Chen Weijie

Fluorinated ligand capped quantum dots are first demonstrated as interface engineering layer for efficient and stable 0D/3D perovskite solar cells, a champion efficiency of 23.42% is obtained together with enhanced ambient storage stability due to reduced trap state density as well as enhanced resistance of moisture.

Abstract

Dimensionality engineering involving the low-dimensional and 3D perovskites has been demonstrated as an efficient promising strategy to modulate interfacial energy loss as well as instability in perovskite solar cells (PSCs). Herein, the use of fluorinated Cesium Lead Iodide (CsPbI₃) perovskite quantum dot (PQD) is first reported as interface modification layer for PSCs. The binding between the CsPbI₃ PQD surface and native oleic acid (OLA)/oleylamine (OAm) ligands is governed by a dynamic adsorption–desorption equilibrium. Perfluorooctanoic acid (PFA) with stronger binding affinity and more hydrophobic nature is explored to partially replace OLA to prepare the fluorinated ligand capped CsPbI₃ PQDs (F-CsPbI₃). Through optimization of the addition of PFA during hot-injection synthesis, the in situ treated F-CsPbI₃ PQDs display reduced surface defect states, higher photoluminescence quantum yields together with improved stability. Subsequently, both CsPbI₃ and F-CsPbI₃ PQDs are utilized as interface engineering layer in PSCs, delivering the best efficiency values of 21.99% and 23.42%, respectively, which is significantly enhanced compared to the control device (20.37%). More importantly, benefiting from its more hydrophobic properties, the F-CsPbI₃ PQD treated device exhibits excellent ambient storage stability (25 °C, relative humidity: 35–45%), retaining over 80% of its initial efficiency after 1500 h aging.

The concept of energy space and 3D space of the trap states is put forward to systematically probe the comprehensive and in-depth mechanism behind the macroscopic properties. The realization of visualization and digitalization can obtain a complete image of energy and spatial distributions of trap states, and further provides powerful technical support for developing targeted passivation strategies.

Abstract

Due to their greater opt electric performance, perovskite photovoltaics (PVs) present huge potential to be commercialized. Perovskite PV’s high theoretical efficiency expands the available development area. The passivation of defects in perovskite films is crucial for approaching the theoretical limit. In addition to creating efficient passivation techniques, it is essential to direct the passivation approach by getting precise and real-time information on the trap states through measurements. Therefore, it is necessary to establish quantitative characterization methods for the trap states in energy and 3D spaces. The authors cover the characterization of the spatial and energy distributions of trap states in this article with an eye toward high-efficiency perovskite photovoltaics. After going over the strategies that have been created for characterizing and evaluating trap states, the authors will concentrate on how to direct the creative development of characterization techniques for trap states assessment and highlight the opportunities and challenges of future development. The 3D space and energy distribution mappings of trap states are anticipated to be realized. The review will give key guiding importance for further approaching the theoretical efficiency of perovskite photovoltaics, offering some future research direction and technological assistance for the development of appropriate targeted passivation technologies.

Several solid additives are designed to regulate the crystallization behavior of nonfullerene acceptors in organic solar cells. The intermolecular interaction between acceptors and additives results in the formation of more ordered molecular packing, thus the exciton diffusion and dissociation become faster, contributing to a power conversion efficiency of 19.3%.

Abstract

Although the advances in organic solar cells (OSCs) have been considerable, their efficiency is still limited by recombination losses. Photogenerated electrons and holes are generally bound as localized excitons in organic semiconductors. The transition from excitons into free charges requires diffusion and dissociation processes, in which parasitic recombination losses exist. Reducing these losses is necessary for highly efficient OSCs. The crystallization behavior of the active layers can influence the efficiency of exciton diffusion and dissociation. In this work, different additives are delicately designed to control the crystallization behavior. It is found that the crystallization quality of active layers can be improved by controlling the aggregation of nonfullerene acceptors. The π–π stacking of blend films becomes compact, meanwhile, the crystallization in the vertical direction is more uniform. These are beneficial to the diffusion and dissociation of excitons. As a consequence, recombination losses are reduced and power convention efficiencies (PCEs) are improved significantly. Meanwhile, the general applicability of the additive is demonstrated in various organic photovoltaic systems, in which a PCE of 19.3% is achieved in D18:BTP-eC9-4F OSCs. This work provides a facile strategy to reduce the recombination losses of excitons for efficient devices.

