Amorphous F-doped TiOx Caulked SnO2 Electron Transport Layer for Flexible Perovskite Solar Cells with Efficiency Exceeding 22.5%

Flexible perovskite solar cells (f-PSCs) show great promise in portable-power applications (e.g., chargers, drones) and low-cost, scalable productions (e.g., roll-to-roll). However, in conventional n–i–p architecture f-PSCs, the low-temperature processed metal oxide electron transport layers (ETLs) usually suffer from high resistance and severe defects that limit the power conversion efficiency (PCE) improvement of f-PSCs. Besides the enhancement in the mobility of metal oxide and passivation for perovskite/ETL interfacial defects reported in previous literature, herein, the electron transport loss between the metal oxide nanocrystallines within the ETL is studied by introducing an amorphous F-doped TiO_x (F-TiO_x) caulked crystalline SnO₂ composite ETL. The F-TiO_x in this novel composite ETL acts as an interstitial medium between adjacent SnO₂ nanocrystallines, which can provide more electron transport channels, effectively passivate oxygen vacancies, and optimize the energy level arrangement, thus significantly enhancing the electron mobility of ETL and reducing the charge transport losses. The composite ETL-based f-PSCs achieve a high PCE of 22.70% and good operational stability. Furthermore, a moderate roughness of the composite ETL endows f-PSCs with superior mechanical reliability by virtue of a strong coupling at the ETL/perovskite interface, by which the f-PSCs can maintain 82.11% of their initial PCE after 4000 bending cycles.     

Electron‐Transport‐Layer‐Assisted Crystallization of Perovskite Films for High‐Efficiency Planar Heterojunction Solar Cells

Crystal engineering of CH₃NH₃PbI₃ perovskite materials through template‐directed nucleation and growth on PbI₂ nuclei dispersed in a polar fullerene (C₆₀ pyrrolidine tris‐acid, CPTA) electron transport layer (ETL) (CPTA:PbI₂) is proposed as a route for controlling crystallization kinetics and grain sizes. Chemical analysis of the CPTA:PbI₂ template confirms that CPTA carboxylic acid groups can form a monodentate or bidentate chelate with Pb(II), resulting in a lower nucleation barrier that promotes rapid formation of the tetragonal perovskite phase. Moreover, it is demonstrated that a uniform CH₃NH₃PbI₃ film with highly crystalline and large domain sizes can be realized by increasing the spacing between nuclei to retard perovskite crystal growth via careful control of the preferred nucleation site distribution in the CPTA:PbI₂ layer. The improved perovskite morphology possesses a long photoluminescence lifetime and efficient photocarrier transport/separation properties to eliminate the hysteresis effect. The corresponding planar heterojunction photovoltaic yields a high power conversion efficiency (PCE) of 20.20%, with a high fill factor (FF) of 81.13%. The average PCE and FF values for 30 devices are 19.03% ± 0.57% and 78.67% ± 2.13%, respectively. The results indicate that this ETL template‐assisted crystallization strategy can be applied to other organometal halide perovskite‐based systems.     

Enhanced Stability of Perovskite Solar Cells with Low‐Temperature Hydrothermally Grown SnO2 Electron Transport Layers

Perovskite solar cells (PSCs) may offer huge potential in photovoltaic conversion, yet their practical applications face one major obstacle: their low stability, or quick degradation of their initial efficiencies. Here, a new design scheme is presented to enhance the PSC stability by using low‐temperature hydrothermally grown hierarchical nano‐SnO₂ electron transport layers (ETLs). The ETL contains a thin compact SnO₂ layer underneath a mesoporous layer of SnO₂ nanosheets. The mesoporous layer plays multiple roles of enhancing photon collection, preventing moisture penetration and improving the long‐term stability. Through such simple approaches, PSCs with power conversion efficiencies of ≈13% can be readily obtained, with the highest efficiency to be 16.17%. A prototypical PSC preserves 90% of its initial efficiency even after storage in air at room temperature for 130 d without encapsulation. This study demonstrates that hierarchical SnO₂ is a potential ETL for fabricating low‐cost and efficient PSCs with long‐term stability.     

