Modification of Energy Level Alignment for Boosting Carbon-Based CsPbI2Br Solar Cells with 14% Certified Efficiency

Hole transfer material (HTM)-free, carbon-based all-inorganic perovskite solar cells (C-PSCs) are promising alternatives to conventional organic–inorganic hybrid PSCs in addressing thermal and moisture instability issues. However, the energy level mismatch between the inorganic perovskite and carbon electrode coupled, together with the incapability of the carbon electrode to reflect incident light for reabsorption, limits the power conversion efficiency (PCE) of C-PSCs. To address these issues, herein, a new strategy of a hexyltrimethylammonium bromide (HTAB)-modified CsPbI₂Br perovskite surface is devised to reduce this energy offset from 0.70 to 0.32 eV and increase the built-in potential by 70 mV for the final devices. Additionally, a CsPbI₂Br perovskite film with a thickness of up to 800 nm is realized via a hot-flow-assisted spin coating approach in an ambient atmosphere with humidity of less than 80%. Reduced energy offset coupled with suppressed charge recombination and thick perovskite layer boosts the champion PCE of CsPbI₂Br C-PSCs to 14.3% (J_sc = 14.1 mA cm⁻², V_oc = 1.26 V, and fill factor = 0.806), and the average PCE to 13.9% under one sun illumination. A new certified efficiency record of 14.0% is obtained for HTM-free inorganic C-PSCs. Meanwhile, the moisture-resistant barrier from the alkyl chain in HTAB improves the stability of the final devices.     

Combined Bulk and Surface Passivation in Dimensionally Engineered 2D-3D Perovskite Films via Chlorine Diffusion

Dimensional engineering of perovskite films is a promising pathway to improve the efficiency and stability of perovskite solar cells (PSCs). In this context, surface or bulk passivation of defects in 3D perovskite film by careful introduction of 2D perovskite plays a key role. Here the authors demonstrate a 2D perovskite passivation scheme based on octylammonium chloride, and show that it provides both bulk and surface passivation of 1.6 eV bandgap 3D perovskite film for highly efficient (≈23.62%) PSCs with open-circuit voltages up to 1.24 V. Surface and depth-resolved microscopy and spectroscopy analysis reveal that the Cl⁻ anion diffuses into the perovskite bulk, passivating defects, while the octylammonium ligands provide effective, localized surface passivation. The authors find that the Cl⁻ diffusion into the perovskite lattice is independent of the 2D perovskite crystallization process and occurs rapidly during deposition of the 2D precursor solution. The annealing-induced evaporation of Cl from bulk perovskite is also inhibited in 2D–3D perovskite film as compared to pristine 3D perovskite, ensuring effective bulk passivation in the relevant film.     

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.     

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.     

Cuprous iodide dose dependent passivation of MAPbI3 perovskite solar cells

MAPbI₃ has been considered as a candidate for the active layer of perovskite solar cells in recent years. We proposed a device model to investigate the contribution of cuprous iodide (CuI) to MAPbI₃ perovskite thin films to power conversion efficiency (PCE) and demonstrated that the dosage of CuI affects the grain size of thin films and the PCE. Through the results of the SEM analysis, we found that the grain boundaries of MAPbI₃ perovskite films decreased with increases in the dosage of CuI and the grain size increased significantly from 164 nm ± 49 nm–299 nm ± 127 nm. In addition, the results of the PL measurement showed that the PL intensity decreased after addition of CuI to the MAPbI₃ perovskite thin films, suggesting a reduction in the charge recombination. The XRD patterns indicated that the addition of CuI did not influence the main structure of the MAPbI₃ perovskite. Interestingly, CuI plays a key role in the passivation of defects in MAPbI₃ perovskite thin films, which can reduce non-radiative recombination and increase the fill factor and open-circuit voltage of the device. In this study, we adjusted the grain size and passivated the MAPbI₃ thin film by controlling the dosage of CuI. We also increased the power conversion efficiency from 10% to 13%. This type of perovskite solar cell provided a simple, low cost preparation process for practical applications.     

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.     