Nature Chemistry, Published online: 28 August 2023; doi:10.1038/s41557-023-01305-y

The α-cyano-4-hydroxycinnamic acid with −COOH, −OH, and −C ≡ N groups was adopted to modify the buried interfaces of perovskite solar cells (PSCs), resulting in an elevated power conversion efficiency (PCE) from 21.26% to 23.02%, and the unsealed PSC maintains 94.2% of original PCE after 1500 h at 15–20% relative humidity.

Minimizing the buried interface carrier nonradiative recombination loss has been a great challenge in the field of perovskite solar cells. Herein, a multifunctional chemical-bridging strategy is reported where the α-cyano-4-hydroxycinnamic acid (CHCA) molecule with multiple functional groups including −COOH, −OH, and −C ≡ N is adopted to manipulate buried interface. Due to simultaneous interaction of multiple groups in CHCA with SnO₂ and perovskite layers, interfacial contact is ameliorated. The double-sided chemical anchoring by CHCA enables interfacial defect passivation, residual tensile strain mitigation, reduced interfacial energy barrier, and improved perovskite crystallization. Through this ingenious chemical-linking strategy, the power conversion efficiency is much increased from 21.26% to 23.02%, which is owing to much suppressed buried interface nonradiative recombination. The unsealed modified devices demonstrate enhanced moisture stability, degrading by less than 6% after 1500 h of aging under the relative humidity range of 15–20%. In this work, a way for minimizing buried interfacial nonradiative recombination losses through the rational design of versatile chemical-bridging molecules with the synergy of multiple functional groups is provided.

Directly adding a cyanoacrylic-acid-based molecular additive, namely BT-T, into the perovskite precursor solution achieves in situ buried-interface passivation. The power-conversion efficiency (PCE) for 1.0 cm² inverted-structure PSCs reaches 23.48%. The encapsulated perovskite solar cell (PSC) retains 95.4% of its initial PCE following 1960 h maximum-power-point tracking under continuous light illumination at 65 °C (i.e., ISOS-L-2I protocol).

Abstract

High-performance perovskite solar cells (PSCs) typically require interfacial passivation, yet this is challenging for the buried interface, owing to the dissolution of passivation agents during the deposition of perovskites. Here, this limitation is overcome with in situ buried-interface passivation—achieved via directly adding a cyanoacrylic-acid-based molecular additive, namely BT-T, into the perovskite precursor solution. Classical and ab initio molecular dynamics simulations reveal that BT-T spontaneously may self-assemble at the buried interface during the formation of the perovskite layer on a nickel oxide hole-transporting layer. The preferential buried-interface passivation results in facilitated hole transfer and suppressed charge recombination. In addition, residual BT-T molecules in the perovskite layer enhance its stability and homogeneity. A power-conversion efficiency (PCE) of 23.48% for 1.0 cm² inverted-structure PSCs is reported. The encapsulated PSC retains 95.4% of its initial PCE following 1960 h maximum-power-point tracking under continuous light illumination at 65 °C (i.e., ISOS-L-2I protocol). The demonstration of operating-stable PSCs under accelerated ageing conditions represents a step closer to the commercialization of this emerging technology.

In this study, a high-quality SnO₂ layer with decreased adsorbed oxygen (O^chem) active sites and oxygen vacancies (O^vac) was fabricated on a flexible substrate by introducing TiCl₄ into the SnO₂ bulk layer. The first evidence of flexible perovskite cells working at low temperature was demonstrated, with efficiency as high as 23.7 % based on an improved SnO₂ layer.

Abstract

Photovoltaic technology with low weight, high specific power in cold environments, and compatibility with flexible fabrication is highly desired for near-space vehicles and polar region applications. Herein, we demonstrate efficient low-temperature flexible perovskite solar cells by improving the interfacial contact between electron-transport layer (ETL) and perovskite layer. We find that the adsorbed oxygen active sites and oxygen vacancies of flexible tin oxide (SnO₂) ETL layer can be effectively decreased by incorporating a trace amount of titanium tetrachloride (TiCl₄). The effective defects elimination at the interfacial increases the electron mobility of flexible SnO₂ layer, regulates band alignment at the perovskite/SnO₂ interface, induces larger perovskite crystal growth, and improves charge collection efficiency in a complete solar cell. Correspondingly, the improved interfacial contact transforms into high-performance solar cells under one-sun illumination (AM 1.5G) with efficiencies up to 23.7 % at 218 K, which might open up a new era of application of this emerging flexible photovoltaic technology to low-temperature environments such as near-space and polar regions.

