The dynamic adsorption affinity of ligands is a surrogate for the passivation of surface defects.

Surface defects in semiconducting materials, though they have been widely studied, remain a prominent source of loss in optoelectronic devices; here we sought a new angle of approach, looking into the dynamic roles played by surface defects under atmospheric stressors and their chemical passivants in the lifetime of optoelectronic materials. We find that surface defects possess properties distinct from those of bulk defects. ab initio molecular dynamics simulations reveal a previously overlooked reversible degradation mechanism mediated by hydrogen vacancies. We find that dynamic surface adsorption affinity (DAA) relative to surface treatment ligands is a surrogate for passivation efficacy, a more strongly-correlated feature than is the static binding strength emphasized in prior reports. This guides us to design targeted passivator ligands with high molecular polarity: for example, 4-aminobutylphosphonic acid exhibits strong DAA and provides defect passivation applicable to a range of perovskite compositions, including suppressed hydrogen vacancy formation, enhanced photovoltaic performances and operational stability in perovskite solar cells.

Intensity-Modulated Photocurrent Spectroscopy Measurements of High-Efficiency Perovskite Solar Cells.

Frequency domain characterization has long served as an important method for the examination of diverse kinetic processes that occur in solar cells. In this study, we investigated the dynamic response of high-efficiency perovskite solar cells utilizing ultra-low-intensity-modulated photocurrent spectroscopy. Distinctive intensity-modulated photocurrent spectroscopy (IMPS) attributes were detected only as a result of this low-intensity modulation, and their evolution under light and voltage bias was investigated in detail. We generally observed only two arcs in the Q-plane plots and attributed the smaller, low-frequency arc to trap-dominated charge transport in the device. Light and voltage bias-dependent measurements confirm this attribution. An equivalent circuit model was used to better understand the features and trends of these measurements and to validate our physical interpretation of the results. Additionally, we tracked the IMPS response of one of the cells over time and showed that slow degradation impacts the size and attributes of the low-frequency arc. Finally, we found that changes in the IMPS response correlate closely with the current versus voltage characteristics of the devices.

On the Durability of Tin-Containing Perovskite Solar Cells.

Tin (Sn)-containing perovskite solar cells (PSCs) have gained significant attention in the field of perovskite optoelectronics due to lower toxicity than their lead-based counterparts and their potential for tandem applications. However, the lack of stability is a major concern that hampers their development. To achieve the long-term stability of Sn-containing PSCs, it is crucial to have a clear and comprehensive understanding of the degradation mechanisms of Sn-containing perovskites and develop mitigation strategies. This review provides a compendious overview of degradation pathways observed in Sn-containing perovskites, attributing to intrinsic factors related to the materials themselves and environmental factors such as light, heat, moisture, oxygen, and their combined effects. The impact of interface and electrode materials on the stability of Sn-containing PSCs is also discussed. Additionally, various strategies to mitigate the instability issue of Sn-containing PSCs are summarized. Lastly, the challenges and prospects for achieving durable Sn-containing PSCs are presented.

Enabling High Activity Catalyst Co3O4@CeO2 for Propane Catalytic Oxidation via Inverse Loading.

Propane catalytic oxidation is an important industrial chemical process. However, poor activity is frequently observed for stable C-H bonds, especially for non-noble catalysts in low temperature. Herein, we reported a controlled synthesis of catalyst Co3O4@CeO2-IE via inverse loading and proposed a strategy of oxygen vacancy for its high catalytic oxidation activity, achieving better performance than traditional supported catalyst Co3O4/CeO2-IM, i.e., the T50 (temperature at 50% propane conversion) of 217 °C vs. 235 °C and T90 (temperature at 90% propane conversion) of 268 °C vs. 348 °C at the propane space velocity of 60,000 mL g-1 h-1. Further investigations indicate that there are more enriched oxygen vacancies in Co3O4@CeO2-IE due to the unique preparation method. This work provides an element doping strategy to effectively boost the propane catalytic oxidation performance as well as a bright outlook for efficient environmental catalysts.

Implications of Electron Transport Layer and Back Metal Contact Variations in Tin-Lead Perovskite Solar Cells Assessed by Spectroscopic Ellipsometry and External Quantum Efficiency.

