On The Scalability of Dye-Sensitized Solar Cells : Effect of Photoelectrode Area on the Photovoltaic and Charge Transport Parameters

This study is aimed to provide new insights on the scalability of dye-sensitized solar cells (DSCs). The DSCs of electrode area up to ~2 cm2 were fabricated using commercially available P25 TiO2 particles, N3 dye, and iodide/triiodide electrolyte and evaluated using voltage - current and electrochemical impedance spectroscopic measurements. The photovoltaic conversion efficiency follows a biexponential decay, the main contributor to which is the short circuit current density(JSC). Interesting features were observed in the electrochemical impedance spectra and charge transport parameters in the devices as the photoelectrode areas are increased. Results show that electrons from an area above a threshold are not collected due to varied choice of diffusion pathways. Furthermore, this study identify that area of the photoelectrode for reporting the efficiency needs to be fixed at ~0.5 cm2 for 25 nm TiO2 particles because below which it strongly vary. On the other hand, the study provides opportunities to build high efficiency dye-sensitized solar cells using the current choice of materials

Charge Transport through Electrospun SnO₂ Nanoflowers and Nanofibers: Role of Surface Trap Density on Electron Transport Dynamics

A larger amount of tin precursor was dispersed in electrospun polyvinyl acetate fibers than that required for SnO₂ fiber formation upon annealing, thereby creating a constraint such that all nuclei formed during annealing could not be accommodated within the fiber, which leads to enhanced reaction kinetics and formation of highly crystalline–cum–higher surface area SnO₂ flowers. The flowers are shown to have a lower density of surface trap states than fibers by combining absorption spectra and open circuit voltage decay (OCVD) measurements. Charge transport through the SnO₂ flowers in the presence of the iodide/triiodide electrolyte was studied by OCVD, electrochemical impedance spectroscopy, and transient photodecay techniques. The study shows that the flowers are characterized by higher chemical capacitance, higher recombination resistance, and lower transport resistance compared with fibers. Photocurrent transients were used to extract the effective electron diffusion coefficient and mobility which were an order of magnitude higher for the flowers than that for the fibers. The flowers are also shown to have an enhanced Fermi energy, on account of which as well as higher electron mobility, dye-sensitized solar cells fabricated using the SnO₂ flowers gave <i>V</i>_OC ∼700 mV and one of the highest photoelectric conversion efficiencies achieved using pure SnO₂