Synergistic Effects of Cation and Anion in an Ionic Imidazolium Tetrafluoroborate Additive for Improving the Efficiency and Stability of Half-Mixed Pb-Sn Perovskite Solar Cells

Narrow-bandgap mixed Pb-Sn perovskite solar cells (PSCs) have great feasibility for constructing efficient all-perovskite tandem solar cells, in combination with wide-bandgap lead halide PSCs. However, the power conversion efficiency of mixed Pb-Sn PSCs still lags behind lead-based counterparts. Here, additive engineering using ionic imidazolium tetrafluoroborate (IMBF₄) is proposed, where the imidazolium (IM) cation and tetrafluoroborate (BF₄) anion efficiently passivate defects at grain boundaries and improve crystallinity, simultaneously relaxing lattice strain, respectively. Defect passivation is achieved by the chemical interaction between the IM cation and the positively charged under-coordinated Pb²⁺ or Sn²⁺ ions, and lattice strain relaxation is realized by lattice expansion with the intercalation of BF₄ anions into the perovskite lattice. As a result, the synergistic effects of the cation and anion in the IMBF₄ additive greatly enhance the optoelectronic performance of half-mixed Pb-Sn perovskites, leading to much longer carrier lifetimes. The best-performing half-mixed Pb-Sn PSC shows an efficiency above 19% with negligible hysteresis, while retaining over 90% of its initial efficiency after 1000 h in a nitrogen-filled glovebox and showing a lifetime to 80% degradation of 53.5 h under continuous illumination.     

Synergistic Crystallization Modulation and Defects passivation via Additive Engineering Stabilize Perovskite Films for Efficient Solar Cells

Organic-inorganic lead halide perovskite are promising photovoltaic materials, but their intrinsic defects and crystalline quality severely deteriorate the solar cells efficiency and stability. Herein, potassium 1,1,2,2,3,3-hexafluoroprop-ane-1,3-disulfonimide (KHFDF) is introduced into PbI₂ precursor solution to passivate various defects and improve the crystalline quality of perovskite films. It is found that KHFDF can inhibit PbI₂ crystallization, thus tuning the crystal orientation and growth of perovskite films. Furthermore, KHFDF with dual-functional sulfonyl group cannot only passivate grain boundaries (GBs), but also passivate the defects at GBs via strong interaction with undercoordinated Pb²⁺ and/or hydrogen bonding with FA⁺, while the K⁺ counter cations allow ionic interaction with undercoordinated I⁻. As a result, the KHFDF-modified films exhibit high quality with a larger grain size and a reduced trap-state density, thereby suppressing the trap-state nonradiative recombination. And the devices show a champion efficiency up to 24.15%, benefiting from a sharp enhancement of open-circuit voltage (V_oc) of 1.183 V and fill factor of 81.78%. In addition, due to the enhanced humidity tolerance and chemical structure stability, the devices exhibit excellent long-term humidity and thermal stability without encapsulation.     

Surface Reconstruction Engineering with Synergistic Effect of Mixed-Salt Passivation Treatment toward Efficient and Stable Perovskite Solar Cells

Surface passivation treatment is a widely used strategy to resolve trap-mediated nonradiative recombination toward high-efficiency metal-halide perovskite photovoltaics. However, a lack of passivation with mixture treatment has been investigated, as well as an in-depth understanding of its passivation mechanism. Here, a systematic study on a mixed-salt passivation strategy of formamidinium bromide (FABr) coupled with different F-substituted alkyl lengths of ammonium iodide is demonstrated. It is obtained better device performance with decreasing chain length of the F-substituted alkyl ammonium iodide in the presence of FABr. Moreover, they unraveled a synergistic passivation mechanism of the mixed-salt treatment through surface reconstruction engineering, where FABr dominates the reformation of the perovskite surface via reacting with the excess PbI₂. Meanwhile, ammonium iodide passivates the perovskite grain boundaries both on the surface and top perovskite bulk through penetration. This synergistic passivation engineer results in a high-quality perovskite surface with fewer defects and suppressed ion migration, leading to a champion efficiency of 23.5% with mixed-salt treatment. In addition, the introduction of the moisture resisted F-substituted groups presents a more hydrophobic perovskite surface, thus enabling the decorated devices with excellent long-term stability under a high humid atmosphere as well as operational conditions.     

