Excitonic photovoltaic effect in a cyanine dye molecular assembly electronically coupled to n- and p-type semiconductors

Article abstract of DOI:10.1016/j.jphotochem.2015.11.018

A solar cell of configuration N-[DDD - - D]-P with a molecularly organized dye J-aggregate [DDD - -D] electronically coupled to n- and p-type semiconductors is illustrated by fabricating a model device with TiO₂ and CuSCN as the n- and p-type s

Full text of DOI:10.1016/j.jphotochem.2015.11.018

Accepted Manuscript
Title: Excitonic Photovoltaic Effect in a Cyanine Dye
Molecular Assembly Electronically Coupled to n- and p-Type
Semiconductors
Author: P.K.D. Duleepa Pitigala Maged M. Henary Eric A.
Owens A.G. UnilPerera Kirthi Tennakone
PII:
S1010-6030(15)00423-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2015.11.018
JPC 10064
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
18-5-2015
2-11-2015
21-11-2015
Please cite this article as: P.K.D.Duleepa Pitigala, Maged M.Henary, Eric
A.Owens, A.G.UnilPerera, Kirthi Tennakone, Excitonic Photovoltaic Effect in
a Cyanine Dye Molecular Assembly Electronically Coupled to n- and p-
Type Semiconductors, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2015.11.018
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EXCITONIC PHOTOVOLTAIC EFFECT IN A CYANINE DYE
MOLECULAR ASSEMBLY ELECTRONICALLY COUPLED TO n-
AND p-TYPE SEMICONDUCTORS
P. K. D. Duleepa Pitigalaa, Maged M. Henaryb,c, Eric A. Owensb,c, A. G. UnilPereraa,c, Kirthi Tennakonea*
ktenne@yahoo.co.uk
aDepartment of Physics and Astronomy, Georgia State University, Atlanta GA.
bDepartment of Chemistry, Center for Biotechnology and Drug Design, Georgia State University, Atlanta
GA.
cCenter for Diagnostics and Therapeutics, 100 Piedmont Ave SE Atlanta, GA.
*Corresponding author.
1

Highlights
 J- aggregated dye film with ends bonded to n- & p- semiconductors is fabricated
 Photoresponse from bulk excitons and direct carrier injection is demonstrated
 System exhibits long distance exciton transport.
2

A solar cell of configuration N-[DDD-----D]-P with a molecularly organized dye J-aggregate [DDD---D]
electronically coupled to N- and P-type semiconductors is illustrated by fabricating a model device with
TiO2 and CuSCN as the N- and P-type substrates and the thiocyanate of a cationic pentamethine cyanine
dye as D. Functional moieties in D anchors to TiO2establishing electronic coupling and serving as a
template for assembly of a J-aggregated film. Bonding of sulfur in thiocyanate anions at the other end of
the aggregate create the electronic coupling needed to facilitates transfer of holes to CuSCN. The cell
exhibits photo-response originating from excitons generated in the bulk of the thick dye film as well as
direct sensitized injection at the first interface. The dye molecular assembly is found to admit exciton
transport over relatively long distances and significant hole mobility.
Keywords: Excitonic Solar Cell; Dye-sensitization; Hole mobility; Cyanine; Titanium dioxide
3

