Balanced Charge Transport Research Articles

Zinc–Tin oxide/C8-BTBT hybrid heterostructures providing balanced ambipolar charge transport behaviors and their applications to pre-state-dependent inverter logic

Zinc–Tin oxide/C8-BTBT hybrid heterostructures providing balanced ambipolar charge transport behaviors and their applications to pre-state-dependent inverter logic

The impact of film deposition and annealing on the nanostructure and dielectric constant of organic semiconductor thin films.

The strategy of using a bulk-heterojunction light-absorbing layer has led to the most efficient organic solar cells. However, optimising the blend morphology to maximise light absorption, charge generation and extraction can be challenging. Homojunction devices containing a single component have the potential to overcome the challenges associated with bulk heterojunction films. A strategy towards this goal is to increase the dielectric constant of the organic semiconductor to ≈10, which in principle would lead to free charge carrier generation upon photoexcitation. However, the factors that affect the thin film dielectric constants are still not well understood. In this work we report an organic semiconductor material that can be solution processed or vacuum evaporated to form good quality thin films to explore the effect of chromophore structure and film morphology on the dielectric constant and other optoelectronic properties. 2,2'-[(4,4,4',4'-Tetrakis{2-[2-methoxyethoxy]ethyl}-4H,4'H-{2,2'-bi[cyclo-penta[2,1-b:3,4-b']dithiophene]}-6,6'-diyl)bis(methaneylylidene)]dimalononitrile [D(CPDT-DCV)] was designed to have high electron-affinity end groups and low ionisation-potential central moieties. It can be processed from solution or be thermally evaporated, with the film morphology changing from face-on to a herringbone arrangement upon solvent or thermal annealing. The glycol solubilising groups led to the static dielectric constant (taken from capacitance measurements) of the films to be between 6 and 7 (independent of processing conditions), while the optical frequency dielectric constant depended on the processing conditions. The less ordered solution processed film was found to have the lowest optical frequency dielectric constant of 3.6 at 2.0 × 1014 Hz, which did not change upon annealing. In contrast, the more ordered evaporated film had an optical frequency dielectric constant 20% higher at 4.2 and thermal annealing further increased it to 4.5, which is amongst the highest reported for an organic semiconductor at that frequency. Finally, the more ordered evaporated films had more balanced charge transport, which did not change upon annealing.

P- and N-dopable ambipolar bulk heterojunction thermoelectrics based on ladder-type conjugated polymers

All-polymer bulk heterojunction (BHJ) thermoelectrics with balanced charge transport were demonstrated for the first time. The BHJ blend can be p- or n-doped, and a full thermoelectric cell was constructed.

Strongly polarized surface electroluminescence from an organic light-emitting transistor

Highly emissive organic semiconductor that supports the combination of balanced ambipolar charge transport, weakened light waveguiding, and strongly polarized surface electroluminescence in organic light-emitting transistors is reported.

A Versatile and Low-Cost Polymer Donor Based on 4-Chlorothiazole for Highly Efficient Polymer Solar Cells.

Benefiting from the emergence of narrow-band-gap small-molecule acceptors (SMAs), especially "Y" series, the power conversion efficiency (PCE) of polymer solar cells (PSCs) is rapidly improved. However, polymer donors with high efficiency, easy synthesis, and good universality are relatively scarce except PBDB-TF and D18. Herein, two polymer donors are designed and synthesized based on 4-chlorothiazole derivatives with simple structures, namely PTz3Cl and PBTTz3Cl. The OSCs based on PBTTz3Cl with slightly weaker intermolecular forces in comparison to PTz3Cl exhibits a decent PCE of 18.38% in blending with SMA L8-BO, owing to its strong donor/acceptor interaction with L8-BO, which shapes suitable phase separation morphology. Further research finds that PBTTz3Cl can exhibit excellent photovoltaic performances with various SMA materials, highlighting its universality. Based on this, ternary PSCs are designed where BTP-eC9 is introduced as a guest into the PBTTz3Cl:L8-BO host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 19.12%, which is among the highest values for PSCs. This work provides a new design of low-cost electron-deficient units for constructing highly versatile, high-performance polymer donors.

