Grazing Incidence Wide-angle X-ray Scattering Research Articles

Judicious Fluorination of Perovskite Quantum Wells Enables Over 25% Efficiency in Inverted Solar Cells

AbstractThe photovoltaic performance of inverted perovskite solar cells (PSCs) is often hindered by trap‐induced non‐radiative recombination and photochemical degradation occurring at the upper interfaces and the grain boundaries of perovskite films. Herein, ortho‐, meta‐, and para‐isomers of fluorophenylethylammonium iodine (F‐PEAI) organic spacer molecules are evaluated for the construction of perovskite quantum wells (2D or quasi‐2D, PQWs) to encapsulate 3D perovskites. Among the three variants, p‐F‐PEAI leads to the most symmetric charge distribution and the weakest steric hindrance which resulting in reinforced interactions with PbI2 and perovskite, the enhanced out‐of‐plane orientation is confirmed by Grazing incidence wide‐angle X‐ray scattering (GIWAXS) results. Density functional theory and crystal orbital Hamilton population (COHP) calculations further confirm that p‐F‐PEAI engages most strongly with the perovskite structure. Moreover, transmission electron microscopy (TEM) characterization is used to illustrate that p‐F‐PEAI‐assisted 2D PQWs effectively passivate both the grain boundaries and surfaces of perovskites. This configuration facilitates effective surface passivation, improves charge carrier transport, and significantly suppresses non‐radiative recombination. The resultant inverted PSCs achieve an excellent power conversion efficiency (PCE) of 25.03% with a fill factor (FF) of 85.11%. The unencapsulated devices exhibit enhanced long‐term stability under ambient environments and continuous light illumination.

Ionic liquid engineering enables 1D/3D perovskite photovoltaics with > 25 % efficiency: A real-time study exploring formation mechanism of 1D perovskites

The advancements in low-dimensional/three-dimensional (LD/3D) perovskite devices represent a promising approach to enhancing both the efficiency and stability of of perovskite solar cells (PSCs). However, while significant efforts have been dedicated to understanding the role of already formed low-dimensional perovskites in passivating defects, refining the work function, little attention has been given to the crystallization process of LD perovskites, particularly when induced by ionic liquids (ILs) as green and stable additive. In this work, the formation process of IL Tetrabutylammonium Thiocyanate (TBASCN)-induced 1D perovskite onto the 3D perovskite under varying annealing conditions were explored by in-situ Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) technology, providing a comprehensive investigation into the distinct impacts of 1D perovskite formed under diverse annealing processes on device performance. Owing to the robust interaction and favorable binding energy between the IL TBASCN and perovskite components, coupled with the low formation energy of 1D perovskites, 1D perovskites exhibit rapid formation on the surface of 3D perovskites and display markedly distinct crystallization processes under distinguished annealing conditions. Achieving full crystallization of 1D perovskites necessitates optimal temperature of 100 °C and adequate duration of 100 s. Once fully crystallized, the highly crystalline 1D perovskites and anions SCN- present in ILs can synergistically passivate defects in 3D perovskites, mitigate ion migration and impeding moisture infiltration. Benefiting from the optimized formation process of 1D perovskites, the champion device achieves a power conversion efficiencie (PCE) of 25.18 % and retains 91.0 % of its initial efficiency after 2000 h under RH=40 %. Our research provides new insights for optimizing the formation process of LD perovskites induced by different materials and their application in high-performance LD/3D PSCs.

Highly efficient and stable organic solar cells achieved by improving exciton diffusion and splitting through a volatile additive-assisted ternary strategy

The ternary and additive strategy, introducing a third component into a binary blend and add suitable additives, opens a simple and promising avenue to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). This study investigates the optimization of OSCs by introducing volatile additives and a third component, L8-BO-X, which tunes the active layer morphology and improves the performance of the devices. Utilizing various characterization techniques, such as the grazing-incidence wide-angle X-ray scattering (GIWAXS), film-depth-dependent light absorption spectroscopy (FLAS), and the femtosecond-resolved transient absorption (fsTA) spectroscopy, the effects of these adjustment on crystallinity, phase separation, exciton generation, and charge transport in photovoltaic device are explored. The incorporation of the third component and volatile additives results in less anisotropy in molecular orientation and thus faster exciton splitting at the D-A interface, enhanced π-π stacking coherence length and longer exciton lifetime, and eventually an enhanced power conversion efficiency (PCE) of 19.6 % (certified as 19.07 % in the National Institute of Metrology in China) and exceptional photostability, with the devices retaining 82 % efficiency after 1200 hours of continuous light exposure.

