Chen Weijie

TiO _x N _y films are demonstrated to be an excellent electron‐selective contact for both crystalline silicon and organic solar cells. Remarkable efficiency of 22.3% and 17.02% is achieved for crystalline silicon and organic solar cells, respectively.

Abstract

High‐quality carrier‐selective contacts with suitable electronic properties are a prerequisite for photovoltaic devices with high power conversion efficiency (PCE). In this work, an efficient electron‐selective contact, titanium oxynitride (TiO _x N _y ), is developed for crystalline silicon (c‐Si) and organic photovoltaic devices. Atomic‐layer‐deposited TiO _x N _y is demonstrated to be highly conductive with a proper work function (4.3 eV) and a wide bandgap (3.4 eV). Thin TiO _x N _y films simultaneously provide a moderate surface passivation and enable a low contact resistivity on c‐Si surfaces. By implementation of an optimal TiO _x N _y ‐based contact, a state‐of‐the‐art PCE of 22.3% is achieved for a c‐Si solar cell featuring a full‐area dopant‐free electron‐selective contact. Simultaneously, conductive TiO _x N _y is proven to be an efficient electron‐transport layer for organic photovoltaic (OPV) devices. A remarkably high PCE of 17.02% is achieved for an OPV device with an electron‐transport TiO _x N _y layer, which is superior to conventional ZnO‐based devices with a PCE of 16.10%. Atomic‐layer‐deposited TiO _x N _y ETL on a large area with a high uniformity may help accelerate the commercialization of emerging solar technologies.

This work provides a simple, effective, and low‐cost method to fabricate a ZnO nanoparticle electron transport layer with a thickness higher than 130 nm. The doping of insulating polymer, polystyrene can not only modify the firm quality of ZnO to improve device performance, but also optimize the reproducibility, mechanical endurance, and ambient stability of the polymer‐based solar cells.

Abstract

The optimization of interfacial layer plays a critical role in the ultimate use of polymer‐based solar cells (PSCs). By introducing an insulating polymer, polystyrene (PS), into the ZnO nanoparticles (NPs) with large particle size, an electron transport layer (ETL) with a thickness of more than 130 nm is produced. The doping of PS not only improves the film quality of ZnO NPs to generate a denser, smoother, and more uniform ETL, but also increases the contact properties between the hydrophilic ZnO and hydrophobic active layer. In comparison to control devices, the power conversion efficiencies (PCEs), short circuit current densities, and fill factors of PSCs with the PS‐modified ETL for a typical fullerene system PTB7‐Th:PC₇₁BM and, also, a nonfullerene system PBDB‐T:ITIC are increased, with PCEs from 8.49% to 9.54% and 10.03% to 11.05%, respectively. The reproducibility, mechanical endurance, and ambient stability of the PSCs with the PS‐modified ZnO NP ETL are significantly improved. The combination of the insulating polymer and ZnO NPs provides a simple, low‐cost way to realize the commercialization of high performance, flexible PSCs.

Highly efficient flexible perovskite solar cells prepared by blade coating are reported. A dual hole transport layer comprised of “PEDOT:PSS/PTAA” is delicately designed, which forms a cascade energy level alignment, enabling markedly enhanced charge extraction. In conjugation with a morphology control by additive engineering, the scalable coated flexible solar cell shows an impressive efficiency of 19.41% with a record fill factor of 81%.

Abstract

Halide perovskites are one of the ideal photovoltaic materials for constructing flexible solar devices due to relatively high efficiencies for low‐temperature solution‐processed devices. However, the overwhelming majority of flexible perovskite solar cells are produced using spin coating, which represents a major hurdle for upscaling. Here, a scalable approach is reported to fabricate efficient and robust flexible perovskite solar cells on a polymer substrate. Thiourea is introduced into perovskite precursor solution to modulate the crystal growth, resulting in dense and uniform perovskite thin films on rough surfaces. As a decisive step, a cascade energy alignment is realized for the hole extraction layer by rationally designing a bilayer interface comprised of PEDOT:PSS/PTAA with a distinct offset in the highest occupied molecular orbital levels, enabling markedly enhanced charge extraction and spectral response. An efficiency as high as 19.41% and a record fill factor up to 81% are achieved for flexible perovskite devices processed by a scalable printing method. Equally important, the bilayer interface reinforces the bendability of the indium tin oxide substrate, leading to enhanced mechanical robustness of the flexible devices. These results underpin the importance of morphology control and interface design in constructing high‐performance flexible perovskite solar cells.

