JIALINGBO

Stabilizing high-efficiency perovskite solar cells (PSCs) at operating conditions remains an unresolved issue hampering its large-scale commercial deployment. Here, we report a star-shaped polymer to improve charge transport and inhibit ion migration at the perovskite interface. The incorporation of multiple chemical anchor sites in the star-shaped polymer branches strongly controls the crystallization of perovskite film with lower trap density and higher carrier mobility and thus inhibits the nonradiative recombination and reduces the charge-transport loss. Consequently, the modified inverted PSCs show an optimal power conversion efficiency of 22.1% and a very high fill factor (FF) of 0.862, corresponding to 95.4% of the Shockley-Queisser limited FF (0.904) of PSCs with a 1.59-eV bandgap. The modified devices exhibit excellent long-term operational and thermal stability at the maximum power point for 1000 hours at 45°C under continuous one-sun illumination without any significant loss of efficiency.

A full-spectrum responsive donor–acceptor supramolecular photocatalyst TPPS/C₆₀ is successfully constructed. The theoretical spectral efficiency of TPPS/C₆₀ is as high as 70%. The TPPS/C₆₀ performs with a highly efficient photocatalytic H₂ evolution rate of 34.57 mmol g⁻¹ h⁻¹, surpassing many reported organic photocatalysts. The D–A structure and the giant internal electric field dramatically promote the charge separation.

Abstract

A full-spectrum (300–850 nm) responsive donor–acceptor (D–A) supramolecular photocatalyst tetraphenylporphinesulfonate/fullerene (TPPS/C₆₀) is successfully constructed. The theoretical spectral efficiency of TPPS/C₆₀ is as high as 70%, offering the possibility of full-solar-spectrum light harvesting. The TPPS/C₆₀ performs a highly efficient photocatalytic H₂ evolution rate of 276.55 µmol h⁻¹ (34.57 mmol g⁻¹ h⁻¹), surpassing many reported organic photocatalysts. The D–A structure effectively promotes electron transfer from TPPS to C₆₀, which is beneficial to the photocatalytic reaction. Specifically, a giant internal electric field in the D–A structure is built via the enhanced molecular dipole, which dramatically promotes the charge separation (CS) efficiency by 2.35 times. Transient absorption spectra results show a long-lived CS state TPPS^•+–C₆₀ ^•− in the D–A structure, which effectively promotes participation of photogenerated electrons in the reduction reaction. Briefly, this work provides a novel approach for designing high-performance photocatalytic materials via enhancing the interfacial electric field.

The catastrophic failure of n-i-p type perovskite solar cells under operation is reported, which is proven by the corrosion of the metal electrode on the edge. After inserting a thin MoO₃, the improved Ag thin film morphology as well as better energy alignment suppress the catastrophic failure of perovskite solar cells.

Abstract

The n-i-p type perovskite solar cells suffer unpredictable catastrophic failure under operation, which is a barrier for their commercialization. The fluorescence enhancement at Ag electrode edge and performance recovery after cutting the Ag electrode edge off prove that the shunting position is mainly located at the edge of device. Surface morphology and elemental analyses prove the corrosion of the Ag electrode and the diffusion of Ag⁺ ions on the edge for aged cells. Moreover, much condensed and larger Ag clusters are formed on the MoO₃ layer. Such a contrast is also observed while comparing the central and the edge of the Ag/Spiro-OMeTAD film. Hence, the catastrophic failure mechanism can be concluded as photon-induced decomposition of the perovskite film and release reactive iodide species, which diffuse and react with the loose Ag clusters on the edge of the cell. The corrosion of the Ag electrode and the migration of Ag⁺ ions into Spiro-OMeTAD and perovskite films lead to the forming of conducting filament that shunts the cell. The more condensed Ag cluster on the MoO₃ surface as well as the blocking of holes within the Spiro-OMeTAD/MoO₃ interface successfully prevent the oxidation of Ag electrode and suppress the catastrophic failure.