1. Introduction
Organic- inorganic hybrid nanostructures have been studied extensively as potential systems for solar cell
and other optical-electronic device applications. The prototype examples of photovoltaic devices of this
category are: dye-sensitized solid-state solar cells [1, 2] (DSSSCs), extremely thin absorber (ETA) cells
[3] and organic-inorganic hybrid (OIH) heterostructures [4, 5]. In a DSSSC, the excited molecules in a
dye monolayer sandwiched between high band-gap n- and p-type semiconductors inject electrons and
holes to respective regions, generating a photocurrent. In the analogous ETA cell, where the dye
monolayer is replaced by a thin film of a low band-gap semiconductor, electrons and holes created in the
thin film are efficiently transferred to n- and p-type regions, provided film thickness does not exceed the
carrier diffusion length [6, 7]. Organic-inorganic hybrid solar cell is basically a heterojunction of these
two types of materials, where light absorbed by the organic component creates excitons that decompose at
the interface, separating the charges [8]. In a thin film heterojunction, the thickness of the organic film is
limited by the exciton diffusion length [9]. As the limitation of the thickness of light harvesting
component restricts the overall light absorption cross-section, the performance of all the above systems
are greatly improved by making the inorganic semiconductor, nanostructured to acquire a large surface
area. Dye-sensitized and bulk heterojunction solar cells resorts to this strategy optimally. Furthermore in
excitonic solar cells, the exciton diffusion length increases greatly if the organic film has an organized
molecular arrangement. Generally, an organized molecular structure in the organic film will also enhance
its carrier mobility, imperative for proper functioning of the cell. An advantage of organic-inorganic thin
film systems is that, they more readily afford organized molecular assembly in the exciton transporting
material compared to bulk heterojunction solar cells. Obviously, it is easier to build an organized
molecular arrangement on a preset solid substrate (inorganic semiconductor) than to achieve such
structure in a blend of two materials. This note describe an organic-inorganic hybrid solar cell of
configuration n-TiO2-[DDD----D]-p-CuSCN, where [DDD----D] is a molecularly organized thick film of
a hole conducting cyanine dye electronically coupled to both TiO2 and CuSCN. The cell exhibits
photocurrents originating from exciton dissociation as well as direct dye-sensitized injection at TiO2/dye,
and dye/CuSCN interfaces. The dye assembly [DDD-----D] is found transport excitons over long
distances of the order of 30 nm.
2. Experimental
Films of TiO2 (1×1 cm2) were deposited on fluorine conducting tin oxide (FTO) glass plates (1×1.5 cm2)
by the method described previously for fabrication of DSSSCs [1]. Briefly, the procedure is hydrolysis of
titanium isopropoxide (5 ml) in 75% propan-2-ol (15 ml) and acetic acid (5 ml) to form a colloidal TiO2
suspension. The hydrolyzed product is mixed with Degussa P25 TiO2 powder and the viscous slurry
spread over the FTO plate. The plate is heated to 120oC for 10 min and after blowing off the loose crust of
TiO2 particles not adhered to the FTO surface, film is sintered at 425oC for 10 min. By repetition of the
process, TiO2 film can be grown to a desired thickness. Such films are largely devoid of interconnected
void channels leading the FTO surface, preventing short-circuiting at the FTO interface when a
heterojunction is formed. The roughness of an 8m thick film estimated by adsorption of a standard dye
is of the order of 500.
The cationic cyanine dye named E-65 (structure 3 shown in Fig. 1a) was synthesized by the scheme
shown in Fig. 1a, which admits ready variation of the auxiliary ligands [10, 11]. The polymethine
precursor 1 (structure 1 shown in Fig. 1a) is synthesized by reacting equimolar amounts of mucobromic
acid with aniline in warm ethanol. In parallel, the heterocyclic moiety is prepared by alkylation of 2, 3, 3-
trimethylindolenine using bromoethanol to generate the cationic intermediate 2 (Fig. 1a). The
condensation reaction between 1 and 2 proceeds effectively in the presence of acetic anhydride and
4

