Unconventional Route to Oxygen-Vacancy-Enabled Highly Efficient Electron Extraction and Transport in Perovskite Solar Cells

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high-efficiency solar energy conversion. Now, a simple yet robust acid-treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron-transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂-coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂-based devices, the perovskite solar cells (PSCs) with acid-treated TiO₂ ETL exhibit an enhanced short-circuit current and power conversion efficiency.     

Unconventional Route to Oxygen‐Vacancy‐Enabled Highly Efficient Electron Extraction and Transport in Perovskite Solar Cells

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high‐efficiency solar energy conversion. Now, a simple yet robust acid‐treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron‐transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂‐coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂‐based devices, the perovskite solar cells (PSCs) with acid‐treated TiO₂ ETL exhibit an enhanced short‐circuit current and power conversion efficiency.     

Eliminating Hysteresis of Perovskite Solar Cells with Hollow TiO2 Mesoporous Electron Transport Layer

Current density-voltage(J-V) hysteresis issue caused by unbalanced charge transport has greatly limited the improvement of power conversion efficiency(PCE) of halide perovskite solar cells(PSCs). Herein, hollow TiO₂ mesoporous electron transport layer(ETL) was used to fabricate PSCs. The structure-dependent charge collection as well as its effect on PCE and hysteresis impactor(HI) of PSC were investigated. The results demonstrate that TiO₂ hollow spheres in a size of around 50 nm (HS-50) can form a high quality perovskite/ETL interface with a less trap density. Moreover, the hollow TiO₂ with the thin shell can help promote the extraction of electrons from perovskite layer to ETL, so as to reduce the charge accumulation and recombination at the perovskite/ETL interface and alleviate the hysteresis behavior. As a result, PSCs with HS-50 TiO₂ delivered a champion PCE of 16.81% with a small HI of 0.0297, indicating a better performance than the commercial P25(PCE of 15.87%, HI of 0.2571).     

Suppressing Interfacial Charge Recombination in Electron-Transport-Layer-Free Perovskite Solar Cells to Give an Efficiency Exceeding 21 %

The performances of electron-transport-layer (ETL)-free perovskite solar cells (PSCs) are still inferior to ETL-containing devices. This is mainly due to severe interfacial charge recombination occurring at the transparent conducting oxide (TCO)/perovskite interface, where the photo-injected electrons in the TCO can travel back to recombine with holes in the perovskite layer. Herein, we demonstrate for the first time that a non-annealed, insulating, amorphous metal oxyhydroxide, atomic-scale thin interlayer (ca. 3 nm) between the TCO and perovskite facilitates electron tunneling and suppresses the interfacial charge recombination. This largely reduced the interfacial charge recombination loss and achieved a record efficiency of 21.1 % for n-i-p structured ETL-free PSCs, outperforming their ETL-containing metal oxide counterparts (18.7 %), as well as narrowing the efficiency gap with high-efficiency PSCs employing highly crystalline TiO₂ ETLs.     

Dual-Interface Engineering in Perovskite Solar Cells with 2D Carbides

Passivating the interfaces between the perovskite and charge transport layers is crucial for enhancing the power conversion efficiency (PCE) and stability in perovskite solar cells (PSCs). Here we report a dual-interface engineering approach to improving the performance of FA_0.85MA_0.15Pb(I_0.95Br_0.05)₃-based PSCs by incorporating Ti₃C₂Cl_x Nano-MXene and o-TB-GDY nanographdiyne (NanoGDY) into the electron transport layer (ETL)/perovskite and perovskite/ hole transport layer (HTL) interfaces, respectively. The dual-interface passivation simultaneously suppresses non-radiative recombination and promotes carrier extraction by forming the Pb−Cl chemical bond and strong coordination of π-electron conjugation with undercoordinated Pb defects. The resulting perovskite film has an ultralong carrier lifetime exceeding 10 μs and an enlarged crystal size exceeding 2.5 μm. A maximum PCE of 24.86 % is realized, with an open-circuit voltage of 1.20 V. Unencapsulated cells retain 92 % of their initial efficiency after 1464 hours in ambient air and 80 % after 1002 hours of thermal stability test at 85 °C.     