Simultaneous Optimization of Efficiency, Stretchability, and Stability in All‐Polymer Solar Cells via Aggregation Control†

Comprehensive SummaryWith the emergence of Y‐series small molecule acceptors, polymerizing the small molecule acceptors with aromatic linker units has attracted significant research attention, which has greatly advanced the photovoltaic performance of all‐polymer solar cells. Despite the rapid increase in efficiency, the unique characteristics (e. g., mechanical stretchability and flexibility) of all‐polymer systems were still not thoroughly explored. In this work, we demonstrate an effective approach to simultaneously improve device performance, stability, and mechanical robustness of all‐polymer solar cells by properly suppressing the aggregation and crystallization behaviors of polymerized Y‐series acceptors. Strikingly, when introducing 50 wt% PYF‐IT (a fluorinated version of PY‐IT) into the well‐known PM6:PY‐IT system, the all‐polymer devices delivered an impressive photovoltaic efficiency of 16.6%, significantly higher than that of the control binary cell (15.0%). Compared with the two binary systems, the optimal ternary blend exhibits more efficient charge separation and balanced charge transport accompanying with less recombination. Moreover, a high‐performance 1.0 cm2 large‐area device of 15% efficiency was demonstrated for the optimized ternary all‐polymer blend, which offered a desirable PCE of 14.5% on flexible substrates and improved mechanical flexibility after bending 1000 cycles. Notably, these are among the best results for 1.0 cm2 all‐polymer OPVs thus far. This work also heralds a bright future of all‐polymer systems for flexible wearable energy‐harvesting applications.

TCTA:Ir(ppy)3 Green Emissive Blends in Organic Light-Emitting Transistors (OLETs).

Organic light-emitting transistors are photonic devices combining the function of an electrical switch with the capability of generating light under appropriate bias conditions. Achieving high-performance light-emitting transistors requires high-mobility organic semiconductors, optimized device structures, and highly efficient emissive layers. In this work, we studied the optoelectronic response of green blends (TCTA:Ir(ppy)3) with varying doping concentrations in the limit of field-effect within a transistor device configuration. Increasing the dye concentration within the blend leads to a quenching of the photoluminescence signal; however, when implemented in a multilayer stack in a transistor, we observed an approximately 5-fold improvement in the light output for a 10% Ir(ppy)3 doping blend. We analyzed our results in terms of balanced charge transport in the emissive layer, which, in the limit of field-effect (horizontal component), leads to an improved exciton formation and decay process. While the performances of our devices are yet to achieve the state-of-the-art diode counterpart, this work demonstrates that engineering the emissive layer is a promising approach to enhance the light emission in field-effect devices. This opens the way for a broader exploitation of organic light-emitting transistors as alternative photonic devices in several fields, ranging from display technology to flexible and wearable electronics.

Benzotriazole‐Based Polymer Acceptor for High‐Efficiency All‐Polymer Solar Cells with High Photocurrent and Low Voltage Loss

AbstractThe power conversion efficiencies (PCEs) of all‐polymer solar cells (all‐PSCs) have already exceeded 17%. However, the limited absorption range of an all‐polymer system results in significantly reduced short‐circuit current density (Jsc), which eventually influences the PCE improvement. To broaden the light absorption of polymer acceptors, herein, benzotriazole is introduced in the core unit of small molecule acceptors and thus two narrow‐bandgap polymer acceptors named PTz‐BO and PTz‐C11 featuring the same molecular backbone and different side‐chain length are synthesized. Compared with PTz‐C11, the PTz‐BO based‐all PSCs deliver a slightly reduced Jsc, a large open‐circuit voltage (Voc) and a low voltage loss below 0.50 V. Moreover, ternary all‐PSCs are constructed by introducing PTz‐C11 as a guest component. Benefiting from the reduced recombination, improved exciton generation and dissociation, and balanced charge transport, a high efficiency of 16.58% is obtained for the ternary all‐PSCs, with a high Jsc over 25 mA cm−2 without sacrificing the Voc. Such result represents the highest efficiency reported for benzotriazole‐based all‐PSCs in the literature thus far. This work demonstrates the great potential of benzotriazole for the synthesis of efficient narrow‐bandgap polymer acceptors.

