Temperature-Dependent Optical Properties of Flexible Donor–Acceptor Polymers

Optical properties of five donor–acceptor polymers of interest for light-harvesting in organic photovoltaic devices have been studied experimentally and computationally. Experimentally recorded absorption spectra in solution of the five polymers are shown to be significantly temperature-dependent. The polymers were subjected to a first-principles computational treatment using density functional theory optimizations and excitation calculations. For two of the polymers, APFO-3 and PTI-1, a methodology that accounts for a thermally induced distribution of conformations based on Boltzmann statistics is applied to produce size- and temperature-converged optical results. This provides a deeper understanding of the temperature dependence of optical properties and improves the computational predictions of absorption wavelength and intensity at experimentally accessible temperatures, as compared to results from traditional quantum chemical calculations based on optimized polymers. Together, the combined experimental and computational temperature studies elucidate and quantify the significant influence of structural flexibility on the optical absorption properties of typical donor–acceptor polymers

Very Low Band Gap Thiadiazoloquinoxaline Donor–Acceptor Polymers as Multi-tool Conjugated Polymers

Here we report on the synthesis of two novel very low band gap (VLG) donor–acceptor polymers (<i>E</i>_g ≤ 1 eV) and an oligomer based on the thia­diazolo­quinoxaline acceptor. Both polymers demonstrate decent ambipolar mobilities, with <b>P1</b> showing the best performance of ∼10^–2 cm² V^–1 s^–1 for p- and n-type operation. These polymers are among the lowest band gap polymers (≲0.7 eV) reported, with a neutral λ_max = 1476 nm (<b>P2</b>), which is the farthest red-shifted λ_max reported to date for a soluble processable polymer. Very little has been done to characterize the electrochromic aspects of VLG polymers; interestingly, these polymers actually show a bleaching of their neutral absorptions in the near-infrared region and have an electrochromic contrast up to 30% at a switching speed of 3 s

Influence of Incorporating Different Electron-Rich Thiophene-Based Units on the Photovoltaic Properties of Isoindigo-Based Conjugated Polymers: An Experimental and DFT Study

A series of novel donor–acceptor conjugated alternating copolymers based on the isoindigo acceptor moiety have been designed, synthesized, and characterized, in order to explore the potential of isoindigo for efficient donor materials with high photovoltages in solar cells. We have systematically investigated and discussed the effect of combining different electron-rich thiophene-based units on the structural, optical, electronic, and photovoltaic properties of the resulting polymers. Morphological studies and quantum-chemical calculations are carried out to gain insights into the different properties. The power conversion efficiencies (PCEs) of the solar cells based on these polymers are increased step by step by over 3-fold through a rational structural modification. Among them, PBDTA-MIM shows a PCE of 5.4%, which is to our knowledge the best result achieved among isoindigo-based polymers for solar cells combined with PC₆₁BM as the acceptor using the conventional device configuration. Our results further emphasize the use of isoindigo as an effective acceptor unit and highlight the importance of carefully choosing appropriate chemical structure to design efficient donor–acceptor polymers for organic solar cells. In addition, the resulting low optical gaps, the promising PCEs with PC₆₁BM as the acceptor, and the good open-circuit voltages (up to 0.8 V) synergistically demonstrate the potential of this class of polymers as donor materials for bottom subcells in organic tandem solar cells