evaluación de la eficiencia de una celda solar...
TRANSCRIPT
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CENTRO DE INVESTIGACIN EN MATERIALES AVANZADOS S.C.
POSGRADO
Evaluacin de la eficiencia de una celda solar sensibilizada por colorante mediante el modelado molecular y la teora de funcionales de la densidad
ARTCULOS PUBLICADOS QUE PARA OBTENER EL GRADO DE DOCTOR EN CIENCIA DE MATERIALES
Presenta: Jess Adrin Baldenebro Lpez
DIRECTOR DE TESIS: Dr. Daniel Glossman Mitnik
Dr. Jos Castorena Gonzlez
CHIHUAHUA, CHIH. JUNIO, 2014
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RESUMEN
Colorantes orgnicos basados en trifenilamina y complejos de cobre con ligandos
de bipiridina y biquinolina han sido analizados y discutidos desde el punto de vista
tericos a travs de la qumica computacional y la teora de funcionales de la
densidad (DFT). Diversos niveles de clculo han sido utilizados con la finalidad de
establecer una metodologa computacional confiable para el clculo de las
propiedades de inters tales como la estructura molecular, espectro de infrarrojo,
orbitales moleculares y sus niveles de energa, sitios y parmetros de reactividad
qumica a travs de DFT conceptual; tambin se incluye la prediccin de espectros
de absorcin y espectros de emisin con DFT dependiente del tiempo (TD-DFT).
Los sistemas moleculares son propuestos con potencial aplicacin en la fotovoltaica
y principalmente en celdas solares sensibilizadas por colorante (DSSC). Los
resultados abren la posibilidad de una proyeccin computacional de diversas
predicciones en la estructura electrnica y la respuesta ptica; pavimentando as el
camino a una ingeniera molecular eficaz de una mayor cantidad de sensibilizadores
reforzando las aplicaciones en celdas solares. Interesante trabajo para los
experimentalistas en el campo de las DSSC.
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AGRADECIMIENTOS
Es importante mencionar la suerte que he tenido de trabajar con un conjunto de
personas a lo largo de mis estudios de doctorado, ya que no solo he podido aprender
a travs de ellos un conocimiento cientfico si no tambin moral y humano.
No puedo ni debo dejar pasar esta oportunidad para agradecer a mi familia todo su
apoyo durante este tiempo. A mi esposa Perla e hijos Esteban, Adriana y Elma
(tambin los llamo mis amores), quienes han sabido entender mis necesidades y
apoyado hasta el cansancio con su nimo. En la misma magnitud de importancia a
mis padres, Esteban Baldenebro y Elma Lpez, a quien debo absolutamente todo,
gracias por todo su cario y amor que siempre me han proporcionado; a mis
hermanos: Mirna, Francisco y Karely; por sus palabras de confianza y amor que en
todo momento me han expresado y que son por mi parte bien correspondidos.
Quiero mencionar de una forma muy especial a mis amigos y directores de tesis,
los doctores Daniel Glossman y Jos Castorena. Muchas gracias a ambos por sus
consejos e indicaciones en el trayecto de este trabajo de investigacin, adems del
entusiasmo que en todo momento han aportado y sabido transmitir a mi persona.
Cada uno de ellos ha contribuido de la mejor manera posible desde su rea de
conocimiento a la realizacin de esta tesis.
El agradecimiento para mi comit de sinodales, por sus valiosas aportaciones en la
revisin y evaluacin de mi trabajo, siempre sealando con mucha crtica
constructiva sus observaciones al respecto: Dra. Norma Flores, Dra. Luz Mara
Rodrguez, Dr Erasmo Orrantia y Dr. Francisco Espinosa.
A mis amigos y compaeros del tan querido NANOCOSMOS: Kathy, Nora, Linda, y
Rody, por su optimismo y comprensin, adems de siempre tener disponibilidad
para ayudarme en todo momento durante esta ardua tarea que representa realizar
estudios de posgrado.
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INTRODUCCIN
Las fuentes convencionales de energa en el mundo (petrleo, gas natural y carbn)
tienen una vida finita y las previsiones actuales indican que las alternativas deben
aportar una contribucin importante en el futuro cercano. La energa solar es uno
de los recursos energticos ms prometedores del futuro. La conversin directa de
luz solar en energa elctrica mediante el uso de celdas solares es de particular
inters ya que se presentan muchas ventajas sobre otros mtodos de generacin
de energa elctrica, adems de evitar el escape de gases de efecto invernadero y
subproductos de residuos nucleares. Las celdas solares sensibilizadas por
colorante (DSSC) han atrado considerablemente la atencin, debido a su eficiencia,
proceso de fabricacin sencillo y bajo costo de produccin. En estas DSSC, un
sensibilizador debe ser qumicamente absorbido en la superficie porosa del xido
nanoestructurado. Tras la absorcin de un fotn, el electrn excitado en la molcula
de sensibilizador se transfiere a la banda de conduccin del xido nanoestructurado,
y esto se produce en un lapso de unos cientos de femtosegundos, seguido de un
proceso en el que el electrn se difunde a travs de la red porosa hasta llegar a un
electrodo. El sensibilizador en su estado oxidado, se reduce al estado original
mediante el suministro de electrones a travs de un electrolito lquido dentro de los
poros. En la actualidad, muchos grupos de investigadores de todo el mundo
participan activamente en la mejora de la eficiencia de cada uno de los procesos
involucrados en las DSSC. La eficiencia en la transferencia de carga de la molcula
de colorante al xido nanoestructurado es de vital importancia en el diseo de la
celda solar. Desde que Regal y Grtzel reportaron en su trabajo pionero, el
entendimiento con xito de este mecanismo ha requerido una investigacin
fundamental sobre los distintos fenmenos fsicos en escalas nanomtricas.
Investigaciones tericas de las propiedades fsicas y qumicas de los colorantes
sensibilizadores son muy importantes para conocer la relacin entre la estructura,
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propiedades, desempeo y ayudar en el diseo y sntesis de nuevos colorantes
sensibilizadores.
La optimizacin de dispositivos como las celdas solares sensibilizadas por colorante
requiere un gran esfuerzo dirigido al desarrollo de nuevos diseos y a la elucidacin
de los procesos fotofsicos que producen la respuesta fotovoltaica. En la actualidad,
se realizan un gran nmero de investigaciones en el campo de este tipo de celdas,
tratando de llevar a cabo un mejor diseo que permita elevar la eficiencia. Sin
embargo, se han encontrado con la problemtica de no poder elevar dicho concepto
y para ese momento ya se ha invertido una cantidad importante de recurso
econmico, humano y de tiempo. Lo que nosotros planteamos en esta investigacin,
fue aplicar el modelado molecular con la finalidad de predecir o inferir tericamente
la eficiencia de una DSSC o establecer los parmetros ms relevantes que influyen
en su valor.
Las herramientas que proporciona la qumica computacional y el modelado
molecular, adems del creciente aumento en las capacidades de los equipos de
cmputo, permiten el desarrollo de objetivos que son cada vez ms demandados
por nuestra sociedad. Estas herramientas sern aplicadas al estudio y
caracterizacin por mtodos de qumica computacional de sensibilizadores
orgnicos y compuestos de coordinacin de complejos de cobre con ligandos de
tipo polipiridina, cuyos resultados obtenidos nos servirn para hacer la comparativa
con los datos experimentales y poder encontrar el concepto que permita evaluar o
inferir la eficiencia de una DSSC.
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LISTA DE ARTCULOS
Baldenebro Lpez, J., Castorena Gonzlez, J., Flores Holgun, N., Caldern
Guilln, J., & Glossman Mitnik, D. (2012). Computational characterization of
the molecular structure and properties of Dye 7 for organic photovoltaics.
Journal of Molecular Modeling, 835-842.
Baldenebro Lpez, J., Castorena Gonzlez, J., Flores-Holgun, N., Almaral
Snchez, J., & Glossman Mitnik, D. (2012). Density Functional Theory
(DFT) Study of Triphenylamine-Based Dyes for Their Use as Sensitizers in
Molecular Photovoltaics. International Journal of Molecular Sciences, 4418-
4432.
Baldenebro Lpez, J., Castorena Gonzlez, J., Flores Holgun, N., Almaral
Snchez, J., & Glossman Mitnik, D. (2012). Computational Molecular
Nanoscience Study of the Properties of Copper Complexes for Dye-
Sensitized Solar Cells. International Journal of Molecular Sciences, 16005-
16019.
