Optical and electronic properties enhancement via chalcogenides: promising materials for DSSC applications

Comparatively, metal-free sensitizers featuring the chalcogen family receive less attention despite known electronic properties for metal-chalcogenide materials. This work reports an array of optoelectronic properties using quantum chemical methods. Observed red-shifted bands within the UV/Vis to NIR regions with absorption maxima > 500 nm were consistent with increasing chalcogenide size. There is a monotonic down-shift in the LUMO and ESOP energy consistent with O 2p, S 3p, Se 4p, to Te 5p atomic orbital energies. Excited-state lifetime and charge injection free energies follow the decreasing order of chalcogenide electronegativity. Adsorption energies of dyes on TiO2 anatase (101) range between − 0.08 and − 0.77 eV. Based on evaluated properties, selenium- and tellurium-based materials show potential use in DSSCs and futuristic device applications. Therefore, this work motivates continued investigation of the chalcogenide sensitizers and their application. The geometry optimization was performed at B3LYP/6–31 + G(d,p) and B3LYP/LANL2DZ level of theory for lighter and heavier atoms, respectively, using Gaussian 09. The equilibrium geometries were confirmed by the absence of imaginary frequencies. Electronic spectra were obtained at CAM-B3LYP/6-31G + (d,p)/LANL2DZ level of theory. Adsorption energies for dyes on a 4 × 5 supercell TiO2 anatase (101) were obtained using VASP. The dye-TiO2 optimizations were employed using GGA and PBE with the PAW pseudo-potentials. The energy cutoff was set at 400 eV and convergence threshold for self-consistent iteration was set to 10−4, and van der Waals were accounted using DFT-D3 model and on-site Coulomb repulsion potential set at 8.5 eV for Ti.


Introduction
Solar energy remains one of the most promising alternatives among renewable energy sources; about ~ 3.8 million exajoules are radiated annually, which is equivalent to 10 4 times the current global demand [1]. However, the development of techno-economic feasible systems for harvesting and storage of solar energy remains to be the subject of interest. Owing to their advantageous characteristics, such as relatively high efficiency, low-energy payback time, lightweight, and good performance even under diffuse light [2], dye-sensitized solar cells (DSSCs) present huge potential to revolutionize the energy sector. DSSCs were recently integrated into wearable self-powered energy electronics [3][4][5][6] and facilely woven into smart textiles [7]. However, despite the recent progress recorded by DSSCs, their power conversion efficiencies are not high enough to compete with the dominant crystalline silicon solar cells.
Following the pioneering work of Grätzel et al. in 1991 where power conversion efficiency (PCE) of ~ 8% and ~ 12% were achieved in simulated solar light and diffuse daylight, respectively, using Ru-based N749 dye and TiO 2 photoanode [5], many investigations are still underway to improve the performance of DSSCs. To date, PCE based on DSSCs has reached ∼12-17% [8,9].
Appropriate functionalization of sensitizers may result in the improvement in the optoelectronic characteristics, consequently resulting in the enhancement of PCE for DSSCs. Depending on the purpose, structural units such as donor (D), π-linkers/bridges, and acceptor (A) can be varied. When variation in structural units is performed, the intentions are not limited to widening the absorption bands, increasing light-harvesting efficiency, or improving molecular stability when the dye interacts with the TiO 2 semiconductor. For example, following their remarkable intrinsic properties such as rapid intersystem crossing (ISC), oligothiophenes have been employed as π-linkers in DSSC materials [10]. The ISC property is vital as it leads to extremely high triplet quantum yields [11][12][13]. Reports show that the trans-conformer of the polythiophene bridges are stable when compared to the cis-counterpart and that ultrafast ISC takes place from the first excited singlet state [10]. The torsional motion around the flexible inter-ring bonds is responsible for the relaxation processes of thiophene oligomers after photo-excitation [14]. A systematic comparison of the optoelectronic properties and their performance for dyes with or without additional auxiliary π-linkers (dithiophene) in the dye's skeleton was carried out by Zhu and co-workers [15]. The overall PCE of ~ 8% was recorded for dithiophene-containing molecules; the recorded PCE was found to be ~ 1.4 times that of the dye without dithiophene. While chalcogens are ubiquitous for metal-based optoelectronic materials such as CdX [16,17], CuX [18,19], CoX [20], and ZnX [17,21,22] for X = O, S, Se, and Te, heteroatom permutations among chalcogens remain quite poorly explored in the field of DSSCs.
In our recent works, we have demonstrated that the optoelectronic properties of sensitizers containing heavier chalcogens (Se and Te) far surpass those of S-containing sensitizers [23][24][25]. Motivated by the observed high PCE (8.15%) for dithiophene-containing molecules and superior optoelectronic characteristics for heavier chalcogenides [15,[23][24][25], it is also of interest to explore a complete series of chalcogenides (O, S, Se, and Te) for the sensitizers presented in Fig. 1. It is also suggested that the proposed materials would exhibit superior optical and electronic characteristics over oxygen-and sulfur-containing dyes due to the semimetal effect of Se or Te.

