Molecular Orientation and Phase Separation by Controlling Chain Segment and Molecule Movement in P3HT/N2200 Blends

Domain size control in all-polymer solar cells

In all polymer solar cells (all-PSCs), the domain size is critical for device performance. In highly crystalline polymer blends, however, precisely adjusting the domain size remains a significant challenge because of the simultaneous crystallization of both components. Herein, a strategy that promotes acceptor and donor to crystallize separately was proposed. Take PBDB-T/N2200 blends for instance; the solution state and confined crystallization were combined, which induced the crystallization of N2200, and PBDB-T occurred during the film-forming process and at thermal annealing stage. This separated crystallization process lowers the driving force of phase separation without affecting the degree of crystallinity of the blends. Thus, an interpenetrating network with high crystallinity and proper domain size was obtained, which boosted the power conversion efficiency to 7.59%. Importantly, the relation between pre-aggregation and domain size was established, which may be a guide to precisely adjust the active layer’s domain size in all-PSCs.

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This strategy decreases domain size without sacrificing crystallinity

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A phase diagram about solution state and domain size was proposed

Fabrication of PCDTBT Conductive Network via Phase Separation

Poly[N-9'-hepta-decanyl-2,7-carbazole-alt-5-5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) is a stable semiconducting polymer with high rigidity in its molecular chains, which makes it difficult to organize into an ordered structure and affects the device performance. Here, a PCDTBT network consisting of aggregates and nanofibers in thin films was fabricated through the phase separation of mixed PCDTBT and polyethylene glycol (PEG). Using atomic force microscopy (AFM), the effect of the blending conditions (weight ratio, solution concentration, and molecular weight) and processing conditions (substrate temperature and solvent) on the resulting phase-separated morphologies of the blend films after a selective washing procedure was studied. It was found that the phase-separated structure's transition from an island to a continuous structure occurred when the weight ratio of PCDTBT/PEG changed from 2:8 to 7:3. Increasing the solution concentration from 0.1 to 3.0 wt% led to an increase in both the height of the PCDTBT aggregate and the width of the nanofiber. When the molecular weight of the PEG was increased, the film exhibited a larger PCDTBT aggregate size. Meanwhile, denser nanofibers were found in films prepared using PCDTBT with higher molecular weight. Furthermore, the electrical characteristics of the PCDTBT network were measured using conductive AFM. Our findings suggest that phase separation plays an important role in improving the molecular chain diffusion rate and fabricating the PCDTBT network.

Solution-State Long-Range Molecular Ordering in Poly(3-hexylthiophene)

A blend of poly(3-hexylthiophene) (P3HT) and poly(n-hexyl isocyanate-block-2-vinylpyridine) (PHIC-b-P2VP) in a common solvent shows the formation of long-range (micrometer-scale) nanowires of P3HT through hydrophobic interactions between the hexyl arms of P3HT and PHIC in a parallel way, which increase the planarity that leads to the generation of vibration bands with a lower free exciton bandwidth (W = 67 meV) in the solution state, which is further decreased to 9 meV after 48 h annealing of the blend film. The resulting nanowires of the P3HT show a 100-fold increase in current in comparison to pristine P3HT.

Phase Behavior of Amorphous/Semicrystalline Conjugated Polymer Blends

We report the phase behavior of amorphous/semicrystalline conjugated polymer blends composed of low bandgap poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene) -alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and poly{(N,N′-bis(2-octyldodecyl)naphthalene -1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)). As usual in polymer blends, these two polymers are immiscible because ΔS_m ≈ 0 and ΔH_m > 0, leading to ΔG_m > 0, in which ΔS_m, ΔH_m, and ΔG_m are the entropy, enthalpy, and Gibbs free energy of mixing, respectively. Specifically, the Flory–Huggins interaction parameter (χ) for the PCPDTBT /P(NDI2OD-T2) blend was estimated to be 1.26 at 298.15 K, indicating that the blend was immiscible. When thermally analyzed, the melting and crystallization point depression was observed with increasing PCPDTBT amounts in the blends. In the same vein, the X-ray diffraction (XRD) patterns showed that the π-π interactions in P(NDI2OD-T2) lamellae were diminished if PCPDTBT was incorporated into the blends. Finally, the correlation of the solid-liquid phase transition and structural information for the blend system may provide insight for understanding other amorphous/semicrystalline conjugated polymers used as active layers in all-polymer solar cells, although the specific morphology of a film is largely affected by nonequilibrium kinetics.

Optimizing the Phase-Separated Domain Size of the Active Layer via Sequential Crystallization in All-Polymer Solar Cells

The proper domain size of the active layer plays a key role in determining the exciton dissociation and charge transport in all-polymer solar cells (all-PSCs). However, fine-tuning the domain size remains challenging due to low glass transition temperature and negligible mixing entropy in polymer blends. Herein, we systematically studied the influence of "crystallization kinetics" on the domain size and proposed that if the donor and acceptor crystallize simultaneously, they are prone to form a large domain, while if sequential crystallization of the donor and acceptor occurs, a fine phase separation structure with the proper domain size can be obtained. Taking PBDB-T/PNDI blends for instance, the domain size was decreased by using sequential crystallization; meanwhile, the crystallinity and molecular orientation were also optimized, boosting the power conversion efficiency from 6.55% to 7.78%. This work provides a novel way to finely tune the heterojunction phase separation structures.

