Long Electron–Hole Diffusion Length in High‐Quality Lead‐Free Double Perovskite Films

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead‐based perovskite solar cells. Here, the first double perovskite (Cs₂AgBiBr₆) solar cells using the planar structure are demonstrated. The prepared Cs₂AgBiBr₆ films are composed of high‐crystal‐quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high‐quality double perovskite films show long electron–hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO₂ exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.     

Pb–Sn–Cu Ternary Organometallic Halide Perovskite Solar Cells

Exploiting organic/inorganic hybrid perovskite solar cells (PSCs) with reduced Pb content is very important for developing environment‐friendly photovoltaics. Utilizing of Pb–Sn alloying perovskite is considered as an efficient route to reduce the risk of ecosystem pollution. However, the trade‐off between device performance and Sn substitution ratio due to the instability of Sn²⁺ is a current dilemma. Here, for the first time, the highly efficient Pb–Sn–Cu ternary PSCs are reported by partial replacing of PbI₂ with SnI₂ and CuBr₂. Sn²⁺ substitution results in a redshift of the absorption onset, whereas worsens the film quality. Interestingly, Cu²⁺ introduction can passivate the trap sites at the crystal boundaries of Pb–Sn perovskites effectively. Consequently, a power conversion efficiency as high as 21.08% in inverted planar Pb–Sn–Cu ternary PSCs is approached. The finding opens a new route toward the fabrication of high efficiency Pb–Sn alloying perovskite solar cells by Cu²⁺ passivation.     

Highly Stable and Efficient FASnI3‐Based Perovskite Solar Cells by Introducing Hydrogen Bonding

Tin‐based perovskites with narrow bandgaps and high charge‐carrier mobilities are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, the crystalline rate of tin‐based perovskites is much faster, leading to abundant trap states and much lower open‐circuit voltage (V_oc). Here, hydrogen bonding is introduced to retard the crystalline rate of the FASnI₃ perovskite. By adding poly(vinyl alcohol) (PVA), the O? H…I^? hydrogen bonding interactions between PVA and FASnI₃ have the effects of introducing nucleation sites, slowing down the crystal growth, directing the crystal orientation, reducing the trap states, and suppressing the migration of the iodide ions. In the presence of the PVA additive, the FASnI₃–PVA PSCs attain higher power conversion efficiency of 8.9% under a reverse scan with significantly improved V_oc from 0.55 to 0.63 V, which is one of the highest V_oc values for FASnI₃‐based PSCs. More importantly, the FASnI₃–PVA PSCs exhibit striking long‐term stability, with no decay in efficiency after 400 h of operation at the maximum power point. This approach, which makes use of the O? H…I^? hydrogen bonding interactions between PVA and FASnI₃, is generally applicable for improving the efficiency and stability of the FASnI₃‐based PSCs.     

Structural and Functional Diversity in Lead‐Free Halide Perovskite Materials

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high‐performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less‐toxic, elements while maintaining the superior properties. Here, the design rules for lead‐free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead‐free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic‐related properties. 3D perovskites, especially A₂B⁺B³⁺X₆‐type double perovskites, demonstrate very promising optoelectronic prospects, while low‐dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure–property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead‐free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead‐free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.     

Efficient and Stable Inverted Perovskite Solar Cells Incorporating Secondary Amines

Large‐bandgap perovskites offer a route to improve the efficiency of energy capture in photovoltaics when employed in the front cell of perovskite–silicon tandems. Implementing perovskites as the front cell requires an inverted (p–i–n) architecture; this architecture is particularly effective at harnessing high‐energy photons and is compatible with ionic‐dopant‐free transport layers. Here, a power conversion efficiency of 21.6% is reported, the highest among inverted perovskite solar cells (PSCs). Only by introducing a secondary amine into the perovskite structure to form MA_1?xDMA_xPbI₃ (MA is methylamine and DMA is dimethylamine) are defect density and carrier recombination suppressed to enable record performance. It is also found that the controlled inclusion of DMA increases the hydrophobicity and stability of films in ambient operating conditions: encapsulated devices maintain over 80% of their efficiency following 800 h of operation at the maximum power point, 30 times longer than reported in the best prior inverted PSCs. The unencapsulated devices show record operational stability in ambient air among PSCs.     

