Enhanced photovoltaic performance of meso-porous SnO2 based solar cells utilizing 2D MgO nanosheets sensitized by a metal-free carbazole derivative

Abstract

Herein, we report the power conversion efficiency (PCE) of ∼3.71%, achieved in a mesoporous SnO₂ based solar cell by introducing 15 wt% of 3D porous hierarchical MgO composed of 2D nanosheets by a simple sonochemical route followed by a mixing process, sensitized with a metal-free carbazole dye, namely, 2-cyano-3-(4-(2-(9-p-tolyl-9H-fluoren-6-yl)vinyl)phenyl)acrylic acid (i.e. SK1 dye). We have optimized the performance of solar cell devices with the addition of MgO and performed a comparative study on photovoltaic performances of the fabricated devices, such as SnO₂–MgO with pristine SnO₂, by employing two different redox mediators, namely, (I⁻/I₃⁻) and [Co(bpy)₃]^2+/3+. We observed a significant improvement in the open circuit voltage (V_oc) and fill factor (FF) for the SnO₂–MgO based dye sensitized solar cell (DSSC) over the pristine SnO₂ device, i.e. from 357 mV to 550 mV and from ∼38% to ∼50% respectively, wherein an ∼60% enhancement in PCE for the SnO₂–MgO device as compared to bare SnO₂ device using the redox mediator (I⁻/I₃⁻) is achieved. Interestingly, in the case of cobalt tris(2,2′-bipyridyl) redox shuttle, we observed further improvement in the PCE value of the photovoltaic device by 74%, i.e. from ∼1% (pristine SnO₂ device, redox mediator I⁻/I₃⁻) to ∼3.71% (SnO₂–MgO device). From electrochemical impedance spectroscopy (EIS) of the devices, we conclude that the charge transfer resistance at the SnO₂–MgO/SK1/electrolyte interfaces is lower as compared to SnO₂/SK1/electrolyte interfaces, which demonstrates faster charge migration across the interfaces and a slower electron–hole (e⁻–h⁺) recombination rate. It was observed that in the presence of MgO, the life time of photoinduced electrons in the conduction band (CB) of SnO₂ microspheres increases to τ_e = 15.9 ms from 7.1 ms (for pristine SnO₂ device), by employing [Co(bpy)₃]^2+/3+ as a redox shuttle, and thus resulting in higher values of open circuit voltage (V_oc) and short circuit current density (J_sc). Therefore, hierarchical MgO not only improves the PCE of the photovoltaic devices by minimizing the leakage of trapped electrons in the conduction band level of SnO₂/electrolyte interface but also provides more surface area for the adsorption of dye molecules.