A novel, rapid, and scalable treatment method that significantly improves perovskite thin film crystallinity is introduced. Treated perovskite solar cells (PSCs) exhibit enhanced efficiencies, increased stabilities, and improved reproducibility. Versatility and universality are demonstrated using: CH₃NH₃PbI₃ (MAPbI₃) PSCs with thicknesses from 500–1300 nm; large-area (>1 cm²) devices; a range of device architectures and compositions including Cs_0.1FA_0.9Pb(I_0.95Br_0.05) devices.

Abstract

Metal-halide perovskite solar cells (PSCs) have had a transformative impact on the renewable energy landscape since they were first demonstrated just over a decade ago. Outstanding improvements in performance have been demonstrated through structural, compositional, and morphological control of devices, with commercialization now being a reality. Here the authors present an aerosol assisted solvent treatment as a universal method to obtain performance and stability enhancements in PSCs, demonstrating their methodology as a convenient, scalable, and reproducible post-deposition treatment for PSCs. Their results identify improvements in crystallinity and grain size, accompanied by a narrowing in grain size distribution as the underlying physical changes that drive reductions of electronic and ionic defects. These changes lead to prolonged charge-carrier lifetimes and ultimately increased device efficiencies. The versatility of the process is demonstrated for PSCs with thick (>1 µm) active layers, large-areas (>1 cm²) and a variety of device architectures and active layer compositions. This simple post-deposition process is widely transferable across the field of perovskites, thereby improving the future design principles of these materials to develop large-area, stable, and efficient PSCs.

Inverted wide-bandgap (Eg = 1.67 eV) perovskite solar cells (PSCs) with 2,3,5,6 tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) incorporation not only reduce the trap states at the band edges and the nonradiative recombination, but also deliver a power conversion efficiency (PCE) of 20%. The unencapsulated devices maintain 88% of their peak PCE under continuous illumination in a nitrogen atmosphere after 840 h.

Trap-induced nonradiative recombination and decomposition are the major limiting factors that hinder the development of mixed-halide wide-bandgap perovskite solar cells. Specifically, the incorporation of formamidinium (FA⁺) and bromide in wide-bandgap (WBG) perovskite materials leads to shallow-energy-level traps and inferior light stability. Herein, the electron-withdrawing molecule 2,3,5,6 tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is used as an effective passivator and stabilizer, which has molecular interaction with the FA⁺ cation for reducing trap densities and enhancing the stability of WBG perovskite solar cells. It is found that the extended π aromatic system in F4-TCNQ can enhance the binding energy and stabilize the FA⁺ cation in perovskite films. Furthermore, the 1.67 eV bandgap inverted perovskite solar cells with a small amount of F4-TCNQ have shallower defect levels, reduced trap density, and decreased nonradiative recombination, therefore giving a remarkably improved power conversion efficiency (PCE) of 20.0%. Most importantly, the unencapsulated devices with F4-TCNQ additive have greatly enhanced stability, maintaining 88% of their peak PCEs under continuous illumination after 840 h, whereas the control devices only retain 48% of their PCEs after 500 h.

The synergistic effect of Li:Cu codoping in various NiO _x hole transporting layers and perovskite photovoltaics were systematically investigated. Codoped NiO _x films exhibit enhanced optical and electrical properties. Codoped NiO _x layers induce high-quality perovskite films, minimizing recombination losses in solar cells. In consequence, the Li:Cu(8:2)-based device exhibited a superior power conversion efficiency of 19.46% in comparison with the pristine device.

Inverted perovskite solar cells (PSCs), which feature an attractive structure for diverse applications such as tandem SCs or flexible devices, continue to be rapidly developed. Among various hole-transporting layer (HTL) materials, nickel oxide (NiO _x ) is widely used as a stable and superior HTL even though it exhibits poor conductivity. Although various methods have been proposed to overcome the low conductivity of NiO _x films, codoping methods have not been extensively studied and remain poorly understood. Herein, Li:Cu:NiO _x is systematically investigated to explore the synergistic effect of codoping in various NiO _x HTLs and PSCs. The optical, chemical, and morphological properties of the films are characterized and the dependence of these properties on the codoping ratio are investigated. A gradual improvement of the electrical properties and a tunable Fermi energy level resulting from the Li:Cu codopants is subsequently demonstrated. Furthermore, the structure of the perovskite films on HTLs and the synergistic effect on the preferred crystal growth behavior are elucidated. An inverted PSC with high efficiency was attained as a result of the enhanced electrical properties of the NiO _x HTLs and the high quality of the perovskite film, which were attributed to the synergistic effect of the Li:Cu codoping method.

