Toward a Rational Design of Poly(2,7-Carbazole) Derivatives for Solar Cells

TLDR

Solar cell performance depends not only on material energy levels but also on polymer molecular weight and overall organization. The study aims to rationally design new symmetric low‑band‑gap poly(2,7‑carbazole) derivatives to enhance solar cell efficiency. The authors employed theoretical modeling and Suzuki coupling to synthesize low‑band‑gap alternating copolymers. DFT calculations correlate well with experimental HOMO, LUMO, and band‑gap energies, revealing that the carbazole unit fixes the HOMO while the electron‑withdrawing comonomer sets the LUMO, and preliminary devices show hole mobilities of ~1×10⁻³ cm² V⁻¹ s⁻¹ and a power‑conversion efficiency up to 3.6 %.

Abstract

On the basis of theoretical models and calculations, several alternating polymeric structures have been investigated to develop optimized poly(2,7-carbazole) derivatives for solar cell applications. Selected low band gap alternating copolymers have been obtained via a Suzuki coupling reaction. A good correlation between DFT theoretical calculations performed on model compounds and the experimental HOMO, LUMO, and band gap energies of the corresponding polymers has been obtained. This study reveals that the alternating copolymer HOMO energy level is mainly fixed by the carbazole moiety, whereas the LUMO energy level is mainly related to the nature of the electron-withdrawing comonomer. However, solar cell performances are not solely driven by the energy levels of the materials. Clearly, the molecular weight and the overall organization of the polymers are other important key parameters to consider when developing new polymers for solar cells. Preliminary measurements have revealed hole mobilities of about 1 x 10(-3) cm2 x V(-1) x s(-1) and a power conversion efficiency (PCE) up to 3.6%. Further improvements are anticipated through a rational design of new symmetric low band gap poly(2,7-carbazole) derivatives.