Amide‐Catalyzed Phase‐Selective Crystallization Reduces Defect Density in Wide‐Bandgap Perovskites

Wide‐bandgap (WBG) formamidinium–cesium (FA‐Cs) lead iodide–bromide mixed perovskites are promising materials for front cells well‐matched with crystalline silicon to form tandem solar cells. They offer avenues to augment the performance of widely deployed commercial solar cells. However, phase instability, high open‐circuit voltage (V_oc) deficit, and large hysteresis limit this otherwise promising technology. Here, by controlling the crystallization of FA‐Cs WBG perovskite with the aid of a formamide cosolvent, light‐induced phase segregation and hysteresis in perovskite solar cells are suppressed. The highly polar solvent additive formamide induces direct formation of the black perovskite phase, bypassing the yellow phases, thereby reducing the density of defects in films. As a result, the optimized WBG perovskite solar cells (PSCs) (E_g ≈ 1.75 eV) exhibit a high V_oc of 1.23 V, reduced hysteresis, and a power conversion efficiency (PCE) of 17.8%. A PCE of 15.2% on 1.1 cm² solar cells, the highest among the reported efficiencies for large‐area PSCs having this bandgap is also demonstrated. These perovskites show excellent phase stability and thermal stability, as well as long‐term air stability. They maintain ≈95% of their initial PCE after 1300 h of storage in dry air without encapsulation.     

Ion‐Migration Inhibition by the Cation–π Interaction in Perovskite Materials for Efficient and Stable Perovskite Solar Cells

Migration of ions can lead to photoinduced phase separation, degradation, and current–voltage hysteresis in perovskite solar cells (PSCs), and has become a serious drawback for the organic–inorganic hybrid perovskite materials (OIPs). Here, the inhibition of ion migration is realized by the supramolecular cation–π interaction between aromatic rubrene and organic cations in OIPs. The energy of the cation–π interaction between rubrene and perovskite is found to be as strong as 1.5 eV, which is enough to immobilize the organic cations in OIPs; this will thus will lead to the obvious reduction of defects in perovskite films and outstanding stability in devices. By employing the cation‐immobilized OIPs to fabricate perovskite solar cells (PSCs), a champion efficiency of 20.86% and certified efficiency of 20.80% with negligible hysteresis are acquired. In addition, the long‐term stability of cation‐immobilized PSCs is improved definitely (98% of the initial efficiency after 720 h operation), which is assigned to the inhibition of ionic diffusions in cation‐immobilized OIPs. This cation–π interaction between cations and the supramolecular π system enhances the stability and the performance of PSCs efficiently and would be a potential universal approach to get the more stable perovskite devices.     

Environmentally Robust Memristor Enabled by Lead‐Free Double Perovskite for High‐Performance Information Storage

Memristors are emerging as a rising star of new computing and information storage techniques. However, the practical applications are severely challenged by their instability toward harsh conditions, including high moisture, high temperatures, fire, ionizing irradiation, and mechanical bending. In this work, for the first time, lead‐free double perovskite Cs₂AgBiBr₆ is utilized for environmentally robust memristors, enabling highly efficient information storage. The memory performance of the typical indium‐tin‐oxide/Cs₂AgBiBr₆/Au sandwich‐like memristors is retained after 1000 switching cycles, 10⁵ s of reading, and 10⁴ times of mechanical bending, comparable to other halide perovskite memristors. Most importantly, the memristive behavior remains robust in harsh environments, including humidity up to 80%, temperatures as high as 453 K, an alcohol burner flame for 10 s, and ⁶⁰Co γ‐ray irradiation for a dosage of 5 × 10⁵ rad (SI), which is not achieved by any other memristors and commercial flash memory techniques. The realization of an environmentally robust memristor from Cs₂AgBiBr₆ with a high memory performance will inspire further development of robust electronics using lead‐free double perovskites.     

