Combining Efficiency and Stability in Mixed Tin-Lead Perovskite Solar Cells by Capping Grains with an Ultrathin 2D Layer

Ammonium Additive Engineering in Antisolvents for Improving Perovskite/Charge-Transport-Layer Interfaces toward Efficient Lead-Tin Alloyed Perovskite Solar Cells

Although significant advancements have been achieved in lead-tin (Pb-Sn) alloyed perovskite solar cells (PSCs), their power conversion efficiency (PCE) remains inferior to that of their Pb-based counterparts, primarily due to higher open-circuit voltage (Voc) losses and lower fill factors (FFs). Herein, we report both perovskite top and bottom interfacial improvements by incorporating a facile fluorophenylethylammonium iodide (p-FPEAI)/ethyl acetate (EA) solution during the film crystal growth. Based on the analysis of perovskite crystallization, film growth, and strain relaxation, the mechanisms behind these interfacial improvements have been well understood. Furthermore, p-FPEAI could reduce the defect density and nonradiative recombination losses, thus attributing to the improved Voc and FF. Finally, the treated device achieved a PCE of 20.14% with a Voc of up to 0.84 V, which is among the highest reported values so far for Pb-Sn alloyed PSCs without additional precursor additives. In addition, the unencapsulated p-FPEAI-treated device maintained its initial efficiency of approximately 92% after being kept in a nitrogen atmosphere for 1 month, in contrast to the control device which retained only 30% of its initial value. Our findings provide a comprehension for understanding the effect of bulky cations as antisolvents on fabricating highly efficient Pb-Sn alloyed perovskite solar cells.

Efficient and Stable Tin-Lead Mixed Perovskite Solar Cells Using Post-Treatment Additive with Synergistic Effects

Inhibiting the oxidation of Sn2+ during the crystallization process of Sn-Pb mixed perovskite film is found to be as important as the oxidation resistance of precursor solution to achieve high efficiency, but less investigated. Considering the excellent reduction feature of hydroquinone and the hydrophobicity of tert-butyl group, an antioxidant 2,5-di-tert-butylhydroquinone (DBHQ) was introduced into Sn-Pb mixed perovskite films using an anti-solvent approach to solve this problem. Interestingly, we find that DBHQ can act as function alterable additive during its utilization. On the one hand, DBHQ can restrict the oxidation of Sn2+ during the crystallization process, facilitating the fabrication of high-quality perovskite film; on the other hand, the generated oxidation product 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) can functionalize as defect passivator to inhibit the charge recombination. As a result, this synergetic effect renders the Sn-Pb mixed PSC a power conversion efficiency (PCE) up to 23.0 %, which is significantly higher than the reference device (19.6 %). Furthermore, the unencapsulated DBQH-modified PSCs exhibited excellent long-term stability and thermal stability, with the devices maintaining 84.2 % and 78.9 % of the initial PCEs after aging at 25 °C and 60 °C for 800 h and 120 h under N2 atmosphere, respectively. Therefore, the functional alterable strategy provides a novel cornerstone for high-performance Sn-Pb mixed PSCs.

Minimizing Interfacial Recombination in 1.8 eV Triple-Halide Perovskites for 27.5% Efficient All-Perovskite Tandems

All-perovskite tandem solar cells show great potential to enable the highest performance at reasonable costs for a viable market entry in the near future. In particular, wide-bandgap (WBG) perovskites with higher open-circuit voltage (VOC ) are essential to further improve the tandem solar cells' performance. Here, a new 1.8 eV bandgap triple-halide perovskite composition in conjunction with a piperazinium iodide (PI) surface treatment is developed. With structural analysis, it is found that the PI modifies the surface through a reduction of excess lead iodide in the perovskite and additionally penetrates the bulk. Constant light-induced magneto-transport measurements are applied to separately resolve charge carrier properties of electrons and holes. These measurements reveal a reduced deep trap state density, and improved steady-state carrier lifetime (factor 2.6) and diffusion lengths (factor 1.6). As a result, WBG PSCs achieve 1.36 V VOC , reaching 90% of the radiative limit. Combined with a 1.26 eV narrow bandgap (NBG) perovskite with a rubidium iodide additive, this enables a tandem cell with a certified scan efficiency of 27.5%.

Recent Advances in Interface Engineering for Enhanced Open-Circuit Voltage Regulation in Perovskite Solar Cells

In recent years, perovskite solar cells (PSCs) have attracted significant attention due to their excellent photoelectric properties. However, several key performance parameters of these devices still fall short of their theoretical limits. Among these parameters, the regulation of open-circuit voltage (VOC ) has been a focal point of intensive research efforts, playing a pivotal role in advancing the efficiency of PSCs. This review first provides an overview of the generation and loss mechanism of VOC . It then discusses the significance of interface engineering in VOC regulation. Recent developments in high-efficiency PSCs realized via interface engineering have been summarized and categorized into three key areas: surface modification, interface structure optimization, and surface dimensional engineering. Finally, a comprehensive summary of past research in this domain and offered insights into the future prospects of enhancing VOC in PSCs is provided.

All-perovskite tandem solar cells with 3D/3D bilayer perovskite heterojunction

All-perovskite tandem solar cells promise higher power-conversion efficiency (PCE) than single-junction perovskite solar cells (PSCs) while maintaining a low fabrication cost1-3. However, their performance is still largely constrained by the subpar performance of mixed lead-tin (Pb-Sn) narrow-bandgap (NBG) perovskite subcells, mainly because of a high trap density on the perovskite film surface4-6. Although heterojunctions with intermixed 2D/3D perovskites could reduce surface recombination, this common strategy induces transport losses and thereby limits device fill factors (FFs)7-9. Here we develop an immiscible 3D/3D bilayer perovskite heterojunction (PHJ) with type II band structure at the Pb-Sn perovskite-electron-transport layer (ETL) interface to suppress the interfacial non-radiative recombination and facilitate charge extraction. The bilayer PHJ is formed by depositing a layer of lead-halide wide-bandgap (WBG) perovskite on top of the mixed Pb-Sn NBG perovskite through a hybrid evaporation-solution-processing method. This heterostructure allows us to increase the PCE of Pb-Sn PSCs having a 1.2-µm-thick absorber to 23.8%, together with a high open-circuit voltage (Voc) of 0.873 V and a high FF of 82.6%. We thereby demonstrate a record-high PCE of 28.5% (certified 28.0%) in all-perovskite tandem solar cells. The encapsulated tandem devices retain more than 90% of their initial performance after 600 h of continuous operation under simulated one-sun illumination.

