Block Junction-Functionalized All-Conjugated Donor–Acceptor Block Copolymers

Abstract

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 junctionfunctionalized, all-conjugated, all-crystalline D-A BCPs. Poly(3-hexylthiophene) (P3HT), a single thienylated diketopyrrolopyrrole (Th x DPPTh x , x=1 or 2) unit and poly{[N,N'-bis(2octyldodecyl)-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-Th x DPPTh x , with x determining its C-H reactivity, is key to the synthesis of various BCPs of type H-P3HT-Th x DPPTh x -block-PNDIT2. Remarkable morphological behavior 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 on a length scale of l p ~60 nm. Charge transfer photoluminescence of H-P3HT-Th x DPPTh x 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.

Significantly Improved Morphology and Efficiency of Nonhalogenated Solvent‐Processed Solar Cells Derived from a Conjugated Donor–Acceptor Block Copolymer

“…Resultantly, the molar ratios of the PBDT2T and N2200 repeating units were determined around 1:1 in PBDT2T and N2200. The PBDT2T‐ b ‐N2200 obtained after a one‐pot synthesis may contain a small amount of individual low molecular weight DB or AB owing to defective polymerization sites, but the final product has a block copolymer structure mainly containing DB and AB …”

“…Over the past two decades, several research groups, including our own, have been studying new materials that covalently anchor the electron donor (D) and acceptor (A) moieties . PSCs bearing above‐mentioned materials have recently been studied in two main forms.…”

“…One is a double‐cabled polymer composed mainly of a small conjugated molecular acceptor bound to the side chains of a main chain conjugated polymer donor . The other is a conjugated block copolymer (CBCP) combining the donor block (DB) and the acceptor block (AB) in a polymer chain . Among them, studies on CBCP have attracted significant attention because they are relatively simple to synthesize and can lead to the design of various types of CBCPs.…”

Insights into the structural complexity of semi-crystalline polymer semiconductors: electron diffraction contributions

“…They can form spontaneously nanostructured bulk heterojunctions via phase separation of donor and acceptor blocks. [77][78][79][80] Preliminary studies show that the alignment methods used for the individual blocks are relevant to orient and crystallize such conjugated copolymers and ultimately study their organization by electron diffraction. In particular, TEM studies demonstrated that the crystallization of the P(NDI2ODT2) block can hamper substantially that of the P3HT block due to a mismatch of lattice parameters of the two blocks at the block junction.…”

Impact of Competing Crystallization Processes on the Structure of All-Conjugated Donor–Acceptor Block Copolymers P3HT-b-PNDIT2 in Highly Oriented Thin Films

“…2 A face-on orientation of the conjugated backbone is ideal for efficient organic photovoltaics (OPV), whereas usually an edge-on orientation allows for two-dimensional in-plane charge transport in field effect transistors. [3][4][5][6][7] The most efficient organic solar cells make use of a donor copolymer fullerene acceptor blend, with power conversion efficiencies beyond 15 %. 8 While these mere numbers are exciting, the issue of unstable, kinetically trapped blend morphologies persists.…”