Publication date: October 2021

Source: Nano Energy, Volume 88

Author(s): Yousheng Wang, Hui Ju, Tahmineh Mahmoudi, Chong Liu, Cuiling Zhang, Shaohang Wu, Yuzhao Yang, Zhen Wang, Jinlong Hu, Ye Cao, Fei Guo, Yoon-Bong Hahn, Yaohua Mai

The synergistic strategy of tuning the solution coordination and crystallization process by introducing ionic liquid is implemented to successfully fabricate pinhole-free tin perovskite films with preferential crystal orientation, which possess improved oxidation repellency for Sn(II) and enhanced hydrophobicity. As a result, the stabilization of high-efficiency lead-free tin halide perovskite solar cells is achieved.

Abstract

Tin halide perovskites attract incremental attention to deliver lead-free perovskite solar cells. Nevertheless, disordered crystal growth and low defect formation energy, related to Sn(II) oxidation to Sn(IV), limit the efficiency and stability of solar cells. Engineering the processing from perovskite precursor solution preparation to film crystallization is crucial to tackle these issues and enable the full photovoltaic potential of tin halide perovskites. Herein, the ionic liquid n-butylammonium acetate (BAAc) is used to tune the tin coordination with specific O…Sn chelating bonds and NH…X hydrogen bonds. The coordination between BAAc and tin enables modulation of the crystallization of the perovskite in a thin film. The resulting BAAc-containing perovskite films are more compact and have a preferential crystal orientation. Moreover, a lower amount of Sn(IV) and related chemical defects are found for the BAAc-containing perovskites. Tin halide perovskite solar cells processed with BAAc show a power conversion efficiency of over 10%. This value is retained after storing the devices for over 1000 h in nitrogen. This work paves the way toward a more controlled tin-based perovskite crystallization for stable and efficient lead-free perovskite photovoltaics.

Modifying the interface between SnO₂ and perovskite by the insertion of a MXene/nanotubes interfacial layer resulted in a remarkable power conversion efficiency of up to 21.42%, which paves the way for employing MXene/nanotubes hybrid structures in next-generation electronic devices.

Abstract

Incorporation of 2D MXenes into the electron transporting layer (ETL) of perovskite solar cells (PSCs) has been shown to deliver high-efficiency photovoltaic (PV) devices. However, the ambient fabrication of the ETLs leads to unavoidable deterioration in the electrical properties of MXene due to oxidation. Herein, sorted metallic single-walled carbon nanotubes (m-SWCNTs) are employed to prepare MXene/SWCNTs composites to improve the PV performance of PSCs. With the optimized composition, a power conversion efficiency of over 21% is achieved. The improved photoluminescence and reduced charge transfer resistance revealed by electrochemical impedance spectroscopy demonstrated low trap density and improved charge extraction and transport characteristics due to the improved conductivity originating from the presence of nanotubes as well as the reduced defects associated with oxygen vacancies on the surface of the SnO₂. The MXene/SWCNTs strategy reported here provides a new avenue for realizing high-performance PSCs.

The MA-free organic-inorganic hybrid perovskite (FAPbI₃) have drawn intense attention. The imidazole bromide functionalized graphene quantum dots is introduced to regulate the interface between SnO₂ layer and FAPbI₃ perovskite layer. The resulting reduced interface defects, better energy level alignment, and better perovskite film achieve a high efficiency of 22.37% with enhanced long-term stability.

