Danielly Cristina Bento

Synthesis and spectroscopic analysis of polydiphenylamine via oxidation with bentonite clay in the solid state.

In this study, solids of polydiphenylamine (PDPA) synthesized mechanochemically by reaction with bentonite (PDPAOB) were studied using Raman spectroscopy. It was possible to identify the chemical species in the PDPA-bentonite compound. The spectra obtained were compared to the spectra of PDPA prepared chemically by oxidation of DPA with K2S2O8 in the solid state, and PDPA produced electrochemically, with the aim of studying the characteristic frequencies of the aromatic segments (DPB), radical cation (DPB(+)) and dication (DPB(2+)) of N,N-diphenylbenzidine (DPB) in the polymer structure of the PDPA. To analyze the segments present, the band characteristic of CC asymmetric stretching of the aromatic ring in the Raman spectra was deconvoluted because of the widening of the band and shifts observed by irradiation at wavelengths of 532 and 785 nm. This procedure showed that there are three distinct contributions in the spectra which facilitate the monitoring of changes in the contributions of the segments in the materials doped with HCl (PDPAOBD) and de-doped with NH4OH, (PDPAOBR).

Complementary study on the electrical and structural properties of poly(3-alkylthiophene) and its copolymers synthesized on ITO by electrochemical impedance and Raman spectroscopy

Abstract Targeting the use of organic conjugated polymers in organic solar cells, this study looks at the electrochemical properties of poly(3-alkylthiophene) films: poly(3-methylthiophene), poly(3-hexylthiophene), poly(3-octylthiophene) and its copolymers electrochemically synthesized on a tin-doped indium oxide (ITO) substrate. Electrochemical impedance spectroscopy (EIS) was used to monitor the change in the electrochemical behavior of these films on the ITO and the charge transfer resistance (RCT) values were determined in open circuit potential (OCP) and at different overpotentials. Together with the EIS, the ex situ and in situ Raman spectroscopy was used to characterize the influence of aromatic radical cation and dication species, present in the polymer matrix of homo and copolymers, in the management processes seen in the Nyquist and Bode-phase diagrams for different systems obtained on ITO. The EIS results in OCP showed an decrease in resistance demonstrating increased conductivity with the copolymerization, and through the Bode-phase diagram and the ex situ Raman spectra, these changes were related to the oscillation of the radical cation and dication along the polymer matrix. By studying Nyquist and Bode-phase diagrams in the overpotentials, an increase in RCT values in higher potentials was seen that could be related to the bipolaronic process, which after the deconvolution of the in situ Raman spectra, was possible to relate these results to the stabilization of the dication species in the homo and copolymers matrix when subjected to high potential.

Characterization of the interaction between P3ATs with PCBM on ITO using in situ Raman spectroscopy and electrochemical impedance spectroscopy

Abstract In this study, phenyl-C61-butyric acid methyl ester (PCBM) was deposited onto indium tin oxide (ITO) using the Langmuir–Schaefer technique, and poly(3-methylthiophene) (P3MT) and poly(3-hexylthiophene) (P3HT) were then electrochemically synthesized onto the ITO/PCBM, producing PCBM/P3MT and PCBM/P3HT films respectively. These films are widely used to create heterojunctions in organic photovoltaic cells, and they were produced in this study in order to monitor the charge transfer process between the P3ATs and PCBM. Cyclic voltammetry (CV) of the ITO/PCBM/P3AT systems demonstrates dislocation of oxidation and reduction peaks and a variation in the current of these peaks in relation to those observed in ITO/P3ATs. Charge transfer resistance (RCT) were determined using electrochemical impedance spectroscopy (EIS) to produce Nyquist plots at open circuit potential and at different applied potentials. ITO/PCBM/P3HT had a lower RCT value than ITO/P3HT and ITO/PCBM/P3MT. Bode phase plots produced using EIS indicated that the charge transfer process in ITO/PCBM/P3HT is associated with polaron conduction (radical cation) and in ITO/PCBM/P3MT with bipolar conduction (dication). This study of the behaviour observed in these systems using CV and EIS techniques was supplemented with ex situ and in situ Raman spectroscopy. Structural variations were observed in segments of the P3HT polymer chain when interacting with PCBM in the ITO/PCBM/P3HT system, but not for ITO/P3MT in the ITO/PCBM/P3MT system. The charge transfer observed between PCBM and P3HT was associated with the stabilization of the radical cation species found in P3HT in this material.

