A computational approach to study the optoelectronic properties of F-BODIPY derivatives at the bulk level for photovoltaic applications

The Dipyrrin compounds are recognized as imminent and hi-tech organic semiconductor materials (OSMs) designed for photovoltaic applications owing to low-cost, flexible, and eco-friendly nature. In a latest study, several optoelectronic properties of conjugated F-BODIPY derivatives were studied by using the density functional theory (DFT) at (GGA/PBE) level. This analysis classifies Comp_1 (5,5-difluoro-1,37,9-tetramethyloctahydro-1H,5H-5l4-dipyrrolo[1,2-c:2′,1′-f][1,3,2] diazaborinine) as a direct band-gap semiconductor with a band-gap of 2.81 eV. The band-gap of Comp_2 (2-ethyl-5,5-difluoro-1,3,7,9-tetramethyloctahydro-1H,5H-5l4-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinine) and Comp_3 (2,8-diethyl-5,5-difluoro-1,3,7,9-tetramethyloctahydro-1H,5H-5l4-dipyrrolo[1,2-c:2′,1′-f] [1,3,2] diazaborinine) are reduced by 1.25 and 1.09 eV, respectively in comparison with parent molecule Comp_1 owing to the influence of p- orbitals of all atoms in crystal structures. The reduction in energy and density of states (DOS) for these compounds in this study validated that band-gap could be improved to a required value for optoelectronic applications by derivatives modeling. Additionally, optoelectronic properties at the solid-state bulk level were also computed. Several features of interest at solid-state bulk level exposed the F-BODIPY derivatives as competent compounds for organic semiconductor devices applications.


Introduction
The small π-conjugated organic semiconductor materials (OSMs) are receiving astonishing responses from the scientific investigators currently, owing to the comparatively high stability, enriched fluorescence, rational bandgap and enhanced conductivity [1][2][3] which are crucial for sophisticated portable optoelectronic applications. The OSMs have become imperious by presenting distinctive functionalities and affluent uses in several high-tech fields such as photovoltaic device, light emitting diodes, thin film transistors, field effect transistors and third-order nonlinear optical application due to their prominent performance, cost-effective process, large area flexible devices, easy manufacturability and eco-friendly nature [4][5][6][7][8][9][10][11]. The direct bandgap OSMs have been of specific consideration, ever since these are proficient, quicker and trustworthy for modern optoelectronic applications [12][13][14][15] and establish many benefits above obsolete forms such as minor energy cost, and enhanced illumination capability [15][16][17].
OSMs using the 4,4-difluoro-4-boro-3a,4adiaza-s-indacene (F-BODIPY) [18][19][20] as basic skeleton accredited owing to high thermal reliability and adjustable fluorescence properties that marks F-BODIPYs essentially suitable basic chunk for OSMs. Recently F-BODIPY derivatives synthesized via a novel and enriched scheme with Dipyrrins and Bis(dipyrrin)s and established as a decent OSMs by excellent fluorescence properties [1]. F-BODIPYs typically produced via Dipyrrin doping as its BF 2 complex by means of a reaction among BF 3 ·OEt 2 and NEt 3 [21,22]. Though many F-BODIPY derivatives were reported in literature [23][24][25] , where the capabilities for photovoltaic applications were described as well. In present investigation, the impact of ethyl group on the optoelectronic properties are explored at the solid-state (bulk) level when assigning ethyl group at positions X and Y in F-BODIPY derivatives 5,5-difluoro-1,37,9-tetramethyloctahydro-1H,5H-5l4-dipyrrolo [1, 2- The optoelectronic properties of F-BODIPY derivatives were investigated by distinctive density functional theory (DFT) through (GGA/PBE) [26][27][28] level. An unchanged experimental data [1] is used to optimized these derivatives. The optoelectronic properties analytically assessed for F-BODIPY derivatives. Though numerous studies were available to examine the scopes of F-BODIPY derivatives for applications in leading-edge tools, the optoelectronic reaction of F-BODIPY derivatives in solid-state bulk level is still needed to probe. This work scrutinizes first-time optoelectronic properties at the solid-state bulk level for F-BODIPY derivatives.
The integration of Brillouin Zone (BZ) is done by adopting the Monkhorst-Pack special k-point approach [32]. The plane-wave expansion with optimized cutoff energy of 330 eV is used to automatically generates the Fast Fourier Transform (FFT) grid (54×72×54), (40×45×48) and (45×60×40) for Comp_1, Comp_2 and Comp_3, respectively. The reciprocal space and density mixing technique are utilized by choosing the Ultra-soft Pseudo-potential and electronic minimization method. The energy convergence is used by program's fine to generate the k-point mesh for Monkhorst-Pack grid at (2×2×2) for Comp_1, Comp_2 and Comp_3, respectively. The energy of the three molecular crystals under study is minimized through the Broyden-Flether-Goldfarb-Shanno (BFGS) hessian updated scheme [33]. The following threshold has been used for the converged structure as: the self-consistent field (SCF) converged tolerance is 2.0×10 −6 eV/atom; the energy change tolerance is 1.0×10 −5 eV/atom; the root mean square (RMS) force on atom tolerance is 0.03 eV/Å; RMS displacement of atoms is 0.001 Å and RMS stress is 0.05 GPa. The optimized structures of Comp_1, Comp_2, and Comp_3 with minimized energy are shown in figures S1-S3 is available online at stacks. iop.org/MRX/6/125110/mmedia of supporting information, respectively.

