Chen Weijie

Nature Communications, Published online: 01 July 2022; doi:10.1038/s41467-022-31326-z

Publication date: October 2022

Source: Nano Energy, Volume 101

Author(s): Xunchang Wang, Cong Xiao, Xiaokang Sun, Aziz Saparbaev, Shiyun Lei, Mingrui Zhang, Tian Zhong, Zhiya Li, Jiayi Zhang, Manxue Zhang, Yun Yu, Biao Xiao, Chunming Yang, Renqiang Yang

A novel and effective steric-hindrance-dependent buried interface defect passivation and stress release strategy is reported. The steric-hindrance-dependent defect passivation and stress release mechanisms are revealed experimentally and theoretically. The ADAA-modified device achieves a seductive power conversion efficiency up to 23.18%.

Abstract

Interface engineering is one feasible and effective approach to minimize the interfacial nonradiative recombination stemming from interfacial defects, interfacial residual stress, and interfacial energy level mismatch. Herein, a novel and effective steric-hindrance-dependent buried interface defect passivation and stress release strategy is reported, which is implemented by adopting a series of adamantane derivative molecules functionalized with CO (i.e., 2-adamantanone (AD), 1-adamantane carboxylic acid (ADCA), and 1-adamantaneacetic acid (ADAA)) to modify SnO₂/perovskite interface. All modifiers play a role in passivating interfacial defects, mitigating interfacial strain, and enhancing device performance. The steric hindrance of chemical interaction between CO in these molecules and perovskites as well as SnO₂ is determined by the distance between CO and bulky adamantane ring, which gradually decreases from AD, ADCA, and ADAA. The experimental and theoretical evidences together confirmed steric-hindrance-dependent defect passivation effect and interfacial chemical interaction strength. The interfacial chemical interaction strength, defect passivation effect, stress release effect and thus device performance are negatively correlated with steric hindrance. Consequently, the ADAA-modified device achieves a seductive efficiency up to 23.18%. The unencapsulated devices with ADAA maintain 81% of its initial efficiency after aging at 60 °C for 1000 h.

Self-assembled metal chelates (Hf(ACBN)₄) have been designed and applied as ultraviolet filterable interface layers in organic solar cells. Benefitting from the defect-filling effect on the SnO₂ surface and the UV-filtering ability of Hf(ACBN)₄, a simultaneous enhancement in power conversion efficiency and device stability has been achieved in the solar cells based on the SnO₂/Hf(ACBN)₄ composite interface layer.

Abstract

Interface engineering plays a vital role in the further improvement of efficiency and stability for organic solar cells (OSCs). Herein, a self-assembly metal chelate based on hafnium and a designed ligand, N-(4-(3-oxobutanoyl)phenyl)acetamide (ACBN) is applied as both interfacial modification layer and UV-light filter in OSCs. The strong hydrogen-bond induced intermolecular interaction enables Hf(ACBN)₄ with the prerequisite of adequate solvent resistance to work as an electron transport layer (ETL) in the inverted OSCs. The self-assembly behavior of Hf(ACBN)₄ on the SnO₂ film surface via constructing compact coordination structure has been verified via systematic theory calculations. In addition to optimizing the energy level alignment, the Hf(ACBN)₄ modification effectively passivates the surface defect of SnO₂ films for less surface charge recombination and a more efficient charge collection process. Thus, the OSCs with Hf(ACBN)₄ layer yield a maximum PCE of 18.1%, better than that based on the bare SnO₂ layer. Moreover, beneficial from the reduced oxygen vacancies via coordination effect and the UV-light filter function of Hf(ACBN)₄, the OSCs based on SnO₂/ Hf(ACBN)₄ composite ETL exhibit preferable stabilities under UV-light irradiation or continuous operational conditions.

Inverted perovskite solar cells (PSCs) attract great attention due to their low-temperature processing, negligible hysteresis, and superior stability. For these devices, hole-transport layers play a decisive role in carrier extraction, transport, and perovskite crystallization. This review provides a comprehensive overview of the structural engineering of organic hole-transport layers utilized in inverted PSCs including conductive polymers, small molecules, and emerging self-assembled monolayers.