Over 24% Efficient Poly(vinylidene fluoride) (PVDF)-Coordinated Perovskite Solar Cells with a Photovoltage up to 1.22 V

Recently, organic–inorganic metal halide perovskite solar cells (PSCs) have achieved rapid improvement, however, the efficiencies are still behind the Shockley–Queisser theory mainly due to their high energy loss (E_LOSS) in open-circuit voltage (V_OC). Due to the polycrystalline nature of the solution-prepared perovskite films, defects at the grain boundaries as the non-radiative recombination centers greatly affect the V_OC and limit the device efficiency. Herein, poly(vinylidene fluoride) (PVDF) is introduced as polymer-templates in the perovskite film, where the fluorine atoms in the PVDF network can form strong hydrogen-bonds with organic cations and coordinate bonds with Pb²⁺. The strong interaction between PVDF and perovksite enables slow crystal growth and efficient defect passivation, which effectively reduce non-radiation recombination and minimize E_LOSS of V_OC. PVDF-based PSCs achieve a champion efficiency of 24.21% with a excellent voltage of 1.22 V, which is one of the highest V_OC values reported for FAMAPb(I/Br)₃-based PSCs. Furthermore, the strong hydrophobic fluorine atoms in PVDF endow the device with excellent humidity stability, the unencapsulated solar cell maintain the initial efficiency of >90% for 2500 h under air ambient of ≈50% humid and a consistently high V_OC of 1.20 V.     

Defects Passivation via Potassium Iodide Post-Treatment for Antimony Selenosulfide Solar Cells with Improved Performance

Antimony selenosulfide (Sb₂(S,Se)₃) has been emerging as a promising light absorber in the past few years owing to tunable bandgap (1.1–1.7 eV), high absorption coefficient (>10⁵ cm⁻¹) and excellent phase and environmental stability. However, the efficiency of Sb₂(S,Se)₃ solar cells lags far behind the Shockley–Queisser limit. One of the critical obstacles originates from various extrinsic and intrinsic defects. They mostly locate in the deep energy levels and are prone to form recombination centers, inhibiting the improvement of device performance. Herein, surface post-treatment via potassium iodide is introduced to fabricate high-quality Sb₂(S,Se)₃ films and solar cells. The surface post-treatment not only manipulates the crystal growth process to form compact films with larger grain size but also forms better band alignment and inhibits the formation of deep-level defects antimony antisite (Sb_Se), thus improving the quality of heterojunction. Consequently, the resultant Sb₂(S,Se)₃ solar cells achieve a champion power conversion efficiency  of 9.22%. This study provides a new strategy of passivating deep-level intrinsic defects via surface post-treatment for high-efficiency Sb₂(S,Se)₃ solar cells.     

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.     

Insights into Fullerene Passivation of SnO2 Electron Transport Layers in Perovskite Solar Cells

Interfaces between the photoactive and charge transport layers are crucial for the performance of perovskite solar cells. Surface passivation of SnO₂ as electron transport layer (ETL) by fullerene derivatives is known to improve the performance of n–i–p devices, yet organic passivation layers are susceptible to removal during perovskite deposition. Understanding the nature of the passivation is important for further optimization of SnO₂ ETLs. X‐ray photoelectron spectroscopy depth profiling is a convenient tool to monitor the fullerene concentration in passivation layers at a SnO₂ interface. Through a comparative study using [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and [6,6]‐phenyl‐C₆₁‐butyric acid (PCBA) passivation layers, a direct correlation is established between the formation of interfacial chemical bonds and the retention of passivating fullerene molecules at the SnO₂ interface that effectively reduces the number of defects and enhances electron mobility. Devices with only a PCBA‐monolayer‐passivated SnO₂ ETL exhibit significantly improved performance and reproducibility, achieving an efficiency of 18.8%. Investigating thick and solvent‐resistant C₆₀ and PCBM‐dimer layers demonstrates that the charge transport in the ETL is only improved by chemisorption of the fullerene at the SnO₂ surface.     

