Organismos, normas y pronunciamientos en torno a lo contable – ambiental

ESR measurement is a powerful microscopic tool to detect charge carriers in CPs. It is well known that nonlinear excitations, such as solitons and polarons, are typical spin/charge carriers in lightly doped CPs reflecting their quasi-one-dimensional structural backbone with strong electron-phonon interactions¹²².

Chapter II. Characterization techniques.

In ESR spectroscopy, absorption of microwaves by molecules causes unpaired electrons within the structure to change spin. Consequently, it has been widely used for identifying and quantifying the presence of polaron charge carriers (which have spin) in CPs, in contrast to bipolaron charge carriers, which are dications with no spin²⁰⁶.

A typical ESR spectrum is shown in Fig. S20, which is for PDPP polymer in chloroform (details and discussion are given in Chapter 3). It shows the spectra of the four samples with different molar doping ratio measured at temperatures of 295 K. The spectra are plotted as a function of the g factor, which is independent of the microwave frequency.

Figure S20. EPR spectra of PDPP polymer doped by CN6CP with different molar dopant ratio.

The intensity of the ESR signal is directly related to the concentration of polaronic species in the sample, and the g value obtained from the spectrum gives insights into the chemical environment of the unpaired electron. ESR spectroscopy is also sensitive tool for identifying changes in a conducting polymer caused by storage and environmental factors.

In this work, EPR measurements were conducted to observe:

 radical nature of Zn-activated NDI and isoindigo monomers for further polymerization;

 radicals formed upon doping process of PDPP. Quantitative measurements were done to estimate charge transfer degree in doped blends.

Chapter III. Results and Discussion.

Results & Discussion

What we should start with is to approach carefully to the choice of material to work with. As it is mentioned in General Introduction, conjugated polymers provide several unique features which are quite valuable to be implemented into mass electronic goods production.

First of all, we focused our attention on rational design of conjugated polymer. In general, CP can be roughly divided into 3 parts: conjugated core (backbone), the side solubilizing chains and functional groups (Figure R1). Each of them provides specific function. The conjugated core is the most important part and affords main physical properties like energy levels, band gap, makes available inter- and intramolecular interactions.

Framework of backbone is quite experiential. Tens, even hundreds of different backbones have been explored so far (See Section 2.3.4. Benchmark solution-processable polymers).

However, only some of them are beneficial for further studies because of different aspects.

Figure R1. Rational design of conjugated polymers. Functional parts of the polymer are illustrated.

Side chain provides solubilizing function as the main role and, subsequently, allows for polymer to be solution processed. Additionally, it improves molecular weight and can adjust intermolecular interactions. At the same time, being saturated alkyl chains are the insulating part of molecule which could bring in steric hindrance or even twist a molecule

Chapter III. Results and Discussion.

thereby distorting conjugation in backbone. Length and branching of alkyl chain usually influence on resulting performance of devices based on such polymers⁵⁶ (See Section 2.1.3.

Solubility and Processing).

Functional groups and substituents usually tune physical properties of the molecule, especially electronic properties²⁰⁷: electron affinity or electronegativity, ionization potential, band gap, energy levels etc.

As it was already mentioned, one can classify conjugated backbone into homo-, quinoid- and donor-acceptor-based according to its repeating unit structure. Homopolymer includes in its structure single or fused aromatic unit. Properties of such polymers are largely determined by intrinsic property of consisting unit. Good examples of such type are well known P3HT, PEDOT, PBTTT etc. Despite rather simple structure, such polymers can show very good stability and high efficiency in OFET and photovoltaics^3,73,74.

Donor-acceptor polymer structure grants intrinsic charge transfer capability provided by interactions between donor and acceptor moiety. The extent of ground-state charge transfer can be tuned by the relative donor and acceptor strengths and character of conjugation in polymer backbone. To afford sufficient semiconducting properties, moieties should be well established. Thus, the repeating unit of DA polymer consists of electron-reach donor unit and electron-deficient acceptor unit resulting in partial charge on each unit (Figure R2a, State I).

The internal CT leads to the observed lower band gap and more preferable double-bond character of the single bond between units along with providing more planar configuration of molecule to improve delocalization of π-electrons along polymer core²⁰⁸. The HOMO and LUMO energy levels of newly formed complex are largely localized on the donor moiety and the acceptor moiety, respectively^164,209. This feature implies an advantage of individually adjusting the band gap and energy levels of polymer. In fact, a smaller band gap can be achieved by copolymerizing more electron-reach moiety and more electron-deficient moiety, whereas the HOMO and LUMO can be tuned by varying the electron-donating ability of the donor and the electron affinity of the acceptor¹⁶⁴ (Figure R2b). Most of the conjugated polymers reported so far are based on DA concept.

DA polymers have to be considered as ambipolar SC materials. Building its framework using strong donor together with weak acceptor would facilitate the hole-conducting behavior and vice versa, stronger acceptor and weak donor would contribute to predominantly electron transport. It is important to note that one should prevent extensive ground-state charge transfer to the radical-cation and radical-anion or intramolecular polaron-pair state (Figure R2, State II).

Chapter III. Results and Discussion.

Page 50 Figure R2. Donor-acceptor polymer structure (a) and generation of new hybrid HOMO and LUMO according to Molecular Orbital theory (b).

In addition, polymers with a relatively planar core, which promotes closer π-π stacking to improve charge transport by hopping¹²² would be beneficial.

Assuming above mentioned considerations and aspects described in State-of-the-Art chapter, we have studied three DA polymers which are the benchmark materials and the subject of intensive research up-to-date. These are diketopyrrolopyrrole, naphthalenediimide- and isoindigo-based copolymers which include thiophene unit as a donor moiety. This chapter is dedicated to the synthesis and study of these copolymers and films, their electrical and morphological research as well as molecular doping of DPP copolymers.