Abstract

Organic–inorganic hybrid perovskites have reached an unprecedented high efficiency in photovoltaic applications, which makes the commercialization of perovskite solar cells (PSCs) possible. In the past several years, particular attention has been paid to the stability of PSC devices, which is a critical issue for becoming a practical photovoltaic technology. In particular, the interface-induced degradation of perovskites should be the dominant factor causing poor stability. Here, imidazole bromide functionalized graphene quantum dots (I-GQDs) are demonstrated to regulate the interface between the electron transport layer (ETL) and formamidinium lead iodide (FAPbI₃) perovskite layer. The incorporation of I-GQDs not only reduces the interface defects for achieving a better energy level alignment between ETL and perovskite, but also improves the film quality of FAPbI₃ perovskite including enlarged grain size, lower trap density, and a longer carrier lifetime. Consequently, the planar FAPbI₃ PSCs with I-GQDs regulation achieve a high efficiency of 22.37% with enhanced long-term stability.

Herein, defect inhibition in two-step solution-processed (FAPbI₃)_1−x (MAPbBr₃) _x films via a zwitterionic ionic liquid (ZIL) with 4-fluoro-phenylammonium (4FB⁺) as cations and tetrafluoroborate (BF₄ ⁻) as anions is demonstrated. The rationally designed ZIL with 4-fluoro-phenylammonium as doping cation and tetrafluoroborate as pseudohalogen anion could achieve effective defect suppression, enabling perovskite solar cells to exceed 22% efficiency with superior stability.

Defect passivation has been a promising route for enhancing the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, defect inhibition in two-step solution-processed (FAPbI₃)_1−x (MAPbBr₃) _x films via a rationally designed zwitterionic ionic liquid (ZIL) with 4-fluoro-phenylammonium (4FB⁺) as cations and tetrafluoroborate (BF₄ ⁻) as anions is demonstrated. First, 4FB⁺ and BF₄ ⁻ can effectively fill vacancy defects caused by the migrated organic A-site cation and halogen anion, confirmed by X-ray photoelectron spectroscopy. Second, the 4FB⁺ with π conjugated benzene ring can donate electrons for carrier extraction, whereas its fluorination of the phenyl ammonium could enhance moisture blocking through the molecular packing of CF bond. The electrical characterization, including space charge limited current and Mott–Schottky measurement, proves the enhanced carrier extraction and photovoltaic performance. Third, the pseudohalogen anion BF₄ ⁻ with high ionic conductivity could significantly enhance the carrier lifetime and reduce the V _OC loss. As a result, the ZIL-modified PSCs can achieve a high PCE of 22.5% with excellent long-term stability maintaining more than 80% of the initial efficiency after storing in an ambient condition for 2000 h. Herein, a new paradigm toward accelerating the development of efficient and stable PSCs is opened up.

A domain control strategy is employed to optimize the crystallization behavior of quasi-2D perovskite films and a narrower n domains distribution is achieved by the synergistic effect of ZrO₂ nanoparticles and Cryptand compound additive. By suppressing the nonradiative recombination related to the higher/lower-n domains in perovskite layer, the external quantum efficiency of perovskite light-emitting diodes is improved to 21.2%.

Abstract

Quasi-2D perovskites with enlarged exciton binding energy and tunable bandgap are appealing for application in perovskite light-emitting diodes (PeLEDs). However, wide n domains distribution is commonly formed in solution-processed quasi-2D perovskite films due to the uncontrollable crystallization behavior, which leads to low device performance. Here, the crystallization process is successfully regulated to narrow the n domains distribution by introducing compound additive of ZrO₂ nanoparticles (NPs) and Cryptand complexant. ZrO₂ NPs can avoid the segregation of organic large and small cations by strengthening the solvent extraction capacity of antisolvent, while Cryptand offsets the poor solubility of PbBr₂ by forming an intermediate state to slow down the crystallization of high-n domains. Consequently, both high photoluminescence quantum yields over 90% and a high external quantum efficiency of 21.2% are obtained in the optimized green quasi-2D PeLEDs. Moreover, the lifetime extends about four times compared with control devices. The strategy of domain controlling by compound additive provides a powerful way to develop high-performance quasi-2D perovskite optoelectrical devices.

One of the challenges in perovskite solar cells is passivating the perovskite surface without hindering charge extraction. In this work, a conjugated ligand is introduced to the interface between perovskite and hole-transporting layer, showing efficient hole extraction with improved energy level alignment and suppressed phase segregation. Therefore, devices with high efficiency and stability are achieved.

