Figure-1: Working principle, mechanism and major components of
a Dye Sensitized Solar Cell
Photosensitizer which play important role in operating cycle of DSSC may
be metals-containing inorganic dyes or metals-free organic dyes. The
community is inclined to build DSSC based on organic dyes due to high
cost and environmental hazards of inorganic dyes
[8, 9].
The organic dyes are environmentally friendly, low cost, having elevated
extinction coefficient, affordable and available due to simple synthesis
strategies. The photosensitizers having D-π-A configuration are known to
offer high efficiency [10]. In this
configuration, Donor (D) must be an electron rich specie that donates
electrons, acceptor (A) is electron deficient specie that accept
electrons and these are connected by π-spacer whereas the acceptor is
directly attached to surface of semiconductors (usually
TiO2). The basic purposes of the dyes are absorption of
light and transfer of charge which are strongly dependent on its
structure and electronic properties
[11]. There have been variety of
donor groups such as phenothiazine [12,
13], Carbazole
[14,
15],
triphenylamine[16] and coumarin
[17]. The efficient acceptors are
alkoxysilane [18] and cyanoacrilic
acid [19] whereas π-spacer could have
units of different organic groups including dioxythiophene
[20], benzene
[21], benzothiadiazole
[22] and thiophene
[23]. The organic sensitizers based
on triphenylamine are proven to offer elevated photoelectric conversion
in DSSC because of its ability to restrict the dye aggregation and to
facilitate the hole transferring
[24].
There have been numerous attempts on tuning the properties of
photosensitizers having different configuration [12-26]. The
modifications in the configuration of the dyes thereby using different
donors, acceptors and spacers appeared to change in their efficiencies.
The π-bridge length of organic dyes greatly affects the molecular
structure which causes the changes in band gaps, emission spectra and
optoelectronic properties [25,
26]. The formation of different
π-spacers is an effective strategy to increase the efficiency of the
dyes due to its role in transferring the charge from donor to acceptor
unit. This consideration predicts that a long chain of oligothiophene in
π-spacer could give improved results. On the other hand, π-π stacking in
π-spacers of organic sensitizers typically occurs due to sturdy
intermolecular interface. The π-π pile is beneficial to light harvesting
efficiency (LHE) due to its role in optoelectronic properties of the
dyes that can be studied using UV-Visible excitation spectrum. Mostly
the π-stacked accumulation results in incompetent electron injection
that leads to inefficient light harvesting
[27]. The proscription of π-π heap
taking place via preservatives in solution of dye is usual method to get
better effectiveness of organic DSSC suffering from π-π stacking
trouble. The co-adsorption of additives along with dye
[28] and structural alteration of
sensitizers [29] have been found
effective to avoid dye aggregation or π-π stacking and therefore lead to
improved efficiency of DSSC. The organic dyes with D-π-A design are
widely studied because of their efficient photosensitizing properties
[30-32]. The hydrogen production and
overall performance of DSSC is greatly affected by the length of
π-spacer [33-38].
The process of photoinjection happening through photoanode is at heart
of DSSC. The incident light is absorbed by dyes to excite electrons from
their HOMO to LUMO and the excited electrons are then injected to
semiconductor. The dyes offering elevated oscillator strength are
beneficial for photoinjection in photoanode of DSSC. The light
harvesting ability of organic dyes can be tuned by suitable
modifications which has been studied in this work. The injection of
electron from dye to semiconductor could be done in two different ways;
direct and indirect. The injection could be direct when electron move
directly from HOMO of dye to conduction band (CB) of semiconductor which
is characterized by occurrence of new peaks in spectrum. The indirect
injection of electron involves shift of electron from the LUMO of
excited dye moves toward the CB of semiconductor that is characterized
by broadening of peaks in spectrum
[39-41]. The adsorption of dyes on
semiconductor is carried out by dissolving the dyes in suitable
solvents. The process of solvation affects the optical properties of
dyes [42,
43] due to which the choice of solvent
should be made carefully. It has been often observed that dyes show
red/blue shifts on solvation in different solvents [44].
This work involves computational investigations of carbazole based new
organic dyes and study on their solvation effects to model the
photoinjection for improvement in the current organic DSSCs. The dyes
studied herein follows the molecular structure (3)D-π-A in which the
donor is oxidized by giving electrons through π-bridge to the acceptor.
The carbazole are electron rich species which are known as organic
photo-conductor(OPC) [45,
46] and act as donor in sensitizers
creating photo-injected electrons having long life time by delaying the
charge recombination at interfaces [47,
48]. Moreover carbazole is highly
stable, starburst like twisted structure which allows efficient
photo-sensitizing parameters[23,
49] and have high molar extinction
coefficient [50].