Fluorinated Black Phosphorene Nanosheets with Robust Ambient Stability for Efficient and Stable Perovskite Solar Cells

Extraordinary electronic and photonic features (e.g., tunable direct bandgap, high ambipolar carrier mobility) render few-layer black phosphorus (BP) nanosheets/quantum dots an important optoelectronic material. However, most of the BP applied in metal halide perovskite solar cells (PSCs) are produced by sonication-assisted liquid exfoliation, which inevitably brings inferior electronic properties, thus leading to limited beneficial effects. Furthermore, this study uncovers that the intrinsic instability of BP nanosheets sandwiched between (CsFAMA)Pb(BrI)₃ perovskite and spiro-OMeTAD has a deleterious effect on the performance stabilization of PSCs. To address the above constraints, a feasible strategy herein is developed by introducing high-quality fluorinated BP (F-BP) nanosheets synthesized by one-step electrochemical delamination. In addition to P-Pb coordination, there is a strong hydrogen bond between F^? and MA⁺/FA⁺ as well as an ionic bond between F^? and Pb²⁺ for the perovskite/F-BP interface, thus leading to fewer interfacial traps than perovskite/BP, which is responsible for the highest power conversion efficiency (22.06%) of F-BP devices. More importantly, F-BP devices exhibit significantly improved humidity and shelf-life stabilities due to the excellent ambient stability of F-BP, resulting from the antioxidation and antihydration behavior of fluorine adatoms. Overall, the findings provide a promising strategy to simultaneously enhance the photovoltaic performance and long-term stability of BP-based PSCs.     

Dopant‐Free Spiro‐Triphenylamine/Fluorene as Hole‐Transporting Material for Perovskite Solar Cells with Enhanced Efficiency and Stability

Chemical doping is often used to enhance electric conductivity of the conjugated molecule as hole‐transporting material (HTM) for the application in optoelectronics. However, chemical dopants can promote ion migration at the electrical field, which deteriorates the device efficiency as well as increases the fabrication cost. Here, two star HTMs, namely 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene (Spiro‐OMeTAD) and poly(triarylamine) are subjeted to chemical combination to yield dopant‐free N2,N2,N2′,N2′,N7,N7,N7′,N7′‐octakis(4‐methoxyphenyl)‐10‐phenyl‐10H‐spiro[acridine‐9,9′‐fluorene]‐2,2′,7,7′‐tetraamine (SAF‐OMe). The power conversion efficiencies (PCEs) of 12.39% achieved by solar cells based on pristine, dopant‐free SAF‐OMe are among the highest reported for perovskite solar cells and are even comparable to devices based on chemically doped Spiro‐OMeTAD (14.84%). Moreover, using a HTM comprised of SAF‐OMe with an additional dopant results in a record PCE of 16.73%. Compared to Spiro‐OMeTAD‐based devices, SAF‐OMe significantly improves stability.     

Efficient and Stable Mesoscopic Perovskite Solar Cells Using PDTITT as a New Hole Transporting Layer

Stability is the main challenge in the field of organic–inorganic perovskite solar cells (PSCs). Finding low‐cost and stable hole transporting layer (HTL) is an effective strategy to address this issue. Here, a new donor polymer, poly(5,5‐didecyl‐5H‐1,8‐dithia‐as‐indacenone‐alt‐thieno[3,2‐b]thiophene) (PDTITT), is synthesized and employed as an HTL in PSCs, which has a suitable band alignment with respect to the double‐A cation perovskite film. Using PDTITT, the hole extraction in PSCs is greatly improved as compared to commonly used HTLs such as 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD), addressing the hysteresis issue. After careful optimization, an efficient PSC is achieved based on mesoscopic TiO₂ electron transporting layer with a maximum power conversion efficiency (PCE) of 18.42% based on PDTITT HTL, which is comparable with spiro‐OMeTAD‐based PSC (19.21%). Since spiro‐based PSCs suffer from stability issue, the operational stability in the PSC with PDTITT HTL is studied. It is found that the device with PDTITT retains 88% of its initial PCE value after 200 h under illumination, which is better than the spiro‐based PSC (54%).     