Abstract

Hole-transporting layers (HTLs) are an essential component in inverted, p–i–n perovskite solar cells (PSCs) where they play a decisive role in extraction and transport of holes, surface passivation, perovskite crystallization, device stability, and cost. Currently, the exploration of efficient, stable, highly transparent and low-cost HTLs is of vital importance for propelling p–i–n PSCs toward commercialization. Compared to their inorganic counterparts, organic HTLs offer multiple advantages such as a tunable bandgap and energy level, easy synthesis and purification, solution processability, and overall low cost. Here, recent progress of organic HTLs, including conductive polymers, small molecules, and self-assembled monolayers, as utilized in inverted PSCs is systematically reviewed and summarized. Their molecular structure, hole-transport properties, energy levels, and relevant device properties and resulting performances are presented and analyzed. A summary of design principles and a future outlook toward highly efficient organic HTLs in inverted PSCs is proposed. This review aims to inspire further innovative development of novel organic HTLs for more efficient, stable, and scalable inverted PSCs.

DB18C6 with both electron-rich π bond and electronegative cavity is introduced to FACsPbI₃ films to realize inhibition of cation migration, enhancing the intrinsic stability of perovskite films. Such DB18C6 modification strategy achieves excellent photo and phase stability of perovskite solar cells.

Formamidinium–cesium (FACs)-based perovskites are potential light-absorbing materials for stable perovskite solar cells (PSCs), while long-term stability deterioration caused by photo- and moisture-induced cation migration and phase separation restrict their further commercial application. Herein, crown ether molecules with both, an electronegative cavity and a negatively charged π bond, dibenzo-18-crown-6 (DB18C6), are introduced to FACsPbI₃ films aiming at gaining better capability of inhibiting cation migration benefiting from the crown ether–cation complexation. Meanwhile, 18-crown-6 (18C6) with an electronegative cavity-only is also employed for comparison. By tracing the changes of UV–vis absorption spectra, steady-state photoluminescence, and Kelvin probe force microscopy of perovskite films under 24 h continuous light soaking, it is observed that the DB18C6-modified FACsPbI₃ film shows remarkably negligible degradation, in comparison with the unmodified and 18C6-modified FACsPbI₃ films. As a result, DB18C6-modified PSCs with the architecture of ITO/SnO₂/FACsPbI₃/Spiro-OMeTAD/Ag achieve an enhanced power conversion efficiency (PCE) exceeding 20.84%, which is a competitive efficiency for methylammonium-free pure-iodine n–i–p PSCs. The resultant unencapsulated DB18C6-modified devices retain up to 94.73% and 90.18% of their original PCEs even after 500 h continuous full sunlight light soaking and 85 °C heating, respectively. Such crown ether modification provides a pathway on long-term operational stabilized photovoltaics.

One small-molecule donor (β-S1) is synthesized by linking the beta position of outer thiophene to inner thiophene with blueshifted absorption. When used in ternary organic solar cells, the PM6:Y6:β-S1-based devices yield a higher efficiency of 17.1% than the binary ones, mainly attributed to the increased external quantum efficiency in the range of 300–500 nm.

Adding a small-molecule donor (SMD) to state-of-the-art nonfullerene organic solar cells (OSCs) is demonstrated as a useful strategy to construct ternary organic solar cells, as SMDs typically have high crystallinity and can tune charge transport properties of OSCs. However, the absorption of most SMDs overlaps with typical donor polymers (e.g., PM6), which is against the general guidelines of adopting materials with complementary absorption in ternary OSCs. Herein, the absorption of state-of-art SMDs (BTR-Cl) by linking the beta position of the outer thiophene to the alpha position of the inner thiophene unit is intentionally blueshifted. The resulting molecule β-S1 shows a maximum absorption peak at 505 nm in the film state, which exhibits wider bandgap and shows complementary absorption with the host system (PM6:Y6). The corresponding ternary OSCs with 20%wt β-S1 show significantly enhanced efficiency from 16.2% to 17.1% due to the increased short-circuit current (J _SC) and improved fill factor (FF). Herein, an effective strategy to design SMDs with both wider bandgaps and higher crystallinity for high-performance ternary OSCs is presented.

PM6: Y6 active layer is prepared under methanol vapor atmosphere; methanol vapor regulates the active layer morphology and optimizes the molecular arrangement during the active layer film-forming process; the enrichment of acceptor material on the active layer surface facilitates charge transport and extraction; an improved short-circuit current density of 26.43 mA cm⁻², fill factor of 77.39% and power conversion efficiency of 17.59% are achieved.