SrCl2 Derived Perovskite Facilitating a High Efficiency of 16% in Hole‐Conductor‐Free Fully Printable Mesoscopic Perovskite Solar Cells

Despite the breakthrough of over 22% power conversion efficiency demonstrated in organic–inorganic hybrid perovskite solar cells (PVSCs), critical concerns pertaining to the instability and toxicity still remain that may potentially hinder their commercialization. In this study, a new chemical approach using environmentally friendly strontium chloride (SrCl₂) as a precursor for perovskite preparation is demonstrated to result in enhanced device performance and stability of the derived hole‐conductor‐free printable mesoscopic PVSCs. The CH₃NH₃PbI₃ perovskite is chemically modified by introducing SrCl₂ in the precursor solution. The results from structural, elemental, and morphological analyses show that the incorporation of SrCl₂ affords the formation of CH₃NH₃PbI₃(SrCl₂)_x perovskites endowed with lower defect concentration and better pore filling in the derived mesoscopic PVSCs. The optimized compositional CH₃NH₃PbI₃(SrCl₂)_0.1 perovskite can substantially enhance the photovoltaic performance of the derived hole‐conductor‐free device to 15.9%, outperforming the value (13.0%) of the pristine CH₃NH₃PbI₃ device. More importantly, the stability of the device in ambient air under illumination is also improved.     

Carbon-based all-inorganic perovskite solar cells: Progress,challenges and strategies toward 20% efficiency

Organic-inorganic hybrid perovskite solar cells (PSCs) are promising next-generation photovoltaic technology. However, their long-term operation is limited due to thermodynamic instability of hybrid perovskites (loss of organics) and severe migration of constituents (ions and dopants). PSCs have to be free of volatile organics and mobile dopants to become commercially relevant. PSCs based on cesium lead halide inorganic perovskites (CsPbI_3−xBr_x, x = 0 ~ 3) and a carbon electrode, abbreviated here as C-IPSCs, fulfill these requirements: CsPbI_3−xBr_x is stable against decomposition to binary halides and the carbon electrode is inherently moisture-resistive and dopant-free. Since the first report of C-IPSCs in 2016, their power conversion efficiencies (PCEs) have doubled, recently reaching 14.84% with an astonishing stability of over 2000 h at 80 °C and 80% relative humidity (RH). Here we review recent progress of C-IPSCs and analyze the remaining critical issues in the field. We then offer our perspective to address these challenges through morphology, interface, spectral and material engineering. Finally, we argue that C-IPSCs have potential to overcome the 20% efficiency milestone, making them – in combination with their already impressive stability – the most promising PSC architecture for commercialization.     

Advances in cesium lead iodide perovskite solar cells: Processing science matters

The prevailing perovskite solar cells (PSCs) employ hybrid organic–inorganic halide perovskites as light absorbers, but these materials exhibit relatively poor environmental stability, which potentially hinders the practical deployment of PSCs. One important strategy to address this issue is replacing the volatile and hygroscopic organic cations with inorganic cesium cations in the crystal structure, forming all-inorganic halide perovskites. In this context, CsPbI₃ perovskite is drawing phenomenal attention, primarily because it exhibits an ideal bandgap of 1.7 eV for the use in tandem solar cells, and it shows significantly enhanced thermal stability that is the key to the long-term device operation. Within only half a decade, the power conversion efficiency (PCE) of CsPbI₃ PSCs has ramped beyond 20%, which has been driven by inventions of numerous processing methods for high-quality CsPbI₃ perovskite thin films. These methods are broadly classified into three categories: vapor deposition, nanocrystals assembly, and solution deposition. Herein we present a systematic review on these methods and related materials sciences. In particular, we comprehensively discuss the dimethylammonium-additive-based solution deposition, which has resulted into the best-performing CsPbI₃ PSCs. We also present the challenges and prospects on future research towards the realization of the full potential of CsPbI₃ PSCs.     

