Acceptor Block Research Articles

Single‐Material Organic Solar Cells Based on Small Molecule Homojunctions: An Outdated Concept or a New Challenge for the Chemistry and Physics of Organic Photovoltaics?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

A conjugated donor-acceptor block copolymer enables over 11% efficiency for single-component polymer solar cells

<h2>Summary</h2> Covalently linking donor (D) and acceptor (A) moieties into single material can be an effective strategy to enable efficient exciton dissociation and charge generation. Herein, we synthesized a conjugated D-A block copolymer (PBDB-T-<i>b</i>-PYT), in which a donor block (PBDB-T) was connected to an acceptor block (PYT). Compared with the bulk heterojunction (BHJ) blend mixture containing PBDB-T and PYT with high molecular weights, more efficient exciton dissociation and charge transfer (CT) were observed in the single-component PBDB-T-<i>b</i>-PYT films. Impressively, a single-material organic solar cell (SMOSC) fabricated from PBDB-T-<i>b</i>-PYT alone exhibited a power conversion efficiency (PCE) of 11.32% and a certificated PCE of 10.8%. In addition, higher morphological stability and lower energy loss were also observed in the PBDB-T-<i>b</i>-PYT SMOSC compared with the control PBDB-T:PYT BHJ devices. The results manifest that our newly designed PBDB-T-<i>b</i>-PYT provides a promising design strategy for D-A block copolymers for highly efficient and stable SMOSCs.

Solid-State Donor-Acceptor Coaxial Heterojunction Nanowires via Living Crystallization-Driven Self-Assembly.

The creation of organic heterojunctions from conjugated polymers on the nanoscale has attracted recent attention as a consequence of their considerable potential in optoelectronic devices. Herein, we report proof-of-concept results on a versatile synthetic strategy to access various linearly segmented nanowire heterojunctions with controlled dimensions using the seeded growth "living crystallization-driven self-assembly" method followed by a secondary crystallization step. Specifically, we describe the creation of coaxial and also segmented coaxial B-A-B and A-B-A nanowires with a solvophilic poly(ethylene glycol) (PEG) corona, an inner crystalline core that consists of poly(di-n-hexylfluorene) (PDHF), which functions as a donor, and an outer crystalline core of poly(3-(2'-ethylhexyl)thiophene) (P3EHT), which acts as an acceptor. The latter is present either along the entire nanowire or solely in the central or terminal segments. These assemblies were created by seeded growth of two types of π-conjugated polymeric building blocks, the triblock copolymer PDHF-b-P3EHT-b-PEG and the diblock copolymer PDHF-b-PEG, by using fiber-like seeds derived from either material. The nanowires with both solid-state donor and acceptor blocks exhibit Förster resonance energy transfer (FRET) from the PDHF inner core to the P3EHT outer core which was characterized by fluorescence spectroscopy and laser confocal scanning fluorescence microscopy (LCSM). The FRET in the solid-state coaxial heterojunctions with an inner PDHF core and an outer P3EHT core was enhanced relative to the directly analogous system in which the P3EHT block was solvated.

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.

Effects of Different Bridges of A Series of c-Donor-nc-Bridge-cf- Acceptor Type Block Copolymers for Potential Solar Cell Applications

A series of c-D-nc-B-cf-A (or DBpfA, p=0,1,2,4,6) type of block copolymers has been designed, synthesized, and systematically studied for photo induced charge separations; where c-D is an electron-donating conjugated block, nc-B is a non-conjugated bridge unit, and cf-A is an electron-accepting conjugated and fluorinated block. A series of photoluminescence quenching measurements with and without different bridge units were evaluated for DBpfA. This study highlights the chemical molecular bridge effects on the optoelectronic property of the DBpfA type block copolymer. Specifically, the DB1fA block copolymer (where only one methylene unit is present in the bridge unit) appears exhibiting the best optoelectronic performance. DfA block copolymer (without any bridge unit) exhibits photoluminescence quenching and optoelectronic property that is between the shortest and longest bridge units, and this can be explained by the calculated four stable state twisting angles (most stable state is at a seventeen degree twist) between conjugated donor and conjugated acceptor blocks. The optoelectronic properties appear following the PL quenching nicely.