Computational details
The 3D molecular structures were generated in the Avogadro package [26], and quick energy minimization was performed using Universal Force Field (UFF) embedded within the package. The geometrical, optical, and electronic properties were obtained using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods as implemented in Gaussian 09 [27]. Geometry optimization was performed using B3LYP functional [28] coupled with 6-31 + G(d,p) basis set for light atoms and LANL2DZ basis set for heavy atoms (Se and Te). Next, frequency calculations were performed and the absence of imaginary frequencies confirmed that the optimized geometries correspond to the minima on the potential energy surface. For all calculations, the default convergent criteria are used for the energy and force constants. Furthermore, the simulation of absorption spectra was performed using CAM-B3LYP maintaining same basis sets for corresponding atoms as for optimization and frequency calculations. The simulated maximum absorption for WS-S was found to be 545 nm, comparable to the experimental value (546 nm) [15]. The dichloromethane solvent was chosen for simulation of electronic spectra within the polarized continuum model (PCM) as the same solvent was used for the experimental UV-Vis spectrum measurements on the WS-S molecule.
Adsorption of dyes on the TiO 2 surface was studied, and the conjugate-gradient optimizations were made using the generalized gradient approximation (GGA) as elaborated by Perdew-Burke-Ernzerhof (PBE) [29], and Fig. 1 The chemical structures of the studied sensitizers containing chalcogenide (O, S, Se, and Te) atoms in the π-linkers the projector augmented plane-wave (PAW) pseudopotentials. The energy cutoff was set at 400 eV for all the adsorption calculations. The convergence threshold for self-consistent iteration was set to 10 −4 . To consider the van der Waals interactions, the DFT-D3 model [30] was performed. The Coulomb repulsion potential (U) ranging between 3.5 eV and 10 eV was found to reproduce experiment band gaps. Previous studies set U = 8.5 eV [31][32][33][34][35]; thus, in the present study the same value was adopted. The Gamma point in the reciprocal space was used in the geometric relaxation process to reduce the computation consumption. The titanium dioxide (TiO 2 ) anatase (101) with 4 × 5 supercell was chosen in this study. where E dye@TiO 2 is the total energy of the adsorbed dyes on TiO 2 , E TiO 2 is the total energy of the TiO 2 pure surface, and E dye is the total energy of the isolated optimized adsorbate. The adsorption properties were calculated using the Vienna Ab initio Simulation Package (VASP) [36,37].

Geometrical properties
The optimized geometries of the 2D molecular structures ( Fig. 1) are presented in Fig. 2; selected geometrical parameters of the investigated dyes are presented in Table 1. Benzothiadiazole (BTZ) units exhibit planarity and are useful for improving intramolecular charge transfer. The other units, however, demonstrate non-planarity, which reduces dye aggregation. The measured bond lengths are N−X (2.800 − 2.925 Å) and H−X (2.486 − 3.099 Å) for optimized structures; the bonds strengthen as the electronegativity of the chalcogen atom increases.
This observation was similar to the findings by Ortiz-Rodríguez and co-workers, it was observed that the bond length decreases with orbital size mismatch between the chalcogen and the H [38]. These substitutions showed minimal effect on the dihedral angle X-C-C-X which remained ~ 180° implying that the substituted chalcogens did not alter the planarity of the sensitizers. For each chalcogen, the bond and angles θ 1 and θ 2 are equivalent (i.e., Δθ ≈ 0); however, the values decreased with increasing chalcogen size. Mulliken charges on the chalcogenides show that O (− 0.294) carries a strong negative charge followed by S  Table 1 The calculated dihedral angle Φ (°), the bond angle θ i = 1&2 (°), and bond lengths

