing perovskite film (pero‐0.1) is prepared in a p‐i‐n planar pero‐SC with the configuration of ITO/PTAA/pero‐0.1/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester/Al, the device delivers a promising power conversion efficiency (PCE) of 20.2% without hysteresis, which is one of the few PCE over 20% for the p‐i‐n planar pero‐SCs. Importantly, the pero‐0.1‐based device shows an excellent stability that can retain 98.4% of its initial PCE after being exposed for 300 h under ambient atmosphere with a high humidity, and the flexible pero‐SCs based on pero‐0.1 also demonstrate a promising PCE of 18.1%.     

Optimal Interfacial Engineering with Different Length of Alkylammonium Halide for Efficient and Stable Perovskite Solar Cells

Recently, two‐dimensional (2D) structure on three‐dimensional (3D) perovskites (graded 2D/3D) has been reported to be effective in significantly improving both efficiency and stability. However, the electrical properties of the 2D structure as a passivation layer on the 3D perovskite thin film and resistance to the penetration of moisture may vary depending on the length of the alkyl chain. In addition, the surface defects of the 2D itself on the 3D layer may also be affected by the correlation between the 2D structure and the hole conductive material. Therefore, systematic interfacial study with the alkyl chain length of long‐chained alkylammonium iodide forming a 2D structure is necessary. Herein, the 2D interfacial layers formed are compared with butylammonium iodide (BAI), octylammonium iodide (OAI), and dodecylammonium iodide (DAI) iodide on a 3D (FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite thin film in terms of the PCE and humidity stability. As the length of the alkyl chain increased from BA to OA to DA, the electron‐blocking ability and humidity resistance increase significantly, but the difference between OA and DA is not large. The PSC post‐treated with OAI has slightly higher PCE than those treated with BAI and DAI, achieving a certified stabilized efficiency of 22.9%.     

Toward Long‐Term Stable and Highly Efficient Perovskite Solar Cells via Effective Charge Transporting Materials

Perovskite solar cells (PSCs) have advanced quickly with their power conversion efficiency approaching the record of silicon solar cells. However, there is still a big challenge to obtain both high efficiency and long‐term stability for future commercialization of PSCs. The major instability issue is associated with the decomposition or phase transition of perovskite materials that are believed to be intrinsically unstable under outdoor working conditions. Herein, the authors review the approaches that marked important progress in developing new functional electron/hole transporting materials that enabled highly efficient and stable PSCs. The findings that accelerate charge diffusion and that suppress the irrevocable loss of ions diffusing out of perovskite materials and other diffusion processes are highlighted. In addition, derivative interface engineering methods to control the diffusion process of charges/ions/molecules are also reviewed. Finally, the authors propose key research issues in charge transporting materials and interface engineering with regard to the important diffusion processes that will be one of the keys to realize highly efficient and long‐term stable PSCs.     

A Review on Additives for Halide Perovskite Solar Cells

Additives are widely adopted for efficient, stable, and hysteresis‐free perovskite solar cells and play an important role in various breakthroughs of perovskite solar cells (PSCs). Herein the various additives adopted for PSCs are reviewed and their functioning mechanism and influence on device performance is described. The main roles of additives, modulating morphology of perovskite films, stabilizing phase of formamidinium (FA) and cesium (Cs)‐based perovskites, adjusting energy level alignment in PSCs, suppressing nonradiative recombination in perovskites, eliminating hysteresis, enhancing operational stability of PSCs, are summarized.     

Carrier Interfacial Engineering by Bismuth Modification for Efficient and Thermoresistant Perovskite Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) have emerged as attractive alternatives to conventional solar cells. It is still a challenge to obtain PSCs with good thermal stability and high permanence, especially at extreme outdoor temperatures. This work systematically studies the effects of Bi³⁺ modification on structural, electrical, and optical properties of perovskite films (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃) and the performance of corresponding PSCs. The results indicate that Bi³⁺ modified PSCs can achieve better thermal stability, photovoltaic response, and reproducibility compared with control cells due to the decreased grain boundaries, enhanced crystallization, and improved electron extraction from perovskite film. As a result, the modified PSC exhibits an optimized power conversion efficiency (PCE) of 19.4% compared with 18.3% for the optimized control device, accompanied by better thermoresistant ability under 100–180 °C and enhanced long‐term stability. The degradation rate of the modified device is reduced by an order of magnitude due to effective structural defect modification in perovskite photoactive layer. It could maintain more than two months at 60 °C. These results shed light on the origin of crystallization and thermal stability of perovskite films, and provide an approach to solve thermal stability issue of PSCs.     