With the continuously increasing power conversion efficiency (PCE) in recent years, organic solar cells (OSCs) have attracted extensive attention as a new clean energy. Further improving PCE is an inevitable requirement for the OSC for commercialization. Herein, PM6: Y6-based non-fullerene organic solar cells are prepared by spin-coating active layer under methanol vapor atmosphere, methanol vapor regulates the active layer morphology and optimizes the molecular arrangement during the active layer film-forming process, and enriches the acceptor material Y6 on the active layer surface, improving the contact interface between active layer and cathode buffer layer. The improved active layer morphology and interface contact facilitates charge transport and extraction, boosts short-circuit current density and fill factor, and results in a better PCE of 17.59%, while the PCE of the control device without methanol vapor atmosphere is 16.37%. The results suggest that preparing an active layer under a solvent vapor atmosphere is a simple and efficient method to optimize the active layer morphology and improve the performance of organic solar cells, and this method is convenient to be extended to prepare large-area and high-efficiency organic photovoltaic devices.

A ternary halogen doping strategy is demonstrated to produce air-processed all-inorganic CsPbI₃-based perovskite solar cells. The optimized halogen doping content could yield the air-fabricated device with a champion power conversion efficiency of 17.51% with improved working stability. This work provides important insights into the compositional engineering for air-fabricating of high-quality all-inorganic perovskites.

All-inorganic CsPbI₃ perovskites have shown great promise in photovoltaic devices owing to their high theoretical power conversion efficiency (PCE) and excellent thermal stability compared to their organic–inorganic counterparts. However, due to the poor phase stability of CsPbI₃ perovskite under ambient conditions, it is still challenging to fabricate the efficient and stable all-inorganic CsPbI₃-based perovskite solar cells (PSCs). Herein, a ternary halogen doping strategy is demonstrated to improve the quality and stability of air-processed all-inorganic CsPbI₃ perovskite films, where CsBr and PbCl₂ are introduced as halide sources. Br and Cl are found to insert into the crystal lattice of the CsPbI₃ perovskite to improve its phase stability while regulating the crystallization process to improve its film morphology with reduced surface roughness and suppressed defect density. After optimizing the doping content of Br and Cl, the air-fabricated PSC device based on ternary halogen doping yields a champion PCE of 17.51% with improved working stability, which retains over 80% of the initial PCE after continuous illumination for 2400 h. This work highlights the key role of halide-doping in improving film quality and phase stability of air-fabricated all-inorganic perovskite, which is expected to promote the development of low-cost and large-scale production of all-inorganic PSCs.

We have introduced the trimercapto-s-triazine trisodium salt (TTTS) in the NiO _x layer is introduced. The TTTS can coordinate with Ni²⁺ in NiO _x via an electron-deficient triazine ring to increase the concentration of Ni³⁺ and intrinsic conductivity of NiO _x . The modified device achieves the best PCE of 22.81% and excellent operational, ambient and thermal stabilities.

Nickel oxide (NiO _x ) is a promising hole transport material in inverted organic-inorganic metal halide perovskite solar cells. However, its low intrinsic conductivity hinders its further improvement in device performance. Here, we employ a trimercapto-s-triazine trisodium salt (TTTS) as a chelating agent of Ni²⁺ in the NiO _x layer to improve its conductivity. Due to the electron-deficient triazine ring, the TTTS complexes with Ni²⁺ in NiO _x via a strong Ni²⁺-N coordination bond and increases the ratio of Ni³⁺:Ni²⁺. The increased Ni³⁺ concentration adjusts the band structure of NiO _x , thus enhancing hole density and mobility, eventually improving the intrinsic conductivity of NiO _x . As a result, the device with TTTS modification displays a champion power conversion efficiency (PCE) of 22.81%. The encapsulated device based on a modified-NiO _x layer maintains 94% of its initial power output at the maximum power point and continuous one-sun illumination for 1000 h at 45 °C. In addition, the unencapsulated target devices also maintain 92% at 60 ± 5% relative humidity and 25 °C in the air for 5000 h; and 91% at 85 °C in a nitrogen atmosphere for 1000 h. The research provides an effective strategy to enhance PCE and stability of inverted PSCs via modifying NiO _x films with triazine molecule.