Optical spectra
The electronic absorption and emission spectra of the dyes containing heteroatoms (O, S, Se, and Te) in the π-linkers are presented in Fig. 3. Furthermore, electronic transitions, maximum absorption, oscillator strength, excited-state lifetime, light-harvesting efficiency, and corresponding molecular orbital contributions are shown in Table 2. Both absorption and emission spectra are red-shifted with increased chalcogenides' atomic size. Absorption spectra are characterized by dual-band absorption profile, the higher energy bands were observed at around 300 nm, and the lower energy bands were found within 500 to 600 nm. Contrary to the lower energy bands, the higher energy bands are nearly equivalent in intensities and positions for all dyes. A noticeable trend was observed in the optical band gap among the investigated dyes, where WS-O (2.37 eV) exhibited the widest energy band gap, followed by WS-S (2.28 eV), WS-Se (2.16 eV), and WS-Te (2.07 eV) which had the narrowest gap; thus, these dyes exhibit maximum absorbption of light at 523 nm, 545 nm, 575 nm, and 600 nm, respectively. The optical band gap energies of the chalcogenide sensitizers decreased systematically from S to Te. A more pronounced red-shift was observed for dyes containing large-size chalcogen atom. The observed hyperchromic response was consistent with the electronegativity of dopant chalcogen atoms. A similar trend was observed for the emission em = 1.78, 1.67. 1.53, and 1.45 (all in eV); the stoke-shift was found to be 172 (O), 197 (S), 234 (Se), and 253 nm (Te). Due to long-trailed absorption, red-shifted emission peaks, and larger Stoke's shift (higher than 100 nm) indicating that doping with heavy chalcogen may permit simultaneous excitation of different fluorescence color through a single excitation source; thus, materials containing these elements may be integrated into the building allowing multi-purpose operation.
The maximum absorption band was characterized by HOMO to LUMO as the main electronic transition followed by HOMO-1 to LUMO; the percentage contributions increase with the increase in the atomic size of the chalcogen atom size where HOMO to LUMO transitions can be found within 55 to 56% and 32 to 35% for HOMO-1 to LUMO transitions.
The intramolecular charge transfer of the studied dyes was assessed through the charge density distribution as depicted in Fig. 4. As one may observe from Fig. 4, the HOMO electron densities were mainly delocalized of the donor part

Light-harvesting efficiency
The performance of DSSC is measured through incident photon-to-current conversion efficiency (IPCE) expressed as where Φ LHE is the function of light-harvesting efficiency which accounts for electron density movement related to both optical absorption intensity and available electron transition, Φ inj is the electron injection efficiency, and Φ CC is the charge collection efficiency. The Φ LHE can be expressed as Φ LHE = 1 − 10 −f , where f is the oscillator strength of the sensitizer corresponding to maximum absorption. The obtained f are higher than a unit leading Φ LHE higher than 99% or higher ( Table 2); it is worth noting that there was no clear trend in the Φ LHE values.

Excited-state lifetime
Excited-state lifetimes (τ) for the sensitizers under consideration were evaluated; τ is an important factor influencing the charge transfer. A sensitizer with a longer τ is expected to be more facile for charge transfer and suppresses recombination, consequently leading to reduced energy loss. It is a requirement that charge injection time should be shorter than the excited state decay to the ground state for efficient charge injection before radiative or photo-chemical reactions occur [40][41][42][43]. The τ (ns) of a sensitizer can be calculated by using the following relation [44][45][46][47][48][49]: where ΔE is the transition energy in cm −1 units of measurements; the first excited-state lifetime corresponds to the 1.499 f × ΔE 2 lowest excitation energies, mostly from S 0 → S 1 . Calculated values are found in the range 1.73-2.61 ns in the order WS-O < WS-S < WS-Se < WS-Te. One may hypothesize that the inclusion of less electronegative chalcogens (Se and Te) may lead to the stabilization of the excited state, possibly supported by the extended valence shell accompanied by the change in hybridization [50].