Baldenebro Lpez, J., Flores Holgun, N., Castorena Gonzlez, J., Almaral
Snchez, J., & Glossman Mitnik, D. (2013). Theoretical Study of Copper
Complexes: Molecular Structure, Properties, and Its Application to Solar
Cells. International Journal of Photoenergy, 1-7.
Baldenebro Lpez, J., Flores Holgun, N., Castorena Gonzlez, J., & Glossman
Mitnik, D. (2013). Molecular design of copper complexes as sensitizers for
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efficient dye-sensitized solar cells. Journal of Photochemistry and
Photobiology A: Chemistry, 1-5.
Baldenebro Lpez, J., Castorena Gonzlez, J., Flores Holgun, N., & Glossman
Mitnik, D. (2014). Quantum chemical study of a new class of sensitisers:
influence of the substitution of aromatic rings on the properties of copper
complexes. Molecular Physics, 987-994.
Baldenebro Lpez, J., Flores Holgun, N., Castorena Gonzlez, J., & Glossman
Mitnik, D. (2014). Comparative study of copper complexes with different
anchoring groups by molecular modeling and its application to dye-
sensitized solar cells. Polyhedron, 33-36.
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ORIGINAL PAPER
Computational characterization of the molecular structureand properties of Dye 7 for organic photovoltaics
Jess Baldenebro-Lpez & Jos Castorena-Gonzlez &Norma Flores-Holguin & Joel Caldern-Guilln &Daniel Glossman-Mitnik
Received: 6 February 2011 /Accepted: 5 May 2011 /Published online: 20 May 2011# Springer-Verlag 2011
Abstract Organic dyes have great potential for its use insolar cells. In this recent work, the molecular structure andproperties of Dye 7 were obtained using density functionaltheory (DFT) and different levels of calculation. Uponcomparing the molecular structure and the ultravioletvisible spectrum with experimental data reported in theliterature, it was found that the M05-2X/6-31G(d) level ofcalculation gave the best approximation. Once the appro-priate methodology had been obtained, the molecule wascharacterized by obtaining the infrared spectrum, dipolemoment, total energy, isotropic polarizability, molecularorbital energies, free energy of solvation in differentsolvents, and the chemical reactivity sites using thecondensed Fukui functions.
Keywords Molecular structure . Polarizability .G(solv) .Chemical reactivity
AbbreviationsDFT Density functional theoryTD-DFT Time-dependent density functional theoryIR InfraredUV-vis Ultraviolet Angstrom
max Wavelength of maximum absorptionTHF TetrahydrofuranHOMO Highest occupied molecular orbitalLUMO Lowest unoccupied molecular orbitalG(solv) Free energy of solvationIEF-PCM Integral equation formalism of the
polarized continuum model
Introduction
Photovoltaic devices have gained wide acceptance as aclean and renewable energy source [1]. These devices arebased on the concept of charge separation at an interfacebetween two materials with different conduction mecha-nisms [2, 3]. One important invention in this field is thephotovoltaic dye-sensitized solar cell (DSSC) [4], whichhas been the subject of intense research due to its ability toconvert solar energy into electrical energy [5, 6], as well asits low cost compared to solar cells that use polycrystallinesilicon [7]. There are four main factors that affect theperformance of a DSSC: the photosensitive dye, the anode,the cathode and the electrolyte. The dye plays a crucial rolein enhancing the efficiency of the cell, which is why it isone of the most intensely studied factors [8]. In the presentwork, a theoretical study of the molecular structure andproperties of a dye (Dye 7) was performed. This dyeconsists of a triphenylamine molecule that serves aselectron donor group [9, 10], a thiophene to adjust theabsorption spectra of the molecules [11], and a cyanoacrylicacid that acts as an acceptor group[12], as shown in Fig. 1.In the investigation described below, different levels oftheory were used in order to establish the most appropriatemethodology to study this dye. Besides optimizing the
J. Baldenebro-Lpez : J. Castorena-Gonzlez :J. Caldern-GuillnFacultad de Ingeniera Mochis, Universidad Autnoma deSinaloa. Prol. ngel Flores y Fuente de Poseidn, S.N,C.P. 81223 Los Mochis, Sinaloa, Mxico
N. Flores-Holguin :D. Glossman-Mitnik (*)Centro de Investigacin en Materiales Avanzados, SC,Complejo Industrial Chihuahua,Miguel de Cervantes 120,Chihuahua 31109, Mxicoe-mail: [email protected]
J Mol Model (2012) 18:835842DOI 10.1007/s00894-011-1120-6
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geometry of Dye 7, the infrared and ultraviolet spectra werepresented, the dipole moment calculated, as were the totalenergy, isotropic polarizability, molecular orbital energies,and the free energy of solvation in different solvents;. Itschemically reactive sites were also discerned using thecondensed Fukui functions.
Molecular modeling
Density functional theory (DFT) was used in this study[13]. DFT was developed by Walter Kohn in the 1960s, andwas implemented in this study using the commercialsoftware Gaussian 03W [14]. The geometry of the moleculein its fundamental state was obtained by the establishedtechnique in Gaussian 03W. The force constants andvibrational frequencies were determined by calculatinganalytical frequencies for stationary points obtained afteroptimizing the geometry. Both calculations were done at thesame level of theory. The basis sets used in this study were3-21G(d) and 6-31G(d) (for more details, see [15]). Thedensity functionals used in this research were: BLYP [16,18], B3LYP [1619], PBE [20, 21], PBE1PBE [21], TPSS[22], TPSSh [23] and M05-2X [24]. A detailed descriptionof these density functionals can be found in the updatedbibliography of computational chemistry [2528]. Thecalculation of the ultraviolet spectrum of the moleculeDye 7 was done via time-dependent DFT equationsaccording to the method implemented in the Gaussianmolecular package 03W [25, 2932]. The equations weresolved for 20 excited states. The infrared (IR) andultraviolet-visible (UV-vis) spectra were analyzed andvisualized using the program SWizard [33]. In all cases,
the displayed spectra show the calculated frequencies andabsorption wavelengths.
The condensed Fukui functions were calculated usingAOMIX molecular analysis software [34, 35], starting fromsingle-point energy calculations.
Results and discussion
The molecular structure of Dye 7 was analyzed at differentlevels of theory, as mentioned above. The bond lengthsobtained from our calculations as well as experimental datareported in the literature on the systems that comprise ourdye are shown in Table 1 for the most representative bonds.It is clear that there is good agreement among the resultsobtained for different models for each bond. However, tocheck which methodology gives the most accurate resultsfor the molecular structure of Dye 7, a statistical techniqueknown as population standard deviation was applied(PSTD) to the results for the bond lengths. When applyingthis technique, the experimental result was used as areference for the average value; in this way, the modelsthat give the lowest deviation will give the best representationsof our study system. It is important to note that we have notconsidered a tolerance level for the deviation, as our prioritywas to establish the model that best fits the experimentalresults. The results from the PSTD are shown in Table 2,which indicates that the most accurate methodology for thetheoretical study is M05-2X/6-31G(d). The interatomic bondlengths () and the angles (in degrees) calculated at thislevel of calculation are shown in Fig. 2.