Structurally Reconstructed CsPbI2Br Perovskite for Highly Stable and Square‐Centimeter All‐Inorganic Perovskite Solar Cells

Although all‐inorganic perovskite solar cells (PSCs) demonstrate high thermal stability, cesium‐lead halide perovskites with high iodine content suffer from poor stability of the black phase (α‐phase). In this study, it is demonstrated that incorporating InCl₃ into the host perovskite lattice helps to inhibit the formation of yellow phase (δ‐phase) perovskite and thereby enhances the long‐term ambient stability. The enhanced stability is achieved by a strategy for the structural reconstruction of CsPbI₂Br perovskite by means of In³⁺ and Cl^? codoping, which gives rise to a significant improvement in the overall spatial symmetry with a closely packed atom arrangement due to the crystal structure transformation from orthorhombic (Pnma) to cubic (Pm‐3m). In addition, a novel thermal radiation heating method that further improves the uniformity of the perovskite thin films is presented. This approach enables the construction of all‐inorganic InCl₃:CsPbI₂Br PSCs with a champion power conversion efficiency of 13.74% for a small‐area device (0.09 cm²) and 11.4% for a large‐area device (1.00 cm²).     

Fabrication of Efficient and Stable CsPbI3 Perovskite Solar Cells through Cation Exchange Process

Inorganic lead halide perovskites have attracted attention due to their tolerance to higher processing temperature and higher bandgap suitable for tandem solar cell application. Not only do they improve cell stability and efficiency, they also reveal many interesting and un‐anticipated material qualities. This work reports a simple cation exchange growth (CEG) method for fabricating inorganic high‐quality cesium lead iodide (CsPbI₃) by adding methylammonium iodide (MAI) additive in the precursor. X‐ray diffraction results reveal a multi‐stage film formation process whereby i) MAPbI₃ perovskite first formed that acts as a perovskite template for ii) subsequent ion exchange whereby the MA⁺ ions in the MAPbI₃ are replaced by Cs⁺ (as temperature ramps up) and iii) form g‐phase perovskite CsPbI₃. Optical microscopy, photoluminescence, and electrical characterizations reveal that the CEG process produces high‐quality film with better absorption, uniform and dense film with better interface, lower defects, and better stability. Using the CEG approach, the power conversion efficiency of the best CsPbI₃ solar cell is significantly increased up to 14.1% for the device fabricated using 1.0 m MAI additive. The outcome is beneficial for further improvement of inorganic perovskite solar cells and their application in perovskite‐silicon tandem devices.     

Synergistic Crystal and Interface Engineering for Efficient and Stable Perovskite Photovoltaics

The presence of bulk and surface defects in perovskite light harvesting materials limits the overall efficiency of perovskite solar cells (PSCs). The formation of such defects is suppressed by adding methylammonium chloride (MACl) as a crystallization aid to the precursor solution to realize high‐quality, large‐grain triple A‐cation perovskite films and that are combined with judicious engineering of the perovskite interface with the electron and hole selective contact materials. A planar SnO₂/TiO₂ double layer oxide is introduced to ascertain fast electron extraction and the surface of the perovskite facing the hole conductor is treated with iodine dissolved in isopropanol to passivate surface trap states resulting in a retardation of radiationless carrier recombination. A maximum solar to electric power conversion efficiency (PCE) of 21.65% and open circuit photovoltage (V_oc) of ≈1.24 V with only ≈370 mV loss in potential with respect to the band gap are achieved, by applying these modifications. Additionally, the defect healing enhances the operational stability of the devices that retain 96%, 90%, and 85% of their initial PCE values after 500 h under continuously light illumination at 20, 50, and 65 °C, respectively, demonstrating one of the most stable planar PSCs reported so far.     