2D MXene Additive‐Induced Treatment Enabling High‐Efficiency Indoor Organic Photovoltaics

AbstractThe surge of Internet‐of‐everything applications over the past decade demands the adoption of novel material design and device engineering strategies for the development of state‐of‐the‐art organic photovoltaics (OPVs) in low‐light indoor environments. Owing to their excellent optoelectronic properties, two‐dimensional MXenes possess outstanding potential in this regard. Herein, an unprecedented indoor power conversion efficiency (PCE) of 33.8% under light‐emitting‐diode (LED) illumination (1000‐lx) is secured by additive‐induced treatment of MXene in polymer‐donor:non‐fullerene‐acceptor‐based organic photoactive layer. The remarkable indoor performance of MXene OPVs mainly originates from the enhanced absorption, compact molecular packing, and smooth surface morphology with a reduced number of grain boundaries in the photoactive layer, resulting in an improved fill factor and balanced charge transport and extraction characteristics with suppressed recombination, thereby producing an impressive indoor PCE. In addition, the presence of MXene in the photoactive layer facilitates polaron‐pair dissociation owing to improved free‐charge generation, leading to enhanced photoconductivity. This performance represents the highest PCE among the OPVs measured under indoor illumination. This work highlights the promising prospect of 2D MXene and its composites for indoor light energy harvesting applications.

Fabrication of ZnO dual electron transport layer via atomic layer deposition for highly stable and efficient CsPbBr3 perovskite nanocrystals light-emitting diodes

Electron transport layers (ETLs) are important components of high-performance all-inorganic perovskite nanocrystals light-emitting diodes (PNCs-LED). Herein, atomic layer deposition (ALD) of inorganic ZnO layer is combined to the organic 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) to form dual ETLs to enhance both the efficiency and stability of PNCs-LED simultaneously. Optimization of ZnO thickness suggested that 10 cycles ALD yields the best performance of the devices. The external quantum efficiency of the device reaches to 7.21% with a low turn-on voltage (2.4 V). Impressively, the dual ETL PNCs-LED realizes maximum T 50 lifetime of 761 h at the initial luminance of 100 nit, which is one of the top lifetimes among PNCs-LEDs up to now. The improved performance of dual ETL PNCs-LED is mainly due to the improved charge transport balance with favorable energy level matching. These findings present a promising strategy to modify the function layer via ALD to achieve both highly efficient and stable PNCs-LED.

High-performance scalable organic photovoltaics with high thickness tolerance from 1 cm2 to above 50 cm2

<h2>Summary</h2> To realize a high-performance large-area organic photovoltaic (OPV) module using printing technology, it is important to produce a uniform and an optimal bulk heterojunction (BHJ) morphology over a large area with a high thickness tolerance. We developed PBDB-T-2F:N3:P(NDI2OD-T2) BHJ film, where P(NDI2OD-T2) forms intricate channels in the donor domain and induces improved exciton dissociation, balanced charge transport, and less charge recombination. Moreover, P(NDI2OD-T2) is effective for forming an optimal morphology with uniform nanocrystallites of the polymer donor over a large area in both cases of optimal and thick active films. Consequently, the patterned-blade-coating OPV module device with a 58.5 cm² active area showed an improved efficiency of 14.04%, compared with 12.59% for the control device, which is among the highest efficiencies reported in OPVs with active areas above 50 cm². Notably, the corresponding module prepared using a scribing method achieved an efficiency of 13.14% with a geometric fill factor of 91.82% from a 22.44 cm² area.

Improving the Molecular Packing Order and Vertical Phase Separation of the P3HT:O-IDTBR Blend by Extending the Crystallization Period of O-IDTBR.