Abstract

Surface passivation is an effective way to boost the efficiency and stability of perovskite solar cells (PSCs). However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination without imposing energy barriers to charge extraction. Here, a novel multifunctional semiconducting organic ammonium cationic interface modifier inserted between the light-harvesting perovskite film and the hole-transporting layer is reported. It is shown that the conjugated cations can directly extract holes from perovskite efficiently, and simultaneously reduce interface non-radiative recombination. Together with improved energy level alignment and the stabilized interface in the device, a triple-cation mixed-halide medium-bandgap PSC with an excellent power conversion efficiency of 22.06% (improved from 19.94%) and suppressed ion migration and halide phase segregation, which lead to a long-term operational stability, is demonstrated. This strategy provides a new practical method of interface engineering in PSCs toward improved efficiency and stability.

Quantitative evaluation of interfacial crystals qualities in perovskite solar cells helps the understanding of charge recombination and their effects on the relationship between the ideality factor/photoluminescence and open-circuit voltage. Furthermore, this simple strategy provides the information of which defect engineering will be more effective in solar cells.

Abstract

The ideality factor (n _id) and photoluminescence (PL) analyses assess charge recombination characteristics in perovskite solar cells (PeSCs). However, their correlations with open-circuit voltage (V _oc) are often found to be complicated depending on the recombination types in the devices. Herein, the correlation of n _id, PL characteristics and V _oc is elucidated depending on the interfacial crystal quality in triple-cation mixed-halide perovskite, Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃, deposited on different hole transport layers (HTLs). In the devices with low quality interfacial crystals, V _oc increases together with n _id, which originates from the light intensity-dependence of majority carrier at the interface. Meanwhile, a negative correlation between V _oc and n _id is observed for devices with high quality interfacial crystals. The authors discuss the cases that PL enhancement by the improvement of overall crystal quality can fail to correlate with a V _oc increase if interfacial crystal quality becomes worse. The study highlights that interfacial crystal quality evaluation can help to understand charge recombination via n _id and PL measurements, and more importantly provide information of which defect engineering between at the interface and in the bulk would be more effective for device optimization.

A linear organic cation 1,3-propanediammonium iodide (PDAI₂) is used to passivate the surface defects of perovskite film and suppress the nonradiative recombination, which is beneficial for reducing the open-circuit voltage ( V OC ) deficit of perovskite solar cells. Consequently, the narrow-bandgap Pb–Sn-alloyed perovskite solar cell achieves a high power conversion efficiency of 20.2% with a small V OC deficit of 0.39 V.

The power conversion efficiency (PCE) of narrow-bandgap Pb–Sn-alloyed perovskite solar cells (PVSCs) is seriously impeded by the large open-circuit voltage ( V OC ) deficit. Finding an effective approach to passivate defects in the perovskite film is critical to reduce the V OC deficit. Herein, a linear organic cation 1,3-propanediammonium iodide (PDAI₂) is used to passivate the surface defects of perovskite film, thus restraining the nonradiative recombination. After treating with PDAI₂, the defect density of perovskite film is decreased to half and the carrier lifetime is prolonged more than 1.5 times. As a result, the champion Pb–Sn-alloyed PVSC based on PDAI₂ treatment exhibits a small V OC deficit of 0.39 V, and a high PCE of 20.2%.

NiO _x is used as the hole transport layer with the synergistic effect of poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyfluorene)] (PFN) to shift the valence band downward, leading to reduced open circuit voltage (V _oc) loss and improved efficiency. The champion perovskite solar cell has a remarkable V _oc of 0.88 V, surpassing the previous results reported for NiO _x -based Sn-Pb PSCs.

