Carbazole-based self-assembled monolayer (SAM) materials as hole transport layers (HTL) have led organic solar cells (OSCs) to state-of-the-art photovoltaic performance. Nonetheless, the impact of the alkyl spacer length of SAMs remains inadequately understood. To improve the knowledge, four dichloride-substituted carbazole-based SAMs (from 2Cl-2PACz to 2Cl-5PACz) with spacer lengths of 2-5 carbon atoms is developed. Single crystal analyses reveal that SAMs with shorter spacers exhibit stronger intermolecular interactions and denser packing. The molecular conformation of SAMs significantly impacts their molecular footprint and coverage on ITO. These factors result in the highest coverage of 2Cl-2PACz and the lowest coverage for 2Cl-3PACz on ITO. OSCs based on PM6:L8-BO with 2Cl-2PACz as HTL achieved high efficiencies of 18.95% and 18.62% with and without methanol rinsing of the ITO/SAMs anodes, corresponding to monolayer and multilayer structures, respectively. In contrast, OSCs utilizing the other SAMs showed decreased efficiencies as spacer length increased. The superior performance of 2Cl-2PACz can be attributed to its shorter spacer, which reduces series resistance, hole tunneling distance, and barrier. This work provides valuable insights into the design of SAMs for high-performance OSCs.

We herein undertook a multiscale approach combining molecular dynamics (MD) simulations of solution-processed polymer bulk with sequential quantum mechanics/molecular mechanics (s-QM/MM) calculations to assess the influence of structural disorder and environmental effects on the electronic structure of conjugated donor-acceptor (D-A) polymers in bulk phase. As a case study, PF5-Y5 polymer bulk formation is modeled via gradual solvent removal under ambient conditions. The electronic structure is analyzed using state-of-the-art electronic structure methods, including optimally tuned range-separated hybrids (OT-DFT), double-hybrid functionals, and the second order algebraic diagrammatic construction (ADC(2)) method as a reference. Environmental effects are accounted for using both implicit and explicit electrostatic embedding models. Our findings reveal that structural disorder at the D-A interfaces reduces frontier orbital overlap and narrows the fundamental gap by localizing the orbitals, primarily due to significant LUMO stabilization on the acceptor unit. This effect enhances the charge-transfer (CT) character of low-lying singlet and triplet states within the OT-DFT approach, while double hybrid methods preserve a more localized nature. Disorder reshapes the energetic gaps between singlet-singlet and singlet-triplet excited states and increases its energetic disorder, with CT-rich states being particularly sensitive. Explicit electrostatic embedding further amplifies CT character and disorder in singlets while preserving triplet localization. These effects contribute to spectral broadening and help explain a shoulder feature in the visible region, linking it to structural disorder and ambient anisotropy alongside CT excitations. The choice of QM method and environment treatment in QM/MM simulations is critical, neglecting anisotropy in the surroundings can influence the excited-state descriptions in D-A materials. This work advances our theoretical understanding of organic photovoltaics by highlighting these interrelated effects.

Conjugated polymers belong to a class of organic semiconductors that are used in a broad range of optoelectronic applications such as organic solar cells and organic light-emitting diodes. Thin films of two or more conjugated polymers or small molecules are coated from a solution that undergoes phase separation during drying and forms multiscale structures. In state-of-the-art bulk heterojunction organic solar cells, electron-donating and electron-accepting molecules form a network of donor-rich and acceptor-rich phases, whose domain sizes, composition, and interconnectivity play an important role in their power conversion efficiency. While these mesoscale structures formed in bulk heterojunction blend films under some circumstances can be observed by conventional scanning probe microscopy techniques, the task of mapping the film morphology becomes increasingly difficult when the donor and acceptor molecules are more chemically similar. Here we use AFM-IR, a combination of AFM (atomic force microscopy) and IR (infrared) spectroscopy, to image, with nanometer resolution, the morphology of a blend film of a donor polymer, TQ1, and an acceptor polymer, N2200, by using their distinct chemical composition contrast. These composition maps expose an interpenetrating network of the polymers that could not be distinguished by topography or phase imaging. Moreover, the dependence of the film structures, visualized by AFM-IR, on the molecular weight of the N2200 acceptor and the donor:acceptor blend ratio could be rationalized using Hansen solubility parameters (HSP).