A series of acceptors featuring conjugation-extended electron-deficient cores were synthesized via a newly developed strategy for organic solar cells, and power conversion efficiencies (PCEs) of up to ≈19 % were achieved. The properties of new acceptors were systematically investigated and elucidated experimentally and computationally.

Abstract

In the molecular optimizations of non-fullerene acceptors (NFAs), extending the central core can tune the energy levels, reduce nonradiative energy loss, enhance the intramolecular (donor-acceptor and acceptor-acceptor) packing, facilitate the charge transport, and improve device performance. In this study, a new strategy was employed to synthesize acceptors featuring conjugation-extended electron-deficient cores. Among these, the acceptor CH-BBQ, embedded with benzobisthiadiazole, exhibited an optimal fibrillar network morphology, enhanced crystallinity, and improved charge generation/transport in blend films, leading to a power conversion efficiency of 18.94 % for CH-BBQ-based ternary organic solar cells (OSCs; 18.19 % for binary OSCs) owing to its delicate structure design and electronic configuration tuning. Both experimental and theoretical approaches were used to systematically investigate the influence of the central electron-deficient core on the properties of the acceptor and device performance. The electron-deficient core modulation paves a new pathway in the molecular engineering of NFAs, propelling relevant research forward.

Recent progress on the strategies to fabricate Pb, Sn, and Pb–Sn perovskite-based solar cells is reviewed. This report discusses the fundamental aspects of crystal structure and perovskite chemistry along with advancements in device performance and operational stability using the crystal lattice modulation, molecular passivation, and carrier transport engineering.

Halide-perovskite-based solar cells (HPSCs) have established themselves as a promising photovoltaic (PV) technology in a remarkably short time. The rapid improvement in HPSCs can be attributed to the unique material and optoelectronic properties of metal halide perovskite semiconductors coupled with a very knowledgeable and experienced PV community. This review briefly summarizes the chemistry of halide perovskites, delving into the fundamental aspects of crystal structure and optical bandgap, followed by a more in-depth report on the advancements in HPSCs efficiencies, thanks to structural regulation, interfacial modulation, and thin-film engineering. It is mainly focused on three metal halide perovskites topics: 1) high-performance Pb-based perovskites, 2) Sn-based perovskites and their associated challenges, and 3) emerging work on mixed composition Pb–Sn perovskites. For each of these domains, the effects stemming from the tuning of the monovalent A-site and the halide site are examined. Additionally, various approaches aimed at passivating defects in the bulk film and at the interface, along with carrier transport engineering, are discussed. The discussions also encompass the broader implications for device performance, stability, and material toxicity. Finally, perspectives on the future directions and the commercial feasibility of perovskite photovoltaic technologies are provided.

Green crystallization produces pure formamidinium lead iodide (FAPbI₃) for efficient perovskite solar cells. Nontoxic solvents, triethylphosphate and acetonitrile, produce pure δ-FAPbI₃ single crystals for α-FAPbI₃ perovskite solar cells. δ-FAPbI₃ crystals lead to 22.61% efficient solar cells with fewer trap states, maintaining stability under continuous 1 sun illumination at 50 °C for 1000 h.

The production of pure formamidinium lead iodide (FAPbI₃) is crucial to ensure high-efficiency perovskite solar cells and such pure FAPbI₃ is obtainable from crystallization process because impurities from the reagents are removable by molecular recognition effect. In addition, it is important to develop a green crystallization process for the production of FAPbI₃ single crystals. Via broad screening of chemicals, here, triethylphosphate and acetonitrile are selected as nontoxic solvent and antisolvent, respectively, for the liquid–liquid reaction of FAI and PbI₂ and the antisolvent crystallization. The new green crystallization process produces pure δ-FAPbI₃ single crystals, which are applicable to fabricate α-FAPbI₃ perovskite solar cells. Due to the reduced trap states in the δ-FAPbI₃ single crystals, the corresponding α-FAPbI₃ perovskite solar cells have 22.61% efficiency (average = 20.65 ± 0.86% across 50 devices) and show good operational stability under continuous 1 sun illumination at 50 °C for 1000 h.

CeO₂ is introduced to passivate the interface for an effective hole transferring via oxygen ion hopping during the conversion of Ce⁴⁺ to Ce³⁺. Moreover, CeO₂ oxidizes spiro-OMeTAD to form a donor-acceptor complex, balancing electron and hole extraction by their respective electrodes for better hole collecting/transferring. Therefore, the CeO₂-based perovskite solar cell achieves a power conversion efficiency over 24% with better stability.