Synchrotron Radiation-Based In Situ GIWAXS for Metal Halide Perovskite Solution Spin-Coating Fabrication.

Solution-processable perovskite-based devices are potentially very interesting because of their relatively cheap fabrication cost but outstanding optoelectronic performance. However, the solution spin-coating process involves complicated processes, including perovskite solution droplets, nucleation of perovskite, and formation of intermediate perovskite films, resulting in complicated crystallization pathways for perovskite films under annealing. Understanding and therefore controlling the fabrication process of perovskites is difficult. Recently, synchrotron radiation-based in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) techniques, which possess the advantages of high collimation, high resolution, and high brightness, have enabled to bridge complicated perovskite structure information with device performance by revealing the real-time crystallization pathways of perovskites during the spin-coating process. Herein, the developments of synchrotron radiation-based in situ GIWAXS are discussed in the study of the crystallization process of perovskites, especially revealing the important crystallization mechanisms of state-of-the-art perovskite optoelectronic devices with high performance. At the end, several potential applications and challenges associated with in situ GIWAXS techniques for perovskite-based devices are highlighted.

Condensation or Desublimation: Nanolevel Structural Look on Two Frost Formation Pathways on Surfaces with Different Wettabilities.

Processes of water condensation and desublimation on solid surfaces are ubiquitous in nature and essential for various industrial applications, which are crucial for their performance. Despite their significance, these processes are not well understood due to the lack of methods that can provide insight at the nanolevel into the very first stages of phase transitions. Taking advantage of synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) and environmental scanning electron microscopy (ESEM), two pathways of the frosting process from supersaturated vapors were studied in real time for substrates with different wettabilities ranging from highly hydrophilic to superhydrophobic. Within GIWAXS, a fully quantitative structural and orientational characterization of the undergoing phase transition reveals the information on degree of crystallinity of the new phase and determines the ordering at the surfaces and inside the films at the initial stages of water/ice nucleation from vapor onto the substrates. The diversity of frosting scenarios, including direct desublimation from the vapor and two-stage condensation-freezing processes, was observed by both GIWAXS and ESEM for different combinations of substrate wettability and vapor supersaturations. The classical nucleation theory straightforwardly predicts the pathway of the phase transition for hydrophobic and superhydrophobic substrates. The case of hydrophilic substrates is more intricate because the barriers in Gibbs free energy for nucleating both liquid and solid embryos are close to each other and comparable to thermal energy kBT. At that end, classical nucleation theory allows concluding a relation between contact angles for ice and water embryos on the basis of the observed frosting pathway.

Solvent-doped PEDOT:PSS: Structural transformations towards enhanced electrical conductivity and transferable electromagnetic shields

The enhanced electrical conductivity of solvent-doped poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) is attributed to the screening effect between PSS and PEDOT chains. Yet, the precise manner in which this effect influences the crystalline structure and conformation of PEDOT:PSS remains unclear. In this study, we utilize Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Grazing-incidence wide-angle X-ray scattering (GIWAXS) to examine the conformational changes and two-dimensional (2D) crystalline structure of PEDOT:PSS when doped with a variety of solvents, including dimethyl sulfoxide (DMSO), ethylene glycol (EG), dimethylformamide (DMF), methanol (MeOH), ethanol (EtOH), tetrahydrofuran (THF), and acetone. Our observations unveil that solvent doping improves the electrical conductivity of PEDOT:PSS by modifying its crystalline structure. This alteration leads to reduced inter-lamellar (d200) and π-π stacking (d010) distances, facilitating the formation of densely packed and well-ordered PEDOT crystallites in both face-on and edge-on orientations. Moreover, this doping process enhances the transferability and mechanical properties of drop-casted films, resulting in a flexible and transferable electromagnetic interference (EMI) shield with exceptional total shielding effectiveness (SET) of 35.70 dB and specific shielding effectiveness (SSE/t) of 7105.34 dB cm2.g−1.

Ammonium Sulfate to Modulate Crystallization for High-Performance Rigid and Flexible Perovskite Solar Cells.

