A Large Bandgap Organic Salt Dopant for Sn-Based Perovskite Thin-Film Transistor

Metal halide perovskite optoelectronic devices have made significant progress over the past few years, but precise control of charge carrier density through doping is essential for optimizing these devices. In this study, the potential of using an organic salt, N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate, as a dopant for Sn-based perovskite devices, is explored. Under optimized conditions, the thin film transistors based on the doped 2D/3D perovskite PEAFASnI₃ demonstrate remarkable improvement in hole mobility, reaching 7.45 cm²V⁻¹s⁻¹ with a low subthreshold swing and the smallest sweep hysteresis (ΔV_hysteresis = 2.27 V) and exceptional bias stability with the lowest contact resistance (2.2 kΩ cm). The bulky chemical structure of the dopant prevents it from penetrating the perovskite lattice and also surface passivation against Sn oxidation due to its hydrophobic nature surface. This improvement is attributed to the bifunctional effect of the dopant, which simultaneously passivates defects and improves crystal orientation. These findings provide new insights into potential molecular dopants that can be used in metal halide perovskite devices.     

Two-Dimensional Metal–Organic Frameworks-Based Grain Termination Strategy Enables High-Efficiency Perovskite Photovoltaics with Enhanced Moisture and Thermal Stability

Perovskite degradation induced by surface defects and imperfect grain boundaries of films seriously damages the performance of perovskite solar cells (PSCs). Meanwhile, conventional organic molecules cannot maintain the long-time passivation effects under the stimulation of external environmental factors. Here, efficient and stable grain passivation in perovskite films is realized by preparing formic acid-functionalized 2D metal–organic frameworks (MOFs) as the terminated agent. Through robust interactions between exposed active sites and PbI₂, the 2D MOFs tightly caps the surface of PbI₂-terminated perovskite grains to stabilize the perovskite phases and aids the adhesion of adjacent grains. The MOFs mainly distributed at the grain boundaries of the perovskite film is directly observed at the microscopic scale. The modified perovskite films have regular morphology, lower defect density, and superior optoelectronic properties. Benefiting from the suppressed charge recombination and faster charge extraction, a power conversion efficiency of 21.28% is achieved for the best-performing PSC device. The unencapsulated PSCs with the MOFs modification maintain 88% and 81% of their initial efficiency after 750 h heating at 85  ° C under N₂ atmosphere and more than 1000 h storage in ambient environment (25  ° C, RH  ≈  40%), respectively.     

Modulation on Electrostatic Potential of Passivator for Highly Efficient and Stable Perovskite Solar Cells

The perovskite layer contains a large number of charged defects that seriously impair the efficiency and stability of perovskite solar cells (PSCs), thus it is essential to develop an effective passivation strategy to heal them. Based on theoretical calculations, it is found that enhancing the electrostatic potential of passivators can improve passivation effect and adsorption energy between charged defects and passivators. Herein, an electrostatic potential modulation (EPM) strategy is developed to design passivators for highly efficient and stable PSCs. With the EPM strategy, 1-phenylethylbiguanide (PEBG) and 1-phenylbiguanide (PBG) are designed. It is found that the charge distribution and electrostatic potential of phenyl- and phenylethyl- substituent on the biguanide are significantly enhanced. The N atom directly bonding to the phenyl group shows larger positive charge than that bonding to the phenylethyl group. The modulated electrostatic potential makes PBG bind stronger with the defects on perovskite surface. Based on the effective passivation of EPM, a champion efficiency of 24.67% is realized and the device retain 91.5% of its initial PCE after ≈1300 h. The promising EPM strategy, which provides a principle of passivator design and allows passivation to be controllable, may advance further optimization and application of perovskite solar cells toward commercialization.     