HOMO-LUMO energy levels
Understanding the energy level of the highest occupied molecular orbital HOMO (E H ) or ground state oxidation potential (GSOP) and LUMO (E L ) is critical to the successful design of sensitizers with improved PCE. The E H and E L are sensitive to changes in the chemical structures such as nature and placement of the functional group. Charge injection to the CB of the semiconductor proceeds from the unrelaxed excited-state dye species to the CB of the semiconductor [51,52]. The energy level corresponding to this is known as excited-state oxidation potential (ESOP) and can be calculated as ESOP = GSOP + ΔE, where ΔE corresponds to the vertical excitation energy/optical band gap which can be obtained from the TD-DFT calculations. The ability of the dye to inject charge into the CB of a semiconductor can be quantified through the free energy of charge injection (ΔG inj ) calculated as ΔG inj = E CB − ESOP, where E CB is the reduction potential of the CB of the TiO 2 semiconductor. The oxidized dye is regenerated by receiving an electron from the I − ∕I − 3 electrolyte; dye regeneration is quantified through free energy of dye regeneration (ΔG reg ) which can be calculated as ΔG reg = GSOP − E I − ∕I − 3 . The energy level alignment diagram with conduction band of TiO 2 semiconductor (− 4.05 eV) at pH = 7 [53] and redox potential of iodide/ triiodide electrolyte and its value is − 4.8 eV [54]. Table 3 shows the calculated GSOP/E H , vertical transition energies ΔE, energy band gaps E g , LUMO, ESOP, ΔG inj , and ΔG reg . Generally, we observe nearly similar HOMO energies; however, the LUMO stabilization (i.e., downward shift) results in the narrowed energy band gaps.
Clear trends were observed in the optical (ΔE) and energy (E g ) gaps between dyes. The studied dyes exhibit ΔE values from 2.07 to 2.37 eV and E g between 1.77 and 1.97 eV; we observe a positive correlation between the ΔE and E g defined by the linear equation: ΔE = 1.3576 × E g − 0.2882 with R 2 = 92%. The decreased energy band gap for dyes containing larger chalcogen atoms is caused by reduced aromaticity in conjugated five-member rings, despite being easier to polarize and having larger atomic radii (O: 0.73 Å; S: 1.02 Å; Se: 1.16 Å; Te: 1.40 Å) [55], exhibit poor orbital overlap with neighboring carbon due to their larger size; aromaticity of the five-member rings decreases in the following order: Te < Se < S < O [56]. It is worth noting that a similar effect in the above sequence of chalcogen atom substitutions was observed recently by DFT calculations and experimental measurements of a series of Cu 2 HgGeQ 4 (Q = Se, Te) compounds [57][58][59]. Figure 5 shows a minimal shift in the E H energies among the investigated dyes ranging between − 5.12 and − 5.10 eV. This observation was consistent with the findings by Planells and co-workers [56], implying that the dyes have comparable ΔG reg within the range − 0.30 and − 0.32 eV. It is interesting to note the monotonic decrease in the ESOP energy level from oxygen to tellurium. This observation is consistent with atomic orbital energies for group VI where the p orbital valence energies increase moving down group VI (from O 2p, S 3p, Se 4p, Te 5p) [60].