Inhibiting Interfacial Diffusion in Heterojunction Perovskite Solar Cells by Replacing Low-Dimensional Perovskite with Uniformly Anchored Quaternized Polystyrene

Surface heterojunction has been regarded as an effective method to improve the device efficiency of perovskite solar cells. Nevertheless, the durability of different heterojunction under thermal stress is rarely investigated and compared. In this work, benzylammonium chloride and benzyltrimethylammonium chloride are utilized to construct 3D/2D and 3D/1D heterojunctions, respectively. A quaternized polystyrene is synthesized to construct a three-dimensional perovskite/amorphous ionic polymer (3D/AIP) heterojunction. Due to the migration and volatility of organic cations, severe interfacial diffusion is found among 3D/2D and 3D/1D heterojunctions, in which the quaternary ammonium cations in the 1D structure are less volatile and mobile than the primary ammonium cations in the 2D structure. 3D/AIP heterojunction remains intact under thermal stress due to the strong ionic bond anchoring at the interface and the ultra-high molecular weight of AIP. Furthermore, the dipole layer formed by AIP can further reduce the voltage loss caused by nonradiative recombination at the interface by 0.088 V. Therefore, the devices based on the 3D/AIP heterojunction achieve a champion power conversion efficiency of 24.27% and maintain 90% of its initial efficiency after either thermal aging for 400 h or wet aging for 3000 h, showing a great promise for polymer/perovskite heterojunction towards real applications.

Perovskite interface defect passivation with poly(ethylene oxide) for improving power conversion efficiency of the inverted solar cells

Inverted perovskite solar cells (PSCs) attract researchers' attention for their potential application due to the low-temperature fabrication, negligible hysteresis and compatibility with multi-junction cells. However, the low-temperature fabricated perovskite films containing excessive undesired defects are not benefit for improving the performance of the inverted PSCs. In this work, we used a simple and effective passivation strategy that Poly(ethylene oxide) (PEO) polymer as an antisolvent additive to modify the perovskite films. The experiments and simulations have shown that the PEO polymer can effectively passivate the interface defects of the perovskite films. The defect passivation by PEO polymers suppressed non-radiative recombination, resulting in an increase in power conversion efficiency (PCE) of the inverted devices from 16.07% to 19.35%. In addition, the PCE of unencapsulated PSCs after PEO treatment maintains 97% of its original stored in a nitrogen atmosphere for 1000 h.

Targeted passivation and optimized interfacial carrier dynamics improving the efficiency and stability of hole transport layer-free narrow-bandgap perovskite solar cells

Narrow-bandgap mixed Sn-Pb perovskite solar cells (PSCs) have showcased great potential to approach the Shockley-Queisser limit. Nevertheless, the practical application and long-term deployment of mixed Sn-Pb PSCs are still largely impeded by the rapid oxidation of Sn²⁺ ions and under-optimized carrier transport layer (CTL)/perovskite interfaces that would inevitably incur serious interfacial charge recombination and device performance degradation. Herein, we successfully removed the hole transport layer (HTL) by incorporating a small amount of organic phosphonic acid molecules into perovskites, which could preferably interact with Sn²⁺ ions (relative to Pb²⁺ analogues) at the grain boundaries (GBs) throughout the perovskite film thickness via coordination bonding, thus effectively retarding the oxidation of Sn²⁺, passivating the defects and suppressing the non-radiative recombination. Targeted modification effectively reinforced built-in potential by ∼100 mV, and favorably induced energy level cascade, thus accelerating spatial charge separation and facilitating the hole extraction from perovskite layer to underlying conductive electrodes even in the absence of HTL. Consequently, enhanced power conversion efficiencies up to 20.21% have been achieved, which is the record efficiency for the HTL-free mixed Sn-Pb PSCs, accompanied by a decent photovoltage of 0.87 V and improved long-term stability over 2400 h.

Bridging the Buried Interface with Piperazine Dihydriodide Layer for High Performance Inverted Solar Cells

Given that it is closely related to perovskite crystallization and interfacial trap densities, buried interfacial engineering is crucial for creating effective and stable perovskite solar cells. Compared with the in-depth studies on the defect at the top perovskite interface, exploring the defect of the buried side of perovskite film is relatively complicated and scanty owing to the non-exposed feature. Herein, the degradation process is probed from the buried side of perovskite films with continuous illumination and its effects on morphology and photoelectronic characteristics with a facile lift-off method. Additionally, a buffer layer of Piperazine Dihydriodide (PDI₂ ) is inserted into the imbedded bottom interface. The PDI₂ buffer layer is able to lubricate the mismatched thermal expansion between perovskite and substrate, resulting in the release of lattice strain and thus a void-free buried interface. With the PDI₂ buffer layer, the degradation originates from the growing voids and increasing non-radiative recombination at the imbedded bottom interfaces are suppressed effectively, leading to prolonged operation lifetime of the perovskite solar cells. As a result, the power conversion efficiency of an optimized p-i-n inverted photovoltaic device reaches 23.47% (with certified 23.42%) and the unencapsulated devices maintain 90.27% of initial efficiency after 800 h continuous light soaking.

