Use of side-chain for rational design of n-type diketopyrrolopyrrole-based conjugated polymers: what did we find out?

Abstract

The primary role of substituted side chains in organic semiconductors is to increase their solubility in common organic solvents. In the recent past, many literature reports have suggested that the side chains play a critical role in molecular packing and strongly impact the charge transport properties of conjugated polymers. In this work, we have investigated the influence of side-chains on the charge transport behavior of a novel class of diketopyrrolopyrrole (DPP) based alternating copolymers. To investigate the role of sidechains, we prepared four diketopyrrolopyrrole-diketopyrrolopyrrole (DPP-DPP) conjugated polymers with varied side-chains and carried out a systematic study of thin film microstructure and charge transport properties in polymer thin-film transistors (PTFTs). Combining results obtained from grazing incidence X-ray diffraction (GIXD) and charge transport properties in PTFTs, we conclude side-chains have a strong influence on molecular packing, thin film microstructure, and the charge carrier mobility of DPP-DPP copolymers. However, the influence of side-chains on optical properties was moderate. The preferential ''edge-on'' packing and dominant n-channel behavior with exceptionally high field-effect electron mobility values of 41 cm 2 V À1 s À1 were observed by incorporating hydrophilic (triethylene glycol) and hydrophobic side-chains of alternate DPP units. In contrast, moderate electron and hole mobilities were observed by incorporation of branched hydrophobic side-chains. This work clearly demonstrates that the subtle balance between hydrophobicity and hydrophilicity induced by side-chains is a powerful strategy to alter the molecular packing and improve the ambipolar charge transport properties in DPP-DPP based conjugated polymers. Theoretical analysis supports the conclusion that the side-chains influence polymer properties through morphology changes, as there is no effect on the electronic properties in the gas phase. The exceptional electron mobility is at least partially a result of the strong intramolecular conjugation of the donor and acceptor as evidenced by the unusually wide conduction band of the polymer.

Effect of Chalcogens on Electronic and Photophysical Properties of Vinylene-Based Diketopyrrolopyrrole Copolymers

“…Compared to the band gaps of DPP polymers without a vinylene linkage (e.g., 2DPP-OD-OD, ΔE g = 1.22 eV), which were previously reported by our group, 48 the optical band gap increases due to inclusion of the vinylene unit in the polymer backbone. 48 2.3. Theoretical Calculations.…”

“…48 Hence, close proximity of a very long and branched alkyl chain appears to be detrimental for molecular packing and consequently for measurable charge transport in the device channel. Here the vinylene group acts as a spacer between two DPP units to reduce the interchain repulsive interaction.…”

Solution-grown small-molecule organic semiconductor with enhanced crystal alignment and areal coverage for organic thin film transistors

“…[29][30][31] As a newly-developed derivative of DPP, SMDPPEH hosts two pendant alkyl chains, which increases its solubility and thermal stability. 32 The molecular structure of SMDPPEH is shown in Figure 1(a).…”

Thiophene-Diketopyrrolopyrrole-Based Quinoidal Small Molecules as Solution-Processable and Air-Stable Organic Semiconductors: Tuning of the Length and Branching Position of the Alkyl Side Chain toward a High-Performance n-Channel Organic Field-Effect Transistor

“…[34][35][36][37][38] Several research groups have reported electron mobility values surpassing 1 cm 2 V -1 s -1 for FETs based on DPPcontaining polymers. [39][40][41][42] In comparison, the development of n-type DPP-based small molecules, which possess advantages over polymers including ease of purification and less variation between batches, has been largely lagged behind. 43 In our search for high-performance n-type OSCs, we focused on DPP-containing quinoidal small molecules with dicyanovinyl terminal groups.…”