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Title: Effect of an Adsorbent on Recombination and Band-Edge Movement in Dye-Sensitized TiO2 Solar Cells: Evidence for Surface Passivation

Abstract

The mechanism by which the adsorbent guanidinium affects the open-circuit photovoltage of dye-sensitized TiO{sub 2} nanocrystalline solar cells was investigated. The influence of the guanidinium cation on the rate of recombination and band-edge movement was measured by transient photovoltage. When guanidinium is present in the electrolyte recombination becomes slower by a factor of about 20. At the same time, the adsorbent causes the band edges to move downward, toward positive electrochemical potentials, by 100 mV. The collective effect of both a downward shift of the band edges and slower recombination, owing to the presence of guanidinium, results in an overall improvement in the open-circuit photovoltage.

Authors:
; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
977299
DOE Contract Number:
AC36-08GO28308
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry B; Journal Volume: 110; Journal Issue: 2006
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 77 NANOSCIENCE AND NANOTECHNOLOGY; ADSORBENTS; CATIONS; ELECTROLYTES; PASSIVATION; RECOMBINATION; SOLAR CELLS; TRANSIENTS; Materials Science and Semiconductors; Solar Energy - Photovoltaics

Citation Formats

Frank, A. J., Kopidakis, N., and Neale, N. R.. Effect of an Adsorbent on Recombination and Band-Edge Movement in Dye-Sensitized TiO2 Solar Cells: Evidence for Surface Passivation. United States: N. p., 2006. Web. doi:10.1021/jp0607364.
Frank, A. J., Kopidakis, N., & Neale, N. R.. Effect of an Adsorbent on Recombination and Band-Edge Movement in Dye-Sensitized TiO2 Solar Cells: Evidence for Surface Passivation. United States. doi:10.1021/jp0607364.
Frank, A. J., Kopidakis, N., and Neale, N. R.. Sun . "Effect of an Adsorbent on Recombination and Band-Edge Movement in Dye-Sensitized TiO2 Solar Cells: Evidence for Surface Passivation". United States. doi:10.1021/jp0607364.
@article{osti_977299,
title = {Effect of an Adsorbent on Recombination and Band-Edge Movement in Dye-Sensitized TiO2 Solar Cells: Evidence for Surface Passivation},
author = {Frank, A. J. and Kopidakis, N. and Neale, N. R.},
abstractNote = {The mechanism by which the adsorbent guanidinium affects the open-circuit photovoltage of dye-sensitized TiO{sub 2} nanocrystalline solar cells was investigated. The influence of the guanidinium cation on the rate of recombination and band-edge movement was measured by transient photovoltage. When guanidinium is present in the electrolyte recombination becomes slower by a factor of about 20. At the same time, the adsorbent causes the band edges to move downward, toward positive electrochemical potentials, by 100 mV. The collective effect of both a downward shift of the band edges and slower recombination, owing to the presence of guanidinium, results in an overall improvement in the open-circuit photovoltage.},
doi = {10.1021/jp0607364},
journal = {Journal of Physical Chemistry B},
number = 2006,
volume = 110,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • The objective of this research is to determine the operational characteristics key to efficient, low-cost, stable solar cells based on dye-sensitized mesoporous films (in collaboration with DOE's Office of Science Program). Toward this end, we have investigated the mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, improves the open-circuit photovoltage (VOC) and short-circuit photocurrent density (JSC). We find that adding chenodeoxycholate not only shifts the TiO2 conduction-band edge to negative potentials but also accelerates the rate of recombination. The net effect of these opposing phenomena is to produce a higher photovoltage. It is also foundmore » that chenodeoxycholate reduces the dye loading significantly but has only a modest effect on JSC. Implications of these results to developing more efficient cells are discussed.« less
  • No abstract prepared.
  • We report on the influence of morphological disorder, arising from bundling of nanotubes (NTs) and microcracks in films of oriented TiO{sub 2} NT arrays, on charge transport and recombination in dye-sensitized solar cells (DSSCs). Capillary stress created during evaporation of liquids from the mesopores of dense TiO{sub 2} NT arrays was of sufficient magnitude to induce bundling and microcrack formation. The average lateral deflection of the NTs in the bundles increased with the surface tension of the liquids and with the film thicknesses. The supercritical CO{sub 2} drying technique was used to produce bundle-free and crack-free NT films. Charge transportmore » and recombination properties of sensitized films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. Transport became significantly faster with decreased clustering of the NTs, indicating that bundling creates additional pathways via intertube contacts. Removing such contacts alters the transport mechanism from a combination of one and three dimensions to the expected one dimension and shortens the electron-transport pathway. Reducing intertube contacts also resulted in a lower density of surface recombination centers by minimizing distortion-induced surface defects in bundled NTs. A causal connection between transport and recombination is observed. The dye coverage was greater in the more aligned NT arrays, suggesting that reducing intertube contacts increases the internal surface area of the films accessible to dye molecules. The solar conversion efficiency and photocurrent density were highest for DSSCs incorporating films with more aligned NT arrays owing to an enhanced light-harvesting efficiency. Removing structural disorder from other materials and devices consisting of nominally one-dimensional architectures (e.g., nanowire arrays) should produce similar effects.« less
  • A lot of research on various aspects of dye solar cells (DSC) has been carried out in order to improve efficiency. This paper analyzes the utilization of TiO{sub 2} passivation layers of different thicknesses by improving the electron transport properties. Four different thicknesses of passivation layers namely 10, 20, 50 and 100 nm were deposited onto the working electrode using r.f sputtering. The electrodes were assembled into TiO{sub 2} based DSC with active area of 1 cm{sup 2}. The solar performance was investigated using 100 mW/cm{sup 2} of AM 1.5 simulated sunlight from solar simulator. The kinetics of the solarmore » cells was investigated using Electrochemical Impedance Spectroscopy (EIS) measurement and the spectral response was measured using Incident Photon to Electron Conversion (IPCE) measurement system. The highest efficiency was found for DSC with 20 nm passivation layer. DSCs with the passivation layer have open circuit voltage, V{sub OC} increased by 57 mV, their current density, J{sub SC} increased by 0.774 mA cm{sup -2} compared to the one without the passivation layer. The quantum efficiency of the 20 nm passivation layer is the highest, peaking at the wavelength of 534 nm, resulting in the highest performance. All DSCs with the passivation layer recorded higher ratio of R{sub BR}/R{sub T} where R{sub T} is the diffusion resistance of the TiO{sub 2} particles in the mesoscopic layer and R{sub BR} is the recombination resistance of the electron to the electrolyte. This implies that the recombination of the electrolyte I{sup -}{sub 3}/3I{sup -} couple at the substrate/electrolyte interface has been effectively reduced resulting in an enhanced efficiency.« less
  • The composition of the electrolyte is known to greatly influence the performance of dye-sensitized solar cells. It has been speculated that some components of the electrolyte passivate the TiO2 surface against recombination; however, this has never been confirmed experimentally. We hereby present the first case of passivation of the TiO2 surface against recombination by an additive in the electrolyte. Even though the additive also causes a downward movement of the TiO2 bands, suppression of recombination prevails and an overall improvement in open-circuit photovoltage is observed. This work was conducted in collaboration with the DOE Office of Science program.