A second validation of the models involved comparingthe theoretical wavelengths of maximum absorption (max)
Fig. 1 Molecular structure of Dye 7: triphenylamine (donor), a thiophene (to adjust the absorption spectra of the molecules) and cyanoacrylic acid(acceptor)
836 J Mol Model (2012) 18:835842
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Table 1 Bond lengths calculated at different levels of theory for Dye 7, and well as experimental data reported in the literature
Model C1C2 C3C4 C4C5 C1C6 C1H7 C13H12 C13C14 C16C19 C15C17 C23C24 C24C26
BLYP/3-21G* 1.415 1.404 1.408 1.404 1.089 1.089 1.415 1.404 1.408 1.421 1.395
B3LYP/3-21G* 1.403 1.394 1.397 1.394 1.082 1.082 1.403 1.394 1.397 1.408 1.387
PBE/3-21G* 1.412 1.401 1.406 1.401 1.092 1.092 1.412 1.402 1.406 1.418 1.393
PBE1PBE/3-21G* 1.400 1.391 1.395 1.391 1.084 1.084 1.400 1.391 1.395 1.405 1.385
TPSS/3-21G* 1.410 1.401 1.405 1.401 1.087 1.087 1.410 1.401 1.405 1.416 1.393
TPSSh/3-21G* 1.406 1.397 1.401 1.397 1.084 1.084 1.406 1.397 1.401 1.411 1.389
M05-2X/3-21G* 1.397 1.390 1.393 1.390 1.079 1.079 1.397 1.390 1.393 1.400 1.386
BLYP/6-31G(d) 1.414 1.404 1.407 1.403 1.092 1.092 1.414 1.403 1.407 1.420 1.395
B3LYP/6-31G(d) 1.403 1.394 1.397 1.394 1.085 1.085 1.403 1.394 1.396 1.408 1.387
PBE/6-31G(d) 1.410 1.400 1.403 1.399 1.094 1.094 1.410 1.400 1.403 1.416 1.392
PBE1PBE/6-31G(d) 1.399 1.390 1.393 1.390 1.086 1.086 1.399 1.390 1.393 1.403 1.384
TPSS/6-31G(d) 1.408 1.399 1.402 1.399 1.088 1.088 1.408 1.399 1.402 1.414 1.391
TPSSh/6-31G(d) 1.404 1.395 1.398 1.395 1.086 1.086 1.404 1.395 1.398 1.409 1.388
M05-2X/6-31G(d) 1.397 1.390 1.392 1.390 1.082 1.082 1.397 1.390 1.392 1.400 1.385
Experimental 1.397 1.397 1.397 1.397 1.084 1.084 1.397 1.397 1.397 1.397 1.397
Model C26-C30 C24-H27 C2-N33 C30-C34 C34-C35 C34-H36 C35-C37 C37-C38 C37-S39 C38-C40 C38-H41
BLYP/3-21G* 1.425 1.089 1.448 1.459 1.371 1.095 1.444 1.412 1.764 1.409 1.088
B3LYP/3-21G* 1.411 1.082 1.434 1.456 1.355 1.088 1.442 1.397 1.741 1.404 1.080
PBE/3-21G* 1.422 1.091 1.436 1.454 1.369 1.098 1.440 1.412 1.748 1.405 1.090
PBE1PBE/3-21G* 1.407 1.083 1.424 1.452 1.352 1.089 1.440 1.395 1.726 1.401 1.081
TPSS/3-21G* 1.421 1.087 1.440 1.455 1.368 1.093 1.441 1.410 1.750 1.405 1.085
TPSSh/3-21G* 1.415 1.084 1.434 1.454 1.361 1.090 1.441 1.403 1.740 1.404 1.082
M05-2X/3-21G* 1.402 1.079 1.426 1.462 1.343 1.084 1.450 1.386 1.721 1.409 1.076
BLYP/6-31G(d) 1.423 1.092 1.439 1.455 1.374 1.097 1.443 1.410 1.766 1.405 1.091
B3LYP/6-31G(d) 1.410 1.085 1.427 1.453 1.358 1.089 1.442 1.396 1.743 1.400 1.084
PBE/6-31G(d) 1.419 1.093 1.428 1.449 1.370 1.098 1.438 1.408 1.748 1.400 1.092
PBE1PBE/6-31G(d) 1.405 1.085 1.417 1.449 1.353 1.090 1.439 1.392 1.727 1.397 1.084
TPSS/6-31G(d) 1.418 1.088 1.431 1.451 1.369 1.092 1.439 1.406 1.748 1.400 1.087
TPSSh/6-31G(d) 1.412 1.085 1.426 1.451 1.362 1.089 1.440 1.400 1.739 1.398 1.084
M05-2X/6-31G(d) 1.401 1.082 1.418 1.459 1.346 1.086 1.450 1.386 1.723 1.404 1.080
Experimental 1.397 1.084 1.418 1.475 1.334 1.099 1.475 1.370 1.714 1.423 1.079
Model C42-S39 C40-C42 C35-H44 C42-C45 C45-C46 C46-C48 C46-C49 C48-N50 C49-O51 C49-052 O52-53H
BLYP/3-21G* 1.781 1.412 1.094 1.425 1.383 1.417 1.488 1.182 1.244 1.400 1.010
B3LYP/3-21G* 1.758 1.397 1.086 1.421 1.367 1.411 1.477 1.168 1.231 1.376 0.996
PBE/3-21G* 1.763 1.411 1.096 1.422 1.380 1.412 1.481 1.183 1.243 1.390 1.008
PBE1PBE/3-21G* 1.742 1.395 1.087 1.419 1.363 1.408 1.472 1.167 1.228 1.366 0.992
TPSS/3-21G* 1.765 1.409 1.091 1.422 1.379 1.413 1.480 1.181 1.243 1.394 1.005
TPSSh/3-21G* 1.756 1.403 1.088 1.421 1.372 1.411 1.476 1.175 1.237 1.383 0.999
M05-2X/3-21G* 1.735 1.387 1.082 1.428 1.353 1.413 1.473 1.159 1.224 1.366 0.991
BLYP/6-31G(d) 1.784 1.410 1.095 1.426 1.389 1.427 1.492 1.179 1.230 1.375 0.986
B3LYP/6-31G(d) 1.761 1.396 1.088 1.423 1.372 1.424 1.483 1.165 1.216 1.353 0.975
PBE/6-31G(d) 1.765 1.408 1.097 1.422 1.385 1.422 1.486 1.179 1.227 1.364 0.985
PBE1PBE/6-31G(d) 1.743 1.393 1.088 1.421 1.367 1.420 1.479 1.164 1.212 1.343 0.972
TPSS/6-31G(d) 1.765 1.406 1.090 1.423 1.384 1.423 1.484 1.176 1.226 1.366 0.983
TPSSh/6-31G(d) 1.756 1.400 1.088 1.422 1.376 1.422 1.481 1.170 1.220 1.356 0.978
M05-2X/6-31G(d) 1.739 1.387 1.084 1.430 1.358 1.428 1.482 1.157 1.209 1.342 0.971
Experimental 1.714 1.370 1.099 1.475 1.334 1.475 1.475 1.172 1.200 1.334 0.970
J Mol Model (2012) 18:835842 837
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for Dye 7 obtained using the models with the valueobtained experimentally in research conducted by Zhanget al. [4]. In their research, they found that max occurred at432 nm in the solvent tetrahydrofuran (THF). Knowing thisfact, it was possible to re-validate which of the models bestrepresented our molecular system. Thus, the UV-visspectrum of Dye 7 in THF solvent was calculated at theB3LYP/6-31G(d), BLYP/6-31G(d), M05-2X/6-31G(d),PBE1PBE/6-31G(d), PBE/6-31G(d), TPSS/6-31G(d) andTPSSh/6-31G(d) levels of theory using time-dependentDFT (TD-DFT). The results obtained are shown in Fig. 3.
The calculated UV-vis spectra are provided in Table 3,and it is quite apparent that the closest theoretical value ofmax to the experimental one is 444.44 nm, which wasafforded by M05-2X/6-31G(d).
The calculated value of max is an important parameterwhich indicates that this molecular system should beconsidered for use as a functional material (a dye in thiscase) in a DSSC, as the value of this parameter for Dye 7 fallswithin the range of the solar spectrum of visible light [36].
At all of the levels of theory tested, the observed signalcorresponded to the HOMO (highest occupied molecularorbital) to LUMO (lowest unoccupied molecular orbital)transition. Table 4 shows the results of TD-DFT calcu-lations performed using the functional M05-2X and thebasis set 6-31G(d), including the electronic state transitions,their corresponding wavelengths (in nm) and energies (ineV), as well as their assignments in terms of the orbitalsinvolved in the transitions.
Taking into account the results for the molecularstructure and UV-vis spectrum calculations, it is clear thatthe functional M05-2X and the basis set 6-31G(d) is the
most appropriate level of calculation to perform the rest ofthe characterization of Dye 7.
The infrared spectrum (IR) for Dye 7 calculated with M05-2X/6-31G(d) is shown in Fig. 4. The vibrational bands wereassigned using the molecular visualization software forWindows ChemCraft. CS stretching is observed as a peakat 584 cm1. At 796 cm1, a peak due to vibrations of carbon-chain hydrogens out of the plane of the aromatic rings can beseen, while another peak due to the corresponding vibrationsfor the bending of CH in thiophene is present at 1091 cm1.Vibrations due to the bending of the bond OH produce apeak at 1236 cm1. Other intense peaks include those due tothe stretching of the CN bond in the amine and the CC inthe thiophene occurring at 1392 cm1 and 1521 cm1
respectively; meanwhile, at 1677 cm1, a peak due to thedouble-bond stretching of C(45)=C(46) is noted. The peak at1872 cm1 represents the stretching of the double bond C=O,while the vibration at 2417 cm1 corresponds to the stretchingof the triple bond CN. The CH vibrations for the aromaticrings occur at 3244 cm1, and stretching of the OH bond isobserved at 3784 cm1.
