Improved molecular architecture of D–π–A carbazole dyes: 9% PCE with a cobalt redox shuttle in dye sensitized solar cells

Abstract

Two new D–π–A dyes (SK2 & SK3) based on carbazole, and vinylene-phenylene (π-bridge) with rhodanine-3-acetic acid and cyanoacrylic acid as electron withdrawing–injecting as well as anchoring groups were designed and synthesised under conditions that were free from precious metal-catalysts and well characterized for dye-sensitized solar cell (DSSC) applications. A high power conversion efficiency (PCE) of 9% (AM 1.5 G, 100 mW cm⁻²) has been achieved using cyanoacrylic acid as an acceptor in D–π–A carbazole dye (SK3) with a cobalt based redox shuttle, while a PCE of 7.1% was exhibited in triiodide based redox mediators. A short-circuit current density, J_sc of ∼18.2 mA cm⁻², an open-circuit voltage, V_oc of ∼725 mV, and a fill factor, FF of ∼67% have been afforded by the SK3 based DSSC incorporating the Co²⁺/Co³⁺ electrolyte as the one-electron redox mediator. In contrast, SK2 dye based DSSCs with a cobalt based redox mediator have shown a J_sc ≈ 8.4 mA cm⁻², a V_oc ≈ 587 mV, and a FF ≈ 48%, yielding a PCE of 2.4%. The devices based on SK3 showed outstanding stability performance without significant degradation even after 1000 h of illumination under standard conditions in the Co²⁺/Co³⁺ electrolyte.