Well-Defined All-Conducting Block Copolymer Bilayer Hybrid Nanostructure: Selective Positioning of Lithium Ions and Efficient Charge Collection

A block copolymerization of nonfunctionalized conducting monomers was developed to enable the successful synthesis of a highly insoluble 3,4-(ethylenedioxy)thienyl-based all-conducting block copolymer (PEDOT-<i>b</i>-PEDOT-TB) that could encapsulate nanocrystalline dyed TiO₂ particles, resulting in the formation of an all-conducting block copolymer bilayer hybrid nanostructure (TiO₂/Dye/PEDOT-<i>b</i>-PEDOT-TB). Lithium ions were selectively positioned on the outer PEDOT-TB surface. The distances through which the positively charged dye and PEDOT-TB(Li⁺) interacted physically or through which the TiO₂ electrode and the Li⁺ centers on PEDOT-TB(Li⁺) interacted ionically were precisely tuned and optimized within <i>ca.</i> 1 nm by controlling the thickness of the PEDOT blocking layer (the block length). The optimized structure provided efficient charge collection in an iodine-free dye-sensitized solar cell (DSC) due to negligible recombination of photoinduced electrons with cationic species and rapid charge transport, which improved the photovoltaic performance (η = 2.1 → 6.5%)

Effects of Regioregularity and Molecular Weight on the Growth of Polythiophene Nanofibrils and Mixes of Short and Long Nanofibrils To Enhance the Hole Transport

Morphological control over polythiophenes has been widely studied; however the impacts of regioregularity (RR) and molecular weight (MW) on their structural development have not been investigated systematically. This study examined a representative polythiophene, poly­(3-hexylthiophene) (P3HT), to reveal that small differences in the RR can produce a large difference in the growth of nanofibrils. Low-RR P3HTs generated neat long nanofibrils (LNFs), whereas high-RR P3HTs formed short nanofibrils (SNFs). This study identified a critical RR (96–98%) depending on their MW, below which P3HT grew into LNFs and above which P3HT grew into SNFs. This study also found that the mixing ratio between high-RR P3HT and a low-RR P3HT in the solution phase is strongly correlated with the relative populations of SNF and LNF in the coated film. This study suggested that mixing high-RR and low-RR polymers may be a good strategy to optimize the electrical properties of polythiophenes for target applications. As an example, a mixture of high-RR (75%) P3HT and low-RR P3HT (25%) improved considerably the power conversion efficiency of bulk heterojunction polymer solar cells compared with the values obtained from the pure high-RR P3HT and the pure low-RR P3HT

Well-Defined Nanostructured, Single-Crystalline TiO₂ Electron Transport Layer for Efficient Planar Perovskite Solar Cells

An electron transporting layer (ETL) plays an important role in extracting electrons from a perovskite layer and blocking recombination between electrons in the fluorine-doped tin oxide (FTO) and holes in the perovskite layers, especially in planar perovskite solar cells. Dense TiO₂ ETLs prepared by a solution-processed spin-coating method (S-TiO₂) are mainly used in devices due to their ease of fabrication. Herein, we found that fatal morphological defects at the S-TiO₂ interface due to a rough FTO surface, including an irregular film thickness, discontinuous areas, and poor physical contact between the S-TiO₂ and the FTO layers, were inevitable and lowered the charge transport properties through the planar perovskite solar cells. The effects of the morphological defects were mitigated in this work using a TiO₂ ETL produced from sputtering and anodization. This method produced a well-defined nanostructured TiO₂ ETL with an excellent transmittance, single-crystalline properties, a uniform film thickness, a large effective area, and defect-free physical contact with a rough substrate that provided outstanding electron extraction and hole blocking in a planar perovskite solar cell. In planar perovskite devices, anodized TiO₂ ETL (A-TiO₂) increased the power conversion efficiency by 22% (from 12.5 to 15.2%), and the stabilized maximum power output efficiency increased by 44% (from 8.9 to 12.8%) compared with S-TiO₂. This work highlights the importance of the ETL geometry for maximizing device performance and provides insights into achieving ideal ETL morphologies that remedy the drawbacks observed in conventional spin-coated ETLs

Gradated Mixed Hole Transport Layer in a Perovskite Solar Cell: Improving Moisture Stability and Efficiency