Crystal Growth Regulation of α-FAPbI3 Perovskite Films for High-Efficiency Solar Cells with Long-Term Stability

The two-step sequentially deposition strategy has been widely used to produce high-performance FAPbI₃-based solar cells. However, due to the rapid reaction between PbI₂ and FAI, a dense perovskite film forms on top of the PbI₂ layer immediately and blocks the FAI diffusion into the bottom of the PbI₂ film for a complete reaction, which results in a low-efficiency and limited reproducibility of perovskite solar cells (PSCs). Here, high-quality α-FAPbI₃ perovskite films by crystal growth regulation with 4-fluorobenzamide additives is fabricated. The additives can interact with FAI to suppress the fast reaction between the FAI and PbI₂ and effectively passivate the under-coordinated Pb²⁺ or I^- defects. As a result, α-FAPbI₃ perovskite films with low trap density and large grain size are prepared. The modified PSCs present a high-power conversion efficiency of 24.08%, maintaining 90% of their initial efficiency after 1400 h in high humidity. This study provides an efficient strategy of synergistic crystallization and passivation to form high-quality α-FAPbI₃ films for high-performance PSCs.     

Anti-Dissociation Passivation via Bidentate Anchoring for Efficient Carbon-Based CsPbI2.6Br0.4 Solar Cells

Molecular passivation on perovskite surface is an effective strategy to inhibit surface defect-assisted recombination and reduce nonradiative recombination loss in perovskite solar cells (PSCs). However, the majority of passivating molecules bind to perovskite surface through weak interactions, resulting in weak passivation effects and susceptible to interference from various factors. Especially in carbon-based perovskite solar cells (C-PSCs), the molecular passivation effect is more susceptible to disturbance in subsequent harsh preparation of carbon electrodes via blade-coating route. Herein, bidentate ligand 2,2′-Bipyridine (2Bipy) is explored to passivate surface defects of CsPbI_2.6Br_0.4 perovskite films. The results indicate that compared with monodentate pyridine (Py), bidentate 2Bipy shows a stronger chelation with uncoordinated Pb(II) defects and exhibits a greater passivation effect on perovskite surface. As a result, 2Bipy-modified perovskite films display a significantly boosted photoluminescence lifetime, accompanied by excellent anchoring stability and anti-dissociation of passivating molecules. Meanwhile, the moisture resistance of the 2Bipy-modified perovskite films is also significantly enhanced. Consequently, the efficiency of C-PSCs is improved to 16.57% (J_sc = 17.16 mA cm⁻², V_oc = 1.198 V, FF = 80.63%). As far as it is known, this value represents a new record efficiency for hole transport material-free inorganic C-PSCs.     

Multi-Site Intermolecular Interaction for In Situ Formation of Vertically Orientated 2D Passivation Layer in Highly Efficient Perovskite Solar Cells

Surface passivation via 2D perovskite is critical for perovskite solar cells (PSCs) to achieve remarkable performances, in which the applied spacer cations play an important role on structural templating. However, the random orientation of 2D perovskite always hinder the carrier transport. Herein, multiple nitrogen sites containing organic spacer molecule (1H-Pyrazole-1-carboxamidine hydrochloride, PAH) is introduced to form 2D passivation layer on the surface of formamidinium based (FAPbI₃) perovskite. Deriving from the interactions between PAH and PbI₂, the defects of FAPbI₃ perovskite are effectively passivated. Interestingly, due to the multiple-site interactions, the 2D nanosheets are found to grow perpendicularly to the substrate for promotion of charge transfer. Therefore, an impressive power conversion efficiency of 24.6% and outstanding long-term stability are achieved for the 2D/3D perovskite devices. The findings further provide a perspective in structure design of novel organic halide salts for the fabrication of efficient and stable PSCs.     

Side-Chain Functionalized Polymer Hole-Transporting Materials with Defect Passivation Effect for Highly Efficient Inverted Quasi-2D Perovskite Solar Cells

Compared with inverted 3D perovskite solar cell (PSCs), inverted quasi-2D PSCs have advantages in device stability, but the device efficiency is still lagging behind. Constructing polymer hole-transporting materials (HTMs) with passivation functions to improve the buried interface and crystallization properties of perovskite films is one of the effective strategies to improve the performance of inverted quasi-2D PSCs. Herein, two novel side-chain functionalized polymer HTMs containing methylthio-based passivation groups are designed, named PVCz-SMeTPA and PVCz-SMeDAD, for inverted quasi-2D PSCs. Benefited from the non-conjugated flexible backbone bearing functionalized side-chain groups, the polymer HTMs exhibit excellent film-forming properties, well-matched energy levels and improved charge mobility, which facilitates the charge extraction and transport between HTM and quasi-2D perovskite layer. More importantly, by introducing methylthio units, the polymer HTMs can enhance the contact and interactions with quasi-2D perovskite, and further passivating the buried interface defects and assisting the deposition of high-quality perovskite. Due to the suppressed interfacial non-radiative recombination, the inverted quasi-2D PSCs using PVCz-SMeTPA and PVCz-SMeDAD achieve impressive power conversion efficiency (PCE) of 21.41% and 20.63% with open-circuit voltage of 1.23 and 1.22 V, respectively. Furthermore, the PVCz-SMeTPA based inverted quasi-2D PSCs also exhibits negligible hysteresis and considerably improved thermal and long-term stability.     

