Theoretical and experimental study of low band gap polymers for organic solar cells

Abstract

A combined theoretical and experimental investigation of the electronic structure and optical properties of poly(3-hexylthiophene) (P3HT), poly[3-(4-octylphenyl)thiophene] (POPT) and poly[3-(4-octylphenoxy)thiophene] (POPOT) is reported. In comparison with P3HT, POPT and POPOT exhibit better stabilities and the presence of an oxygen atom and/or a phenyl ring in the side chains enhances conjugation. Quantum chemical calculations have been performed on oligomers of increasing chain length to establish the changes in the electronic and optical properties when going from P3HT to the new derivative POPOT. The knowledge of the structure of these polymers is of utmost importance in understanding their optical properties in different phases (solution and condensed phase). The calculations indicate that, in opposition to P3HT and POPT polymers where the introduction of alkyl chains and the pendant phenyl disturbs the planarity of the backbone of the conjugated segment, POPOT has a better degree of organization in both states: the conjugated chain remains planar even in the presence of the phenoxy groups. Finally, the exciton binding energy is evaluated for these polymers and allows us to conclude that the POPOT is a promising polymer for photovoltaic applications when compared to P3HT and POPT.

Synthesis and characterization of poly(3-hexylthiophene): improvement of regioregularity and energy band gap

“…The increase in the conjugation length of polymer results in an increase in the HOMO energy level while LUMO energy is decreased at the same time. 28 Therefore, the energy band gap decreased with increase in number of repeat units in the conjugated polymer. The band gap of polymer having 12 repeat units is 3.073 eV, large for OSC application.…”

“…Among available quantum chemical methods, density functional theory (DFT) has shown significant promise for monitoring changes in electronic structure. 28 In density functional theory, DFT, the energy of the fundamental state of a polyelectronic system can be expressed as the total electronic density, therefore, it is used as electron density instead of a wave function for calculating the energy, constitutes the fundamental basis of DFT. Different parameters during polymerization such as sonication, stirring, and external electric field (EEF) were evaluated for their effects on regioregularity of the synthesized polymers.…”

Effect of the length of alkyl side chains in the electronic structure of conjugated polymers

“…The most popular are Kuhn 14,18 , Meier 8,9,42 , linear 34 , and polynomial 12,15 . However, it should be noted that, in studies conducted with oligomers with up to eight monomer units, the use of linear or polynomial fitting in relation to the inverse quantity of monomeric units (1/n) produced reasonable results 15,34 . Thus, for our extrapolations, we adopted linear fits.…”

“…E b can be estimated as the difference between E g and the vertical transition energy from the ground state to first dipole-allowed excited state (E vert ), i.e., E b = E g -E vert 34,35 . Such data can be obtained by calculation of electronic structure.…”

A comparative study of electronic properties of disordered conjugated polymers

“…There is great interest to understand the electronic properties of semiconducting conjugated polymers -the constituent materials of organic electronic devices. [1][2][3][4][5][6][7][8][9][10][11] This is a significant challenge due to the complex structure of these materials. Depending on the processing conditions, polymer chains are arranged either in an amorphous spaghetti-like structure or form ordered domains with amorphous regions at their boundaries.…”