Improved Performance of Dye-Sensitized Solar Cells Fabricated from a Coumarin NKX-2700 Dye-Sensitized TiO2/MgO Core–Shell Photoanode with an HfO2 Blocking Layer and a Quasi-Solid-State Electrolyte

Dye sensitized solar cells (DSSC) were fabricated from a coumarin NKX-2700 dye-sensitized core–shell photoanode and a quasi-solid-state electrolyte, sandwiched together, with a cobalt sulfide-coated counter electrode. The core–shell photoanode consisted of a composite mixture of 90% TiO 2 nanoparticles and 10% TiO 2 nanowires (TNPW) as core layer and MgO nanoparticles (MNP) as shell layer. Hafnium oxide (HfO 2 ) was applied to the core–shell photoanode film as a blocking layer. TiO 2 nanoparticles, TiO 2 nanowires, and TNPW/MNP were characterized by x-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. It was apparent from the UV–visible spectrum of the sensitizing dye coumarin NKX-2700 that its absorption was maximum at 525 nm. Power conversion efficiency (PCE) was greater for DSSC-1, fabricated with a core–shell TNPW/MNP/HfO 2 photoanode, than for the other DSSC; its photovoltaic properties were: short circuit photocurrent J sc = 19 mA/cm 2 , open circuit voltage ( V oc ) = 720 mV, fill factor ( FF ) = 66%, and PCE ( η ) = 9.02%. The charge-transport and charge-recombination behavior of the DSSC were investigated by electrochemical impedance spectroscopy; the results showed that the composite core–shell film resulted in the lowest charge-transfer resistance ( R CE ) and the longest electron lifetime ( τ eff ). Hence, the improved performance of DSSC-1 could be ascribed to the core–shell photoanode with blocking layer, which increased electron transport and suppressed recombination of charge carriers at the photoanode/dye/electrolyte interface.