Condensed Matter

Organic semiconductors offer a potential class of materials for organic photovoltaic (OPV) applications due to their tunable optoelectronic properties and low-cost processing. A methodical DFT/TD-DFT study of a library of organic donor–π–acceptor (D–π–A) compounds based on triphenylamine donors, thiophene-based π-bridges, and benzothiadiazole/malononitrile acceptors is presented in this work, with the goal of rationalizing the structure–property relationships governing their photovoltaic behavior. CAM-B3LYP calculations were used to analyze the role of donor, bridge, and acceptor units in modulating frontier-orbital alignment, charge-transfer character, and optical absorption properties, as well as to evaluate the active-layer thickness in the estimation of the light-harvesting efficiency. The results, which are intended as internal comparative descriptors rather than predictive device efficiencies, reveal that the most pronounced bathochromic shifts and most favorable optical responses are not simply associated with the strongest donor or acceptor moieties, but rather arise from an optimal balance between frontier-orbital delocalization and charge-transfer character across the molecular framework. A preliminary assessment of photovoltaic descriptors suggests that the proposed computational workflow may provide useful guidelines for the descriptor-guided design and screening of next-generation organic photovoltaic materials.

The theory of hole superconductivity has developed over more than three decades through a sequence of steps addressing distinct physical problems. This paper identifies and documents a recurring structural pattern in that development: ideas introduced to solve one problem were only later recognized as being required by independent physical constraints. By tracing a series of such delayed conceptual unifications, spanning pairing mechanism, charge expulsion, electrodynamics, spin structure, rotation, relativity, thermodynamics and momentum conservation, we highlight that the framework evolves by constraint tightening rather than by ad hoc embellishment. While this does not establish the correctness of the theory, it provides evidence that it is responding to real physical requirements uncovered progressively, in contrast to theories that accommodate discrepancies or new constraints primarily through auxiliary assumptions and ultimately fail.

Charge order driven by electron–phonon coupling is well understood at equilibrium but pump-probe experiments raise a new question: how does this order melt and recover after strong photoexcitation? A pump pulse promotes carriers across the charge-order gap and creates a nonequilibrium high-energy electronic population. In a closed system, the subsequent dynamics are constrained by energy conservation. In an ‘open system’—where the system is coupled to a thermal bath at some temperature $T_{\mathrm{bath}}$—there are new fluctuation and dissipation processes at play. One can attempt a computational scheme that incorporates coupling of electrons to a laser pump, the coupling of system phonons to a thermal bath, and the Holstein interaction that couples electrons and phonons. We attempt an approximation where the pump-induced electronic excitations are modeled by a slowly time-varying ‘electron temperature’, $T_{\mathrm{el}}\left(t\right)$, indicative of a quasi-equilibrium electronic state. We solve the problem for different combinations of $T_{\mathrm{el}}$ and $T_{\mathrm{bath}}$, probing the order parameter dynamics, the static properties and excitations in the long-time ‘quasi-steady state’, and establish a ‘phase diagram’ in terms of bath temperature and electron temperature.