Electron donor materials: conjugated polymers

The backbone of a semiconducting,π-conjugated polymer is composed of alternat- ing single and double bonds between carbon atoms with possible aromatic and het- eroaromatic rings. Due to conjugated backbones, this kind of polymers absorb in the visible spectral range, which make them suitable light absorbing material for so- lar cell applications[63]. Conjugated polymers used in PSCs can be classified by the type of their CRUs into homopolymers, D–A copolymers, quinoid polymers, and other types of polymers[6]. The first polymers employed in BHJ PSCs were polyphenylenevinylenes[64]. However, their limited light absorption due to the relatively large bandgaps (ca. 2.2 eV) and low carrier mobilities led to the PCEs of only 2–3%. Thus, the attention shifted towards polythiophenes, including a widely employed homopolymer poly(3-hexylthiophene) (P3HT), which had smaller band gap (ca. 1.9 eV), higher hole mobility, and broader spectral coverage compared to polyphenylenevinylenes[6, 65]. However, the PCEs of the P3HT–[6,6]-phenyl- C₆₁-butyric acid methyl ester (PC₆₁BM) system were still limited to 4–5% due to large IE of P3HT. Nevertheless, P3HT has been a subject of a tremendous amount of studies and a benchmark material in the many PSC studies[66].

Although the homopolymer P3HT had promising features, its performance could not be improved further and it did not meet all the requirements set for an "ideal"

polymer. That is, a strong absorption in both the visible and near-infrared regions, IE and EA matching well with those of the eA material, and planar backbone to enable closely packed, parallel polymer chains[6, 59, 67]. In 1993, Havinga et al.

[68, 69]introduced the concept of a D–A approach, where the CRU of a conjugated polymer consists of alternating electron-donating (donor) and electron-withdrawing (acceptor) units (Figures 2.2a and 2.2b). When considering this concept at the level of one-electron molecular orbitals (MOs), the MOs of the donor and acceptor units

Donor Acceptor n (a)

D–A

Donor Acceptor EH–L

HOMO LUMO

HOMO LUMO

E HOMO

LUMO

(c) (b)

Figure 2.2 Schematic illustrations of (a) a CRU of a D–A copolymer, (b) a CRU of the PBDT- TPD copolymer, and (c) frontier molecular orbital (FMO) levels of separate donor and acceptor units and resulting hybridized FMO levels.

units. Thus, at the molecular level, the selection of the donor unit with a small IE (i.e. high-lying HOMO) and the acceptor unit with a large EA (i.e. low-lying LUMO) will lead to a smaller fundamental (HOMO–LUMO) gap than that of either unit. Consequently, the properties of the D–A copolymer, such as the IE, EA, band gap, optical gap, and transition dipole moments, can be fine-tuned by selecting the donor and acceptor units with suitable electron-donating and electron-withdrawing strengths[59, 70].

Different design strategies have been employed for obtaining D–A copolymers with desired characteristics. The optoelectronic properties, e.g. band gap, IE, EA, charge carrier mobility, and conductivity of the polymer are mainly governed by its con- jugated backbone [59]. Typically, a planar backbone has been aimed at, as it en- hances the delocalization of the electrons andπ-stacking of the polymer chains.

This has been achieved by using fused conjugated units consisting of three or more rings, such as benzene, thiophene, or other heterorings[6]. For example, benzo[1,2- b:4,5-b’]dithiophene (BDT) (see Figure 2.2b), which is included in most of the D–

A copolymers studied in this work, has been a successful donor unit. Among the

acceptor units, thieno[3,4-c]pyrrole-4,6-dione (TPD) and quinoxaline, which have been employed also in the copolymers examined in this thesis, have been promising candidates. Even higher levels of fusion have been pursued via ladder-type donor units, where several adjacent aromatic units have been covalently fastened together.

However, too rigid and coplanar backbones may lead to increased intermolecular π-πstacking interactions, which will hinder the solubility and processibility of the polymer and miscibility of the polymers with the eA materials, e.g. fullerenes[59].

This is also supported by MD simulations of Jackson et al. [13], where they have showed that some of the high-performing conjugated polymers do not necessarily have rigid, planar backbones, but rather disordered, twisted ones.

The side chains of the polymer may impact its molecular weight, solubility, and pro- cessibility[59]. They also control the intermolecular interactions,π–πstacking of polymer chains, and intercalation of fullerene derivatives between the polymer side chains[71]. However, they should be selected carefully, as long and branched side chains may improve solubility, but on the downside, introduce steric hindrance and increased torsional twists in the backbone leading to poorer photovoltaic properties of the polymer[6, 59]. This can be avoided by placing a spacer unit, e.g. thiophene, between the donor and acceptor units, which provides additional degree of freedom to the backbone[3].

Finally, the properties of the copolymer can be further fine-tuned by attaching differ- ent substituents, such as electron-donating methoxy groups and nitrogen or electron- withdrawing fluorine, chlorine, and cyano group, to its backbone units[6]. The in- clusion of the electron-donating groups result in the decreased IEs (i.e. destabilized oxidation potentials), whereas the electron-withdrawing groups may increase both IEs and EAs (i.e. stabilize redox potentials), which has resulted in improved short- circuit current densities, open circuit voltages, fill-factors, and PCEs of the devices.

Furthermore, it has been shown both experimentally[72, 73]and theoretically[15]

that even small changes in these substituents can impact the blend morphology.