In this research, the relationship between chain aggregation and poly(9,9-dioctylfluorene) (PFO) β-phase content with a change of the PFO solution concentration is investigated. Photoluminescence and UV–vis absorption spectra are used to explore the PFO β-phase and its content. Chain aggregation is explored by dynamic light scattering (DLS). It is found that the chain aggregation size of the starting to form PFO β-phase is approximately 100 nm. The β-phase content increases with the chain aggregation degree. When the β-phase content is in the range of 44% ± 2%, the maximum value of the content (saturation content) of the β-phase is reached and cannot be changed by any external field that could induce β-phase formation. Therefore, the concept of the saturation content of the PFO β-phase is first proposed. The mechanism from PFO β-phase formation to saturation content is explored by DLS and atomic force microscopy. This research is significant to understand and control the molecular chain condensed-state process as well as phase transition from solution to film to achieve a photoelectric device with high charge carrier mobility, stability, and efficiency.

In this work, the quantitative relationship in the heredity of β-phase from a solution to a thin film based on poly(9,9-dioctylfluorene) (PFO), the mechanism of β-phase formation, and the effects of β-phase contents on hole mobility were investigated. The heredity based on PFO β-phase from the solution to the thin film was characterized through UV–vis absorption. Results indicated that β-phase can be completely transferred from solutions to films during drying to form films. PFO β-phase was stable and could manage the dynamic changes from a liquid state to a thin-film state. The β-phase content was higher in the diluted solutions, and the reason was revealed through dynamic light scattering. Thus, a new structure model was constructed, and polymer chain aggregation was rendered unnecessary during PFO β-phase formation. The energy status of the β-phase was lower than that of the α-phase. Consequently, PFO chains were autonomously assembled to become orderly. The chemical environment of the low-concentration solution was more suitable than that of the high-concentration solution. The polymer chains in the former could more freely adapt to a flat geometry than those in the latter to facilitate interchain stacking. Chain aggregation was then observed through transmission electron microscopy. Photoinduced charge extraction with a linear increase in voltage was also performed to examine the charge density and hole mobility of PFO. Hole mobility could be enhanced by an order of magnitude when β-phase was increased from 0% to 5.4%. Thus, the presence of a small amount of ordered domains that can form interconnected channels could strongly enhance the carrier transport of materials in poorly ordered organic thin films, such as PFO. This condition is possibly beneficial for photoelectronic devices, and the adaptive nature of PFO chains in solutions to form a flat geometry is the main factor that promotes the order of the system.

A Pauli master equation method is adopted for the simulation of polymer bulk heterojunction (BHJ) solar cells with vinylidene fluoride–trifluoroethylene copolymer (P(VDF–TrFE)) films as interfacial layers. According to previous reports, using highly crystalline P(VDF–TrFE) films as interfacial layers can highly enhance the efficiency of polymer BHJ solar cells, and possible mechanisms for the enhancement by two different groups were given as the dipole induced permanent internal electric field or simply the electrode improvement which implied that the origin is the barrier lowering effect. The correlation between the appearance of S-shaped current density–voltage (J–V) characteristics and the energy barrier is studied first, and then further results indicate that the previous electrode improvement model provides a consistent explanation for the origin of performance enhancement due to the insertion of polarized P(VDF–TrFE) interfacial layers. Moreover, the phenomenon of an increase of the internal electric field observed before can be attributed to better contact conditions which help reduce the bimolecular recombination rate. Comparatively speaking, the electrode improvement model can give a more rational explanation for the origin of performance enhancement experimentally found. In contrast, the dipole induced permanent internal electric field model was not complete enough.

