Pd(II)/Pd(IV) redox shuttle to suppress vacancy defects at grain boundaries for efficient kesterite solar cells.

Charge loss at grain boundaries of kesterite Cu2ZnSn(S, Se)4 polycrystalline absorbers is an important cause limiting the performance of this emerging thin-film solar cell. Herein, we report a Pd element assisted reaction strategy to suppress atomic vacancy defects in GB regions. The Pd, on one hand in the form of PdSex compounds, can heterogeneously cover the GBs of the absorber film, suppressing Sn and Se volatilization loss and the formation of their vacancy defects (i.e. VSn and VSe), and on the other hand, in the form of Pd(II)/Pd(IV) redox shuttle, can assist the capture and exchange of Se atoms, thus contributing to eliminating the already-existing VSe defects within GBs. These collective effects have effectively reduced charge recombination loss and enhanced p-type characteristics of the kesterite absorber. As a result, high-performance kesterite solar cells with a total-area efficiency of 14.5% (certified at 14.3%) have been achieved.

Controlling Selenization Equilibrium Enables High-Quality Kesterite Absorbers for Efficient Solar Cells.

Kesterite Cu2ZnSn(S, Se)4 is considered one of the most competitive photovoltaic materials due to its earth-abundant and nontoxic constituent elements, environmental friendliness, and high stability. However, the preparation of high-quality Kesterite absorbers for photovoltaics is still challenging for the uncontrollability and complexity of selenization reactions between metal element precursors and selenium. In this study, we propose a solid-liquid/solid-gas (solid precursor and liquid/vapor Se) synergistic reaction strategy to precisely control the selenization process. By pre-depositing excess liquid selenium, we provide the high chemical potential of selenium to facilitate the direct and rapid formation of the Kesterite phase. The further optimization of selenium condensation and subsequent volatilization enables the efficient removal of organic compounds and thus improves charge transport in the absorber film. As a result, we achieve high-performance Kesterite solar cells with total-area efficiency of 13.6% (certified at 13.44%) and 1.09 cm2-area efficiency of 12.0% (certified at 12.1%).

Constructing an Interfacial Gradient Heterostructure Enables Efficient CsPbI3 Perovskite Solar Cells and Printed Minimodules.

Severe nonradiative recombination originating from interfacial defects together with the pervasive energy level mismatch at the interface remarkably limits the performance of CsPbI3 perovskite solar cells (PSCs). These issues need to be addressed urgently for high-performance cells and their applications. Herein, an interfacial gradient heterostructure based on low-temperature post-treatment of quaternary bromide salts for efficient CsPbI3 PSCs with an impressive efficiency of 21.31% and an extraordinary fill factor of 0.854 is demonstrated. Further investigation reveals that Br- ions diffuse into the perovskite films to heal undercoordinated Pb2+ and inhibit Pb cluster formation, thus suppressing nonradiative recombination in CsPbI3 . Meanwhile, a more compatible interfacial energy level alignment resulting from Br- gradient distribution and organic cations surface termination is also achieved, hence promoting charge separation and collection. Consequently, the printed small-size cell with an efficiency of 20.28% and 12 cm2 printed CsPbI3 minimodules with a record efficiency of 16.60% are also demonstrated. Moreover, the unencapsulated CsPbI3 films and devices exhibit superior stability.

Initializing film homogeneity to retard phase segregation for stable perovskite solar cells.

The mixtures of cations and anions used in hybrid halide perovskites for high-performance solar cells often undergo element and phase segregation, which limits device lifetime. We adapted Schelling's model of segregation to study individual cation migration and found that the initial film inhomogeneity accelerates materials degradation. We fabricated perovskite films (FA1-xCsxPbI3; where FA is formamidinium) through the addition of selenophene, which led to homogeneous cation distribution that retarded cation aggregation during materials processing and device operation. The resultant devices achieved enhanced efficiency and retained >91% of their initial efficiency after 3190 hours at the maximum power point under 1 sun illumination. We also observe prolonged operational lifetime in devices with initially homogeneous FACsPb(Br0.13I0.87)3 absorbers.