Highly Efficient and Stable Wide-Bandgap Perovskite Solar Cells via Strain Management

Wide-bandgap (WBG) perovskite solar cells (PSCs) with high performance and stability are in considerable demand to boost tandem solar cell efficiencies. Perovskite bandgap broadening results in a high barrier for enhancing the efficiency of PSCs and phase segregation in perovskite. In this study, it is shown that the residual strain is the key factor affecting the WBG perovskite device efficiency and stability. The dimethyl sulfoxide addition helps lead halide with opening the layer spacing to form intermediate phases that provide more nucleation sites to eliminate lattice mismatch with organic components, which dominates the strain effects on the WBG perovskite growth in a sequential deposition. By minimizing the strain, 1.67 and 1.77 eV nip devices with record efficiencies of 22.28% and 20.45%, respectively, can be achieved. The greatly suppressed phase segregation enables the devices with retained 90–95% of initial efficiency over 4000 h of damp stability and 80–90% of initial efficiency over 700 h of maximum-power-point (MPP) stability. Besides, the 1.67 eV pin devices can achieve a competitive 22.3% efficiency with considerable damp-heat, pre-ultraviolet (pre-UV) aging and MPP tracking stability according to IEC 61215. The final efficiency of more than 28.3% for the perovskite/Si tandem is obtained.     

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.     

Spidermen Strategy for Stable 24% Efficiency Perovskite Solar Cells

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

Imide-Functionalized Triarylamine-Based Donor-Acceptor Polymers as Hole Transporting Layers for High-Performance Inverted Perovskite Solar Cells

Dopant-free hole-transporting layers (HTLs) are highly desired for realizing efficient and stable perovskite solar cells (PVSCs), but only very few of them can enable power conversion efficiencies (PCEs) over 20%. Herein, two imide-functionalized triarylamine-based donor-acceptor (D-A) type copolymers, PBTI-TPA and PTTI-TPA, are developed and applied as dopant-free HTLs in inverted PVSCs. The combination of a classic redox-active triphenylamine donor unit and an electron-withdrawing oligothiophene imide co-unit with rigid and planar backbone furnishes the two polymers with quasi-planar backbone, suitable frontier molecular orbital (FMO) energy levels, favorable thermal stability, appropriate film morphology, and passivation effect. More importantly, the greatly improved hole mobility renders them as promising HTLs for PVSCs. As a result, the undoped PTTI-TPA-based inverted PVSCs deliver a remarkable PCE up to 21% as well as negligible hysteresis and substantial long-term stability, outperforming the devices based on PBTI-TPA and PTAA. The performance also represents one of the highest PCEs reported to date for PVSCs based on dopant-free polymeric HTLs. The results highlight the great potentials of oligothiophene imides for constructing donor-acceptor polymeric HTLs for enabling high-performance dopant-free PVSCs.     

Dual Additive for Simultaneous Improvement of Photovoltaic Performance and Stability of Perovskite Solar Cell

Additive engineering is one of the most efficient approaches to improve not only photovoltaic performance but also phase stability of formamidinium (FA)-based perovskite. Chlorine-based additives, such as methylammonium chloride (MACl), have been in general used to improve phase stability of FAPbI₃, which however often leads to loss of open-circuit voltage V_oc, accompanied by instability of the perovskite phase due to the volatile nature of the MA cation. A dual additive strategy for improving V_oc and thereby the overall efficiency are reported here. The mixing ratio of MACl to CsCl is varied from [MACl]/[CsCl] = 4 to 1, where V_oc increases with decreasing the ratio and best performance is achieved from [MACl]/[CsCl] = 2. As compared to the single source of MACl, the addition of CsCl reduces trap density and increases resistance against charge recombination, which is responsible for the increased V_oc. Moreover, defect passivation achieved by dual additive enables better stability than the single additive MACl as confirmed by long-term stability tests with unencapsulated devices for 50 days under relative humidity of about 40% at room temperature. The best power conversion efficiency of 23.22% is achieved by dual additive, which is higher than that for single additive of MACl or CsCl.     

