Molecular organization relationship of low-bandgap polymers at the air-water interface and in solid films

Highlights

• Low-bandgap polymers, PCPDTBT and Si-PCPDTBT, mixed with stearic acid were spread onto water surface at Langmuir trough.

• Compressibility modulus showed the presence of liquid-expanded and liquid-condensed phases in the isotherms.

• Langmuir-Blodgett and Langmuir-Schaefer techniques are suitable to fabricate organized thin films of low-bandgap polymers.

• The energy levels for the films of the low-bandgap polymers were determined from cyclic voltammetry and UV-Visible spectra.

• The low-bandgap polymer films showed photoconductivity effect.

Abstract

Low-bandgap organic polymers, poly[(4,4‑bis(2‑ethylhexyl)cyclopenta‑[2,1‑b:3,4‑b′]dithiophene)2,6‑diyl‑al‑(2,1,3‑benzothiadiazole)‑4,7‑diyl](PCPDTBT), and poly [(4,4′‑dioctyldithieno[3,2‑b:2′,3′d]silol‑2,6‑diyl)‑alt‑(2,1,3‑benzothiadiazole)‑4,7‑diyl)], (Si-PCPDTBT) were analyzed at the air-water interface forming a Langmuir monolayer. In order to form stable monolayers and to transfer to solid supports, amphiphilic molecules of stearic acid (SA) were mixed with them. For the pristine polymers, the floating monolayers were transferred onto solid substrates via the Langmuir-Schaefer (LS) technique. Surface pressure-area isotherms and compressibility modulus curves demonstrated that the SA incorporation to the polymers at the air-water interface modified the rheological properties of the Langmuir films, since the films became less compressible at higher pressures and there is clear conformational reorganization taking place at intermediary pressures. The UV–Vis absorption also depicted the changes on the overall film morphology by the shift on the maximum absorption bands, and along with cyclic voltammetry curves the absorption spectra made it possible to estimate the energy diagrams for the polymers. Photoconductivity effects were observed for all the sample, among which the pristine polymers fabricated by LS showed better results, suggesting that the organization provided by the Langmuir-Blodgett (LB) technique was not enough to overcome the insulating characteristic of the SA molecules in this specific configuration.

Introduction

The emergence of conducting polymers began the process of many studies, and the search for a comprehension of the electric conduction through the conjugation of chains, understanding that has enabled the creation of a new class of conductive polymers, the so-called low-bandgap polymers [1]. These materials have a low gap energy compared to existing polymers with gap energy above 1.5 eV [2,3], besides a molecular structure with donor part and another acceptor of electrons, being a hybrid material with great potential for organic photonic devices [4,5].

Low-bandgap polymers are commonly used as the active layer of solar cells or field-effect transistors [6,7], however is still unwell explored by techniques that study and control its molecular stability that can contribute to a more favorable conformation of the film for its application in various devices [8,9]. The efficiency of organic devices based on low-bandgap polymers has been optimized with the use of polymers such as PCPDTBT and Si-PCPDTBT, due to its extensive optical absorption band widely used in the literature [3,10].

The study of molecular monolayers and its organization onto liquid subphases has expanded due to diverse technological interests in the areas of engineering, chemistry, physics and biology. These interests stimulated a search for a better understanding of the researchers, with respect to the interaction of molecules in liquid subphases; and how the manipulation of these materials as conductive polymers can have their optical and electrical properties optimized and possibly applied in organic devices, such as: diodes, photodetectors or organic photovoltaic devices. Despite the vast theory and knowledge in the field of inorganic materials, there is still much to be discovered in relation to the processing of organic materials [11].

The optical and electronic properties of conductive polymer thin films, processed from solution, are normally associated with the way that the molecules are organized in the solid form. It is a determinant factor to control the organization process of the studied polymer, from a disordered state in solution to a state of better molecular ordering, as an attempt to achieve the most possible organized phase [12]. Hence, an effective way to study the assembly of conjugated polymers in an aqueous subphase is the Langmuir technique that allows the study of molecules under controlled and defined conditions enabling the analysis of mechanical properties, and molecular organization and interactions [[13], [14], [15]].

The assembled molecules over the water surface are called Langmuir films and they can be transferred onto solid substrates forming Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films with controlled thicknesses, that are well-ordered and made using a small amount of material due to low concentration of the polymer solution required in these methods [2,4,5].

Currently, analyzes by UV–Vis optical spectroscopy of low-bandgap polymers have been performed, making it possible to observe through the optical spectrum its extensive range of absorption and energy band around 900 nm [9,17,18]. UV–Vis spectroscopy is also used to determine the optical bandgap, which value is usually close to the electronic gap reported [9,[17], [18], [19]]. As well electrochemical analysis, that provides the values of oxidation/reduction obtained by cyclic voltammetry (CV), are efficient in providing values that allow estimating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Due to these properties that can be monitored and improved using distinct deposition techniques, these materials are widely used in photoconductive applications [20,21].

The present study addresses the influence of the deposition technique and the addition of stearic acid to the polymeric system, and compares this impact in two different low-bandgap polymers. Thereby, isotherm and compressibility modulus analysis of the pristine polymers and their mixture with the fatty acid were investigated. For further examination, CV and UV–Vis absorption measurements were carried out to evaluate conformational and electronic features of the materials and estimate orbital levels during the compression of the lateral barriers that were moving. Moreover, through electrical characterization, the polymer solid films were tested for photoconductive effects.