Manipulating the Migration of Iodine Ions via Reverse-Biasing for Boosting Photovoltaic Performance of Perovskite Solar Cells.

Perovskite solar cells (PSCs) are being developed rapidly and exhibit greatly potential commercialization. Herein, it is found that the device performance can be improved by manipulating the migration of iodine ions via reverse-biasing, for example, at -0.4 V for 3 min in dark. Characterizations suggest that reverse bias can increase the charge recombination resistance, improve carrier transport, and enhance built-in electric field. Iodine ions including iodine interstitials in perovskites are confirmed to migrate and accumulate at the SnO2 /perovskite interface under reverse-basing, which fill iodine vacancies at the interface and interact with SnO2 . First-principles calculations suggest that the SnO2 /perovskite interface with less iodine vacancies has a stronger interaction and higher charge transfer, leading to larger built-in electric field and improved charge transport. Iodine ions that may pass through the SnO2 /perovskite interface are also confirmed to be able to interact with Sn4+  and passivate oxygen vacancies on the surface of SnO2 . Consequently, an efficiency of 23.48% with the open-circuit voltage (Voc ) of 1.16 V is achieved for PSCs with reverse-biasing, as compared with the initial efficiency of 22.13% with a Voc  of 1.10 V. These results are of great significance to reveal the physics mechanism of PSCs under electric field.

A Versatile Molten-Salt Induction Strategy to Achieve Efficient CsPbI3 Perovskite Solar Cells with a High Open-Circuit Voltage >1.2 V.

All-inorganic CsPbI3 perovskite has emerged as an important photovoltaic material due to its high thermal stability and suitable bandgap for tandem devices. Currently, the cell performance of CsPbI3 solar cells is mainly subject to a large open-circuit voltage (VOC ) deficit. Herein, a multifunctional room-temperature molten salt, dimethylamine acetate (DMAAc) is demonstrated, which not only directly acts as a solvent for precursor solutions, but also regulates the phase conversion process of the CsPbI3 film for high-efficiency photovoltaics. DMAAc can stabilize the DMAPbI3 structure and eliminate the Cs4 PbI6 intermediate phase, which is easily spatially segregated. Meanwhile, a new homogeneous intermediate phase DMAPb(I,Ac)3 is formed, which finally affords high-quality CsPbI3 films. With this approach, the charge capture activity of defects in the CsPbI3 film is significantly suppressed. Consequently, a VOC of 1.25 V and >21% power conversion efficiency are achieved, which is the record highest reported thus far. This intermediate phase-regulation strategy is believed to be applicable to other perovskite material systems.

Simultaneously Enhancing Exciton/Charge Transport in Organic Solar Cells by an Organoboron Additive.

Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).

Ge Bidirectional Diffusion to Simultaneously Engineer Back Interface and Bulk Defects in the Absorber for Efficient CZTSSe Solar Cells.

Aiming at a large open-circuit voltage (VOC ) deficit in Cu2 ZnSn(S,Se)4 (CZTSSe) solar cells, a new and effective strategy to simultaneously regulate the back interface and restrain bulk defects of CZTSSe absorbers is developed by directly introducing a thin GeO2 layer on Mo substrates. Power conversion efficiency (power-to-efficiency) as high as 13.14% with a VOC of 547 mV is achieved for the champion device, which presents a certified efficiency of 12.8% (aperture area: 0.25667 cm2 ). Further investigation reveals that Ge bidirectional diffusion simultaneously occurs toward the CZTSSe absorber and MoSe2 layer at the back interface while being selenized. That is, some Ge element from the GeO2 diffuses into the CZTSSe absorber layer to afford Ge-doped absorbers, which can significantly reduce the defect density and band tailing, and facilitate quasi-Fermi level split by relatively higher hole concentration. Meanwhile, a small amount of Ge element also participates in the formation of MoSe2 at the back interface, thus enhancing the work function of MoSe2 and effectively separating photoinduced carriers. This work highlights the synergistic effect of Ge element toward the bulk absorber and the back interface and also provides an easy-handling way to achieve high-performance CZTSSe solar cells.

