Donor Acceptor Block Copolymers Research Articles

Unraveling the Phase Behavior of All-Conjugated Donor–Acceptor Block Copolymers through Molecular Engineering

Unraveling the Phase Behavior of All-Conjugated Donor–Acceptor Block Copolymers through Molecular Engineering

Fully conjugated block copolymers enhance thermal stability of polymer blend solar cells

Fully conjugated donor-acceptor block copolymers are composed of covalently-linked conjugated polymers that can leverage self-assembly to realize an ordered microstructure in the active layer of photovoltaic devices. Providing a thermodynamic driving force for mesoscale structure formation may also provide enhanced morphological stability. In this work, a new donor-acceptor block copolymer is synthesized and added into polymer blend solar cells to examine their photovoltaic performance under thermal stress. Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [W3,4-b]thiophene)-2-carboxylate-2-6-diyl)]-block-poly{[N,N′-bis(2-octyldodecyl) naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PTB7Th-b-PNDI) was synthesized through sequential Stille reactions. Photovoltaic performance of ternary blends comprised of PTB7Th and PNDI homopolymers with various amounts of block copolymers were examined. The addition of the block copolymer improves thermal stability, but with an initial drop in power conversion efficiency (PCE) that follows block copolymer content. Nevertheless, inclusion of 5 % of PTB7Th-b-PNDI by weight in the active layer can effectively reduce 30 % of thermal degradation of the photovoltaic devices under thermal stress at 110 °C for 168 h, without sacrificing initial PCE. Donor-acceptor block copolymers are expected to provide a broad, generalizable approach to enhance thermal stability of polymer blend solar cells.

Efficient and Nonhalogenated Solvent-Processed Organic Solar Cells Enabled by Conjugated Donor–Acceptor Block Copolymers Containing the Same Benzodithiophene Unit

Organic solar cells (OSCs) based on conjugated block copolymers (CBCs) have gained considerable attention owing to their simple one-pot solution process. However, their power conversion efficiencies (PCEs) require significant improvement. Furthermore, the majority of efficient CBC-based OSCs are processed using environmentally toxic halogenated solvents. Herein, we develop a new CBC (PBDB-T-b-PY5BDT) and demonstrate efficient and stable OSCs achieved by a halogen-free solution process. We design a (D1-A1)-b-(D1-A2)-type CBC (PBDB-T-b-PY5BDT) that shares the same benzodithiophene (BDT) units in donor and acceptor blocks. This alleviates unfavorable molecular interactions between the blocks at their interfaces. The PBDB-T-b-PY5BDT-based devices exhibit a high PCE (10.55%), and they show good mechanical, thermal, and storage stabilities. Importantly, we discuss the potential of our OSCs by preparing two different control systems: one based on a binary polymer blend (PBDB-T:PY5BDT) and another based on a conjugated random copolymer (CRC, PBDB-T-r-PY5BDT). We demonstrate that the photovoltaic performance, device stability, and mechanical robustness of the CBC-based OSCs exceed those of the binary all-polymer solar cells and CRC-based OSCs.

Polymer solar cells made with photocrosslinkable conjugated donor–acceptor block copolymers: improvement in the thermal stability and morphology with a single-component active layer

New photocrosslinkable conjugated donor–acceptor block copolymer bearing oxetane side chains is synthesized by one-pot polymerization to improve the thermal and morphological properties.

Single‐Material Organic Solar Cells Based on Small Molecule Homojunctions: An Outdated Concept or a New Challenge for the Chemistry and Physics of Organic Photovoltaics?

