Topical PerspectivesA theoretical approach of star-shaped molecules with triphenylamine core as sensitizer for their potential application in dye sensitized solar cells
Graphical abstract
This work is supplying an in-depth investigation of the optical, electronic, and charge transfer properties for heteroatom effects on the WD8-c-1. The introducing of heteroatom could improve the JSC and VOC properties for investigated molecules.
Introduction
Dye sensitized solar cells (DSSCs) have received a lot of investigator attentions due to their potentially low cost and easy fabrication [1]. Many researches and developments have been dedicated to this field over the past years [[2], [3], [4], [5]]. In DSSCs, the sensitizer could absorb sunlight and transfer the generated photoelectrons to the TiO2 semiconductor. Thus, it plays a key role as one of the significant components, which could control the efficiency of equipment in comparison with the electrolyte and semiconductor [6,7]. Generally, the sensitizer can be classified into two types, including metal-containing and metal-free dyes. The metal-containing sensitizers, such as the black dye [8], N719 [9], ruthenium dyes [10,11], Zn-porphyrins [12], TiO2 [13], and perovskites [14], obtain excellent power conversion efficiency (PCE). However, the practical uses of metal-containing sensitizers are still limited because of their expensive cost, complicated synthetic process, low yield, and environmental pollution. On the other hand, the researchers focused on metal-free dyes in order to replacing of costlier metal-containing dyes. It is because that the metal-free dyes have high coefficients owning to its intramolecular π−π∗ transitions [15]. Moreover, metal-free dyes are fascinating because they own many advantages, such as low cost, flexible molecular design, easier tunability of energy levels, versatile structure tailoring along with superior light harvesting efficiency, and easy synthetic process [16,17]. Recently, lots of metal-free dyes have been prepared, for example, diketopyrrolopyrrole [18], indoline [19], phenothiazine [20], pyrazine [21], and triphenylamine (TPA) [[22], [23], [24]].
Among the various metal-free dyes, special attentions have been paid to TPA and it derivatives. It is attributed to these molecules, possessing three-dimensional geometry, could constitute amorphous materials with interesting optical and charge transport properties for a variety of DSSCs devices [[25], [26], [27]]. The different π-conjugated ring could be used to extend the π-conjugated system with TPA as core of molecule. Star-shaped molecules containing TPA core and thiophene derivatives have yielded interesting results in DSSC applications [28]. Moreover, the star-shaped 2D-π-A structure of the metal-free dyes will not only avoid charge recombination, but also extend the absorption region and enhance the molar extinction coefficient [29]. Although numerous metal-free dyes have been developed, there is still a huge gap between actual power conversion efficiency (PCE) (14.3%) [30] and the Schokley-Queisser theoretical PCE (30%) [31], and also the PCE of commercial silicon-based solar cells (26%) [32]. Thus, improving the PCE of DSSC is a significative and imperative challenge. The PCE of DSSCs on the basis of metal-free dyes leaves a lot of room for improvements.
Theoretical predictions have become an accurate scientific and inexpensive tool to provide deeply investigation at atomistic scale and forge ahead in designing new metal-free dyes as sensitizers. Zhang et al. provided a computational protocol for precise prediction of dye-sensitized solar cell in short-circuit current density (JSC), open-circuit photovoltage (VOC), fill factor (FF), and PCE [33]. Yang et al. studied the intramolecular charge separation of D-A-π-A organic sensitizers with different linker groups by DFT calculations [34]. Bahers et al. gave a step-by-step theoretical protocol on the basis of density functional theory (DFT) and time-dependent DFT (TD-DFT) at both the molecular and periodic levels, which was proposed for designing DSSC including dyes and electrolyte additives [35]. Estrella et al. designed and characterized porphyrin-based sensitizers for applications in DSSC via DFT and TD-DFT calculations [36]. Xie et al. investigated the spectral complementary composite dye molecules for design high performance DSSCs [37]. Wang et al. supplied theoretical insights into the rigidified dithiophene effects on the performances of cis-squaraine-based DSSCs with panchromatic absorption [38]. However, theoretical prediction has not played a due role in development of new metal-free dyes. The reason may be by the fact that there is rarely quantitative calculation and inaccurate estimated values for JSC and VOC, especially for VOC.