Impact of substrate digestibility on microbial community stability in methanogenic digestors: The mechanism and solution.

This study investigated the temporal dynamics of digestion efficiency and community stability in digesters fed with waste activated sludge (WAS), straw (STR-AD), food waste (FW-AD) and mixture of straw-and-food waste (STR-FW-AD). Results showed that carbon removals of recalcitrant substrates (i.e., 48.2 ± 3.9% in WAS-AD and 57.8 ± 4.9% in STR-AD) were lower than that of labile substrates (i.e., 70.7 ± 4.0% in FW-AD). Nonetheless, carbon removal of recalcitrant substrates was largely improved through co-digestion (70.3 ± 3.2% in STR-FW-AD). In contrast to monopoly communities (e.g., the highly enriched Paludibacter) fed with the labile substrates, recalcitrant substrates supported highly diverse communities. Accordingly, the medians of negative/positive cohesions of communities in WAS-AD, STR-AD, STR-FW-AD and FW-AD decreased from 0.86 to 0.63, suggesting their decreasing community stability. Microbial source tracking analyses showed the major contribution of the STR-AD community to the co-digestion community. This study provided unprecedented mechanistic insight into stability improvement of substrate co-digestion on the methanogenic digestion microbiome.

Temperature-Reliable Low-Dimensional Perovskites Passivated Black-Phase CsPbI3 toward Stable and Efficient Photovoltaics.

Low-dimensional (LD) perovskites can effectively passivate and stabilize 3D perovskites for high-performance perovskite solar cells (PSCs). Regards CsPbI3 -based PSCs, the influence of high-temperature annealing on the LD perovskite passivation effect has to be taken into account due to fact the black-phase CsPbI3 crystallization requires high-temperature treatment, however, which has been rarely concerned so far. Here, the thermal stability of LD perovskites based on three hydrophobic organic ammonium salts and their passivation effect toward CsPbI3 and the whole device performance, have been investigated. It is found that, phenyltrimethylammonium iodide (PTAI) and its corresponding LD perovskites exhibit excellent thermal stability. Further investigation reveals that PTAI-based LD perovskites are mainly distributed at grain boundaries, which not only enhances the phase stability of CsPbI3 but also effectively suppresses non-radiative recombination. As a consequence, the champion PSC device based on CsPbI3 exhibits a record efficiency of 21.0 % with high stability.

Organic carbon and eukaryotic predation synergistically change resistance and resilience of aquatic microbial communities.

With rapid global urbanization, anthropogenic activities alter aquatic biota in urban rivers through inputs of dissolved organic carbon (DOC) and nutrients. Microorganisms-mediated global element cycles provide functions in maintaining microbial ecology stability. The DOC (bottom-up control) and microbial predation (top-down control) may synergistically drive the competition and evolution of aquatic microbial communities, as well as their resistance and resilience, for which experimental evidences remain scarce. In this study, laboratory sediment-water column experiments were employed to mimic the organic carbon-driven water blackening and odorization process in urban rivers and to elucidate the impact of DOC on microbial ecology stability. Results showed that low (25-75 mg/L) and high DOC (100-150 mg/L) changed the aquatic microbial community assemblies in different patterns: (1) the low DOC enriched K-selection microorganisms (e.g., C39, Tolumonas and CR08G) with low biomass and low resilience, as well as high resistance to perturbations in changing microbial community assemblies; (2) the high DOC was associated with r-selection microorganisms (e.g., PSB-M-3 and Clostridium) with high biomass and improved resilience, together with low resistance detrimental to microbial ecology stability. Overall, this study provided new insight into the impact of DOC on aquatic microbial community stability, which may help guide sustainable urban river management.