A comprehensive review on the current development and advanced understanding of Sn‐based perovskite solar cell (PSCs) from the viewpoint of A‐site cation engineering is demonstrated. The key challenges and current opportunities in the field of Sn‐based PSCs are discussed. This review highlights the significant promise of Sn‐based metal halide perovskites in the application of PSCs as well as many other potential opotoelectronic devices.

Abstract

Pb‐based metal halide perovskites (MHPs) have emerged as efficient light absorbers in third‐generation photovoltaic devices, and the latest certified power conversion efficiency (PCE) of Pb‐based perovskite solar cells (PSCs) has reached 25.2%. Despite great progress, Pb‐based MHPs are affected by toxicity, which hinders their market entry in a potential future large‐scale commercialization effort. Therefore, the exploration of Pb‐free MHPs has become one of the alternative solutions sought in the community. Among all the Pb‐free MHPs, Sn‐based MHPs show great promise owing to their similar or even superior theoretical optoelectronic characteristics. After several years of development, the PCE of Sn‐based PSCs has recently been approaching 10%, with the breakthroughs mainly coming from A‐site cation engineering of Sn‐based MHPs. In this review, the crucial status of A‐site cation engineering strategies in the research of Sn‐based PSCs is highlighted. First, the way the features of A‐site cation influence the structure and characteristics of MHPs is systematically demonstrated. Then, the state‐of‐the‐art developments, focusing on A‐site cation engineering of Sn‐based MHPs, are comprehensively reviewed. Subsequently, the current challenges and opportunities for further boosting the performance of Sn‐based PSCs are discussed. Finally, conclusions and perspectives on the promising Sn‐based optoelectronic devices are discussed.

A systematic study of a series of polymer:non‐fullerene acceptor blends is conducted to unify the cumulative effects of voltages losses, charge generation efficiencies, non‐geminate recombination and extraction dynamics, and nuanced morphological differences to the device performance. Deconvolution of the major loss processes in these blends and their connections to the nuanced bulk‐heterojunction morphology and energetics are established.

Abstract

Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non‐fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non‐geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid‐state NMR (for atomic level insights on the local ordering and donor:acceptor ππ interactions) and resonant soft X‐ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

By using a solvent‐mediated phase transformation process, a record certified 21.8% power conversion efficiency in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based solar cells is achieved.

Abstract

Composition and film quality of perovskite are crucial for the further improvement of perovskite solar cells (PSCs), including efficiency, reproducibility, and stability. Here, it is demonstrated that by simply mixing 50% of formamidinium (FA⁺) into methylammonium lead iodide (MAPbI₃), a highly crystalline, stable phase, and compact, polycrystalline grain morphology perovskite is formed by using a solvent‐mediated phase transformation process via the synergism of dimethyl sulfoxide and diethyl ether, which shows long carrier lifetime, low trap state density, and a record certified 21.8% power conversion efficiency (PCE) in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based PSCs. These PSCs show very high operational stability, with 85% PCE retention upon 1000 h 1 Sun intensity illumination. A 17.33% PCE module (6.5 × 7 cm²) is also demonstrated, attesting to the scalability of such devices.

Conjugated polyelectrolytes (CPEs) are studied as interlayers in perovskite‐based solar cells. By modulating the ionic density in CPEs, wetting, perovskite crystal growth, and interfacial defect passivation are optimized, achieving 18.38% efficiency for a large‐area (1 cm²) device with negligible hysteresis and stable power output.

Abstract

A series of anionic conjugated polyelectrolytes (CPEs) is synthesized based on poly(fluorene‐co‐phenylene) by varying the side‐chain ionic density from two to six per repeat units (MPS2‐TMA, MPS4‐TMA, and MPS6‐TMA). The effect of MPS2, 4, 6‐TMA as interlayers on top of a hole‐extraction layer of poly(bis(4‐phenyl)‐2,4,6‐trimethylphenylamine (PTAA) is investigated in inverted perovskite solar cells (PeSCs). Owing to the improved wettability of perovskites on hydrophobic PTAA with the CPEs, the PeSCs with CPE interlayers demonstrate a significantly enhanced device performance, with negligible device‐to‐device dependence relative to the reference PeSC without CPEs. By increasing the ionic density in the MPS‐TMA interlayers, the wetting, interfacial defect passivation, and crystal growth of the perovskites are significantly improved without increasing the series resistance of the PeSCs. In particular, the open‐circuit voltage increases from 1.06 V for the PeSC with MPS2‐TMA to 1.11 V for the PeSC with MPS6‐TMA. The trap densities of the PeSCs with MPS2,4,6‐TMA are further analyzed using frequency‐dependent capacitance measurements. Finally, a large‐area (1 cm²) PeSC is successfully fabricated with MPS6‐TMA, showing a power conversion efficiency of 18.38% with negligible hysteresis and a stable power output under light soaking for 60 s.

