Trace Water in Lead Iodide Affecting Perovskite Crystal Nucleation Limits the Performance of Perovskite Solar Cells

The experimental replicability of highly efficient perovskite solar cells (PSCs) is a persistent challenge faced by laboratories worldwide. Although trace impurities in raw materials can impact the experimental reproducibility of high-performance PSCs, the in situ study of how trace impurities affect perovskite film growth is never investigated. Here, light is shed on the impact of inevitable water contamination in lead iodide (PbI2 ) on the replicability of device performance, mainly depending on the synthesis methods of PbI2 . Through synchrotron-based structure characterization, it is uncovered that even slight additions of water to PbI2 accelerate the crystallization process in the perovskite layer during annealing. However, this accelerated crystallization also results in an imbalance of charge-carrier mobilities, leading to a degradation in device performance and reduced longevity of the solar cells. It is also found that anhydrous PbI2 promotes a homogenous nucleation process and improves perovskite film growth. Finally, the PSCs achieve a remarkable certified power conversion efficiency of 24.3%. This breakthrough demonstrates the significance of understanding and precisely managing the water content in PbI2 to ensure the experimental replicability of high-efficiency PSCs.

A Practical Approach Toward Highly Reproducible and High-Quality Perovskite Films Based on an Aging Treatment

Solution processing of hybrid perovskite semiconductors is a highly promising approach for the fabrication of cost-effective electronic and optoelectronic devices. However, challenges with this approach lie in overcoming the controllability of the perovskite film morphology and the reproducibility of device efficiencies. Here, a facile and practical aging treatment (AT) strategy is reported to modulate the perovskite crystal growth to produce sufficiently high-quality perovskite thin films with improved homogeneity and full-coverage morphology. The resulting AT-films exhibit fewer defects, faster charge carrier transfer/extraction, and suppressed non-radiative recombination compared with reference. The AT-devices achieve a noticeable improvement in the reproducibility, operational stability, and photovoltaic performance of devices, with the average efficiency increased by 16%. It also demonstrates the feasibility and scalability of AT strategy in optimizing the film morphology and device performance for other perovskite components including MAPbI3 , (MAPbBr3 )15 (FAPbI3 )85 , and Cs0.05 (MAPbBr3 )0.17 (FAPbI3 )0.83 . This method opens an effective avenue to improve the quality of perovskite films and photovoltaic devices in a scalable and reproducible manner.

Influence of the Sequence Motive Repeating Number on Protein Folding in Spider Silk Protein Films

Like multiblock copolymers, spider silk proteins are built of repetitive sequence motives. One prominent repetitive motif is based on the consensus sequence of spidroin 4 of the spider Araneus diadematus ADF4. The number x of the repeating sequence motives (C) determines the molecular weight of the recombinant ADF4-based, engineered spider silk protein denoted as eADF4(Cx). eADF4(Cx) can be used as a model for intrinsically disordered proteins (IDP) and to elucidate their folding. Herein, the influence of the variation of the sequence motive repeating number x (x = 1, 2, 4, 8, 16) on the protein folding within eADF4(Cx) films was investigated. eADF4(Cx) films were cast from 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) solutions onto planar silicon model substrates, revealing mainly helical or random coil structure. Upon treatment with methanol vapor (ptm), the formation of crystalline β-sheets was triggered. Dichroic Fourier-transform infrared (FTIR) spectroscopy, circular dichroism, spectroscopic ellipsometry, atomic force microscopy, grazing-incidence small-angle X-ray scattering (GISAXS), grazing-incidence wide-angle X-ray scattering (GIWAXS), and electrokinetic and contact angle measurements were used to get information concerning the secondary structure and folding kinetics, orientation of β-sheets, the ratio of parallel/antiparallel β-sheets, domain sizes and distributions, surface topography, surface potential, hydrophobicity and the film integrity under water. Significant differences in the final β-sheet content, the share of antiparallel β-sheet structures, film integrity, surface potential, and isoelectric points between eADF4(Cx) with x = 1, 2 and eADF4(Cx) with x = 4, 8, 16 gave new insights in the molecular weight-dependent structure formation and film properties of IDP systems. GISAXS and kinetic measurements confirmed a relation between β-sheet crystal growth rate and final β-sheet crystal size. Further, competing effects of reduced diffusibility hindering accelerated crystal growth and enhanced backfolding promoting accelerated crystal growth with increasing molecular weight were discussed.

