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Role of energy level alignment in solar cells sensitized with a metal-free organic dye: A combined experimental and theoretical approach

Domenii publicaţii > Fizica + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Corneliu I. Oprea, Anca Dumbrava , Irina Enache , Jeanina Lungu, Adrian Georgescu, Florin Moscalu, Camelia Oprea, Mihai A. Girtu

Editorial: Phys. Status Solidi A , 208 (10), p.2467–, 2011.


We report results of combined experimental and theoretical
studies of dye-sensitized solar cells (DSSCs) using 5-(4sulfophenylazo)salicylic
acid disodium salt, known as Mordant
Yellow 10 (MY-10), as TiO2
sensitizer. We focus on a single
dye but vary the solvent and the pH of the solution as well as the
photoelectrode preparation conditions to determine the conditions
for best photovoltaic conversion efficiency. We found
experimentally that the efficiency, measured under standard air
mass 1.5 global (AM 1.5G) conditions, was higher in solutions
of ethanol than of water, but still small (up to 0.174%), although
the fill factor (FF) was large (up to 0.73). Of the dyes in ethanol,
MY-10 in alkaline solution showed the best matching of the
solar spectrum but displayed the lowest efficiency. Density
functional theory (DFT) calculations provided the optimized
geometry, electronic structure, and electronic spectrum of the
dye in fully protonated as well as partially and totally
deprotonated forms, in solution. The calculated optical spectra
are consistent with the experimental data, with strong
absorption in the visible range only for the alkaline dye solution.
The low device efficiency is very likely related to the
weak optical absorption in the visible range. The much higher
photovoltaic conversion efficiency of the DSSCs fabricated
using acid or roughly neutral pH solutions, corresponding to the
protonated and partially deprotonated forms of MY-10,
respectively, is likely caused by the better alignment of the
ground state of the dye with the redox level of the electrolyte.
The decrease with pH of the dye solution of the short-circuit
current was linked to a weaker charge injection from the excited
state of the dye to the conduction band of the oxide, which is
correlated with the shifting of the excited state of the dye deeper
into the CB edge of the semiconductor. The variation of the
open-circuit voltage with the pH of the solution was linked to
the adjustment of the conduction band edge of TiO2, depending
on the number of protons transferred from the dye to the oxide
surface. Based on our results, we analyze the relative
importance of the main criteria that should be met by a dye to
be used in a DSSC.

Cuvinte cheie: celule solare de tip DSSC // conversion efficiency, dye-sensitized solar cells, optical absorption, TD-DFT

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