High-efficiency (>20%) planar carbon-based perovskite solar cells through device configuration engineering.

Carbon-based perovskite solar cells (C-PSCs) have attracted widespread research interest because of their excellent stability. However, the power conversion efficiency (PCE) of C-PSCs, especially planar C-PSCs, lags far behind the certified efficiency (25.5%) of metal-based PSCs. The simple architecture of planar C-PSCs imparts stringent requirements for device configuration. In this study, we fabricated high-performance planar C-PSCs through device configuration engineering in terms of the perovskite active layer and carbon electrode. Through the combination of component and additive engineering, the crystallization and absorption profiles of perovskite active layer have been improved, which afforded sufficient photogenerated carriers and decreased nonradiative recombination. Furthermore, the mechanical and physical properties of carbon electrode were evaluated comprehensively to regulate the back-interface contact. Based on the compromise of the flexibility and conductivity of carbon film, an excellent back-interface contact has been formed, which promoted fast interface charge transfer, thereby decreasing interface recombination and improving carrier collection efficiency. Finally, the as-prepared devices achieved a remarkable PCE of up to 20.04%, which is a record-high value for planar C-PSCs. Furthermore, the as-prepared devices exhibited excellent long-term stability. After storage for 1000 h at room temperature and 25% relative humidity without encapsulation, the as-prepared device retained 94% of its initial performance.

Two-Step Annealing CZTSSe/CdS Heterojunction to Improve Interface Properties of Kesterite Solar Cells.

The post-heating treatment of the CZTSSe/CdS heterojunction can enhance the interfacial properties of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this regard, a two-step annealing method was developed to enhance the heterojunction quality for the first time. That is, a low-temperature (90 °C) process was introduced before the high-temperature treatment, and 12.3% efficiency of CZTSSe solar cells was achieved. Further investigation revealed that the CZTSSe/CdS heterojunction band alignment with a smaller spike barrier can be realized by the two-step annealing treatment, which assisted in carrier transportation and reduced the charge recombination loss, thus enhancing the open-circuit voltage (VOC) and fill factor (FF) of the devices. In addition, the two-step annealing could effectively avoid the disadvantages of direct high-temperature treatment (such as more pinholes on CdS films and excess element diffusion), improve the CdS crystallization, and decrease the defect densities within the device, especially interfacial defects. This work provides an effective method to improve the CZTSSe/CdS heterojunction properties for efficient kesterite solar cells.

Efficient (>20 %) and Stable All-Inorganic Cesium Lead Triiodide Solar Cell Enabled by Thiocyanate Molten Salts.

Besides widely used surface passivation, engineering the film crystallization is an important and more fundamental route to improve the performance of all-inorganic perovskite solar cells. Herein, we have developed a urea-ammonium thiocyanate (UAT) molten salt modification strategy to fully release and exploit coordination activities of SCN- to deposit high-quality CsPbI3 film for efficient and stable all-inorganic solar cells. The UAT is derived by the hydrogen bond interactions between urea and NH4 + from NH4 SCN. With the UAT, the crystal quality of the CsPbI3 film has been significantly improved and a long single-exponential charge recombination lifetime of over 30 ns has been achieved. With these benefits, the cell efficiency has been promoted to over 20 % (steady-state efficiency of 19.2 %) with excellent operational stability over 1000 h. These results demonstrate a promising development route of the CsPbI3 related photoelectric devices.

CdS Induced Passivation toward High Efficiency and Stable Planar Perovskite Solar Cells.