Molecular aggregates in conjugated polymer (CP) solution can propagate into mesoscale morphology of the relevant film and further dominate the optoelectronic property. Herein, we probed the aggregation behavior of poly(9,9- dioctylfluorene-2,7-diyl) (PFO) and studied its influence on the photophysical property in 1,2-dichloroethane (DCE) solution, where the contents of β-phase or -aggregates increased with prolonged aging time. Thereinto, high quality β-film was fabricated from DCE solution with critical aggregate time of 6 min. The film exhibited excellent surface morphology and characteristic emission of β-phase. Meanwhile, films prepared from aged DCE solutions exhibited high crystallinity, which was promising to obtain higher photoluminance efficiency and charge transport ability simultaneously. Therefore, it is significant to get deep insight into the aggregation behavior of CP, which is involved not only with the solution-processing technology of plastic device, but also with the optoelectronic property of CP.

•The forming condition and mechanism of β conformation in PFO solution were studied.•The β conformation was formed by both interchain and intrachain interactions.•The study has a potential meaning to control the content of β conformation in films via solution.In this work, the forming condition and mechanism of the β conformation in poly (9,9-dioctylfluorene) (PFO) solution are studied. We find the forming condition of β conformation is strongly associated with the proportion of the poor solvent (ethanol) in mixed solution (toluene/ethanol). When the proportion of poor solvent is less than 40% (volume ratio), there is no β conformation in solution; but if the proportion of poor solvent increases to 40%, the β conformation starts to appear. Static and Dynamic Light Scattering research reveal that the molecular chains become collapsed; meanwhile the degree of aggregation is also enhanced when the ethanol proportion increases and proved by TEM. By changing the PFO concentration, we find the concentrations have great influence on the chains aggregation but little influence on the content of β conformation. So we infer the β conformation was formed not in solution but in aggregation, the forming reason was proved to be both intrachain interaction in single chain itself as well as interchain intraction with neighbors in aggregation, which were all enhanced by adding ethanol proportion. The study has a potential meaning to make optoelectronic films whose the condensed state structure was mostly decided by the precursor solution with high charge carrier mobility and devices efficiency by controlling the content of β conformation in PFO precursor solution.

•The relationship between molecular weight and PFO β phase was revealed.•The β phase was formed by the fold of chain itself.•This study is significant to control β phase content in solution.In the work, the effect of PFO molecular weight change on the single chains, aggregation and β phase in the solution was investigated. Some regularity of the chain size and shape with the molecular weight change in the dynamics process were revealed and related mechanism was explored. It was found that the β phase content of high molecular weight was higher than that of low molecular weight in the range of 47,000 to 145,000 g/mol. The main reason was the intrachain interaction which led to the fold of chain itself. When PFO molecular weight was higher, the β phase can be easier formed as the intrachain interaction was stronger to fold itself to overcome the steric repulsion. Besides, single chain with lower molecular weight was more rigid than that of higher molecular weight in pure chloroform, but formed aggregation with low molecular weight was bigger and denser from the results of SLS/DLS, fractal dimension (df) and TEM. It was inferred that the reason for this phenomenon was the fold of chain itself and the β phase. For chains with high molecular weight, they were comparatively long and flexible; it was easy for them to be folded to enhance the steric hindrance between different chains, so they were loosely packed. The ordered β phase occupied less volume compared to the α phase as the β conformation was more planar and packed tightly, and the proportion of β phase was more in aggregation of high molecular weight, so the aggregation size of high molecular weight was smaller than that of low molecular weight. This study is significant to control the β phase content in solution so as to increase the charge carrier mobility of the photoelectric films and the devices efficiency.Figure optionsDownload full-size imageDownload as PowerPoint slide