Structural stability of poly(3-methylthiophene) and polydiphenylamine blend as an interface applied to hole injector

In this study, interfaces between poly(3-methylthiophene) (P3MT) and polydiphenylamine (PDPA) were prepared on an electrode of indium tin oxide (ITO) (ITO/PDPA:P3MT) or Pt (Pt/PDPA:P3MT) in LiClO4–acetonitrile (ACN) and in diphenyl phosphate (DPHP)–LiClO4–ACN. These systems were compared with those consisting of homopolymers (ITO/PDPA and ITO/P3MT) with the aim of studying the stability of quinone and semiquinone segments in the matrices of blends capable of enhancing the efficiency of organic photovoltaic cells. Raman spectroscopy and electrochemical impedance spectroscopy (EIS) were used as a function of time after preparation of the systems studied. The behavior of charged species in the blend matrix as a function of the applied potential were verified by taking in situ Raman spectra of the Pt/PDPA:P3MT system, also facilitated by the identification of the charge transfer processes of the different P3MT segments by means of EIS. Electron paramagnetic resonance spectroscopy was used to quantify, as time progressed, the radical cation segments present in the polymer matrices after preparation. Ex situ Raman spectroscopy of the ITO/PDPA:P3MT system synthesized in LiClO4–ACN was used to compare it with the ITO/P3MT interface, which revealed that as time progressed the radical cation segments lost stability more slowly, subsequently boosting the dication segments. However, for the ITO/PDPA:P3MT system in DPHP–LiClO4–ACN, this process occurred more rapidly.

Changes induced by electrochemical oxidation of poly(9,9-dioctylfluorene- alt -thiophene): towards a correlation between charge transport, molecular structure modifications and degradation

In this work, an alternating copolymer of fluorene and thiophene, the poly(9,9-dioctylfluorene-alt-thiophene) (PDOF-alt-Th), was synthesized by the Suzuki coupling reaction. To confirm its molecular structure, the copolymer was characterized by means of photoelectron, vibrational and optical spectroscopies. Raman scattering and infrared absorption spectra were fully interpreted, with the support of DFT modeling. It was found that the incorporation of thiophene units along the poly(9,9-dioctylfluorene) chain caused a red shifted emission, and thus the UV–Vis spectroscopy was also used to determine the value of the optical band gap and electron affinity. Electrochemical and electrical properties were then investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The values of ionization potential (Ip) and the charge transfer resistance (RCT) at different potentials were determined. The EIS results showed an increase of conductivity at the first oxidation potential peak of the compound which was related with the formation of the thiophene radical cation. Consequently, the electroactive behavior of the copolymer was complemented by means of UV–Vis ex situ and Raman-spectroelectrochemical analysis. Finally, electrical, electrochemical and optical changes induced by electrochemical oxidation were presented, and the relationship between molecular structure modifications and charge generation are discussed.

Thin films of poly[(9,9-dioctylfluorene)-co-thiophene] deposited on ITO by the Langmuir–Schaefer and Langmuir–Blodgett techniques

The copolymer, poly[(9,9-dioctylfluorene)-co-thiophene] (PDOF-co-Th), prepared by the Suzuki reaction, was deposited as thin films on ITO substrates using the Langmuir–Blodgett and Langmuir–Schaefer techniques. The optical properties of the films were studied by UV–Vis reflectance, photoluminescence and time-resolved photoluminescence, and the electrical properties by electrochemical impedance spectroscopy as a function of film thickness. Raman spectroscopy and atomic force microscopy (AFM) were also used with the aim of elucidating the interactions between layers and the morphology of the PDOF-co-Th thin films at the interface with the ITO. The absorption and emission spectra showed shifts and alterations in the intensities of the bands as a function of film thickness, related to the formation of aggregates and increases in the radiation self-absorption effect. The Nyquist and Bode phase diagrams revealed that the charge transfer process at the ITO/PDOF-co-Th/electrolyte interfaces was enhanced as the number of layers increased. Raman spectra of monolayer films revealed a preferential interaction between the fluorine rings and the electrode surface that was not strong enough to alter the optical and electrical properties of the copolymer. However, as shown in the AFM images of the PDOF-co-Th film surfaces, there is an increase in roughness and compaction that proves that the material agglomerates as the number of layers increases, suggesting that the thiophene rings also come closer together on the substrate surface, enhancing the charge transfer process at this interface.