Interface defect is a limiting factor of the charge dynamics and stability of perovskite solar cells (PSCs). Herein, a rare earth metal oxide cerium oxide is introduced into the interface between perovskite and spiro-OMeTAD [2,2,7,7-tetrakis (N,N -di-p-methoxyphenyl-amine) 9,9-spirobifluorene] to passivate interfacial defects. Due to the nearest-neighbor interaction of CeO₂ with spiro-OMeTAD, it can accelerate the oxidization process of spiro-OMeTAD to form a donor–acceptor complex at this interface, which can overcome the interface barrier for the high hole collecting ability. The bonding formation between lead and oxygen makes this heterojunction show metal conduction behavior at this interface. The insertion of CeO₂ between perovskite and spiro-OMeTAD, can improve hole transferring to balance the extraction of electron and hole by their respective electrodes, for the decreased device hysteresis with the higher efficiency and improved stability. The results show that the CeO₂-based PSC device achieve a power conversion efficiency (PCE) of over 24%, retaining more than 87% of the initial PCE after 2570 h of storage at 20 ≈ 30% humidity.

This article describes a double-layer modification strategy to enhance the power conversion efficiency (PCE) of perovskite solar cells (PSCs). The “spiderman” nitrosonium tetrafluoroborate (NOBF₄) compound is introduced between the tin dioxide (SnO₂) and perovskite layers to passivate defects and improve interfacial bonding. The synergistic effect of NO⁺ and BF₄ ⁻ improves photoelectric performance, resulting in a champion PCE of 24.04%.

Abstract

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

2-Methylbenzimidazole (MBI) molecular ferroelectric with low coercive field and high Curie-temperature is introduced to construct an additional ferroelectric field in FASnI₃-based perovskite solar cells (PSCs), where the directional polarization of MBI can upgrade the built-in electric field and improve carrier dynamics to enhance the photovoltaic performance of PSCs.

Abstract

Due to the relatively inferior dielectric constant, Sn-based perovskites exhibit lower defect tolerance and insufficient dielectric shielding effect compared with Pb-perovskites. Upgrading built-in electric field (BEF) in Sn-based perovskite solar cells (PSCs) can be effective to reduce large voltage deficit and improve poor performance caused by the low defect tolerance resulting from the intrinsic inferior dielectric of Sn-based perovskites. Herein, 2-methylbenzimidazole (MBI) molecular ferroelectric with low coercive field and high Curie-temperature is introduced to construct an additional ferroelectric field in FASnI₃-based PSCs. The ferroelectric effect of MBI can promote exciton dissociation, enhance carrier population, and suppress the adverse effect of the residual defects on carriers, and the directional polarization of MBI in FASnI₃ film can be driven by the BEF in PSCs to broaden the width of depletion region. Additionally, the MBI molecules with amine functional groups effectively regulate perovskite crystallization, passivate Sn-related defects, and enhance the oxidation barrier. Profiting from the above advantages, the MBI-modified device achieves a champion power conversion efficiency (PCE) of 12.91%, keeping over 94% average PCE after 1056 h in N₂ glovebox for the unencapsulated device. This study highlights the significant role of molecular ferroelectrics in perovskite photovoltaics.

Sodium cyanoborohydride (NaBH₃CN, SC) is introduced to modify the upper surface of Pb-Sn perovskite. This is the first report of reducing agent as modification material of Pb-Sn perovskite. SC can suppress Sn²⁺ oxidation, reduce Sn⁴⁺, optimize perovskite morphology, and release stress. Consequently, optimized perovskite solar cells achieve a power conversion efficiency of 21.3% with good stability.

Abstract

Pb-Sn mixed perovskite has received extensive attention because of its high research value in environmental protection and theoretical prospects. However, the film defects brought on by the quick crystallization of Pb-Sn mixed perovskites and the facile oxidation of Sn²⁺ to Sn⁴⁺ are detrimental to the performance of Pb-Sn perovskite solar cells (PSCs). Here, aiming at the interface of Pb-Sn PSCs, which has been less studied, a reducing agent is first introduced on the top of Pb-Sn mixed perovskite interface. The results show that in NaBH₃CN(SC), B─H can suppress the oxidation of Sn²⁺ and C≡N can enhance the perovskite crystallinity and passivate the surface defects. Benefitting from this synergistic effect, a champion power conversion efficiency of 21.3% is obtained for Pb-Sn PSCs. In addition, the encapsulated device can maintain good stability under maximum power point tracking conditions.