Tuning Perovskite Surface Polarity via Dipole Moment Engineering for Efficient Hole-Transport-Layer-Free Sn-Pb Mixed-Perovskite Solar Cells

Post-treatment has been recognized as one of the effective methods for passivating the underlying defects in perovskite solar cells (PSCs), but little attention has been paid to how to pick suitable passivation agents with diverse isomers for efficient PSCs, particularly for the tin-lead (Sn-Pb) mixed PSCs. Here, we introduce the dependence of the power conversion efficiency (PCE) on a dipole moment for surface passivator screening, in which we chose three trifluoromethyl-phenylethylamine hydroiodide (CF₃-PEAI) isomers as surface-treatment materials for hole-transport-layer-free (HTL-free) Sn-Pb mixed PSCs. The different positions of the -CF₃ group for the CF₃-PEAI isomer result in different dipole moments, which influences the interaction between CF₃-PEAI and lead iodide. The para position CF₃ with the highest dipole moment exhibits a higher PCE than the ortho-position with a lower dipole moment, which is attributed to the large dipole moment on the surface that could tune the surface polarity from p-type to n-type, facilitating electron charge transport in the HTL-free Sn-Pb mixed PSCs. An ultrathin 2D layer is formed on the perovskite surface to passivate the surface defects, which is responsible for the enhancement of the PCE and stability of the PSCs. As a result, the open-circuit voltage (V_OC) of the device is improved from 0.775 to 0.824 V, yielding a champion PCE of 20.17%, which is one of the highest PCEs among the reported HTL-free Sn-Pb mixed PSCs. The device also shows improved stability with remaining 75% of its initial PCEs after storage in N₂ for 700 h.

Atomic Model for Alkali Metal-Doped Tin-Lead Mixed Perovskites: Insight from Quantum Dynamics

Defects such as metal vacancies act as nonradiative recombination centers to deteriorate the photoelectric properties of metal halide perovskites. Nonadiabatic molecular dynamics has demonstrated that alkali metal dopants markedly improve the performance of mixed tin-lead perovskites. Alkali dopants increase the formation energy of tin vacancies to 1 eV, so that the defect concentration is decreased. When tin vacancies exist, alkali metals are easily doped into perovskites. Tin vacancies produce iodine trimers that create midgap states and cause rapid electron-hole recombination. Alkali metal additives eliminate the trap state, weaken nonadiabatic coupling, and decelerate charge recombination with a coefficient of ≤5.5 compared with the performance of the defective tin-lead mixed perovskite. Our research has constructed a theoretical model at the atomic level for alkali metal passivation that enhances defect tolerance of tin-lead mixed perovskites, generating valuable inspiration for optimizing high-performance perovskites.

Highly efficient and stable near-infrared photodetectors enabled from passivated tin-lead hybrid perovskites

Tin-lead perovskite-based photodetectors have a wide light-absorption wavelength range, which spans 1000 nm. However, the preparation of the mixed tin-lead perovskite films faces two great obstacles, namely easy oxidation of Sn²⁺to Sn⁴⁺and fast crystallization from tin-lead perovskite precursor solutions, thus further resulting in poor morphology and high density of defects in tin-lead perovskite films. In this study, we demonstrated a high-performance of near-infrared photodetectors prepared from a stable low-bandgap (MAPbI₃)_0.5(FASnI₃)_0.5film modified with 2-fluorophenethylammonium iodide (2-F-PEAI). The addition engineering can efficiently improve the crystallization of (MAPbI₃)_0.5(FASnI₃)_0.5films through the coordination binding between Pb²⁺and N atom in 2-F-PEAI, and resulting in a uniform and dense (MAPbI₃)_0.5(FASnI₃)_0.5film. Moreover, 2-F-PEAI suppressed Sn²⁺oxidation and effectively passivated defects in the (MAPbI₃)_0.5(FASnI₃)_0.5film, thereby significantly reducing the dark current in the PDs. Consequently, the near-infrared photodetectors showed a high responsivity with a specific detectivity of over 10¹²Jones at 800 to near-1000 nm. Additionally, the stability of PDs incorporated with 2-F-PEAI has been significantly improved under air conditions, and the device with the 2-F-PEAI ratio of 400:1 retained 80% of its initial efficiency after 450 h storage in air without encapsulation. Finally, 5 × 5 cm²photodetector arrays were fabricated to demonstrate the potential utility of the Sn-Pb perovskite photodetector in optical imaging and optoelectronic applications.

Synergistic Surface Modification of Tin-Lead Perovskite Solar Cells

Interfaces in thin-film photovoltaics play a pivotal role in determining device efficiency and longevity. In this work, the top surface treatment of mixed tin-lead (≈1.26 eV) halide perovskite films for p-i-n solar cells is studied. Charge extraction is promoted by treating the perovskite surface with piperazine. This compound reacts with the organic cations at the perovskite surface, modifying the surface structure and tuning the interfacial energy level alignment. In addition, the combined treatment with C₆₀ pyrrolidine tris-acid (CPTA) reduces hysteresis and leads to efficiencies up to 22.7%, with open-circuit voltage values reaching 0.90 V, ≈92% of the radiative limit for the bandgap of this material. The modified cells also show superior stability, with unencapsulated cells retaining 96% of their initial efficiency after >2000 h of storage in N₂ and encapsulated cells retaining 90% efficiency after >450 h of storage in air. Intriguingly, CPTA preferentially binds to Sn²⁺ sites at film surface over Pb²⁺ due to the energetically favored exposure of the former, according to first-principles calculations. This work provides new insights into the surface chemistry of perovskite films in terms of their structural, electronic, and defect characteristics and this knowledge is used to fabricate state-of-the-art solar cells.

A Roadmap for Efficient and Stable All-Perovskite Tandem Solar Cells from a Chemistry Perspective

Multijunction tandem solar cells offer a promising route to surpass the efficiency limit of single-junction solar cells. All-perovskite tandem solar cells are particularly attractive due to their high power conversion efficiency, now reaching 28% despite being made with relatively easy fabrication methods. In this review, we summarize the progress in all-perovskite tandem solar cells. We then discuss the scientific and engineering challenges associated with both absorbers and functional layers and offer strategies for improving the efficiency and stability of all-perovskite tandem solar cells from the perspective of chemistry.