In perovskite solar cells, the halide vacancy defects on the perovskite film surface/interface will instigate charge recombination, leading to a decrease in cell performance. In this study, cadmium sulfide (CdS) has been introduced into the precursor solution to reduce the halide vacancy defects and improve the cell performance. The highest efficiency of the device reaches 21.62%. Density functional theory calculation reveals that the incorporated Cd2+ ions can partially replace Pb2+ ions, thus forming a strong Cd-I bond and effectively reducing iodide vacancy defects (VI); at the same time, the loss of the charge recombination is significantly reduced because VI is filled by S2- ions. Besides, the substitution of Cd2+ for Pb2+ could increase the generation of PbI2, which can further passivate the grain boundary. Therefore, the stability of the cells, together with the efficiency of the power conversion efficiencies (PCEs), is also improved, maintaining 87.5% of its initial PCEs after being irradiated over 410 h. This work provides a very effective strategy to passivate the surface/interface defects of perovskite films for more efficient and stable optoelectronic devices.

Eliminating the electric field response in a perovskite heterojunction solar cell to improve operational stability.

Intrinsic and extrinsic ion migration is a very large threat to the operational stability of perovskite solar cells and is difficult to completely eliminate due to the low activation energy of ion migration and the existence of internal electric field. We propose a heterojunction route to help suppress ion migration, thus improving the operational stability of the cell from the perspective of eliminating the electric field response in the perovskite absorber. A heavily doped p-type (p+) thin layer semiconductor is introduced between the electron transporting layer (ETL) and perovskite absorber. The heterojunction charge depletion and electric field are limited to the ETL and p+ layers, while the perovskite absorber and hole transporting layer remain neutral. The p+ layer has a variety of candidate materials and is tolerant of defect density and carrier mobility, which makes this heterojunction route highly feasible and promising for use in practical applications.

Sludge digestibility and functionally active microorganisms in methanogenic sludge digesters revealed by E. coli-fed digestion and microbial source tracking.

Methanogenic sludge digestion plays a pivotal role in attenuating and hygienizing the massively-produced waste activated sludge (WAS), which is predominantly composed of microbial cells and extracellular polymeric substances (EPS). The efficient sludge digestion requires a variety of functionally active microorganisms working together closely to convert sludge organic matter into biogas. Nonetheless, the digestion efficiency (or digestibility quantified as carbon removal efficiency) of major sludge constituents (i.e., microbial cells and EPS) and associated functionally active microorganisms in sludge digesters remain elusive. In this study, we identified the digestibility of sludge microbial cells and the associated functionally active microorganisms by using Escherichia coli (E. coli)-fed digestion and microbial source tracking. The average carbon removals in four digesters fed with fresh WAS (WAS-AD), thermal pretreated WAS (Thermal-WAS-AD), E. coli cells (E.coli-AD) and thermal pretreated E. coli cells (Thermal-E.coli-AD) were 30.6 ± 3.4%, 45.8 ± 2.9%, 69.0 ± 3.4% and 68.9 ± 4.6%, respectively. Compared to WAS-AD and Thermal-WAS-AD, the significantly higher carbon removals in E. coli-AD and Thermal-E. coli-AD suggested the remarkably higher digestibility of microbial cells than EPS, and releasing organic matter from EPS might be a rate-limiting step in sludge digestion. Functionally active microorganisms for microbial cell digestion predominantly included fermenters (e.g., Petrimonas and Lentimicrobium), syntrophic acetogens (e.g., Synergistaceae) and methanogens (e.g., Methanosaeta and Methanosarcina). Microbial source tracking estimation showed that the microbial cell-digesting populations accounted for 35.6 ± 9.1% and 70.3 ± 10.1% of total microbial communities in the WAS-AD and Thermal-WAS-AD, respectively. Accordingly, the functionally active microorganisms for digestion of both microbial cells and EPS accounted for 64.5 ± 12.1% and 97.3 ± 2.0% of total digestion sludge microbiome in WAS-AD and Thermal-WAS-AD, respectively. By contrast, feeding WAS-derived microorganisms accounted for 23.2 ± 4.4% and 2.3 ± 1.2% of total microbial communities in the WAS-AD and Thermal-WAS-AD, respectively.

Genome-Centric Metagenomic Insights into the Impact of Alkaline/Acid and Thermal Sludge Pretreatment on the Microbiome in Digestion Sludge.