The structural and optical properties of hybrid organic-inorganic metal halide perovskite solar cells are measured by spectroscopic ellipsometry to reveal an optically distinct interfacial layer among the back contact metal, charge transport, and absorber layers. Understanding how this interfacial layer impacts performance is essential for developing higher performing solar cells. This interfacial layer is modeled by Bruggeman effective medium approximations (EMAs) to contain perovskite, C60, BCP, and metal. External quantum efficiency (EQE) simulations that consider scattering, electronic losses, and the formation of nonparallel interfaces are created with input derived from ellipsometry structural-optical models and compared with experimental EQE to estimate optical losses. This nonplanar interface causes optical losses in short circuit current density (JSC) of up to 1.2 mA cm-2. A study of glass/C60/SnO2/Ag or Cu and glass/C60/BCP/Ag film stacks shows that C60 and BCP mix, but replacing BCP with SnO2 can prevent mixing between the ETLs to prevent contact between C60 and back contact metal and enable the formation of a planar interface between ETLs and back contact metals.

Operando Characterizations of Light-Induced Junction Evolution in Perovskite Solar Cells.

Light-induced performance changes in metal halide perovskite solar cells (PSCs) have been studied intensively over the last decade, but little is known about the variation in microscopic optoelectronic properties of the perovskite heterojunctions in a completed device during operation. Here, we combine Kelvin probe force microscopy and transient reflection spectroscopy techniques to spatially resolve the evolution of junction properties during the operation of metal-halide PSCs and study the light-soaking effect. Our analysis showed a rise of an electric field at the hole-transport layer side, convoluted with a more reduced interfacial recombination rate at the electron-transport layer side in the PSCs with an n-i-p structure. The junction evolution is attributed to the effects of ion migration and self-poling by built-in voltage. Device performances are correlated with the changes of electrostatic potential distribution and interfacial carrier dynamics. Our results demonstrate a new route for studying the complex operation mechanism in PSCs.

Suppressed phase segregation for triple-junction perovskite solar cells.

The tunable bandgaps and facile fabrication of perovskites make them attractive for multi-junction photovoltaics1,2. However, light-induced phase segregation limits their efficiency and stability3-5: this occurs in wide-bandgap (>1.65 electron volts) iodide/bromide mixed perovskite absorbers, and becomes even more acute in the top cells of triple-junction solar photovoltaics that require a fully 2.0-electron-volt bandgap absorber2,6. Here we report that lattice distortion in iodide/bromide mixed perovskites is correlated with the suppression of phase segregation, generating an increased ion-migration energy barrier arising from the decreased average interatomic distance between the A-site cation and iodide. Using an approximately 2.0-electron-volt rubidium/caesium mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3 per cent (23.3 per cent certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21 volts. This is, to our knowledge, the first reported certified efficiency for perovskite-based triple-junction solar cells. The triple-junction devices retain 80 per cent of their initial efficiency following 420 hours of operation at the maximum power point.

Rational design of Lewis base molecules for stable and efficient inverted perovskite solar cells.

Lewis base molecules that bind undercoordinated lead atoms at interfaces and grain boundaries (GBs) are known to enhance the durability of metal halide perovskite solar cells (PSCs). Using density functional theory calculations, we found that phosphine-containing molecules have the strongest binding energy among members of a library of Lewis base molecules studied herein. Experimentally, we found that the best inverted PSC treated with 1,3-bis(diphenylphosphino)propane (DPPP), a diphosphine Lewis base that passivates, binds, and bridges interfaces and GBs, retained a power conversion efficiency (PCE) slightly higher than its initial PCE of ~23% after continuous operation under simulated AM1.5 illumination at the maximum power point and at ~40°C for >3500 hours. DPPP-treated devices showed a similar increase in PCE after being kept under open-circuit conditions at 85°C for >1500 hours.

Urbach Energy and Open-Circuit Voltage Deficit for Mixed Anion-Cation Perovskite Solar Cells.