Perovskite Solar Cells Consisting of PTAA Modified with Monomolecular Layer and Application to All-Perovskite Tandem Solar Cells with Efficiency over 25%

This study is on the enhancement of the efficiency of wide bandgap (FA_0.8Cs_0.2PbI_1.8Br_1.2) perovskite solar cells (PSCs) used as the top layer of the perovskite/perovskite tandem solar cell. Poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) and the monomolecular layer called SAM layer are effective hole collection layers for APbI₃ PSCs. However, these hole transport layers (HTL) do not give high efficiencies for the wide bandgap FA_0.8Cs_0.2PbI_1.8Br_1.2 PSCs. It is found that the surface-modified PTAA by monomolecular layer (MNL) improves the efficiency of PSCs. The improved efficiency is explained by the improved FA_0.8Cs_0.2PbI_1.8Br_1.2 film quality, decreased film distortion (low lattice disordering) and low density of the charge recombination site, and improves carrier collection by the surface modified PTAA layer. In addition, the relationship between the length of the alkyl group linking the anchor group and the carbazole group is also discussed. Finally, the wide bandgap lead PSCs (E_g = 1.77 eV) fabricated on the PTAA/monomolecular bilayer give a higher power conversion efficiency of 16.57%. Meanwhile, all-perovskite tandem solar cells with over 25% efficiency are reported by using the PTAA/monomolecular substrate.     

Combination of Hybrid CVD and Cation Exchange for Upscaling Cs‐Substituted Mixed Cation Perovskite Solar Cells with High Efficiency and Stability

Mixed cation hybrid perovskites such as Cs_xFA_1?xPbI₃ are promising materials for solar cell applications, due to their excellent photoelectronic properties and improved stability. Although power conversion efficiencies (PCEs) as high as 18.16% have been reported, devices are mostly processed by the anti‐solvent method, which is difficult for further scaling‐up. Here, a method to fabricate Cs_xFA_1?xPbI₃ by performing Cs cation exchange on hybrid chemical vapor deposition grown FAPbI₃ with the Cs⁺ ratio adjustable from 0 to 24% is reported. The champion perovskite module based on Cs_0.07FA_0.93PbI₃ with an active area of 12.0 cm² shows a module PCE of 14.6% and PCE loss/area of 0.17% cm^?2, demonstrating the significant advantage of this method toward scaling‐up. This in‐depth study shows that when the perovskite films prepared by this method contain 6.6% Cs⁺ in bulk and 15.0% at the surface, that is, Cs_0.07FA_0.93PbI₃, solar cell devices show not only significantly increased PCEs but also substantially improved stability, due to favorable energy level alignment with TiO₂ electron transport layer and spiro‐MeOTAD hole transport layer, increased grain size, and improved perovskite phase stability.     

A Universal Strategy to Utilize Polymeric Semiconductors for Perovskite Solar Cells with Enhanced Efficiency and Longevity

In this contribution, a facile and universal method is successfully reported to fabricate perovskite solar cells (PSCs) with enhanced efficiency and stability. Through dissolving functional conjugated polymers in antisolvent chlorobenzene to treat the spinning CH₃NH₃PbI₃ perovskite film, the resultant devices exhibit significantly enhanced efficiency and longevity simultaneously. In‐depth characterizations demonstrate that thin polymer layer well covers the top surface of perovskite film, resulting in certain surface passivation and morphology modification. More importantly, it is shown that through rational chemical modification, namely molecular fluorination, the air stability and photostability of the perovskite solar cells are remarkably enhanced. Considering the vast selection of conjugated polymer materials and easy functional design, promising new results are expected in further enhancement of device performance. It is believed that the findings provide exciting insights into the role of conjugated polymer in improving the current perovskite‐based solar cells.     

Improved Air Stability of Tin Halide Perovskite Solar Cells by an N-Type Active Moisture Barrier

Tin halide perovskite solar cells are promising for the next generation of highly efficient photovoltaics. Their commercialization can be accelerated by increasing their stability in moisture and oxygen. Herein, an n-type organic molecule (IO-4Cl) is applied as an interlayer between the perovskite films and electron transport layers in p-i-n structured devices. The electron-rich indacenodithieno-[3,2-b]thiophene enhances electron transport, while the hydrocarbon side chains and rigid conjugated backbone isolate air. It is also shown that the C═O in IO-4Cl can coordinate with Sn²⁺ on perovskite films' surface and grain boundaries to enhance perovskite crystal stability. In addition, IO-4Cl slows down crystallization dynamics, resulting in lower non-radiation recombination. The moisture ingress in the perovskite films is tracked under high relative humidity (RH) and it is found that IO-4Cl can mitigate moisture infiltration. Finally, the devices with IO-4Cl maintain 95% of the initial power conversion efficiency after 1200 h of storage in a nitrogen-filled glovebox, and their stability in ambient air (60–80% RH) is significantly improved against pristine devices, thus demonstrating the beneficial effects of IO-4Cl interlayer on device stability.     

