Resurgence of DSCs with copper electrolyte: a detailed investigation of interfacial charge dynamics with cobalt and iodine based electrolytes

Abstract

Deploying earth abundant copper as a redox mediator in dye-sensitized solar cells (DSCs) has been found to be a very promising strategy to achieve higher photovoltage and power conversion efficiencies in full sun (100 mW cm⁻²) and in low/diffuse light conditions. Achieving higher photovoltage without compromising photocurrent helped copper electrolyte attract considerable attention among alternate electrolytes currently employed in DSCs. The very small reorganization energy between Cu(I) and Cu(II) and small molecular size helped copper achieve unit regeneration efficiency, with a driving force as low as 100 mV and a high diffusion coefficient (D_n), leading to better diffusion length (L_n) and charge collection efficiency (η_cc). Mass transport issues were also found to be improved for copper electrolytes in comparison with cobalt electrolytes. As it is inert to silver and other electrical contacts used in DSCs and possesses higher mobility even in solid state, copper-based electrolyte is a promising candidate to spearhead the commercialization of dye solar technology. In this regard, a detailed evaluation of internal electron transfer dynamics is highly essential to understand the limiting processes in these devices. In the present study, we performed a comparison between copper, cobalt and iodine electrolytes using the same dye (LEG4), semiconductor (TiO₂) and additive concentrations to understand in detail the charge transfer processes leading to higher photoconversion efficiencies and also probe the various deleterious processes taking place in copper devices that provide opportunities to further improve its performance in future.