Incorporating silver atoms into the inorganic halide perovskite Cs₃Bi₂Br₉ to form Cs₂AgBiBr₆ eliminates the strong localization of electron–hole pairs, makes the electronic band distribution more dispersible, and further changes the photoelectric properties including band structure, exciton binding energy, charge carrier mobility, and carrier relaxation lifetime, leading to a remarkable enhancement in photocatalytic hydrogen evolution under visible light.

Abstract

Lead‐free inorganic halide perovskites have triggered appealing interests in various energy‐related applications including solar cells and photocatalysis. However, why perovskite‐structured materials exhibit excellent photoelectric properties and how the unique crystalline structures affect the charge behaviors are still not well elucidated but essentially desired. Herein, taking inorganic halide perovskite Cs₃Bi₂Br₉ as a prototype, the significant derivation process of silver atoms incorporation to induce the structural transformation from Cs₃Bi₂Br₉ to Cs₂AgBiBr₆, which brings about dramatic differences in photoelectric properties is unraveled. It is demonstrated that the silver incorporation results in the co‐operated orbitals hybridization, which makes the electronic distributions in conduction and valence bands of Cs₂AgBiBr₆ more dispersible, eliminating the strong localization of electron–hole pairs. As consequences of the electronic structures derivation, exhilarating changes in photoelectric properties like band structure, exciton binding energy, and charge carrier dynamics are verified experimentally and theoretically. Using photocatalytic hydrogen evolution activity under visible light as a typical evaluation, such crystalline structure transformation contributes to a more than 100‐fold enhancement in photocatalytic performances compared with pristine Cs₃Bi₂Br₉, verifying the significant effect of structural derivations on the exhibited performances. The findings will provide evidences for understanding the origin of photoelectric properties for perovskites semiconductors in solar energy conversion.

Iron pyrite (FeS₂) is predicted to be the lowest‐cost material for solar electricity production. However, its solar energy conversion efficiency is below 3% because of low photovoltage (<0.2 V). The photovoltage loss mechanisms associated with FeS₂ as absorber materials in solar cell applications are discussed, followed by recommendation for future research.

Abstract

Considering the natural abundance, the optoelectronic properties, and the electricity production cost, iron pyrite (FeS₂) has a strong appeal as a solar cell material. The maximum conversion efficiency of FeS₂ solar cells demonstrated to date, however, is below 3%, which is significantly below the theoretical efficiency limit of 25%. This poor conversion efficiency is mainly the result of the poor photovoltage, which has never exceeded 0.2 V with a device having appreciable photocurrent. Several studies have explored the origin of the low photovoltage in FeS₂ solar cells, and have improved understanding of the photovoltage loss mechanisms. Fermi level pinning, surface inversion, ionization of bulk donor states, and photocarrier loss have been suggested as the underlying reasons for the photovoltage loss in FeS₂. Given the past and more recent scientific data, together with contradictory results to some extent, it is timely to discuss these mechanisms to give an updated view of the present status and remaining challenges. Herein, the current understanding of the origin of low photovoltage in FeS₂ solar cells is critically reviewed, preceded by a succinct discussion on the electronic structure and optoelectronic properties. Finally, suggestions of a few research directions are also presented.

SnO₂ and perovskite have been bridged with multifunctional graphdiyne. Such delicate interface modification boosted the performance of solar cells in energy band alignment, electron mobility improvement, controllable perovskite growth inducement, and interface defect passivation.