Intrinsic Instability of Inorganic–Organic Hybrid Halide Perovskite Materials

Hybrid lead halide perovskite materials are used in solar cells and show efficiencies greater than 23%. Furthermore, they are applied in light‐emitting diodes, X‐ray detectors, thin‐film transistors, thermoelectrics, and memory devices. Lead trihalide hybrid materials contain methylammonium (MA) or formamidinium (FA) (or a mixture), or long alkylammonium halides, as alternative organic cations. However, the intrinsic stability of hybrid lead halide perovskites is not very high, and they are chemically unstable when exposed to moisture, light, or heat because of their organic contents and low formation energies. Therefore, although improvements in the chemical stability are crucial, changing the material composition is challenging because it is directly related to the desired application requirements. Fortunately, hybrid lead halide perovskites have a very high tolerance toward changes in physical properties arising from doping or addition of different cations and anions, in many cases showing improved properties. Here, the intrinsic instability of hybrid lead halide perovskites is reviewed in relation to the crystal phase and chemical stability. It is suggested that FA should be used for lead halide perovskites for chemical stability instead of MA. Furthermore, additives that stabilize the crystal phase with α‐FAPbI₃ should eschew MA.     

Suppressing Defects-Induced Nonradiative Recombination for Efficient Perovskite Solar Cells through Green Antisolvent Engineering

Organic–inorganic hybrid perovskites have attracted considerable attention due to their superior optoelectronic properties. Traditional one-step solution-processed perovskites often suffer from defects-induced nonradiative recombination, which significantly hinders the improvement of device performance. Herein, treatment with green antisolvents for achieving high-quality perovskite films is reported. Compared to defects-filled ones, perovskite films by antisolvent treatment using methylamine bromide (MABr) in ethanol (MABr-Eth) not only enhances the resultant perovskite crystallinity with large grain size, but also passivates the surface defects. In this case, the engineering of MABr-Eth-treated perovskites suppressing defects-induced nonradiative recombination in perovskite solar cells (PSCs) is demonstrated. As a result, the fabricated inverted planar heterojunction device of ITO/PTAA/Cs_0.15FA_0.85PbI₃/PC₆₁BM/Phen-NADPO/Ag exhibits the best power conversion efficiency of 21.53%. Furthermore, the corresponding PSCs possess a better storage and light-soaking stability.     

Extending the Photovoltaic Response of Perovskite Solar Cells into the Near‐Infrared with a Narrow‐Bandgap Organic Semiconductor

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.     

Hybrid PbS Quantum‐Dot‐in‐Perovskite for High‐Efficiency Perovskite Solar Cell

In this study, a facile and effective approach to synthesize high‐quality perovskite‐quantum dots (QDs) hybrid film is demonstrated, which dramatically improves the photovoltaic performance of a perovskite solar cell (PSC). Adding PbS QDs into CH₃NH₃PbI₃ (MAPbI₃) precursor to form a QD‐in‐perovskite structure is found to be beneficial for the crystallization of perovskite, revealed by enlarged grain size, reduced fragmentized grains, enhanced characteristic peak intensity, and large percentage of (220) plane in X‐ray diffraction patterns. The hybrid film also shows higher carrier mobility, as evidenced by Hall Effect measurement. By taking all these advantages, the PSC based on MAPbI₃‐PbS hybrid film leads to an improvement in power conversion efficiency by 14% compared to that based on pure perovskite, primarily ascribed to higher current density and fill factor (FF). Ultimately, an efficiency reaching up to 18.6% and a FF of over ≈0.77 are achieved based on the PSC with hybrid film. Such a simple hybridizing technique opens up a promising method to improve the performance of PSCs, and has strong potential to be applied to prepare other hybrid composite materials.     

Defect‐Engineering‐Enabled High‐Efficiency All‐Inorganic Perovskite Solar Cells

The emergence of cesium lead iodide (CsPbI₃) perovskite solar cells (PSCs) has generated enormous interest in the photovoltaic research community. However, in general they exhibit low power conversion efficiencies (PCEs) because of the existence of defects. A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared by defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, while the intrinsic defect concentration is largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere and thus a glovebox is not required. By completely eliminating the labile and expensive components in traditional PSCs, the all‐inorganic PSCs based on CsPbI₃:Br:InI₃ and carbon electrode exhibit PCE and open‐circuit voltage as high as 12.04% and 1.20 V, respectively. More importantly, they demonstrate excellent stability in air for more than two months, while those based on CsPbI₃ can survive only a few days in air. The progress reported represents a major leap for all‐inorganic PSCs and paves the way for their further exploration in order to achieve higher performance.     