A Universal Surface Treatment for p-i-n Perovskite Solar Cells

Perovskite interfaces critically influence the final performance of the photovoltaic devices. Optimizing them by reducing the defect densities or improving the contact with the charge transporting material is key to further enhance the efficiency and stability of perovskite solar cells. Inverted (p-i-n) devices can particularly benefit here, as evident from various successful attempts. However, every reported strategy is adapted to specific cell structures and compositions, affecting their robustness and applicability by other researchers. In this work, we present the universality of perovskite top surface post-treatment with ethylenediammonium diiodide (EDAI₂) for p-i-n devices. To prove it, we compare devices bearing perovskite films of different composition, i.e., Sn-, Pb-, and mixed Sn-Pb-based devices, achieving efficiencies of up to 11.4, 22.0, and 22.9%, respectively. A careful optimization of the EDAI₂ thickness indicates a different tolerance for Pb- and Sn-based devices. The main benefit of this treatment is evident in the open-circuit voltage, with enhancements of up to 200 mV for some compositions. In addition, we prove that this treatment can be successfully applied by both wet (spin-coating) and dry (thermal evaporation) methods, regardless of the composition. The versatility of this treatment makes it highly appealing for industrial application, as it can be easily adapted to specific processing requirements. We present a detailed experimental protocol, aiming to provide the community with an easy, universal perovskite post-treatment method for reliably improving the device efficiency, highlighting the potential of interfaces for the field.

Improved p-i-n MAPbI3 perovskite solar cells via the interface defect density suppression by PEABr passivation

Organic-inorganic hybrid perovskite solar cells (PSCs) are promising candidates for next-generation photovoltaics due to their excellent optoelectronic properties and process compatibility. In this report, numerical simulations show the effect of perovskite surface defect density on the inverted MAPbI₃ perovskite device. The Phenethylammonium bromide (PEABr) is introduced to passivate the MAPbI₃ layer surface of the perovskite solar cell devices, PEA⁺ diffuses into the grain boundaries of the 3D perovskite to form 2D/3D hybrid structure during the thermal annealing process, thus improve the surface morphology and decrease the interface defects between MAPbI₃ layer and PCBM layer. The power conversion efficiency (PCE) of the PSCs increased from 17.95% to 19.24% after PEABr treatment. In addition, the 2D/3D hybrid structure can also hinder the intrusion of water and oxygen, the stability of perovskite devices has been greatly improved.

Data-driven design of high-performance MASnxPb1-xI3 perovskite materials by machine learning and experimental realization

The photovoltaic performance of perovskite solar cell is determined by multiple interrelated factors, such as perovskite compositions, electronic properties of each transport layer and fabrication parameters, which makes it rather challenging for optimization of device performances and discovery of underlying mechanisms. Here, we propose and realize a novel machine learning approach based on forward-reverse framework to establish the relationship between key parameters and photovoltaic performance in high-profile MASn_xPb_1-xI₃ perovskite materials. The proposed method establishes the asymmetrically bowing relationship between band gap and Sn composition, which is precisely verified by our experiments. Based on the analysis of structural evolution and SHAP library, the rapid-change region and low-bandgap plateau region for small and large Sn composition are explained, respectively. By establishing the models for photovoltaic parameters of working photovoltaic devices, the deviation of short-circuit current and open-circuit voltage with band gap in defective-zone and low-bandgap-plateau regions from Shockley-Queisser theory is captured by our models, and the former is due to the deep-level traps formed by crystallographic distortion and the latter is due to the enhanced susceptibility by increased Sn⁴⁺ content. The more difficulty for hole extraction than electron is also concluded in the models and the prediction curve of power conversion efficiency is in a good agreement with Shockley-Queisser limit. With the help of search and optimization algorithms, an optimized Sn:Pb composition ratio near 0.6 is finally obtained for high-performance perovskite solar cells, then verified by our experiments. Our constructive method could also be applicable to other material optimization and efficient device development.

Quasi-2D Bilayer Surface Passivation for High Efficiency Narrow Bandgap Perovskite Solar Cells

The combination of comprehensive surface passivation and effective interface carriers transfer plays a critical role in high-performance perovskite solar cells. A 2D structure is an important approach for surface passivation of perovskite film, however, its large band gap could compromise carrier transfer. Herein, we synthesize a new molecule 2-thiopheneethylamine thiocyanate (TEASCN) for the construction of bilayer quasi-2D structure precisely on a tin-lead mixed perovskite surface. This bilayer structure can passivate the perovskite surface and ensure effective carriers transfer simultaneously. As a result, the open-circuit voltage (V_oc ) of the device is increased without sacrificing short-circuit current density (J_sc ), giving rise to a high certified efficiency from a credible third-party certification of narrow band gap perovskite solar cells. Furthermore, theoretical simulation indicates that the inclusion of TEASCN makes the bilayer structure thermodynamically more stable, which provides a strategy to tailor the number of layers of quasi-2D perovskite structures.

A Selective Targeting Anchor Strategy Affords Efficient and Stable Ideal-Bandgap Perovskite Solar Cells

Mixed lead-tin perovskite solar cells (LTPSCs) with an ideal bandgap are demonstrated as a promising candidate to reach higher power conversion efficiency (PCE) than their Pb-counterparts. Herein, a Br-free mixed lead-tin perovskite material, FA_0.8 MA_0.2 Pb_0.8 Sn_0.2 I₃ , with a bandgap of 1.33 eV, as a perovskite absorber, is selected. Through density functional theory calculations and optoelectronic techniques, it is demonstrated that both Pb- and Sn-related A-site vacancies are pushed into deeper energetic depth, causing severe nonradiative recombination. Hence, a selective targeting anchor strategy that employs phenethylammonium iodide and ethylenediamine diiodide as co-modifiers to selectively anchor with Pb- and Sn-related active sites and passivate bimetallic traps, respectively, is established. Furthermore, the selectivity of the molecular oriented anchor passivation is demonstrated through energetic depth specificity of Pb- and Sn-related traps. As a result, a substantially enhanced open-circuit voltage (V_OC ) from 0.79 to 0.90 V for the LTPSCs is achieved, yielding a champion PCE of 22.51%, which is the highest PCE among the reported ideal-bandgap PSCs. The V_OC loss is reduced to 0.43 V.