Perovskite solar cells (PSCs) are attracting widespread research and attention as highly promising candidates in the field of electronic photovoltaics owing to their exceptional power conversion efficiency (PCE). However, rigid or flexible PSCs still face challenges in preparing full-coverage and low-defect perovskite films, as well as achieving highly reproducible and highly stable devices. Herein, a multifunctional additive 2-aminoethyl hydrogen sulfate (AES) is designed to regulate the film crystallization and thereby form flat and pinhole-free perovskite films. It is found that the introduction of AES can effectively passivate defects, restrain charge carrier recombination, and then achieve a higher fill factor. As seen with grazing incidence wide-angle X-ray scattering (GIWAXS), this approach does not affect the crystal orientation distribution. It is observed that AES addition shows a universality across different perovskite components since the PCE is improved up to 20.7% for FA0.97MA0.03Pb(I0.97Br0.03)3-AES, 22.85% for Cs0.05FA0.95PbI3-AES, 22.23% for FAPbI2.7Br0.3-AES, and 23.32% for FAPI-AES rigid devices. Remarkably, the non-encapsulated flexible Cs0.05 (FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 device with AES additive delivers a PCE of 20.1% and maintains over 97% of its initial efficiency under ambient conditions (25±5% relative humidity) over 2280h of aging.

Influence of MACl on the Crystallization Kinetics of Perovskite via a Two-Step Method

The addition of methylammonium chloride (MACl) significantly improves the performance and stability of perovskite fabricated by two-step processes. However, its role in crystallization dynamics has not been thoroughly studied. In this work, a comparison study is carried out using different additions of MACl to investigate the impact of the perovskite crystallization dynamics. In situ grazing incidence wide-angle X-ray scattering (GIWAXS) observations during the annealing process of perovskite revealed that the amount of MACl significantly influences the crystallinity and orientation of the perovskite. Increasing the MACl addition enhances the crystallinity of the perovskite in the wet film‘s intermediate phase and strengthens the out-of-plane orientation of the FAPbI3 perovskite α-phase (001) planes during annealing. Moreover, it was found that both excessive and insufficient amounts of MACl introduce defects into the perovskite, which are detrimental to device performance. In contrast, an optimal ratio of MACl-9 mg leads to the formation of uniform and large-grained FAPbI3 perovskite films, with the longest carrier lifetimes (163.7 ns) compared to MACl-5 mg (68.4 ns) and MACl- 13 mg (120.1 ns). As a result, the fabricated MACl-9 mg-based solar cell achieved the highest efficiency (22.63%), which is higher than those of MACl-5 mg (21.47%) and MACl-13 mg (20.07%).

Two-Step Perovskite Solar Cells with > 25% Efficiency: Unveiling the Hidden Bottom Surface of Perovskite Layer.

While significant efforts in surface engineering have been devoted to the conversion process of lead iodide (PbI2) into perovskite and top surface engineering of perovskite layer with remarkable progress, the exploration of residual PbI2 clusters and the hidden bottom surface on perovskite layer have been limited. In this work, a new strategy involving 1-butyl-3-methylimidazolium acetate (BMIMAc) ionic liquid (IL) additives is developed and it is found that both the cations and the anions in ILs can interact with the perovskite components, thereby regulating the crystallization process and diminishing the residue PbI2 clusters as well as filling vacancies. The introduction of BMIMAc ILs induces the formation of a uniform porous PbI2 film, facilitating better penetration of the second-step organic salt and fostering a more extensive interaction between PbI2 and the organic salt. Surprisingly, the oversized residual PbI2 clusters at the bottom surface of the perovskite layer completely diminish. In addition, advanced depth analysis techniques including depth-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) and bottom thinning technology are employed for a comprehensive understanding of the reduction in residual PbI2. Leveraging effective PbI2 management and regulation of the perovskite crystallization process, the champion devices achieve a power conversion efficiency (PCE) of 25.06% with long-term stability.

Determining Structures of Layer-by-Layer Spin-Coated Zinc Dicarboxylate-Based Metal-Organic Thin Films.