Results and discussion
3.1. Optical properties 3.1.1. Dielectric function Table 1 and figure 2 demonstrates real and imaginary dielectric functions (ε 1 and ε 2 ) of Comp_1, Comp_2, and Comp_3. The ε 1 unveils high peaks in the small energy value among 1 to 3 eV as 4.48, 3.30, and 5.42 for Comp_1, Comp_2, and Comp_3, respectively. The Comp_1, Comp_2, and Comp_3 crystal structures displayed the static ε (ε (0)) driven from the real part of dielectric function as 3.27, 2.66 and 4.01 in (010) direction, respectively. The ε (0), ε 1 (max) and ε 2 (max) has been summarized in table 1. The Comp_3 displayed larger ε (0) and ε 1 follows by Comp_1 and Comp_2, which reveals that the polarizability is higher in Comp_3 among three studied compounds hence would be good to transport the electron through the crystal. The ε (0) at zero photon energy having excellent values disclosed the substantial aptitude for optical transitions of electron through the crystals of these compounds.
The (010) direction is dominating ε 2 for all compounds, while Comp_3 (5.12) poses greater ε 2 as compared to Comp_1 (3.9) and Comp_2 (2.5). The distribution of ε 2 designate electron transitions (optical transitions) between valance bands and conduction bands as well as the transitions within the valance band from lower energy to higher energy states. The figure 2 indicates that the optical transitions in the F-BODIPY derivatives mainly occures in the visible range of electromagnetic spectrum. The compounds Comp_1, Comp_2, and Comp_3 with these values of dielectric functions revealed that these compounds might be worthy for charge transport inside the crystals.
Compounds  power swiftly at 10 eV, though displaying some little peaks among 10 -15 eV. The σ 1 for Comp_1, Comp_2, and Comp_3 presenting identical conductivity afterward 16 eV having nil strength. The unchanged spectra disclosed the decent chemical strength of F-BODIPY derivatives that were ultimately originating huge ability for electron transference in the crystal, illuminating improved charge transport in the F-BODIPY derivatives.

Refractive indexes and extinction co-efficient
Calculated highest refractive indexes (n) are 2.16, 1.84, and 2.38 for the studied compounds in (010) direction have been demonstrated in figure 4. The computed n is clarifying that Comp_1, Comp_2, and Comp_3 would be superior to deflect a photon at minor energies. The Comp_1, Comp_2, and Comp_3 display decent n at smallest energy between 0 to 3 eV, hitherto the spectra began tumbling after 5 eV and made unaffected spectra of same power afterward 7 eV. The figure 4 exposed that these compounds would be capable for generating decent yields at smaller energy responses of 3 eV.