AbstractSingle‐material organic solar cells (SMOSCs) are on the forefront of research on organic photovoltaics (OPV). The generic term of SMOSCs encompasses a large variety of chemical structures implying very different basic concepts. Polydisperse «double cable» polymers and oligomers with acceptor groups linked to the conjugated backbone by a flexible spacer and donor–acceptor block copolymers are at present, the most investigated and efficient systems with spectacular progress in conversion efficiency achieved in the past 2–3 years. However, besides this mainstream SMOSCs research, a few recent publications describe OPV cells constituted of homojunctions based on small π‐conjugated molecules. While the process of charge generation in such systems is still a matter of debate due in particular to the possible direct photogeneration of charge–carriers, devices with significant performance have been recently reported. After a brief overview of the most recent advances on the various types of SMOSCs, recent remarkable results on homojunction OPV cells based on small π‐conjugated molecules are discussed in order to highlight the potential fundamental and technological interest of this emerging field of research.

Single-component organic solar cells with over 11% efficiency

Achieving stable high-efficiency single-component devices is a challenging problem in the field of organic photovoltaics. Recently in <i>Joule</i>, Min and co-workers reported a single-component organic solar cell using a conjugated donor-acceptor block copolymer (PBDB-T-<i>b</i>-PYT); a remarkable efficiency of 11.32% was realized with impressive photostability and storage stability.

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

A highly crystalline conjugated donor (D)–acceptor (A) block copolymer (PBDT2T‐b‐N2200) that has good solubility in nonhalogenated solvents is successfully synthesized. PBDT2T‐b‐N2200 shows a broad complementary absorption behavior owing to a wide‐band gap donor (PBDT2T) present as a D‐block and a narrow‐band gap acceptor (N2200) present as an A‐block. Polymer solar cells (PSCs) with conjugated block copolymer (CBCP) are fabricated using a toluene solution and PSC created with an annealed film showing the highest power conversion efficiency of 6.43%, which is 2.4 times higher than that made with an annealed blend film of PBDT2T and N2200. Compared to the blend film, the PBDT2T‐b‐N2200 film exhibits a highly improved surface and internal morphology, as well as a faster photoluminescence decay lifetime, indicating a more efficient photoinduced electron transfer. In addition, the PBDT2T‐b‐N2200 film shows high crystallinity through an effective self‐assembly of each block during thermal annealing and a predominant face‐on chain orientation favorable to a vertical‐type PSC. Moreover, the CBCP‐based PSCs exhibit an excellent shelf‐life time of over 1020 h owing to their morphological stability. From these results, a D–A block copolymer system is one of the efficient strategies to improve miscibility and morphological stability in all polymer blend systems.

All-conjugated donor–acceptor block copolymers featuring a pentafulvenyl-polyisocyanide-acceptor

A fulvenyl-functionalized polyisocyanide (PIC2) with a high electron mobility of μe = 10−2 cm2 V−1 s−1 has been incorporated into donor–acceptor block copolymers. Their self-assembly and bulk-morphology have been studied, and potential device applications have been explored.

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

Mastering orientation and crystallization of all-crystalline, all-conjugated donor–acceptor diblock copolymers is of high interest for photovoltaic applications, as electronically beneficial nanost...

Influence of the molecular weight in P3HT block on fully conjugated block copolymers

All-conjugated donor-acceptor block copolymers (BCP) were synthesized in a one-pot reaction by a Stille coupling polycondensation through a terminal functional polymer copolymerization process. Mono bromo end-functionalized P3HT samples with various molecular weights (P3HT-Br, Mn: 5, 9, and 26 kg/mol) were copolymerized with AA (N,Nʹ-Bis(2-decyl-1-tetradecyl)-2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic acid bisimide, NDI) and BB (5,5′-bis(trimethylstannyl)-2,2′-bithiophene) type monomers. Further purification by preparative GPC removed excess P3HT to yield block copolymers of low polydispersities with different donor and acceptor block chain lengths. The block copolymers were characterized by 1H-NMR and UV–vis absorption and FTIR spectroscopy. The optical properties, film morphologies, and optoelectronic properties of the block copolymer were strongly influenced by the donor and acceptor block chain lengths. Furthermore, the fully conjugated D-A block copolymer films exhibited fine structures, smooth film morphologies, and better OPV performance than those features of P3HT/PNDIT2 polymer blends. In polymer solar cells based on the various block copolymer as active layers, the device power convention efficiency was improved by maintaining the acceptor block content at 40–50 kg/mol and increasing the P3HT chain from 5 to 26 kg/mol.

