•We detailedly investigated the effect of the nucleating agent of donor material on the performance parameters of P3AT/PCBM BHJ OPV cells and explore the mechanism. We find that the addition of MDBS (nucleating agent) decreases the VOC but increases the FF and JSC, finally improves the PCE. The most fundamental reason of the varies of performance parameters of BHJ OPV cells with MDBS is that the addition of MDBS accelerate the crystallization kinetics, leads to the decrease of the crystalline size and crystallinity, and the increase of the connectivity between P3AT ordered regions.•The PCE of P3OT/PCBM cells reaches 1.59% after adding 5.0 wt% MDBS, which is slight higher than the reported highest PCE (1.53%) of P3OT/PCBM cell without post-treatment. MDBS is cheap and easily accessible. It can stably and uniformly disperse in many organic solvents used for preparing polymer BHJ OPV solar cells.•Our works verify that it is a new and easy way to design and improve the performance parameters of BHJ OPV devices by adding the nucleating agent which modulates the crystallization kinetics of crystalline donor materials.Differential scanning calorimetry (DSC) and atomic force microscopy (AFM) measurements indicate that 1,2,3,4-bis(p-methylbenzylidene) sorbitol (MDBS) is a nucleating agent for both poly(3-alkylthiophene)s (P3ATs) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), but the intermolecular interaction between P3AT and MDBS is stronger than that between PCBM and MDBS. MDBS regulates the crystallization kinetics of P3AT in P3AT/PCBM blend films, resulting in the acceleration of the crystallization rate, and the decrease of the crystallinity and crystallite size of P3AT. This directly causes the decrease of the open circuit voltage (VOC) and the increase of both the fill factor (FF) and the short circuit current (JSC) with the addition of MDBS. Based on the variation of VOC, FF, and JSC, the power conversion efficiency (PCE) of P3AT/PCBM BHJ OPV devices improves with the addition of MDBS. Our work verifies that this is a new way to modulate and improve the performance parameters of BHJ OPV devices, i.e., by adding a nucleating agent that modulates the crystallization kinetics of crystalline donor materials.1,2,3,4-bis(p-methylbenzylidene) sorbitol (MDBS) is a nucleating agent for both poly(3-alkylthiophene)s (P3ATs) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), but the intermolecular interaction between P3AT and MDBS is stronger than that between PCBM and MDBS. MDBS regulates the crystallization kinetics of P3AT in P3AT/PCBM blend films, resulting in the acceleration of the crystallization rate, and the decrease of the crystallinity and crystallite size of P3AT. This directly causes the decrease of the open circuit voltage (VOC) and the increase of both the fill factor (FF) and the short circuit current (JSC) with the addition of MDBS. Based on the variation of VOC, FF, and JSC, the power conversion efficiency (PCE) of P3AT/PCBM BHJ OPV devices improves with the addition of MDBS.

•We investigated the effects of nucleating agents with tiny structural differences on the crystallization of P3OT, and explored the relationship between the structure of the sorbitol derivatives and the performances of P3OT/PCBM BHJ OPV devices.•The best PCE as 1.77% is obtained in the P3OT/PCBM/DMDBS device, which is the highest PCE of P3OT/PCBM BHJ OPV without post-treatment.•It verifies that the nucleation efficiency and the interaction between the nucleating agent and P3OT greatly affects the performance of P3OT/PCBM/sorbitol derivative BHJ OPV devices.In this work, we compare the effects of sorbitol derivatives (1,3:2,4-dibenzylidene sorbitol (DBS), 1,3:2,4-di(p-methylbenzylidene) sorbitol (MDBS) and 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS)) on the performances of poly(3-octyl thiophene)/[6,6]-phenyl C61-butyric acid methyl ester (P3OT/PCBM) bulk heterojunction (BHJ) organic photovoltaic (OPV) devices and explore the mechanism. Differential scanning calorimetry (DSC) and atomic force microscopy (AFM) measurements indicate that DBS, MDBS and DMDBS are nucleating agents of P3OT. DMDBS has the strongest molecular polarizability and exhibits the best propensity for self-assembly in 1,2-dichlorobenzene (ODCB). The strong π-π stacking of aromatic benzylidene group and the high density of the fibrillary aggregates supply more nucleation surfaces for P3OT, leading DMDBS has the highest nucleation efficiency (NE). Sorbitol derivatives accelerate the crystallization rate (G) of P3OT with the order as GP3OT/DMDBS > GP3OT/MDBS > GP3OT/DBS > GP3OT. The acceleration of the crystallization increases the number of tie molecules, causing the improvement of the connectivity between ordered regions, resulting dramatically increasing the carrier transport of P3OT. GP3OT/DMDBS is highest, the connectivity between ordered regions is best in P3OT with DMDBS. UV–vis measurement indicates that the intra-chain order of P3OT reduces with the addition of sorbitol derivative, and the intra-chain order of P3OT with DMDBS is lowest. The P3OT/PCBM/sorbitol derivative BHJ OPV devices were fabricated and show that the short circuit current JSC P3OT/DMDBS > JSC P3OT/MDBS > JSC P3OT/DBS > JSC P3OT. It hints that the connectivity of tie molecules plays a significant role in defining semiconducting polymer transport characteristics, and is perhaps more important than molecular level interactions (inter- and intra-chain order) for efficient macroscopic charge carrier transport. Finally, it shows that adding sorbitol derivatives can improve the power conversion efficiency (PCE) of P3OT/PCBM BHJ OPV device, the best PCE as 1.77% is obtained in the P3OT/PCBM/DMDBS device.Download high-res image (318KB)Download full-size image

