Hopping transport in triphenylamine and indoline based semiconductors in the context of dye sensitized solar cells: A DFT Study

Unprecedented human population growth, technological advances in industrial sectors related to developmental activities have led to an enormous increase in global energy demand. To meet the global energy demand and environmental issues researchers are motivated to search new eco friendly and clean energy sources [1], [2]. Solar energy technology, basically silicon-based solar cell, is a promising renewable energy technique as they possess the highest power conversion efficiency (PCE). Silicon-based solar cells are ideal, although they have limitations like difficulties in manufacturing process, high cost, less mechanical flexibility, high operation temperature limits etc [3], [4]. Therefore, it is an imperative need to replace Si with a better and efficient material for photovoltaic applications. On the other hand, the second generation solar cells based on copper indium gallium selenide (CIGS), Cadmium telluride (CdTe), p-GaAs/n-GaAs, and ZnO/CdS etc. have 20% efficiency. These types of solar cells also have some difficulties such as they are highly expensive, and have elements that are less abundant in earth and toxic for environment [5]. Researchers have made considerable efforts on approaching the third-generation solar cells, such as organic/inorganic perovskite solar cells [6], inorganic solar cells [7], organic tandem solar cells [8], quantum dot solar cells [9], small molecule organic solar cell [10] and dye-sensitized solar cells (DSSCs) [11]. Among them perovskite solar cells offer the highest efficiency upto 25% [12], [13], [14]. However, these types of solar cells are less stable, and have difficulty in the production process [15]. Further, organic solar cells based on fullerene acceptors also played a major role in past decades. However, this type of solar cells have some disadvantages such as limited chemical and energetic tunabilities, poor light absorption, high-cost purification, morphology instability etc [16], [17].

Recent study reveals that the environment friendly DSSC is considered as the best way to meet our energy requirements as its PCE gets increased from 7% to 14% [18]. In 1991, pioneering work of O’Regan and Gratzel involves the introduction of DSSC, which have received considerable interest due to their flexibility, low material cost, easy manufacturing processes, multi color choice and low toxicity [19]. A convenient DSSC is mainly composed of three major components viz., photo-anode, a counter-electrode, and a redox couple I−/I3−. The working mechanism of DSSC involves a dye which is adsorbed on mesoporous wide band gap TiO2 cluster and a redox shuttle to return the electrons to the oxidized dye [20]. With these building blocks, the dye is liable for light absorption followed by electron injection into the TiO2 cluster which plays a key role in the device performance. Consequently, considerable exploration were carried out based on the dyes to look after the light harvesting capacity, charge generation, and charge recombination processes [21]. In this respect, three different classes of DSSC have been explored as the potential light harvester, viz., ruthenium complex sensitizer, [22], [23] porphyrin based sensitizer, [24], [25] and metal free organic dye sensitizer [18] showing the certified PCE values of 12%, 13% and 14% respectively. Out of these, three classes of DSSCs, ruthenium complex sensitizer and porphyrin based sensitizer are difficult to explore owing to non-availability of metals on earth, challenges to prepare with high production yields, hazardous to the environment, and cost issues. Hence, significant researches have been carried out to explore enormously some new metal free organic dyes of donor(D)-π-acceptor(A) type architecture for improving the performance of DSSCs [26], [27]. Various D-π-A-substituted organic dyes have been prepared and used as sensitizers in DSSCs as they exhibit numerous advantages such as chemical versatility and facile synthetic approaches to distinct molecular structures, ease of purification, low material cost, high molar extinction coefficients, and high levels of solar spectral absorption within the visible region [28], [29]. This type of architecture can promote efficient intramolecular charge transfer (ICT) from donor moiety of the dye to its acceptor unit through the π-conjugated spacer, which is essential for efficient injection of electrons to the semiconductor. Till date, notable PCE have been achieved due to structural disposition of the electron donor, acceptor, and π-spacer, as it affects both the HOMO and LUMO energies of the designed material. Moreover, the performance of DSSCs can be affected by the π-electron delocalization from donor to acceptor through the π-conjugated spacer. Therefore, in D-π-A dyes, the π-conjugation plays a significant role in charge transfer process and photophysical property [30]. This kind of structural disposition is regarded as the necessary step for better management of the optoelectronic properties [31].