The molecular dipole moment is an experimentalmeasure of the charge distribution in a molecule. Theprecision of the global distribution of electrons in amolecule is difficult to quantify, since it involves allmultipoles. In this calculation, the values of the totalenergy of the system, the total dipole moment and theisotropic polarizability in the fundamental state obtained atthe M05-2X/6-31G(d) level of calculation are 1736.99 a.u.,6.7374 debye and 433.06 bohr3. Moreover, the calculatedenergies of the HOMO and LUMO are 6.29 eV and 1.85 eV,respectively. These results are of great importance, since theycan be used during synthesis to determine the solubility andchemical reactivity of the molecule, and they can also beemployed in organic electronics and photovoltaics, asreported in different works [3739].
The free energy of solvation G(solv) of the moleculewas calculated for Dye 7 using M05-2X/6-31G(d) coupledwith the integral equation formalism of the polarizedcontinuum model (IEF-PCM) for different solvents. Thesolubility of a molecule depends on several kinetic andthermodynamic factors. However, the magnitude and signof G(solv) can be used as an approximate index ofsolubility. In this sense, a negative sign and a largemagnitude indicates increased solubility. The results ofthis calculation for the studied molecule can besummarized as follows: cyclohexane=1.85 kcal mol1,chloroform=4.68 kcal mol1, water=5.02 kcal mol1,THF=5.30 kcal mol1, acetone=12.86 kcal mol1,ethanol=14.43 kcal mol1 and methanol=15.03 kcalmol1. Based on these results, it appears that the moleculeunder investigation will be most soluble in methanol andethanol.
Table 2 Results of the population standard deviation for the bondlengths obtained with different models
Model Population standard deviation
BLYP/3-21G* 0.0258
B3LYP/3-21G* 0.0189
PBE/3-21G* 0.0236
PBE1PBE/3-21G* 0.0175
TPSS/3-21G* 0.0232
TPSSh/3-21G* 0.0203
M05-2X/3-21G* 0.0150
BLYP/6-31G(d) 0.0238
B3LYP/6-31G(d) 0.0171
PBE/6-31G(d) 0.0214
PBE1PBE/6-31G(d) 0.0159
TPSS/6-31G(d) 0.0204
TPSSh/6-31G(d) 0.0179
M05-2X/6-31G(d) 0.0130
838 J Mol Model (2012) 18:835842
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The HOMO and LUMO orbitals of Dye 7 calculated atthe M05-2X/6-31G(d) level of theory are shown in Fig. 5.The HOMO orbital density is located over the double bondsof the carbon chain and the nitrogen (N33); meanwhile, thedensity of the LUMO orbital is concentrated over the CCsingle bonds. This provides a good idea of the reactivity ofthe molecule.
The reactive sites can be identified through theseorbital densities. The calculated HOMO and LUMO
densities shown in Fig. 5 indicate that electrophilic attackmay occur preferentially at the C=C double bonds or atN33, while nucleophilic attack occurs at CC singlebonds.
The condensed Fukui functions can also be used todetermine the reactivity of each atom in the molecule. Thecorresponding condensed Fukui functions are f k qk N 1 % qkN (for nucleophilic attack), f %k qkN % qk N % 1 (for electrophilic attack) and f 0k
Fig. 2 Interatomic bond distances () and bond angles (in degrees) for Dye 7 obtained at the M05-2X/6-31G(d) level of calculation
J Mol Model (2012) 18:835842 839
-
qk N 1 % qk N % 1 '=2 (for radical attack), where qk isthe effective Mulliken charge of atom k in the molecule.
The calculations of the condensed Fukui functions fornucleophilic and electrophilic attacks were performed usingAOMIX (a molecular analysis program), which gave thefollowing results: f k 0:1876 and f %k 0:1906.
Electrophilic attack will occur at atoms that produce anegative charge, and where the Fukui function f %k is amaximum. This value confirms that the most probable siteof electrophilic attack is N33. Nucleophilic attacks, on theother hand, will occur at atoms that produce a positivecharge and where the Fukui function f k is a maximum.
Fig. 3 Ultraviolet-visible (UV-vis) spectrum of Dye 7 calculated using time-dependent DFT (TD-DFT) with the basis set 6-31G(d) and thefunctionals used in this research
Table 3 Values of the wavelength of maximum absorption by Dye 7calculated using the various models tested
Model max (nm)
BLYP/6-31G(d) 500.00
B3LYP/6-31G(d) 598.80
PBE/6-31G(d) 495.05
PBE1PBE/6-31G(d) 564.97
TPSS/6-31G(d) 480.77
TPSSh/6-31G(d) 666.67
M05-2X/6-31G(d) 444.44
Experimental 432.00
Table 4 Electronic transition states of Dye 7 (calculated with TD-DFT at the M05-2X/6-31G(d) level of theory)
State Wavelength (nm) Energy (eV) f Assignment (H = HOMO,L = LUMO)
1 444.1 2.79 1.7821 S H-0L+0(+73%) H-1L+0(11%)
2 325.2 3.81 0.0338 S H-1L+0(+61%) H-0L+1(+12%) H-0L+0(+8%)
3 295.5 4.2 0.2897 S H-0L+1(+64%) H-0L+0(12%) H-1L+0(9%)
4 280.2 4.42 0.0564 S H-0L+2(+79%)
5 271.3 4.57 0.2703 S H-0L+3(+81%) H-1L+3(+8%)
6 268.1 4.62 0.0851 S H-7L+0(+71%) H-4L+0(+12%)
7 252.1 4.92 0.0215 S H-0L+5(+33%) H-0L+4(9%) H-6L+0(+8%) H-1L+1(+8%) H-3L+0(+6%)
8 250.8 4.94 0.0052 S H-1L+1(+18%) H-4L+0(12%) H-0L+5(12%) H-7L+0(+9%) H-0L+4(6%)
9 243.4 5.09 0.0001 S H-9L+0(+67%) H-9L+8(12%) H-9L+1(9%)
10 241 5.15 0.0261 S H-0L+6(+57%) H-4L+0(+10%) H-1L+6(+8%) H-3L+3(5%)
11 239.2 5.18 0.0028 S H-1L+1(+24%) H-4L+0(+23%) H-2L+0(14%) H-0L+4(11%) H-0L+6(8%)
12 234.1 5.3 0.0824 S H-3L+0(+40%) H-0L+5(24%) H-6L+0(+14%)
13 227.8 5.44 0.001 S H-0L+7(+47%) H-1L+7(39%)
14 223.2 5.55 0.0095 S H-0L+4(+49%) H-1L+1(+26%) H-0L+1(+7%)
15 221 5.61 0.0054 S H-2L+0(+68%) H-4L+0(+17%)
16 215.4 5.76 0.0469 S H-6L+0(+51%) H-3L+0(33%)
17 209.6 5.92 0.0093 S H-5L+0(+88%) H-5L+1(+6%)
18 206.6 6 0.0012 S H-1L+10(+17%) H-15L+0(16%) H-0L+10(12%) H-9L+8(6%)
19 206 6.02 0.0678 S H-8L+0(+42%) H-4L+1(+15%) H-10L+0(13%) H-4L+0(+6%)
20 203.1 6.1 0.0158 S H-1L+2(+35%) H-2L+3(10%) H-2L+2(+6%) H-0L+5(+6%)
840 J Mol Model (2012) 18:835842
-
Thus, the atom most likely to suffer a nucleophilic attack isC45.
Conclusions
In this work, a general comparison of the optimizations ofthe molecular structure and the ultraviolet spectrum in THFsolvent achieved with different density functionals andbasis sets was performed. This comparison indicated thatthe functional that gave results that were closest to theexperimental results was M05-2X, along with the basis set6-31G(d), so this level of theory was then used to studyDye 7 molecule, which is intended for use in photovoltaicdevices. The total energy of this system, its dipole moment,
its isotropic polarizability, its molecular orbitals and itsinfrared spectrum were calculated using M05-2X/6-31G(d).
The free energy of solvation G(solv) of the molecule,calculated using the same level of theory along with theintegral equation formalism of the polarized continuummodel (IEF-PCM), indicates that the molecule is potentiallysoluble in methanol and ethanol.