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.     

Multifunctional Organic Potassium Salt Additives as the Efficient Defect Passivator for High-Efficiency and Stable Perovskite Solar Cells

Despite the rapid developments are achieved for perovskite solar cells (PSCs), the existence of various defects in the devices still limits the further enhancement of the power conversion efficiency (PCE) and the long-term stability of devices. Herein, the efficient organic potassium salt (OPS) of para-halogenated phenyl trifluoroborates is presented as the precursor additives to improve the performance of PSCs. Studies have shown that the 4-chlorophenyltrifluoroborate potassium salt (4-ClPTFBK) exhibits the most effective interaction with the perovskite lattice. Strong coordination between  BF₃⁻/halogen in anion and uncoordinated Pb²⁺/halide vacancies, along with the hydrogen bond between F in  BF₃⁻ and H in FA⁺ are observed. Thus, due to the synergistic contribution of the potassium and anionic groups, the high-quality perovskite film with large grain size and low defect density is achieved. As a result, the optimal devices show an enhanced efficiency of 24.50%, much higher than that of the control device (22.63%). Furthermore, the unencapsulated devices present remarkable thermal and long-term stability, maintaining 86% of the initial PCE after thermal test at 80 °C for 1000 h and 95% after storage in the air for 2460 h.     

Pre-buried Interface Strategy for Stable Inverted Perovskite Solar Cells Based on Ordered Nucleation Crystallization

The buried interface has important effect on carrier extraction and nonradiative recombination of perovksite solar cells (PSCs). Herein, to inactivate the buried interfacial defects of perovskite and boost the crystallization quality of perovskite film, 3-amino-1-adamantanol (AAD) serves as a pre-buried interface modifier on nickel oxide (NiO_x) surface to regulate the nucleation and crystallization process of perovskite precursor. The amino and hydroxyl groups in AAD molecule can synchronously coordinate with nickel ion (Ni³⁺) in NiO_x and lead ion in perovskite, respectively. The dual action favors the ordered arrangement of AAD molecules between NiO_x and perovskite, which not only enhances hole extraction in hole transport layer, but also provides active sites for homogeneous nucleation. Furthermore, AAD modifier blocks the unfavorable reaction between Ni³⁺ and perovskite, and effectively passivates the buried interfacial defects. The optimal inverted PSCs achieve a champion power conversion efficiency of 22.21% with negligible hysteresis, favorable thermal, optical, and long-term stability. Thus, this strategy of modulating perovskite nucleation and crystallization by pre-buried modifier is feasible for achieving efficient and stable inverted perovskite solar cells.     

Tailoring the Functionality of Additives for Enhancing the Stability of Perovskite Solar Cells

In a two-step procedure for fabricating perovskite films, the PbI₂ layer formed on the substrate is converted to perovskite by reacting PbI₂ with organic iodide. Excess PbI₂ left after forming perovskite composition, however, might have an ill effect on device stability and current–voltage hysteresis, although it positively affects efficiency improvement.  Additive engineering is reported here on to control the residual PbI₂ in a two-step procedure. A series of organic multi-ammonium chloride derivatives are introduced into the PbI₂ precursor solution for the first-step coating, which results in an increase in the perovskite grain size. In addition, carrier lifetime is elongated due to the reduced trap density and the energetics are adjusted to facilitate the extraction of photogenerated carriers. The aminoguanidinium-containing precursor leads to an improved power conversion efficiency (PCE) as compared to the bare PbI₂ precursor mainly due to the significantly enhanced open-circuit voltage and fill factor. Consequently, a PCE of 23.46% is achieved from the hysteresis-less photovoltaic parameters and 93% of the initial PCE is maintained after aging for 1000 h in ambient conditions.     

