Structure-Property Relationships of Diketopyrrolopyrrole-Based Organic Semiconductors for Field Effect Transistors and Solar Cells

Leonardi, Matthew John

doi:https://doi.org/10.21985/n2-2tjy-4w39

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Structure-Property Relationships of Diketopyrrolopyrrole-Based Organic Semiconductors for Field Effect Transistors and Solar Cells

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Organic semiconductors are an active area of research with great promise for delivering next generation electronics and clean energy technologies. As the field matures, understanding the connection between molecular structure, materials’ properties, and device performance will be critical in finding the right material for an intended application. An effective strategy for studying structure-property relationships is to synthesize a new material that is a derivative of an existing one with a specific modification. By comparing the properties of the new material and the existing one, the effect of the modification can be elucidated and used to develop design rules for new materials. Diketopyrrolopyrrole (DPP) is a structural motif that has been utilized to create a number of successful organic semiconducting materials with high charge mobility in organic field effect transistors and high power conversion efficiency in solar cells. Many derivatives of the DPP structure have been synthesized and studied, with varying degrees of success in device applications. One underexplored area of DPP functionalization is fluorination. Fluorination has been shown to benefit organic semiconductor device performance by increasing molecular planarity and material crystallinity, as well as decreasing highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies. The result of these alterations to the materials properties is an increase in overall charge mobility, increased n-type character, and increased power conversion efficiency. Applying this concept to DPP, derivatives of thienyl-flanked DPP (TDPP) containing one or two fluorine atoms in the structure were synthesized and incorporated into semiconducting copolymers. These materials were characterized to understand the effect of the degree of fluorination on the materials properties. Expected decreases in HOMO and LUMO energies were observed as well as increases in material crystallinity and tighter π-π stacking when compared to the unfluorinated materials. Stable ambipolar mobilities were observed for the difluorinated TDPP (FTDPP) containing copolymer, with electron transport persisting even after storage in air, highlighting its promise in semiconducting materials. Building on the results from the degree of fluorination study, a series of FTDPP containing copolymers incorporating a range of comonomers and solubilizing side chains were synthesized and studied. These comonomers and side chains were chosen to attempt to modulate material crystallinity and energy levels, with the goal of studying the behavior of the FTDPP monomer unit in polymers and improving the overall charge mobility. All materials exhibited crystalline order and ambipolar charge mobility, but the size of the comonomer and side chain had a noticeable impact on the materials properties, with the best charge mobilities observed in those polymers with the highest and lowest degrees of crystalline order. This suggests that FTDPP-based polymers can be effectively tuned with judicious choice of side chain and comonomer and could be used to realize future high-performance materials. TDPP has previously been shown to give modest solar cell PCE values when incorporated into acceptor-donor-acceptor (A-D-A) small molecules as the A unit. This class of p-type molecules has historically shown low fill factor (FF) values, limiting the performance that can be achieved. By increasing the atomic number (Z) of the chalcogen atom in the side chain of the central benzodithiophene (BDT) unit in a series of TDPP-BDT-TDPP small molecules, the FF value could be reliably increased. Characterization of the small molecules and their thin films revealed that a more ordered morphology and reduced charge recombination were responsible for the FF increase. The finding suggests a possible materials design rule for new small molecule semiconductors.

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