Tin–lead (Sn–Pb) binary low-bandgap perovskites are more environmentally friendly than conventional Pb-based perovskites and promise to deliver high photovoltaic performance by constructing tandem solar cells. However, the energy-level mismatch between functional layers and tremendous trap states in perovskite films make it challenging to reduce the high open-circuit voltage (V _oc) loss in Sn–Pb binary perovskite solar cells (PSCs). Herein, energy loss reduction at the hole collection interface in Sn–Pb binary PSCs is demonstrated using nickel oxide (NiO _x ) as the hole transport material (HTM) with optimal poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyfluorene)] (PFN) modification, which enables a significantly enhanced V _oc compared to the traditional poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)-based devices. The NiO _x /PFN bilayer has a downward-shifted valence band compared to PEDOT:PSS, providing well-matched energy-level alignment with the perovskite material, resulting in more fluent charge transfer and reduced V _oc losses. The optimized device has a high V _oc of 0.88 V and an efficiency of 19.80%, surpassing the previous results reported for NiO _x -based Sn–Pb PSCs. Moreover, the robust NiO _x /PFN substrate and the high-quality perovskite film grown on it make the device less vulnerable to ambient exposure. This work highlights the significance of ideal hole conductors and interface engineering in efficient and stable Sn–Pb low-bandgap PSCs.

The formation of lead iodide sol–gel intermediate phase would intrude on orientation by in situ synchrotron-based grazing incident X-ray diffraction measurement. The incorporation of methylammonium thiocyanate inhibits the formation of the intermediate phase in the quasi-2D precursor. Accordingly, the vertical-oriented perovskite films with reduced trap density and higher carrier mobility are obtained and yield an efficiency of 15.2%.

Abstract

Quasi-2D perovskites are enchanting alternative materials for solar cells due to their intrinsic stability. The manipulation of crystal orientation of quasi-2D perovskites is indispensable to target efficient devices, however, the origin of orientation during the film fabrication process still lacks in-depth understanding and convincing evidence yet, which hinders further boosting the performance of photovoltaic devices. Herein, the crystallizing processes during spin-coating and annealing are probed by in situ grazing-incidence wide-angle X-ray scattering (GIWAXS), and the incident-angle-dependent GIWAXS is conducted to unveil the phase distribution in the films. It is found that undesirable lead iodide sol–gel formed intermediate phase would disturb oriented crystalline growth, resulting in random crystal orientation in poor quasi-2D films. A general strategy is developed via simple additive agent incorporation to suppress the formation of the intermediate phase. Accordingly, highly oriented perovskite films with reduced trap density and higher carrier mobility are obtained, which enables the demonstration of optimized quasi-2D perovskite solar cells with a power conversion efficiency of 15.2% as well as improved stability. This work paves a promising way to manipulate the quasi-2D perovskites nucleation and crystallization processes via tuning nucleation stage.

Here, the effect of Al-CuS is investigated as the hole-transport layer (HTL) for perovskite solar cells. It is shown that the sulfur in the HTL can interact with the perovskite film and lead to the better interface crystallization of perovskite. Due to this improved interface quality, sulfide-HTL-based devices outperform the oxide-HTL-based devices in terms of stability.

Abstract

Inorganic hole-transport layers (HTLs) are widely investigated in perovskite solar cells (PSCs) due to their superior stability compared to the organic HTLs. However, in p–i–n architecture when these inorganic HTLs are deposited before the perovskite, it forms a suboptimal interface quality for the crystallization of perovskite, which reduces device stability, causes recombination, and limits the power conversion efficiency of the device. The incorporation of an appropriate functional group such as sulfur-terminated surface on the HTL can enhance the interface quality due to its interaction with perovskite during the crystallization process. In this work, a bifunctional Al-doped CuS film is wet-deposited as HTL in p–i–n architecture PSC, which besides acting as an HTL also improves the crystallization of perovskite at the interface. Urbach energy and light intensity versus open-circuit voltage characterization suggest the formation of a better-quality interface in the sulfide HTL–perovskite heterojunction. The degradation behavior of the sulfide-HTL-based perovskite devices is studied, where it can be observed that after 2 weeks of storage in a controlled environment, the devices retain close to 95% of their initial efficiency.

Herein, the contruction of tunable graphitic carbon nitride (g–C₃N₄) into perovskite is introduced. g–C₃N₄ is obtained by thermal polymerization from different precursors (dicyandiamide, cyanamide, and urea), which act as a template for modifying the perovskite nucleation rate. Urea-derived g–C₃N₄ reduces the defects of the perovskite layer, improving the stability and boosting the efficiency up to 20.03%.