Picolylamine isomers as A-site cations are proven to dominate the crystallization kinetics via hydrogen bonding and conjugate π–π stacking, and ultimately determine the perovskite dimensionality. The as-fabricated 1D–3D inorganic perovskite solar cells demonstrate remarkably improved carrier transfer capability and long-term moisture stability through passivating the iodine vacancy defect (V _I) drifting inside the perovskite layer, which would promote the application of inorganic perovskite photovoltaics.

Although the CsPbI _x Br_3−x (0 ≤ x ≤ 3) inorganic perovskite has emerged as excellent candidate for advanced photovoltaic technologies, the long-term stability against moisture is a detrimental issue that lags their power conversion efficiency (PCE). Herein, picolylamine isomers triggered multidimension coupling strategy is experimentally and theoretically proved to be capable of stabilizing black phase with hydrogen bonding and establishing new-fangled charge carrier transport channel by means of the π–π stacking interaction between conjugating A-site organic groups. Equally importantly, the presence of functional A-site organic group stemming from electron-rich benzene ring in 1D perovskite, i.e., 4-picolylamine (4-PA), is also able to passivate the iodine vacancy defect (V_I) drifting in the 3D perovskite layer. Accordingly, the optimized CsPbI_2.85Br_0.15 perovskite solar cells (PSCs) allow to yield a PCE as high as 19.75% under AM1.5G light intensity, and the unencapsulated devices remain 94% of their original PCE after 3000 h aging upon 1.5 AM light irradiation with a relative humidity of 25%, exhibiting remarkable enhancement in comparison to their 3D counterparts. This study provides an in-depth insight of the multidimension coupling strategy in developing state-of-the-art inorganic PSCs.

Herein, a high-quality perovskite film is fabricated using a metal alloying approach, which results in stable devices with an efficiency of up to 21.2%.

Perovskite materials with ABX₃ structure (A: organic, B: metal, and X: halides) have attracted tremendous attention due to their outstanding optoelectronic properties. Herein, a novel approach is developed using chemical vapor deposition (CVD), i.e., metal alloying of halide-perovskite domain via ion-transfer (MAHDI) for the growth of high-quality perovskite films, grown directly from a metal precursor. This technique easily enables us to replace the toxic Pb metal (B site) with other metals using alloying approach. Using the proposed approach, we fabricated stable and efficient Pb–In perovskite solar cells (PSCs) with a maximum power conversion efficiency (PCE) of 21.2%, which is more efficient than the pure Pb-based PSCs (19.23%). Our characterization results reveal that In-doping improves the crystallinity and photoluminescence (PL) of the perovskite film, resulting in higher photovoltaic properties in the device. To demonstrate the potential of our proposed method for other alloys, we also fabricated PSCs based on Pb–25%Sn alloy and obtained PCE of up to 15.2%. Overall, MAHDI technique opens up a new direction in the field of perovskite devices demonstrating great advantages such as lower price, higher performance, scalability, and fabrication flexibility.

Herein, 6-aminonicotinic acid is incorporated to “cure” the defects among the fabricated perovskite film. The strong molecular interaction can effectively tailor the crystallization process and passivate the defects to achieve the high-performance perovskite solar cells (PSCs). The simple doping method would provide the pioneering investigation for the commercialization of PSCs.

The disordered distribution of defects in perovskite structures seriously disrupts the carrier transmission rates and limits the power conversion efficiency (PCE) in perovskite-based solar cells. Defect passivation is an effective strategy to eliminate defects in perovskites and suppress the process of nonradiative recombination. Herein, 6-aminonicotinic acid (6-ANA, C₆H₆N₂O₂), which contains both amino and carboxyl groups, has been used, as a crosslinking agent, between perovskite grain boundaries, which allows for the development of efficient and stable perovskite solar cells (PSCs). The passivation mechanism of 6-ANA that enables the fabrication of highly efficient and stable PSCs has systematically been investigated in this study. The results show that the crystallinity of the perovskite film improves with the addition of 6-ANA in the perovskite films and the trap density is suppressed. The optimized efficiency achieved for the PSC device is as high as 19.71% while maintaining an initial efficiency of 75% after 500 h of shelf storage. This work provides a simple and effective strategy to reduce electronic defects, present in perovskite films, as well as at the interface between the perovskite films and the hole transport layers.