The Trapped Charges at Grain Boundaries in Perovskite Solar Cells

The performance of perovskite solar cells is greatly affected by the crystallization of the perovskite active layer. Perovskite crystal grains should neatly arrange and penetrate the entire active layer for an ideal perovskite crystallization. These kinds of crystallized perovskite films exhibit fewer defects and longer carrier lifetime, which is beneficial to enhance the performance of perovskite solar cells. Here, by testing the residual charge of perovskite solar cells with different crystallization conditions, it is demonstrated that the residual charge exists widely at the grain boundary, which is parallel to the device, and the residual charge is related to the performance of the perovskite solar cells. Single crystal grains neatly arranged and penetrate the entire active layer can generate less residual charge and improve device performance of the perovskite solar cells. The results also show that the long decay time of open-circuit voltage comes from the detrapping of trapped carriers. The residual charge testing technology provides a new idea for the investigation of carrier trap and detrap characteristics in photovoltaic devices.     

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.     

Phenylhydrazinium Iodide for Surface Passivation and Defects Suppression in Perovskite Solar Cells

In recent years, hybrid perovskite solar cells (HPSCs) have received considerable research attention due to their impressive photovoltaic performance and low‐temperature solution processing capability. However, there remain challenges related to defect passivation and enhancing the charge carrier dynamics of the perovskites, to further increase the power conversion efficiency of HPSCs. In this work, the use of a novel material, phenylhydrazinium iodide (PHAI), as an additive in MAPbI₃ perovskite for defect minimization and enhancement of the charge carrier dynamics of inverted HPSCs is reported. Incorporation of the PHAI in perovskite precursor solution facilitates controlled crystallization, higher carrier lifetime, as well as less recombination. In addition, PHAI additive treated HPSCs exhibit lower density of filled trap states (10¹⁰ cm^?2) in perovskite grain boundaries, higher charge carrier mobility (≈11 × 10^?4 cm² V^?1 s), and enhanced power conversion efficiency (≈18%) that corresponds to a ≈20% improvement in comparison to the pristine devices.     

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.     

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.     

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

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

Carbon Nanotube Based Inverted Flexible Perovskite Solar Cells with All‐Inorganic Charge Contacts

Organolead halide perovskite solar cells (PSC) are arising as promising candidates for next‐generation renewable energy conversion devices. Currently, inverted PSCs typically employ expensive organic semiconductor as electron transport material and thermally deposited metal as cathode (such as Ag, Au, or Al), which are incompatible with their large‐scale production. Moreover, the use of metal cathode also limits the long‐term device stability under normal operation conditions. Herein, a novel inverted PSC employs a SnO₂‐coated carbon nanotube (SnO₂@CSCNT) film as cathode in both rigid and flexible substrates (substrate/NiO‐perovskite/Al₂O₃‐perovskite/SnO₂@CSCNT‐perovskite). Inverted PSCs with SnO₂@CSCNT cathode exhibit considerable enhancement in photovoltaic performance in comparison with the devices without SnO₂ coating owing to the significantly reduced charge recombination. As a result, a power conversion efficiency of 14.3% can be obtained on rigid substrates while the flexible ones achieve 10.5% efficiency. More importantly, SnO₂@CSCNT‐based inverted PSCs exhibit significantly improved stability compared to the standard inverted devices made with silver cathode, retaining over 88% of their original efficiencies after 550 h of full light soaking or thermal stress. The results indicate that SnO₂@CSCNT is a promising cathode material for long‐term device operation and pave the way toward realistic commercialization of flexible PSCs.