Organic–inorganic hybrid perovskite solar cell (PSC) demonstrates outstanding photovoltaic characteristics. However, the instability under high temperature and high relative humidity remains an obstacle that needs to be overcome. Furthermore, the rapid growth of perovskite crystals causes a lot of defect formation in the perovskite active layer, leading to insufficient stability. Two-dimensional g–C₃N₄ is a typical lewis base, providing electron pairs for bonding and is suitable as a template for controlling the nucleation of the perovskite active layer. Herein, tunable g–C₃N₄ by calcining the precursors (cyanamide, dicyandiamide, and urea) at different conditions to control the grain size and to passivate perovskite crystal is constructed. The Kelvin probe force microscopy study reveals that the successful defect passivation by urea-polymerized g–C₃N₄ (UCN) at grain boundaries could induce the suppression of non-radiative recombination. Adding the least polymerized UCN into a perovskite film allows for uniform surface morphology and a reduced trap density. UCN coordinated with the perovskite provides an efficient path for electron injection to the electron transport layer. The unencapsulated PSCs with UCN additive achieve a maximum power conversion efficiency of 20.03% and retain ≈93% of their initial efficiency values after aging for 960 h at 25 °C and relative humidity of 30%.

Flexible perovskite solar cells deliver a 21.1% power conversion efficiency (certified 20.5%) with good bending resistance and long-term stability via creating a dual-functional 2D perovskite layer at the interface.

Abstract

Flexible perovskite solar cells (f-PSCs) have been attracting tremendous attention due to their potentially commercial prospects in flexible energy system and mobile energy system. Reducing the energy barriers and charge extraction losses at the interfaces between perovskite and charge transport layers is essential to improve both efficiency and stability of f-PSCs. Herein, 4-trifluoromethylphenylethylamine iodide (CF₃PEAI) is introduced to form a 2D perovskite at the interface between perovskite and hole transport layer (HTL). It is found that the 2D perovskite plays a dual-functional role in aligning energy band between perovskite and HTL and passivating the traps in the 3D perovskite, thus reducing energy loss and charge carrier recombination at the interface, facilitating the hole transfer from perovskite to the Spiro-OMeTAD. Consequently, the photovoltaic performance of f-PSCs is significantly improved, leading to a power conversion efficiency (PCE) of 21.1% and a certified PCE of 20.5%. Furthermore, the long-term stability of f-PSCs is greatly improved through the protection of 2D perovskite layer to the underlying 3D perovskite. This work provides an excellent strategy to produce efficient and stable f-PSCs, which will accelerate their potential applications.

Polyaromatic passivator 4-hydroxybiphenyl substituted naphthalene-1,8-dicarboximide provides chemical passivation (protonic/Lewis-base groups system) and energetic passivation (creating benign midgap states) effects. The Lewis-base/polyaromatic conjugation/protonic system reduces defects efficiently and avoids superoxide anions in perovskite solar cells.

Abstract

As game-changers in the photovoltaic community, perovskite solar cells are making unprecedented progress while still facing grand challenges such as improving lifetime without impairing efficiency. Herein, two structurally alike polyaromatic molecules based on naphthalene-1,8-dicarboximide (NMI) and perylene-3,4-dicarboximide (PMI) with different molecular dipoles are applied to tackle this issue. Contrasting the electronically pull–pull cyanide-substituted PMI (9CN-PMI) with only Lewis-base groups, the push–pull 4-hydroxybiphenyl-substituted NMI (4OH-NMI) with both protonic and Lewis-base groups can provide better chemical passivation for both shallow- and deep-level defects. Moreover, combined theoretical and experimental studies show that the 4OH-NMI can bind more firmly with perovskite and the polyaromatic backbones create benign midgap states in the excited perovskite to suppress the damage by superoxide anions (energetic passivation). The polar and protonic nature of 4OH-NMI facilitates band alignment and regulates the viscosity of the precursor solution for thicker perovskite films with better morphology. Consequently, the 4OH-NMI-passivated perovskite films exhibit reduced grain boundaries and nearly three-times lower defect density, boosting the device efficiency to 23.7%. A more effective design of the passivator for perovskites with multi-passivation mechanisms is provided in this study.