Phase Diagrams of n-Type Low Bandgap Naphthalenediimide-Bithiophene Copolymer Solutions and Blends

Phase diagrams of n-type low bandgap poly{(N,N′-bis(2-octyldodecyl)naphthalene -1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′,-(2,2′-bithiophene)} (P(NDI2OD-T2)) solutions and blends were constructed. To this end, we employed the Flory–Huggins (FH) lattice theory for qualitatively understanding the phase behavior of P(NDI2OD-T2) solutions as a function of solvent, chlorobenzene, chloroform, and p-xylene. Herein, the polymer–solvent interaction parameter (χ) was obtained from a water contact angle measurement, leading to the solubility parameter. The phase behavior of these P(NDI2OD-T2) solutions showed both liquid–liquid (L–L) and liquid–solid (L–S) phase transitions. However, depending on the solvent, the relative position of the liquid–liquid phase equilibria (LLE) and solid–liquid phase equilibria (SLE) (i.e., two-phase co-existence curves) could be changed drastically, i.e., LLE > SLE, LLE ≈ SLE, and SLE > LLE. Finally, we studied the phase behavior of the polymer–polymer mixture composed of P(NDI2OD-T2) and regioregular poly(3-hexylthiophene-2,5-dyil) (r-reg P3HT), in which the melting transition curve was compared with the theory of melting point depression combined with the FH model. The FH theory describes excellently the melting temperature of the r-reg P3HT/P(NDI2OD-T2) mixture when the entropic contribution to the polymer–polymer interaction parameter (χ = 116.8 K/T − 0.185, dimensionless) was properly accounted for, indicating an increase of entropy by forming a new contact between two different polymer segments. Understanding the phase behavior of the polymer solutions and blends affecting morphologies plays an integral role towards developing polymer optoelectronic devices.

Efficient Nonhalogenated Solvent-Processed Ternary All-Polymer Solar Cells with a Favorable Morphology Enabled by Two Well-Compatible Donors

The rational design of the morphology of ternary all-polymer solar cells (all-PSCs) having broadened photon harvesting is crucial to achieve high device performance. However, multicomponent blends often illustrated an unfavorable morphology such as large-sized phase separation due to their complicated interaction. Herein, we proposed to solve these problems by employing two donors with good miscibility (J51 and PTB7-Th), which also have similar compatibility with the acceptor (N2200). The resultant ternary blend films of J51:PTB7-Th:N2200 feature a uniform phase separation morphology due to the reduced competitive effect of intermolecular interactions. As an additional polymer donor, PTB7-Th could not only enhance the absorption of the binary blend but also act as a crystallization regulator to boost the face-on orientation in ternary blends. Accordingly, the J51:PTB7-Th:N2200 ternary blends exhibited improved sunlight absorption and higher and well-balanced carrier mobility accompanied by enhanced carrier extraction. With the nonhalogenated cyclopentyl methyl ether as the processing solvent, the ternary all-PSCs showed outstanding power conversion efficiency (PCE) higher than 9% when varying the PTB7-Th weight ratio in donors from 20 to 50%. Due to the PTB7-Th content holding a 30% weight ratio in donors, the ternary all-PSCs demonstrated the optimal PCE of 9.60%, which perform better than those of binary all-PSCs (PCE = 7.58 or 5.63%).

Efficient and thermally stable organic solar cells based on small molecule donor and polymer acceptor

Efficient organic solar cells (OSCs) often use combination of polymer donor and small molecule acceptor. Herein we demonstrate efficient and thermally stable OSCs with combination of small molecule donor and polymer acceptor, which is expected to expand the research field of OSCs. Typical small molecule donors show strong intermolecular interactions and high crystallinity, and consequently do not match polymer acceptors because of large-size phase separation. We develop a small molecule donor with suppressed π-π stacking between molecular backbones by introducing large steric hindrance. As the result, the OSC exhibits small-size phase separation in the active layer and shows a power conversion efficiency of 8.0%. Moreover, this OSC exhibits much improved thermal stability, i.e. maintaining 89% of its initial efficiency after thermal annealing the active layer at 180 °C for 7 days. These results indicate a different kind of efficient and stable OSCs.