In this work, the solvent field and temperature are used to explore the mutual transformation dynamic process and mechanism between the α-conformation and β-conformation in poly(9,9-dioctylfluorene) (PFO) precursor solution. The conformational transformation of the PFO chain is researched by UV-vis absorption spectra and the proportions of the β-conformation are quantitatively calculated. The corresponding variation trend of the aggregation structure is researched using a static and dynamic light scattering (SLS/DLS) method. It is found that the mutual transformation processes between the α-conformation and β-conformation are reversible in essence. Especially in the transformation processes, the complicated relationship between the β-conformation and the aggregation structure is understood, that is the aggregation structure promotes formation of the β-conformation under solvent field, then the conformational transformation of the β-conformation promotes the dissociation of the aggregation structure under temperature. The above results give an insight into the β-conformation and the aggregation structure of PFO in theory. Furthermore, under the temperature, we find that both two transformation steps have good linear correlations, which indicates that using temperature can be considered as a good method to accurately control the proportion of β-conformation in actual applications, and it will help us to get the desired proportion of the β-conformation in PFO precursor solution so as to make the charge carrier mobility of optoelectronic films increased and device performance better.

In this work, the α-conformation (individual locally separated chain) of the conjugated polymer poly(9,9-dioctylfluorene) (PFO) in dilute solution is studied by the following three points: the Mark–Houwink exponent a, the fractal dimension D, and the form factor Rg/Rh. From the result of a, the α-conformation of PFO is considered to have a semirigid chain conformation in dilute solution. We establish the mathematical relationship between the Mark–Houwink exponent a and the fractal dimension D in dilute solution. To prove the rationality of this mathematical relationship, the classic polymer polystyrene (PS) is used for comparison. According to the result of D, it can be known that the α-conformation of PFO has a relatively loose and extended chain conformation. Moreover, we use light scattering to get the form factor Rg/Rh of the PFO solution in which the α-conformation and β-conformation coexist. It is found that Rg/Rh increases with the proportion of α-conformation, which indicates that the α-conformation of PFO has a loose and extended chain conformation, and this result agrees well with the conclusions drawn from a and D. On the basis of the fact that the β-conformation (ordered conformation) and α-conformation of PFO can transform into each other in solution, this work is significant for the control of the β-conformation and will have a potential meaning to increase the charge carrier mobility and efficiency from the PFO solution to films.

In this work, the effects of solvation and desolvation on the β phase of poly(9,9-dioctylfluorene) (PFO) are studied. The content of β phase is approximately calculated for comparison. The content of β phase can be enhanced up to 40% by the solvation effect and become a metastable state; the desolvation effect is a dynamic process and can enhance the content of β phase remarkably by 18%age units. It is found that the contents of β phase are always changing with the aggregation degrees of PFO chains. To fully understand it, the concepts of mesoscopic aggregates and macroscopic aggregates are proposed and well proved by the filtration experiment. In the solution (the ethanol content less than 30%), the mesoscopic aggregates are beneficial to enhance the content of β phase; in the solution (the ethanol content more than 40%), which is close to the condensed state of fabricated optoelectronic film, the macroscopic aggregates can make the content of β phase not only much higher but also stable. The content of β phase can be controlled by changing the aggregation characteristics of PFO chains in solution. This work will be significant in fabricating the optoelectronic devices from solutions to films with high carrier mobility and good stability.

A Pauli master equation method is adopted for the simulation of polymer bulk heterojunction (BHJ) solar cells with vinylidene fluoride–trifluoroethylene copolymer (P(VDF–TrFE)) films as interfacial layers. According to previous reports, using highly crystalline P(VDF–TrFE) films as interfacial layers can highly enhance the efficiency of polymer BHJ solar cells, and possible mechanisms for the enhancement by two different groups were given as the dipole induced permanent internal electric field or simply the electrode improvement which implied that the origin is the barrier lowering effect. The correlation between the appearance of S-shaped current density–voltage (J–V) characteristics and the energy barrier is studied first, and then further results indicate that the previous electrode improvement model provides a consistent explanation for the origin of performance enhancement due to the insertion of polarized P(VDF–TrFE) interfacial layers. Moreover, the phenomenon of an increase of the internal electric field observed before can be attributed to better contact conditions which help reduce the bimolecular recombination rate. Comparatively speaking, the electrode improvement model can give a more rational explanation for the origin of performance enhancement experimentally found. In contrast, the dipole induced permanent internal electric field model was not complete enough.