The morphology with strong molecular packing order and gradient vertical composition distribution associated with efficient charge transport and collection is critical to achieve high performance in nonfullerene solar cells. However, the rapid solidification process of the active layer upon the fast removal of solvent usually results in a kinetically trapped state with undesired morphology. Herein, we proposed a strategy to extend the crystal growth time of the acceptor via a high-boiling-point additive that selectively dissolved the acceptor. This was enabled by adding dibenzyl ether (DBE) to the poly(3-hexylthiophene) (P3HT):O-IDTBR blend in chlorobenzene (CB) solution. The combination of the kinetic study by time-resolved ultraviolet-visible (UV-vis) absorption spectra and detailed morphological characterization allows us to correlate the crystallization kinetics with the microstructural transition. The results show that the crystal growth time of O-IDTBR increases from 3 to 60 s upon the addition of 0.75% DBE, leading to further evolution of the molecular order of O-IDTBR during the DBE-dominated drying period. Meanwhile, O-IDTBR has more time to migrate toward the substrate owing to the larger surface energy. In addition, the onset of the crystallization process of P3HT is brought forward from 8 to 6 s due to the reduced solvent quality, which favors P3HT to crystallize into a fibril network. As a result, an optimized morphology that features the enhanced molecular packing order of P3HT and O-IDTBR as well as the vertical compositional gradient of O-IDTBR is obtained. Devices based on the optimized blend show more balanced charge transport and suppressed bimolecular recombination, giving rise to an improved power conversion efficiency (PCE) from 4.29 ± 0.04 to 7.30 ± 0.12%.

High‐Performance Perovskite Photovoltaics by Heterovalent Substituted Mixed Perovskites

Abstract2D perovskites are relatively stable but possess poor charge transport compared to 3D perovskites. To boost charge transport, novel 2D perovskites mixed with 3D perovskites are developed, where Pb2+ are partially substituted by the heterovalent neodymium cations (Nd3+) within both 2D and 3D perovskites (termed Nd3+‐substituted 2D:3D mixed perovskites. Systematical studies reveal that the Nd3+‐substituted 2D:3D mixed perovskites possess larger crystals, superior crystallinity, suppressed non‐radiative charge recombination, and enhanced and balanced charge transport compared to the 2D:3D mixed perovskites. As a result, perovskite photovoltaics based on the Nd3+‐substituted 2D:3D mixed perovskites exhibit a power conversion efficiency of 22.11%, a photoresponsibility of over 700 mA W−1, a photodetectivity of 4.29 × 1014 cm Hz1/2 W−1, a linear dynamic range of 165 dB at room temperature, and dramatically boosted stability. These results demonstrate that, a facile way is developed to realize high‐performance perovskite photovoltaics through partially heterovalent substituted Pb2+ by Nd3+ within 2D:3D mixed perovskites.

Multicomponent Solar Cells with High Fill Factors and Efficiencies Based on Non-Fullerene Acceptor Isomers.

Multicomponent organic solar cells (OSCs), such as the ternary and quaternary OSCs, not only inherit the simplicity of binary OSCs but further promote light harvesting and power conversion efficiency (PCE). Here, we propose a new type of multicomponent solar cells with non-fullerene acceptor isomers. Specifically, we fabricate OSCs with the polymer donor J71 and a mixture of isomers, ITCF, as the acceptors. In comparison, the ternary OSC devices with J71 and two structurally similar (not isomeric) NFAs (IT-DM and IT-4F) are made as control. The morphology experiments reveal that the isomers-containing blend film demonstrates increased crystallinity, more ideal domain size, and a more favorable packing orientation compared with the IT-DM/IT-4F ternary blend. The favorable orientation is correlated with the balanced charge transport, increased exciton dissociation and decreased bimolecular recombination in the ITCF-isomer-based blend film, which contributes to the high fill factor (FF), and thus the high PCE. Additionally, to evaluate the generality of this method, we examine other acceptor isomers including IT-M, IXIC-2Cl and SY1, which show same trend as the ITCF isomers. These results demonstrate that using isomeric blends as the acceptor can be a promising approach to promote the performance of multicomponent non-fullerene OSCs.

Effect of cyano substituent on the functional properties of blue emitting Imidazo[1,2-a]pyridine derivatives