Thin films of crystalline solids with substantial free volume built from organic chromophores and metal secondary building units (SBUs) are promising for engineering new optoelectronic properties through control of interchromophore coupling. Zn-based SBUs are especially relevant in this case because they avoid quenching the chromophore's luminescence. We find that layer-by-layer spin-coating using Zn acetate dihydrate and benzene-1,4-dicarboxylic acid (H2BDC) and biphenyl-4,4'-dicarboxylic acid (H2BPDC) linkers readily produces crystalline thin films. However, analysis of the grazing-incidence wide-angle X-ray scattering (GIWAXS) data reveals the structures of these films vary significantly with the linker, and with the metal-to-linker molar ratio used for fabrication. Under equimolar conditions, H2BPDC creates a type of structure like that proposed for SURMOF-2, whereas H2BDC generates a different metal-hydroxide-organic framework. Large excess of Zn2+ ions causes the growth of layered zinc hydroxides, irrespective of the linker used. Density functional theory (DFT) calculations provide structural models with minimum total energy that are consistent with the experimentally observed diffractograms. In the broader sense, this work illustrates the importance in this field of careful structure determination, e. g., by utilizing GIWAXS and DFT simulations to determine the structure of the obtained crystalline metal-organic thin films, such that properties can be rationally engineered and explained.

Axial Phenoxylation of Aluminum Phthalocyanines for Improved Cannabinoid Sensitivity in OTFT Sensors.

Cannabis producers, consumers, and regulators need fast, accurate, point-of-use sensors to detect Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) from both liquid and vapor source samples, and phthalocyanine-based organic thin-film transistors (OTFTs) provide a cost-effective solution. Chloro aluminum phthalocyanine (Cl-AlPc) has emerged as a promising material due to its unique coordinating interactions with cannabinoids, allowing for superior sensitivity. This work explores the molecular engineering of AlPc to tune and enhance these interactions, where a series of novel phenxoylated R-AlPcs are synthesized and integrated into OTFTs, which are then exposed to THC and CBD solution and vapor samples. While the R-AlPc substituted molecules have a comparable baseline device performance to Cl-AlPc, their new crystal structures and weakened intermolecular interactions increase sensitivity to THC. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) are used to investigate this film restructuring, where a significant shift in the crystal structure, grain size, and film roughness is detected for the R-AlPc molecules that do not occur with Cl-AlPc. This significant crystal reorganization and film restructuring are the driving force behind the improved sensitivity to cannabinoids relative to Cl-AlPc and demonstrate that analyte-semiconductor interactions can be enhanced through chemical modification to create more responsive OTFT sensors.

Effect of Aggregation Structure on Capacitive Energy Storage in Conducting Polymer Films.

Conducting polymers (CPs), a significant class of electrochemical capacitor electrode materials, exhibit exceptional capacitive energy storage performance in aqueous electrolytes. Current research primarily concentrates on enhancing the electrical conductivity and capacitive performance of CPs via molecular design and structural control. However, the absence of a comprehensive understanding of the impact of molecular chain spatial order on ion/electron transport and capacitive performance impedes the development and optimization of advanced electrode materials. Here, a solvent treatment strategy is employed to modulate the molecular chain spatial order of PEDOT : PSS films. The results of electrochemical performance tests and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) show that Poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonic acid) (PEDOT : PSS) films with both face-on and edge-on orientations exhibit exceptional electronic conductivity and ion diffusion efficiency, with capacitive performance 1.33 times higher than that of PEDOT : PSS films with only edge-on orientation. Consequently, molecular chain orientations conducive to charge transport not only enhance inter-chain coupling, but also effectively reduce ion transport resistance, enabling efficient capacitive energy storage. This research provides novel insights for the design and development of higher performance CPs-based electrode materials.

The Role of Side Chains and Hydration on Mixed Charge Transport in n-Type Polymer Films.

Introducing ethylene glycol (EG) side chains to a conjugated polymer backbone is a well-established synthetic strategy for designing organic mixed ion-electron conductors (OMIECs). However, the impact that film swelling has on mixed conduction properties has yet to be scoped, particularly for electron-transporting (n-type) OMIECs. Here, the authors investigate the effect of the length of branched EG chains on mixed charge transport of n-type OMIECs based on a naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene backbone. Atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and scanning tunneling microscopy (STM) are used to establish the similarities between the common-backbone films in dry conditions. Electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) and in situ GIWAXS measurements reveal stark changes in film swelling properties and microstructure during electrochemical doping, depending on the side chain length. It is found that even in the loss of the crystallite content upon contact with the aqueous electrolyte, the films can effectively transport charges and that it is rather the high water content that harms the electronic interconnectivity within the OMIEC films. These results highlight the importance of controlling water uptake in the films to impede charge transport in n-type electrochemical devices.