PbTiO3 as Electron‐Selective Layer for High‐Efficiency Perovskite Solar Cells: Enhanced Electron Extraction via Tunable Ferroelectric Polarization

PbTiO₃ (PTO) is explored as a versatile and tunable electron‐selective layer (ESL) for perovskite solar cells. To demonstrate effectiveness of PTO for electron–hole separation and charge transfer, perovskite solar cells are designed and fabricated in the laboratory with the PTO as the ESL. The cells achieve a power conversion efficiency (PCE) of ≈12.28% upon preliminary optimization. It is found that the PTO ferroelectric layer can not only increase the PCE, but also tune the photocurrent via tuning PTO's ferroelectric polarization. Moreover, to understand the physical mechanism underlying the carrier transport by the ferroelectric polarization, the electronic structure of PTO/CH₃NH₃PbI₃ heterostructure is computed using the first‐principles methods, for which the triplet state is used to simulate charge transfer in the heterostructure. It is shown that the synergistic effect of type II band alignment and the specific ferroelectric polarization direction provide the effective extraction of electrons from the light absorber, while minimize recombination of photogenerated electron–hole pairs. Overall, the ferroelectric PTO is a promising and tunable ESL for optimizing electron transport in the perovskite solar cells. The design offers a different strategy for altering direction of carrier transport in solar cells.     

Improving the Extraction of Photogenerated Electrons with SnO2 Nanocolloids for Efficient Planar Perovskite Solar Cells

Great attention to cost‐effective high‐efficiency solar power conversion of trihalide perovskite solar cells (PSCs) has been hovering at high levels in the recent 5 years. Among PSC devices, admittedly, TiO₂ is the most widely used electron transport layer (ETL); however, its low mobility which is even less than that of CH₃NH₃PbI₃ makes it not an ideal material. In principle, SnO₂ with higher electron mobility can be regarded as a positive alternative. Herein, a SnO₂ nanocolloid sol with ≈3 nm in size synthesized at 60 °C was spin‐coated onto the fuorine‐doped tin oxide (FTO) glass as the ETL of planar CH₃NH₃PbI₃ perovskite solar cells. TiCl₄ treatment of SnO₂‐coated FTO is found to improve crystallization and increase the surface coverage of perovskites, which plays a pivotal role in improving the power conversion efficiency (PCE). In this report, a champion efficiency of 14.69% (J_sc = 21.19 mA cm^?2, V_oc = 1023 mV, and FF = 0.678) is obtained with a metal mask at one sun illumination (AM 1.5G, 100 mW cm^?2). Compared to the typical TiO₂, the SnO₂ ETL efficiently facilitates the separation and transportation of photogenerated electrons/holes from the perovskite absorber, which results in a significant enhancement of photocurrent and PCE.     

Rubidium Fluoride Modified SnO2 for Planar n-i-p Perovskite Solar Cells

Regulating the electron transport layer (ETL) has been an effective way to promote the power conversion efficiency (PCE) of perovskite solar cells (PSCs) as well as suppress their hysteresis. Herein, the SnO₂ ETL using a cost-effective modification material rubidium fluoride (RbF) is modified in two methods: 1) adding RbF into SnO₂ colloidal dispersion, F and Sn have a strong interaction, confirmed via X-ray photoelectron spectra and density functional theory results, contributing to the improved electron mobility of SnO₂; 2) depositing RbF at the SnO₂/perovskite interface, Rb⁺ cations actively escape into the interstitial sites of the perovskite lattice to inhibit ions migration and reduce non-radiative recombination, which dedicates to the improved open-circuit voltage (V_oc) for the PSCs with suppressed hysteresis. In addition, double-sided passivated PSCs, RbF on the SnO₂ surface, and p-methoxyphenethylammonium iodide on the perovskite surface, produces an outstanding PCE of 23.38% with a V_oc of 1.213 V, corresponding to an extremely small V_oc deficit of 0.347 V.     

Bifunctional SnO2 Colloid Offers No Annealing Effect Compact Layer and Mesoporous Scaffold for Efficient Perovskite Solar Cells