High fluorescence efficiency and balanced charge transportation in organic emitters are desirable characteristics for application in OLEDs. Generally, donor-acceptor compounds showing pronounced intramolecular charge transfer exhibit low fluorescence quantum yield. Here, we report D-A fluorophores containing CN-substituted 2-phenylimidazo[1,2-a]pyridine acceptor unit which exhibit good fluorescence characteristics. The positions of donors (triphenylamine/carbazole) and cyano substituents are respectively interchanged to get a series of isomers. All the fluorophores exhibit high fluorescence quantum yield (Фf≥ 0.70) and excellent thermal stability (Td10≥ 390 °C). A dye comprising triphenylamine unit at para-position of C2 phenyl ring exhibited red-shifted emission maxima with larger Stokes shift in the series. Moreover, all the isomers displayed excited-state solvatochromism attributable to the photo-induced intramolecular charge transfer between donors and cyano-imidazo[1,2-a]pyridine acceptor. Addition of the cyano group on imidazopyridine restrained the formation of aggregates in the solid-state and stabilized the lowest unoccupied molecular orbital. Among the all, doped devices constructed with 3 wt% of triphenylamine containing dye revealed excellent performance in the series with a maximum EQE of 6.9%, 2759 cd/m2 luminance, and CIE coordinates (0.16, 0.08) which is very close to the NTSC blue standard.

Nonfullerene acceptors with cyano-modified terminal groups for organic solar cells: Effect of substitution position on photovoltaic properties

Two kinds of nonfullerene acceptors based on cyano-modified 1,1-dicyanomethylene-3-indanone (IC) as end groups and the dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (BTP) unit as a core are developed. Both electron acceptors exhibit strong intramolecular charge transfer absorption with the maximum absorption peak at 832 nm, and the absorption edge extends to 953 nm, corresponding to the narrow bandgap down to 1.30 eV. Comparing with the electron acceptor with cyano groups at mixed γ/δ position, the acceptor with cyano substituted at the β position shows more twisted structure and slightly higher energy level. When a wide bandgap polymer PM6 is adopted as the donor, the blend film with β-cyano substituted acceptor shows more suitable phase separation morphology and more balanced charge transport. The device based on the β-cyano substituted acceptor achieves a larger open-circuit voltage (VOC) of 0.88 V than the acceptor with cyano groups at mixed γ/δ position (VOC = 0.78 V), and the energy loss was calculated to be 0.45 eV, leading to a higher power conversion efficiency (PCE) of 12.41% comparing with the devices with the γ/δ-cyano substituted acceptor (PCE = 11.08%).

Improved Performance of Organic Solar Cells by Utilizing Green Non‐Halogen Additive to Modulate Active‐Layer Morphology

Using solvent additives to optimize the morphology of the blend films in organic solar cells (OSCs) is a simple and effective method. Here, methyl salicylate (MeSA) is used as a non‐halogen additive for inverted OSCs, and the impact of this additive on the blend film and photovoltaic performance is carefully investigated. The significant increase in short‐circuit current density (JSC) and fill factor (FF) leads to a significant improvement in device performance, which is caused by bicontinuous interpenetrating phase separation and balanced charge transport. The results indicate that MeSA modulates the phase distribution and promotes the accumulation of ordered molecules in the blend film, thus exhibiting an efficiency of 9.45% and improved FF (&gt;70%) with a 7% MeSA additive. Most importantly, MeSA can be added in large doses (7%) compared to other traditional solvent additives (e.g., 1,8‐diiodooctane, 1‐chloronaphthalene, etc.), indicating that its concentration variation has little effect on performance and is conducive to repeatable, large‐scale production, which is of great importance for industrialization.

Binary Organic Solar Cells Breaking 19% via Manipulating the Vertical Component Distribution.

The variation of the vertical component distribution can significantly influence the photovoltaic performance of organic solar cells (OSCs), mainly due to its impact on exciton dissociation and charge-carrier transport and recombination. Herein, binary devices are fabricated via sequential deposition (SD) of D18 and L8-BO materials in a two-step process. Upon independently regulating the spin-coating speeds of each layer deposition, the optimal SD device shows a record power conversion efficiency (PCE) of 19.05% for binary single-junction OSCs, much higher than that of the corresponding blend casting (BC) device (18.14%). Impressively, this strategy presents excellent universality in boosting the photovoltaic performance of SD devices, exemplified by several nonfullerene acceptor systems. The mechanism studies reveal that the SD device with preferred vertical components distribution possesses high crystallinity, efficient exciton splitting, low energy loss, and balanced charge transport, resulting in all-around enhancement of photovoltaic performances. This work provides a valuable approach for high-efficiency OSCs, shedding light on understanding the relationship between photovoltaic performance and vertical component distribution.