Cd alloying pure sulfide kesterite solar cells by regulating the alloying concentration and the grain growth process systematically from Sn⁴⁺-DMSO solution approach makes a large-grain spanning absorber monolayer and significantly suppresses deep-level defects. At the optimal concentration, a champion device with an efficiency of 12.3% is achieved with a record low open-circuit voltage loss even without post heat treatment.

Abstract

Cd alloying has been theoretically proved to be an effective strategy to suppress Cu-Zn antisite defects and related defect cluster for improving device performance of pure sulfide kesterite Cu₂ZnSnS₄ (CZTS) thin film solar cells. However, the potential of Cd alloying has not been fully realized by solely doping without further post heat-treatment. Here, Cd alloying CZTS (Cu₂(Zn,Cd)SnS₄, CZCTS) is reported through dimethyl sulfoxide (DMSO) solution and how alloying concentration affects reaction path, grain growth, and electronic properties of the CZCTS absorbers is investigated. This study found that Cd can be incorporated into CZTS through direct phase transformation grain growth, which sufficiently suppresses band tailing. High quality CZCTS absorber films and efficient solar cells are fabricated within a wide range of alloy concentration. A champion CZCTS device with a power conversion efficiency of 12.3% is achieved at 35% Cd concentration without any post heat treatment, improved by over 70% compared to 7.0% of CZTS. This device exhibits a high V _OC gain to the Shockley–Queisser (Voc/Voc^SQ = 59.7%), the lowest V _OC deficit achieved in pure sulfide kesterite solar cells. The results demonstrate the importance of the Cd alloying strategy for mitigating band tailing and achieving high efficiency pure sulfide kesterite solar cells.

The oligomeric acceptors with different flexible alkyl chains (DOY-C2, DOY-C4, and TOY-C4) are synthesized. When doped into D18:N3 system, outstanding power conversion efficiency (PCE) of 19.01% and 17.91% are achieved in rigid and flexible devices based on D18:N3:DOY-C4, respectively. In addition, the mechanical properties of ternary films are also successfully improved due to form tie and entangled chain networks.

Abstract

High power conversion efficiency (PCE) and mechanical robustness are key requirements for wearable applications of organic solar cells (OSCs). However, almost all highly efficient photoactive films comprising polymer donors (P_D) and small molecule acceptors (SMAs) are mechanically brittle. In this study, highly efficient (PCE = 17.91%) and mechanically robust (crack-onset strain [COS] = 11.7%) flexible OSCs are fabricated by incorporating a ductile oligomeric acceptor (DOA) into the P_D:SMA system, representing the most flexible OSCs to date. The photophysical, mechanical, and photovoltaic properties of D18:N3 with different DOAs are characterized. By introducing DOA DOY-C4 with a longer flexible alkyl linker and lower polymerization, the D18:N3:DOY-C4-based flexible OSCs exhibit a significantly higher PCE (17.91%) and 50% higher COS (11.7%) than the D18:N3-based device (PCE = 17.06%, COS = 7.8%). The flexible OSCs based on D18:N3:DOY-C4 retain 98% of the initial PCE after 2000 consecutive bending cycles, showing greater mechanical stability than the reference device (maintaining 89% of initial PCE). After careful investigation, it is hypothesized that the enhancement in mechanical properties is mainly due to the formation of tie chains or entanglement in the ternary blend films. These results demonstrate that DOAs have great potential for achieving high-performance flexible OSCs.

Tryptamine as an additive is introduced in the hole transport layer. The molecular interactions of tryptamine with perovskite and lithium cation result in a drastic improvement of V _OC from 1.192 to 1.251 V, yielding a high device power conversion efficiency of 21.8%, together with greatly enhanced moisture stability.

