Interfacial phenomena play a crucial role in the performance of organic electronic devices. In the research described herein, we have studied the junction between regioregular poly(3-hexylthiophene-2,5-diyl) (R-P3HT), a model organic semiconductor, and the insulator poly(4-vinylpyridine) (P4VP). Pyridine rings in the chemical structure of P4VP possess a large dipole moment and can easily form complexes with d-block metals. We have composed R-P3HT and P4VP systems in two main mutual arrangements: (i) layer-by-layer systems and (ii) laterally spread domains resulting from phase separation. In the bilayer arrangement, we added an additional layer of Al2O3 that was thick enough to eliminate contact interactions between the materials. We have demonstrated that the presence of P4VP has a positive influence on the semiconductor in-plane conductance, even without direct molecular contact. The effect can be further enhanced by the formation of Br2Co(py)2 complexes on the P4VP surface. This is direct evidence of the importance of molecular dipoles and the influence of their electric fields on the conductivity of R-P3HT in a field-transistor-like insulator-semiconductor junction. The second system allows us to study the physicochemical properties of direct interactions between the two materials. Due to the secondary phase separation, we were able to investigate two different ranges of molecular contact: (i) at the interface between P4VP-rich and R-P3HT-rich regions and (ii) within the P4VP-rich domain where R-P3HT is intermixed. Detailed analysis with a combination of conductive atomic force microcopy, Raman spectroscopy, and other techniques shows no evidence of doping of the R-P3HT through the contact with P4VP, even though the intermixing of R-P3HT into the P4VP matrix results in high conductance within the domain, which is most a likely consequence of altered molecular ordering. Our results point to the primacy of the dipole-derived interactions dominating the conductance increase in this system.

The molecular orientation is crucial for the efficiency of organic solar cells. A face-on orientation, in which the pi-pi stacking direction is oriented perpendicular to the substrate, is typically preferred because it enhances vertical charge transport to the electrodes and can additionally modify the position of energy levels. In this study, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy was employed to investigate the molecular orientation of the acceptor polymers PYT and PF5-Y5 and the donor polymer PBDB-T in spin-coated blend films with different donor: acceptor ratios. From the comparison of NEXAFS spectra acquired in partial electron yield (PEY), total electron yield (TEY), and fluorescence yield (FY) modes, depth-dependent information about the orientation of the components in the films can be extracted. We found that the absorption resonances in the PEY carbon K-edge spectra of all the blend films resembled the spectral signatures of PBDB-T, indicating that the surface of these blend films is PBDB-T-rich, even at a 1:10 donor-to-acceptor ratio. To identify the acceptor component in the carbon spectra, deeper subsurface probing was required using TEY and FY modes, alongside analysis of the angular dependence of these spectra. Nitrogen K-edge NEXAFS spectra were employed to selectively probe the acceptor orientation in the blend films, revealing that generally the polymer acceptors retain their face-on orientation observed in neat acceptor films. However, in one blend, a decrease in the dichroic ratio suggests that the donor polymer influences the molecular orientation of the acceptor at the film's surface. This work demonstrates a novel strategy to probe molecular orientation in all-polymer blend films. The approach exploits dichroism at selective absorption edges to access detailed information on the molecular orientation of one component within the blend film.

The Rashba effect is a manifestation of spin-orbit coupling in systems with structural inversion asymmetry, resulting in a spin-dependent splitting of energy bands in low-dimensional systems. This gives rise to diverse spin-dependent phenomena in semiconductor heterostructures, offering significant potential for spintronic applications. However, a comprehensive theoretical characterization remains incomplete, since most existing approaches are restricted to relatively small structures. In this work, we combine the well-established eight-band Kane model and envelope-function formalism with a recently developed Green's function approach. The framework allows to obtain analytical expressions for the spin-dependent coherent transport in semiconductor heterostructures with an arbitrary number N of cells exhibiting the Rashba effect. In addition, we propose guidelines for enhancing spin polarization and spin-miniband separation in extended semiconductor heterostructures by tuning structural inversion asymmetry. Furthermore, we find that as a geometric parameter is varied, the spin-splitting dynamics present the quantum avoided-crossing behavior. We finally show that, for suitably designed heterostructures, the polarization bands can exhibit step-like (rectangular-wave) profiles. Although our examples focus on GaAs/In-based systems, the results are expected to hold for other semiconductor materials as well.