Gradient Doping in Sn-Pb Perovskites by Barium Ions for Efficient Single-Junction and Tandem Solar Cells

Narrow-bandgap (NBG) tin (Sn)-lead (Pb) perovskites generally have a high density of unintentional p-type self-doping, which reduces the charge-carrier lifetimes, diffusion lengths, and device efficiencies. Here, a p-n homojunction across the Sn-Pb perovskite is demonstrated, which results from a gradient doping by barium ions (Ba²⁺ ). It is reported that 0.1 mol% Ba²⁺ can effectively compensate the p-doping of Sn-Pb perovskites or even turns it to n-type without changing its bandgap. Ba²⁺ cations are found to stay at the interstitial sites and work as shallow electron donor. In addition, Ba²⁺ cations show a unique heterogeneous distribution in perovskite film. Most of the barium ions stay in the top 600 nm region of the perovskite films and turn it into weakly n-type, while the bottom portion of the film remains as p-type. The gradient doping forms a homojunction from top to bottom of the perovskite films with a built-in field that facilitates extraction of photogenerated carriers, resulting in an increased carrier extraction length. This strategy enhances the efficiency of Sn-Pb perovskite single-junction solar cells to over 21.0% and boosts the efficiencies of monolithic perovskite-perovskite tandem solar cells to 25.3% and 24.1%, for active areas of 5.9 mm²  and 0.94 cm² , respectively.

All-perovskite tandem solar cells with improved grain surface passivation

All-perovskite tandem solar cells hold the promise of surpassing the efficiency limits of single-junction solar cells^1-3; however, until now, the best-performing all-perovskite tandem solar cells have exhibited lower certified efficiency than have single-junction perovskite solar cells^4,5. A thick mixed Pb-Sn narrow-bandgap subcell is needed to achieve high photocurrent density in tandem solar cells⁶, yet this is challenging owing to the short carrier diffusion length within Pb-Sn perovskites. Here we develop ammonium-cation-passivated Pb-Sn perovskites with long diffusion lengths, enabling subcells that have an absorber thickness of approximately 1.2 μm. Molecular dynamics simulations indicate that widely used phenethylammonium cations are only partially adsorbed on the surface defective sites at perovskite crystallization temperatures. The passivator adsorption is predicted to be enhanced using 4-trifluoromethyl-phenylammonium (CF3-PA), which exhibits a stronger perovskite surface-passivator interaction than does phenethylammonium. By adding a small amount of CF3-PA into the precursor solution, we increase the carrier diffusion length within Pb-Sn perovskites twofold, to over 5 μm, and increase the efficiency of Pb-Sn perovskite solar cells to over 22%. We report a certified efficiency of 26.4% in all-perovskite tandem solar cells, which exceeds that of the best-performing single-junction perovskite solar cells. Encapsulated tandem devices retain more than 90% of their initial performance after 600 h of operation at the maximum power point under 1 Sun illumination in ambient conditions.

Advancing 2D Perovskites for Efficient and Stable Solar Cells: Challenges and Opportunities

Perovskite solar cells (PSCs) have rapidly emerged as one of the hottest topics in the photovoltaics community owing to their high power-conversion efficiencies (PCE), and the promise to be produced at low cost. Among various PSCs, typical 3D perovskite-based solar cells deliver high PCE but they suffer from severe instability, which restricts their practical applications. In contrast to 3D perovskites, 2D perovskites that incorporate larger, less volatile, and generally more hydrophobic organic cations exhibit much improved thermal, chemical, and environmental stability. 2D perovskites can have different roles within a solar cell, either as the primary light absorber (2D PSCs), or as a capping layer atop a 3D perovskite absorbing layer (2D/3D PSCs). Tradeoffs between PCE and stability exist in both types of PSCs-2D PSCs are more stable but exhibit lower efficiency while 2D/3D PSCs deliver exciting efficiency but show relatively poor stability. To address this PCE/stability tradeoff, the challenges both the 2D and 2D/3D PSCs face are identified and select works the community has undertaken to overcome them are highlighted in this review. It is ended with several recommendations on how to further improve PSCs so their performance and stability can be commensurate with application requirements.

23.7% Efficient inverted perovskite solar cells by dual interfacial modification

Despite remarkable progress, the performance of lead halide perovskite solar cells fabricated in an inverted structure lags behind that of standard architecture devices. Here, we report on a dual interfacial modification approach based on the incorporation of large organic cations at both the bottom and top interfaces of the perovskite active layer. Together, this leads to a simultaneous improvement in both the open-circuit voltage and fill factor of the devices, reaching maximum values of 1.184 V and 85%, respectively, resulting in a champion device efficiency of 23.7%. This dual interfacial modification is fully compatible with a bulk modification of the perovskite active layer by ionic liquids, leading to both efficient and stable inverted architecture devices.

Enhancing the stability of CsPbX3 (X = Br, I) through combination with Y-zeolites for WLED application

The stability of perovskite quantum dots (PQDs) plays a vital role in practical devices. Besides silica coating, embedding PQDs in zeolites is another strategy to improve their stability significantly. Although the zeolite rigid framework has been reported to protect PQDs, there are few reports on the in situ synthesis of PQDs in zeolites. In this work, cubic phase CsPbX₃ (X = Br, I) nanocrystals were successfully prepared by the ion exchange method combined with a non-polar organic trigger. Dropping a certain amount of ZnM₂ (M = Br, I) solution into the intermediate product Cs₄PbCl₆ nanocrystals resulted in the formation of the final CsPbX₃ nanocrystals. The ZnM₂ solutions were prepared in non-polar solvents (hexane, octane, or chloroform). The highest photoluminescence quantum yield (PLQY) of the synthesized CsPbX₃@zeolite composites can reach 83%, with a lifetime of 1.37 μs. The stability of the CsPbX₃@zeolite composites thin film against damage from air and light is significantly improved. We fabricated white light-emitting diodes (WLEDs) using CsPbBr₃@zeolite as the green light source and CsPbI₃@zeolite as the red light source to further emphasize the practical application effect of the CsPbX₃@zeolite composites. This work not only provides a new method for the synthesis of CsPbX₃ nanocrystals but also involves the in situ synthesis of high stability CsPbX₃@zeolite composites within the zeolite.