Defect and doping concentration study with series and shunt resistance influence on graphene modified perovskite solar cell: A numerical investigation in SCAPS-1D framework

Perovskite solar cells (PSCs) have the potential to be highly efficient, low-cost next-generation solar cells. By raising open circuit voltage (V_oc), the interfacial recombination kinetics can further improve device performance. In this study, we used simulation concept to elucidate the influence of using graphene as a surface passivation material in perovskite solar cells. Graphene works well as an interlayer to promote hole extraction and reduce interfacial recombination. In order to evaluate the effect of graphene in PSCs, the simulation was done in the SCAPS-1D framework to compare the performance of a device with and without graphene. Three interface layers were included to the model: TiO₂/MAPbI₃, MAPbI₃/Graphene, and Graphene/Spiro-OMeTAD, in order to account for the impacts of interface defect density on device performance. The impacts of absorber doping concentration, absorber defect density, ETL doping concentration, HTL doping concentration, series resistance, and shunt resistance were also evaluated for the modelled PSC. Without any optimization, the control device with power conversion efficiency (PCE) of 20.677% was outperformed by the graphene-modified device with PCE of 20.911%. This difference is mostly due to the lower recombination losses and more effective suppression of interfacial non-radiative recombination. With optimization, the modified graphene-based device has a PCE of 26.667%. This result shows an enhancement of ∼1.28 times over that of the pristine graphene-based device. The outcomes have opened the way for the development of cost-effective and comparable state-of-the-art, high-efficiency perovskite solar cells with graphene interlayer by eliminating defects and managing non-radiative recombination.     

A Modified Sequential Deposition Route for High-Performance Carbon-Based Perovskite Solar Cells under Atmosphere Condition

Carbon-based hole transport material (HTM)-free perovskite solar cells have exhibited a promising commercialization prospect, attributed to their outstanding stability and low manufacturing cost. However, the serious charge recombination at the interface of the carbon counter electrode and titanium dioxide (TiO₂) suppresses the improvement in the carbon-based perovskite solar cells’ performance. Here, we propose a modified sequential deposition process in air, which introduces a mixed solvent to improve the morphology of lead iodide (PbI₂) film. Combined with ethanol treatment, the preferred crystallization orientation of the PbI₂ film is generated. This new deposition strategy can prepare a thick and compact methylammonium lead halide (MAPbI₃) film under high-humidity conditions, which acts as a natural active layer that separates the carbon counter electrode and TiO₂. Meanwhile, the modified sequential deposition method provides a simple way to facilitate the conversion of the ultrathick PbI₂ capping layer to MAPbI₃, as the light absorption layer. By adjusting the thickness of the MAPbI₃ capping layer, we achieved a power conversation efficiency (PCE) of 12.5% for the carbon-based perovskite solar cells.     

High-Efficiency Perovskite Solar Cells Enabled by Anatase TiO2 Nanopyramid Arrays with an Oriented Electric Field

One-dimensional (1D) nanostructured oxides are proposed as excellent electron transport materials (ETMs) for perovskite solar cells (PSCs); however, experimental evidence is lacking. A facile hydrothermal approach was employed to grow highly oriented anatase TiO₂ nanopyramid arrays and demonstrate their application in PSCs. The oriented TiO₂ nanopyramid arrays afford sufficient contact area for electron extraction and increase light transmission. Moreover, the nanopyramid array/perovskite system exhibits an oriented electric field that can increase charge separation and accelerate charge transport, thereby suppressing charge recombination. The anatase TiO₂ nanopyramid array-based PSCs deliver a champion power conversion efficiency of approximately 22.5 %, which is the highest power conversion efficiency reported to date for PSCs consisting of 1D ETMs. This work demonstrates that the rational design of 1D ETMs can achieve PSCs that perform as well as typical mesoscopic and planar PSCs.     

