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Dependence of Dye Regeneration and Charge Collection on the Pore‐Filling Fraction in Solid‐State Dye‐Sensitized Solar Cells
Author(s) -
Weisspfennig Christian T.,
Hollman Derek J.,
Menelaou Christoper,
Stranks Sam D.,
Joyce Hannah J.,
Johnston Michael B.,
Snaith Henry J.,
Herz Laura M.
Publication year - 2014
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201301328
Subject(s) - materials science , chemical engineering , photocurrent , nanoporous , dye sensitized solar cell , porosity , energy conversion efficiency , analytical chemistry (journal) , porous medium , ultrafast laser spectroscopy , spectroscopy , composite material , electrolyte , optoelectronics , nanotechnology , chemistry , organic chemistry , electrode , physics , quantum mechanics , engineering
Solid‐state dye‐sensitized solar cells rely on effective infiltration of a solid‐state hole‐transporting material into the pores of a nanoporous TiO 2 network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole‐transfer yield from the dye to the hole‐transporting material 2,2′,7,7′‐tetrakis( N,N ‐di‐p‐methoxyphenylamine)‐9,9'‐spirobifluorene (spiro‐OMeTAD) is shown to rise rapidly with higher pore‐filling fractions as the dye‐coated pore surface is increasingly covered with hole‐transporting material. Once a pore‐filling fraction of ≈30% is reached, further increases do not significantly change the hole‐transfer yield. Using simple models of infiltration of spiro‐OMeTAD into the TiO 2 porous network, it is shown that this pore‐filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole‐transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole‐transporting material. Comparison of these results with device parameters shows that improvements of the power‐conversion efficiency beyond ≈30% pore filling are not caused by a higher hole‐transfer yield, but by a higher charge‐collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power‐conversion efficiencies with increasing pore‐filling fraction correlate well with percolation theory, predicting the points of cohesive pathway formation in successive spiro‐OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO 2 with 20 nm pore size, the photocurrent should show no further improvement beyond an ≈83% pore‐filling fraction.