The M05-2X/6-31G(d) methodology can be used as anuseful tool for studying the molecular structure andelectronic properties of Dye 7, as well as other structuresderived from it.
Acknowledgments This work was made possible by the support ofUniversidad Autnoma de Sinaloa through the Facultad de IngenieraMochis, by the PROFAPI2010/033 project, Centro de Investigacinen Materiales Avanzados, S.C. (CIMAV), and Consejo Nacional deCiencia y Tecnologa.
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Int. J. Mol. Sci. 2012, 13, 4418-4432; doi:10.3390/ijms13044418
International Journal of
Molecular Sciences ISSN 1422-0067
www.mdpi.com/journal/ijms
Article
Density Functional Theory (DFT) Study of
Triphenylamine-Based Dyes for Their Use as Sensitizers
in Molecular Photovoltaics
Jess Baldenebro-Lpez 1,2
, Jos Castorena-Gonzlez 2, Norma Flores-Holgun
1,
Jorge Almaral-Snchez 2 and Daniel Glossman-Mitnik
1,*
1 NANOCOSMOS Virtual Lab, Advanced Materials Research Center (CIMAV),
Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua 31190, Mxico;
E-Mails: [email protected] (J.B.-L.); [email protected] (N.F.-H.) 2 Faculty of Engineering Mochis, Autonomous University of Sinaloa, Prol. ngel Flores y Fuente de
Poseidn, S.N., Los Mochis, Sinaloa 81223, Mxico; E-Mails: [email protected] (J.C.-G.);
[email protected] (J.A.-S.)
* Author to whom correspondence should be addressed; E-Mail: [email protected];
Tel.: +52-614-439-1151; Fax: +52-614-439-1130.
Received: 6 February 2012; in revised form: 4 March 2012 / Accepted: 20 March 2012 /
Published: 10 April 2012
Abstract: In this work we studied three dyes which are proposed for potential photovoltaic
applications and named Dye7, Dye7-2t and Dye7-3t. The Density Functional Theory (DFT)
was utilized, using the M05-2X hybrid meta-GGA functional and the 631+G(d,p) basis
set. This level of calculation was used to find the optimized molecular structure and to
predict the main molecular vibrations, the absorption and emission spectra, the molecular
orbitals energies, dipole moment, isotropic polarizability and the chemical reactivity
parameters that arise from Conceptual DFT. Also, the pKa values were calculated with the
semi-empirical PM6 method.
Keywords: molecular structure; absorption spectrum; polarizability; chemical reactivity;
dipole moment; triphenylamine; dye sensitizers
OPEN ACCESS
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Int. J. Mol. Sci. 2012, 13 4419
1. Introduction
The worlds traditional energy sources (oil, natural gas and coal) have a finite life, and actual
predictions indicate that alternative sources must provide an important contribution in the near
future [1]. Solar energy is one of the most promising sources of energy in the future. The direct
conversion of sunlight into electric energy using solar cells is particularly interesting because it has a
lot of advantages over other methods, for example, it does not produce greenhouse gases nor nuclear
byproducts [2]. The dye sensitized solar cells (DSSC) have attracted attention due to their efficiency,
simple manufacturing and low cost [310]. In these DSSC, an organic sensitizer must be chemically
absorbed on the porous surface of the nanocrystalline oxide. After absorbing a photon, the excited
electron in the dye-sensitized molecule is transferred into the conduction band of nanocrystalline oxide,
followed by a process in which the electron diffuses through the electrode. The sensitizer in this
oxidized state is reduced to its normal state gaining electrons through a liquid electrolyte [1113].
Nowadays, many research groups from all over the world actively participate to improve the efficiency
of every single process involved in the DSSC [1416]. The charge transfer efficiency from the dye
molecule to the nanocrystalline oxide is extremely important in the solar cell design. Since Regal and
Grtzel published their pioneer study [17], the understanding of the mechanism has required
fundamental research about the diverse physical phenomena at nanometric scale [18].
Theoretical studies on physical and chemical properties of dye-sensitizers are very important to
understand the relationship between the structure, properties and performance in order to design and
synthesize new molecules for this purpose [1923]. To be useful in DSSC a sensitizer must meet
important requirements in its structure, such as: the electron-donating part [24,25], a unit to adjust the
absorption spectrum [26] and the electron-acceptor part [27].
In the synthesis of new DSSC dyes, the triphenylamine based structures have been widely
employed to build metal-free organic dyes and they have been successfully proven to show high
conversion efficiency in DSSC devices [2832] since they can both enhance the hole transporting
ability of the materials and inhibit the aggregation of the dyes with their non-planar structure [33]. The
tiophene molecule is included in the structure of the proposed systems due is one of the most
frequently used -spacer in organic dyes having a donor-(-conjugated bridge)-acceptor (D--A)
system in dye-sensitized solar cells (DSSCs) due to its high structural stability, ease of structural
modification, excellent optical and electronic properties and controllable electrochemical
behavior [3439]; the -conjugated derivates are generally efficient fluorophores, and as such, useful
for the fabrication of nanobiosensors; they can be used as an attractive building block for Organic
Molecular Materials [40]. The absorption spectra of the dye molecules can be easily tuned by changing
the length of the oligothiophene bridge [24], this modification of the size of the thiophene chain not
only will allow to evaluate the influence of these changes on the properties of absorption, but also on
the emission spectra and band gap, among others. Moreover, the electron acceptor part has significant
influences on the photovoltaic properties due to the fact that the excited electrons from the dye
molecules are injected into the semiconductor film through this component [41]; a viable candidate
that can efficiently act as a linker moiety is cyanoacrylic acid, since carboxylic acid binds strongly to
the TiO2 surface through a bridging which is not easily removed by rinsing, and the presence of the
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Int. J. Mol. Sci. 2012, 13 4420
cyano group enhances the electron-withdrawing nature of the linker moiety [39]. Based on this, we
studied three sensitizers, which were named Dye7, Dye7-t2 and Dye7-t3 (Figure 1).
Figure 1. Sensitizer diagrams studied in this research (Dye7, Dye7-2t and Dye7-3t).
O
N
S
N
OH
N
S S
O
N
OH
N
S S
S
O
N
OH
Dye7
Dye7-2t
Dye7-3t
The aim of this work is to report the results of our research using molecular structure calculations
and properties of the three dyes according to the density functional theory [42], developed by Walter
Kohn in the 1960s. This method is implemented in the Gaussian 09W program package [43].
2. Results and Discussion
Once the molecular structures were proposed, the geometry optimization followed, which allowed
the lowest energy configurations to be obtained, which are shown in Figure 2 including the numbers of
atoms and symbols.
A selection of geometric parameters was made to clearly visualize how the geometric structures are
similar regarding both conformation and geometric data when they are optimized using the M05-2X
functional and the 631+G(d,p) basis set. Table 1 shows the selected values for bond length (), bond
angles and dihedral angles (in degrees).
Table 1. Dye 7, Dye7-2t and Dye7-3t selected bond lengths (angstroms), bond angles and
dihedral angles (degrees).
Dye7 Value Dye7-2t Value Dye7-3t Value
C2-N33 1.418 C55-N86 1.416 C125-N146 1.417
C14-N33 1.418 C67-N86 1.417 C115-N146 1.416
C23-N33 1.404 C76-N86 1.407 C139-N149 1.408
C2-C3 1.398 C55-C56 1.399 C125-C126 1.399
C15-C17 1.394 C70-C72 1.394 C118-C119 1.394
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Int. J. Mol. Sci. 2012, 13 4421
Table 1. Cont.
C30-C34 1.460 C83-C87 1.463 C140-C147 1.463
C35-C37 1.452 C88-C90 1.454 C148-C151 1.454
C42-C45 1.430 C101-C105 1.427 C166-C172 1.431
C2-N33-C23 120.3 C55-N86-C76 120.4 C125-N146-C139 120.2
C14-N33-C23 120.6 C67-N86-C76 120.1 C115-N146-C139 120.3
C30-C34-C35 126.6 C83-C87-C88 126.6 C140-C147-C148 126.3
C42-C45-C46 129.2 C101-C105-C106 130.2 C166-C172-C173 129.0
C2-N33-C23-C25 35.16 C55-N86-C76-C78 37.71 C125-N146-C139-C137 37.52
C23-N33-C14-C16 44.61 C66-C67-N86-C76 44.26 C116-C115-N146-C139 42.86
C45-C46-C49-O51 0.21 C105-C106-C109-O111 0.22 C172-C173-C177-O178 0.27
Figure 2. Optimized molecular structures of Dye7, Dye7-2t and Dye7-3t.
