Donor Functionalized Perylene and Different π-Spacers Based Sensitizers for DSSC Applications A Theoretical Approach

Perylene based novel organic sensitizers for the Dye sensitized solar cell applications are investigated by using Density functional theory (DFT) and time dependant density functional theory (TD-DFT).The designed sensitizers have perylene and dimethylamine (DM) and N-N-dimethylaniline(DMA) functionalized perylene for the dssc applications.π-spacers are thiophene andcyanovinyl groups and cyanoacrylic acid is chosen as the acceptor for the designed sensitizers. The studied sensitizers were fully optimized by density functional theory at B3LYP/6-311G basis set on gas phase and DMF phase. The electronic absorption of the sensitizers is analyzed by TD-DFT at B3LYP/6-311G basis set in both gas and DMF phase.

acceptor. Cyano acrylic acid is the e cient electron withdrawing moiety for the dye sensitized solar cell applications.
In the present study, perylene is the donor molecule and cyanoacrylic acid is electron acceptor groups for all the studied sensitizers and the thiophene and cyanovinyl groups are π-spacers in the different con gurations. The six con gurations are studied in different positions of thiophene and cyanovinyl groups. Dimethylamine (DM) and N-N-dimethylaniline (DMA) are adding the perylene donor for each six con gurations. These sensitizers are investigated by studies of frontier molecular orbital analysis, absorption spectra, light harvesting e ciency, molar extinction coe cient, electron injection and regeneration, natural bonding orbital, Nonlinear optical properties.

Theoretical Background
The power conversion e ciency (PCE) of the dye sensitized solar cell were calculated from the following factors: open circuit voltage (V oc ),short circuit current (J sc ), ll factor (FF) and incident light energy (P in ) [26,27], …………………………. 1 In dye sensitized solar cell the V oc is calculated from disparity between Fermi level of the semiconductor to the redox potential to the redox electrolyte. The short circuit current is depends the factor of photon harvesting ability and injection of electron to the semiconductor. The J sc and V oc should be high possess that the PCE of the solar cell will be high. The short circuit current of the solar cell was calculated to the equation, [28,29] ………………………….2 Where LHE is the light harvesting e ciency and ϕ inject is the electron injection e ciency of the sensitizers.
for DSSC's the electrode is same and dye sensitizers only differs. So the η collect of the DSSC is assumed to be constant.

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The ϕ inject were calculated from electron driving force ΔG inject . The ΔG inject of the sensitizers were calculated from the equation, [30,31] ………………………….. 4 Where, is the excited state oxidation potential of the sensitizers and is the ground state reduction potential of the conduction band edge of the TiO 2 . The excited state oxidation potential were calculated from the equation as, ………………………….. 5 Where, E ox is the redox potential of the ground state of the sensitizers and vertical transition energy corresponding to the maximum absorption (λ max ).

Computational Details
The entire work, the designed sensitizers were ground state optimized on both gas and DMF phase by density functional theory method of B3LYP with 6-311G(d,p) [34][35][36] basis set using Gaussian 09 program [37] Gauss view visualization program [38] was used for designed the the molecular structure of the sensitizers and Gausssum 3.0 software [39] were to analysis the electronic excitation properties. The solvent effect and projectionof the absorption properties of the sensitizers DMF phase were analyzed from polarizable continuum model (PCM) [40]. The absorption spectrum vertical excitation energy of the sensitizers were investigated by TD-DFT with CAM-B3LYP/ Perylene based D-π-A structured sensitizers are investigated by DFT/ B3LYP-6-311G basis set. All con gurations of π-spacers and acceptor is reported at pounraj et al. [42] These sensitizes have 6 con gurations of π-spacers positions and mono anchoring group with cyano acrylic acid. Thiophene and cyanovinyl groups are π-spacers in the different con gurationsand con gurations of the sensitizers have different positions of same groups. Thiophene and cyanovinyl group already provide good PCE of the DSSC applications. Especially thiophene based sensitizers have lower energy gap and broad absorption spectrum. In the present work, mainly focused the perylene donor and dimethylamine and N-Ndimethylaniline functionalized perylene combined with above π-spacers and acceptor con guration.
Perylene is the polycyclic aromatic hydrocarbon and their intrinsic π -conjugation and chargew delocalization properties exhibits there is used for optoelectronocapplications Perylene and its derivatives with huge absorption coe cient, high uorescence quantum yield and outstanding photo chemical stability have been widely employed as luminous probes in cytochemistry, dopants in organic light emitting diodes [43]. Perylene structure is shown in Fig. 1.
Perylene have twelve functionalizable positions in which 3,4,9,10 positions are known as peri, the 1,6,7,12 positions are known as bay and the 2,5,8,11 are ortho positions. The π-spacers are connected to the 10th positions of the perylene donor. The π-spacers are transfer electron from donor to acceptor.Moveable πbonds is the key constraint for π-bridge .More number of π-spacers is minimizing the band gap and improves absorption spectrum. The six con gurations of the spacer are substituted in the perylene and donor functionalized perylene. The donor modi ed perylene donor has dimethylamine and N-Ndimethylaniline donor group.In the study dimethylamine andN-N-dimethylaniline are indicates that DM and DMA andperylene donor sensitizer are indicates that PER and donor functionalized perylene sensitizers are indicates DM-PER and DMA-PER. DM and DMA groups are connected to the 2nd, 5th position of each perylene sensitizers.The chemical structure of the sensitizers is illustrated in Fig. 2(a-c) andstructural arrangement is summarized in Table 1. Con gurations 2 and 3 have similar molecular formula and different π-spacer setup and also, con gurations 4, 5 and 6 have similar molecular formula and different π-spacer setup. The geometrical structure of the PER, DM-PER, and DMA-PER sensitizers are in illustrated in Fig. 3. The PER sensitizers have perylene as a donor. The six π-spacers con gurations are connected to the perylenering in the 10th position and due to this connection, the sensitizers are named as PER-1,PER-2, PER-3,PER-4,PER-5 and PER-6.The optimized structure of the PER sensitizers is shown in Fig. 3.

