Oxalate Pushes Efficiency of CsPb0.7 Sn0.3 IBr2 Based All-Inorganic Perovskite Solar Cells to over 14

Effect of ABX3 site changes on the performance of tin-lead mixed perovskite solar cells

Tin-lead mixed perovskite solar cells (TLMPSCs), with the advantage of approaching the Shockley-Queisser (S-Q) limit for photovoltaic applications, have been rapidly developed and achieved a power conversion efficiency (PCE) of 23.7%. Although the low toxicity of TLMPSCs is conducive to sustainable development, the oxidation of Sn2+ could destroy the perovskite structure easily. Thus, most researchers are devoted to improving the photoelectric performance and stability through additive engineering, interface engineering, device structure optimization, solvent engineering, etc. However, TLMPs with different A-sites and X-sites in the ABX3 model and an optimal ratio of Sn : Pb still need to be investigated; this is the basis of mechanistic analysis. In this paper, we introduce TLMPSCs with different A-sites, X-sites, and Sn-Pb ratios. The mechanism and properties of the cations are analyzed based on the performance of TLMPSCs. Finally, a series of prospects for optimizing ABX3 are put forward, with the hope of attracting the attention and interest of researchers.

The Effect of Component Defects on the Performance of Perovskite Devices and the Low-Cost Preparation of High-Purity PbI2

The efficiency and reproducibility of perovskite solar cells (PSCs) are significantly influenced by the purity of lead iodide (PbI2) in the raw materials used. Pb(OH)I has been identified as the primary impurity generated from PbI2 in water-based synthesis. Consequently, a comprehensive investigation into the impact of Pb(OH)I impurities on film and device performance is essential. In this study, PbI2, with varying stoichiometries, was synthesized to examine the effects of different Pb(OH)I levels on perovskite device performance. The characterization results revealed that even trace amounts of Pb(OH)I impede the formation of precursor prenucleation clusters. These impurities also increase the energy barrier of the α-phase and facilitate the transition of the intermediate phase to the δ-phase. These effects result in poor perovskite film morphology and sub-optimal photovoltaic device performance. To address these issues, a cost-effective method for preparing high-stoichiometry PbI2 was developed. The formation of Pb(OH)I was effectively inhibited through several strategies: adjusting solution pH and temperature, modifying material addition order, simplifying the precipitation-recrystallization process, and introducing H3PO2 as an additive. These modifications enabled the one-step synthesis of high-purity PbI2. PSCs prepared using this newly synthesized high-stoichiometry PbI2 demonstrated photovoltaic performance comparable to those fabricated with commercial PbI2 (purity ≥ 99.999%). Our novel method offers a cost-effective alternative for synthesizing high-stoichiometry PbI2, thereby providing a viable option for the production of high-performance PSCs.

High-entropy alloy screening for halide perovskites

As the concept of high-entropy alloying (HEA) extends beyond metals, new materials screening methods are needed. Halide perovskites (HP) are a prime case study because greater stability is needed for photovoltaics applications, and there are 322 experimentally observed HP end-members, which leads to more than 1057 potential alloys. We screen HEAHP by first calculating the configurational entropy of 106 equimolar alloys with experimentally observed end-members. To estimate enthalpy at low computational cost, we turn to the delta-lattice parameter approach, a well-known method for predicting III-V alloy miscibility. To generalize the approach for non-cubic crystals, we introduce the parameter of unit cell volume coefficient of variation (UCV), which does a good job of predicting the experimental HP miscibility data. We use plots of entropy stabilization versus UCV to screen promising alloys and identify 102 HEAHP of interest.

Reductive Sn2+ Compensator for Efficient and Stable Sn-Pb Mixed Perovskite Solar Cells

Tin-lead (Sn-Pb) mixed perovskite with a narrow bandgap is an ideal candidate for single-junction solar cells approaching the Shockley-Queisser limit. However, due to the easy oxidation of Sn2+, the efficiency and stability of Sn-Pb mixed perovskite solar cells (PSCs) still lag far behind that of Pb-based solar cells. Herein, highly efficient and stable FA0.5MA0.5Pb0.5Sn0.5I0.47Br0.03 compositional PSCs are achieved by introducing an appropriate amount of multifunctional Tin (II) oxalate (SnC2O4). SnC2O4 with compensative Sn2+ and reductive oxalate group C2O4 2- effectively passivates the cation and anion defects simultaneously, thereby leading to more n-type perovskite films. Benefitting from the energy level alignment and the suppression of bulk nonradiative recombination, the Sn-Pb mixed perovskite solar cell treated with SnC2O4 achieves a power conversion efficiency of 21.43%. More importantly, chemically reductive C2O4 2- effectively suppresses the notorious oxidation of Sn2+, leading to significant enhancement in stability. Particularly, it dramatically improves light stability.