sodium acetate which also affords the acylation of hydroxyl groups to yield the final compound 3 (E-65).
The dye in the bromide form is converted to the thiocyanate by boiling the dye powder in saturated
solution of KSCN and separation of the dye thiocyante precipitating on cooling the solution.
Dye was coated over the TiO2 film by the following procedure. The film is thoroughly cleaned with
alkaline 50% propan-2-ol followed by water and 99% propan-2-ol and plate dried at 120oC. Dried plate is
positioned vertically at the bottom of a glass tube (diameter ~ 1.3 cm) and the dye solution (M in 90%
ethanol) is poured over the plate to a height of about 4 cm. The vessel was left in a constant temperature
(15oC) environment for several days to adsorb the dye. The monolayer thickness of the dye film was
estimated by extracting dye from sample films and spectrophotometric estimation of the dye content. The
film is dried in air at ambient temperature and p-CuSCN is drop coated over its surface from a solution in
n-propyl sulfide and the film thickness is controlled by repeating the procedure. The performance of the
cell decreases with the increase of the thickness of the CuSCN layer above the TiO2 film. However, to
avoid scratching of the CuSCN surface, during graphite coating, the thickness of CuSCN above the TiO2
surface maintained ~ 1m. The back electrode is formed by pressing a FTO glass plate to secure the
contact. Painting graphite over the CuSCN surface improves the back contact. However, for comparison
of the cell performance in front-wall (light incident on TiO2) and back-wall (light incident on CuSCN)
modes of illumination, graphite painting was avoided. The active area of the cells were 1 cm2 as 0.5 cm
in the 1.5 × 1 cm2 conducting glass plate has been left bare to connect the leads. The I-V characteristics
of the cells were measured using a source meter at 1.5 AM one sun illumination and photocurrent action
spectra were recorded using a monochromator set-up.
The carrier mobility of the dye was determined by measuring the space-charge limited current. For this
measurement, vacuum dried dye is pressed into a thin pellet between two stainless steel electrodes in a
glass tube and connected to a d.c. voltage supply, sufficient to obtain a measurable current avoiding
degradation of the material due to heating. Mobility is calculated from the Mott-Gurney equation[12],
9V 2
8L
J 
(1)
3
where J = current density, mobility,dielectric constant, V = applied voltage, L = pellet length.
The sign of the charge carrier was ascertained by field-assisted thermoelectric tests using the same
experimental set-up. Alternative heating of two electrodes and noticing direction of biasing which yields a
higher current in the circuit enables determination of the sign of carrier. Method does not require heating
of the electrodes to an excessive temperature as in direct thermoelectric tests. FTIR spectra of dye lightly
coated to TiO2 and the non-adsorbing substrate GaAS were recorded to ascertain the mode bonding the
dye to TiO2.
Results and Discussion
When the TiO2 film is exposed to a relatively dilute (~ 10-3 M) solution of the dye E-65 for 30-45 min, the
dye coverage approximates to a monolayer formed by chelation of dye molecules to the TiO2 surface. The
two functional groups in the dye anchor bidentately to TiO2 as shown in Fig. 1b. Comparison of the FTIR
spectra of the dye adsorbed TiO2, bare dye and bare TiO2 shows changes in the characteristic C-O-H
bending and C-O-H stretching vibrations [13] in the region suggestive of dye binding to TiO2in this
manner. The absence of the C-O-H bending band at ~1100 cm-1 and drastic reduction in the C-O-H
stretching band at 1738 cm-1 in the dye adsorbed onto TiO2, indicate bidentate bonding of the dye.
5

