Dielectric Constant Engineering of Organic Semiconductors: Effect of Planarity and Conjugation Length

Abstract

Bulk heterojunction organic solar cells continue to show steady photoconversion efficiency improvements. However, single component organic solar cells are a particularly attractive alternative due to the relative simplicity of device manufacture. It has been proposed that organic semiconductors with a high dielectric constant (≈10) could give rise to spontaneous free charge carrier generation upon photoexcitation. In this manuscript, factors are explored that affect the dielectric constant of organic semiconductors, particularly the optical‐frequency dielectric constant. The properties of monomers, dimers and trimers of two isoelectronic families of materials that have acceptor units composed of one or two dicyanovinylbenzothiadiazole moieties and one to three donor units are compared. The donor components are composed of either fluorenyl or cyclopentadithiophene moieties with the same glycol‐based solubilizing groups. It is found that chromophore planarity and orientation with respect to the substrate, and film density affect the optical and electronic properties of the materials, especially the high‐frequency dielectric constant. The results also indicate that delocalization of the highest occupied and lowest unoccupied molecular orbitals is a critical factor. The dimer with two dicyanovinylbenzothiadiazole moieties and two dithienocyclopentadiene units is found to have the highest optical frequency dielectric constant and overall performance.

Understanding the Role of Removable Solid Additives: Selective Interaction Contributes to Vertical Component Distributions

“…[5][6][7] Blending electron donor and electron acceptor in BHJ layer can facilitate exciton dissociation at the donor-acceptor interfaces with suitable frontier molecular orbital energy levels. [8][9][10] However, organic semiconductors usually have much lower charge-carrier mobility than inorganic materials due to their semicrystalline nature and large energetic disorder. [11][12][13] Theorefore, the competition between charge transport and charge recombination in BHJ film is quite severe especially at the thickness beyond one hundred nanometers, limiting the efficiency of OSCs.…”

Developing Y‐Branched Polymer Acceptor with 3D Architecture to Reconcile Between Crystallinity and Miscibility Yielding >15% Efficient All‐Polymer Solar Cells

“…The intermolecular packing is also correlated intimately with thin film density, which can be determined from X‐ray reflectometry (XRR) profiles (Figure S6 , Supporting Information). [ 27 , 28 ] XRR characterization is so sensitive to thin film roughness that the larger surface roughness, the lower XRR intensity, and hence the stronger background noise. Therefore, the surface roughness of the film is required to be small enough when using XRR to characterize the film density.…”

Molecular Packing and Dielectric Property Optimization through Peripheral Halogen Swapping Enables Binary Organic Solar Cells with an Efficiency of 18.77%

“…It is well known that the relative dielectric constants of organic semi‐conductors can be affected intrinsically by a series of structurally determining factors, including dipole moment, chromophore conjugation length, packing orientation, and molecular packing density, etc. [ 28–30 ] Fortunately, despite the huge gap of ε r between organic and inorganic materials, great potentials for improving dielectric properties to further evaluate the FF and J SC of OSCs have been attached by the remarkable structural diversity of organic molecules. [ 26,31 ]…”

“…It is well known that the relative dielectric constants of organic semi-conductors can be affected intrinsically by a series of structurally determining factors, including dipole moment, chromophore conjugation length, packing orientation, and molecular packing density, etc. [28][29][30] Fortunately, despite the huge gap of ε r between organic and inorganic materials, great…”