n-Doping of bio-inspired electron transporting materials: the influence of charge-transfer complexation†

Abstract

Interest in sustainable and bio-inspired materials for optoelectronic applications is burgeoning, driven by the prospect of greener production, compatibility with large-scale manufacturing and potential biocompatibility. This study introduces two analogues of the biological redox co-factor flavin (BFG, BFA) as bioinspired electron-transporting materials featuring solubilizing ethylene glycol and alkyl side chains. These materials demonstrated a conductivity of ∼5.6 × 10⁻⁷ S cm⁻¹ in their pristine form which compares favourably with widely employed PCBM (6.8 × 10⁻⁸ S cm⁻¹). To enhance the conductivity of the material the chemical dopant N-DMBI was added. UV-vis absorption and electron spin resonance measurements confirmed radical anion formation, while glycol-functionalized derivative BFG shows faster reactivity toward the dopant due to increased polarity of the acceptor molecule conferred by the more polar side chain. Surprisingly, these materials did not exhibit the expected enhancement effect in terms of conductivity or increased power conversion efficiency in perovskite solar cells. DFT calculations correlated to features in the absorption spectra of the compounds indicates the formation of stable charge-transfer complexes upon the addition of the dopant. We hypothesise that this inhibits electron transfer of the reduced species in the film to its undoped neighbour and thereby prevents effective doping. Our results highlight the significance of charge-transfer complexation in the design of future electron transporting materials for perovskite solar cells and advocates the use of low cost DFT modelling early on in the design of these species and their dopants.