Progress in Perovskite Solar Cells towards Commercialization-A Review

In recent years, perovskite solar cells (PSCs) have experienced rapid development and have presented an excellent commercial prospect as the PSCs are made from raw materials that are readily and cheaply available depending on simple manufacturing techniques. However, the commercial production and utilization of PSCs remain immature, leading to substantial efforts needed to boost the development of scalable fabrication of PSCs, pilot scale tests, and the establishment of industrial production lines. In this way, the PSCs are expected to be successfully popularized from the laboratory to the photovoltaic market. In this review, the history of power conversion efficiency (PCE) for laboratory-scale PSCs is firstly introduced, and then some methods for maintaining high PCE in the upscaling process is displayed. The achievements in the stability and environmental friendliness of PSCs are also summarized because they are also of significance for commercialization. Finally, this review evaluates the commercialization prospects of PSCs from the economic view and provides a short outlook.

Two-/Three-Dimensional Perovskite Bilayer Thin Films Post-Treated with Solvent Vapor for High-Performance Perovskite Photovoltaics

Perovskite photovoltaics (PPVs) using three-dimensional (3D) perovskites incorporated with two-dimensional (2D) perovskites have drawn great concentration in both academic and industrial sectors. Here, we report high performance of PPVs based on the 2D/3D perovskite bilayer thin film post-annealed with solvent vapor. The 2D/3D perovskite bilayer thin film post-annealed with solvent vapor possesses enlarged crystal size and crystallinity and blue-shifted photoluminescence compared to a 3D MAPbI₃ thin film. Moreover, compared to the PPVs based on a 3D perovskite thin film, enlarged built-in potential, suppressed charge carrier recombination, boosted charge transport, and reduced charge carrier extraction time are observed from the PPVs based on the 2D/3D perovskite bilayer thin film post-annealed with solvent vapor. As a result, perovskite solar cells exhibit a power conversion efficiency of 22.13% and dramatically enhanced stability, and perovskite photodetectors show a photoresponsivity of 1.38 AW^-1, detectivity of 6.52 × 10¹⁴ cm Hz^1/2 W^-1, and linear dynamic range of over 167 dB at room temperature. These results demonstrate that we develop a simple method to approach high-performance PPVs by the 2D/3D perovskite bilayer thin film.

Mixed lead-tin perovskite films with >7 μs charge carrier lifetimes realized by maltol post-treatment

Mixed lead-tin (Pb-Sn) halide perovskites with optimum band gaps near 1.3 eV are promising candidates for next-generation solar cells. However, the performance of solar cells fabricated with Pb-Sn perovskites is restricted by the facile oxidation of Sn(ii) to Sn(iv), which induces self-doping. Maltol, a naturally occurring flavor enhancer and strong metal binding agent, was found to effectively suppress Sn(iv) formation and passivate defects in mixed Pb-Sn perovskite films. When used in combination with Sn(iv) scavenging, the maltol surface treatment led to high-quality perovskite films which showed enhanced photoluminescence intensities and charge carrier lifetimes in excess of 7 μs. The scavenging and surface treatments resulted in highly reproducible solar cell devices, with photoconversion efficiencies of up to 21.4% under AM1.5G illumination.

Defect Passivation in Lead-Halide Perovskite Nanocrystals and Thin Films: Toward Efficient LEDs and Solar Cells

Lead-halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge carriers into light and vice versa. However, despite the defect-tolerance of LHPs, defects at the surface of colloidal NCs and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the PLQYs, device efficiency, and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes are critical for achieving advances in performance. This Review presents the current understanding of defects in halide perovskites and their influence on the optical and charge-carrier transport properties. Passivation strategies toward improving the efficiencies of perovskite-based LEDs and solar cells are also discussed.

Antioxidation and Energy-Level Alignment for Improving Efficiency and Stability of Hole Transport Layer-Free and Methylammonium-Free Tin-Lead Perovskite Solar Cells

Tin-lead (Sn-Pb) perovskites have shown great potential in applications of single-junction perovskite solar cells (PSCs) and tandem devices due to outstanding photoelectrical properties and low band gaps. Currently, Sn-Pb PSCs typically have a p-i-n structure, but choices of hole transport layer (HTL) materials are very limited and there are different concerns in each of them. Eliminating the HTL is a direct and promising strategy to address the concerns, but is rarely studied. In this work, we demonstrate HTL-free and MA-free based Sn-Pb PSCs and a synergistic integration strategy of simultaneously introducing a reducing agent and in situ surface passivation. With the integration strategy, Sn-Pb perovskite films with enhanced antioxidation, reduced trap density, prolonged carrier lifetime, and improved energy-level alignment are achieved. Consequently, final HTL-free PSCs exhibit a champion power conversion efficiency (PCE) of 17.4%, which is a new record for HTL-free and MA-free Sn-Pb PSCs. Meanwhile, the integration strategy-based HTL-free device maintains excellent stability with efficiency unchanged for the first 200 h, and finally retaining 81% of the efficiency after 480 h aging in the air. This study shows the potential of achieving desirable HTL-free and MA-free Sn-Pb PSCs and offers more opportunities for tandem solar cells and other photovoltaic devices.

MA Cation-Induced Diffusional Growth of Low-Bandgap FA-Cs Perovskites Driven by Natural Gradient Annealing

Low-bandgap formamidinium-cesium (FA-Cs) perovskites of FA_1-xCs_xPbI₃ (x < 0.1) are promising candidates for efficient and robust perovskite solar cells, but their black-phase crystallization is very sensitive to annealing temperature. Unfortunately, the low heat conductivity of the glass substrate builds up a temperature gradient within from bottom to top and makes the initial annealing temperature of the perovskite film lower than the black-phase crystallization point (~150°C). Herein, we take advantage of such temperature gradient for the diffusional growth of high-quality FA-Cs perovskites by introducing a thermally unstable MA⁺ cation, which would firstly form α-phase FA-MA-Cs mixed perovskites with low formation energy at the hot bottom of the perovskite films in the early annealing stage. The natural gradient annealing temperature and the thermally unstable MA⁺ cation then lead to the bottom-to-top diffusional growth of highly orientated α-phase FA-Cs perovskite, which exhibits 10-fold of enhanced crystallinity and reduced trap density (~3.85 × 10¹⁵ cm⁻³). Eventually, such FA-Cs perovskite films were fabricated into stable solar cell devices with champion efficiency up to 23.11%, among the highest efficiency of MA-free perovskite solar cells.

