Block junction-functionalized all-conjugated donor-acceptor block copolymers.

Junction-functionalized donor-acceptor (D-A) block copolymers (BCPs) enable spatial and electronic control over interfacial charge dynamics in excitonic devices such as solar cells. Here we present the design, synthesis, morphology and electronic characterization of block junction-functionalized, all-conjugated, all-crystalline D-A BCPs. Poly(3-hexylthiophene) (P3HT), a single thienylated diketopyrrolopyrrole (ThxDPPThx, x=1 or 2) unit and poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDIT2) is used as donor, interfacial unit and acceptor, respectively. Almost all C-C coupling steps are accomplished by virtue of C-H activation. Synthesis of the macroreagent H-P3HT-ThxDPPThx, with x determining its C-H reactivity, is key to the synthesis of various BCPs of type H-P3HT-ThxDPPThx-block-PNDIT2. Morphology is determined from a combination of calorimetry, transmission electron microscopy (TEM) and thin film scattering. Block copolymer crystallinity of P3HT and PNDIT2 is reduced indicating frustrated crystallization. A long period lp, is invisible from TEM, but shows up in resonant soft X-ray scattering experiments at a length scale of lp~ 60 nm. Photoluminescence of H-P3HT-ThxDPPThx indicates efficient transfer of the excitation energy to the DPP chain end, but is quenched in BCP films. Transient absorption and pump-push-photocurrent spectroscopies reveal geminate recombination (GR) as the main loss channel in as-prepared BCP films independent of junction functionalization. Melt-annealing increases GR as a result of the low degree of crystallinity and poorly defined interfaces, and additionally changes backbone orientation of PNDIT2 from face-on to edge-on. These morphological effects dominate solar cell performance and cause an insensitivity to the presence of the block junction