Abstract

Metal halide inorganic perovskite solar cells (PSCs) have great potential to achieve high efficiency with excellent thermal stability. However, the surface defect traps restrain the achievement of high open circuit voltage (V _OC) and power conversion efficiency (PCE) of the devices due to the severe nonradiative charge recombination. Moreover, the state-of-the-art hole transporting layer (HTL) significantly hampers device moisture stability, even though it renders the highest solar cell efficiency. Herein, a one-stone-two-birds strategy is proposed using a biocompatible material tryptamine (TA) as an additive in HTL. First, TA bearing electron rich moieties can favorably passivate the surface defects of inorganic perovskite films, significantly reducing trap density and prolonging charge lifetime. It results in a drastic improvement of V _OC from 1.192 to 1.251 V, with a V _OC loss of 0.48 V. The corresponding PSCs achieve a 21.8% PCE under 100 mW cm⁻² illumination. Second, TA in HTL can coordinate with lithium cations, retarding their reaction with moisture and increasing the moisture stability of HTL. Consequently, the black phase of inorganic perovskite films is well preserved, and the corresponding PSCs maintain 90% of the initial PCE after 800 h storage at relative humidity of 25–35%, much higher than the control devices.

Two-dimensional (2D) carbides Ti₃C₂Cl _x Nano-MXene and o-TB-GDY NanoGDY are successfully employed to passivate the perovskite/electron transport layer (ETL) and perovskite/hole transport layer (HTL) interfaces, respectively. Due to significantly inhibited non-radiative recombination, enhanced energy band alignment, and improved charge-carrier extraction, the n-i-p devices obtain a high efficiency of 24.86 % with improved long-term stability.

Abstract

Passivating the interfaces between the perovskite and charge transport layers is crucial for enhancing the power conversion efficiency (PCE) and stability in perovskite solar cells (PSCs). Here we report a dual-interface engineering approach to improving the performance of FA_0.85MA_0.15Pb(I_0.95Br_0.05)₃-based PSCs by incorporating Ti₃C₂Cl _x Nano-MXene and o-TB-GDY nanographdiyne (NanoGDY) into the electron transport layer (ETL)/perovskite and perovskite/ hole transport layer (HTL) interfaces, respectively. The dual-interface passivation simultaneously suppresses non-radiative recombination and promotes carrier extraction by forming the Pb−Cl chemical bond and strong coordination of π-electron conjugation with undercoordinated Pb defects. The resulting perovskite film has an ultralong carrier lifetime exceeding 10 μs and an enlarged crystal size exceeding 2.5 μm. A maximum PCE of 24.86 % is realized, with an open-circuit voltage of 1.20 V. Unencapsulated cells retain 92 % of their initial efficiency after 1464 hours in ambient air and 80 % after 1002 hours of thermal stability test at 85 °C.

The effect of lattice tensile strain on the optoelectronic properties of Sn–Pb perovskite film is investigated, and a moderate tensile strain is beneficial for obtaining high-quality film. The optimized film produces a device with champion efficiency of 19.3% and improved storage stability. This work provides new insight into controlling tensile strain in Sn–Pb perovskite solar cells.

Pb–Sn perovskites (PSPs) with narrow bandgap and low toxicity show great promise in next-generation perovskite-based photovoltaics, especially in constructing all-perovskite tandem solar cells as the rare subcell. However, partially replacing Pb by Sn with smaller ionic size in PSPs is shown to reduce their crystal lattice symmetry, which leads to intrinsic compressed lattice strain and reduced structural stability, resulting in the poor device performance of PSPs-based solar cells. Herein, the effect of lattice tensile strain on the optoelectronic properties of FA_0.7MA_0.3Pb_0.5Sn_0.5I₃ film is investigated and the tensile strain is finely adjusted by introducing monovalent-cation chlorides with different cation sizes as additive. It is found that a moderate tensile strain in the film is beneficial for stabilizing the crystal structure, promoting oriented crystal growth and reducing the defect density. After optimization, FA_0.7MA_0.3Pb_0.5Sn_0.5I₃-based perovskite solar cells using FACl as additive can produce the device with champion power conversion efficiency (PCE) of 19.30% together with improved device reproducibility. Importantly, the FACl-based device shows robust stability that exhibits no PCE loss after being stored in N₂ for 1000-h. This work provides new insight into the key role of tensile strain in PSPs, which can facilitate the development of PSP-based optoelectronic devices.

A non-halide ionic salt 1-naphthylmethylammonium formate (NMACOOH) is synthesized for the surface passivation of perovskites. Owing to the strong coordination between HCOO⁻ and Pb²⁺, the formation of 2D perovskite is inhibited, and a thermally stable PbI₂-NMACOOH adduct is formed on the perovskite surface. The passivated devices deliver a champion efficiency of 24.75% with a high open-circuit voltage of 1.19 V.