Abstract

The matching of charge transport layer and photoactive layer is critical in solar energy conversion devices, especially for planar perovskite solar cells based on the SnO₂ electron‐transfer layer (ETL) owing to its unmatched photogenerated electron and hole extraction rates. Graphdiyne (GDY) with multi‐roles has been incorporated to maximize the matching between SnO₂ and perovskite regarding electron extraction rate optimization and interface engineering towards both perovskite crystallization process and subsequent photovoltaic service duration. The GDY doped SnO₂ layer has fourfold improved electron mobility due to freshly formed C−O σ bond and more facilitated band alignment. The enhanced hydrophobicity inhibits heterogeneous perovskite nucleation, contributing to a high‐quality film with diminished grain boundaries and lower defect density. Also, the interfacial passivation of Pb−I anti‐site defects has been demonstrated via GDY introduction.

Trial separation : 2D layered Pb‐ and Sn‐halide perovskites show surprising optical properties, such as dual excitonic emissions. Such unusual properties are shown to arise through to the interaction between the 2D inorganic layers. Separating the inorganic layers either by molecular intercalation or by increasing the length organic spacer ion reversibly switches the optical properties.

Abstract

In layered hybrid perovskites, such as (BA)₂PbI₄ (BA=C₄H₉NH₃), electrons and holes are considered to be confined in atomically thin two dimensional (2D) Pb–I inorganic layers. These inorganic layers are electronically isolated from each other in the third dimension by the insulating organic layers. Herein we report our experimental findings that suggest the presence of electronic interaction between the inorganic layers in some parts of the single crystals. The extent of this interaction is reversibly tuned by intercalation of organic and inorganic molecules in the layered perovskite single crystals. Consequently, optical absorption and emission properties switch reversibly with intercalation. Furthermore, increasing the distance between inorganic layers by increasing the length of the organic spacer cations systematically decreases these electronic interactions. This finding that the parts of the layered hybrid perovskites are not strictly electronically 2D is critical for understanding the electronic, optical, and optoelectronic properties of these technologically important materials.

This work reports a compatible strategy to enhance the efficiency of planar n–i–p Sb₂Se₃ solar cells through Sb₂Se₃ surface modification and an architecture with oriented 1D van der Waals material, trigonal selenium (t‐Se). The p‐type t‐Se layer functionally works as a surface passivation and hole transport material. The all‐inorganic device structure enables high efficiency and superb stability.

Abstract

Environmentally benign and potentially cost‐effective Sb₂Se₃ solar cells have drawn much attention by continuously achieving new efficiency records. This article reports a compatible strategy to enhance the efficiency of planar n–i–p Sb₂Se₃ solar cells through Sb₂Se₃ surface modification and an architecture with oriented 1D van der Waals material, trigonal selenium (t‐Se). A seed layer assisted successive close spaced sublimation (CSS) is developed to fabricate highly crystalline Sb₂Se₃ absorbers. It is found that the Sb₂Se₃ absorber exhibits a Se‐deficient surface and negative surface band bending. Reactive Se is innovatively introduced to compensate the surface Se deficiency and form an (101) oriented 1D t‐Se interlayer. The p‐type t‐Se layer promotes a favored band alignment and band bending at the Sb₂Se₃/t‐Se interface, and functionally works as a surface passivation and hole transport material, which significantly suppresses interface recombination and enhances carrier extraction efficiency. An efficiency of 7.45% is obtained in a planar Sb₂Se₃ solar cell in superstrate n–i–p configuration, which is the highest efficiency for planar Sb₂Se₃ solar cells prepared by CSS. The all‐inorganic Sb₂Se₃ solar cell with t‐Se shows superb stability, retaining ≈98% of the initial efficiency after 40 days storage in open air without encapsulation.

An optimal power conversion efficiency (PCE) of 17.4% is achieved in the optimized ternary organic photovoltaics (OPVs) with two well‐compatible acceptors (BTP‐4F‐12 and MeIC) and one wide bandgap donor (PM6), resulting from simultaneously improved J _SC, fill factor (FF), and V _OC. The energy loss of ternary OPVs is minimized compared with the two binary OPVs, which is an important development for PCE improvement of ternary OPVs.

