A method for impressive efficiency enhancement in TiO$_2$-based nanoparticle (NP) dye-sensitized solar cells (DSSCs) is demonstrated by using a co-sensitized triple layered photoanode, comprising a nanofibre (NF) layer of TiO$_2$ sandwiched between two TiO$_2$ P25 NP layers. Rose Bengal (RB) and Eosin-Y (EY) dyes are used for the co-sensitization. DSSCs with conventional TiO$_2$ (P25) NP bi-layer photoanode (NP/NP), sensitized with EY, showed an overall power conversion efficiency (${\eta}$) of 0.89% under the illumination of 100 mW cm$^{–2}$ (AM 1.5) with iodide-based liquid electrolyte. Whereas DSSCs fabricated with triple layered photoanode (NP/NF/NP) with the same total thickness and sensitized with EY yielded 1.77% efficiency under the same illumination conditions, showing an impressive ${\sim}$99% enhancement in the overall power conversion efficiency. The DSSCs fabricated with RB-sensitized NP/NP and NP/NF/NP photoanodes showed 0.25 and 0.73% efficiencies, respectively. Upon optimization, DSSCs fabricated with co-sensitized NP/NP bilayer and NP/NF/NP triple layer photoanodes showed 1.04 and 2.09% efficiencies, respectively, showing again an impressive ${\sim}$100% enhancement in ${\eta}$ due to the co-sensitized triple layer photoanode structure. Increase in the short circuit photocurrent density, UV–visible absorptions measurements, incident photon to current efficiency and electrochemical impedance spectroscopic measurements confirmed that this enhancement is very likely due to the enhanced light harvesting and reduction of recombination of photoelectrons combined with the enhanced spectral responses of the cosensitized triple layered photoanode.

Influence of citric acid on the photovoltaic properties of the CdS quantum dot-sensitized TiO$_2$ solar cells (QDSSCs) was studied. Tethering of citric acid molecules with both TiO$_2$ and CdS quantum dots (QDs) was confirmed by Fourier transform infrared spectroscopy technique. High-resolution transmission electron microscopic studies revealed that QDs with average size of ${\sim}$4.5 nm, were tethered with TiO$_2$ nanoparticles of diameter ${\sim}$40 nm. Presence of Cd, S,C, Ti and O elements in the composite photoanode and their uniform distribution throughout the photoanode were confirmed by energy dispersive X-ray spectroscopy measurements. QDSSCs fabricated with pristine TiO$_2$ photoanode exhibited a short circuit current density ($J_{SC}$) of 5.80 mA cm$^{-2}$ and an overall power conversion efficiency (${\eta}$) of 1.10%, whereas solar cells made with citric acid-treated, photoanode-exhibited a $J_{SC}$ of 8.20 mA cm$^{-2}$ with 1.50% efficiency under 100 mW cm$^{-2}$ (AM 1.5) light illumination. This is an impressive 60% increase in the $J_{SC}$ and ${\sim}$36% enhancement in the overall power conversion efficiency. Interfacial resistance of QDSSCs is estimated by using electrochemical impedance spectroscopy revealed that citric acid treatment enhanced both the electron injection to the conduction band of the TiO$_2$ from the CdS as well as the overall charge transfer of the device, while decreasing the recombination of the photo-generated electrons with their holes in the electrolyte.

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) electrospun polymer nanofibre-based quasisolid or gel electrolytes were successfully fabricated by incorporating a liquid electrolyte within the nanofibre membrane. The dye-sensitized solar cells (DSSCs) fabricated with gel and with liquid electrolyte were characterized by photocurrent– voltage measurements and electrochemical impedance spectroscopy measurements. The maximum efficiency (${\eta}$) of 6.79% was observed for the DSSC fabricated with optimized nanofibre membrane thickness, corresponding to 4 min of electrospinning time. The optimized PVdF-HFP nanofibre gel electrolyte shows an ionic conductivity of 7.16 ${\times}$ 10$^{-3}$ S cm$^{–1}$ at 25°C, while the corresponding liquid electrolyte exhibits an ionic conductivity of 11.69 ${\times}$ 10$^{–3}$ S cm$^{–1}$ at the same temperature. The open circuit voltage ($V_{oc}$), short circuit current density ($J_{sc}$) and fill factor were recorded as 801.40 mV, 12.70 mA cm$^{–2}$, and 66.67%, respectively, at an incident light intensity of 100 mW cm$^{–2}$ with a 1.5 AM filter. The nanofibre gel electrolyte-based cell showed an efficiency of 6.79%, whereas the efficiency of the conventional liquid electrolyte-based cell was 7.28% under the same conditions. Furthermore, nanofibre gel electrolyte-based cells exhibited better stability, maintaining 85.40% of initial efficiency after 120 h. These results show that the optimized, polymer nanofibre-based gel electrolyte can be used successfully to replace the liquid electrolyte in DSSCs without much loss of efficiency but improving the stability while minimizing most of the drawbacks associated with liquid electrolytes.