boosting the photoelectric conversion efficiency of dsscs

5814 | Mater. Chem. Front., 2021, 5, 5814–5825 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2021

Cite this: Mater. Chem. Front.,

2021, 5, 5814

Boosting the photoelectric conversion efficiencyof DSSCs through graphene quantum dots:insights from theoretical study†

Yuanchao Li,a Wenyuan Zhang,a Xin Li *a and Yanling Xu b

The introduction of graphene quantum dots (GQDs) into the photoactive layer of dye-sensitized solar

cells (DSSCs) has achieved encouraging experimental results. However, the best linkage configuration

between the sensitizer and graphene quantum dots to enhance the photoelectric conversion efficiency

of DSSCs is still unclear. Hence, three different covalent bindings (amidation reaction, cycloaddition

reaction and free radical addition) were comprehensively investigated from a theoretical perspective.

The theoretical investigation indicated that covalent bonding of GQD is beneficial to the optical and

electrical properties, especially via the amidation reaction, greatly enhancing the absorption intensity,

which further improves the utilization of light and leads to a better photocurrent response. Covalent

bonding of GQD via the amidation reaction can improve the maximum photon generated current

compared to other covalent behaviors, exhibiting better intermolecular charge transfer character. Based

on the analysis of the intermolecular interaction between the dye and I2, covalent bonding of GQD via

the amidation reaction can decrease the electron recombination rate. Therefore, covalent bonding of

GQD via the amidation reaction is regarded as the most promising approach to enhance the

photoelectric conversion efficiency of DSSCs. In addition, various graphene oxide and doped graphene

QDs were also studied to provide possibilities for improving the efficiency of DSSCs.

1. Introduction

Over the past few decades, fossil fuels have been regarded asthe primary energy source to promote social development.However, with the increasing demands of industries, the tradi-tional fossil resources are being depleted, causing severe envir-onmental issues. Hence, the development of renewable andclean energy technologies is a promising strategy to overcomethis problem.1,2 Solar energy as an almost limitless, abundantand pollution-free resource has received considerable interestworldwide, especially for photovoltaic devices, which directlyconvert solar energy to electricity.3,4 However, the high cost ofmaterials and complex fabrication processes are the mainfactors limiting their large-scale applications. Among variousemerging technologies, dye-sensitized solar cells (DSSCs) areconsidered the most promising photovoltaic equipment due to

their low cost, easy assembly, environment-friendly featuresand relatively high power conversion efficiency.5 A typical DSSCis composed of four components, i.e., sensitizer, TiO2 photo-anode, electrolyte and counter electrode.6,7 The sensitizer is acrucial component in a DSSC, which is responsible for thecapture of photons and injection of electron. The metal sensi-tizers (such as Zn-porphyrins and Ru-complexes) have achievedhigh efficiency,8,9 but their development is still limited by theirscarcity and tedious purification. In contrast, metal-freeorganic dyes are fascinating due to their flexible moleculardesign, abundant raw materials and easily tunable opticalproperties.10,11 Thus, to further improve the photoelectric con-version efficiency (PCE) of DSSCs, continued efforts have beendevoted to searching for novel sensitizers.12–17 Currently, thehighest efficiency achieved by a metal-free organic dye-basedDSSC is 14.3% using a cobalt complex as the electrolyte.18

The D-p-A framework is the most typical structure for metal-free organic dyes in DSSCs, which is beneficial for the chargeseparation and transfer upon photo excitation. However,organic dyes with the D-p-A configuration usually possess asingle and narrow absorption band in the UV-visible region.Accordingly, poor light harvesting ability is the primary reasonwhy the efficiencies of DSSCs based on the D-p-A configurationdyes are still low. Therefore, improving the light absorption

a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion

and Storage, School of Chemistry and Chemical Engineering, State Key Lab of

Urban Water Resource and Environment, Harbin Institute of Technology, Harbin

150090, China. E-mail: [email protected]; Tel: +86-0451-86282153b School of Chemistry and Chemical Engineering, Harbin Institute of Technology,

Harbin 150090, China

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00580d

Received 14th April 2021,Accepted 14th June 2021

DOI: 10.1039/d1qm00580d

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RESEARCH ARTICLE

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ability has become a hot topic in DSSC research. To overcomethe shortcomings of individual sensitizers, the co-sensitizationtechnique, which involves mixing two or more dyes in thephotoactive layer, is a promising method to improve the lightharvesting efficiency.19,20 Over the past few decades, various co-sensitization systems, such as porphyrin and ruthenium com-plexes with organic dyes, i.e., two different organic dyes, havebeen proposed and improved the efficiency of DSSCs comparedwith the devices based on individual sensitizers.21,22 In addi-tion, numerous efforts have also been devoted to developingnew photoactive materials to enhance the photovoltaic perfor-mances of DSSCs.23–27