Controlling the pore size in conjugated polymer films via crystallization-driven phase separation

A wide range of possible applications in sensors and optoelectronic devices have focused considerable attention on porous membranes made of semi-conducting polymers. In this study, porous films of poly(3-hexylthiophene) (P3HT) were conveniently constructed through spin-coating of solutions of a blend of P3HT and polyethylene glycol (PEG). Pores were formed by phase separation driven simultaneously by incompatibility and crystallization. The influence of the polymer concentration (c), molecular weight (Mn) and spin-coating temperature (Tsp) on the pore size and structure was investigated. With increasing c from 0.5 to 5.0 wt%, the pore diameter (d) varied from ≈1.3 μm to ≈38 μm. Similarly, we observed a substantial increase of d with increasing Mn of PEG, while changing Mn of P3HT did not affect d. Micron- and nano-scale pores coexisted in porous P3HT films. While incompatibility of P3HT and PEG caused the formation of nano-pores, micron-scale pores resulted from crystallization in the PEG-rich domains by forcing PEG molecules to diffuse from the surrounding PEG-P3HT blend region to the crystal growth front.

Solution-Processed Nonvolatile Organic Transistor Memory Based on Semiconductor Blends

Solution-processed nonvolatile organic transistor memory devices are fabricated by employing semiconductor blends of p-channel 6,13-bis(triisopropylsilylethynyl)pentacene and n-channel poly{[ N, N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-2T); N2200) on polystyrene-brush as a polymer electret. Electret-based memory characteristics are significantly changed depending on the frontier molecular orbitals of the active semiconductors because the charge-trapping efficiency is mainly determined by the energy barrier to transfer electrons and holes from the active channel to the electret layer. A semiconductor mixture with an optimized blending ratio results in an efficient programming and erasing process. Thus, we obtained a remarkably high ratio of ON/OFF current (memory ratio) about 10⁷ and a large amount of shifts in the threshold voltage (memory window) between the programmed and erased states of 55 V, while single-component N2200 showed only writing-once-read-many (WORM)-type memory. Especially, the programmed data can be stably retained more than 10 years with a sufficient memory ratio of 10³. Furthermore, our semiconductor blend system leads to preferable vertical phase separation, which affords good reliability under a sequential memory operation condition as well as stability in ambient air. It is expected that our memory devices can be applied for versatile data storage in printed and flexible electronic applications.

Modulation of the Molecular Orientation at the Bulk Heterojunction Interface via Tuning the Small Molecular Donor-Nonfullerene Acceptor Interactions

The interfacial molecular packing orientation of the nonfullerene systems at the donor-acceptor interface is considered as one of the key parameters in fabricating high-performance devices because of the anisotropic molecular characteristics of conjugated donors (D) and nonfullerene acceptors (A). However, regulating the interfacial orientation for the nonfullerene systems is still scarcely studied. Herein, modulation of the interfacial molecular packing orientation of bulk heterojunction layer is successfully realized via tuning the D-A interactions. The results indicate that the molecule with relatively shorter alkyl side chain (2F-C₄C₆) because of weak D-A interactions is unable to influence the molecular orientation of the active layer, as compared to their longer alkyl side-chain counterpart (2F-C₆C₈), which demonstrates strong D-A interactions and thus efficiently modulates the overall packing orientation. The power conversion efficiencies of 6.41 and 8.23% are obtained for the relatively short and long alkyl side-chain donors with IDIC acceptor, respectively. Hence strong D-A interactions because of long enough alkyl side chain on a donor small molecule can modify the interfacial molecular packing orientation of the system, leading to a better performing device.

Temperature controlled tensiometry using droplet microfluidics

We develop a temperature controllable microfluidic device for the accurate measurement of temperature dependent interfacial tensions between two immiscible liquids. A localized temperature control system is integrated with the microfluidic platform to maintain an accurate temperature inside the device. The temperature uniformity and sensitivity are verified by both simulation and experimental results. Temperature dependent interfacial tensions are measured dynamically and rapidly, relying on quantitative analysis of the deformation and retraction dynamics of droplets under extensional flow. Our microfluidic tensiometry offers the capability of measuring temperature dependent interfacial tensions with precise and systematic temperature control in the range of room temperature to 70 °C, which is valuable for studying transient interfacial dynamics, interfacial reactions, and the surfactant adsorption process.

New Wide Band Gap Donor for Efficient Fullerene-Free All-Small-Molecule Organic Solar Cells

A new organic small molecule, DRTB-T, that incorporates a two-dimensional trialkylthienyl-substituted benzodithiophene core building block was designed and synthesized. DRTB-T has a band gap (E_g^opt) of 2.0 eV with a low-lying highest occupied molecular orbital (HOMO) level of -5.51 eV. Nonfullerene small-molecule solar cells consisting of DRTB-T and a nonfullerene acceptor (IC-C6IDT-IC) were constructed, and the morphology of the active layer was fine-tuned by solvent vapor annealing (SVA). The device showed a record 9.08% power conversion efficiency (PCE) with a high open-circuit voltage (V_oc = 0.98 V). This is the highest PCE for a nonfullerene small-molecule organic solar cell (NFSM-OSC) reported to date. Our notable results demonstrate that the molecular design of a wide band gap (WBG) donor to create a well-matched donor-acceptor pair with a low band gap (LBG) nonfullerene small-molecule acceptor, as well as subtle morphological control, provides great potential to realize high-performance NFSM-OSCs.