Probing into Reverse Bias Dark Current in Perovskite Photodiodes: Critical Role of Surface Defects

Minimizing reverse bias dark current density (J_dark) while retaining high external quantum efficiency is crucial for promising applications of perovskite photodiodes, and it remains challenging to elucidate the ultimate origin of J_dark. It is demonstrated in this study that the surface defects induced by iodine vacancies are the main cause of J_dark in perovskite photodiodes. In a targeted way, the surface defects are thoroughly passivated through a simple treatment with butylamine hydroiodide to form ultrathin 2D perovskite on its 3D bulk. In the passivated perovskite photodiodes, J_dark as low as 3.78 × 10^-10 A cm^-2 at -0.1 V is achieved, and the photoresponse is also enhanced, especially at low light intensities. A combination of the two improvements realizes high specific detectivity up to 1.46 × 10¹² Jones in the devices. It is clarified that the trap states induced by the surface defects can not only raise the generation-recombination current density associated with the Shockley–Read–Hall mechanisms in the dark (increasing J_dark), but also provide additional carrier recombination paths under light illumination (decreasing photocurrent). The critical role of surface defects on J_dark of perovskite photodiodes suggests that making trap-free perovskite thin films, for example, by fine preparation and/or surface engineering, is a top priority for high-performance perovskite photodiodes.     

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.     

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.     

Surface Defects Management by In Situ Etching with Methanol for Efficient Inverted Inorganic Perovskite Solar Cells

Inorganic perovskite solar cells (IPSCs) have developed rapidly due to their good thermal stability and the bandgap suitable for perovskite/silicon tandem solar cells. However, the large open-circuit voltage (V_OC) deficit derived from the surface defects and the energy level structure mismatch impede the development of device performance, especially in the P-I-N structure IPSCs. Herein, an innovative in situ etching (ISE) treatment method is proposed to reduce surface defects through methanol without additional passivator. It is found that the perovskite films treated with methanol result in a slight excess of PbI₂ on the surface and inserted into the grain boundaries. Therefore, the successful decrease of surface defects by methanol and the passivation of grain boundary defects by PbI₂ greatly reduce the trap density of perovskite films. And the larger work function of PbI₂ contributes to the energy band of perovskite surface bending downward and forms gradient energy level alignment at the I/N interface, which accelerates extraction of charge carriers. As a result, the efficiency of CsPbI_2.85Br_0.15 inverted IPSC is enhanced from 16.00% to 19.34%, which is one of the mostly efficient IPSCs. This work provides an original idea without additional passivator to manage the defects of inorganic perovskite.     

Synergistic Passivation and Down-Conversion by Imidazole-Modified Graphene Quantum Dots for High Performance and UV-Resistant Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PVSCs) have achieved stunning progress during the past decade, which has inspired great potential for future commercialization. However, tin dioxide (SnO₂) as a commonly used electron transport layer with varied defects and energy level mismatch with perovskite contributes to the energy loss and limitation of charge extraction. Herein, imidazole-modified graphene quantum dots (IGQDs) are introduced as the interlayer, which plays a significant role in three aspects: 1) dually passivating the defects of SnO₂ and buried interface of perovskite by first-principles calculations; 2) accelerating the carrier extraction and transfer owing to ideal band alignment; and 3) improving light utilization through down-conversion proved by light intensity measurement. Consequently, the devices based on IGQDs/SnO₂ not only exhibit the champion power conversion efficiency (PCE) of 24.11%, but display a significantly enhanced ultraviolet (UV) stability retaining about 81% of their initial PCEs after continuous UV irradiation (365 nm, 20 mW cm⁻²) for 300 h. Moreover, the unencapsulated modified device remains 82% after storing for 1650 h in air (20–30 °C, RH 45–55%). This work furnishes a novel method for the combination of interfacial passivation and photon management, which holds out for the prospect of employment in other optoelectronic applications.     