In this study, we successfully developed a novel method to create [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoscale aggregates using supercritical carbon dioxide (scCO2) annealing and fabricated bulk heterojunction (BHJ) solar cells with the nanoscale PCBM to improve device performance. PCBM forms nanoscale aggregates with a size of approximately 70 nm after scCO2 annealing at 11 MPa and 50 °C for 60 min. However, PCBM remains amorphous after thermal annealing (TA) at 150 °C for 5 min. The morphology, structure, and crystallinity of poly(3-hexylthiophene) (P3HT) in the scCO2-treated P3HT film are nearly the same as those in the TA-treated P3HT film. In the P3HT/PCBM blend, the formation of PCBM nanoscale aggregates by scCO2 treatment decreases the disturbance for P3HT crystallization and improves diffusion and regular packing of P3HT molecular chains. This increases the crystallinity of P3HT so that it becomes higher than that in the TA-treated blend film. The nanoscale aggregates of PCBM and the higher crystallinity of P3HT give the scCO2-treated P3HT/PCBM BHJ solar cells a maximum power conversion efficiency (PCE) of 2.74%, which is much higher than that of the as-cast device (PCE is 1.70%) and a little higher than the highest PCE (2.64%) of thermally annealed devices. These results indicate that scCO2 is an effective, mild, and environmental method to modulate the nanoscale aggregates of PCBM and to improve the PCE of BHJ solar cells. However, the size of the PCBM aggregates is a little larger than the most suitable size of the exciton diffusion length, leading to limited improvement of the PCE.

Poly(3-hexylthiophene) (P3HT)/1,2,3,4-bis(p-methylbenzylidene) sorbitol (MDBS) hybrid shish-kebab nanostructures were prepared by spin-coating their hot o-dichlorobenzene (ODCB) solutions. The P3HT nanofibrils grow perpendicularly to the MDBS nanowire surface with uniform width and height. The lengths of these P3HT nanofibrils are controlled by solvent evaporation rate and P3HT solubility in ODCB at different temperatures during spin-coating. The P3HT nanofibril length also can be tailored through variations in spin-coating temperatures, spin-coating rates, P3HT concentrations, and solvent quality or substrates. The method is shown to be general to several other poly(3-alkylthiophene)s (P3ATs) and sorbitol derivatives. The mobility of the P3HT/MDBS thin-film organic field-effect transistor (OFET) with the hybrid shish-kebab morphology is 3.80 × 10−2 cm2 V−1 s−1, which is one order of magnitude higher than the mobility (2.91 × 10−3 cm2 V−1 s−1) of the OFET using only P3HT. In this work, we provide a simple and robust route to form hybrid shish-kebab nanostructures of organic conjugated polymers. Our findings suggest that this nanostructure has the benefit of improving the carrier transport of organic semiconductor materials.