In this consideration, here, we introduce oxygen, sulfur, and nitrogen atoms into the molecule WD8-c-1 which has been studied in our previous work [39]. The investigated molecules are shown in Scheme 1. The frontier molecular orbitals (FMOs) including the highest occupied molecular (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies, the HOMO–LUMO gaps (Eg) of these molecules were calculated by the DFT method. The TD-DFT method was employed to simulate all the absorption spectra of these molecules. The carrier mobility is also a key parameter which could evaluate the performance of sensitizer. Thus, we simulated the reorganization energies of all derivatives. We also simulated the related parameters of JSC and VOC for investigated molecules.
Section snippets
Computational details
All the calculations were carried out via Gaussion 09 [40]. The DFT method at the B3LYP/6-31G (d,p) level was used for optimization and the TD-DFT method at the PBE0/6-31 + G (d,p) level was used to calculate the absorption spectra of all the investigated molecules. It is because that those methods successfully predicted the electronic and optical properties of molecule WD8-c-1 in our previous work [39]. The charge transfer rate can be described by Marcus theory via the following equation:K = V2
Frontier molecular orbitals
The optical and electronic properties of molecules are much concerned with the distribution patterns of the FMOs. Thus, the distribution and characteristic of FMOs should be analyzed in detail. Here, the corresponding diagrams of HOMO and LUMO are depicted in Fig. 1.
In Fig. 1, for the nitrogen atom substituent molecules, the distribution patterns of FMO are similar with those of WD8-c-1, except molecules WD8-c-1-N1 and WD8-c-1-N3. The electronic cloud of HOMO for WD8-c-1-N1 and WD8-c-1-N3 are
Conclusion
In present work, a theoretical study of simulating the heteroatom effects on optical, electronic, charge transport, JSC, and VOC properties for the WD8-c-1 has been reported. The calculated results show that the oxygen and sulfur atom substituents affect the distributions of FMOs for WD8-c-1 slightly. The introducing of nitrogen and sulfur atoms affect the EHOMO, ELUMO, and Eg values of WD8-c-1 significantly. Moreover, the electron could transfer from excited sensitizer into the CB of TiO2,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Thanks for the supported by the Inner Mongolia Key Laboratory of Photoelectric Functional Materials and the Science and Technology Project fromEducation Department of Jilin Province (No. JJKH20190906KJ).
References (58)
- et al.
Triphenylamine-based starburst dyes with carbazole and phenothiazine antennas for dye-sensitized solar cells
J. Power Sources
(2012) - et al.
Silicon-bridged triphenylamine-based organic dyes for efficient dye sensitised solar cells
Sol. Energy
(2018) - et al.
Synthesis and photovoltaic properties of a star-shaped molecule with triphenylamine as core and benzo[1,2,5]thiadiazol vinylene as arms
Sol. Energy Mater. Sol. Cells
(2009) - et al.
Theoretical design and characterization of NIR porphyrin-based sensitizers for applications in dye-sensitized solar cells
Sol. Energy
(2019) - et al.
Study on the spectral complementary composite dye molecules designed for high performance dye-sensitized solar cells: a theoretical investigation
Comput. Theor. Chem.
(2019) - et al.
Theoretical insights on the rigidified dithiophene effects on the performance of near-infrared cis-squaraine-based dye-sensitized solar cells with panchromatic absorption
J. Photochem. Photobiol. Chem.
(2019) - et al.
The substituent effects of naphthalene diimide as acceptor for organic solar cells: a theoretical study
J. Taiwan. Inst. Chem. E.
(2017) - et al.
Electronic structures and optical properties of organic dye sensitizer NKX derivatives for solar cells: a theoretical approach
J. Mol. Graph. Model.
(2012) - et al.
Theoretical study of triphenylamine-based organic dyes with mono-, di-, and tri-anchoring groups for dye-sensitized solar cells
Org. Electron.
(2018) - et al.
Organic molecules based on dithienyl-2,1,3-benzothiadiazole as new donor materials for solution-processed organic photovoltaic cells
Sol. Energy Mater. Sol. Cell.
(2010)