The limited operational lifetime of organic solar cells remains an obstacle to their commercial development and is largely due to the poor intrinsic photostability of the conjugated molecules that constitute the photoactive layer. Here, we selected a series of state-of-the-art donor and acceptor materials including PBDB-T, Y5, PF5-Y5, and PYT to study their photostability under AM1.5 simulated sunlight in ambient conditions. Their properties are monitored over time, using various spectroscopy techniques, including UV-Vis absorption, Fourier-transform infrared (FTIR), and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). We found that the absorption spectra of Y5 and PYT films remain almost intact even after 30 hours of light exposure in air, while the PF5-Y5 and PBDB-T films undergo rapid photobleaching. The absorption losses observed in blend films of PBDB-T with Y5 and with PF5-Y5 can be understood as composed of contributions from the separate blend components that are similar to the absorption losses in neat films. The new peaks emerging in the FTIR spectra of PBDB-T, PF5-Y5, and their blend films witness the formation of new carbonyl groups, while these are absent in the spectra of the Y5 and PYT films. The XPS C 1s spectra of the PF5-Y5 and PBDB-T films confirm this carbonyl formation and the S 2p spectra reveal that sulphone groups are formed after 30 hours of exposure of these films. These results confirm that films of Y5 and the copolymer PYT are significantly more resistant to photooxidation, compared to the copolymer PF5-Y5. The comparison of these results suggests that the benzo[1,2-b:4,5-b ']dithiophene moiety with alkylated thiophenes as side chains (BDT-T) accelerates the photodegradation of PBDB-T and PF5-Y5. The replacement of the BDT-T unit by thiophene contributes to the enhanced stability of PYT, demonstrating that the nature of the co-monomer has a significant effect on the intrinsic photostability of Y5-based copolymers. These new insights are expected to stimulate the design of stable donors and acceptor polymers for the development of long-lived OPV devices. Absorption spectra show the photobleaching of acceptor copolymer PF5-Y5. The replacement of BDT-T by thiophene strongly improves the photostability.

Mixed-halide perovskites enable bandgap engineering for tandem solar cell and light-emitting diode applications. However, photoinduced halide phase segregation introduces a compositional instability, that is, formation of I-rich and Br-rich phases, which compromises photovoltaic efficiency and stability. While optical and structural studies of the photoinduced phase segregation in mixed-halide perovskites have been reported, its impact on the material stability is missing. Here, a detailed compositional analysis of mixed-halide perovskite films using x-ray and ultraviolet photoelectron spectroscopy (UPS) was carried out to determine how their stability in various environments depends on the halide ratio. A series of perovskite thin films were fabricated with the composition CH3NH3Pb(IxBr1−x)3, where x = 0.00, 0.25, 0.50, 0.75, and 1.00, and analyzed under different conditions, such as exposure to light in ambient and in nitrogen atmosphere, as well as storage in the dark. From the spectroscopy results, complemented with structural and optical properties, it was found that the deletion of halide ions from the surface is facilitated in mixed-halide perovskites in comparison with pure halide perovskites. A higher stability was found for the mixed-halide perovskite containing less than 25% Br, and it decreases with increasing Br content. This study also established the effect of the Br/I ratio on the energy landscape of the materials. The UPS spectra reveal that photoinduced degradation results in a mismatch of the energy levels at the perovskite/transport layer interface, which may limit the collection of charge carriers. These findings correlate well with the photovoltaic device stability under similar degradation conditions.

In this study, a rational processing strategy is presented, aiming to achieve well-ordered thin films of the molecular electron acceptors Y5 and Y6 by the choice of solvent as a key parameter. The thin films were spin-coated from chlorobenzene (CB), chloroform (CF), and ortho-xylene (o-XYL) solutions. The film morphology and molecular orientation were investigated by atomic force microscopy (AFM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, respectively. A homogeneous and smooth morphology was achieved when CF was used as the processing solvent. However, using CB and o-XYL resulted in significantly rougher films with larger structures. The dichroism observed in NEXAFS spectra using a linearly polarized incident X-ray beam and recorded in total electron yield (TEY) mode is indicative of a preferential face-on molecular orientation at the surface of Y5 and Y6 thin films processed from CF solution. In contrast, NEXAFS spectra of thin films processed from CB and o-XYL do not show any dependence on the electric field polarization direction of the incident X-ray beam, implying the absence of molecular orientation in those films. To understand the nature of the electronic transitions responsible for the absorption resonances in the NEXAFS spectra at the N and C K-edges, the natural transition orbitals corresponding to these electronic transitions were determined by time-dependent density functional theory (TD-DFT) calculations, confirming the face-on orientation of the molecules in the films processed from the CF solution. The fact that face-on oriented films are only achieved using CF is attributed to the superior solubility of Y5 and Y6 in this solvent and the lower degree of preaggregation in solution.