The Urbach energy indicating the width of the exponentially decaying sub-bandgap absorption tail is commonly used as the indicator of electronic quality of thin-film materials used as absorbers in solar cells. Urbach energies of hybrid inorganic-organic metal halide perovskites with various anion-cation compositions are measured by photothermal deflection spectroscopy. The variation in anion-cation composition has a substantial effect on the measured Urbach energy and hence the electronic quality of the perovskite. Depending upon the compositions, the Urbach energy varies from 18 to 65 meV for perovskite films with similar bandgap energies. For most of the perovskite compositions studied here including methylammonium (MA) + formamidinium (FA)-based Pb iodides, mixed Sn + Pb narrow-bandgap perovskites with low or intermediate Sn contents, and wide-bandgap FA + Cs- and I + Br-based perovskites, the correlation between the Urbach energy of the perovskite thin film and open-circuit voltage (VOC) deficit for corresponding solar cells shows a direct relationship with reduction of the Urbach energy occurring with a beneficial decrease in the VOC deficit. However, due to issues related to material quality, impurity phases and stability in laboratory ambient air, and unoptimized film processing techniques, the solar cells incorporating Cs-based inorganic and mixed Sn + Pb perovskites with a higher than optimum Sn content show a higher VOC deficit even though the corresponding films show a lower Urbach energy.

Perovskite Solar Cells Go Bifacial-Mutual Benefits for Efficiency and Durability.

Bifacial solar cells hold the potential to achieve a higher power output per unit area than conventional monofacial devices without significantly increasing manufacturing costs. However, efficient bifacial designs are challenging to implement in inorganic thin-film solar cells because of their short carrier lifetimes and high rear surface recombination. The emergence of perovskite photovoltaic (PV) technology creates a golden opportunity to realize efficient bifacial thin-film solar cells, owing to their outstanding optoelectronic properties and unique features of device physics. More importantly, transparent conducting oxide electrodes can prevent electrode corrosion by halide ions, mitigating one major instability issue of the perovskite devices. Here, the theory of bifacial PV devices is summarized and the advantages of bifacial perovskite solar cells, such as high power output, enhanced device durability, and low economic and environmental costs, are reviewed. The limitations and challenges for bifacial perovskite solar cells are also discussed. Finally, the awareness of bifacial solar cells as a feasible commercialization pathway of perovskite PV for mainstream solar power generation and building-integrated PV is advocated and future research directions are suggested.

Effects of Cu Precursor on the Performance of Efficient CdTe Solar Cells.

Copper (Cu) incorporation is a key process for fabricating efficient CdTe-based thin-film solar cells and has been used in CdTe-based solar cell module manufacturing. Here, we investigate the effects of different Cu precursors on the performance of CdTe-based thin-film solar cells by incorporating Cu using a metallic Cu source (evaporated Cu) and ionic Cu sources (solution-processed cuprous chloride (CuCl) and copper chloride (CuCl2)). We find that ionic Cu precursors offer much better control in Cu diffusion than the metallic Cu precursor, producing better front junction quality, lower back-barrier heights, and better bulk defect property. Finally, outperforming power conversion efficiencies of 17.2 and 17.5% are obtained for devices with cadmium sulfide and zinc magnesium oxide as the front window layers, respectively, which are among the highest reported CdTe solar cells efficiencies. Our results suggest that an ionic Cu precursor is preferred as the dopant to fabricate efficient CdTe thin-film solar cells and modules.

Impact of Humidity and Temperature on the Stability of the Optical Properties and Structure of MAPbI3, MA0.7FA0.3PbI3 and (FAPbI3)0.95(MAPbBr3)0.05 Perovskite Thin Films.

In situ real-time spectroscopic ellipsometry (RTSE) measurements have been conducted on MAPbI3, MA0.7FA0.3PbI3, and (FAPbI3)0.95(MAPbBr3)0.05 perovskite thin films when exposed to different levels of relative humidity at given temperatures over time. Analysis of RTSE measurements track changes in the complex dielectric function spectra and structure, which indicate variations in stability influenced by the underlying material, preparation method, and perovskite composition. MAPbI3 and MA0.7FA0.3PbI3 films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited on soda lime glass directly. (FAPbI3)0.95(MAPbBr3)0.05 films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition.

Mesoporogen-Free Strategy to Construct Hierarchical TS-1 in a Highly Concentrated System for Gas-Phase Propene Epoxidation with H2 and O2.

Hierarchical TS-1 has attracted enormous attention from both academia and industry due to its remarkable catalytic performance in epoxidation reactions. However, sustainable synthesis of hierarchical-nanosized TS-1 without mesoporogens is still challenging. In this work, we report a facile and mesoporogen-free strategy to simultaneously manipulate pore structure and particle size of TS-1 employing the concentrated system. Taking advantage of the suspended nuclei in the concentrated system as confirmed by the DLS-PSD and atomic force microscopy, the novel TS-1 is demonstrated to have higher Ti concentration on surface, higher surface area (539 m2/g), abundant mesopores, and reduced crystal size (ca. 150 nm). Moreover, this Au-Ti bifunctional catalyst shows a good PO formation rate with enhanced catalytic stability due to the hierarchical structure. This strategy opens a novel way for the green synthesis of hierarchical-nanosized TS-1 and facilitates industrial development of the Au/TS-1 catalyst for propene epoxidation.