In IR spectra calculations there also appears to be a similarity in molecular structure due to the
presence of vibrations with wave numbers close to each other. The O-H bond stretching vibration
appears from 3836 cm1
to 3843 cm1
. The vibration of the aromatic ring C-H bond is observed in the
range from 3237 cm1
to 3248 cm1
, and the range from 2406 cm1
to 2416 cm1
corresponds to the
stretching of the triple bond CN. Other intense peaks that correspond to the stretching of the double
bond carboxylic acid C=O and to the double bond between the thiophene molecule and the acid (C=C),
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Int. J. Mol. Sci. 2012, 13 4422
appear in the ranges from 1835 cm1
to 1842 cm1
and from 1664 cm1
to 1678 cm1
, respectively. The
double bond C=C in the thiophene has a symmetric stretching vibration which presents from 1513
cm1
to 1517 cm1
. The vibration of the bond C-N in the amine occurs in the range from 1380 cm1
to
1391 cm1
. The corresponding vibration for the bending of the bond O-H produces a peak from 1201
cm1
to 1223 cm1
. The bending of C-H in thiophene is present from 1086 cm1
to 1093 cm1
. The out-
of-plane bending vibration associated with the aromatic rings appears from 721 cm1
to 726 cm1
. The
C-S bond stretching vibration is shown in the wavenumbers ranging from 581 cm1
to 598 cm1
.
Absorption spectra for the proposed dye molecules are shown in Figure 3. The calculated value of
max is an important parameter, which indicates that these molecular systems should be considered for
use as a functional material (as dye in this case) in a DSSC, the value of this parameter for Dye7,
Dye7-2t and Dye7-3t meets the requirements established in the literature [44].
Figure 3. Ultraviolet-visible (UV-vis) spectra of Dye7, Dye7-2t and Dye7-3t.
In all the UV-Vis spectra studied, the observed signal corresponds to the highest occupied
molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition. Tables 24
show the results of TD-DFT calculations performed using the M05-2X functional and the
631+G(d,p) basis set; including the electronic excited states, the corresponding wavelengths (in nm),
the energies (in eV), the oscillator strength (f) and the orbitals involved in the transitions.
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Int. J. Mol. Sci. 2012, 13 4423
Table 2. Dye7 electronic excited states, showing wavelengths (nm), energies (eV),
oscillator strength (f) and the orbitals involved in the transitions; calculated with
Time-dependent density functional theory (TD-DFT) at M05-2X/631+G(d,p). Only
excited states with f > 0.03 are shown.
Number nm eV (f) Assignment; H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.
1 425.5 2.91 1.6808 S H-0- > L + 0 (+73%) H-1- > L + 0 (8%)
2 320.5 3.87 0.0361 S H-1- > L + 0 (+55%) H-0- > L + 1 (19%)
3 301.4 4.11 0.0797 S H-0- > L + 2 (+42%) H-0- > L + 1 (+27%)
H-1- > L + 0 (+10%) H-0- > L + 0 (+7%)
4 294.4 4.21 0.1563 S H-0- > L + 2 (+40%) H-0- > L + 1 (30%)
H-0- > L + 0 (7%) H-1- > L + 0 (6%)
5 281.2 4.41 0.2058 S H-0- > L + 3 (+80%) H-1- > L + 3 (+9%)
6 266.4 4.65 0.0517 S H-6- > L + 0 (+44%) H-7- > L + 0 (14%)
H-4- > L + 0 (+13%) H-1- > L + 1 (10%)
H-0- > L + 4 (+6%)
7 259.4 4.78 0.0365 S H-0- > L + 5 (+40%) H-0- > L + 4 (15%) H-7- > L + 0 (11%)
Table 3. Dye7-2t electronic excited states, showing wavelengths (nm), energies (eV),
oscillator strength (f) and the orbitals involved in the transitions; calculated with TD-DFT
at M05-2X/6-31+G(d,p). Only excited states with f > 0.03 are shown.
Number nm eV (f) Assignment; H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.
1 446.4 2.78 1.8564 S H-0- > L + 0 (+67%) H-1- > L + 0 (22%) H-0- > L + 1 (+7%)
2 322.8 3.84 0.2720 S H-0- > L + 1 (+41%) H-0- > L + 0 (25%)
H-1- > L + 0 (19%) H-1- > L + 1 (6%)
3 299.8 4.14 0.0368 S H-0- > L + 2 (+65%) H-0- > L + 3 (14%) H-1- > L + 2 (+8%)
4 286.8 4.32 0.0514 S H-1- > L + 1 (+36%) H-2- > L + 0 (22%)
H-8- > L + 0 (14%) H-0- > L + 3 (11%) H-5- > L + 0 (+6%)
5 283.2 4.38 0.2221 S H-0- > L + 4 (+77%) H-1- > L + 4 (+17%)
6 258.6 4.79 0.0496 S H-0- > L + 5 (+28%) H-8- > L + 0 (+12%)
H-5- > L + 0 (+11%) H-0- > L + 7 (7%)
7 248.9 4.98 0.0668 S H-0- > L + 6 (+42%) H-0- > L + 3 (12%) H-1- > L + 6 (+10%)
8 239.1 5.19 0.1066 S H-1- > L + 3(+15%) H-2- > L + 1(+11%) H-0- > L + 8(8%)
H-0- > L + 7(+8%) H-9- > L + 0(8%) H-0- > L + 9(+8%)
9 231.1 5.36 0.0553 S H-4- > L + 0(+32%) H-4- > L + 1(+15%)
H-0- > L + 7(+10%) H-7- > L + 0(10%) H-7- > L + 1(6%)
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Int. J. Mol. Sci. 2012, 13 4424
Table 4. Dye7-3t electronic excited states, showing wavelengths (nm), energies (eV),
oscillator strength (f) and the orbitals involved in the transitions; calculated with TD-DFT
at M05-2X/6-31+G(d,p). Only excited states with f > 0.03 are shown.
Number nm eV (f) Assignment; H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.
1 448.4 2.76 2.1948 S H-0- > L + 0 (+47%) H-1- > L + 0 (26%) H-0- > L + 1 (+12%)
2 364.3 3.40 0.1536 S H-0- > L + 1 (+49%) H-1- > L + 0 (+19%)
H-0- > L + 2 (6%) H-2- > L + 0 (+6%)
3 339.3 3.65 0.3332 S H-0- > L + 0 (+47%) H-1- > L + 0 (+21%)
H-0- > L + 1 (12%) H-2- > L + 0 (+9%) H-1- > L + 1 (+5%)
4 309.6 4.00 0.0758 S H-1- > L + 1 (+39%) H-0- > L + 2 (19%) H-2- > L + 0 (12%)
5 300.2 4.13 0.0462 S H-0- > L + 3 (+70%) H-1- > L + 3 (+13%)
H-1- > L + 1 (+12%)
6 284.2 4.36 0.2165 S H-0- > L + 4 (+69%) H-1- > L + 4 (+18%)
7 278.2 4.46 0.0348 S H-0- > L + 2 (+29%) H-1- > L + 2 (27%)
H-1- > L + 1 (+15%) H-0- > L + 1 (+6%)
8 255.8 4.85 0.0597 S H-10- > L + 0 (+32%) H-9- > L + 0 (13%) H-6- > L + 0 (8%)
9 243.6 5.09 0.0752 S H-0- > L + 10 (13%) H-2- > L + 1 (+12%)
H-11- > L + 0 (+11%) H-0- > L + 5 (+10%)
H-5- > L + 0 (+9%) H-6- > L + 0 (+6%)
10 234.0 5.30 0.0496 S H-2- > L + 2 (+13%) H-1- > L + 5 (+10%)
H-0- > L + 11 (10%) H-9- > L + 0 (+6%)
To calculate the fluorescence spectra, the excited state optimization was carried out using the
CIS/6-31+G(d,p) model [45]. The TD-DFT results are shown in Figure 4. The wavelength
corresponding to the HOMO-LUMO transition is 483 nm for Dye7, 518 nm for Dye7-2t and 476 nm
for Dye7-3t. These results indicate that the studied molecules have fluorescence in the visible region,
and for this reason they constitute potential application in organic light emission diodes (OLEDs) [46].
Figure 4. Fluorescence (fluo) spectra of sensitizers calculated using time-dependent DFT
(TD-DFT) with the M05-2X/631+G(d,p) level.