Polarized Soft X-ray Scattering Reveals Chain Orientation within Nanoscale Polymer Domains

Fully conjugated donor–acceptor block copolymers can serve as the active layer in organic photovoltaics and other organic electronic devices. Self-assembly into periodic domains and crystallization of the constituent blocks are crucial to enable control of the multiscale structure and consequently electronic properties. Resonant soft X-ray scattering (RSoXS) is an invaluable tool to characterize such materials, where tuning the X-ray energy and polarization reveals molecular orientation and domain spacing. Here, anisotropic soft X-ray scattering data reveal the type and degree of orientation within conjugated block copolymers composed of poly(3-hexylthiophene-2,5-diyl) and various push–pull copolymers. The maximum anisotropy is observed at the scattering vector corresponding to the spacing between block copolymer domains, which scales with the end-to-end distance of the blocks, as expected. Furthermore, the anisotropy in RSoXS data reveals that the crystalline P3HT blocks orient, on average, parallel or n...

Fully Conjugated Donor-Acceptor Block Copolymers for Organic Photovoltaics via Heck-Mizoroki Coupling.

The development of facile routes to prepare fully conjugated block copolymers (BCPs) from diverse monomers is an important goal for advancing robust bulk-heterojunction (BHJ) organic photovoltaics (OPVs). Herein we introduce a synthetic strategy for step-growth BCPs employing 1,2-bis(trialkylstannyl)ethene as one monomer, which, in addition to offering improved backbone planarity, directly yields a vinylene-terminated macromonomer suitable for Heck-Mizoroki coupling. The benefits of our strategy, which facilitates the preparation of functionalized macromonomers suitable for BCP synthesis, are demonstrated with a representative BCP based on a diketopyrrolopyrrole (DPP) copolymer coded pBDTTDPP as the donor block and a perylenediimide (PDI) copolymer coded as pPDIV as the acceptor block. Feed ratio optimization affords control over the macromonomer chain-end functionalities and allows for the selective formation of a tri-BCP consisting of pPDIV-b-pBDTTDPP-b-pPDIV, which is employed in a single-component BHJ OPV. Devices achieved a power conversion efficiency of 1.51% after thermal stress at 150 °C compared to 0.02% for a control device consisting of a comparable blend of pBDTTDPP and pPDIV. The difference in performance is ascribed to the morphological stability of the BHJ when using the BCP.

Facile one-pot polymerization of a fully conjugated donor–acceptor block copolymer and its application in efficient single component polymer solar cells

A single component polymer solar cell with a CDABP film annealed at 180 °C showed a maximum power conversion efficiency of 5.28%, which is much higher than that (2.62%) of the device with the as-cast film.

Modulation of molecular orientation enabling high photovoltaic performance of block copolymer nanostructures

In this work, by means of theoretical simulations we show that more than 150% improvement in the photovoltaic performance of donor–acceptor block copolymer thin films could be achieved through regulating the molecular orientation.

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 (Th xDPPTh x, 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) 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-Th xDPPTh x, with x determining its C-H reactivity, is key to the synthesis of various BCPs of type H-P3HT-Th xDPPTh x- 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-Th xDPPTh 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.

Conjugated Block Copolymers as Model Systems to Examine Mechanisms of Charge Generation in Donor–Acceptor Materials

AbstractFully conjugated donor–acceptor block copolymers are established as model systems to elucidate fundamental mechanisms of photocurrent generation in organic photovoltaics. Using analysis of steady‐state photoluminescence quenching, exciton dissociation to a charge transfer state within individual block copolymer chains is quantified. By making a small adjustment to the conjugated backbone, the electronic properties are altered enough to disrupt charge transfer almost entirely. Strong intermolecular coupling of the electron donor is introduced by synthesizing block copolymer nanoparticles. Transient absorption spectroscopy is used to monitor charge generation in block copolymer isolated chains and nanoparticles. While efficient charge transfer is observed in isolated chains, there is no indication of complete charge separation. In the nanoparticles, long‐lived polarons are observed as early as ≈15 ns. Thus, aggregation of electron donors can facilitate efficient charge generation.