Abstract

The non-radiative recombination at the interfaces of perovskite solar cells (PSCs) is a crucial issue that limits the efficiency and stability of the devices. State-of-the-art surface passivation strategies usually utilize alkyl ammonium halides to suppress the non-radiative recombination of PSCs, but their high surface reactivity leads to the transformation into 2D perovskites under working conditions, limiting the passivation effect and the charge transport of PSCs. Herein, a non-halide ionic salt 1-naphthylmethylammonium formate (NMACOOH) is synthesized for surface passivation of perovskite films. In contrast to the traditional 1-naphthylmethylammonium iodide, NMACOOH treatment hinders the formation of 2D perovskite and forms a thermally stable PbI₂-NMACOOH adduct on the perovskite surface. Surface characterization reveals that NMA⁺ can passivate the cation vacancies of the 3D perovskite while HCOO⁻ passivates the metallic Pb⁰ and halide-vacancy defects. Therefore, the non-radiative recombination of PSCs is dramatically suppressed and a high open-circuit voltage of 1.19 V is obtained. Finally, PSCs with high efficiency of 24.75% and improved long-term stability (98% of the initial efficiency after 1800-h storage) are obtained. Moreover, the NMACOOH-passivated devices also show robust operational stability, retaining 83% of the initial efficiency after working for 658 h under continuous one-sun illumination.

The iodine-rich 2D perovskite materials work as an innovative iodine cathode for high-performance iodine-based batteries, which are well suited to diversified electrolytes and two-electron redox modes, delivering large capacity, high output voltage, and high energy density.

Abstract

Although conversion-type iodine-based batteries are considered promising for energy storage systems, stable electrode materials are scarce, especially for high-performance multi-electron reactions. The use of tin-based iodine-rich 2D Dion–Jacobson (DJ) ODASnI₄ (ODA: 1,8-octanediamine) perovskite materials as cathode materials for iodine-based batteries is suggested. As a proof of concept, organic lithium-perovskite and aqueous zinc-perovskite batteries are fabricated and they can be operated based on the conventional one-electron and advanced two-electron transfer modes. The active elemental iodine in the perovskite cathode provides capacity through a reversible I⁻/I⁺ redox pair conversion at full depth, and the rapid electron injection/extraction leads to excellent reaction kinetics. Consequently, high discharge plateaus (1.71 V vs Zn²⁺/Zn; 3.41 V vs Li⁺/Li), large capacity (421 mAh g⁻¹ _I), and a low decay rate (1.74 mV mAh⁻¹ g⁻¹ _I) are achieved for lithium and zinc ion batteries, respectively. This study demonstrates the promising potential of perovskite materials for high-performance metal-iodine batteries. Their reactions based on the two-electron transfer mechanism shed light on similar battery systems aiming for decent operational stability and high energy density.

A series of terminally chlorinated and thiophene-linked three-dimensional (3D) dimerized acceptors were reported to explore the effect of chlorination by precisely tuning the position and number of chlorine atoms. The decent performance of PM6:CH8-4 on small-area devices, non-halogen solvent and air-processed devices, and 2.88 cm² solar cell modules highlights the versatile processing capability of our 3D acceptors.

Abstract

To exploit the potential of our newly developed three-dimensional (3D) dimerized acceptors, a series of chlorinated 3D acceptors (namely CH8-3/4/5) were reported by precisely tuning the position of chlorine (Cl) atom. The introduction of Cl atom in central unit affects the molecular conformation. Whereas, by replacing fluorinated terminal groups (CH8-3) with chlorinated terminal groups (CH8-4 and CH8-5), the red-shift absorption and enhanced crystallization are achieved. Benefiting from these, all devices received promising power conversion efficiencies (PCEs) over 16 % as well as decent thermal/photo-stabilities. Among them, PM6:CH8-4 based device yielded a best PCE of 17.58 %. Besides, the 3D merits with multi alkyl chains enable their versatile processability during the device preparation. Impressive PCEs of 17.27 % and 16.23 % could be achieved for non-halogen solvent processable devices prepared in glovebox and ambient, respectively. 2.88 cm² modules also obtained PCEs over 13 % via spin-coating and blade-coating methods, respectively. These results are among the best performance of dimerized acceptors. The decent performance of CH8-4 on small-area devices, modules and non-halogen solvent-processed devices highlights the versatile processing capability of our 3D acceptors, as well as their potential applications in the future.

