Optimizing photovoltaic performance in CuInS2 and CdS quantum dot-sensitized solar cells by using an agar-based gel polymer electrolyte

Quantum dot-sensitized solar cells (QDSSCs) offer new opportunities to address the clean energy challenge, being one of the top candidates for third generation photovoltaics. Like dye-sensitized solar cells (DSSCs), QDSSCs normally use liquid electrolytes that suffer from issues such as evaporation or leakage. In this study a gel polysulfide electrolyte was prepared containing a natural polymer, agar, and was used as a quasi-solid-state electrolyte in solar cells to replace the conventional liquid electrolytes. This gel electrolyte shows almost the same conductivity as the liquid one. The solar cells were fabricated using CuInS2 quantum dots (QDs), previously synthesized, deposited on TiO2 photoanodes by electrophoretic deposition (EPD). CdS was deposited on TiO2 by successive ionic layer adsorption and reaction (SILAR). Reduced graphene oxide (RGO)-Cu2S, brass, and thin film CuxS were used as counter electrodes. Compared to a liquid polysulfide water based electrolyte, solar cells based on CuInS2 and CdS using gel polymer electrolyte (GPE) exhibit greater incident photon to current conversion efficiency (IPCE = 51.7% at 520 nm and 72.7% at 440 nm), photocurrent density (J(sc) = 10.75 and 13.51 mA cm(-2)), and power conversion efficiency (eta - 2.97 and 2.98%) while exhibiting significantly enhanced stability. The solar cells employing the agar-based gel polymeric electrolyte are about a factor of 0.20 more stable than using a liquid electrolyte. The higher photovoltaic performance is due to the good conductivity and high wettability as well as the superior permeation capability of the gel electrolyte into the mesoporous matrix of a TiO2 film.