High‐Performance Flexible Perovskite Solar Cells via Precise Control of Electron Transport Layer

Flexible perovskite solar cells (f‐PSCs) have attracted great attention due to their promising commercial prospects. However, the performance of f‐PSCs is generally worse than that of their rigid counterparts. Herein, it is found that the unsatisfactory performance of planar heterojunction (PHJ) f‐PSCs can be attributed to the undesirable morphology of electron transport layer (ETL), which results from the rough surface of the flexible substrate. Precise control over the thickness and morphology of ETL tin dioxide (SnO₂) not only reduces the reflectance of the indium tin oxide (ITO) on polyethylene 2,6‐naphthalate (PEN) substrate and enhances photon collection, but also decreases the trap‐state densities of perovskite films and the charge transfer resistance, leading to a great enhancement of device performance. Consequently, the f‐PSCs, with a structure of PEN/ITO/SnO₂/perovskite/Spiro‐OMeTAD/Ag, exhibit a power conversion efficiency (PCE) up to 19.51% and a steady output of 19.01%. Furthermore, the f‐PSCs show a robust bending resistance and maintain about 95% of initial PCE after 6000 bending cycles at a bending radius of 8 mm, and they present an outstanding long‐term stability and retain about 90% of the initial performance after >1000 h storage in air (10% relative humidity) without encapsulation.     

Stability and Dark Hysteresis Correlate in NiO‐Based Perovskite Solar Cells

In perovskite solar cells (PSCs), the interfaces are a weak link with respect to degradation. Electrochemical reactivity of the perovskite's halides has been reported for both molecular and polymeric hole selective layers (HSLs), and here it is shown that also NiO brings about this decomposition mechanism. Employing NiO as an HSL in p–i–n PSCs with power conversion efficiency (PCE) of 16.8%, noncapacitive hysteresis is found in the dark, which is attributable to the bias‐induced degradation of perovskite/NiO interface. The possibility of electrochemically decoupling NiO from the perovskite via the introduction of a buffer layer is explored. Employing a hybrid magnesium‐organic interlayer, the noncapacitive hysteresis is entirely suppressed and the device's electrical stability is improved. At the same time, the PCE is improved up to 18% thanks to reduced interfacial charge recombination, which enables more efficient hole collection resulting in higher V_oc and FF.     

In Situ Grain Boundary Functionalization for Stable and Efficient Inorganic CsPbI2Br Perovskite Solar Cells

The phase instability and large energy loss are two obstacles to achieve stable and efficient inorganic‐CsPbI_3?xBr_x perovskite solar cells. In this work, stable cubic perovskite (α)‐phase CsPbI₂Br is successfully achieved by Pb(Ac)₂ functioning at the grain boundary under low temperature. Ac^? strongly coordinates with CsPbI₂Br to stabilize the α‐phase and also make the grain size smaller and film uniform by fast nucleation. PbO is formed in situ at the grain boundary by decomposing Pb(Ac)₂ at high‐temperature annealing. The semiconducting PbO effectively passivates the surface states, reduces the interface recombination, and promotes the charge transport in CsPbI₂Br perovskite solar cells. A 12% efficiency and good stability are obtained for in situ PbO‐passivated CsPbI₂Br solar cells, while Pb(Ac)₂‐passivated device exhibits 8.7% performance and the highest stability, much better than the control device with 8.5% performance and inferior stability. This article highlights the extrinsic ionic grain boundary functionalization to achieve stable and efficient inorganic CsPbI_3?xBr_x materials and the devices.     

Aquointermediate Assisted Highly Orientated Perovskite Thin Films toward Thermally Stable and Efficient Solar Cells

The stability of single‐crystalline/monocrystalline‐like perovskite film is expected to be better than its microcrystalline counterparts. In the present work, highly orientated perovskite thin films (CH₃NH₃PbI_3–xCl_x) are prepared by means of aquointermediates assisted solution process. It displays super‐duper preferred‐orientation along <110> direction that is close to the single crystal, and its diffraction intensity ratio of (110)/(310) is nearly two orders of magnitude higher in contrast to the films that prepared by traditional way. Owing to its superior performances, e.g., highly crystallized quality, stress‐free inside films, longer electron lifetime, faster temporal response time, etc., the highly orientated perovskite‐based solar cells accordingly allow realizing high efficiency while improving its thermal stability.