Reflectivity and energy loss function
The reflectivity for Comp_1, Comp_2, and Comp_3 has been computed and presented in figure 5 to explore these compounds precisely. The Comp_1, Comp_2, and Comp_3 revealed higher reflectivity as 0.224, 0.148 and 0.314, respectively, that grows amongst 1 to 4 eV in the (010) direction, respectively. The Comp_1, Comp_2, and Comp_3 demonstrate peaks at lesser energy in reflectivity spectra, then starts to grasp identical heights successively after 9 eV as might be seen in figure 5. These values of reflectivity unveil that these compounds might be effective for OPV applications at lower energy. The energy loss function is a crucial feature that reveals the loss of electron energy when traveling within the compound [34][35][36]. The loss function for studied compounds is presented graphically via

Absorption
We calculate the absorption wavelengths for Comp_1, Comp_2, and Comp_3 molecular crystals and displayed in figure 6. It is clear from figure 6

Electronic properties
The electronic properties are equally essential to estimate the charge transport capability of OSMs.

Density of states (DOS)
TDOS and PDOS have estimated for Comp_1, Comp_2, and Comp_3 crystals to distinguish electronic properties and displayed in figures 7 and 8. The contribution of carbon (C), nitrogen (N), florin (F) and boron (B) atoms in the formation of PDOS has shown in figure 8 for Comp_1; whereas for Comp_2 and _3, the involvement from C, N, F and B atoms have shown in Figures S4 and S5 of supporting information. A careful inspection of figure 7 displays the impact of s-orbitals are leading in lowermost energy states; however, it is almost immaterial in high energy levels. Conversely, the input of p-orbitals is considerably excessive for higher energy states that are perhaps due to the valance electronic behavior of p-orbitals contributions. For Comp_1, sorbitals denote the energy bands in the lower valance bands (VB) from −7 to −3.5 eV, though, no partaking in conduction bands (CB) clarifying the minor effect of s-orbitals in electro-optical properties. The p-orbitals control the TDOS along-with the PDOS in VB and CB. The TDOS displays the domination of p-orbitals adjacent the Fermi level of Comp_1. Similarly, the participation from s-and p-orbitals have been originated for the Comp_2 and Comp _3. The evaluation of electronic outline reveals that these compounds might be a consequent competitor for OPV applications having remarkable electro-optical properties.

Electronic band structure
The electronic band structure for studied compounds has been evaluated at GGA/PBE level of DFT in BZ at the symmetrical point as revealed in figure 9. The zoomed energy spectra of studied compounds can be found in figure 10 for a more precise idea of the bandgap. The TDOS and PDOS have been revealed in figures 7 and 8 for total energy states that expressed a vibrant involvement in the electro-optical properties of a OSMs. The electronic band structures of Comp_1 , Comp_2, and Comp_3 adjacent to Fermi level are shown in figure 9 to

Conclusions
Hence the analysis at the bulk level, we found the following fascinating findings in this study. The static dielectric functions at 0 eV with exceptional values revealed remarkable capability for electron transportation in the crystals of F-BODIPY derivatives. The imaginary dielectric function specifies the transmission of an electron from VB to CB. These outcomes verified that Comp_1, Comp_2, and Comp_3 may be superior for charge transport inside the crystals. The unchanged spectra of conductivity disclosed the decent strength of Comp_1, Comp_2, and Comp_3 that ultimately originating enormous capability for conducting the electrons through the crystals. The calculated refractive indexes are enlightening that Comp_1, Comp_2, and Comp_3 might be worthy of refracting a photon at minor energy. The smaller reflectivity finds that F-BODIPY derivatives might be proficient compounds for OPV application.
Our evaluation demonstrates that F-BODIPY derivatives unveiled the bandgap of scale 2.81, 1.56, and 1.78 eV, respectively. The exploration of electronic configuration establishes that F-BODIPY derivatives having remarkable electro-optical properties might be a successive candidate for OPV application. These features at bulk level disclosed that F-BODIPY and its derivatives might be efficient materials and for multifunctional organic microelectronic applications.