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 diketo-pyrrolopyrrole (ThxDPPThx, x = 1 or 2) unit, and poly{[N,N'-bis(2-octyldodecy1)-naphthalene-1,4,5,8-bis-(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDIT2) are 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 l(p) is invisible from TEM, but shows up in resonant soft X-ray scattering experiments at a length scale of l(p) similar to 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.