Efficiency-Tunable Single-Component White-Light Emission Realized in Hybrid Halides Through Metal Co-Occupation

Organic-inorganic hybrid metal halides have attracted widespread attention as emerging optoelectronic materials, especially in solid-state lighting, where they can be used as single-component white-light phosphors for white light-emitting diodes. Herein, we have successfully synthesized a zero-dimensional (0D) organic-inorganic hybrid mixed-metal halide (Bmpip)2PbxSn1-xBr4 (0 < x < 1, Bmpip+ = 1-butyl-1-methyl-piperidinium, C10H22N+) that crystallizes in a monoclinic system in the C2/c space group. Pb2+ and Sn2+ form a four-coordinate seesaw structure separated by organic cations forming a 0D structure. For different excitation wavelengths, (Bmpip)2PbxSn1-xBr4 (0 < x < 1) exhibits double-peaked emission at 470 and 670 nm. The emission color of (Bmpip)2PbxSn1-xBr4 can be easily tuned from orange-red to blue by adjusting the Pb/Sn molar ratio or excitation wavelength. Representatively, (Bmpip)2Pb0.16Sn0.84Br4 exhibits approximately white-light emission with high photoluminescence quantum yield up to 39%. Interestingly, the color of (Bmpip)2PbxSn1-xBr4 can also be easily tuned by temperature, promising its potential for application in temperature measurement and indication. Phosphor-converted light-emitting diodes are fabricated by combining (Bmpip)2PbxSn1-xBr4 and 365 nm near-UV LED chips and exhibit high-quality light output.

Exploring the Synthesis, Band Edge Insights, and Photoelectrochemical Water Splitting Properties of Lead Vanadates

Exploring the ideal and stable semiconductor material for the efficient photoelectrochemical (PEC) overall water splitting reaction has remained a major challenge. Herein, we develop a facile hydrothermal method for the fabrication of monoclinic Pb₃[VO₄]₂ and orthorhombic PbV₂O₆ thin films for the efficient and stable PEC overall water splitting applications. Detailed characterization was performed to study the crystal structure and optical, electrical, and electrochemical properties. The band edge positions of Pb₃[VO₄]₂ and PbV₂O₆ are determined using spectroscopic data, revealing the conduction band edge positioned near the water reduction potential [∼0 V vs reversible hydrogen electrode (RHE)] and the valence band edge positioned well above the water oxidation potential, indicating the possible utilization of photogenerated electrons and holes for efficient water reduction and oxidation, respectively. With the optimized PbV₂O₆ thin films, a maximum photocurrent of 0.35 mA cm^-2 was obtained at 1.23 V versus RHE and the stable production of both O₂ and H₂ is observed with >90% Faradaic efficiency. Importantly, this work demonstrates the possibility of utilizing lead vanadate materials for PEC water splitting applications.

Halogen Functionalization in the 2D Material Flatland: Strategies, Properties, and Applications

Given the electronegativity and bonding environment of halogen elements, halogenation (i.e., fluorination, chlorination, bromination, and iodination) serves as a versatile strategy for chemical modifications of materials. The combination of halogens and 2D materials has triggered extensive interests since the first report on graphene fluorination in 2008. Subsequently, scholars consistently conduct pre-, in-process, or posthalogenation modifications of emerging 2D materials to achieve desired properties and broad device applications. They also continuously explore the role of halogens in 2D material functionalization. The multiple advantages introduced by halogen decoration make 2D materials outstanding from each subclass. In this review, an overall retrospect is provided on the research advances in the area of 2D material halogenation, including experimental halogenation strategies, halogen-triggered novel physics and properties, and advanced applications across the studied objects. Future research directions in this area are also proposed.

Two-Terminal Perovskite-Based Tandem Solar Cells for Energy Conversion and Storage

The organic-inorganic hybrid perovskite solar cells present a rapid improvement on power conversion efficiency from 3.8% to 25.5% in the past decades. Owing to the tuneable bandgaps, low-cost, and ease of fabrication, perovskites become ideal candidate materials for fabricating tandem solar cells, especially for efficient and high-voltage monolithic two-terminal devices. In this review, an overview of recent advances in various monolithic perovskite-based tandem solar cells with a focus on the key challenges is provided. Subsequently, the recombination layer materials, construction of wide-bandgap perovskite layer, stability of narrow-bandgap, and current matching principle in tandems are highlighted in order to optimize the output voltage and conversion efficiency of tandem solar cells. Finally, the recent progress is summarized with a focus on potential applications of tandem solar cells for energy conversion and storage, including hydrogen production by water splitting, CO₂ reduction, supercapacitors, and rechargeable batteries, benefiting from the adjustable output voltage of tandem solar cells. It is hoped that this work can offer a feasible strategy to explore more possibilities for fabricating new two-terminal tandem solar cells with high voltage and high conversion efficiency for energy conversion and storage.

The Fascinating Properties of Tin-Alloyed Halide Perovskites

Tin-alloyed halide perovskites are progressively becoming more popular as slowly their optoelectronic properties start to rival those of the potentially risky pure lead analogues. However, to push this attractive semiconductor toward realistic applications, several major issues need to be solved. This Perspective will start with a description of the fundamental properties of tin-alloyed halide perovskites, continue discussing their weak points with special attention on the structural and electronic instabilities, and conclude examining the effects of the above-mentioned properties on devices. Finally we propose a plausible roadmap to further boost tin-alloyed halide perovskite devices to practical applications. We believe this roadmap should start from an understanding of this family of semiconductors from an atomistic viewpoint, proceeding to the control of thin-film fabrication, the structural properties, and finally the device optimization. We hope this Perspective can help to inject new enthusiasm and facilitate the progress in tin-alloyed halide perovskites, catalyzing their transition from the cradle of the laboratories to the reality of their fabrication.