Random Copolymers Allow Control of Crystallization and Microphase Separation in Fully Conjugated Block Copolymers

Thin films of fully conjugated donor–acceptor block copolymers composed of an electron donating block and an electron accepting block can be used as single component photoactive layers in organic photovoltaic (OPV) devices. In order to realize their full potential, control over microphase separation and thin-film morphology are critical. In conjugated block copolymer systems where one or more blocks can crystallize, the morphological evolution is governed by the competition between microphase separation and crystallization. In this work, we control crystallization of fully conjugated block copolymers with a random copolymer block. We suppress the crystal packing of poly(3-hexylthiophene-2,5-diyl) (P3HT) through the insertion of a small number of 3-octylthiophene (3OT) units within the chains, yielding poly(3-hexylthiophene-2,5-diyl-random-3-octylthiophene-2,5-diyl) (P[3HT-r-3OT]). While crystallization of P3HT dominates the morphology and prevents microphase separation in poly(3-hexylthiophene-2,5-diyl)-b...

Modular Synthesis and Structure Analysis of P3HT-b-PPBI Donor–Acceptor Diblock Copolymers

While donor–acceptor block copolymers (BCPs) are widely discussed to be optimal model materials for studying the morphology-dependent processes in organic photovoltaics, the observation of the desired microphase-separated structures is rare. We present a novel modular approach for the synthesis of donor–acceptor block copolymers offering a fast and flexible way for chemical modifications leading to microphase separation accompanied by confined crystallization of the individual blocks. Two donor–acceptor BCPs of poly(3-hexylthiophene)-b-polyperylene bisimide (P3HT-b-PPBI 1 and 2) were synthesized incorporating P3HT as a donor and a polystyrene with two different pendant perylene bisimides (PBI-N3 1 and 2) as acceptor blocks to show the flexibility of the synthetic approach. The synthesis combines Kumada catalyst transfer polymerization (KCTP), controlled radical polymerization, and click chemistry to obtain highly comparable polymers via a modular approach. We synthesized poly(3-hexylthiophene) (P3HT) with...

Studying the mechanism of the organic electrochromic effects in donor-acceptor block copolymers

To demonstrate the effects of donor and acceptor units on the structure-property of electrochromism, we design and synthesize new block copolymers in this work. It is found that these units play key roles on the redox behavior, band gap, neutral state color and the electrochromic performance. Through changing the ratio between donor and acceptor units of copolymer, electrochromic performance and the response time of electrochromism are changed. More important, the mechanism and insightful understanding on electrochromic effects in block copolymers based devices are studied, and more information is provided for further electrochromic block copolymers design and devices fabrication.

All-Conjugated, All-Crystalline Donor–Acceptor Block Copolymers P3HT-b-PNDIT2 via Direct Arylation Polycondensation

The synthesis and characterization of all-conjugated, all-crystalline donor–acceptor block copolymers (BCPs) containing poly(3-hexylthiophene) (P3HT) 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 presented. Direct arylation polycondensation (DAP) of dibromonaphthalenediimide and bithiophene is carried out in the presence of P3HT end-cappers to allow the in situ formation of BCPs P3HT-b-PNDIT2. As-prepared, well-defined H-P3HT-Br with hydrogen and bromine chain termini shows nonoptimal reactivity under the DAP conditions used. Therefore, H-P3HT-Br is converted into either H-P3HT-Th (thiophene) or H-P3HT-Mes (mesitylene), giving α,ω-hetero-C–H functionalized P3HT with modulated C–H reactivity. The influence of the different C–H chain termini of P3HT on the ability to act as end-capper and the resulting block structures is investigated in detail using wavelength-dependent size exclusion chromatography (SEC) and NMR spectroscopy. Di...