In more concentrated solutions (> 0.05 M), the first monolayer forms almost instantly by surface
chelation of dye cations and electrostatic bonding of the SCN anions (Fig. 2a). The subsequent slower
growth of dye film proceeds as consecutive bonding of dye cations and SCN anions via dipole-ion and
electrostatic forces respectively (Fig. 2b). An initial fast absorption to form a monolayer and subsequent
slow assembly has also been observed in J-aggregate formation on metallic [14] and other semiconductor
substrates [9]. The firm anchorage of the first monolayer serves as a template for growth of an organized
film. Slow evaporation of the solvent during storage (determined by ambient temperature and height of
the air column and diameter of the tube) assists maintenance of dye concentration at a sufficient level to
continue the film growth via forward progression of the dye adsorption, i.e.
X++ SCN- ↔ [XSCN]ab
(2)
where X+ denote the dye (E-65) cation and [XSCN]ab dye molecules adsorbed onto the substrate.
Obviously incorporation of SCN– into the solution by addition of KSCN, accelerates the film growth via
common ion effect. Highly organized J or H aggregated films of ionic dyes can be deposited by the above
technique. Nature of the dye and substrate and deposition conditions determines the type of the
aggregation (i.e. J, H or a mixture of J and H) and extent of disorder.
A schematic diagram illustrating construction of the cell and an element of the heterojunction interface
are shown in Fig. 3(a) and 3(b) respectively. Bonding of the functional groups of the dye, anchors it
firmly to TiO2, facilitating the necessary electronic coupling. As sulfur in SCN readily binds to Cu sites in
solid copper compounds [9, 15, 16], the electronic coupling of the organized dye film to the hole
collecting CuSCN is also secured.
The short-circuit photocurrent density (Jsc) depended on the thickness of the dye film and in both front
and back wall modes of illumination, Jsc was found to be optimum in films much thicker than one
monolayer, contrary to familiar dye-sensitized solar cells where the best photo-response is generally seen
at monolayer coverage of the dye. The observed effect in the thick film can be understood as a result of an
excitonic contribution to the photocurrent.
Like most cyanine dyes solid E-65 exhibits hole conduction [17], with a carrier mobility significantly
high compared to familiar dyes. Possibly, the Br atom in methine chain acts as an electron acceptor
releasing holes. The measurement of the variation of the space charge limited current with voltage bias is
in good agreement with eqn.1, giving a mobility × 10-3 cm2 V-1s-1. When the cell is
illuminated, HOMOs in dye molecules bonded directly to TiO2 excite electrons to LUMOs and transfer
them to conduction band of TiO2 as in a normal DSSSC. As the dye is a p-type conductor, the hole
released is transported along the thick dye layer to CuSCN. Again the excitons generated in the bulk
decompose at the TiO2/dye interface, separating electrons and holes to TiO2 and dye respectively and
holes are transported across the dye layer to CuSCN. The photocurrent action spectra of a cell with thick
(~ 11 monolayers) dye film, in front and back wall modes of illumination are presented in Fig. 4. Both
spectra have profiles extending from 500-775 nm with distinct differences. The front wall spectrum has a
peak at 681 nm red-shifted from the absorption spectrum of the dye solution by ~ 31 nm. The cause of
this peak is more intense direct sensitization at the TiO2/dye interface combined with the excitonic
contribution from light adsorbed in the bulk of the film. Whereas the back-wall spectrum displays a peak
at ~706 nm with a narrow profile red-shifted by 56 nm and a largely broadened blue-shifted peak
structure at ~ 579 nm. It is interesting note that the back wall spectrum also shows a peak at ~ 635 nm
corresponding to direct sensitization at the dye/CuSCN interface. The back-wall mode spectrum could be
understood as an effect originating from presence of J and H aggregated domains in the dye film. J-
aggregates have narrow and red-shifted absorption peaks compared to the monomer [18]. H-aggregates
display the opposite behavior with broadened and blue-shifted absorption peaks [18]. In the front-wall
6

