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].