Adsorption on the TiO 2 surface
To understand the adsorption capability of the four dyes on TiO 2 anatase (1 0 1), geometrical parameters and adsorption energies of adsorbed systems were studied using PBE + U level. Carboxylic acid groups can be anchored to the TiO 2 surface through three modes: mono-dentate, bidentatechelating, and bi-dentate bridging [63]. In previous theoretical studies, the adsorption of carboxylic acid on TiO 2 by bidentate was found to be the more stable mode than the other two modes [64,65]. Unlike the mono-dentate mode, the bidentate was observed to have shortened bond lengths that support increased electron injection rates. Hence, this study considered the bi-dentate bridging mode. Figure 6 presents the optimized geometries of four dye adsorbed systems.
Using Eq. 1, the adsorption energy ( E ads ) can be determined for the four adsorbed systems, tabulated in Table 4 along with geometrical parameters (bond lengths and dihedral angles) using PBE + U functional. Each of the four dyes is adsorbed on TiO 2 by bonding between oxygen atoms of the anchoring group and surface atoms, and the bond lengths fall in the range of 2.031-2.122 Å. Based on the literature [66], those values indicated strong interactions between the investigated dyes and the surface. The four investigated adsorption systems are stable based on their negative adsorption energies, which indicates that the exothermic processes occurred. It can also be noted the adsorption energies are relatively small and negative ranging from − 0.08 to − 0.77 eV.
This means that dyes may be adsorbed onto the TiO 2 surface. Among the four adsorbed systems, WS-Te@TiO 2 has the strongest interaction with the TiO 2 surface due to its higher adsorption energy (the most negative value), leading to faster charge transfer rates and enhanced DSSC performances [66,67]. The opposite note has been observed for WS-O@TiO 2 . On the other hand, the Fermi energies are negative and they increase with increased electronegative of chalcogenides (i.e., Table 3 The calculated optical band gap ΔE, energy band gap E g , the highest occupied molecular orbital (HOMO) energies, the lowest unoccupied molecular orbital (LUMO) energies, excited-state oxida-tion potential (ESOP), free energies of charge injection ΔG inj , and free energy of dye regeneration ΔG reg (all values in eV)   [61,62] from Te to O atoms) in the order WS-Te@TiO 2 < WS-Se@ TiO 2 < WS-S@TiO 2 < WS-O@TiO 2 . Generally, these results indicate that the dyes were tightly adsorbed on the semiconductor surface, which might result in improved photo-voltaic performance and long-term stability of the devices.

Conclusion
In the present study, we have reported the optoelectronic properties of dyes featuring five-membered ring π-linkers containing chalcogenides (O, S, Se, and Te). It was observed that the chalcogenide π-linkers influence both geometrical and optoelectronic properties. On the other hand, the absorption and emission spectra are red-shifted in the order of increasing heteroatom size (O < S < Se < Te). The red-shifted absorption between 523 and 600 nm resulted in narrowed energy band gaps (E g < 2 eV). This observation was consistent with decreasing electronegativity of chalcogenides. LHE for all dyes reaches 99%, therefore, are expected to produce higher short-circuit current density (J sc ) . The sensitizers exhibit comparable HOMO energy levels, which imply that the dyes may have comparable regeneration ability; there is a down-shift in the LUMO energy level from oxygen to tellurium-containing dyes. This observation is due to the increased atomic orbital energies for the group in the following order: O > S > Se > Te. Based on the results, it can be concluded that the four sensitizers have the appropriate HOMO and LUMO energy levels as well as matching as required for injection and recombination process. There is the stabilization of the excite-state with an increase in chalcogen atomic size; the evaluated excited-state lifetime are 1.73, 1.96, 2.30, and 2.61 ns for oxygen-, sulfur-, selenium-, and tellurium-containing dyes, respectively, which leads to increase V oc . Consequently, resulting in improved power conversion efficiency of the solar cell. On the other hand, the periodic DFT calculations were carried out, and it found the same trend which the stabilization of systems increases;  that is, dyes interact stronger with the surface with decreasing chalcogenides' electronegativity. Our evaluation of the reported properties shows that consideration of heavier chalcogen atoms (Se and Te) in sensitizers improves the characteristics of interest that led to (i) longer absorption wavelengths to NIR region, (ii) increase the light-harvesting ability, and (iii) stabilization of the adsorption systems; therefore, sensitizers containing chalcogenides could be a key in accelerating power conversion efficiencies for DSSCs enabling renewable energy transformation and future technological advances.