Graphene, as a popular sp2-hybridized material, hasattracted widespread attention in recent years due to itsremarkable physical and chemical properties, such as highelectron mobility, easy surface modification, excellent flexibil-ity, and extraordinary conductive properties, which has beenwidely researched in the fields of sensors, supercapacitors,photocatalysis, Li-ion batteries and other areas.28–32 Liu et al.reported that covalently functionalized graphene is beneficialfor improving the optical absorption property, which mayprovide a new development in optoelectronic devices. There-fore, researchers have synthesized various nanocomposites toenhance the light absorption property of DSSCs via differentchemical modification approaches.33 Kiessling et al. demon-strated the applicability of nanographene/porphyrin hybrids inDSSCs.34 Guarracino et al. synthesized a graphene-dye hybridmaterial as a stable photosensitizer in DSSCs via free radicaladdition. They observed fast quenching of the dye excited statein the isolated hybrid and efficient electron transfer to the N-doped TiO2 nanopowder.35 Mandal et al. systematically studiedthe feasibility of a porphyrin-sensitized graphene quantum dotsolar cell through an amide linkage. They found that the hybridnanocomposite of GQD and porphyrin may be a good candidatefor application in solar cells.36 Gao et al. performed a theore-tical investigation on the photoelectric properties of atetraphenylporphyrin-GQD nanohybrid material as a sensitizer.Their results indicated that GQD-3TPPs are potential candi-dates for application in solar cells due to their enhanced lightharvesting capability, obvious charge spatial separation and

low recombination rate of electrons and holes.37 Thus, it isclear that the covalent bonding of GQD via the chemicalmodification approach is a promising strategy to improve thephotoelectric performance of DSSCs.

The common covalent method can be classified into threecategories (see Fig. 1): (i) amidation reaction (AR), (ii) cycload-dition reaction (CR) and (iii) free radical addition (FRA). How-ever, to date, which covalent behavior is the best way to improvethe efficiency of DSSCs has not been reported in the literature.Hence, the purpose of the current work was to systematicallyinvestigate the influence of different linkage types between thesensitizer and GQD on the photoelectric performance of DSSCs,and then try to find a promising approach to enhance thephotoelectric conversion efficiency.

2. Models and methods

In this research, the organic molecule DM with a D-p-A frame-work was employed as the dye model, according to Dhar’swork.38 The model of cir-coronene C54H18 was selected as thepristine GQD (Fig. 2), which has been widely reported bytheoretical studies.39–41 In addition, graphene oxide and dopedgraphene QDs are popular and have been extensively investi-gated in recent years.42,43 For the graphene oxide quantum dot(GOQD), three reference models were used (Fig. 2). In the firstmodel, an additional O radical appeared on the basal plane(GO1) through a double C–O–C linkage. In the second and thirdmodels, an additional OH radical was anchored on the basalplane and the edges of the thin film sheet (GO2 and GO3,respectively). For the doped graphene quantum dot, type Iwas dual-doped with the same heteroatoms (e.g., N/N-doped,

Fig. 1 Common methods for the covalent bonding of graphene quantumdots with sensitizer.

Fig. 2 Theoretical models of pristine and functionalized graphene quan-tum dots.

Research Article Materials Chemistry Frontiers


B/B-doped and P/P-doped, which were labeled as NNG, BBGand PPG, respectively). Type II was co-doped with differentheteroatoms (e.g., N/B-doped, B/P-doped and N/P-doped, whichwere designated as NBG, BPG and NPG, respectively).

The ground-state geometries of the nanocomposites andisolated dye in ethanol solution were fully optimized usingthe density functional theory (DFT) method at the B3LYP/6-31G(d) level employing the conductor-like polarized conti-nuum model (C-PCM).44 Frontier molecular orbitals and energylevels were obtained based on the optimized configurations.This level of theory has been widely used to calculate thegeometrical properties of organic molecules in related theore-tical research and provides reliable results comparable toexperimental data.45–47 Moreover, the anionic and cationicenergies were calculated at the aforementioned level of theoryto estimate the ionization potentials (IP), electron affinities(EA), chemical reactivity parameters, and hole (lh) and electron(le) reorganization energies. The optical absorption propertiesfor the lowest 20 singlet–singlet excitations of the nanocompo-sites were calculated via the time-dependent density functionaltheory (TD-DFT) method. To ensure the reliability of theTD-DFT calculation, several exchange correlation functionalscontaining B3LYP, CAM-B3LYP, MPW1PW91, B3PW91 andM062X with the 6-31G(d) basis set were selected to simulatethe absorption spectra of DM and the results are presented inTable S1 (ESI†). The maximum absorption wavelength andabsorption shape showed that the CAM-B3LYP functional isin good agreement with the experimental value with a deviationof 6 nm. Therefore, the optical properties of nanocompositeswere calculated using the CAM-B3LYP/6-31G(d) level, which isconsistent with previous research.48–50 In addition, the analysisof the intramolecular charge transfer parameters and electrondensity difference maps (EDDs) was performed using theMultiwfn software.51 All calculations were performed usingthe Gaussian 09 package.52