Low-energy room-temperature optical switching in mixed-dimensionality nanoscale perovskite heterojunctions.

Long-lived photon-stimulated conductance changes in solid-state materials can enable optical memory and brain-inspired neuromorphic information processing. It remains challenging to realize optical switching with low-energy consumption, and new mechanisms and design principles giving rise to persistent photoconductivity (PPC) can help overcome an important technological hurdle. Here, we demonstrate versatile heterojunctions between metal-halide perovskite nanocrystals and semiconducting single-walled carbon nanotubes that enable room-temperature, long-lived (thousands of seconds), writable, and erasable PPC. Optical switching and basic neuromorphic functions can be stimulated at low operating voltages with femto- to pico-joule energies per spiking event, and detailed analysis demonstrates that PPC in this nanoscale interface arises from field-assisted control of ion migration within the nanocrystal array. Contactless optical measurements also suggest these systems as potential candidates for photonic synapses that are stimulated and read in the optical domain. The tunability of PPC shown here holds promise for neuromorphic computing and other technologies that use optical memory.

Optical and Electronic Losses Arising from Physically Mixed Interfacial Layers in Perovskite Solar Cells.

Perovskite solar cell device performance is affected by optical and electronic losses. To minimize these losses in solar cells, it is important to identify their sources. Here, we report the optical and electronic losses arising from physically mixed interfacial layers between the adjacent component materials in highly efficient two terminal (2T) all-perovskite tandem, single-junction wide-bandgap, and single-junction narrow-bandgap perovskite-based solar cells. Physically mixed interfacial layers as the sources of optical and electronic losses are identified from spectroscopic ellipsometry measurements and data analysis followed by comparisons of simulated and measured external quantum efficiency spectra. Parasitic absorbance in the physically mixed regions between silver metal electrical contacts and electron transport layers (ETLs) near the back contact and a physical mixture of commercial indium tin oxide and hole transport layers (HTL) near the front electrical contact lead to substantial optical loss. A lower-density void + perovskite nucleation layer formed during perovskite deposition at the interface between the perovskite absorber layer and the HTL causes electronic losses because of incomplete collection of photogenerated carriers likely originating from poor coverage and passivation of the initially nucleating grains.

Reversing Titanium Oligomer Formation towards High-Efficiency and Green Synthesis of Titanium-Containing Molecular Sieves.

Green and efficient synthesis of titanium-containing molecular sieves is limited by the quantity of environmentally unfriendly additives and complicated synthesis procedures required. Oligomerization of Ti monomers into anatase TiO2 is the typical outcome of such procedures because of a mismatch between hydrolysis rates of Si and Ti precursors. We report a simple and generic additive-free route for the synthesis of Ti-containing molecular sieves (MFI, MEL, and BEA). This approach successfully reverses the formation of Ti oligomers to match hydrolysis rates of Ti and Si species with the assistance of hydroxyl free radicals generated in situ from ultraviolet irradiation. Moreover, fantastic catalytic performance for propene epoxidation with H2 and O2 was observed. Compared with the conventional hydrothermal method, this approach opens up new opportunities for high-efficiency, environmentally benign, and facile production of pure titanium-containing molecular sieves.

Narrow-Bandgap Mixed Lead/Tin-Based 2D Dion-Jacobson Perovskites Boost the Performance of Solar Cells.

The advent of the two-dimensional (2D) family of halide perovskites and their demonstration in 2D/three-dimensional (3D) hierarchical film structures broke new ground toward high device performance and good stability. The 2D Dion-Jacobson (DJ) phase halide perovskites are especially attractive in solar cells because of their superior charge transport properties. Here, we report on 2D DJ phase perovskites using a 3-(aminomethyl)piperidinium (3AMP) organic spacer for the fabrication of mixed Pb/Sn-based perovskites, exhibiting a narrow bandgap of 1.27 eV and a long carrier lifetime of 657.7 ns. Consequently, solar cells employing mixed 2D DJ 3AMP-based and 3D MA0.5FA0.5Pb0.5Sn0.5I3 (MA = methylammonium, FA = formamidinium) perovskite composites as light absorbers achieve enhanced efficiency and stability, giving a power conversion efficiency of 20.09% with a high open-circuit voltage of 0.88 V, a fill factor of 79.74%, and a short-circuit current density of 28.63 mA cm-2. The results provide an effective strategy to improve the performance of single-junction narrow-bandgap solar cells and, potentially, to give a highly efficient alternative to bottom solar cells in tandem devices.