Designing a Perylene Diimide/Fullerene Hybrid as Effective Electron Transporting Material in Inverted Perovskite Solar Cells with Enhanced Efficiency and Stability

Electron transport materials (ETM) play an important role in the improvement of efficiency and stability for inverted perovskite solar cells (PSCs). This work reports an efficient ETM, named PDI‐C₆₀, by the combination of perylene diimide (PDI) and fullerene. Compared to the traditional PCBM, this strategy endows PDI‐C₆₀ with slightly shallower energy level and higher electron mobility. As a result, the device based on PDI‐C₆₀ as electron transport layer (ETL) achieves high power conversion efficiency (PCE) of 18.6 %, which is significantly higher than those of the control devices of PCBM (16.6 %) and PDI (13.8 %). The high PCE of the PDI‐C₆₀‐based device can be attributed to the more matching energy level with the perovskite, more efficient charge extraction, transport, and reduced recombination rate. To the best of our knowledge, the PCE of 18.6 % is the highest value in the PSCs using PDI derivatives as ETLs. Moreover, the device with PDI‐C₆₀ as ETL exhibits better device stability due to the stronger hydrophobic properties of PDI‐C₆₀. The strategy using the PDI/fullerene hybrid provides insights for future molecular design of the efficient ETM for the inverted PSCs.     

High‐Efficiency Perovskite Solar Cells Enabled by Anatase TiO2 Nanopyramid Arrays with an Oriented Electric Field

One‐dimensional (1D) nanostructured oxides are proposed as excellent electron transport materials (ETMs) for perovskite solar cells (PSCs); however, experimental evidence is lacking. A facile hydrothermal approach was employed to grow highly oriented anatase TiO₂ nanopyramid arrays and demonstrate their application in PSCs. The oriented TiO₂ nanopyramid arrays afford sufficient contact area for electron extraction and increase light transmission. Moreover, the nanopyramid array/perovskite system exhibits an oriented electric field that can increase charge separation and accelerate charge transport, thereby suppressing charge recombination. The anatase TiO₂ nanopyramid array‐based PSCs deliver a champion power conversion efficiency of approximately 22.5 %, which is the highest power conversion efficiency reported to date for PSCs consisting of 1D ETMs. This work demonstrates that the rational design of 1D ETMs can achieve PSCs that perform as well as typical mesoscopic and planar PSCs.     

Enhanced Lifetime and Photostability with Low‐Temperature Mesoporous ZnTiO3/Compact SnO2 Electrodes in Perovskite Solar Cells

Perovskite solar cells (PSCs) with power conversion efficiencies (PCEs) of 25 % mainly have SnO₂ or TiO₂ as electron‐transporting layers (ETLs). Now, zinc titanate (ZnTiO₃, ZTO) is proposed as mesoporous ETLs owing to its weak photo‐effect, excellent carrier extraction, and transfer properties. Uniform mesoporous films were obtained by spinning coating the ZTO ink and annealed below 150 °C. Photovoltaic devices based on Cs_0.05FA_0.81MA_0.14PbI_2.55B_r0.45 perovskite sandwiched between SnO₂‐mesorporous ZTO electrode and Spiro‐OMeTAD layer achieved the PCE of 20.5 %. The PSCs retained more than 95 % of their original efficiency after 100 days lifetime test without being encapsulated. Additionally, the PSCs retained over 95 % of the initial performance when subjected at the maximum power point voltage for 120 h under AM 1.5 G illumination (100 mW cm^?2), demonstrating superior working stability. The application of ZTO provides a better choice for ETLs of PSCs.     

Tetra‐ammonium Zinc Phthalocyanine to Construct a Graded 2D–3D Perovskite Interface for Efficient and Stable Solar Cells

The crystallographic defects inevitably incur during the solution processed organic‐inorganic hybrid perovskite film, especially at surface and the grain boundaries (GBs) of perovskite film, which can further result in the reduced cell performance and stability of perovskite solar cells (PSCs). Here, a simple defect passivation method was employed by treating perovskite precursor film with a hydrophobic tetra‐ammonium zinc phthalocyanine (ZnPc). The results demonstrated that a 2D‐3D graded perovskite interface with a capping layer of 2D (ZnPc)_0.5MA_n ? 1Pb_nI_3n + 1 perovskite together with 3D MAPbI₃ perovskite was successfully constructed on the top of 3D perovskite layer. This situation realized the efficient GBs passivation, thus reducing the defects in GBs. As expected, the corresponding PSCs with modified perovskite revealed an improved cell performance. The best efficiency reached 19.6%. Especially, the significantly enhanced long‐term stability of the responding PSCs against humidity and heating was remarkably achieved. Such a strategy in this work affords an efficient method to improve the stability of PSCs and thus probably brings the PSCs closer to practical commercialization.     