-
Int. J. Mol. Sci. 2012, 13 4425
In this work, a summary has been made based on the total dipole moment (for the ground state), the
isotropic polarizability, and pKa values (in water as solvent). This information is shown in Table 5.
Figure 5 shows the HOMO-LUMO molecular orbitals energetic position. These results are of great
importance, since they can be used during synthesis to determine the solubility and chemical reactivity
of the molecule, and they can also be employed in organic electronics and photovoltaics, as reported in
different studies [4749].
Table 5. Dipole moment (), polarizability () and pKa values.
Molecule (Debye) (Bohr3) pKa
Dye7 7.15 479.04 0.17
Dye7-2t 6.42 578.60 0.24
Dye-3t 7.13 684.24 0.39
Figure 5. Molecular orbitals energy levels diagram.
The energy values of the orbitals indicate that these dyes can be used in a DSSC; this is a result of the
sensitizer LUMO level and the conduction band of nanocrystalline oxides that are commonly used in
such devices.
The molecular orbitals HOMO and LUMO calculated at M05-2X/631+G(d,p) level of theory are
shown in Figure 6. The reactive sites can be identified via orbital densities. The HOMO orbital density
is located over the double bonds of the carbon chain and the nitrogen of the triphenylamine molecule,
which indicates that in these sites an electrophilic attack can occur. Meanwhile, the density of the
LUMO orbital is concentrated over the C-C single bonds; therefore, these are most likely sites for a
nucleophilic attack.
-
Int. J. Mol. Sci. 2012, 13 4426
Figure 6. HOMO and LUMO orbitals of Dye 7, Dye7-2t and Dye7-3t calculated at the
M05-2X/631G(d) level of theory.
The site for electrophilic attack will occur at atoms that produce a negative charge, and where the
Fukui function fk is a maximal. The sites for nucleophilic attack will be those atoms that produce a
positive charge and where the Fukui function fk is a maximal. The condensed Fukui function results
for nucleophilic and electrophilic attacks were obtained with the AOMIX (a molecular analysis
program) and are shown in Table 6.
Table 6. Dye7, Dye7-2t and Dye7-3t nucleophilic and electrophilic attack sites.
Molecule Site for Electrophilic Attack Site for Nucleophilic Attack
Dye7 N33 C45
Dye7-2t N86 C105
Dye7-3t N146 C172
As can be seen, these values confirm that the nitrogen atom in the triphenylamine molecule is the
most likely site for the electrophilic attack, and the carbon atom that joins the thiophene chain to the
acid is the site for the nucleophilic attack.
Chemical reactivity parameters such as ionization potential (I), electron affinity (A),
electronegativity (), chemical hardness () and electrophilic index () for the studied molecular
systems (Table 7) were obtained by energy calculations (neutral and ionic state), taking into account
the ground state geometry optimization.
-
Int. J. Mol. Sci. 2012, 13 4427
Table 7. Dye7, Dye7-2t and Dye7-3t chemical reactivity parameters calculated with
M05-2X/631+G(d,p) using DFT descriptors.
Molecule Conceptual DFT
I (eV) A (eV) (eV) (eV) (eV)
Dye7 6.844 1.672 4.258 2.586 3.506
Dye7-2t 6.693 1.748 4.220 2.473 3.602
Dye7-3t 6.625 1.826 4.226 2.399 3.721
3. Experimental Section
Computational calculations were carried out with the Gaussian 09W molecular modeling software
obtaining the ground state molecular geometry for each dye. The strength constants and vibrational
frequencies were determinated via analytic frequency calculations on stationary points obtained after
geometry optimization. Both calculations were carried out at the same theory level. The chemical
model utilized for the calculations was the 631+G(d,p) basis set [5053] and the M05-2X hybrid
meta-GGA functional [54], which have been proved to yield good results when modeling these kinds
of structures.
Ultraviolet and fluorescence spectra in gas phase were carried out using Time-dependent density
functional theory (TD-DFT) equations according to the method implemented in the molecular package
Gaussian 09W [5558]. The equations were solved for 20 excited states. The infrared (IR),
ultraviolet-visible (UV-vis) and fluorescence (fluo) spectra were analyzed using the program SWizard [59].
The wavelength of maximum absorption and emission are shown for UV-vis and fluo spectra.
In this work we calculated the total dipole moment () and the isotropic polarizability (). The pKa for
the hydrogen atom attached to the oxygen atom was calculated using the MOPAC 2009 program [60].
In this program, the pKa is calculated using the O-H distance calculated using PM6 [61]. Molecular
dipole moment is an experimental measure of the charge distribution in a molecule. It is difficult to
evaluate accurately the global electron distribution in a molecule because it involves all the multipoles.
The polarizability contributes in a significant way to the understanding of the response of the system
facing an external field.
The reactive sites can be identified using orbital densities. The condensed Fukui functions can also
be used to determine the reactivity of each atom in the molecule. The corresponding condensed
Fukui functions are given by NqkNqkfk 1 (nucleophilic attack), 1 NqkNqkfk (electrophilic attack) y 2/110 NqkNqkfk (radical attack), where qk is the effective
Mulliken charge of atom k in the molecule. The condensed Fukui functions were evaluated with the
AOMix molecular analysis program [62,63].
On the other hand, using the DFT framework makes it possible to find the chemical reactivity
descriptors values, such as: electron affinity, ionization potential, electronegativity, hardness and
electrophilic index. All these values were obtained using system energy calculations.
-
Int. J. Mol. Sci. 2012, 13 4428
4. Conclusions
In this work, the molecular structure and properties of three molecules proposed as sensitizers in
solar cells have been calculated. The applied methodology for this study is based on the density
functional theory, using the M05-2x hybrid meta-GGA functional and the 631+G(d,p) basis set.
The molecular systems characterization includes the calculation of vibrations of functional groups,
ultraviolet-visible and fluorescence spectra, total dipole moment, isotropic polarizability, pKa values,
molecular orbitals, chemical reactivity parameters and attack sites. The molecular orbitals energy
indicates a smaller energy gap in the Dye7-3t sensitizer. For all three sensitizers, the LUMO orbital
energies are similar, the difference being approximately 0.14 eV. Analyzing these data makes it
possible to find potential applications for these dyes in photovoltaic devices.
Acknowledgments
This work has been supported by Consejo Nacional de Ciencia y Tecnologa (CONACYT) and
Centro de Investigacin en Materiales Avanzados, S.C. (CIMAV), and Universidad Autnoma de
Sinaloa and Direccin de Investigacin y Posgrado by PROFAPI 2011/043. J.B.L. gratefully
acknowledges a fellowship from CONACYT. D.G.M and N.F.H. are researchers of CIMAV
and CONACYT.
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2012 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
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Int. J. Mol. Sci. 2012, 13, 16005-16019; doi:10.3390/ijms131216005
International Journal of
Molecular Sciences ISSN 1422-0067
www.mdpi.com/journal/ijms
Article
Computational Molecular Nanoscience Study of the Properties
of Copper Complexes for Dye-Sensitized Solar Cells
Jess Baldenebro-Lpez 1,2
, Jos Castorena-Gonzlez 2, Norma Flores-Holgun
1,
Jorge Almaral-Snchez 2 and Daniel Glossman-Mitnik
1,*
1 Centro de Investigacin en Materiales Avanzados, S.C., Miguel de Cervantes 120,
Complejo Industrial Chihuahua, Chihuahua 31190, Mxico;
E-Mails: [email protected] (J.B.-L.); [email protected] (N.F.-H.) 2 Universidad Autnoma de Sinaloa, Prol. ngel Flores y Fuente de Poseidn, S.N., Los Mochis,
Sinaloa 81223, Mxico; E-Mails: [email protected] (J.C.-G.); [email protected] (J.A.-S.)
* Author to whom correspondence should be addressed; E-Mail: [email protected];
Tel.: +52-614-439-1151; Fax: +52-614-439-1130.
Received: 20 August 2012; in revised form: 9 October 2012 / Accepted: 12 November 2012 /
Published: 28 November 2012
Abstract: In this work, we studied a copper complex-based dye, which is proposed for
potential photovoltaic applications and is named Cu (I) biquinoline dye. Results of electron
affinities and ionization potentials have been used for the correlation between different
levels of calculation used in this study, which are based on The Density Functional Theory
(DFT) and time-dependent (TD) DFT. Further, the maximum absorption wavelengths of
our theoretical calculations were compared with the experimental data. It was found that
the M06/LANL2DZ + DZVP level of calculation provides the best approximation. This
level of calculation was used to find the optimized molecular structure and to predict the
main molecular vibrations, the molecular orbitals energies, dipole moment, isotropic
polarizability and the chemical reactivity parameters that arise from Conceptual DFT.
