Dynamics in Electronically Excited States of Diketopyrrolopyrrole–Thiophene Conjugated Polymer Thin Films

Abstract

Conjugated polymers based on diketopyrrolopyrrole (DPP) are a versatile basis for a variety of high-performance organic semiconductors. Two conjugated polymers with DPP as an electron acceptor were prepared. The DPP-based polymers were completed with quaterthiophene (PDPP4T) as a donor in the first case; for the second donor−acceptor conjugated polymer, two thiophenes have been replaced by thieno-thiophene (PDPPTTT). Time-resolved two-photon photoemission spectroscopy is employed to study the lifetimes of electronically excited states. On the basis of the 2PPE spectra, we propose four unoccupied states between the determined valence band maximum and LUMO of the thin films. Electron kinetic energy resolved lifetimes range from a few 100 fs up to 1.4 ps and show also a slower component around 10 ps. After annealing, the ultrafast decay of the lowest-energy electrons in the upper polaron state terminated after about 20 ps. This happens in PDPPTTT for the high-energy level as well. We suggest that it is connected with improved charge transport properties of the film.

Unsubstituted thiophene–diketopyrrolopyrrole conjugated polymer thin films via oxidative chemical vapor deposition – electronic behavior

“…In accordance with previous solution-based studies, the grafting of long alkyl chains to the axial positions of the TDPP motif is foreseen to confer solubility to the formed polymeric chains by reducing π–π intermolecular interactions between the aromatics systems, and by preventing hydrogen bonding between amide functional groups. 11,12,14–17 In addition to the 2-ethylhexyl group, we also investigated the oCVD reaction of TDPP substituted with one or two tert -butyloxycarbonyl (boc) protecting groups. However, the low-pressure (10 −3 mbar) and temperature (200 °C) operating conditions required to supply the TDPP-based monomers from the vapor phase in the proposed CVD approach yield to the removal of the Boc group during sublimation step (Fig.…”

“…Among the commercially available thienyl core substituted DPP derivatives, TDPP constitutes the simplest motif, bearing no substituent at its axial positions (N-positions). Alkyl chains, which are commonly used as axial substituents to grant solubility to TDPP-based polymers (blocking H-bonding and reducing p-p stacking), 11,12,[14][15][16][17] are not required in oCVD since the synthesis, deposition and integration are performed simultaneously. 18,21,23,30,32 Iron(III) chloride (FeCl 3 ) was selected due to its ability to promote the dehydrogenative coupling of both thiophene compounds 54 and heterocyclic macrocycles.…”

“…In accordance with previous solution-based studies, the grafting of long alkyl chains to the axial positions of the TDPP motif is foreseen to confer solubility to the formed polymeric chains by reducing p-p intermolecular interactions between the aromatics systems, and by preventing hydrogen bonding between amide functional groups. 11,12,[14][15][16][17] In addition to the 2-ethylhexyl group, we also investigated the oCVD reaction of TDPP substituted with one or two tert-butyloxycarbonyl (boc) protecting groups. However, the low-pressure (10 À3 mbar) and Similarly to what is observed for TDPP, the oCVD reaction of BEHTDPP with FeCl 3 (Table S1 and Scheme S2, ESI †) yields the formation of coatings (denoted pBEHTDPP) whose color (brownish to greyish) contrasts with the color (pinkish) of the coatings prepared from the sole sublimation of BEHTDPP (denoted sBEHTDPP) (Fig.…”

“…p-type, n-type or ambipolar polymeric semiconductors for organic field effect transistors (OFETs). 11,12 Several studies have highlighted the cooperative effect promoted by H-bonds 13 or conjugated covalent bonds 11,12,[14][15][16][17] between DPP derivatives on their functional properties. Particularly, DPP-based conjugated polymers (CPs) are benchmark CPs for high-performance organic semiconductors.…”

“…7 However, N-alkylation, which can significantly increase the production costs, is required to prevent p-p stacking and ensure the solubility and processability of DPP-based CPs. 11,12,[14][15][16][17] In addition, the synthesis of DPP-based CPs implies multistep processes catalysed by expensive and toxic derivatives (e.g. tin and palladium-based compounds).…”