Asymmetric Tris-Heteroleptic Cyclometalated Phosphorescent Iridium(III) Complexes: An Emerging Class of Metallophosphors

ConspectusThe rapid developments of cutting-edge research on photofunctional organic semiconductor materials are greatly promoting the progress of science and technology. As one of the most important organic semiconductor materials, phosphorescent iridium(III) complexes are a promising class of organometallic emitters because of their rich emissive excited states together with excellent chemical stability, which have been widely used in various fields, such as organic electroluminescence, solar cells, photocatalysis, biosensing, bioimaging, cancer therapy, etc. The exploration of highly efficient phosphorescent iridium(III) complexes showing various structural features is blooming quickly. In general, the traditional iridium(III) phosphors usually contain at least two identical cyclometalating bidentate ligands. These iridophosphors with two identical bidentate ligands are termed as the bis-heteroleptic complex Ir((L1)2L2), where L denotes the free ligand or the deprotonated form of the free ligand. Those with three identical ligands are called homoleptic complex Ir(L)3. Recently, the iridium(III) complexes Ir(L1L2L3) supported by three different (cyclometalating) ligands are emerging as a novel and interesting category of phosphors. This class of iridophosphors usually refers to tris-heteroleptic cyclometalated phosphorescent iridium(III) complexes, and they usually show the asymmetry in their molecular structures. Compared with the case for the traditional iridium(III) phosphors, the independent ligand control provides tris-heteroleptic iridium(III) phosphors with more flexible molecular design/synthesis and excited-state fine-tuning ability, thereby resulting in more rich and appealing excited states and potential multifunctional applications.In this Account, we will highlight our recent efforts on the asymmetric tris-heteroleptic cyclometalated iridium(III) phosphors including the molecular design strategies, chemical synthesis, excited-state tuning, and structure–property relationships as well as their applications, especially in organic electroluminescence. Specifically, the molecular design of tris-heteroleptic iridium(III) phosphors focuses on the independent ligand design including the following three aspects: (1) the substituent functionalization engineering (SFE); (2) the ligand skeleton engineering (LSE); (3) the double metalation engineering (DME). For SFE, we mainly introduce the substituent based on the main-group element into the ligand of iridophosphors and also investigate the influence of the substituent position on the emissive excited states of iridophosphors. For instance, the main-group elements show unique electronic effects, which could contribute to the manipulation of the photoelectric properties of complexes (e.g., balanced charge transport ability, improved utilization of excitons, etc.). For LSE, different types of aromatic skeleton or various lengths of π conjugation of the ligands are also examined for the effective tuning of their excited states. For DME, the type and spatial orientation of the bridging/franking ligand will be considered. At the same time, the related optoelectronic applications (e.g., electroluminescence, optical power limiting, etc.) of this novel class of versatile iridophosphors are also discussed. Finally, we give some perspectives on this fascinating topic and also try to provide some potential research opportunities based on the current stage of asymmetric tris-heteroleptic cyclometalated phosphorescent iridium(III) complexes. It is believed that the emerging asymmetric tris-heteroleptic cyclometalated iridophosphors will open an important avenue for designing novel metallophosphor-based materials with tunable and appealing photophysical properties, thus offering new probabilities for potential multifunctional applications.

Non-fullerene acceptors with alkoxy side chains and different halogenated end groups for efficient organic solar cells

Two polycyclic non-fullerene acceptors BTMe-OEH-2F and BTMe-OEH-2Cl were constructed by connecting the fused ring core with 4-(2-ethylhexyloxy) phenyl side chain to two kind of end groups 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2F-IC) and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2Cl-IC), respectively. Compared with BTMe-OEH-2F, BTMe-OEH-2Cl showed a more red-shifted absorption and lower energy level. When the two non-fullerene acceptors were blended with the donor PM6, a fiber distribution and continuous nanophase separation can be observed in the PM6:BTMe-OEH-2Cl blend film, and more balanced charge transport and less charge recombination were exhibited in the organic solar cell (OSC) device based on BTMe-OEH-2Cl, which achieved a higher PCE of 13.23% with a Voc of 0.98 V, a Jsc of 18.76 mA cm−2 and a FF of 72.23%. These results proved that the synergistic effect between side chains and end groups on the properties of fused-ring electron acceptors (FREAs) can be exerted by choosing appropriate end groups to match with alkoxy side chains.