All-Inorganic Perovskite Solar Cells: Defect Regulation and Emerging Applications in Extreme Environments

All-inorganic perovskite solar cells (PSCs), such as CsPbX3, have garnered considerable attention recently, as they exhibit superior thermodynamic and optoelectronic stabilities compared to the organic-inorganic hybrid PSCs. However, the power conversion efficiency (PCE) of CsPbX3 PSCs is generally lower than that of organic-inorganic hybrid PSCs, as they contain higher defect densities at the interface and within the perovskite light-absorbing layers, resulting in higher non-radiative recombination and voltage loss. Consequently, defect regulation has been adopted as an important strategy to improve device performance and stability. This review aims to comprehensively summarize recent progresses on the defect regulation in CsPbX3 PSCs, as well as their cutting-edge applications in extreme scenarios. The underlying fundamental mechanisms leading to the defect formation in the crystal structure of CsPbX3 PSCs are firstly discussed, and an overview of literature-adopted defect regulation strategies in the context of interface, internal, and surface engineering is provided. Cutting-edge applications of CsPbX3 PSCs in extreme environments such as outer space and underwater situations are highlighted. Finally, a summary and outlook are presented on future directions for achieving higher efficiencies and superior stability in CsPbX3 PSCs.

Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin-lead (Sn-Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn-Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn-Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn-Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.

B-Site Co-Doping Coupled with Additive Passivation Pushes the Efficiency of Pb-Sn Mixed Inorganic Perovskite Solar Cells to Over 17

Pb-Sn mixed inorganic perovskite solar cells (PSCs) have garnered increasing interest as a viable solution to mitigate the thermal instability and lead toxicity of hybrid lead-based PSCs. However, the relatively poor structural stability and low device efficiency hinder its further development. Herein, high-performance manganese (Mn)-doped Pb-Sn-Mn-based inorganic perovskite solar cells (PSCs) are successfully developed by introducing Benzhydroxamic Acid (BHA) as multifunctional additive. The incorporation of smaller divalent Mn cations contributes to a contraction of the perovskite crystal, leading to an improvement in structural stability. The BHA additive containing a reductive hydroxamic acid group (O═C-NHOH) not only mitigates the notorious oxidation of Sn2+ but also interacts with metal ions at the B-site and passivates related defects. This results in films with high crystallinity and low defect density. Moreover, the BHA molecules tend to introduce a near-vertical dipole moment that parallels the built-in electric field, thus facilitating charge carrier extraction. Consequently, the resulting device delivers a champion PCE as high as 17.12%, which represents the highest reported efficiency for Pb-Sn-based inorganic PSCs thus far. Furthermore, the BHA molecule provides an in situ encapsulation of the perovskite grain boundary, resulting in significant enhancement of device air stability.

2D Ruddlesden-Popper Sn-Based Perovskite Weak Light Detector for Image Transmission and Reflection Imaging

2D Ruddlesden-Popper Sn-based perovskite has excellent optoelectronic properties and weak halide ion migration characteristics, making it an ideal candidate for weak light detection, which has great potential in light communication, and medical applications. Although Sn-based perovskite photodetectors are developed, weak light detection is not demonstrated yet. Herein, a high-performance self-powered photodetector with the capability to detect ultra-weak light signals is designed based on vertical PEA2 SnI4 /Si nanowires heterojunction. Due to the low dark current and high light absorption efficiency, the devices present a remarkable responsivity of 42.4 mA W-1 , a high detectivity of 8 × 1011 Jones, and an ultralow noise current of 2.47 × 10-13 A Hz-1/2 . Especially, the device exhibits a high on-off current ratio of 18.6 at light signals as low as 4.60 nW cm-2 , revealing the capacity to detect ultra-weak light. The device is applied as a signal receiver and realized image transmission in light communication system. Moreover, high-resolution reflection imaging and multispectral imaging are obtained using the device as the sensor in the imaging system. These results reveal that 2D PEA2 SnI4 -based self-powered photodetectors with low-noise current possess enormous potential in future weak light detection.