A quantitative analysis of the Pb leaching dynamics is performed by investigating five types of state-of-the-art perovskite solar cells (PSCs). It is found that Pb leaching occurs rapidly, with more than 60% of total Pb amount leaching within the first 120 s upon aqueous exposure. The Pb leaching rate is likely dependent upon the types of PSCs.

Lead halide perovskite solar cells (PSCs) have emerged as a highly promising next-generation photovoltaic (PV) technology that combines high device performance with ease of processing and low cost. However, the potential leaching of lead is recognized as a major environmental concern for their large-scale commercialization, especially for application areas with significant overlap with human life. Herein, a quantitative kinetic analysis of the Pb leaching behavior of five types of benchmark PSCs, namely, MAPbI₃, FA_0.95MA_0.05Pb(I_0.95Br_0.05)₃, Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃, CsPbI₃, and CsPbI₂Br, under laboratory rainfall conditions is reported. Strikingly, over 60% of the Pb contained in the unencapsulated perovskite devices is leached within the first 120 s under rainfall exposure, suggesting that very rapid leaching of Pb can occur when indoor and outdoor PV devices are subject to physical damage or failed encapsulation. The initial Pb leaching rate is found to be strongly dependent on the types of PSCs, pointing to a potential route toward Pb leaching reduction through further optimization of their materials design. The findings offer kinetic insights into the Pb leaching behavior of PSCs upon aqueous exposure, highlighting the urgency to develop robust mitigation methods to avoid a potentially catastrophic impact on the environment for their large-scale deployment.

The synergistic effect of ethyl acetate as antisolvent and CsBr as the additive in the MAPbI₃ precursor solution to increase perovskite grain size, reduce defect density, improve quality of the perovskite film, and finally, enhance efficiency and stability of the device is demonstarted. Especially, the indium tin oxide/SnO₂/CsBr–MAPbI₃/carbon device achieves a champion photoelectric conversion efficiency of 16.45%.

Defect passivation is a crucial process for achieving high-performance perovskite solar cells (PSCs). Herein, the synergistic effect of anti-solvent and component engineering for effective passivation to attain highly stable PSCs is demonstrated, specifically, for ethyl acetate as the anti-solvent and CsBr as the additive in the MAPbI₃ precursor solution. It is found that the rapid solvent evaporation results in fast nucleation, and the CsBr assists the perovskite grain growth. The synergistic effect of anti-solvent and additive engineering leads to increased perovskite grain size, reduced defect density, improved quality of the perovskite thin film, and finally, enhanced efficiency and stability of the indium tin oxide/SnO₂/perovskite/carbon device. The influence of this synergistic effect on the morphology and photovoltaic performance is systematically investigated. Printable PSCs with hole-transport-layer-free carbon electrodes are designed and constructed, which achieve a champion photoelectric conversion efficiency of 16.45%, fill factor of 72.63%, short circuit current of 19.90 mA cm^−2, and open circuit voltage of 1.14 V. Herein, a facile and low-cost approach is demonstrated to obtain highly stable C-PSCs and a promising strategy for future commercial application is provided.

Asymmetrical perovskite single crystals featuring nearly vertical Ruddlesden–Popper phases are synthesized, allowing effective charge collection upon solar irradiation. The high crystallinity and preferred orientation of the 2D layers minimize the impact of the bulky side chains of the ammonium cations and facilitate charge transport in the perovskite solar cells.

2D perovskites are receiving increasing amounts of attention because of their superior device stability. The framework is “cut” into corner-sharing PbX₆ layers, known as Ruddlesden–Popper (RP) phases, by employing bulky cations that cannot fit into the previous A-site interspaces. Unfortunately, the insulating moieties of the cations can significantly affect charge transport in 2D perovskites, thereby limiting the power conversion efficiencies (PCEs) of perovskite solar cells. Herein, a vapor venting space-limited crystallization method is developed for growing asymmetric 2D perovskite single crystals (SCs) featuring nearly vertically oriented RP phases. The high crystallinity and preferred orientation of the 2D layered structures minimize the impact of the bulky side chains of the ammonium cations. By using grazing incidence small-angle X-Ray scattering, it is found that the RP phases in the 2D SCs are aligned with a tilt angle with respect to the substrate normal. It is suspected that the strong π–π interaction between the benzene rings of the ammonium cations and the surface of the hole-transport layer plays important roles in determining the 2D crystal structure. The PCE is improved to greater than 16% after surface passivation to lower the degree of surface defects.