Exploring structure-property landscape of non-fullerene acceptors for organic solar cells.

We present a comprehensive analysis of the structure-property relationship in small molecule non-fullerene acceptors (NFAs) featuring an acceptor-donor-acceptor configuration employing state-of-the-art quantum chemical computational methods. Our focus lies in the strategic functionalization of halogen groups at the terminal positions of NFAs as an effective means to mitigate non-radiative voltage losses and augment photovoltaic and photophysical properties relevant to organic solar cells. Through photophysical studies, we observe a bathochromic shift in the visible region for all halogen-functionalized NFAs, except type-2, compared to the unmodified compound. Most of these functionalized compounds exhibit exciton binding energies below 0.3 eV and ΔLUMO less than 0.3 eV, indicating their potential as promising candidates for organic solar cells. Selected candidate structures undergo an analysis of charge transport properties using the semi-classical Marcus theory based on hopping transport formalism. Molecular dynamics simulations followed by charge transport simulations reveal an ambipolar nature of charge transport in the investigated NFAs, with equivalent hole and electron mobilities compared to the parent compound. Our findings underscore the crucial role of end-group functionalization in enhancing the photovoltaic and photophysical characteristics of NFAs, ultimately improving the overall performance of organic solar cells. This study advances our understanding of the structure-property relationships in NFAs and provides valuable insights into the design and optimization of organic solar cell materials.

Polarity and orbital driven reduction in contact resistance in organic devices with functionalized electrodes.

Interlayers at electrode interfaces have been shown to reduce contact resistance in organic devices. However, there still needs to be more clarity regarding the role of microscopic properties of interlayer functionalized interfaces on device behavior. Here, we show that the impact of functionalized electrodes on device characteristics can be predicted by a few critical computationally derived parameters representing the interface charge distribution and orbital interactions. The significant influences of interfacial orbital interactions and charge distribution over device and interface properties are exhibited. Accordingly, a function is developed based on these parameters that capture their effect on the interface resistance. A strong correlation is observed, such that enhanced orbital interactions and reduced charge separation at the interface correspond to low resistance regardless of the individual molecules utilized as the interlayer. The charge distribution and orbital interactions vary with the molecular structure of the interlayer, allowing the tuning of device characteristics. Hence, the proposed function serves as a guideline for molecular design and selection for interlayers in organic devices.

Understanding the Microscopic Origin of the Contact Resistance at the Polymer-Electrode Interface.

Contact resistance (RC) in organic devices originates from a mismatch in energy levels between injecting electrodes and organic semiconductors (OSCs). However, the microscopic effects governing charge transfer between electrodes and the OSCs have not been analyzed in detail. We fabricated transistors with different OSCs (PTB7, PCDTBT, and PTB7-Th) and electrodes (MoO3, Au, and Ag) and measured their contact resistance. Regardless of the electrodes, devices with PTB7-Th exhibit the lowest values of RC. To explain the trends observed, first-principles computations were performed on contact interfaces based on the projector operator diabatization method. Our results revealed that differences in energy levels and the electronic couplings between OSCs' highest occupied molecular orbitals and vacant states on the electrodes influence device RC. Further, based on values obtained from the first-principles, the rate of charge transfer between OSCs and electrodes is calculated and found to correlate strongly with trends in RC for devices with different OSCs. We thus show that device RC is governed by the feasibility of charge transfer at the contact interface and hence determined by energy levels and electronic coupling among orbitals and states located on OSCs and electrodes.

Ligand-mediated electron transport channels enhance photocatalytic activity of plasmonic nanoparticles.

Photoexcitation of noble metal nanoparticles creates surface plasmons which further decay to form energetic charge carriers. These charge carriers can initiate and/or accelerate various chemical processes at nanoparticle surfaces, although the efficiency of such processes remains low as a large fraction of these carriers recombine before they can reach the reaction sites. Thus efficient utilization of these charge carriers requires designing nanostructures that promote the separation of charges and their transport toward the reaction sites. Here we demonstrate that covalently bound surface-coating ligands with suitable orbital alignment can provide electron transport channels boosting hot electron extraction from a gold nanostructure leading to a huge enhancement in the rate of hydrogen evolution reaction (HER) under NIR excitation. A (p)Br-Ph-SH substituted gold nanoprism (AuTP) substrate produced ∼4500 fold more hydrogen compared to a pristine AuTP substrate under 808 nm excitation. Further experimental and theoretical studies on a series of substituted benzene-thiol bound AuTP substrates showed that the extent of the ligand-mediated HER enhancement depends not only on the polarity of the ligand but on the interfacial orbitals interactions.