DM-PER sensitizers:
The DM-PER donors have perylene and dimethylamine molecules. The dimethylamine molecule is attached the perylene in the positions of 2nd and 5th. The DM molecule has the planer structure and higher electron donating ability. The planer structures of the molecules were help to effective electron transfer process. Due to the substitution, the sensitizers were named as, DM-PER-1,DM-PER-2, DM-PER-3,DM-PER-4,DM-PER-5 and DM-PER-6. The optimized structure of the PER sensitizers is shown in Fig. 3.

DMA-PER sensitizers:
The DMA-PER donors have the perylene substituted with theN-N-dimethylanilinemolecule. N-Ndimethylaniline (DMA) molecule was dimethyl amino group connected to a phenyl group. The DMA dyes have simpler structures and are strong electron donating ability compared to coumarin(r). Due to the substitution, the sensitizers were named as, DMA-PER-1,DMA-PER-2, DMA-PER-3,DMA-PER-4,DMA-PER-5 and DMA-PER-6. The optimized structure of the PER sensitizers is shown in Fig. 3.

NBO analysis:
Natural Bonding Orbital values are determining the population of charges on the molecules and intramolecular charge transfer to the acceptor from donor through π-spacers [44][45][46][47][48]. NBO values are positive and negative, which is indicating that electron donating ability and accepting ability. q donnor values are positive indicates that, donor molecules areelectron donating groups. A negative value of the q acceptor represents that, electron accepting from π-spacers effectively. Positive q π−spacers values are illustrated by electron donating ability and π-spacers acts as donor molecules. Cyano acrylic acid has more negative values are indicating that strong electron acceptor. Charge variation between natural charges of the donor and acceptor molecules represented as q D−A . The higher values of q D−A are illustrated as stronger charge separation.The charge population values of dye molecules are listed in Table 2. All the molecules have positive q D−A values and the con guration 2 has the maximum values of the alldonor groups. From Table 2, con guration 5 of the all donor groups and DM-PER-2 has the negative value of q π−spacers values indicates that, the π-spacers are acting as an acceptor. Con gurations 2 and 3 have different spacers con guration and same molecular formula but different q D−A values. Similarly con guration 4, 5 and 6 has the same. The above results show that, the con guration 2 of the dimethylamine and N-N-dimethylaniline functionalized donors based sensitizers have the higher q donnor values comparatively other dye sensitizers and more donating ability then others.