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.

Eco-friendly solution processing and the low-cost synthesis of photoactive materials are important requirements for the commercialization of organic solar cells (OSCs). Although varieties of aqueous-soluble acceptors have been developed, the availability of aqueous-processable polymer donors remains quite limited. In particular, the generally shallow highest occupied molecular orbital (HOMO) energy levels of existing polymer donors limit further increases in the power conversion efficiency (PCE). Here, we design and synthesize two water/alcohol-processable polymer donors, poly[(thiophene-2,5-diyl)-alt-(2-((13-(2,5,8,11-tetraoxadodecyl)-2,5,8,11-tetraoxatetradecan-14-yl)oxy)-6,7-difluoroquinoxaline-5,8-diyl)] (P(Qx8O-T)) and poly[(selenophene-2,5-diyl)-alt-(2-((13-(2,5,8,11-tetraoxadodecyl)-2,5,8,11-tetraoxatetradecan-14-yl)oxy)-6,7-difluoroquinoxaline-5,8-diyl)] (P(Qx8O-Se)) with oligo(ethylene glycol) (OEG) side chains, having deep HOMO energy levels (similar to-5.4 eV). The synthesis of the polymers is achieved in a few synthetic and purification steps at reduced cost. The theoretical calculations uncover that the dielectric environmental variations are responsible for the observed band gap lowering in OEG-based polymers compared to their alkylated counterparts. Notably, the aqueous-processed all-polymer solar cells (aq-APSCs) based on P(Qx8O-T) and poly[(N,N '-bis(3-(2-(2-(2-methoxyethoxy)-ethoxy)ethoxy)-2-((2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-methyl)propyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-(2,5-thiophene)] (P(NDIDEG-T)) active layer exhibit a PCE of 2.27% and high open-circuit voltage (V-OC) approaching 0.8 V, which are among the highest values for aq-APSCs reported to date. This study provides important clues for the design of low-cost, aqueous-processable polymer donors and the fabrication of aqueous-processable OSCs with high V-OC.

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.

A deep understanding of the factors influencing the morphology of thin films based on conjugated polymers is essential to boost their performance in optoelectronic devices. Herein, we investigated the electronic structure and morphology of thin films of the copolymer poly(9,9-dioctyl-fluorenyl-co-bithiophene) (F8T2) in its pristine form as well as samples processed with the solvent additive 1,8-diiodooctane (DIO) or post-processed through thermal annealing treatment. Measurements were carried out using angle-resolved S K-edge NEXAFS (near-edge X-ray absorption fine structure) in total electron yield (TEY) and fluorescence yield (FY) detection modes. Two main transitions were observed at the S 1s NEXAFS spectra: S 1s -> pi* and S 1s -> sigma* (S-C). The observed dichroism pointed to a face-on orientation of the conjugated backbone, which was significantly increased for F8T2 films processed with DIO. Resonant Auger decay spectra were obtained and analyzed using the core-hole clock (CHC) method. An enhancement in the charge transfer process was observed for thermally annealed films, especially for samples processed with DIO, corresponding to an increase in film ordering. Furthermore, the investigated films were characterized using X-ray photoelectron spectroscopy, attesting to the presence of the thiophene unit in the samples and demonstrating that some of its sulfur atoms were positively polarized in the F8T2 films. All these experimental findings were compared with molecular dynamics (MD) simulations of film evaporation with and without DIO. The use of MD, together with mathematical modeling, was able to explain the major effects found in the experiments, including the polarization of sulfur atoms. The simultaneous use of powerful spectroscopic techniques and theoretical methods shed light on key aspects linking film morphology with fabrication procedures.

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.

New materials for electrochemical energy storage and conversion are the key to the electrification and sustainable development of our modern societies. Molecular modelling based on the principles of quantum mechanics and statistical mechanics as well as empowered by machine learning techniques can help us to understand, control and design electrochemical energy materials at atomistic precision. Therefore, this roadmap, which is a collection of authoritative opinions, serves as a gateway for both the experts and the beginners to have a quick overview of the current status and corresponding challenges in molecular modelling of electrochemical energy materials for batteries, supercapacitors, CO2 reduction reaction, and fuel cell applications.

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.