•We design the experiments to understand interrelationships between morphology and performance parameters.•FF increases with the increased P3HT phase purity and the decreased PCBM aggregate thickness.•VOC increases with the decrease of P3HT conjugate length.•JSC relates to the interfacial region area and the phase purity of P3HT.•The devices having large PCBM crystals can obtain good PCE if they have enough interfacial areas.Poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) solar cells are one of archetypal polymer photovoltaic devices. Understanding the relationship between electronic properties and active layer morphology is essential to obtain high-performance electronic devices. The magnitudes of the short circuit current (JSC), the fill factor (FF) and the open circuit voltage (VOC) will vary on the basis of changes in phase purity, interfacial region area and domain size of the active layer. We investigated the device characteristics of the samples having comparable phase purity and found that the performance parameters were better in the device having larger interfacial region area. In another case the phase purity decreases, the interfacial area increases and the recombination rate increases, resulting FF and VOC increase, JSC first increases then decreases. Power conversion efficiency (PCE) increases with the increase of interfacial region area, although at the same time associates with the decrease of the phase purity. The device efficiency reaches optimal value by balance the phase purity and the interfacial area. We finally investigated the two devices where, in spite of significant difference between domain dimensions, the PCEs were quite similar. Especially, the devices having large micrometer scale PCBM crystals also obtain good PCEs if they have enough interfacial areas.The performance parameters of OPV devices have been determined by the combinative effects of phase purity, interfacial area and domain size. For the devices having comparable phase purity, the performance parameters are better in the device having larger interfacial region area. The devices can obtain the similar PCEs in spite of having significant difference between domain dimensions: one has large micrometer scale PCBM crystals, and another has nanoscale phase separation.

The poor solvent (hexanol)-induced crystallization of poly(ε-caprolactone) (PCL) ultrathin film on poly(vinylpyrrolidone) (PVPY) substrate is studied by using atomic force microscopy (AFM). PCL single crystals with screw dislocations melt and wet on PVPY substrate, forming a PCL ultrathin film with continuously changed film thickness. The effect of film thickness on the PCL crystallization morphology is directly observed by using the wetted PCL ultrathin film. Morphology transition from compact seaweed (CS) to fractal dendrite (FD) occurs with the decrease of the film thickness. A lot of dispersed FDs appear when PCL ultrathin film recrystallizes in the presence of minute amount of hexanol. We denote it as a “multi-nuclei” phenomenon. The content of residual hexanol is regulated by changing sample drying time under vacuum or sample melting temperature. Results show that the condition to obtain the highest nuclei density (or crystal density) is that the sample is dried under vacuum at room temperature for 24 h during sample preparation and melts at 200 °C for 10 min before recrystallization. If hexanol evaporates completely, the large area of FD grows continuously and no multi-nuclei phenomenon occurs. The interaction between PCL and PVPY substrate is excluded as the reason of the induction of multi-nuclei phenomenon. PVPY which blended in the PCL ultrathin film during the sample preparation is immiscible with PCL and inhibits the nucleation and crystallization of PCL. In this work, poor solvent provides us a new method to find nucleating agent for polymer thin films.

The effects of lithium perchlorate (LiClO4) on the nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) in the PEO–PCL–LiClO4 ternary blend were studied by using a polarizing optical microscope (POM), differential scanning calorimetry (DSC) and Fourier transformation infrared spectroscopy (FTIR). LiClO4 has a heterogeneous nucleation effect on PEO in the blend, irrespective of the presence of PCL. The coordination interaction between PEO and LiClO4 decreases with the increase of the PCL content, so the heterogeneous nucleation ability of LiClO4 on PEO decreases with the increase of the PCL content in the ternary blends. LiClO4 has no heterogeneous nucleation effect on PCL. In the ternary blend, the heterogeneous nucleation effect of PEO on PCL is remarkably promoted with the addition of LiClO4. POM observations show that the miscibility of PEO and PCL improves after the addition of LiClO4, leading to an increase in the interface volume of PEO and PCL phase domains. The increased concentration-fluctuation at the interface promotes the interfacial nucleation of PCL in the ternary blend. The combination of “fluctuation-assisted crystallization” and “interface-assisted crystallization” mechanisms of the interfacial nucleation of PCL in the PEO–PCL binary blend is also suitable for the interfacial nucleation of PCL in the PEO–PCL–LiClO4 ternary blend.