Frontier Molecular Orbital analysis
The electron density charge distribution of the sensitizers is the vital parameter of the photo excitation process. The photo excitation is the fundamental process of the mechanism of DSSC. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, energy gap (E g ) of sensitizers crucial factor for the charge separation process. During the charge separation process the energy gap of the HOMO and LUMO energy levels should be minimum and it's provide better e ciency of the solar cell[10]. The HOMO and LUMO energy values and energy gap of the PER, DM-PER and DMA-PER sensitizers in gas phase and DMF phase is summarized in the Table 3 and energy gap of the sensitizers is illustrated in Fig. 4. The HOMO values of all the sensitizers are above the CBE of semiconductor (-4.0 eV) and LUMO values are below the redox electrolyte (-4.8eV) [49,50]. The π-spacer of the sensitizers is extended due to inclusion of thiophene and cyanovinyl group and change the con guration of the π-spacer is decrease the energy gap. The con guration 2 and 3 has the similar molecular formula and differentpositions and con guration 4,5 and 6 also same. PER, DM-PER and DMA-PER sensitizers are optimized in DMF phase in using polarizable continuum model (PCM). The energy gap of the sensitizers is minimum as compared to Gas phase due solvent polarization. The solvent polarization is stabilizes the energy levels soE g has been reduced. The energy gap of the PER sensitizers in gas phase in the decreasing order: PER-1 > PER-2 > PER-3 > PER-4 > PER-5 > PER-6. The PER-6 has the minimumE g value of 1.830eVin the six con gurations. The  Fig. 5 (a-c). From Fig. 5 (a-c), the electron density of HOMO is localized in donor and LUMO is localized in π-spacer and acceptor. HOMO is localized in the entire molecule of con guration-1 of the all donors.
The LUMO of the all sensitizers are localized in the acceptor part this is help to electron injection of the Cyanoacrylic acid effectively. From results shows that, the N-N-dimethylaniline functionalized DMA-PER sensitizers have minimum energy gap compared the other donor groups and Con guration-6 better than others of the all donorsespecially DMA-PER-6 have the lowest Eg value and more suitable for the DSSC applications. Absorption spectra of the sensitizers have the extensive spectrum of absorption in the visible to Near IR. Intramolecular charge transfer process of the sensitizer take place while during photo excitation process. The ICT process is crucial parameter for improve the J sc . The studied sensitizers is calculated by TD-DFT with B3LYP-6-311G basis set in gas phase and DMF phase. The absorption spectra of the sensitizers are illustrated as Fig. 6 and absorption maximum (λ max ), Molar extinction coe cient(ε) of the sensitizers are encapsulated in Tables6,7,8and also oscillator strength (f), transition assignment of the sensitizers are summarized in Table 4. Absorption maximum wavelengths of parent con guration perylene based sensitizers are in the range of 454nm to 541nm. The maximum value of 541nm having con guration-4. The con guration 2 and 3 have similar molecular formula and con guration disparate from the π-spacers the maximum absorption is decreased. The con guration 4, 5 and 6 has also same as above. These All the designed sensitizers have higher value of (ε) in DMF phase compared to gas phase since of due to solvent effect. The con guration 6 has the higherε of the all the sensitizers and more e cient to DSSC applications.  Light harvesting e ciency is anothervitalconstraint for intensity of the absorption spectra of the sensitizers. The LHE values of the sensitizers are calculated by Eq. 3 and tabulated in Table 5. From the Table 5, the LHE values are functionalized perylene sensitizers are higher compared to perylene donor. The con guration 1 has the minimum LHE value of the designed sensitizers. The con guration 2 and 3 has the similar molecular formula and different π-spacer positions, but different LHE values. The results of Con guration 4,5,6 as same as con guration 2 and 3. The con guration 6 has the maximum value of LHE, it seen that it is better candidate for DSSC applications. Dimethylamine and N-N-dimethylaniline functionalized DM-PER-6 and DMA-PER-6 has the higher LHE values which indicates, these are better candidate compare to other sensitizers.       The electron injection and electron regeneration of the perylene based donor and donar functionalized perylene donor based sensitizers were analyzed from DFT and TDDFT functions and calculated using Eq. 4,6 and tabulated at Table 6.From the Table 6,

Conclusion
The designed sensitizers have optimized through DFT with B3LYP/6-311G basis set in both gas and DMF phase. Absorption spectra and molar extinction coe cient of the sensitizer analyzed through TD-DFT with B3LYP/6-311G basis set in both gas and DMF phase. From NBO analysis of the designed dye sensitizers, Con guration 2 of DM and DMA functionalized perylene donors has maximum q donnor values indicated that, more electron donating ability compared to other studied sensitizers. The FMO analysis results shows that, the DMA functionalized perylene donor molecules have minimum energy gap of the sensitizers compared to the other sensitizers. The con guration 6 is better than other con gurations, especially DMA-PER-6 have low energy gap and more suitable for DSSC applications. In absorption spectrum, LHE and molar extinction coe cient results is illustrated DMA functionalizedperylene donor have maximum LHE and ε values compared to the other sensitizers and especially DMA-PER-6 have the higher LHE and ε values for e cient sensitizers to the PCE performance. The electron injection and electron regeneration properties of the sensitizers are investigated that, con guration 4 for DMA functionalizedperylene donor have lower negative ΔG inject value and lower positive ΔG reg values. For NLO properties, the DMA-PER sensitizers especially con guration 6 have the maximum dipolemoment and static polarizability. From the results, the DMA-PER-6 exhibit the active NLO performance compared to the other studied sensitizers. the Frontier molecular orbitals, light harvesting e ciency, molar extinction coe cient and NLO performance results are indicated that, the DMA functionalized perylene donor is the better performer, especially DMA-PER-6 is the better candidate for the DSSC applications.

Declarations
Funding: No funding received

Con icts of interest/Competing interests:
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.
The authors declare the following nancial interests/personal relationships which may be considered as potential competing interests: Availability of data and material: Not Applicable  c. the geometrical structure of the DMA-PER sensitizers.