Recent Progress of Low-Toxicity Poor-Lead All-Inorganic Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved an impressive certified efficiency of 25.7%, which is comparatively higher than that of commercial silicon solar cells (23.3%), showing great potential toward commercialization. However, the low stability and high toxicity due to the presence of volatile organic components and toxic metal lead in the perovskites pose significant challenges. To obtain robust and low-toxicity PSCs, substituting organic cations with pure inorganic cations, and partially or fully replacing the toxic Pb with environmentally benign metals, is one of the promising methods. To date, continuous efforts have been made toward the construction of highly performed low-toxicity inorganic PSCs with astonishing breakthroughs. This review article provides an overview of recent progress in inorganic PSCs in terms of lead-reduced and lead-free compositions. The physical properties of poor-lead all-inorganic perovskites are discussed to unveil the major challenges in this field. Then, it reports notable achievements for the experimental studies to date to figure out feasible methods for efficient and stable poor-lead all-inorganic PSCs. Finally, a discussion of the challenges and prospects for poor-lead all-inorganic PSCs in the future is presented.

Molecular modification of MAPbI3 surface: insights from first-principles theory studies

Molecular surface modification has been widely used to improve the stability and the power conversion efficiency of perovskite solar cells. First-principles studies have played a crucial role in the mechanism of surface modification. However, the design of surface modification molecules lacks theoretical guidelines. Herein, we studied the surface modifications of a series of typical small molecules based on first-principles calculations. The relevance of the calculated properties and experimental performance has been investigated. It was found that molecules with nitrogen-containing groups, including amino, π-conjugated N-heterocycle, and (thio)amide groups, could have strong adsorption energies, and may be suitable modifiers. Molecules such as oxygen-containing six-membered rings and 1,2,4-triazine may induce defect states. Based on our calculations, design guidelines for perovskite surface modification molecules have been proposed based on three aspects: interfacial buffering, defect avoidance, and energy level alignment. This work may shed light on the development of perovskite surface modification molecules towards higher power conversion efficiency and more stable perovskite solar cells.

Surface Passivation of Perovskite Solar Cells with Oxalic Acid: Increased Efficiency and Device Stability

Solution processed perovskite films usually exhibit numerous defect states on the surfaces of the films. Here in this work, oxalic acid (H2 C2 O4 ), which has two C=O groups, is selected and used to passivate the surface defects of the two-step deposited perovskite films via post-treatment. Strong interaction between H2 C2 O4 molecule and the Pb2+ ions located on the surface of perovskite film has been confirmed via Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, which can result in an effective suppress of the surface defects. Furthermore, time-resolved PL spectrum indicates that carrier lifetime is prolonged in the H2 C2 O4 passivated perovskite film. After optimizing the H2 C2 O4 concentration, the target perovskite solar cells can demonstrate superior power conversion efficiencies (21.67 % from reverse measurement and 21.54 % from forward measurement) and superior device-stability.

Surface Modification in CsPb0.5Sn0.5I2Br Inorganic Perovskite Solar Cells: Effects of Bifunctional Dipolar Molecules on Photovoltaic Performance

Inorganic tin-lead binary perovskites have piqued the interest of researchers as effective absorbers for thermally stable solar cells. However, the nonradiative recombination originating from the surface undercoordinated Sn²⁺ cations and the energetic offsets between different layers cause an excessive energy loss and deteriorate the perovskite device's performance. In this study, we investigated two thioamide derivatives that differ only in the polar part connected to their common benzene ring, namely, benzenecarbothioamide and 4-fluorophenylcarbothioamide (F-TBA). These two molecules were implemented as modifiers onto the inorganic tin-lead perovskite (CsPb_0.5Sn_0.5I₂Br) surface in the perovskite solar cells. Modifiers that carry C═S and NH₂ functional groups, equipped with lone electron pairs, can autonomously associate with surface Sn²⁺ through coordination and electrostatic attraction mechanisms. This interaction serves effectively to passivate the surface. In addition, due to the permanent dipole moment of the intermediate layer, an interfacial dipole field appears at the PCBM/CsPb_0.5Sn_0.5I₂Br interface, reducing the electron extraction potential barrier. Consequently, the planar solar cell with an ITO/PEDOT:PSS/CsPb_0.5Sn_0.5I₂Br/PCBM/BCP/Ag layered structure featuring an F-TBA surface post-treatment demonstrated a noteworthy power conversion efficiency of 14.01%. Simultaneously, after being stored for 1000 h in an inert atmosphere glovebox, the non-encapsulated CsPb_0.5Sn_0.5I₂Br solar cells managed to preserve 94% of their original efficiency.