Advancing MR‐TADF OLED Toward True‐Blue CIE Value via Asymmetric Host Materials with Amorphous Morphology

AbstractThe new host materials are reported to boost the multiple resonance‐induced thermally activated delayed fluorescence (MR‐TADF) based pure deep‐blue organic light emitting diodes (OLEDs) toward further shortening the emission full width at the half maximum (FWHM). Based on an unusual asymmetric design concept, two new host compounds Bz2cb and Bz2cbz are synthesized. Despite possessing a compact and rigid molecular packing in crystal, Bz2cbz shows pure amorphous morphology via vapor deposition, as confirmed by the grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) analysis. Via incorporating a promising MR‐TADF emitter, ν‐DABNA‐O‐Me, into Bz2cb and Bz2cbz films, highly pure deep‐blue OLEDs are successfully demonstrated. Remarkably, the Bz2cbz device exhibits a 464 nm electroluminescence (EL) with narrow FWHM of 22 nm, accompanied by the reduction of the 0–1 vibronic sideband, and a maximum external quantum efficiency (EQEmax) of 28.2%, which, overall, reaches the true‐blue Commission Internationale de l'Eclairage coordinates (CIE) of (0.13, 0.07) and the high blue index of 253. The amorphous film formation turns out to be an important factor that is previously unrecognized to externally fine‐tune the MR‐TADF OLEDs toward even higher color purity.

Aqueous inks for ecofriendly processing of organic solar cells: Investigation of morphological changes

Aqueous dispersions of organic semiconducting nanoparticles (NPs) are particularly attractive as inks for the environmentally friendly preparation of organic solar cells. The internal morphology of the NPs, which depends on their elaboration process, is a key parameter, which has a significant influence on the final morphology of the active layer and therefore its effectiveness. In the present study, core-shell (PF2:PC71BM) NPs were prepared by miniemulsion. Their internal morphology including the composition of the two phases was characterized by scanning transmission X-ray microscopy (STXM) showing a core composed of 77% PC71BM and a shell composed of 75% PF2. It was found that thermal annealing promotes PC71BM diffusion from the core to the shell, increasing its proportion in the shell from 25% to 42%. This annealing, when applied after NPs deposition by spincoating, allows partial coalescence of the NPs, reducing the roughness of the active layer, and increases electron mobility, thus demonstrating the formation of PC71BM percolation paths for electron transport. A PCE of 1.6% could thus be obtained after 10 min of thermal annealing at 100°C. At higher temperature, Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS) analyses demonstrate the modification of the PF2 structuration from randomly oriented lamella after deposition to edge-on orientation after annealing, leading to an unfavorable decrease of the hole mobility in the direction perpendicular to the substrate, while increasing the hole mobility in the substrate plane. This study demonstrates the need to systematically characterize the internal morphology of NPs in order to rationalize the morphology of the active layer and optimize its properties.

Study of the Crystal Structure and Hydrogen Bonding during Cold Crystallization of Poly(trimethylene 2,5-furandicarboxylate).

Here, we present a detailed description of the in situ isothermal crystallization of poly(trimethylene 2,5-furandicarboxylate)(PTF) as revealed by real-time Fourier transform infrared spectroscopy (FTIR) and grazing incidence wide-angle X-ray scattering (GIWAXS). From FTIR experiments, the evolution of hydrogen bonding with crystallization time can be monitored in real time, while from GIWAXS, crystal formation can be followed. Density functional theory (DFT) calculations have been used to simulate FTIR spectra for different theoretical structures, enabling a precise band assignment. In addition, based on DFT ab initio calculations, the influence of hydrogen bonding on the evolution with crystallization time can be understood. Moreover, from DFT calculations and comparison with both FTIR and GIWAXS experiments, a crystalline structure of poly(trimethylene 2,5-furandicarboxylate) is proposed. Our results demonstrate that hydrogen bonding is present in both the crystalline and the amorphous phases and its rearrangement can be considered as a significant driving force for crystallization of poly(alkylene 2,5-furanoate)s.

Charge Transport in Sub‐Monolayer Networks of a Naphthalene‐Diimide‐Based Copolymer

AbstractSub‐monolayer networks of the electron‐transporting semiconducting polymer poly([N,N’‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)−2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)) (P(NDI2OD‐T2)) are realized through judicious choice of spin‐coating solvent and control of solution concentration. These sub‐monolayer networks provide a platform to study the effects of surface coverage, network connectivity, and orientational order on charge transport. It is found that charge transport through semiconducting polymer networks depends not only on the coverage/concentration of the material but also on the nature of the conduction paths themselves. Down to 40% surface coverage, current measured in a field‐effect architecture decreases in line with the decrease in surface coverage, but drops precipitously below a surface coverage of 40%. Below 40% surface coverage, there is a marked decrease in network connectivity (quantified through the branching point density) and a decrease in orientational order. Furthermore, an increase in the density of dangling branches (dead ends) is also seen with decreasing surface coverage. Additional supporting data in the form of grazing incidence wide‐angle X‐ray scattering (GIWAXS), optical spectroscopy, and charge modulation spectroscopy are presented, which help to establish that long‐range interconnectivity rather than local morphology dominates charge transport in P(NDI2OD‐T2) sub‐monolayers as well as thin films.