SnO₂ compact layer (c-SnO₂) frequently suffers from degradation in high temperature processes (HTP) such as crack, worse interfacial contact, and electrical properties, that is, annealing effect. To solve this problem, a kind of bifunctional SnO₂ colloid is developed by using small molecular oxalate whose organic components can be removed clearly at a low temperature process (LTP). The c-SnO₂ and SnO₂ mesoporous layer (m-SnO₂) derived from the fresh and aged sols with the same colloid show no annealing effect, decreasing oxygen vacancy, and adsorbing water on increasing annealing temperature. The champion devices of LTP and HTP SnO₂ planar perovskite solar cells (PSCs) achieve, respectively, stabilized photoelectric conversion efficiencies (PCEs) of 20.74% and 20.70%. In contrast, the performance of champion devices of their mesoporous counterparts is significantly improved, showing nearly hysteresis free character with stabilized PCEs of 22.40% and 22.37%, respectively. The inclusion of m-SnO₂ plays a role of an energy bridge, improving electrons collection efficiency, which is supported by photoluminescence and transient photoluminescence characterizations. HTP SnO₂ mesoporous PSCs can preserve 97.6% and 80% of their initial PCEs after aging for 25 weeks and 8-h irradiated/16-h dark cycle within 104 h. The high stability of HTP SnO₂ PSCs may ascribe to low oxygen vacancy and adsorbed water of HTP SnO₂.     

Achieving 20% Efficiency for Low‐Temperature‐Processed Inverted Perovskite Solar Cells

Low‐temperature‐processed inverted perovskite solar cells (PVSCs) attract increasing attention because they can be fabricated on both rigid and flexible substrates. For these devices, hole‐transporting layers (HTLs) play an important role in achieving efficient and stable inverted PVSCs by adjusting the anodic work function, hole extraction, and interfacial charge recombination. Here, the use of a low‐temperature (≤150 °C) solution‐processed ultrathin film of poly[(9,9‐dioctyl‐fluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐secbutylphenyl) diphenylamine)] (TFB) is reported as an HTL in one‐step‐processed CH₃NH₃PbI₃ (MAPbI₃)‐based inverted PVSCs. The fabricated device exhibits power conversion efficiency (PCE) as high as 20.2% when measured under AM 1.5 G illumination. This PCE makes them one of the MAPbI₃‐based inverted PVSCs that have the highest efficiency reported to date. Moreover, this inverted PVSC also shows good stability, which can retain 90% of its original efficiency after 30 days of storage in ambient air.     

Scalable Fabrication of Stable High Efficiency Perovskite Solar Cells and Modules Utilizing Room Temperature Sputtered SnO2 Electron Transport Layer

Stability and scalability have become the two main challenges for perovskite solar cells (PSCs) with the research focus in the field advancing toward commercialization. One of the prerequisites to solve these challenges is to develop a cost‐effective, uniform, and high quality electron transport layer that is compatible with stable PSCs. Sputtering deposition is widely employed for large area deposition of high quality thin films in the industry. Here the composition, structure, and electronic properties of room temperature sputtered SnO₂ are systematically studied. Ar and O₂ are used as the sputtering and reactive gas, respectively, and it is found that a highly oxidizing environment is essential for the formation of high quality SnO₂ films. With the optimized structure, SnO₂ films with high quality have been prepared. It is demonstrated that PSCs based on the sputtered SnO₂ electron transport layer show an efficiency up to 20.2% (stabilized power output of 19.8%) and a T₈₀ operational lifetime of 625 h. Furthermore, the uniform and thin sputtered SnO₂ film with high conductivity is promising for large area solar modules, which show efficiencies over 12% with an aperture area of 22.8 cm² fabricated on 5 × 5 cm² substrates (geometry fill factor = 91%), and a T₈₀ operational lifetime of 515 h.     

Hydrothermally Treated SnO2 as the Electron Transport Layer in High‐Efficiency Flexible Perovskite Solar Cells with a Certificated Efficiency of 17.3%

Perovskite solar cells (PSCs) are one of the most promising solar energy conversion technologies owing to their rapidly developing power conversion efficiency (PCE). Low‐temperature solution processing of the perovskite layer enables the fabrication of flexible devices. However, their application has been greatly hindered due to the lack of strategies to fabricate high‐quality electron transport layers (ETLs) at the low temperatures (≈100 °C) that most flexible plastic substrates can withstand, leading to poor performances for flexible PSCs. In this work, through combining the spin‐coating process with a hydrothermal treatment method, ligand‐free and highly crystalline SnO₂ ETLs are successfully fabricated at low temperature. The flexible PSCs based on this SnO₂ ETL exhibit an excellent PCE of 18.1% (certified 17.3%). The flexible PSCs maintained 85% of the initial PCE after 1000 bending cycles and over 90% of the initial PCE after being stored in ambient air for 30 days without encapsulation. The investigation reveals that hydrothermal treatment not only promotes the complete removal of organic surfactants coated onto the surface of the SnO₂ nanoparticles by hot water vapor but also enhances crystallization through the high vapor pressure of water, leading to the formation of high‐quality SnO₂ ETLs.     