Three-dimensional solids were developed to optimize the nanoscale structure of the active layer in organic solar cells by offering ample assembly space during the phase-transition process, yielding a high power-conversion efficiency of over 19 %. This research not only established a valuable pre-selection approach for solid additives but also deepened our understanding of the underlying working mechanism.

Abstract

Fine-tuning the thermodynamic self-assembly of molecules via volatile solid additives has emerged to be an effective way to construct high-performance organic solar cells. Here, three-dimensional structured solid molecules have been designed and applied to facilitate the formation of organized molecular assembly in the active layer. By means of systematic theory analyses and film-morphology characterizations based on four solid candidates, we preselected the optimal one, 4-fluoro-N,N-diphenylaniline (FPA), which possesses good volatility and strong charge polarization. The three-dimensional solids can induce molecular packing in active layers via strong intermolecular interactions and subsequently provide sufficient space for the self-reassembly of active layers during the thermodynamic transition process. Benefitting from the optimized morphology with improved charge transport and reduced energy disorder in the FPA-processed devices, high efficiencies of over 19 % were achieved. The strategy of three-dimensional additives inducing ordered self-assembly structure represents a practical approach for rational morphology control in highly efficient devices, contributing to deeper insights into the structural design of efficient volatile solid additives.

The α-phase formamidinium lead triiodide perovskite single crystal redissolution strategy facilitates direct α-phase formation with inhibition of complex intermediate phases due to the larger-sized colloidal clusters present in the precursor solution. The resultant film demonstrates significantly improved crystallization and mitigated defects. This strategy relaxes the lattice strain through isotropic orientation phase growth, contributing to the prolonged α-phase stability.

Abstract

Perovskite single-crystal redissolution (PSCR) strategy is highly desired for efficient formamidinium lead triiodide (FAPbI₃) perovskite photovoltaics with enhanced phase purity, improved film quality, low trap-state density, and good stability. However, the phase transition and crystallization dynamics of FAPbI₃ remain unclear in the PSCR process compared to the conventional fabrication from the mixing of precursor materials. In this work, a green-solvent-assisted (GSA) method is employed to synthesize centimeter-sized α-FAPbI₃ single crystals, which serve as the high-purity precursor to fabricate perovskite films. The α-FAPbI₃ PSCR strategy facilitates direct α-phase formation and inhibits the complex intermediate phases monitored by in situ grazing-incidence wide-angle X-ray scattering. Moreover, the α-phase stability is prolonged due to the relaxation of the residual lattice strain through the isotropic orientation phase growth. Consequently, the GSA-assisted PSCR strategy effectively promotes crystallization and suppresses non-radiative recombination in perovskite solar cells, which boosts the device efficiency from 22.08% to 23.92% with significantly enhanced open circuit voltage. These findings provide deeper insight into the PSCR process in terms of its efficacy in phase formation and lattice strain release. The green low-cost solvent may also offer a new and ideal solvent candidate for large-scale production of perovskite photovoltaics.

Using low-temperature device measurements of bilayer and bulk heterojunction blends, it is shown that different open-circuit voltage behavior results from the different molecular packing of the non-fullerene acceptor (NFA) ITIC in neat (crystalline) and blend (disordered) films. Material parameters extracted from neat films of NFAs like ITIC may not correspond to the properties of such NFAs in blends.

To understand the limitations placed on the open-circuit voltage of bulk heterojunction (BHJ) organic solar cells, the energy levels of neat donor and acceptor samples are often characterized and applied to study BHJ blends. However, energy levels derived from neat samples may not necessarily reflect those at the donor:acceptor interface in blends. The properties of organic semiconductors are sensitive to microstructural changes, with non-fullerene acceptors (NFAs) in particular known to exhibit different thin-film polymorphs. To investigate the influence of differences in molecular packing in neat and blend films, temperature-dependent current–voltage characteristics are measured for bilayer (BL) and BHJ devices. Herein, the fullerene acceptor PC₇₁BM is compared—whose energy levels are expected to be less sensitive to molecular packing—with the NFA ITIC, paired with the same donor polymer PTB7-Th. It is found that the interfacial energy levels differ for BL and BHJ devices for the PTB7-Th:ITIC system but remain the same for the PTB7-Th:PC₇₁BM system. Furthermore, X-ray scattering measurements identify that ITIC exhibits a different packing mode in neat films and in BHJ blends. Such microstructure-dependent differences between neat and blend samples need to be considered when studying energy losses in NFA BHJ solar cells.

Nature Materials, Published online: 14 August 2023; doi:10.1038/s41563-023-01631-z