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

The development of narrow-bandgap (E_g ≈ 1.2 eV) mixed tin–lead (Sn–Pb) halide perovskites enables all-perovskite tandem solar cells. Whereas pure-lead halide perovskite solar cells (PSCs) have advanced simultaneously in efficiency and stability, achieving this crucial combination remains a challenge in Sn–Pb PSCs. Here, Sn–Pb perovskite grains are anchored with ultrathin layered perovskites to overcome the efficiency-stability tradeoff. Defect passivation is achieved both on the perovskite film surface and at grain boundaries, an approach implemented by directly introducing phenethylammonium ligands in the antisolvent. This improves device operational stability and also avoids the excess formation of layered perovskites that would otherwise hinder charge transport. Sn–Pb PSCs with fill factors of 79% and a certified power conversion efficiency (PCE) of 18.95% are reported—among the highest for Sn–Pb PSCs. Using this approach, a 200-fold enhancement in device operating lifetime is achieved relative to the nonpassivated Sn–Pb PSCs under full AM1.5G illumination, and a 200 h diurnal operating time without efficiency drop is achieved under filtered AM1.5G illumination.     

Molecule‐Doped Nickel Oxide: Verified Charge Transfer and Planar Inverted Mixed Cation Perovskite Solar Cell

Both conductivity and mobility are essential to charge transfer by carrier transport layers (CTLs) in perovskite solar cells (PSCs). The defects derived from generally used ionic doping method lead to the degradation of carrier mobility and parasite recombinations. In this work, a novel molecular doping of NiO_x hole transport layer (HTL) is realized successfully by 2,2′‐(perfluoronaphthalene‐2,6‐diylidene)dimalononitrile (F6TCNNQ). Determined by X‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, the Fermi level (E_F) of NiO_x HTLs is increased from ?4.63 to ?5.07 eV and valence band maximum (VBM)‐E_F declines from 0.58 to 0.29 eV after F6TCNNQ doping. The energy level offset between the VBMs of NiO_x and perovskites declines from 0.18 to 0.04 eV. Combining with first‐principle calculations, electrostatic force microscopy is applied for the first time to verify direct electron transfer from NiO_x to F6TCNNQ. The average power conversion efficiency of CsFAMA mixed cation PSCs is boosted by ≈8% depending on F6TCNNQ‐doped NiOx HTLs. Strikingly, the champion cell conversion efficiency of CsFAMA mixed cations and MAPbI₃‐based devices gets to 20.86% and 19.75%, respectively. Different from passivation effect, the results offer an extremely promising molecular doping method for inorganic CTLs in PSCs. This methodology definitely paves a novel way to modulate the doping in hybrid electronics more than perovskite and organic solar cells.     

Efficient Room‐Temperature Phosphorescence from Organic–Inorganic Hybrid Perovskites by Molecular Engineering

Solution‐processed organic–inorganic hybrid perovskites are promising emitters for next‐generation optoelectronic devices. Multiple‐colored, bright light emission is achieved by tuning their composition and structures. However, there is very little research on exploring optically active organic cations for hybrid perovskites. Here, unique room‐temperature phosphorescence from hybrid perovskites is reported by employing novel organic cations. Efficient room‐temperature phosphorescence is activated by designing a mixed‐cation perovskite system to suppress nonradiative recombination. Multiple‐colored phosphorescence is achieved by molecular design. Moreover, the emission lifetime can be tuned by varying the perovskite composition to achieve persistent luminescence. Efficient room‐temperature phosphorescence is demonstrated in hybrid perovskites that originates from the triplet states of the organic cations, opening a new dimension to the further development of perovskite emitters with novel functional organic cations for versatile display applications.