Efficient Passivation Strategy on Sn Related Defects for High Performance All-Inorganic CsSnI3 Perovskite Solar Cells

Despite remarkable progress in hybrid perovskite solar cells (PSCs), the concern of toxic lead ions remains a major hurdle in the path towards PSC's commercialization; tin (Sn)-based PSCs outperform the reported Pb-free perovskites in terms of photovoltaic performance. However, it is of a particularly great challenge to develop effective passivation strategies to suppress Sn(II) induced defect densities and oxidation for attaining high-performance all-inorganic CsSnI₃ PSCs. Herein, a facile yet effective thioamides passivation strategy to modulate defect state density at surfaces and grain boundaries in CsSnI₃ perovskites is reported. The thiosemicarbazide (TSC) with SC N functional groups can make strong coordination interaction with charge defects, leading to enhanced electron cloud density around defects and increased vacancy formation energies. Importantly, the surface passivation can reduce the deep level trap state defect density originated from undercoordinated Sn²⁺ ion and Sn²⁺ oxidation, significantly restraining nonradiative recombination and elongating the carrier lifetime of TSC treated CsSnI₃ PSCs. The surface passivated all-inorganic CsSnI₃ PSCs based on an inverted configuration delivers a champion power conversion efficiency (PCE) of 8.20%, with a prolonged lifetime over 90% of initial PCE, after 500 h of continuous illumination. The present strategy sheds light on surface defect passivation for achieving highly efficient all-inorganic lead-free Sn-based PSCs.     

Chemical Reduction of Iodine Impurities and Defects with Potassium Formate for Efficient and Stable Perovskite Solar Cells

The performance of perovskite solar cells (PSCs) is negatively affected by iodine (I₂) impurities generated from the oxidation of iodide ions in the perovskite precursor powder, solution, and perovskite films. In this study, the use of potassium formate (HCOOK) as a reductant to minimize the presence of detrimental I₂ impurities is presented. It is demonstrated that HCOOK can effectively reduce I₂ back to I⁻ in the precursor solution as well as in the devices under external conditions. Furthermore, the introduced formate anion (HCOO⁻) and alkali metal cation (K⁺) can reduce the defect density within the perovskite film by modulating perovskite growth and passivating electronic defects, significantly prolonging the carrier lifetime and reducing the J–V hysteresis. Consequently, the maximum efficiency of the HCOOK-doped planar n–i–p PSCs reaches 23.8%. After 1000 h of operation at maximum power point tracking under continuous 1 sun illumination, the corresponding encapsulated devices retain 94% of their initial efficiency.     

3D Polydentate Complexing Agents for Passivating Defects and Modulating Crystallinity for High-Performance Perovskite Solar Cells

The grain boundaries (GBs)/surface defects within perovskite film directly impede the further improvement of photoelectric conversion efficiency (PCE) and stability of planar perovskite solar cells (PSCs). Herein, 3D phytic acid (PA) and phytic acid dipotassium (PAD) with polydentate are explored to synchronously passivate the defects of perovskite absorber directly in multiple spatial directions. The strong electron-donating groups ( H₂PO₄) in the PA molecule afford six anchor sites to bind firmly with uncoordinated Pb²⁺ at the GBs/surface and modulate perovskite crystallization, thus enhancing the quality of perovskite film. Particularly, PAD containing an additional (K→PO) push–pull structure promotes the dominant coordination of phosphate group (PO) with Pb²⁺ and passivates halide anion defects due to the complexation of potassium ions (K⁺) with iodide ions (I^-). Consequently, the PAD-complexed PSCs deliver a champion PCE of 23.18%, which is remarkably higher than that of the control device (19.94%). Meanwhile, PAD-complexed PSCs exhibit superior moisture and thermal stability, remaining 79% of their initial PCE after 1000 h under continuous illumination, while the control device remain only 48% of their PCE after 1000 h. This work provides important insights into designing multifunctional 3D passivators for the purpose of simultaneously enhancing the efficiency and stability of devices.     