CuSCN as the Back Contact for Efficient ZMO/CdTe Solar Cells.

The replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl2 treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges. To realize the full potential of the ZMO buffer layer, plenty of investigations need to be accomplished. Here, copper thiocyanate (CuSCN) is demonstrated to be a suitable back-contact material with multi-advantages for ZMO/CdTe solar cells. Particularly, ammonium hydroxide as the solvent for CuSCN deposition shows no detrimental impact on the ZMO layer during the post heat treatment. The post annealing temperature as well as the thickness of CuSCN films are investigated. Finally, a champion power conversion efficiency of 16.7% is achieved with an open-circuit voltage of 0.857 V, a short-circuit current density of 26.2 mA/cm2, and a fill factor of 74.0%.

Correlating Hysteresis and Stability with Organic Cation Composition in the Two-Step Solution-Processed Perovskite Solar Cells.

The two-step solution-based process has demonstrated substantial success in fabricating high-efficiency perovskite solar cells in recent years. Despite the high performance, the underlying mechanisms that govern the formation of perovskite films and corresponding device performance are yet to be fully understood. Particularly, organic cation composition used in the two-step solution processing of mixed-cation lead halide perovskite solar cells plays a critical role in the perovskite film formation and the resultant device performance. However, little is understood about the impacts of organic cation composition on the current density-voltage (J-V) hysteretic behavior and stability of perovskite solar cells. To address this need, here, we study the effect of mixed organic cations, that is, the fraction of formamidinium (FA) and methylammonium (MA) contents, used for the two-step solution-processed perovskite thin films on solar cell performance, including efficiency, J-V hysteresis, and stability. In addition to the efficiency variations, we find that perovskite solar cells based on FA-rich and MA-rich stoichiometries show distinct characteristics in J-V hysteresis and stability. The origins of such a discrepancy are attributed to the thermodynamically driven conversion from lead iodide to perovskites, which is determined by the combination of organic cations. The perovskite solar cells based on the mixed cation FA0.6MA0.4PbI3 composition show a champion power conversion efficiency of over 21% and robust stability (retaining more than 90% of initial efficiency) under maximum power-point tracking in dry nitrogen for more than 500 h. Our work provides insights on understanding the formation of perovskite films in the two-step process, which may benefit further investigation on perovskite solar cells.

Effects of intrinsic and atmospherically induced defects in narrow bandgap (FASnI3)x(MAPbI3)1-x perovskite films and solar cells.

Narrow bandgap mixed tin (Sn) + lead (Pb) perovskites are necessary for the bottom sub-cell absorber in high efficiency all-perovskite polycrystalline tandem solar cells. We report on the impact of mixed cation composition and atmospheric exposure of perovskite films on sub-gap absorption in films and performance of solar cells based on narrow bandgap mixed formamidinium (FA) + methylammonium (MA) and Sn + Pb halide perovskites, (FASnI3)x(MAPbI3)1-x. Structural and optical properties of 0.3 ≤ x ≤ 0.8 (FASnI3)x(MAPbI3)1-x perovskite thin film absorbers with bandgaps ranging from 1.25 eV (x = 0.6) to 1.34 eV (x = 0.3) are probed with and without atmospheric exposure. Urbach energy, which quantifies the amount of sub-gap absorption, is tracked for pristine perovskite films as a function of composition, with x = 0.6 and 0.3 demonstrating the lowest and highest Urbach energies of 23 meV and 36 meV, respectively. Films with x = 0.5 and 0.6 compositions show less degradation upon atmospheric exposure than higher or lower Sn-content films having greater sub-gap absorption. The corresponding solar cells based on the x = 0.6 absorber show the highest device performance. Despite having a low Urbach energy, higher Sn-content solar cells show reduced device performances as the amount of degradation via oxidation is the most substantial.