Oriented Growth for Efficient and Scalable Perovskite Solar Cells by Vapor–Solid Reaction

AbstractThe precise control and understanding of crystal orientation in perovskite polycrystalline films are crucial for the development of efficient and stable devices. However, achieving this control remains a significant challenge. Herein, a PbI2 oriented growth strategy is developed, combined with in situ vapor–solid reaction transformations, to achieve oriented growth of full‐vacuum perovskite films. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) analysis revealed the general pattern of PbI2 oriented vapor growth and in situ vapor–solid reaction transformation in this two‐step process. The resulting preferred orientation has effectively reduced trap state density, optimizing the carrier dynamics. As a result, champion efficiencies of 22.11% (0.148 cm2), 20.60% (1 cm2), and 19.41% (5 × 5 cm2 mini‐modules) are achieved, which are the highest value for perovskite based on vapor–solid reaction. Additionally, the oriented growth method is applicable to a variety of guide layers and is extended to 30 × 30 cm2 film, demonstrating the universality and scalability of the method.

Methylation of Si(111) Modulates Molecular Orientation in Perylenediimide Thin Films.

Singlet fission produces a pair of low-energy spin-triplet excitons from a single high-energy spin-singlet exciton. While this process offers the potential to enhance the efficiency of silicon solar cells by ∼30%, meeting this goal requires overlayer materials that can efficiently transport triplet excitons to an underlying silicon substrate. Herein, we demonstrate that the chemical functionalization of silicon surfaces controls the structure of vapor-deposited thin films of perylenediimide (PDI) dyes, which are prototypical singlet fission materials. Using a combination of atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), we find terminating Si(111) with either a thin, polar oxide layer (SiOx) or with hydrophobic methyl groups (Si-CH3) alters the structures of the resulting PDI films. While PDI films grown on SiOx are comprised of small crystalline grains that largely adopt an "edge-on" orientation with respect to the silicon surface, films grown on Si-CH3 contain large grains that prefer to align in a "face-on" manner with respect to the substrate. This "face-on" orientation is expected to enhance exciton transport to silicon. Interestingly, we find that the preferred mode of growth for different PDIs correlates with the space group associated with bulk crystals of these compounds. While PDIs that inhabit a monoclinic (P21/c) space group nucleate films by forming tall and sparse crystalline columns, PDIs that inhabit triclinic (P1̅) space groups afford films comprised of uniform, lamellar PDI domains. The results highlight that silicon surface functionalization profoundly impacts PDI thin film growth, and rational selection of a hydrophobic surface that promotes "face-on" adsorption may improve energy transfer to silicon.

In Situ Study of Purified Phase Transition Path for α‐FAPbI3 Crystallization

AbstractPhase transition during annealing in the two‐step sequential deposition has drawn intensive attention as its significance in fabricating superior perovskite films. However, previous works have not paid enough attention to the importance of the purified phase transition path in the crystallization process. Herein, different from the mixed paths in the conventional cognition, purified phase transition path for α‐FAPbI3 crystallization is achieved by introducing dimethylurea (DMU) into lead iodide (PbI2) precursor solution. The multifunctional molecule is found to design a penetrable porous PbI2 film and fundamentally regulate the perovskite crystallization by forming single phase transition path via the complete δ‐phase during annealing of perovskite. The role of DMU in purified transition path for α‐FAPbI3 crystallization is unraveled with in situ photoluminescence and grazing incidence wide‐angle x‐ray scattering (GIWAXS) investigation. The crystal quality of perovskite films is significantly improved, resulting in single crystal‐like film. The best‐performing perovskite solar cells (PSCs) deliver a power conversion efficiency of 24.75%, resulting from the higher FF of 83.25% and an improved long‐term stability up to 3600 h. This work highlights the importance of purified phase transition path for the superior crystal quality toward high‐performance perovskite photovoltaics.