Bidirectional Anions Gathering Strategy Afford Efficient Mixed PbSn Perovskite Solar Cells

Mixed lead-tin (PbSn) perovskite solar cells (PSCs) possess low toxicity and adjustable bandgap for both single-junction and all-perovskite tandem solar cells. However, the performance of mixed PbSn PSCs still lags behind the theoretical efficiency. The uncontrollable crystallization and the resulting structural defect are important reasons. Here, the bidirectional anions gathering strategy (BAG) is reported by using Methylammonium acetate (MAAc) and Methylammonium thiocyanate (MASCN) as perovskite bulk additives, which Ac^- escapes from the perovskite film top surface while SCN^- gathers at the perovskite film bottom in the crystallization process. After the optoelectronic techniques, the bidirectional anions movement caused by the top-down gradient crystallization is demonstrated. The layer-by-layer crystallization can collect anions in the next layer and gather at the broader, enabling a controllable crystallization process, thus getting a high-quality perovskite film with better phase crystallinity and lower defect concentration. As a result, PSCs treated by the BAG strategy exhibit outstanding photovoltaic and electroluminescent performance with a champion efficiency of 22.14%. Additionally, it demonstrates excellent long-term stability, which retains ≈92.8% of its initial efficiency after 4000 h aging test in the N₂ glove box.

All-inorganic perovskite solar cells featuring mixed group IVA cations

All-inorganic perovskites are promising for solar cells owing to their potentially superior tolerance to environmental factors, as compared with their hybrid organic-inorganic counterparts. Over the past few years, all-inorganic perovskite solar cells (PSCs) have seen a dramatic improvement in certified power conversion efficiencies (PCEs), demonstrating their great potential for practical applications. Pb, Sn, and Ge are the most studied group IVA elements for perovskites. These group IVA cations share the same number of valence electrons and similarly exhibit the beneficial antibonding properties of lone-pair electrons when incorporated in the perovskite structure. Meanwhile, mixing these cations in all-inorganic perovskites provides opportunities for stabilizing the photoactive phase and tailoring the bandgap structure. In this mini-review, we analyze the structural and bandgap design principles for all-inorganic perovskites featuring mixed group IVA cations, discuss the updated progress in the corresponding PSCs, and finally provide perspectives on future research efforts faciliating the continued development of high-performance Pb-less and Pb-free all-inorganic PSCs.

Monovalent Copper Cation Doping Enables High-Performance CsPbIBr2-Based All-Inorganic Perovskite Solar Cells

Organic-inorganic perovskite solar cells (PSCs) have delivered the highest power conversion efficiency (PCE) of 25.7% currently, but they are unfortunately limited by several key issues, such as inferior humid and thermal stability, significantly retarding their widespread application. To tackle the instability issue, all-inorganic PSCs have attracted increasing interest due to superior structural, humid and high-temperature stability to their organic-inorganic counterparts. Nevertheless, all-inorganic PSCs with typical CsPbIBr2 perovskite as light absorbers suffer from much inferior PCEs to those of organic-inorganic PSCs. Functional doping is regarded as a simple and useful strategy to improve the PCEs of CsPbIBr2-based all-inorganic PSCs. Herein, we report a monovalent copper cation (Cu+)-doping strategy to boost the performance of CsPbIBr2-based PSCs by increasing the grain sizes and improving the CsPbIBr2 film quality, reducing the defect density, inhibiting the carrier recombination and constructing proper energy level alignment. Consequently, the device with optimized Cu+-doping concentration generates a much better PCE of 9.11% than the pristine cell (7.24%). Moreover, the Cu+ doping also remarkably enhances the humid and thermal durability of CsPbIBr2-based PSCs with suppressed hysteresis. The current study provides a simple and useful strategy to enhance the PCE and the durability of CsPbIBr2-based PSCs, which can promote the practical application of perovskite photovoltaics.