Spidermen Strategy for Stable 24% Efficiency Perovskite Solar Cells

The interface energetics-modification plays an important role in improving the power conversion efficiency (PCE) among the perovskite solar cells (PSCs). Considering the low carrier mobility caused by defects in PSCs, a double-layer modification engineering strategy is adopted to introduce the “spiderman” NOBF₄ (nitrosonium tetrafluoroborate) between tin dioxide (SnO₂ and perovskite layers. NO⁺, as the interfacial bonding layer, can passivate the oxygen vacancy in SnO₂, while BF₄⁻ can optimize the defects in the bulk of perovskite. This conclusion is confirmed by theoretical calculation and transmission electron microscopy (TEM). The synergistic effect of NO⁺ and BF₄⁻ distinctly heightens the carrier extraction efficiency, and the PCE of PSCs is 24.04% with a fill factor (FF) of 82.98% and long-term stability. This study underlines the effectiveness of multifunctional additives in improving interface contact and enhancing PCE of PSCs.     

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.     

Multifunctional Cross-Linked Polyurethane Polymer as Interface Layer for Efficient and Stable Perovskite Solar Cells

In the past decade, perovskite solar cells (PSCs) have made remarkable progress in improving power conversion efficiency (PCE). In order to further improve the photovoltaic performance and long-term stability of PSCs, the interface layer is essential. A multifunctional cross-linked polyurethane (CLPU) is designed and synthesized via the spontaneous quaternization of polyurethane and 1, 6-diiodohexane on the surface of the perovskite layer. CLPU layer cannot only effectively induce secondary crystallization and passivate the surface defects of perovskite, reduce the non-radiative recombination, but also effectively block the moisture invasion. By this strategy, Cs_0.05FA_0.95PbI₃ PSCs with excellent reproducibility, is realized, achieving a PCE of 23.14% with an open-circuit voltage of 1.11 V, a short-circuit current density of 25.69 mA cm⁻², and a fill factor of 0.81. In addition, the unencapsulated devices show enhanced stability in 35 ± 5% relative humidity (RH) near 3000 h and in 65 ± 5% RH over 700 h. This study provides valuable insights into the role of CLPU interface layer in PSCs, which are essential for the design of high-performance devices.     

A Review on Encapsulation Technology from Organic Light Emitting Diodes to Organic and Perovskite Solar Cells

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

Mitigating Surface Deficiencies of Perovskite Single Crystals Enables Efficient Solar Cells with Enhanced Moisture and Reverse-Bias Stability

Metal halide perovskite single crystals are promising for diverse optoelectronic applications due to their outstanding properties. In comparison to the bulk, the crystal surface suffers from high defect density and is moisture sensitive; however, surface modification strategies of perovskite single crystals are relatively deficient. Herein, solar cells based on methylammonium lead triiodide (MAPbI₃) thin single crystals are selected as a prototype to improve single-crystal perovskite devices by surface modification. The surface trap passivation and protection against moisture of MAPbI₃ thin single crystals are achieved by one bifunctional molecule 3-mercaptopropyl(dimethoxy)methylsilane (MDMS). The sulfur atom of MDMS can coordinate with bare Pb²⁺ of MAPbI₃ single crystals to reduce surface defect density and nonradiative recombination. As a result, the modified devices show a remarkable efficiency of 22.2%, which is the highest value for single-crystal MAPbI₃ solar cells. Moreover, MDMS modification mitigates surface ion migration, leading to enhanced reverse-bias stability. Finally, the cross-link of silane molecules forms a protective layer on the crystal surface, which results in enhanced moisture stability of both materials and devices. This work provides an effective way for surface modification of perovskite single crystals, which is important for improving the performance of single-crystal perovskite solar cells, photodetectors, X-ray detectors, etc.     

Surface Passivation with Diaminopropane Dihydroiodide for p-i-n Perovskite Solar Cells with Over 25% Efficiency

At present, one of the major factors limiting the further improvement of inverted (p-i-n) perovskite solar cells (PSCs) is trap-assisted non-radiative recombination at the perovskite/electron transporting layer (ETL) interface. Surface passivation with organic ammonium salt is a powerful strategy to improve the performance of PSCs. Herein, an effective method by using propylamine hydroiodide (PAI) and 1,3-diaminopropane dihydroiodide (PDADI) is reported to modify the perovskite/ETL interface. These two ammonium salts do not form new perovskite but directly passivate the defects on the perovskite surface after annealing. The results show that the PDADI-modified perovskite films possess a lower surface defect density and less non-radiative recombination as well as improved charge carrier transport. Based on this strategy, the PDADI-modified p-i-n PSCs deliver an impressive efficiency of 25.09% (certified 24.58%) with an open-circuit voltage of 1.184 V. Furthermore, the unencapsulated PDADI-modified PSCs also exhibit good storage stability, retaining 91% of initial PCE at 65 °C in a N₂ glove box for 1300 h. This strategy provides an efficient route to fabricate highly efficient and stable inverted p-i-n structured PSCs.     