Low-Toxicity Perovskite Applications in Carbon Electrode Perovskite Solar Cells—A Review

Perovskite solar cells (PSCs) with earth-abundant carbon as an effective replacer for unstable hole-transporting materials and expensive electrodes is a recently proposed structure promising better air and moisture stability. In this review paper, we report on the latest advances and state of the art of Pb-free and low-Pb-content perovskites, used as absorbers in carbon-based perovskite solar cells. The focus is on the implementation of these, environmentally friendly and non-toxic, structures in PSCs with a carbon electrode as a replacement of the noble metal electrode typically used (C-PSCs). The motivation for this study has been the great potential that C-PSCs have shown for the leap towards the commercialization of PSCs. Some of their outstanding properties include low cost, high-stability, ambient processability and compatibility with most up-scaling methods (e.g., printing). By surpassing the key obstacle of toxicity, caused by the Pb content of the highest-performing perovskites, and by combining the advantages of C-PSCs with the Pb-free perovskites low toxicity, this technology will move one step further; this review summarizes the most promising routes that have been reported so far towards that direction.

Advances in Metal Halide Perovskite Film Preparation: The Role of Anti-Solvent Treatment

In the past decade, hybrid organic-inorganic perovskite solar cells (PSCs) have attracted significant attention. Since then, the power conversion efficiency has astonishingly reached to 25.5%, situating perovskites at the forefront of all reported solution-processed photovoltaic materials. The research of PSCs has reached a stage where efficiency, stability, and cost need to be simultaneously considered before reaching the threshold for large-scale commercialization. In this article, the recent progress in fabricating high-quality perovskite thin-films adopting "anti-solvent" strategy is reviewed and the established nucleation and crystal growth mechanisms during the treatment process is discussed. In addition, present challenges and further opportunities of the anti-solvent methodology toward efficient and large-scale PSCs are highlighted. The continuous efforts dedicated to the development of anti-solvent treatment for fabricating high-performance large-area devices may pave the way toward commercial applications of PSCs in the near future.

GABr Post-Treatment for High-Performance MAPbI3 Solar Cells on Rigid Glass and Flexible Substrate

Perovskite solar cells have exhibited astonishing photoelectric conversion efficiency and have shown a promising future owing to the tunable content and outstanding optoelectrical property of hybrid perovskite. However, the devices with planar architecture still suffer from huge Voc loss and severe hysteresis effect. In this research, Guanidine hydrobromide (GABr) post-treatment is carried out to enhance the performance of MAPbI3 n-i-p planar perovskite solar cells. The detailed characterization of perovskite suggests that GABr post-treatment results in a smoother absorber layer, an obvious reduction of trap states and optimized energy level alignment. By utilizing GABr post-treatment, the Voc loss is reduced, and the hysteresis effect is alleviated effectively in MAPbI3 solar cells. As a result, solar cells based on glass substrate with efficiency exceeding 20%, Voc of 1.13 V and significantly mitigated hysteresis are fabricated successfully. Significantly, we also demonstrate the effectiveness of GABr post-treatment in flexible device, whose efficiency is enhanced from 15.77% to 17.57% mainly due to the elimination of Voc loss.

The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency

Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

Simultaneously Passivating Cation and Anion Defects in Metal Halide Perovskite Solar Cells Using a Zwitterionic Amino Acid Additive

Ionic defects (e.g., organic cations and halide anions), preferably residing along grain boundaries (GBs) and on perovskite film surfaces, are known to be a major source of the notorious environmental instability of perovskite solar cells (PeSCs). Although passivating ionic defects is desirable, previous approaches using Lewis base or acid molecules as additives suppress only the negatively or positively charged defects, thus leaving oppositely charged defects. In this work, both the cationic and anionic defects inside methyl ammonium lead tri-iodide (MAPbI₃ ) are simultaneously passivated by introducing a zwitterionic form of the amino acid, L-alanine, into the precursor solution as an additive. L-alanine has both positive (NH₃⁺ ) and negative (COO^- ) functional groups at a specific solvent pH, thereby passivating both the cation and anion defects in MAPbI₃ . The addition of L-alanine increases the grain size of the perovskite crystals and lengthens the charge carrier lifetime (τ > 1 µs), leading to improved power conversion efficiencies (PCEs) of 20.3% (from 18.3% without an additive) for small-area (4.64 mm² ) devices and 15.6% (from 13.5%) for large-area submodules (9.06 cm² ). More importantly, the authors' approach also significantly enhances the shelf storage and photoirradiation stabilities of PeSCs.

Layered perovskite materials: key solutions for highly efficient and stable perovskite solar cells

Metal halide perovskites having three-dimensional crystal structures are being applied successfully in various optoelectronic applications. To address their most challenging issues-instability and toxicity-without losing efficiency, lower-dimensional perovskites appear to be promising alternatives. Recently, two-dimensional (2D) perovskite solar cells have been developed exhibiting excellent photostability and moisture-stability, together with moderate device efficiency. This review summarizes the photophysical properties and operating mechanisms of 2D perovskites as well as recent advances in their applications in solar cell devices. Also presented is an agenda for the next-stage development of stable perovskite materials for solar cell applications, highlighting the issues of stability and toxicity that require further study to ensure commercialization.

Sensitive and Stable Tin-Lead Hybrid Perovskite Photodetectors Enabled by Double-Sided Surface Passivation for Infrared Upconversion Detection

Tin(Sn)-based perovskite is currently considered one of the most promising materials due to extending the absorption spectrum and reducing the use of lead (Pb). However, Sn²⁺ is easily oxidized to Sn⁴⁺ in atmosphere, causing more defects and degradation of perovskite materials. Herein, double-sided interface engineering is proposed, that is, Sn-Pb perovskite films are sandwiched between the phenethylammonium iodide (PEAI) in both the bottom and top sides. The larger organic cations of PEA+ are arranged into a perovskite surface lattice to form a 2D capping layer, which can effectively prevent the water and oxygen to destroy bulk perovskite. Meanwhile, the PEA+ can also passivate defects of iodide anions at the bottom of perovskite films, which is always present but rarely considered previously. Compared to one sided passivation, Sn-Pb hybrid perovskite photodetectors contribute a significant enhancement of performance and stability, yielding a broadband response of 300-1050 nm, a low dark current density of 1.25 × 10^-3 mA cm^-2 at -0.1 V, fast response speed of 35 ns, and stability beyond 240 h. Furthermore, the Sn-Pb broadband photodetectors are integrated in an infrared up-conversion system, converting near-infrared light into visible light. It is believed that a double-sided passivation method can provide new strategies to achieving high-performance perovskite photodetectors.