Metal-free dye sensitizers are of considerable interest due to their high molar absorption coefficient, greater structural flexibility, and ease of production and purification. In general, metal-free dye sensitizers are designed based on donor-π-acceptor architecture [32]. To date, numerous electron donor(D), acceptor(A) and π linker groups have been studied experimentally and theoretically [33], [34]. Common examples are triphenylamine, indoline, coumarin, bis-dimethyl-fluorenylamino, phenothiazine, which are known for its excellent electron donating ability [35], [36], [37]. Among these donor materials, the strong electron donating triphenylamines (TPA) are found to be the best candidates due to their better hole transporting abilities [38], [39], [40], [41], [42]. On the other hand, cyanoacrylic acid, carboxylic acid, and rhodanine-3-acetic acid are widely used as efficient acceptor unit [43], [44], [45]. Common π-spacers used are methane chains, 8H-thieno [2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]pyrrole, thiophene, furan, pyrrole, benzene, etc [46], [47], [48]. The key parameters governing high performance of donor acceptor based DSSC includes dihedral angle, dipole moment (μ), energies of the HOMO and LUMO, energy difference between the HOMO and LUMO (ΔH−L values), ionization potential (IP), electron affinity (EA), ground state oxidation potential (GSOP), excited state oxidation potential (ESOP), light harvesting capacity (LHC), open circuit voltage (Voc), absorption properties, reorganization energies (λ), charge transfer rate (kCT), Jsc, PCE etc. [49]. Additionally, variation of the D, π and A moieties make the dyes easy to tune their HOMO and LUMO levels as well as their absorbance spectra. Triphenylamine based dyes act as promising candidates for DSSCs as it hold the efficiency of over 10.3% [50].

To gather the information regarding the tuning of optoelectronic properties of DSSCs, density functional theory (DFT) now-a-days become a hot tool for researchers. Zaier et al. reported a computational study based on the electronic, photo-physical and charge transport properties of novel designed small and planar molecules by means of DFT and time dependent density functional theory (TD-DFT) methods [51]. Moreover, Divya et al. reported a DFT study of DSSCs based on donor-π-acceptor dyes and further studied their adsorption on TiO2 semiconductor. They also studied various parameters such as molecular electrostatic potential analysis, absorption maximum, open-circuit voltage, etc [52]. Besides, Haroon et al. carried out an analysis based on D-A-π-A framework and investigated the photo-physical, optoelectronic and PV characteristics with the aid of DFT and TD-DFT approaches [53].

The objective of the present study is to design effective organic dyes for DSSCs based on D-π-A architecture. To attain better performance of the designed dyes, we have modified the π-bridging unit of the already reported D-π-A type dyes (i.e., dyes 1 and 3) by introducing different electron donating and withdrawing substituents at the π-bridging unit [46]. In this work, we have used methoxy substituted triphenyl amine(TPA) and methyl substituted indoline(IND) as the donor units and cyanoacrylic acid (CA) as the acceptor unit along with the 8H-thieno [2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]pyrrole (TTP) unit as π-bridge. Besides,we have tried to tune the performance of DSSCs by molecular engineering. In this regard, different electron donating groups viz. single bondH, single bondCH3, single bondOCH3, single bondCH2 double bondCH2, single bondSH, and single bondOH and different electron withdrawing groups viz. single bondCF3, single bondCOCH3, single bondCOOH and single bondCN have been incorporated into the π-moieties. These groups are denoted numerically in suffix as D1, D2, D3, D4, D5, D6, D7, D8, D9 and D10 respectively for TPA-CA dyes and D11, D12, D13, D14, D15, D16, D17, D18, D19 and D20 respectively for IND-CA dyes. Moreover, we have used TiO2 as the semiconducting surface. The optimized geometries of the designed dyes have been presented in Fig. S1 (in the Supporting information) and their structures have been presented in Fig. 1.

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