A Versatile Fluoro‐Containing Low‐Bandgap Polymer for Efficient Semitransparent and Tandem Polymer Solar Cells

The versatility of a fluoro‐containing low band‐gap polymer, poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b’]dithiophene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothia‐diazole)] (PCPDTFBT) in organic photovoltaics (OPVs) applications is demonstrated. High boiling point 1,3,5‐trichlorobenzene (TCB) is used as a solvent to manipulate PCPDTFBT:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) active layer morphology to obtain high‐performance single‐junction devices. It promotes the crystallization of PCPDTFBT polymer, thus improving the charge‐transport properties of the active layer. By combining the morphological manipulation with interfacial optimization and device engineering, the single‐junction device exhibits both good air stability and high power‐conversion efficiency (PCE, of 6.6%). This represents one of the highest PCE values for cyclopenta[2,1‐b;3,4‐b’]dithiophene (CPDT)‐based OPVs. This polymer is also utilized for constructing semitransparent solar cells and double‐junction tandem solar cells to demonstrate high PCEs of 5.0% and 8.2%, respectively.     

Solution‐Processed Metal Oxide Nanocrystals as Carrier Transport Layers in Organic and Perovskite Solar Cells

There has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.     

A 2.20 eV Bandgap Polymer Donor for Efficient Colorful Semitransparent Organic Solar Cells

Semitransparent organic solar cells (ST-OSCs) have attracted increasing attention due to their promising prospect in building-integrated photovoltaics. Generally, efficient ST-OSCs with good average visible transmittance (AVT) can be realized by developing active layer materials with light absorption far from the visible light range. Herein, the development of ultrawide bandgap polymer donors with near-ultraviolet absorption, paired with near-infrared acceptors, is proposed to achieve high-performance ST-OSCs. The key points for the design of ultrawide bandgap polymers include constructing donor–donor type conjugated skeleton, suppressing the quinoidal resonance effect, and minimizing the twist of conjugated skeleton via noncovalent conformational locks. As a proof of concept, a polymer named PBOF with an optical bandgap of 2.20 eV is synthesized, which exhibited largely reduced overlap with the human eye photopic response spectrum and afforded a power conversion efficiency (PCE) of 16.40% in opaque device. As a result, ST-OSCs with a PCE over 10% and an AVT over 30% are achieved without optical modulation. Moreover, colorful ST-OSCs with visual aesthetics can be achieved by tuning the donor/acceptor weight ratio in active layer benefiting from the ultrawide bandgap nature of PBOF. This study demonstrates the great potential of ultrawide bandgap polymers for efficient colorful ST-OSCs.     

One-Source Strategy Boosting Dopant-Free Hole Transporting Layers for Highly Efficient and Stable CsPbI2Br Perovskite Solar Cells

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

Concurrent Top and Buried Surface Optimization for Flexible Perovskite Solar Cells with High Efficiency and Stability

Although much progress is made toward enhancing the efficiency of perovskite solar cells (PSCs), their operational reliability, particularly their mechanical stability, which is a crucial factor for flexible PSCs (f-PCSs), has not attracted sufficient attention. The defects in the perovskite layer, especially on the top and the buried surface of the perovskite layer, can induce perovskite fracture, highly limiting the performance of f-PSCs. Herein, a novel multifunctional organic salt, metformin hydrochloride, which can passivate cationic and anionic defects, is incorporated on both the top and buried surfaces of perovskite layer to suppress defects. As a result, a power conversion efficiency (PCE) of 24.40% for rigid PSCs and a PCE of 22.04% for f-PSCs are achieved. Simultaneously, the device can retain 90% and 80% of the initial efficiency after 1000 h of light illumination and 10 000 bending cycles, respectively, showing excellent operational stability. This study may provide a global way to design a passivation strategy and fabricate flexible perovskite solar cells with high efficiency and stability.     

Construction of Polyoxometalate-Based Material for Eliminating Multiple Pb-Based Defects and Enhancing Thermal Stability of Perovskite Solar Cells

The high-quality perovskite film with few defects plays an important role in the power conversion efficiency (PCE) and long-term stability of perovskite solar cells. Here, an efficient strategy is proposed to eliminate Pb⁰ and passivate Pb²⁺ simultaneously by employing a stable polyoxometalate-based material CoW₁₂@MIL-101(Cr) in the precursor solution of perovskite. The controllable oxidation ability of CoW₁₂ is optimized through the interaction with metal–organic frameworks, resulting in a doped perovskite film with regular morphology, large grain size, and low defects density. The solvent effects and formation of intermediate materials in the precursor solution are further investigated by an in situ thermogravimetry-Fourier transform infrared spectroscopy analysis. In addition, the champion doped-device showed enhanced PCE to 21.39% and excellent stability, maintaining 85% and 89% of the original PCE after heating at 85 °C in N₂ atmosphere and stored in ambient conditions (25 °C, 40% humidity) for 1000 h, respectively.