We demonstrate a simple and facile way to improve the efficiency and moisture stability of perovskite solar cells using commercially available hole transport materials, 2,2′,7,7′-tetrakis-(<i>N</i>,<i>N</i>-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) and poly­(3-hexylthiophene) (P3HT). The hole transport layer (HTL) composed of mixed spiro-OMeTAD and P3HT exhibited favorable vertical phase separation. The hydrophobic P3HT was more distributed near the surface (the air atmosphere), whereas the hydrophilic spiro-OMeTAD was more distributed near the perovskite layer. This vertical separation resulted in improved moisture stability by effectively blocking moisture in air. In addition, the optimized composition of spiro-OMeTAD and P3HT improved the efficiency of the solar cells by enabling fast intramolecular charge transport. In addition, a suitable energy level alignment facilitated charge transfer. A device fabricated using the mixed HTL exhibited enhanced performance, demonstrating 18.9% power conversion efficiency and improved moisture stability

Synthesis and Self-Assembly of Thiophene-Based All-Conjugated Amphiphilic Diblock Copolymers with a Narrow Molecular Weight Distribution

A series of amphiphilic poly­(3-hexylthiophene-<i>b</i>-3-(2-(2-{2-[2-(2-methoxy–ethoxy)-ethoxy]-ethoxy}-ethyl))­thiophene) (P­(3HT-<i>b</i>-3EGT)) polymers was synthesized via a nickel-catalyzed quasi-living polymerization. Size exclusion chromatograms (SEC) revealed that the molecular weight distributions of the <b>P3HT</b> blocks in the block copolymers were comparable with those of the polystyrene standard (monodisperse). ¹H NMR spectra revealed that the <b>P3HT</b> and <b>PEGT</b> units in the block copolymers were well-defined and did not form compositionally mixed regions at the boundary between the blocks and the highly regioregular <b>P3HT</b> units. The correlations among the block ratio, the amphiphilicity, and the self-assembled nanostructures of the block copolymers in thin films and in solution were examined. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies revealed that the crystallinity of the <b>BP93</b> composed of 93 mol % <b>P3HT</b> blocks was higher than the crystallinity of the <b>P3HT</b> alone due to the packing effects caused by repulsion among the hydrophobic hexyl and hydrophilic ethylene glycol oligomer side chains. A long relaxation time was required to observe the ordering among <b>P3HT</b> blocks in the <b>BP26</b> composed of 26 mol % P3HT blocks, suggesting that self-assembly could occur if induced on the molecular level. We demonstrated that the molecular-level self-assembly of <b>BP26</b> (at dilute concentrations) via a slow dialysis method produced highly ordered polymer vesicles 200–250 nm in size under thermodynamic control. The size could be tuned via competitive hydrophobic interactions using polystyrene. In contrast, kinetic control via a rapid precipitation method yielded 5–20 nm micelles

3,6-Carbazole Incorporated into Poly[9,9-dioctylfluorene-<i>alt</i>-(bisthienyl)benzothiadiazole]s Improving the Power Conversion Efficiency

A novel concept of D–A-type copolymers based on fluorene polymer incorporated with 3,6-carbazole unit enhances the device performance for organic photovoltaic cells. <b>P­(F</b>_<b>45</b><b>C</b>_<b>5</b><b>-DTBT)</b>, incorporating 5 mol % 3,6-carbazole into <b>P­(2,7F-DTBT)</b>, shows an almost 2-fold improvement (5.1%) in power conversion efficiency relative to <b>P­(2,7F-DTBT)</b> (2.6%). This results is ascribed to the good balance between electron and hole mobilities in the devices (μ_e/μ_h ∼ 1.8 for <b>P­(F</b>_<b>45</b><b>C</b>_<b>5</b><b>-DTBT)</b> vs 152 for <b>P­(2,7F-DTBT)</b>), and the formation of a nanoscale morphology in the blend of the copolymer and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM)

Role of Disorder in the Extent of Interchain Delocalization and Polaron Generation in Polythiophene Crystalline Domains