Physicochemical approaches for optimization of perovskite solar cell performance

Perovskite solar cells (PSCs) with photovoltaic parameters improved using a number of physicochemical approaches for optimization of structure and properties of their components were fabricated and studied under both standard illumination conditions AM1.5G and reduced illumination intensity. Photoelectrodes based on mesoscopic TiO₂ layers with different content of anatase and rutile particles were constructed, as well as the perovskite material and the TiO₂—perovskite interface were modified. As a result, the optimized PSCs had increased stability in a humid atmosphere and showed high efficiencies (10–14%) in a wide range of illumination intensities of 10–1000 W m^?2.

     

Effect of hole-transporting materials on the photovoltaic performance and stability of all-ambient-processed perovskite solar cells

High-efficiency perovskite solar cells(PSCs) reported hitherto have been mostly prepared in a moisture and oxygen-free glove-box atmosphere, which hampers upscaling and real-time performance assessment of this exciting photovoltaic technology. In this work, we have systematically studied the feasibility of allambient-processing of PSCs and evaluated their photovoltaic performance. It has been shown that phasepure crystalline tetragonal MAPbI_3 perovskite films are instantly formed in ambient air at room temperature by a two-step spin coating process, undermining the need for dry atmosphere and post-annealing.All-ambient-processed PSCs with a configuration of FTO/TiO_2/MAPbI_3/Spiro-OMeTAD/Au achieve opencircuit voltage(990 mV) and short-circuit current density(20.31 mA/cm~2) comparable to those of best reported glove-box processed devices. Nevertheless, device power conversion efficiency is still constrained at 5% by the unusually low fill-factor of 0.25. Dark current–voltage characteristics reveal poor conductivity of hole-transporting layer caused by lack of oxidized spiro-OMe TAD species, resulting in high seriesresistance and decreased fill-factor. The study also establishes that the above limitations can be readily overcome by employing an inorganic p-type semiconductor, copper thiocyanate, as ambient-processable hole-transporting layer to yield a fill-factor of 0.54 and a power conversion efficiency of 7.19%. The present findings can have important implications in industrially viable fabrication of large-area PSCs.     

Interfacial Strain Release from the WS2/CsPbBr3 van der Waals Heterostructure for 1.7 V Voltage All-Inorganic Perovskite Solar Cells

Perovskite lattice distortion induced by residual tensile strain from the thermal expansion mismatch between the electron-transporting layer (ETL) and perovskite film causes a sluggish charge extraction and transfer dynamics in all-inorganic CsPbBr₃ perovskite solar cells (PSCs) because of their higher crystallization temperatures and thermal expansion coefficients. Herein, the interfacial strain is released by fabricating a WS₂/CsPbBr₃ van der Waals heterostructure owing to their matched crystal lattice structure and the atomically smooth dangling bond-free surface to act as a lubricant between ETL and CsPbBr₃ perovskite. Arising from the strain-released interface and condensed perovskite lattice, the best device achieves an efficiency of 10.65 % with an ultrahigh open-circuit voltage of 1.70 V and significantly improved stability under persistent light irradiation and humidity (80 %) attack over 120 days.     

Hole‐Boosted Cu(Cr,M)O2 Nanocrystals for All‐Inorganic CsPbBr3 Perovskite Solar Cells

The all‐inorganic CsPbBr₃ perovskite solar cell (PSC) is a promising solution to balance the high efficiency and poor stability of state‐of‐the‐art organic–inorganic PSCs. Setting inorganic hole‐transporting layers at the perovskite/electrode interface decreases charge carrier recombination without sacrificing superiority in air. Now, M‐substituted, p‐type inorganic Cu(Cr,M)O₂ (M=Ba²⁺, Ca²⁺, or Ni²⁺) nanocrystals with enhanced hole‐transporting characteristics by increasing interstitial oxygen effectively extract holes from perovskite. The all‐inorganic CsPbBr₃ PSC with a device structure of FTO/c‐TiO₂/m‐TiO₂/CsPbBr₃/Cu(Cr,M)O₂/carbon achieves an efficiency up to 10.18 % and it increases to 10.79 % by doping Sm³⁺ ions into perovskite halide, which is much higher than 7.39 % for the hole‐free device. The unencapsulated Cu(Cr,Ba)O₂‐based PSC presents a remarkable stability in air in either 80 % humidity over 60 days or 80 °C conditions over 40 days or light illumination for 7 days.     