2D wide-angle X-ray diffraction (2D-WAXD) measurement was performed to investigate the effects of both oscillatory shear and the nucleating agent on the crystalline structure distribution and orientation of isotactic polypropylene (iPP). 1,3:2,4-bis(p-methylbenzylidene) sorbitol (MDBS) and 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS) can induce α-PP and β-PP simultaneously. The presence of MDBS (or DMDBS) and oscillatory strain (oscillatory frequency is fixed) exhibits a synergistic interaction on increasing the content of β-crystals of iPP. Under the oscillatory shear field at the fixed oscillatory strain, the β-crystal content and the orientation of iPP with and without MDBS (or DMDBS) change slightly with the increase of the oscillatory frequency. Comparing with MDBS (or DMDBS) nucleated iPP crystallization under shear field, the periodically changed flow direction of the oscillatory shear field leads to the shorter α-row nuclei, weaker orientation but more β-crystals of the nucleated iPP.

The nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) in the PEO/PCL blends have been investigated by means of optical microscopy (OM) and differential scanning calorimetry (DSC). During the isothermal or nonisothermal crystallization process, when the adjacent PEO is in the molten state, PCL nucleation preferentially occurs at the PEO and PCL interface; after the crystallization of the adjacent PEO, much more PCL nuclei form on the surface of the PEO crystal. However, PEO crystallizes normally and no interfacial nucleation occurs in the blend. The concentration fluctuation caused by liquid–liquid phase separation (LLPS) induces the motion of PEO and PCL chains through interdiffusion and possible orientation of chain segments. The oriented PEO chain segments can assist PCL nucleation, and the heterogeneous nucleation ability of PEO increases with the orientation of PEO chains. Oriented PCL chain segments have no heterogeneous nucleation ability on PEO. It is postulated that the interfacial nucleation of PCL in the PEO/PCL blend follows the combination of “fluctuation-assisted crystallization” and “interface-assisted crystallization” mechanisms.

Sorbitol derivatives, the conventional α-nucleating agents of isotactic polypropylene (iPP), are discovered to induce β-phase iPP under normal crystalline conditions. Combined effects of shear flow and sorbitol derivatives on the crystallization of iPP were investigated by using differential scanning calorimetry, wide-angle X-ray diffraction, and small-angle X-ray scattering. In the nucleation stage, sorbitol derivatives induce both α- and β-nuclei, while shear flow and the interactions between shear and sorbitol derivatives enhance the amount of α-nuclei. In the growth stage, the epitaxial growth of β-crystals on shear-induced α-row nuclei occurs. As the shear rate increases, more epitaxial β-crystals form due to the increase of α-row nuclei, further increasing the content of β-crystals. Under high shear rate, the presence of sorbitol derivatives and shear flow exhibit a synergistic interaction on increasing the content of β-crystals. Moreover, α-nuclei, which arise from the interaction between shear and sorbitol derivatives, emerge earlier than shear-induced α-row nuclei.

Poly(3-hexylthiophene) (P3HT)/1,2,3,4-bis(p-methylbenzylidene) sorbitol (MDBS) hybrid shish-kebab nanostructures were prepared by spin-coating their hot o-dichlorobenzene (ODCB) solutions. The P3HT nanofibrils grow perpendicularly to the MDBS nanowire surface with uniform width and height. The lengths of these P3HT nanofibrils are controlled by solvent evaporation rate and P3HT solubility in ODCB at different temperatures during spin-coating. The P3HT nanofibril length also can be tailored through variations in spin-coating temperatures, spin-coating rates, P3HT concentrations, and solvent quality or substrates. The method is shown to be general to several other poly(3-alkylthiophene)s (P3ATs) and sorbitol derivatives. The mobility of the P3HT/MDBS thin-film organic field-effect transistor (OFET) with the hybrid shish-kebab morphology is 3.80 × 10−2 cm2 V−1 s−1, which is one order of magnitude higher than the mobility (2.91 × 10−3 cm2 V−1 s−1) of the OFET using only P3HT. In this work, we provide a simple and robust route to form hybrid shish-kebab nanostructures of organic conjugated polymers. Our findings suggest that this nanostructure has the benefit of improving the carrier transport of organic semiconductor materials.