Pretreatment of waste-activated sludge (WAS) is an effective way to destabilize sludge floc structure and release organic matter for improving sludge digestion efficiency. Nonetheless, information on the impact of WAS pretreatment on digestion sludge microbiomes, as well as mechanistic insights into how sludge pretreatment improves digestion performance, remains elusive. In this study, a genome-centric metagenomic approach was employed to investigate the digestion sludge microbiome in four sludge digesters with different types of feeding sludge: WAS pretreated with 0.25 mol/liter alkaline/acid (APAD), WAS pretreated with 0.8 mol/liter alkaline/acid (HS-APAD), thermally pretreated WAS (thermal-AD), and fresh WAS (control-AD). We retrieved 254 metagenome-assembled genomes (MAGs) to identify the key functional populations involved in the methanogenic digestion process. These MAGs span 28 phyla, including 69 yet-to-be-cultivated lineages, and 30 novel lineages were characterized with metabolic potential associated with hydrolysis and fermentation. Interestingly, functional populations involving carbohydrate digestion were enriched in APAD and HS-APAD, while lineages related to protein and lipid fermentation were enriched in thermal-AD, corroborating the idea that different substrates are released from alkaline/acid and thermal pretreatments. Among the major functional populations (i.e., fermenters, syntrophic acetogens, and methanogens), significant correlations between genome sizes and abundance of the fermenters were observed, particularly in APAD and HS-APAD, which had improved digestion performance.IMPORTANCE Wastewater treatment generates large amounts of waste-activated sludge (WAS), which consists mainly of recalcitrant microbial cells and particulate organic matter. Though WAS pretreatment is an effective way to release sludge organic matter for subsequent digestion, detailed information on the impact of the sludge pretreatment on the digestion sludge microbiome remains scarce. Our study provides unprecedented genome-centric metagenomic insights into how WAS pretreatments change the digestion sludge microbiomes, as well as their metabolic networks. Moreover, digestion sludge microbiomes could be a unique source for exploring microbial dark matter. These results may inform future optimization of methanogenic sludge digestion and resource recovery.

Intermolecular π-π Conjugation Self-Assembly to Stabilize Surface Passivation of Highly Efficient Perovskite Solar Cells.

Surface passivation is an effective approach to eliminate defects and thus to achieve efficient perovskite solar cells, while the stability of the passivation effect is a new concern for device stability engineering. Herein, tribenzylphosphine oxide (TBPO) is introduced to stably passivate the perovskite surface. A high efficiency exceeding 22%, with steady-state efficiency of 21.6%, is achieved, which is among the highest performances for TiO2 planar cells, and the hysteresis is significantly suppressed. Further density functional theory (DFT) calculation reveals that the surface molecule superstructure induced by TBPO intermolecular π-π conjugation, such as the periodic interconnected structure, results in a high stability of TBPO-perovskite coordination and passivation. The passivated cell exhibits significantly improved stability, with sustaining 92% of initial efficiency after 250 h maximum-power-point tracking. Therefore, the construction of a stabilized surface passivation in this work represents great progress in the stability engineering of perovskite solar cells.

Enhanced Perovskite Solar Cell Efficiency Via the Electric-Field-Induced Approach.

The stability issue hinders the commercialization of the perovskite solar cells (PSCs), which is widely recognized. The efficiency generally decreases over time during the working condition. Here, we report an efficiency enhancement phenomenon of PSCs in the stability test at the maximum power point, which is speculated to be related to the electric-field-induced ion migration. The defect density and efficiency-related parameters were traced in situ by admittance spectroscopy and transient photovoltage when the cell works under bias voltage. The performance enhancement was revealed to be attributed to the reduction of the cell defects owing to ion migration. An efficiency of 22.3% can be achieved after the bias voltage was kept for 8 h. These findings suggest that ion migration is a double-edged sword that affects the electrical stability of PSCs, which presents a potential approach to improve the device's stability by appropriately controlling the defect states.