Morphological Insights into the Degradation of Perovskite Solar Cells under Light and Humidity

Perovskite solar cells (PSCs) have achieved competitive power conversion efficiencies compared with established solar cell technologies. However, their operational stability under different external stimuli is limited, and the underlying mechanisms are not fully understood. In particular, an understanding of degradation mechanisms from a morphology perspective during device operation is missing. Herein, we investigate the operational stability of PSCs with CsI bulk modification and a CsI-modified buried interface under AM 1.5G illumination and 75 ± 5% relative humidity, respectively, and concomitantly probe the morphology evolution with grazing-incidence small-angle X-ray scattering. We find that volume expansion within perovskite grains, induced by water incorporation, initiates the degradation of PSCs under light and humidity and leads to the degradation of device performance, in particular, the fill factor and short-circuit current. However, PSCs with modified buried interface degrade faster, which is ascribed to grain fragmentation and increased grain boundaries. In addition, we reveal a slight lattice expansion and PL redshifts in both PSCs after exposure to light and humidity. Our detailed insights from a buried microstructure perspective on the degradation mechanisms under light and humidity are essential for extending the operational stability of PSCs.

Revealing Surface and Interface Evolution of Molybdenum Nitride as Carrier-Selective Contacts for Crystalline Silicon Solar Cells

Molybdenum nitride (MoN_x) was perceived as carrier-selective contacts (CSCs) for crystalline silicon (c-Si) solar cells due to having proper work functions and excellent conductivities. However, the poor passivation and non-Ohmic contact at the c-Si/MoN_x interface endow an inferior hole selectivity. Here, the surface, interface, and bulk structures of MoN_x films are systematically investigated by X-ray scattering, surface spectroscopy, and electron microscope analysis to reveal the carrier-selective features. Surface layers with the composition of MoO_2.51N_0.21 form upon air exposure, which induces the overestimated work function and explains the origin of inferior hole selectivities. The c-Si/MoN_x interface is confirmed to adopt long-term stability, providing guidance for designing stable CSCs. A detailed evolution of the scattering length density, domain sizes, and crystallinity in the bulk phase is presented to elucidate its superior conductivity. These multiscale structural investigations offer a clear structure-function correlation of MoN_x films, providing key inspiration for developing excellent CSCs for c-Si solar cells.

Superlattice deformation in quantum dot films on flexible substrates via uniaxial strain

The superlattice in a quantum dot (QD) film on a flexible substrate deformed by uniaxial strain shows a phase transition in unit cell symmetry. With increasing uniaxial strain, the QD superlattice unit cell changes from tetragonal to cubic to tetragonal phase as measured with in situ grazing-incidence small-angle X-ray scattering (GISAXS). The respective changes in the optoelectronic coupling are probed with photoluminescence (PL) measurements. The PL emission intensity follows the phase transition due to the resulting changing inter-dot distances. The changes in PL intensity accompany a redshift in the emission spectrum, which agrees with the Förster resonance energy transfer (FRET) theory. The results are essential for a fundamental understanding of the impact of strain on the performance of flexible devices based on QD films, such as wearable electronics and next-generation solar cells on flexible substrates.

In Situ Observation of Morphological and Oxidation Level Degradation Processes within Ionic Liquid Post-treated PEDOT:PSS Thin Films upon Operation at High Temperatures

Organic thermoelectric thin films are investigated in terms of their stability at elevated operating temperatures. Therefore, the electrical conductivity of ethyl-3-methylimidazolium dicyanamide (EMIM DCA) post-treated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films is measured over 4.5 h of heating at 50 or 100 °C for different EMIM DCA concentrations. The changes in the electrical performance are correlated with changes in the film morphology, as evidenced with in situ grazing-incidence small-angle X-ray scattering (GISAXS). Due to the overall increased PEDOT domain distances, the resulting impairment of the interdomain charge carrier transport directly correlates with the observed electrical conductivity decay. With in situ ultraviolet-visible (UV-Vis) measurements, a simultaneously occurring reduction of the PEDOT oxidation level is found to have an additional electrical conductivity lowering contribution due to the decrease of the charge carrier density. Finally, the observed morphology and oxidation level degradation is associated with the deterioration of the thermoelectric properties and hence a favorable operating temperature range is suggested for EMIM DCA post-treated PEDOT:PSS-based thermoelectrics.