mode, this effect is partly masked by the contribution to the photocurrent from direct sensitization by the
monomer anchored to the TiO2 surface. In the back wall mode, light incident on TiO2/dye interface is not
sufficiently intense to mask the photocurrent generated by excitons produced in the bulk of the thick film
and decomposing at the TiO2/dye interface and injecting electrons to TiO2 and conducting holes to
CuSCN. A cell with a monolayer thick dye film shows no distinction in front and back wall action
spectra, both have peaks at ~ 650 nm, almost same as the peak absorption of dye in solution (Fig. 5).
Again if the thick dye film of similar thickness is deposited by drop coating of the dye over the warmed
TiO2 film instead of slow growth as in the previous experiment, the photocurrent action spectra in front
and back wall modes do not demonstrate conspicuous distinction and the observed photo-response is
weaker. Clearly the organized molecular assembly of the dye has been cause of the observed distinction
in front and back wall action spectra and the enhanced photo-response. Generally, molecular organization
greatly enhances fast diffusion of excitons [19]. The Fig. 6 shows the I-V curves of the cells with
organized and disorganized thick dye layers and a monolayer. I-V parameters and peak wavelengths of
the action spectra of different cells are summarized in the Table.1.
A severe problem in conventional DSSSCs is recombination at the points where inorganic n- and p-type
semiconductors are in direct contact. Imperfections in the dye monolayer frequently lead to this situation
lowering the efficiency. In the present system, the thick dye layer naturally resolves this problem. It is
important to note that distinction in the charge transfer processes originating from light absorbed by the
first monolayer anchored to TiO2 and those in the bulk of the thick film. The bonding of the dye to TiO2
creates a charge transfer complex and kinetics of electron transfer from a dye molecule in the monolayer
is a characteristic of the entirety of dye molecule and the charge transfer complex. Whereas light absorbed
by dye molecules in the bulk emit exctions which decompose at the TiO2/monolayer interface. There is
evidence that such interfaces involving charge transfer couplings efficiently decompose excitons and the
mobility of the hole in the J-aggregate suppresses back electron transfer [20].
We have not yet succeeded in elucidating the morphological arrangement of J and H domains in the dye
film. The simultaneous occurrence J- and H-aggregates has been reported in solid films with other dyes
[21]. Two possibilities exist, i.e. regions composed entirely of either J or H aggregates connecting the two
surfaces (TiO2 and CuSCN) or separated distributions across the thickness of the film. The latter
morphology is somewhat detrimental to cell performance, for the following reason. During J-aggregation
HOMO is moved upwards and LMUO downwards relative to their positions in the monomer, agreeing
with observed red-shift in absorption spectrum [22-24]. The opposite would happen in an H-aggregate, as
this form molecular assembly leads to a blue-shift in absorption [22, 23]. Consequently, electron or hole
transfer from an H to a J-region is permitted, but not the reverse. Thus an H-aggregated region in the
middle of J-aggregate connecting TiO2 and CuSCN would impede carrier transport. The dye E-65
deposited on TiO2 is predominantly J-aggregated. The Br atom in the methine chain seems to enhance J-
aggregation by facilitation of appropriate molecular stacking. A cyanine analogous to E65 without a
halogen atom in the methine chain was found to prefer H-aggregation when deposited on TiO2 by the
same procedure. The fact the action spectrum display greatly blue shifted broad peak, confirms that
excitons generated in the H-aggregated regions also participate in photocurrent generation, implying the
presence of H-aggregated domains connecting TiO2 and CuSCN interfaces.
The film thickness dependence of the photocurrent can be understood as follows: If P = photon flux
incident on TiO2/dye interface (front-wall mode). Flux at a distance x from the interface is P exp(-
x/where  mean free path of light. The exciton flux emanating from light absorbed in this element
is P P exp(- x/dx, where P = probability that an excited dye molecule will emit an exciton (Fig.
7).
7

If a one dimensional model is assumed [8], half of excitons generated will travel in the backward
direction and strike TiO2/dye interface. The other half of excitons travelling in the forward direction will
get reflected at dye/CuSCN (R = reflection coefficient) and the strike the dye/TiO2 interface. As the
exciton flux also attenuate in traversing through the dye film, the photocurrent dI generated by the light
absorbed in the element dx can be written as,
1
x



dI  e1DPP exp




2
(3)

x

x
2L 






exp
 Rexp
exp
dx


















Where exciton diffusion length, L = thickness of the dye film, e = electronic charge, and D =
probability of an exciton decomposing and injecting electrons to TiO2 and holes to CuSCN, R = excition
reflection coefficient at the dye/CuSCN interface. Instead of reflection, excitons could also decompose at
this interface injecting holes to CuSCN. However, as the dye is not conducting electrons, holes will
recombine with electrons residing in the dye at the interface. Integrating (3), the photocurrent I is
calculated as,
1

L 





I  eDPP 1 exp





2

(4)

2L 


1 Rexp









in the approximation Total photocurrent is obtained by adding the contribution from direct
sensitization by dye monolayer anchored to TiO2, which is independent of L. Similarly, the back wall
mode photocurrent can be expressed as (Fig. 7),
1
I  eDPP (1 R)
2
(5)

L  
L 







1exp
exp








 