Aryl Diammonium Iodide Passivation for Efficient and Stable Hybrid Organ‐Inorganic Perovskite Solar Cells

Surface passivation is increasingly one of the most prominent strategies to promote the efficiency and stability of perovskite solar cells (PSCs). However, most passivation molecules hinder carrier extraction due to poorly conductive aggregation between perovskite surface and carrier transportation layer. Herein, a novel molecule: p‐phenyl dimethylammonium iodide (PDMAI) with ammonium group on both terminals is introduced, and its passivation effect is systematically investigated. It is found that PDMAI can mitigate defects at the surface and promote carrier extraction from perovskite to the hole transporting layer, leading to a lift of open‐circuit voltage of 40 mV. Profiting from superior PDMAI passivation, the average efficiency of PSCs has been elevated from 19.69% to 20.99%. As demonstrated with density functional theory calculations, PDMAI probably tends to anchor onto the perovskite surface with both ? NH₃I tails, and enhances the adhesion and contact to perovskite layer. The exposed hydrophobic aryl core protects perovskite against detrimental environmental factors. In addition, the alkyl component between aryl and ammonium groups is demonstrated to be essentially vital in triggering passivation function, which offers the guidance for the design of passivation molecules.     

Impact of 2D Ligands on Lattice Strain and Energy Losses in Narrow-Bandgap Lead–Tin Perovskite Solar Cells

Mixed lead and tin (Pb/Sn) hybrid perovskites exhibit a great potential in fabricating all-perovskite tandem devices due to their easily tunable bandgaps. However, the energy deficit and instability in Pb/Sn perovskite solar cells (PSCs) constrain their practical applications, which renders defect passivation engineering indispensable to develop highly efficient and long-term stable PSCs. Herein, the mechanisms of strain tailoring and defect passivation in Pb/Sn PSCs by 2D ligands are investigated. The 2D ligands include electroneutral cations with long alkyl chain (LAC), iodates with relatively short alkyl chain (SAC) and their mixtures. This study reveals that LAC ligands facilitate the relaxation of tensile strain in perovskite films while SAC ligands cause strain buildup. By mixing LAC/SAC ligands, tensile strain in perovskite films can be balanced which improves solar cell performance. PSCs with admixed β-guanidinopropionic acid (GUA)/phenethylammonium iodide (PEAI) exhibit enhanced open circuit voltage and fill factor, which is attributed to reduced nonradiative recombination losses in the bulk and at the interfaces. Furthermore, the operational stability of PSCs is slightly improved by the mixed 2D ligands. This work reveals the mechanisms of 2D ligands in strain tailoring and defect passivation toward efficient and stable narrow-bandgap PSCs.     

Molecularly Tailored Surface Defect Modifier for Efficient and Stable Perovskite Solar Cells

Surface defects cause non-radiative charge recombination and reduce the photovoltaic performance of perovskite solar cells (PSCs), thus effective passivation of defects has become a crucial method for achieving efficient and stable devices. Organic ammonium halides have been widely used for perovskite surface passivation, due to their simple preparation, lattice matching with perovskite, and high defects passivation ability. Herein, a surface passivator 2,4,6-trimethylbenzenaminium iodide (TMBAI) is employed as the interfacial layer between the spiro-OMeTAD and perovskite layer to modify the surface defect states. It is found that TMBAI treatment suppresses the nonradiative charge carrier recombination, resulting in a 60 mV increase of the open-circuit voltage (V_oc) (from 1.11 to 1.17 V) and raises the fill factor from 76.3% to 80.3%. As a result, the TMBAI-based PSCs device demonstrates a power conversion efficiency (PCE) of 23.7%. Remarkably, PSCs with an aperture area of 1 square centimeter produce a PCE of 21.7% under standard AM1.5 G sunlight. The unencapsulated TMBAI-modified device retains 92.6% and 90.1% of the initial values after 1000 and 550 h under ambient conditions (humidity 55%–65%) and one-sun continuous illumination, respectively.     