Surface‐Modified Metallic Ti3C2Tx MXene as Electron Transport Layer for Planar Heterojunction Perovskite Solar Cells

MXenes are a large and rapidly expanding family of 2D materials that, owing to their unique optoelectronic properties and tunable surface termination, find a wide range of applications including energy storage and energy conversion. In this work, Ti₃C₂T_x MXene nanosheets are applied as a novel type of electron transport layer (ETL) in low‐temperature processed planar‐structured perovskite solar cells (PSCs). Interestingly, simple UV‐ozone treatment of the metallic Ti₃C₂T_x that increases the surface Ti? O bonds without any change in its bulk properties such as high electron mobility improves its suitability as an ETL. Improved electron transfer and suppressed recombination at the ETL/perovskite interface results in augmentation of the power conversion efficiency (PCE) from 5.00% in the case of Ti₃C₂T_x without UV‐ozone treatment to the champion PCE of 17.17%, achieved using the Ti₃C₂T_x film after 30 min of UV‐ozone treatment. As the first report on the use of pure MXene layer as an ETL in PSCs, this work shows the great potential of MXenes to be used in PSCs and displays their promise for applications in photovoltaic technology in general.     

SnO2‐C60 Pyrrolidine Tris‐Acid (CPTA) as the Electron Transport Layer for Highly Efficient and Stable Planar Sn‐Based Perovskite Solar Cells

For solar cell applications, Sn‐based hybrid perovskites have drawn particular interest due to their environmental friendliness. Here, a thin layer of C₆₀ pyrrolidine tris‐acid (CPTA) is found essential for achieving high efficiency with planar solar cells of Sn‐based perovskites. As a result, a power conversion efficiency of 7.40% is achieved for {en}FASnI₃ solar cells with a planar n–i–p architecture, and the device exhibits excellent stability in air. For the first time, highly efficient Sn‐based hybrid perovskite solar cells on n–i–p architecture are achieved. A V_oc of 0.72 V is highlighted as the highest V_oc ever reported for FASnI₃ solar cells.     

A Band‐Edge Potential Gradient Heterostructure to Enhance Electron Extraction Efficiency of the Electron Transport Layer in High‐Performance Perovskite Solar Cells

As the key component in efficient perovskite solar cells, the electron transport layer (ETL) can selectively collect photogenerated charge carriers produced in perovskite absorbers and prevent the recombination of carriers at interfaces, thus ensuring a high power conversion efficiency. Compared with the conventional single‐ or dual‐layered ETLs, a gradient heterojunction (GHJ) strategy is more attractive to facilitate charge separation because the potential gradient created at an appropriately structured heterojunction can act as a driving force to regulate the electron transport toward a desired direction. Here, a SnO₂/TiO₂ GHJ interlayer configuration inside the ETL is reported to simultaneously achieve effective extraction and efficient transport of photoelectrons. With such an interlayer configuration, the GHJs formed at the perovskite/ETL interface act collectively to extract photogenerated electrons from the perovskite layer, while GHJs formed at the boundaries of the interconnected SnO₂ and TiO₂ networks throughout the entire ETL layer can extract electron from the slow electron mobility TiO₂ network to the high electron mobility SnO₂ network. Devices based on GHJ ETL exhibit a champion power conversion efficiency of 18.08%, which is significantly higher than that obtained from the compact TiO₂ ETL constructed under the comparable conditions.     

Multifunctional Aminoglycoside Antibiotics Modified SnO2 Enabling High Efficiency and Mechanical Stability Perovskite Solar Cells