Abstract

A power conversion efficiency (PCE) of 16.2% is achieved in PM6:BTP‐4F‐12 based organic photovoltaics (OPVs). On the basis of efficient binary OPVs, a series of ternary OPVs are constructed by incorporating MeIC as the third component. The open circuit voltages (V _OCs) of ternary OPVs can be gradually increased along with the incorporation of MeIC, suggesting the formation of an alloy state between BTP‐4F‐12 and MeIC with good compatibility. The energy loss (E _loss) of ternary OPVs can be decreased compared with that of two binary OPVs, contributing to the V _OC improvement of ternary OPVs. The short circuit current density (J _SC) and fill factor (FF) of ternary OPVs can also be simultaneously enhanced with MeIC content up to 10 wt% in acceptors, leading to 17.4% PCE of the optimized ternary OPVs. The J _SC and FF improvement of ternary OPVs is thought to result from the optimized ternary active layers with more efficient photon harvesting, exciton dissociation and charge transport. The 17.4% PCE and 79.2% FF is among the top values of ternary OPVs. This work indicates that a ternary strategy is an emerging method to simultaneously minimize E _loss and optimize photon harvesting as well as improve the morphology of active layers for realizing performance improvement for OPVs.

Di‐n‐propylamine solution in methyl acetate is successfully demonstrated as an efficient solid‐state treatment for CsPbI₃ perovskite quantum dot (PQD) solar cells, and a record power conversion efficiency of ≈15% and high reproducibility are achieved for CsPbI₃ PQD solar cells.

Abstract

Lead‐halide perovskite quantum dots (PQDs) or more broadly, nanocrystals possess advantageous features for solution‐processed photovoltaic devices. The nanocrystal surface ligands play a crucial role in the transport of photogenerated carriers and ultimately affect the overall performance of PQD solar cells. Significantly improved CsPbI₃ PQD synthetic yield and solar‐cell performance through surface ligand management are demonstrated. The treatment of a secondary amine, di‐n‐propylamine (DPA), provides a mild and efficient approach to control the surface ligand density of PQDs, which has an apparently different working mechanism compared to previously reported surface treatments. Using an optimal DPA concentration, the treatment can simultaneously remove both long‐chain insulating surface ligands of oleic acid and oleylamine, even for unpurified PQDs with high ligand density. As a result, the electrical coupling between PQDs is enhanced, leading to improved charge transport, reduced carrier recombination, and a high power conversion efficiency approaching 15% for CsPbI₃‐PQD‐based solar cells. In addition, the production yield of CsPbI₃ PQDs can be increased by a factor of 8. These results highlight the importance of developing new ligand‐management strategies, specifically for emerging PQDs to achieve scalable and high‐performance perovskite‐based optoelectronic devices.

The article studies SnO₂'s role in the stability of air‐processed planar perovskite solar cells. UV treatment of sub‐cells (500 h N₂ environment) speeds up the depletion of perovskite films, leading to excess PbI₂ formation at the perovskite surfaces. This inadvertently leads to full device stabilization through passivation as seen in maximum power point (MPP) measurements of perovskite solar cells incorporating UV‐treated sub‐cells.

SnO₂ is nowadays the widely preferred material as an electron transport layer (ETL) in most n‐i‐p planar perovskite solar cells (PSCs) due to its facility for ambient, low temperature processing, and ultraviolet (UV) stability. Most reports published so far study device stability on full cells. Herein, the role of slot‐die‐coated SnO₂ on air‐processed planar PSCs by analyzing sub‐cells (indium tin oxide [ITO]/SnO₂/perovskite) under UV exposure is investigated. Results from UV–vis spectroscopy, depth profiling using X‐ray diffraction measurement in grazing incidence mode (GIXRD), X‐ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy measurements show that UV treatment of ITO/SnO₂/perovskite leads to a reduced electron transfer to the SnO₂ layer and a gradual increase in the amount of PbI₂ toward the perovskite surfaces. Subsequently, hole transport layer (HTL) and electrodes are applied on SnO₂/perovskite interfaces (UV‐treated and non‐UV‐treated) and complete devices are fabricated. Device performance is compared and analyzed through J–V curves and maximum power point (MPP) tracking. Results show that devices built on a UV‐treated SnO₂/perovskite interface show better stability attributed to the presence of excess PbI₂ resulting in a passivation effect. Challenges in uniform film formation of slot‐die‐coated SnO₂ and potential solutions using a polymeric additive are also highlighted.

Dilution by high‐concentration of insulators in organic semiconducting donor: acceptor blends yields enhanced solar cells. The diluted ternary solar cells show improved mobility and decreased radiative and nonradiative recombination. In addition, both thermal and environmental stability of the diluted ternary solar cells are improved, and the scalability of the diluted ternary solar cell is enhanced.