Dual-Phase Stabilized Perovskite Nanowires for Reduced Defects and Longer Carrier Lifetime

1D perovskite materials are of significant interest to build a new class of nanostructures for electronic and optoelectronic applications. However, the study of colloidal perovskite nanowires (PNWs) lags far behind those of other established perovskite materials such as perovskite quantum dots and perovskite thin films. Herein, a dual-phase passivation strategy to synthesize all-inorganic PNWs with minimized surface defects is reported. The local phase transition from CsPbBr₃ to CsPb₂Br₅ in PNWs increases the photoluminescence quantum yield, carrier lifetime, and water-resistivity, owing to the energetic and chemical passivation effect. In addition, these dual-phase PNWs are employed as an interfacial layer in perovskite solar cells (PSCs). The enhanced surface passivation results in an efficient carrier transfer in PSCs, which is a critical enabler to increase the power conversion efficiency (PCE) to 22.87%, while the device without PNWs exhibits a PCE of 20.74%. The proposed strategy provides a surface passivation platform in 1D perovskites, which can lead to the development of novel nanostructures for future optoelectronic devices.     

High performance inverted perovskite solar cells using PEDOT:PSS/KCl hybrid hole transporting layer

PEDOT:PSS is one of the most widely used hole transporting layer for inverted perovskite solar cells. Yet the performances of the corresponding perovskite solar cells are not satisfactory. Here, we demonstrate that KCl modified PEDOT:PSS film can promote the crystallization of perovskite film and enlarge the perovskite crystals. At the same time, KCl can diffuse into the perovskite film and effectively passivate the defects. As a result, inverted perovskite solar cells fabricated on 10 mg mL⁻¹ PEDOT:PSS/KCl films exhibit an average power conversion efficiency of 16.24 %, which is enhanced by 17.77 % compared with the reference perovskite solar cells. Open circuit voltage of 1.009 V and power conversion efficiency of 17.09 % have also been demonstrated using the optimized 10 mg mL⁻¹ PEDOT:PSS/KCl films.     

Highly Sensitive Self-Powered 2D Perovskite Photodiodes with Dual Interface Passivations

2D perovskites have attracted intensive attention by virtue of their excellent optical and electrical properties along with good stabilities. Herein, a highly sensitive self-powered photodiode based on (PEA)₂(MA)₄Pb₅I₁₆ (PEA=C₆H₅(CH₂)NH₃, MA=CH₃NH₃) 2D perovskite is demonstrated by dual interface passivations. The Al₂O₃ bottom passivation can reduce the pinhole defects in the 2D perovskite film and suppress the trap-related recombination loss, bringing forward much reduced dark current and increased photocurrent. The poly (methyl methacrylate) (PMMA) top passivation can encapsulate the 2D perovskite film and thus improve the stability of the device. These results show that the 2D perovskite-based photodiode with dual interface passivations exhibits a large photo-to-dark current ratio of 10⁷, a fast response speed of 597 ns and a linear dynamic range of 160 dB without bias. Responsivity (R) and detectivity (D*) respectively reach 0.36 A W⁻¹ and 5.4 × 10¹² Jones under 532 nm laser illumination at a power density of 1.5 nW cm⁻². Moreover, the dual interface passivated device exhibits good stabilities. This study paves the road for developing low-cost, low-power, solution processed image sensors.     

An Easily Developed Difunctional Molecule Enabling Highly Efficient and Stable Light-Emitting Diodes

The surface treatment for solution-processed perovskite films using molecular passivation agents has been reported to be critical for improving the performance of perovskite light-emitting diodes (PeLEDs). Generally, most studies emphasized the binding effect between passivation molecules and perovskite surface, while halide perovskites are dominated by multiple defects and need more comprehensive passivation, which is often neglected. Here, a molecule of 5-ammonium valeric acid trifluoroacetate (5-AVATFA) with multi-functional groups is rationally developed to modify the interface of advanced PeLEDs. Combining experimental and density functional theory analysis, the synergism effects of reduced surface defects and increased steric hindrance of 5-AVATFA, which can greatly suppress the ion migration of PeLEDs, are demonstarated. Because of the passivation effect of multi-functional 5-AVATFA, an optimal device with an external quantum efficiency of 22.32%, a radiance of 558.46 W sr⁻¹ m⁻², and an extended half-lifetime of 103.3 h is obtained.     