SnO₂ as an electron transport layer (ETL) has been widely used in regular planar perovskite solar cells (PSCs) owing to its high optical transmittance, less photocatalytic activity, and low-temperature processing. However, SnO₂-based PSCs still face many challenges which greatly impair their efficiency and stability of PSCs. Herein, a novel and effective multifunctional modification strategy is proposed by incorporating streptomycin sulfate (STRS) molecules with multiple functional groups into SnO₂ ETL. STRS can significantly suppress SnO₂ nanoparticle agglomeration, improve the electronic property of SnO₂, as well as reduce nonradiative recombination. At the same time, interfacial residual tensile stress is released and the interfacial energy level alignment becomes more matched. As a result, the STRS-modified PSCs achieve a higher efficiency of 22.89% compared to 20.61% of the control device and exhibit a hysteresis-free feature. The humidity and thermal stability of PSCs based on STRS-SnO₂ are significantly improved. Furthermore, the efficiency of flexible devices increased from 19.74% to 20.79%, and the devices still maintain >80% of initial PCE after 4500 bending cycles with a bend radius of 5 mm. This study provides a low-cost, facile, and efficient strategy for achieving high efficiency and stability in PSCs.     

Controllable Perovskite Crystallization by Water Additive for High‐Performance Solar Cells

A key issue for perovskite solar cells is the stability of perovskite materials due to moisture effects under ambient conditions, although their efficiency is improved constantly. Herein, an improved CH₃NH₃PbI_3?xCl_x perovskite quality is demonstrated with good crystallization and stability by using water as an additive during crystal perovskite growth. Incorporating suitable water additives in N,N‐dimethylformamide (DMF) leads to controllable growth of perovskites due to the lower boiling point and the higher vapor pressure of water compared with DMF. In addition, CH₃NH₃PbI_3?xCl_x · nH₂O hydrated perovskites, which can be resistant to the corrosion by water molecules to some extent, are assumed to be generated during the annealing process. Accordingly, water additive based perovskite solar cells present a high power conversion efficiency of 16.06% and improved cell stability under ambient conditions compared with the references. The findings in this work provide a route to control the growth of crystal perovskites and a clue to improve the stability of organic–inorganic halide perovskites.     

Review on the Application of SnO2 in Perovskite Solar Cells

SnO₂ has been well investigated in many successful state‐of‐the‐art perovskite solar cells (PSCs) due to its favorable attributes such as high mobility, wide bandgap, and deep conduction band and valence band. Several independent studies show the performances of PSCs with SnO₂ are higher than that with TiO₂, especially in device stability. In 2015, the first planar PSCs were reported with a power conversion efficiency over 17% using a low temperature sol‐derived SnO₂ nanocrystal electron transport layer (ETL). Since then, many other groups have also reported high performance PSCs based on SnO₂ ETLs. SnO₂ planar PSCs show currently the highest performance in planar configuration devices (21.6%) and are close to the record holder of TiO₂ mesoporous PSCs, suggesting their high potential as ETLs in PSCs. The main concerns with the application of SnO₂ as ETL are that it suffers from degradation in high temperature processes and that its much lower conduction band compared to perovskite may result in a voltage loss of PSCs. Here, notable achievements to date are outlined, the unique attributes of SnO₂ as ETLs in PSCs are described, and the challenges facing the successful development of PSCs and approaches to the problems are discussed.     

Self‐Crystallized Multifunctional 2D Perovskite for Efficient and Stable Perovskite Solar Cells

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.     

Passivated Perovskite Crystallization via g‐C3N4 for High‐Performance Solar Cells

Organometallic halide perovskite films with good surface morphology and large grain size are desirable for obtaining high‐performance photovoltaic devices. However, defects and related trap sites are generated inevitably at grain boundaries and on surfaces of solution‐processed polycrystalline perovskite films. Seeking facial and efficient methods to passivate the perovskite film for minimizing defect density is necessary for further improving the photovoltaic performance. Here, a convenient strategy is developed to improve perovskite crystallization by incorporating a 2D polymeric material of graphitic carbon nitride (g‐C₃N₄) into the perovskite layer. The addition of g‐C₃N₄ results in improved crystalline quality of perovskite film with large grain size by retarding the crystallization rate, and reduced intrinsic defect density by passivating charge recombination centers around the grain boundaries. In addition, g‐C₃N₄ doping increases the film conductivity of perovskite layer, which is beneficial for charge transport in perovskite light‐absorption layer. Consequently, a champion device with a maximum power conversion efficiency of 19.49% is approached owing to a remarkable improvement in fill factor from 0.65 to 0.74. This finding demonstrates a simple method to passivate the perovskite film by controlling the crystallization and reducing the defect density.     

Multifunctional Synthesis Approach of In:CuCrO2 Nanoparticles for Hole Transport Layer in High‐Performance Perovskite Solar Cells

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.