Novel conjugated copolymers with dithienyl and cyclopentadithienyl substituted dicyanoethene acceptor blocks

Novel bifunctional acceptor monomer, 2-[bis(5-bromothiophen-2-yl)methylidene]malononitrile, is obtained in three steps in a total yield of 79%. The Stille cross-coupling between this monomer and 2,6-ditin derivative of 4,4-didecyl-4H-silolo[3,2-b: 4,5-b′]dithiophene afforded donor–acceptor polymer whose properties were compared to the analogue containing a planar cyclopenta[2,1-b: 3,4-b′]dithiophene acceptor block. The copolymers have low narrow optical band gaps and effectively absorb visible light, however the former possesses improved solubility and thermal stability as compared to the latter.

C60-small arylamine push-pull dyads for single-material organic solar cells

The synthesis of two dyads involving small arylamine-based push-pull systems as donor block and C60 fullerene as acceptor connected by a flexible insulating linker formed by esterification is described. After analysis of their electronic properties by cyclic voltammetry and UV–Vis absorption spectroscopy, the potential of these molecules as active material for single-material organic solar cells has been evaluated in simple devices of direct structure. Both dyads lead to modest conversion efficiencies of ∼0.40% due essentially to poor fill factors. These results are discussed in terms of insufficient nano-phase separation of the donor and acceptor blocks and of fall of hole-mobility of the donor block consecutive to the linking to C60.

Well‐defined structures and nanoscale morphology for all‐conjugated BCPs

A series of conjugated donor–acceptor (D–A) block copolymers (BCPs) were synthesised using a one-pot Stille coupling polycondensation reaction. This involved reaction between a series of mono-bromo-functionalised Poly3-hexylthiophene (P3HT) polymers (P3HT-Br, Mn: 17, 21 and 43 kg/mol) and [N, N ′-bis (2-decyl-tetradecyl)-1,7-dibromo-3,4,9,10-perylene diimide (PBI) and [2,5-bis(trimethylstannyl)-thiophene] (T) monomers. Purification using preparative gel permeation chromatography (GPC) removed any excess P3HT and resulted in BCPs with low polydispersity index values. The P3HT-b-PBIT BCPs were characterised using 1 H-NMR and Fourier-transform infrared spectroscopy. When compared to a P3HT/PBIT polymer blend, the D–A BCP films exhibited a remarkably fine structure with a nanoscale morphology. These results indicated that these D–A BCPs have the potential for use as nanostructured active layers in polymer solar cells.

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...

Synthesis and photovoltaic properties of alkylthiothiophene modified benzodithiophene polymer

Here, we report a facile molecular engineering approach with alkylthio and fluorine atom modified the conjugated side chain of quinoxaline-based polymer, namely DTFFQx-TS, which afforded lower highest occupied molecular orbital energy level and enhanced interchain interactions. The conventional devices based on DTFFQx-TS:ITIC exhibited the power conversion efficiency (PCE) of 5.26% with a remarkably high open circuit voltage (Voc) of 0.99 V. This work not only provides a newly efficient candidate of quinoxaline acceptor block modified with fluorinated conjugated side chain, but also achieves high Voc copolymers for polymer solar cells via the synergistic effect of fluorination and side chain engineering strategies.

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.

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...

Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration.

Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constantsthat governthe driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we concludethat changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.

W-Band ENDOR of Light-Induced PPerAcr Anion Radicals in Double-Crystalline Donor–Bridge–Acceptor P3HT-b-PPerAcr Block Copolymer in Frozen Solution: Experimental and DFT Study

Light-induced W-band electron nuclear double resonance and density functional theory of a spin density distribution study of poly-perylene bisimide acrylate (PPerAcr) anion radicals in double-crystalline donor–bridge–acceptor block copolymers (BCPs) are under consideration. Evidence of spin density sharing in the electron acceptor block, mainly distributed on the perylenediimide frame of PPerAcr without delocalization along the π-stack direction of the PPerAcr block in BCPs, was found at the low (50 K) temperature in frozen o-1,2-dichlorobenzene solution. The electron-hopping frequency between the anion and the nearby neutral PPerAcr of assembled π-stacks in P3HT-b-PPerAcr BCPs is limited by a frequency of around 108 Hz at temperatures below 50 K in the steady-state regime of continuous-wave light illumination.