Thomas S. van der Poll

Non‐Basic High‐Performance Molecules for Solution‐Processed Organic Solar Cells

A new small molecule, p-DTS(FBTTh(2))(2), is designed for incorporation into solution-fabricated high-efficiency organic solar cells. Of primary importance is the incorporation of electron poor heterocycles that are not prone to protonation and thereby enable the incorporation of commonly used interlayers between the organic semiconductor and the charge collecting electrodes. These features have led to the creation of p-DTS(FBTTh(2))(2)/PC(71)BM solar cells with power conversion efficiencies of up to 7%.

A High‐Performing Solution‐Processed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell

By combining the molecular donor p-DTS(FBTTh2 )2 with a readily produced perylene diimide acceptor we are able to achieve a power conversion efficiency of 3.0%, making this one of the most efficient non-fullerene organic solar cells to date. The reduced power conversion efficiency of the present system compared to the use of phenyl-C71 -butyric acid methyl ester as an electron acceptor is shown to primarily be related to a significant reduction in the internal quantum efficiency. These results indicate the potential of small-molecule:non-fullerene bulk-heterojunction organic photovoltaics.

Pyridalthiadiazole-Based Narrow Band Gap Chromophores

π-Conjugated materials containing pyridal[2,1,3]thiadiazole (PT) units have recently achieved record power conversion efficiencies of 6.7% in solution-processed, molecular bulk-heterojunction (BHJ) organic photovoltaics. Recognizing the importance of this new class of molecular systems and with the aim of establishing a more concrete path forward to predict improvements in desirable solid-state properties, we set out to systematically alter the molecular framework and evaluate structure-property relationships. Thus, the synthesis and properties of 13 structurally related D(1)-PT-D(2)-PT-D(1) compounds, where D represents a relatively electron-rich aromatic segment compared to PT, are provided. Physical properties were examined using a combination of absorption spectroscopy, cyclic voltammetry, thermal gravimetric analysis, differential scanning calorimetry, and solubility analysis. Changes to end-capping D(1) units allowed for fine control over electronic energy levels both in solution and in the bulk. Substitution of different alkyl chains on D(2) gives rise to controllable melting and crystallization temperatures and tailored solubility. Alterations to the core donor D(2) lead to readily identifiable changes in all properties studied. Finally, the regiochemistry of the pyridal N-atom in the PT heterocycle was investigated. The tailoring of structures via subtle structural modifications in the presented molecular series highlights the simplicity of accessing this chromophore architecture. Examination of the resulting materials properties relevant for device fabrication sets forth which can be readily predicted by consideration of molecular structure and which lack a systematic understanding. Guidelines can be proposed for the design of new molecular frameworks with the possibility of outperforming the current state of the art OPV performance.

Solvent Additive Effects on Small Molecule Crystallization in Bulk Heterojunction Solar Cells Probed During Spin Casting

Solvent additive processing can lead to drastic improvements in the power conversion efficiency (PCE) in solution processable small molecule (SPSM) bulk heterojunction solar cells. In situ grazing incidence wide-angle X-ray scattering is used to investigate the kinetics of crystallite formation during and shortly after spin casting. The additive is shown to have a complex effect on structural evolution invoking polymorphism and enhanced crystalline quality of the donor SPSM.

Temperature Dependence of Exciton Diffusion in a Small‐Molecule Organic Semiconductor Processed With and Without Additive

The temperature dependence of exciton diffusion in a small-molecule organic semiconductor processed with and without additive is investigated. As-cast and 1,8-diiodooctane-processed films yield exciton diffusion lengths of 6.8 and 4.9 nm, respectively. Using a Monte Carlo simulation, it is shown that processing with 1,8-diiodooctane increases the excitonic trap density, which directly reduces the exciton diffusion length.

Polymorphism of Crystalline Molecular Donors for Solution-Processed Organic Photovoltaics

Using ab initio calculations and classical molecular dynamics simulations coupled to complementary experimental characterization, four molecular semiconductors were investigated in vacuum, solution, and crystalline form. Independently, the molecules can be described as nearly isostructural, yet in crystalline form, two distinct crystal systems are observed with characteristic molecular geometries. The minor structural variations provide a platform to investigate the subtlety of simple substitutions, with particular focus on polymorphism and rotational isomerism. Resolved crystal structures offer an exact description of intermolecular ordering in the solid state. This enables evaluation of molecular binding energy in various crystallographic configurations to fully rationalize observed crystal packing on a basis of first-principle calculations of intermolecular interactions.