To understand how disorder within conjugated polymer aggregates influences the polaron generation process, we investigated poly­(3-hexylthiophene) (P3HT) and a congeneric random copolymer incorporating 33 mol % substituent-free thiophene units (RP33). Steady-state absorption and fluorescence spectra showed that increasing the intrachain torsional disorder in aggregates increases the energy and breadth of the density of states (DOS). By extracting polaron dynamics in the transient absorption spectra, we found that an activation energy barrier of 0.05 eV is imposed on the charge separation process in P3HT, whereas that in RP33 is essentially barrierless. We also found that a significant amount of excitons in P3HT are deactivated by traps, while no trapped excitons are generated in RP33. This efficient polaron generation in RP33 was attributed to the excess energy and enhanced interchain delocalization of precursor states provided by the intrachain torsional disorder and the close-packing structure in the absence of hexyl substituents

Morphological Control of Donor/Acceptor Interfaces in All-Polymer Solar Cells Using a Pentafluorobenzene-Based Additive

We report a pentafluorobenzene-based additive (FPE) to control the donor/acceptor (D/A) interfacial morphology via quadrupolar electrostatic interactions between donor and acceptor polymers in all-polymer solar cells (all-PSCs). The morphology changes are investigated using a combination of atomic force microscopy, grazing incidence wide-angle X-ray scattering, and near-edge X-ray absorption fine-structure spectroscopy. Unlike a conventional solvent additive, such as 1,8-diiodooctane, a bicontinuous interpenetrating morphology without large-scale phase separation and an enhanced π–π stacking with face-on orientation are found in the FPE processed blended films. These morphology changes improve the charge carrier extraction and charge transport between D/A interfaces to achieve an increase in the photovoltaic performance of all-PSCs

High-Field-Effect Mobility of Low-Crystallinity Conjugated Polymers with Localized Aggregates

Charge carriers typically move faster in crystalline regions than in amorphous regions in conjugated polymers because polymer chains adopt a regular arrangement resulting in a high degree of π–π stacking in crystalline regions. In contrast, the random polymer chain orientation in amorphous regions hinders connectivity between conjugated backbones; thus, it hinders charge carrier delocalization. Various studies have attempted to enhance charge carrier transport by increasing crystallinity. However, these approaches are inevitably limited by the semicrystalline nature of conjugated polymers. Moreover, high-crystallinity conjugated polymers have proven inadequate for soft electronics applications because of their poor mechanical resilience. Increasing the polymer chain connectivity by forming localized aggregates via π-orbital overlap among several conjugated backbones in amorphous regions provides a more effective approach to efficient charge carrier transport. A simple strategy relying on the density of random copolymer alkyl side chains was developed to generate these localized aggregates. In this strategy, steric hindrance caused by these side chains was modulated to change their density. Interestingly, a random polymer exhibiting low alkyl side chain density and crystallinity displayed greatly enhanced field-effect mobility (1.37 cm²/(V·s)) compared with highly crystalline poly­(3-hexylthiophene)

Requirements for Forming Efficient 3‑D Charge Transport Pathway in Diketopyrrolopyrrole-Based Copolymers: Film Morphology vs Molecular Packing

To achieve extremely high planarity and processability simultaneously, we have newly designed and synthesized copolymers composed of donor units of 2,2′-(2,5-dialkoxy-1,4-phenylene)­dithieno­[3,2-<i>b</i>]­thiophene (TT-P-TT) and acceptor units of diketopyrrolopyrrole (DPP). These copolymers consist of a highly planar backbone due to intramolecular interactions. We have systematically investigated the effects of intermolecular interactions by controlling the side chain bulkiness on the polymer thin-film morphologies, packing structures, and charge transport. The thin-film microstructures of the copolymers are found to be critically dependent upon subtle changes in the intermolecular interactions, and charge transport dynamics of the copolymer based field-effect transistors (FETs) has been investigated by in-depth structure–property relationship study. Although the size of the fibrillar structures increases as the bulkiness of the side chains in the copolymer increases, the copolymer with the smallest side chain shows remarkably high charge carrier mobility. Our findings reveal the requirement for forming efficient 3-D charge transport pathway and highlight the importance of the molecular packing and interdomain connectivity, rather than the crystalline domain size. The results obtained herein demonstrate the importance of tailoring the side chain bulkiness and provide new insights into the molecular design for high-performance polymer semiconductors