Influence of Electrical Potential Distribution, Charge Transport, and Recombination on the Photopotential and Photocurrent Conversion Efficiency of Dye-Sensitized Nanocrystalline TiO₂ Solar Cells:  A Study by Electrical Impedance and Optical Modulation Techniques

Abstract

The role of electrical potential, charge transport, and recombination in determining the photopotential and photocurrent conversion efficiency (IPCE) of dye-sensitized nanocrystalline solar cells was studied. Electrostatic arguments and electrical impedance spectroscopy (EIS) are used to obtain information on the electrical and electrochemical potential distribution in the cell. It is shown that on the macroscopic level, no significant electrical potential drop exists within the porous TiO2 when it contacts the electrolyte and that the electrical potential drop at the transparent conducting oxide substrate (TCO)/TiO2 interface occurs over a narrow region, one or two layers of TiO2. Analyses of EIS and other data indicate that both the photopotential of the cell and the decrease of the electrical potential drop across the TCO/TiO2 interface are caused by the buildup of photoinjected electrons in the TiO2 film. The time constants for the recombination and collection of the photoinjected electrons are measured by EIS and intensity-modulated photocurrent spectroscopy (IMPS). As the applied bias is varied from short-circuit to open-circuit conditions at 1 sun light intensity, recombination becomes faster, the collection of electrons becomes slower, and the IPCE decreases. The decrease of IPCE correlates directly with the decline of the charge-collection efficiency ηcc, which is obtained from the time constants for the recombination and collection of the photoinjected electrons. Significantly, at open circuit, η cc is only 45% of its short-circuit value, indicating that the dye-sensitized nanocrystalline TiO2 solar cell behaves as a nonideal photodiode.

Maximizing omnidirectional light harvesting in metal oxide hyperbranched array architectures

“…Figure 5b shows the Nyquist plots of the DSSCs based on various TiO 2 architectures. According to a simulated equivalent circuit, charge transfer resistance (R 2 ) at the TiO 2 /dye/electrolyte and corresponding electron lifetime (t r , t r ¼ R 2 Â CPE2 (CPE2, chemical capacitance)) values are calculated 48 and then listed in Supplementary Table 5. The R 2 values decrease significantly from 158.1 O for TNW to 65.08 O for TNW-NS, and then further decline to 30.98 O for TNW-NS-NR-based DSSCs, illustrating a faster recombination rate within DSSCs based on the latter two photoanodes with hierarchical structures.…”

Effect of the self-assembled gel network formed from a low molecular mass organogelator on the electron kinetics in quasi-solid-state dye-sensitized solar cells

“…According to the linear sweep voltammograms (Fig. 2) and the apparent diffusion coefficients (Dapp) which were calculated by Equation (1), the diffusions of I − and I3 − in the electrolytes were investigated in detail [29].…”

Efficiency of FTO/Graphene-based Dye-Sensitized Solar Cell

“…The difference in efficiency is the performance of DSSC. While some reports emphasize that there have not been an acceptable explanation and theories on graphene enhancing role in DSSC, ultrahigh theoretical surface area of 2600 which allows for perfect electron of both FTO/graphene and FTO alone photoanode Furthermore, [41][42] opined that there are many grain boundaries in TiO 2 nanoparticle, leading to faster charge recombination at the TiO 2 /electrolyte interface. Therefore, low dimensional materials such as carbon nanostructures (carbon nanotube (CNT)) and graphene sheets incorporated into semiconductor electrodes could improve the charge collection and photovoltaic performance of DSSC [43][44].This is because they possess band gaps which could align with those of TiO 2 and FTO to produce a more efficient pathway for collection and transport of photogenerated electrons [45][46].…”