Keywords: molecular structure; absorption spectra; polarizability; chemical reactivity;
dipole moment; copper complex; dye-sensitized
OPEN ACCESS
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Int. J. Mol. Sci. 2012, 13 16006
1. Introduction
The current warming of the global climate is the result of an increase in greenhouse gas (GHG)
emissions, particularly CO2. Global average atmospheric CO2 has increased from 280 ppm in the
1750s to 389 ppm in 2010 [13]. An increasing demand for energy in the emerging economies and
energy crisis worldwide has stimulated a growing number of researches on renewable energy, in that
the utilization of renewable energy can help reduce fossil fuel consumption and alleviate
environmental problems. Renewables-based power systems provide an opportunity to generate cleaner
electricity with a lower cost of energy [4]. It is thought that the transition from fossil fuels to a
diversified energy matrix can be accelerated by governments by means of adequate policies and
instruments that support the creation of incentives for mitigation of greenhouse gases (GHG) emissions
and investments in renewable energy technology research and development. Solar energy is one of the
most promising sources of energy in the future and one of the renewable energy resources that has long
played a dominant role in the field of energy research with its wide application and great potential [5,6].
Recently, dye sensitized solar cells (DSSC) [79], considered as a credible alternative to conventional
inorganic silicon-based solar cells, have attracted attention due to their efficiency, simple
manufacturing and low cost [1013]. In these DSSC, an organic sensitizer must be chemically
absorbed on the porous surface of the nanocrystalline oxide. After absorbing a photon, the excited
electron in the dye-sensitized molecule is transferred into the conduction band of nanocrystalline
oxide, followed by a process in which the electron diffuses through the electrode. The sensitizer in this
oxidized state is reduced to its normal state gaining electrons through a liquid electrolyte [1416].
Nowadays, many research groups from all over the world actively participate to improve the efficiency
of every single process involved in the DSSC [1719]. The charge transfer efficiency from the dye
molecule to the nanocrystalline oxide is extremely important in the solar cell design. Since Regal and
Grtzel published their pioneer study [7], the understanding of the mechanism has required
fundamental research about the diverse physical phenomena at nanometric scale [20]. Theoretical
studies on physical and chemical properties of dye-sensitizers are very important to understand the
relationship between the structure, properties and performance in order to design and synthesize new
molecules for this purpose [2124].
Ruthenium(II) complexes as dyes have been extensively used as sensitizers in DSSCs owing to their
strong absorption in the visible range and relatively long-lived excited states [25]. The strong
absorptivity of these complexes is due to a metal-to-ligand charge transfer (MLCT) transition [26].
These complexes have reached over 12% power conversion efficiency [27]; but the rarity and high
cost of the Ru may limit their practical usage. Since initial reports of a [Ru(bpy)3]2+
-based
(bpy = 2,2'-bipyridine) photosensitizer, the dyes reported in the literature have predominantly been
ruthenium(II) complexes. We have recently become interested in the study and optimization of
copper(I)-based DSSC, literature reports of which are scarce. Sauvage and co-workers [28] discovered
that copper(I) complexes have similar photo-physical properties with Ru complexes, indicating that the
iterative chemical optimization of common metal complexes sensitizers can be comparable to that of
Ru complexes [29]. Copper(I) complexes display a wide variety of excited states and especially
photophysical and photochemical processes. Particularly, copper(I)poly-pyridine complexes exhibit
low-lying MLCT transitions that can participate, among others, in electron transfer processes [30]. In
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Int. J. Mol. Sci. 2012, 13 16007
this research, we propose the study of a molecular system of this type, such as [Cu(LL)2]+
(LL = 2,2'-biquinoline-4,4'-dicarboxylic acid) which is shown in Figure 1 (Cu(I) biquinoline), in order
to define from the theoretical point of view a suitable calculation methodology for obtaining structural
parameters, as well as electrical and optical properties using the density functional theory (DFT) [3133]
and time-dependent (TD) DFT [3436]. These methods are implemented in the Gaussian 09W
program package [37]. In this paper, we have found very interesting properties with the proposed
ligand to increase the level of conjugation, which has not been reported as an article in the DSSC field.
Figure 1. Molecular structure of copper complex.
2. Results and Discussion
Once the molecular structure was proposed, the geometry optimization was calculated, followed by
the frequency analysis to confirm that the species had the minimum energy conformation. These
calculations were performed in the presence of methanol as solvent.
The electron affinity (A) and the ionization potential (I) were obtained by energy calculations
(neutral and ionic state), taking into account the ground state geometry optimization. The aim of this
was to establish the correlation of results between the different levels of calculation, since there are no
experimental results reported for this system; this is a contribution of our research. Table 1 shows the
values obtained and where it seems as if some of these have variations between them.
Figure 2 helps to visualize more clearly the dispersion of these values. M06-HF/LANL2DZ-DZVP,
M06-2X/6-31G(d) + DZVP and M06-HF/6-31G(d) + DZVP levels of theory show the greatest
dispersion; therefore, the values are ignored for the calculation of some measures of central tendency,
such as the mean and median. The red line represents the mean value, in the case of electron affinity, it
is located at 4.75 eV and for the ionization potential it is equal to 9.75 eV; meanwhile, the medium has
the values of 4.79 eV and 9.80 eV, respectively. This led to results of the population standard deviation
of 0.1555 (in A) and 0.1768 (in I) with a coefficient of variation of 3.27% and 1.81%, which are values
of high quality. On this basis, we can establish that the model chemistry M06/LANL2DZ + DZVP and
PBE0/LANL2DZ + DZVP represent excellent approximations in this context.
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Int. J. Mol. Sci. 2012, 13 16008
Table 1. Electron affinity and the ionization potential of Cu(I) biquinoline with different
levels of theory.
Basis set Functional Electron affinity (eV) Ionization potential (eV)
LANL2DZ + DZVP M06 4.79 9.80
M06-2X 4.65 9.93
M06-HF 4.58 11.86
M06-L 4.98 9.55
B3LYP 4.88 9.84
PBE0 4.79 9.79
LANL2DZ M06 4.82 9.90
M06-2X 4.64 10.01
M06-HF 4.57 9.63
M06-L 5.00 9.48
6-31G(d) + DZVP M06 4.49 9.87
M06-2X 4.26 11.01
M06-HF 4.12 11.61
M06-L 4.69 9.49
Figure 2. Electron affinity and ionization potential for the levels of theory used in this
study and the dispersion between the values. The red line represents the mean value, in the case
of electron affinity it is located at 4.75 eV and in the case of ionization potential it is equal
to 9.75 eV.
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Int. J. Mol. Sci. 2012, 13 16009
Another fundamental property as a potential sensitizer for DSSC is the maximum absorption
wavelength (max); according to the levels of theory selected, the UV-Vis spectrum and the max were
calculated. The results of max are shown in Table 2.
The results in Table 2 show that max varies from a high value to a low value when the functional
increases the percentage of Hartree-Fock exchange. The experimental result indicates that max is
553 nm [38]. Comparing this result with our calculation, we can conclude that the calculation
methodology with more precision is M06/LANL2DZ + DZVP. Similarly, by comparing the results of
A, I and max, this methodology will be used to study other properties of the dye under investigation.
Another aspect to consider is that using a LANL2DZ basis set better describes the behavior of the
excited states of the ligand molecule.
Table 2. Maximum absorption wavelength of Cu(I) biquinoline dye using various models.
Model chemistry max (nm) Model chemistry max (nm)
M06/LANL2DZ + DZVP 556 M06-HF/LANL2DZ 282
M06-2X/LANL2DZ + DZVP 386 M06/LANL2DZ 543
M06-HF/LANL2DZ + DZVP 279 M06-2X/LANL2DZ 378
M06-L/LANL2DZ + DZVP 641 M06-L/6-31G(d) + DZVP 645
B3LYP/LANL2DZ + DZVP 614 M06-HF/6-31G(d) + DZVP 279
PBE0/LANL2DZ + DZVP 578 M06/6-31G(d) + DZVP 488
M06-L/LANL2DZ 629 M06-2X/6-31G(d) + DZVP 328
Figure 3 shows the UV-Vis spectra at the M06/LANL2DZ-DZVP level of calculation and its
corresponding max