Highly-Stable CsPbI3 Perovskite Solar Cells with an Efficiency of 21.11% via Fluorinated 4-Amino-Benzoate Cesium Bifacial Passivation

The poor interface quality between cesium lead triiodide (CsPbI₃) perovskite and the electron transport layer limits the stability and efficiency of CsPbI₃ perovskite solar cells (PSCs). Herein, a 4-amino-2,3,5,6-tetrafluorobenzoate cesium (ATFC) is designed as a bifacial defect passivator to tailor the perovskite/TiO₂ interface. The comprehensive experiments demonstrate that ATFC can not only optimize the conductivity, electron mobility, and energy band structure of the TiO₂ layer by passivation of the undercoordinated Ti⁴⁺, oxygen vacancy (V_O), and free  OH defects but also promote the yield of high-quality CsPbI₃ film by synergistic passivation of undercoordinated Pb²⁺ defects with the  CO group and F atom, and limiting I⁻ migration via F···I interaction. Benefiting from the above interactions, the ATFC-modified CsPbI₃ device yields a champion power conversion efficiency (PCE) of 21.11% and an excellent open-circuit voltage (V_OC) of 1.24 V. Meanwhile, the optimized CsPbI₃ PSC maintains 92.74% of its initial efficiency after aging 800 h in air atmosphere, and has almost no efficiency attenuation after tracking at maximum power point for 350 h.     

Surface Defect Suppression for High Color Purity Light-Emitting Diode of Free-Standing Single-Crystal Perovskite Film

High color purity is one of the important features for single-crystal metal halide perovskite light-emitting diode (LED). Despite single-crystal perovskite showing low bulk defect concentration, single-crystal perovskite LEDs do not exhibit high color purity advantage due to the absence of effective surface defect passivation. Herein, one fully wrapped structure is proposed to passivate the surface of the free-standing CsPbBr₃ single-crystal films. The surface of CsPbBr₃ single-crystal films is wrapped by ultra-thin polymethyl methacrylate, precisely controlling the thickness. The single-crystal perovskite film device can achieve high color purity with a full width at half maximum (FWHM) of 15.8 nm) and a large luminescent area of 2.25 mm². It is observed that surface passivation is due to interaction of CO bond in polymer chains with the Lewis acid PbBr₂. The passivated perovskite single-crystal films significantly improve carrier lifetime and suppress surface defects. It is noteworthy that the passivated free-standing single-crystal perovskite films are feasible to build up a vertical LED device structure, avoiding the edge glowing and short-circuiting of the LED device. This study demonstrates the large luminescent area of the high-quality millimetre-scale free-standing single-crystal films for wide color gamut display and vertical optoelectronic devices.     

Double‐Sided Surface Passivation of 3D Perovskite Film for High‐Efficiency Mixed‐Dimensional Perovskite Solar Cells

Defect‐mediated carrier recombination at the interfaces between perovskite and neighboring charge transport layers limits the efficiency of most state‐of‐the‐art perovskite solar cells. Passivation of interfacial defects is thus essential for attaining cell efficiencies close to the theoretical limit. In this work, a novel double‐sided passivation of 3D perovskite films is demonstrated with thin surface layers of bulky organic cation–based halide compound forming 2D layered perovskite. Highly efficient (22.77%) mixed‐dimensional perovskite devices with a remarkable open‐circuit voltage of 1.2 V are reported for a perovskite film having an optical bandgap of ≈1.6 eV. Using a combination of experimental and numerical analyses, it is shown that the double‐sided surface layers provide effective defect passivation at both the electron and hole transport layer interfaces, suppressing surface recombination on both sides of the active layer. Despite the semi‐insulating nature of the passivation layers, an increase in the fill factor of optimized cells is observed. The efficient carrier extraction is explained by incomplete surface coverage of the 2D perovskite layer, allowing charge transport through localized unpassivated regions, similar to tunnel‐oxide passivation layers used in silicon photovoltaics. Optimization of the defect passivation properties of these films has the potential to further increase cell efficiencies.