Light‐enhanced surface passivation of TiO2‐coated silicon

Titanium dioxide is shown to afford good passivation to non‐diffused silicon surfaces and boron‐diffused surfaces after a low‐temperature anneal. The passivation most likely owes to the significant levels of negative charge instilled in the films, and passivation is enhanced by illumination—advantageous for solar cells—indicating that a titanium dioxide photoreaction is at least partly responsible for the low surface recombination. We demonstrate a surface recombination velocity of less than 30 cm/s, on a 5‐Ω cm n‐type silicon, and an emitter saturation current density of 90 fA/cm² on a 200‐Ω/sq boron diffusion. If these titanium dioxide passivated boron‐diffused surfaces were employed in a crystalline silicon solar cell, an open‐circuit voltage as high as 685 mV could be achieved. Given that TiO₂ has a high refractive index and was deposited with atmospheric pressure chemical vapour deposition, an inexpensive technique, it has the potential as a passivating antireflection coating for industrial boron‐diffused silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

In-Situ Hot Oxygen Cleansing and Passivation for All-Inorganic Perovskite Solar Cells Deposited in Ambient to Breakthrough 19% Efficiency

All-inorganic perovskite CsPbI₃ has attracted extensive attention recently because of its excellent thermal and chemical stability. However, its photovoltaic performance is hindered by large energy losses (E_loss) due to the presence of point defects. In addition, hydroiodic acid (HI) is currently employed as a hydrolysis-derived precursor of intermediate compounds, which often leads to a small amount of organic residue, thus undermining its chemical stability. Herein, an in-situ hot oxygen cleansing with superior passivation (HOCP) for the triple halide-mixed CsPb(I_2.85Br_0.149Cl_0.001) perovskite solar cells (abbreviated as CsPbTh₃) deposited in an ambient atmosphere to reduce the E_loss to as low as 0.48 eV for the power conversion efficiency (PCE) to reach 19.65% is demonstrated. It is found that the hot oxygen treatment effectively removes the organic residues. Meanwhile, it passivates halide vacancies, hence reduces the trap states and nonradiative recombination losses within the perovskite layer. As a result, the PCE is increased significantly from 17.15% to 19.65% under 1 sun illumination with an open-circuit voltage enlarged to 1.23 from 1.14 V, which corresponds to an E_loss reduction from 0.57 to 0.48 eV. Also, the HOCP-treated devices exhibit better long-term stability. This insight should pave a way for decreasing nonradiative charge recombination losses for high-performance inorganic perovskite photoelectronics.     

Experimental evidence of parasitic shunting in silicon nitride rear surface passivated solar cells

Many solar cells incorporating SiN_x films as a rear surface passivation scheme have not reached the same high level of cell performance as solar cells incorporating high‐temperature‐grown silicon dioxide films as a rear surface passivation. In this paper, it is shown by direct comparison of solar cells incorporating the two rear surface passivation schemes, that the performance loss is mainly due to a lower short‐circuit current while the open‐circuit voltage is equally high. With a solar cell test structure that features a separation of the rear metal contacts from the passivating SiN_x films, the loss in short‐circuit current can be reduced drastically. Besides a lower short‐ circuit current, dark I–V curves of SiN_x rear surface passivated solar cells exhibit distinct shoulders. The results are explained by parasitic shunting of the induced floating junction (FJ) underneath the SiN_x films with the rear metal contacts. The floating junction is caused by the high density of fixed positive charges in the SiN_x films. Other two‐dimensional effects arising from the injection level dependent SRV of the Si/SiN_x interfaces are discussed as well, but, are found to be of minor importance. Pinholes in the SiN_x films and optical effects due to a different internal rear surface reflectance can be excluded as a major cause for the performance loss of the SiN_x rear surface passivated cells. Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

Polymer additive assisted crystallization of perovskite films for high-performance solar cells

Perovskite solar cells (PSCs) have attracted much attention as a novel photoelectric converter. The quality of perovskite films plays a key role in the efficiency and stability. Among them, defects in the films surface restrict the performance of solar cells. Surface passivation is an effective route to eliminate defect of perovskite films. In this paper, we introduce PVB as a novel polymer additive, it can assist perovskite films with better crystallinity and morphology, as well as less defects. Perovskite solar cells with 1.5 mg/mL optimized concentration PVB exhibit power conversion efficiency (PCE) of 19.04% than 16.34% of control cells. Meanwhile, the cells with PVB demonstrate less hysteresis than that without additive, as well as excellent reproducibility. Additionally, perovskite solar cells based on PVB can retain around 90% of its original efficiency under fully ambient air of 65 ± 5% relative humidity or under 65 °C after aging for 30 days. The finding provides a potential additive candidate for fabrication of higher performance devices.