Steric Mixed‐Cation 2D Perovskite as a Methylammonium Locker to Stabilize MAPbI3

The reduced dimension perovskite including 2D perovskites are one of the most promising strategies to stabilize lead halide perovskite. A mixed‐cation 2D perovskite based on a steric phenyltrimethylammonium (PTA) cation is presented. The PTA‐MA mixed‐cation 2D perovskite of PTAMAPbI₄ can be formed on the surface of MAPbI₃ (PTAI‐MAPbI₃) by controllable PTAI intercalation by either spin coating or soaking. The PTAMAPbI₄ capping layer can not only passivate PTAI‐MAPbI₃ perovskite but also act as MA⁺ locker to inhibit MAI extraction and significantly enhance the stability. The highly stable PTAI‐MAPbI₃ based perovskite solar cells exhibit a reproducible photovoltaic performance with a champion PCE of 21.16 %. Such unencapsulated devices retain 93 % of initial efficiency after 500 h continuous illumination. This steric mixed‐cation 2D perovskite as MA⁺ locker to stabilize the MAPbI₃ is a promising strategy to design stable and high‐performance hybrid lead halide perovskites.     

Remarkable Charge Separation and Photocatalytic Efficiency Enhancement through Interconnection of TiO2 Nanoparticles by Hydrothermal Treatment

Although tremendous effort has been directed to synthesizing advanced TiO₂, it remains difficult to obtain TiO₂ exhibiting a photocatalytic efficiency higher than that of P25, a benchmark photocatalyst. P25 is composed of anatase, rutile, and amorphous TiO₂ particles, and photoexcited electron transfer and subsequent charge separation at the anatase–rutile particle interfaces explain its high photocatalytic efficiency. Herein, we report on a facile and rational hydrothermal treatment of P25 to selectively convert the amorphous component into crystalline TiO₂, which is deposited between the original anatase and rutile particles to increase the particle interfaces and thus enhance charge separation. This process produces a new TiO₂ exhibiting a considerably enhanced photocatalytic efficiency. This method of synthesizing this TiO₂, inspired by a recently burgeoning zeolite design, promises to make TiO₂ applications more feasible and effective.     

Managing Excess Lead Iodide with Functionalized Oxo-Graphene Nanosheets for Stable Perovskite Solar Cells

Stability issues could prevent lead halide perovskite solar cells (PSCs) from commercialization despite it having a comparable power conversion efficiency (PCE) to silicon solar cells. Overcoming drawbacks affecting their long-term stability is gaining incremental importance. Excess lead iodide (PbI₂) causes perovskite degradation, although it aids in crystal growth and defect passivation. Herein, we synthesized functionalized oxo-graphene nanosheets (Dec-oxoG NSs) to effectively manage the excess PbI₂. Dec-oxoG NSs provide anchoring sites to bind the excess PbI₂ and passivate perovskite grain boundaries, thereby reducing charge recombination loss and significantly boosting the extraction of free electrons. The inclusion of Dec-oxoG NSs leads to a PCE of 23.7 % in inverted (p-i-n) PSCs. The devices retain 93.8 % of their initial efficiency after 1,000 hours of tracking at maximum power points under continuous one-sun illumination and exhibit high stability under thermal and ambient conditions.     

Fluorine-Containing Passivation Layer via Surface Chelation for Inorganic Perovskite Solar Cells

Minimizing surface defect is vital to further improve power conversion efficiency (PCE) and stability of inorganic perovskite solar cells (PSCs). Herein, we designed a passivator trifluoroacetamidine (TFA) to suppress CsPbI_3−xBr_x film defects. The amidine group of TFA can strongly chelate onto the perovskite surface to suppress the iodide vacancy, strengthened by additional hydrogen bonds. Moreover, three fluorine atoms allow strong intermolecular connection via intermolecular hydrogen bonds, thus constructing a robust shield against moisture. The TFA-treated PSCs exhibit remarkably suppressed recombination, yielding the record PCEs of 21.35 % and 17.21 % for 0.09 cm² and 1.0 cm² device areas, both of which are the highest for all-inorganic PSCs so far. The device also achieves a PCE of 39.78 % under indoor illumination, the highest for all-inorganic indoor photovoltaic devices. Furthermore, TFA greatly improves device ambient stability by preserving 93 % of the initial PCE after 960 h.