Incorporation of insulating polymers or molecules into organic semiconductor films, creating so‐called diluted organic semiconductors, has been successfully used both in organic field‐effect transistors and organic light‐emitting diodes to reduce sensitivity to charge traps. However, application of this strategy in organic photovoltaics is challenging due to the complex requirements on the light‐absorbing blend layer. Herein, diluted donor–acceptor–insulator ternary organic solar cells are developed to improve mobility and decrease radiative and nonradiative recombination in the active layer. In addition, both thermal and environmental stability of the diluted ternary solar cells are enhanced. Finally, the diluted semiconductor approach enables large‐area solar cells to be fabricated where the loss in power density upon cell area upscaling is five times lower than for the equivalent binary cells.

In this work, two novel A‐DA₁D‐A‐type asymmetric SMAs are developed, namely C‐shaped BDTP‐4F and S‐shaped BTDTP‐4F . As a result, C‐shape BDTP‐4F‐based device yields a higher PCE (15.24%) than that of S‐shape BTDTP‐4F‐based device (13.12%), while for traditional A‐D‐A type SMAs, IDTP‐4F with S‐shape conformation is better than that of C‐shape IDTTP‐4F.

Abstract

Understanding the conformation effect on molecular packing, miscibility, and photovoltaic performance is important to open a new avenue for small‐molecule acceptor (SMA) design. Herein, two novel acceptor–(donor‐acceptor1‐donor)–acceptor (A‐DA1D‐A)‐type asymmetric SMAs are developed, namely C‐shaped BDTP‐4F and S‐shaped BTDTP‐4F . The BDTP‐4F ‐based polymer solar cells (PSCs) with PM6 as donor, yields a power conversion efficiency (PCE) of 15.24%, significantly higher than that of the BTDTP‐4F ‐based device (13.12%). The better PCE for BDTP‐4F ‐based device is mainly attributed to more balanced charge transport, weaker bimolecular recombination, and more favorable morphology. Additionally, two traditional A‐D‐A‐type SMAs (IDTP‐4F and IDTTP‐4F ) are also synthesized to investigate the conformation effect on morphology and device performance. Different from the device result above, here, IDTP‐4F with S‐shape conformation outperforms than IDTTP‐4F with C‐shape conformation. Importantly, it is found that for these two different types of SMA, the better performing binary blend has similar morphological characteristics. Specifically, both PM6:BDTP‐4F and PM6:IDTP‐4F blend exhibit perfect nanofibril network structure with proper domain size, obvious face‐on orientation and enhance donor‐acceptor interactions, thereby better device performance. This work indicates tuning molecular conformation plays pivotal role in morphology and device effciciency, shining a light on the molecular design of the SMAs.

A comprehensive review on the current development and advanced understanding of Sn‐based perovskite solar cells (PSCs) from the viewpoint of A‐site cation engineering is demonstrated. The key challenges and current opportunities in the field of Sn‐based PSCs are discussed. This review highlights the significant promise of Sn‐based metal halide perovskites in the application of PSCs as well as many other potential optoelectronic devices.

Abstract

Pb‐based metal halide perovskites (MHPs) have emerged as efficient light absorbers in third‐generation photovoltaic devices, and the latest certified power conversion efficiency (PCE) of Pb‐based perovskite solar cells (PSCs) has reached 25.2%. Despite great progress, Pb‐based MHPs are affected by toxicity, which hinders their market entry in a potential future large‐scale commercialization effort. Therefore, the exploration of Pb‐free MHPs has become one of the alternative solutions sought in the community. Among all the Pb‐free MHPs, Sn‐based MHPs show great promise owing to their similar or even superior theoretical optoelectronic characteristics. After several years of development, the PCE of Sn‐based PSCs has recently been approaching 10%, with the breakthroughs mainly coming from A‐site cation engineering of Sn‐based MHPs. In this review, the crucial status of A‐site cation engineering strategies in the research of Sn‐based PSCs is highlighted. First, the way the features of A‐site cation influence the structure and characteristics of MHPs is systematically demonstrated. Then, the state‐of‐the‐art developments, focusing on A‐site cation engineering of Sn‐based MHPs, are comprehensively reviewed. Subsequently, the current challenges and opportunities for further boosting the performance of Sn‐based PSCs are discussed. Finally, conclusions and perspectives on the promising Sn‐based optoelectronic devices are discussed.