We develop a mesoscopic lattice model to study the morphology formation in inter-acting ternary mixtures with the evaporation of one component. As concrete potentialapplication of our model, we wish to capture morphologies as they are typically arisingduring the fabrication of organic solar cells. In this context, we consider an evaporatingsolvent into which two other components are dissolved, as a model for a 2-componentcoating solution that is drying on a substrate. We propose a 3-spins dynamics to describethe evolution of the three interacting species. As main tool, we use a Monte CarloMetropolis-based algorithm, with the possibility of varying the system’s temperature,mixture composition, interaction strengths, and evaporation kinetics. The main novelty isthe structure of the mesoscopic model – a bi-dimensional lattice with periodic boundaryconditions, divided into square cells to encode a mesoscopic range interaction amongthe units. We investigate the effect of the model parameters on the structure of theresulting morphologies. Finally, we compare the results obtained with the mesoscopicmodel with corresponding ones based on an analogous lattice model with a short rangeinteraction among the units, i.e. when the mesoscopic length scale coincides with themicroscopic length scale of the lattice.

In all-polymer solar cells, high performance is attributed to the fine-grained morphology of the film in the active layer. However, the mechanism by which this fine-grained morphology is achieved remains unknown. Polymeric non-fullerene acceptors have the potential to restrict the self-aggregation, typical of non-fullerene small molecule acceptors. Here we employed a blend of the polymeric acceptor PF5-Y5 and the donor polymer PBDB-T to investigate the balance between molecular interactions in solution. Temperature-dependent absorption spectra show evidence of temperature-induced disaggregation of both donor and acceptor polymers, where the donor polymer disaggregation depends on the solvent polarity. Concentration-dependent fluorescence spectra of blend solutions display blue-shifted acceptor emission upon dilution, similar to that observed in acceptor solutions, and a decreased tendency for charge transfer from donor to acceptor upon dilution. Excitation spectra of dilute blend solutions contain an increased contribution to the long-wavelength acceptor emission, as compared to pure acceptor solutions, from a chromophore that absorbs in a region where the donor does not absorb. These observations can be explained by donor-acceptor complexation in dilute blend solutions, that is stabilized in more polar solvents. Moreover, the near IR-region of the absorption spectrum could be matched with the calculated electronic excitations of donor-acceptor complexes of PBDB-T and PF5-Y5 oligomers. The results corroborate that the interaction between segments of the donor and acceptor polymer chains favours the formation of donor-acceptor charge transfer complexes, stabilized by hybridization of the molecular orbitals, which reduces the electronic energy. The proposed donor-acceptor complex formation competes with the donor and acceptor self-aggregation and is influenced by the solvent environment. These pre-formed donor-acceptor complexes in low-concentration solutions can be expected to have important consequences on the film morphology of all-polymer blends. The results from this joint experimental-theoretical spectroscopy study provide insights that can guide the design of compatible donor and acceptor polymers for future high-performance organic solar cells.

Oxyhydrides of rare-earth metals (REMOHs) exhibit notable photochromic behaviors. Among these, yttrium oxyhydride (YHO) stands out for its impressive transparency and swift UV-responsive color change, positioning it as an optimal material for self-cleaning window applications. Although semiconductor photocatalysis holds potential solutions for critical environmental issues, optimizing the photocatalytic efficacy of photochromic substances has not been adequately addressed. This research advances the study of REMOHs, focusing on the properties of gadolinium oxyhydride (GdHO) both theoretically and experimentally. The electronic and structural characteristics of GdHO, vital for ceramic technology, are thoroughly examined. Explicitly determined work functions for GdH2, GdHO, and Gd2O3 stand at 3.4 eV, 3.0 eV, and 4.3 eV, respectively. Bader charge analysis showcases GdHO’s intricate bonding attributes, whereas its electron localization function majorly presents an ionic nature. The charge neutrality level is situated about 0.33 eV below the top valence band, highlighting these materials’ inclination for acceptor-dominant electrical conductivity. Remarkably, this research unveils GdHO films’ photocatalytic capabilities for the first time. Even with their restricted surface due to thinness, these films follow the Langmuir–Hinshelwood degradation kinetics, ensuring total degradation of methylene blue in a day. It was observed that GdHO’s work function diminishes with reduced deposition pressure, and UV exposure further decreases it by 0.2 eV—a change that reverts post-UV exposure. The persistent stability of GdHO films, hinting at feasible recyclability, enhances their potential efficiency, underlining their viability in practical applications. Overall, this study accentuates GdHO’s pivotal role in electronics and photocatalysis, representing a landmark advancement in the domain.