Strategies for the preparation of high-performance inorganic mixed-halide perovskite solar cells

Inorganic halide perovskites have attracted significant attention in the field of photovoltaics (PV) in recent years due to their superior intrinsic thermal stability and excellent theoretical power conversion efficiency (PCE). CsPbI3 with a bandgap of ∼1.7 eV is considered to be the most potential candidate for PV application. However, bulk CsPbI3 films exhibit poor phase stability. The substitution of some iodide ions with bromide/chloride in CsPbI3 results in the formation of mixed-halide CsPbX3 perovskites, which exhibit a good balance between phase stability and efficiency. The halogen-tunable mixed-halide inorganic perovskites have a bandgap matching the sunlight region and show great potential for application in multi-junction tandem and semitransparent solar cells. Herein, the progress of mixed-halide CsPbX3 PSCs is systematically reviewed, including CsPbI x Br y Cl3-x-y - and CsPbIBr2-based IPSCs. In the case of CsPbIBr2 IPSCs, we introduce the low-temperature deposition of CsPbIBr2 films, doping methods for the preparation of high-quality CsPbIBr2 films and strategies for improving the performance of solar cells. Furthermore, the mechanism of crystallization/interface engineering for the preparation of high-quality CsPbIBr2 films and efficient solar cells devices is emphasized. Finally, the development direction of further improving the PV performance and commercialization of mixed-halide IPSCs are summarized and prospected.

Anchoring Vertical Dipole to Enable Efficient Charge Extraction for High-Performance Perovskite Solar Cells

Perovskite solar cells (PSCs) via two-step sequential method have received great attention in recent years due to their high reproducibility and low processing costs. However, the relatively high trap-state density and poor charge carrier extraction efficiency pose challenges. Herein, highly efficient and stable PSCs via a two-step sequential method are fabricated using organic-inorganic (OI) complexes as multifunctional interlayers. In addition to reduce the under-coordinated Pb2+ ions related trap states by forming interactions with the functional groups, the complexes interlayer tends to form dipole moment which can enhance the built-in electric field, thus facilitating charge carrier extraction. Consequently, with rational molecular design, the resulting devices with a vertical dipole moment that parallels with the built-in electric field yield a champion efficiency of 23.55% with negligible hysteresis. More importantly, the hydrophobicity of the (OI) complexes contributes to an excellent ambient stability of the resulting device with 91% of initial efficiency maintained after 3000 h storage.

Multifunctional Two-Dimensional Benzodifuran-Based Polymer for Eco-Friendly Perovskite Solar Cells Featuring High Stability

Perovskite solar cells (PSCs) have been regarded as an exceptional renewable energy conversion technology due to their rapidly increasing photovoltaic efficiency, while their practical application is highly retarded by their intrinsic instability and potential lead ion leakage. Here, a two-dimensional (2D) π-conjugated benzodifuran-based polymer, PBDFP-Bz, is adopted to modify the perovskite film. Note that PBDFP-Bz could neutralize surface defects, fine-tune interfacial energetics, and hamper moisture ingression into the perovskite film. Therefore, high-quality perovskite films featuring reduced trap state density and enhanced moisture tolerance could be obtained after modification via PBDFP-Bz. Consequently, PBDFP-Bz-modified devices deliver a higher efficiency of 21.73% versus 19.55% of control ones. Meanwhile, PBDFP-Bz-modified devices can preserve 82.7 and 90.8% of their initial efficiency under continuous heating at 85 °C or light soaking for 500 h. However, the corresponding retained values of control devices are only 56.4 and 70.2%, respectively. Moreover, PBDFP-Bz can effectively prevent the leakage of lead ions in modified devices relative to control ones. This work not only reveals that PBDFP-Bz features high potential for fabricating high-performance and robust PSCs but also indicates that 2D π-conjugated benzodifuran-based polymers can endow PSCs with great security for sustainable development without the concern of lead ion leakage.