A dual‐functionalized bidentate molecule 2‐(2′‐thienyl)pyridine (2‐ThPy) is introduced to modulate perovskite crystallization and passivate halogen vacancy defects. Compared with monodentate counterparts, 2‐ThPy can anchor Pb²⁺ sites via S and N atomic bonding simultaneously. Consequently, 2‐ThPy‐treated CsPbI₂Br perovskite solar cells achieve a champion power conversion efficiency of 12.69% with negligible hysteresis and exhibit prominent moisture stability.

All inorganic mixed‐halide CsPbI₂Br perovskites with suitable bandgap and superior thermal durability have ignited rising interests in the field of perovskite solar cells (PSCs). However, the serious energy losses derived from deleterious trap‐assisted defects–induced notorious nonradiative recombination and inferior moisture durability are still the primary hindrance on the way to develop high‐performance CsPbI₂Br PSCs. Herein, a novel passivation strategy is presented by introducing dual‐functionalized bidentate molecule 2‐(2′‐thienyl)pyridine (2‐ThPy) to modulate perovskite crystallization and passivate halogen vacancy defects. Compared with monodentate counterparts, 2‐ThPy can anchor Pb²⁺ sites via S and N atomic bonding simultaneously, and the synthesized CsPbI₂Br films exhibit enlarged grain size, show advantages to passivate defect states, and dramatically reduce trap density, thereby lessening the detrimental carrier recombination. Consequently, a champion power conversion efficiency (PCE) of 12.69% with negligible hysteresis is delivered for the fabricated CsPbI₂Br PSCs treated with 2‐ThPy. Moreover, the moisture stability of CsPbI₂Br PSCs with 2‐ThPy is also greatly enhanced, and the device without encapsulation retains 92% of initial PCE value after 30 days aging under 25 °C and 40% relative humidity in ambient environment. The bidentate molecules passivation strategy paves a promising avenue to implement efficient and stable inorganic PSCs.

A new series of dopant‐free hole‐transporting materials (HTMs) YZT1–YZT4 featuring porphyrin backbone is achieved in which the best power conversion efficiency (PCE) of YZT4 is 14.95% for doped and that of YZT1 is 13.10% for dopant‐free perovskite solar cells (PSCs) based on TiO₂ semiconductors.

A new series of structurally simple and easily accessible hole‐transporting materials (HTMs) YZT1–YZT4 using porphyrin backbone is devised for high‐performance perovskite solar cells (PSCs) with and without the aid of doping. The YZT‐series HTMs have either push–push or push–pull type planar linear molecular geometry with substitution of linear or branched alkylamine. UV–vis absorption, photoluminance (PL) quenching experiments, and theoretical studies all suggest a different pattern of molecular packing induced by molecular geometry and/or substituted chains. Nonetheless, both types of porphyrin HTMs perform well in TiO₂‐based PSCs when doped YZT4 with a power conversion efficiency (PCE) of 14.95% and undoped YZT1 with a PCE of 13.10%. The results clearly reveal the potential for porphyrin‐based HTMs for use in dopant‐free PSCs.

Highly efficient flexible perovskite solar cells prepared by blade coating are reported. A dual hole transport layer comprised of “PEDOT:PSS/PTAA” is delicately designed, which forms a cascade energy level alignment, enabling markedly enhanced charge extraction. In conjugation with a morphology control by additive engineering, the scalable coated flexible solar cell shows an impressive efficiency of 19.41% with a record fill factor of 81%.

Abstract

Halide perovskites are one of the ideal photovoltaic materials for constructing flexible solar devices due to relatively high efficiencies for low‐temperature solution‐processed devices. However, the overwhelming majority of flexible perovskite solar cells are produced using spin coating, which represents a major hurdle for upscaling. Here, a scalable approach is reported to fabricate efficient and robust flexible perovskite solar cells on a polymer substrate. Thiourea is introduced into perovskite precursor solution to modulate the crystal growth, resulting in dense and uniform perovskite thin films on rough surfaces. As a decisive step, a cascade energy alignment is realized for the hole extraction layer by rationally designing a bilayer interface comprised of PEDOT:PSS/PTAA with a distinct offset in the highest occupied molecular orbital levels, enabling markedly enhanced charge extraction and spectral response. An efficiency as high as 19.41% and a record fill factor up to 81% are achieved for flexible perovskite devices processed by a scalable printing method. Equally important, the bilayer interface reinforces the bendability of the indium tin oxide substrate, leading to enhanced mechanical robustness of the flexible devices. These results underpin the importance of morphology control and interface design in constructing high‐performance flexible perovskite solar cells.