 
Here, the photocurrent decrease more rapidly after film thickness reaches a maximum when L ~
ln(Knowledge of the extinction coefficient dye yield nm and as the photocurrent is
found to be optimum when the film is 11 monolayers corresponding to film thickness of ~11 nm, we
obtain nm.
3. Conclusion
A pentamethine cationic cyanine dye with two functional moieties binding to TiO2and a Br atom in the
methine chain is synthesized. The functional moities surface chelate bidentately to n-TiO2 establishing
strong electronic coupling to the conduction band states. Bonding of dye to p-CuSCN is facilitated by
replacement halide anion by thiocyanate. Nanocrystalline TiO2 films exposed to a solution of dye under
ambient conditions by mere control of solvent evaporation rate, admits deposition of predominantly J-
aggregated thick films exhibiting fast exciton diffusion and detectable hole mobility. Model solar cell of
the configuration n-TiO2-[DDD---D]-p-CuSCN with the thick organized dye film [DDD----D]
8

demonstrate contributions to the photocurrent originating from excitons generated in bulk of the thick
film and direct sensitized injection at the TiO2/Dye interface. Low efficiency of the cell is attributed to
insufficient hole mobility of the dye and/or exciton recombination at the interface. More stringent
purification of the dye and improvement of film deposition technique to eliminate H–aggregated domains
will improve the efficiency. However, in order to attain orders of magnitude enhancement of the mobility,
understanding of the mechanism of hole conduction and how it relates to the dye structure would be
necessary to aim synthesis of more prospective dyes. Alternatively strategies may also be developed to
enhance electronic carrier collection by modulating the inorganic film architecture [24]. Devices based on
thick multi-layers organized molecules could turn out be promising systems for solar energy conversion
[25, 26]. The present system demonstrates that long distance exciton transport is possible, but
unfortunately, the efficiency is severely limited by the poor carrier mobility.
The I-V parameters of the cell are rather low and several factors contribute to result. Thick dye layers are
resistive because of the poor conductivity of the dye used. Again we have conducted measurements with
cell of active area 1 cm2, compared to 0.25 cm2, generally used in most measurements. Because of the
complexity of the system, reliable results cannot be obtained using smaller cells.
Acknowledgements
This material is based upon work supported by, or in part by, the U.S. Army Research Laboratory and the
U. S. Army Research Office under contract/grant number W911NF-12-2-0035 and NSF grant # ECCS-
1232184.Synthetic work was supported by the Georgia Research Alliance grant to MH. MH and AGUP
also appreciate the Georgia State University Center for Diagnostics and Therapeutics for their support.
9

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Figure Captions
Fig. 1(a) Synthesis scheme of the dye E-65 and (b) its bidentate bonding to TiO2.
12

Fig.2 (a) Formation of the first monolayer of dye on TiO2 by surface chelation of the dye cation X+ and electrostatic bonding of
SCN- (b) subsequent growth of the film via bonding of dye cations and SCN anions by ion-dipole and electrostatic forces.
13

Fig.3 Schematic diagram indicating construction of the cell (a) and an element of the TiO2/dye/CuSCN interfaces schematically
depicting coupling of the organized dye aggregate to TiO2 and CuSCN (b).
14

Fig.4 Normalized photocurrent action spectrum of the cell with a thick dye film (a) front-wall illumination (b) back-wall
illumination and (c) absorption spectrum of 0.001M solution of the dye.
15

Fig.5 Normalized photocurrent action spectrum of a cell with a dye-monolayer in front and back wall modes
16

Fig. 6 I-V curves of the cells with (a) organized thick dye layer (b) disorganized thick dye layer (c) dye monolayer (Front wall
illumination)
17

Fig. 7 Geometry of the front and back wall illumination
18

Tables
Table.1 I-V parameters and peak photoresponse wavelengths of different cells (Front–wall illumination)
Film
I
V
 %
Max
(nm)
687
(mAcm-2) (mV)
Grown
organized
Drop coated
(disorganized)
Monolayer
2.3
1.1
0.3
374
0.4
331 < 0.15 675
350 <0.05 658
19