Correlation of Thermoelectric Performance, Domain Morphology and Doping Level in PEDOT:PSS Thin Films Post-Treated with Ionic Liquids

Ionic liquid (IL) post-treatment of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films with ethyl-3-methylimidazolium dicyanamide (EMIM DCA), allyl-3-methylimidazolium dicyanamide (AMIM DCA), and 1-ethyl-3-methylimidazolium tetracyanoborate (EMIM TCB) is compared. Doping level modifications of PEDOT are characterized using UV-Vis spectroscopy and directly correlate with the observed Seebeck coefficient enhancement. With conductive atomic force microscopy (c-AFM) the authors investigate changes in the topographic-current features of the PEDOT:PSS thin film surface due to IL treatment. Grazing incidence small-angle X-ray scattering (GISAXS) demonstrates the morphological rearrangement towards an optimized PEDOT domain distribution upon IL post-treatment, directly facilitating the interconductivity and causing an increased film conductivity. Based on these improvements in Seebeck coefficient and conductivity, the power factor is increased up to 236 µW m^-1 K^{- 2} . Subsequently, a model is developed indicating that ILs, which contain small, sterically unhindered ions with a strong localized charge, appear beneficial to boost the thermoelectric performance of post-treated PEDOT:PSS films.

Salt-Dependent Phase Transition Behavior of Doubly Thermoresponsive Poly(sulfobetaine)-Based Diblock Copolymer Thin Films

The water vapor-induced swelling, as well as subsequent phase-transition kinetics, of thin films of a diblock copolymer (DBC) loaded with different amounts of the salt NaBr, is investigated in situ. In dilute aqueous solution, the DBC features an orthogonally thermoresponsive behavior. It consists of a zwitterionic poly(sulfobetaine) block, namely, poly(4-(N-(3'-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate) (PSBP), showing an upper critical solution temperature, and a nonionic block, namely, poly(N-isopropylmethacrylamide) (PNIPMAM), exhibiting a lower critical solution temperature. The swelling kinetics in D₂O vapor at 15 °C and the phase transition kinetics upon heating the swollen film to 60 °C and cooling back to 15 °C are followed with simultaneous time-of-flight neutron reflectometry and spectral reflectance measurements. These are complemented by Fourier transform infrared spectroscopy. The collapse temperature of PNIPMAM and the swelling temperature of PSBP are found at lower temperatures than in aqueous solution, which is attributed to the high polymer concentration in the thin-film geometry. Upon inclusion of sub-stoichiometric amounts (relative to the monomer units) of NaBr in the films, the water incorporation is significantly increased. This increase is mainly attributed to a salting-in effect on the zwitterionic PSBP block. Whereas the addition of NaBr notably shifts the swelling temperature of PSBP to lower temperatures, the collapse temperature of PNIPMAM remains unaffected by the presence of salt in the films.

Chemically fueled materials with a self-immolative mechanism: transient materials with a fast on/off response

There is an increasing demand for transient materials with a predefined lifetime like self-erasing temporary electronic circuits or transient biomedical implants. Chemically fueled materials are an example of such materials; they emerge in response to chemical fuel, and autonomously decay as they deplete it. However, these materials suffer from a slow, typically first order decay profile. That means that over the course of the material's lifetime, its properties continuously change until it is fully decayed. Materials that have a sharp on-off response are self-immolative ones. These degrade rapidly after an external trigger through a self-amplifying decay mechanism. However, self-immolative materials are not autonomous; they require a trigger. We introduce here materials with the best of both, i.e., materials based on chemically fueled emulsions that are also self-immolative. The material has a lifetime that can be predefined, after which it autonomously and rapidly degrades. We showcase the new material class with self-expiring labels and drug-delivery platforms with a controllable burst-release.

Spray-Deposited Anisotropic Ferromagnetic Hybrid Polymer Films of PS-b-PMMA and Strontium Hexaferrite Magnetic Nanoplatelets

Spray deposition is a scalable and cost-effective technique for the fabrication of magnetic hybrid films containing diblock copolymers (DBCs) and magnetic nanoparticles. However, it is challenging to obtain spray-deposited anisotropic magnetic hybrid films without using external magnetic fields. In the present work, spray deposition is applied to prepare perpendicular anisotropic magnetic hybrid films by controlling the orientation of strontium hexaferrite nanoplatelets inside ultra-high-molecular-weight DBC polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films. During spray deposition, the evolution of DBC morphology and the orientation of magnetic nanoplatelets are monitored with in situ grazing-incidence small-angle X-ray scattering (GISAXS). For reference, a pure DBC film without nanoplatelets is deposited with the same conditions. Solvent-controlled magnetic properties of the hybrid film are proven with solvent vapor annealing (SVA) applied to the final deposited magnetic films. Obvious changes in the DBC morphology and nanoplatelet localization are observed during SVA. The superconducting quantum interference device data show that ferromagnetic hybrid polymer films with high coercivity can be achieved via spray deposition. The hybrid films show a perpendicular magnetic anisotropy before SVA, which is strongly weakened after SVA. The spray-deposited hybrid films appear highly promising for potential applications in magnetic data storage and sensors.