3. Results and discussionGraphene quantum dots

The nanocomposites of GQD with DM were constructed via theamidation reaction, cycloaddition reaction and free radicaladdition, which were termed G-AR, G-CR and G-FRA, respec-tively. The optimized ground state structures of the nanocom-posites and DM are illustrated in Fig. S1 (ESI†). It is well knownthat the degree of geometric distortion plays an important rolein the charge separation and the coplanar configuration facil-itates electron transfer. The critical positions of the dye mole-cule are marked in Scheme 1 and the corresponding values arelisted in Table S2 (ESI†). It is clear that the covalent GQD hasnegligible effects on the bond lengths and dihedral angles,which will not change the molecular configuration. The F5

values of all the nanocomposites are close to �1801, which isbeneficial for electron transfer from the acceptor to the con-duction band (CB) of TiO2. According to the working principleof DSSCs, the dye should possess an appropriate energy level to

ensure dye regeneration and electron injection. As shown inFig. 3(a), the LUMO levels of all the nanocomposites lie abovethe conduction band edge of TiO2,53 which guarantee sufficientdriving force for electron injection from the excited state dyeinto TiO2. In contrast, the HOMO levels are more negative thanthe redox potential of I�/I3

�,54 indicating that the excited dyescan be regenerated effectively by getting an electron from theelectrolyte. According to Fig. 3(a), covalent bonding of GQD viadifferent covalent methods has negligible effects on the LUMOenergy. It is worth noting that the HOMO energy can beregulated by the covalent bonding of GQD, which is up-shifted by 0.23 eV (G-AR), 0.51 eV (G-CR) and 0.48 eV (G-FRA)compared to the isolated dye (DM) (see Table S3, ESI†). It isevident that the HOMO�2 and LUMO+2 energy levels are closerto that of the HOMO and LUMO after covalent bonding of GQD,which may favor an easier electron transition from HOMO�2/HOMO to LUMO/LUMO+2, respectively. Generally, a smallerenergy gap means that less energy is needed for electronexcitation from the ground state to excited state, therebyharvesting more sunlight. The energy gaps follow the order ofDM (2.37 eV) 4 G-AR (2.13 eV) 4 G-FRA (1.89 eV) 4 G-CR(1.86 eV), indicating that the covalent bonding of GQD isbeneficial to promote electron excitation, and subsequentlyimprove the light absorption property.

In general, significant intramolecular charge transfer (ICT)can induce effective electron–hole separation, which is bene-ficial for interfacial electron injection, achieving a high photo-current conversion efficiency. The electronic density difference(Dr) was used to investigate the ICT characteristic, as shown in

Scheme 1 Critical positions of the dye molecule (F1 � F5 represent thedihedral angle and d1 � d5 represent the bond length).

Fig. 3 Energy levels and band gaps of DM and nanocomposites.

Materials Chemistry Frontiers Research Article


Fig. 4, where the green and pink regions denote the electrondensity depletion and increment, respectively. A decrease inelectron density occurs in the electron-rich regions, whereas anincrease in electron density occurs in the electron-poor regions.With the aim of visualizing the ICT process more insightfully,the centroids of charge associated with the positive and nega-tive electron density zones were calculated. Apparently, all thenanocomposites show ideal charge transfer character fromleft-to-right, which consistent with the discussion above. TheICT properties can be quantified using the index of the spatialextent method. The spatial indices such as the charge transferdistance (DCT), H and t are presented in Fig. 4. DCT representsthe distance difference between the barycenter density distri-bution of the positive and negative parts upon photo-excitation.H represents half of the sum of the centroid axis along theelectron transfer direction. The t index is related to t = DCT � H,which estimates the through-space character with CT excita-tion. It is clear that the covalent bonding of GQD is beneficial toincrease the transfer distance. G-AR exhibits the largest DCT

among the nanocomposites, indicating that the amidationreaction between GQD and the sensitizer can enhance theICT properties. The H values follow the order of G-FRA 4G-CR 4 DM 4 G-AR. This suggests that the covalent bondingof GQD via the cycloaddition reaction and free radical additionwill increase the distance of the centroid axis along the chargetransfer direction. A larger t indicates smaller orbital overlapbetween the electron-donating and electron-accepting regions.The estimated values of t imply that the covalent bonding ofGQD via the amidation reaction can effectively facilitate chargeseparation in the ICT process, leading to favorable interfacialelectron injection. In summary, the covalent bonding of GQDvia the amidation reaction can enhance the ICT properties.

Generally, electron and hole transport ability have an impor-tant impact on the performance of DSSC devices. The electron(le) and hole (lh) reorganization energy can be approximatelydescribed as follows:

le = (E0� � E�) + (E�

0 � E0) (1)

lh = (E0+ � E+) + (E+

0 � E0) (2)

where E�0 is the energy of the anion (cation) based on theoptimized geometries of the neutral molecule, E� is the energyof the anion (cation) calculated based on the optimized geo-metries of the anion (cation), E0

� is the energy of the neutralmolecule calculated at the optimized cationic (anionic) stateand E0 is the energy of the neutral molecule in its ground state.It can be seen from Fig. 5 that the lh of the nanocomposites aresmaller than le, revealing that these nanocomposites have agood hole transport performance. DM and the nanocompositesexhibit similar le. However, there is a significant drop in the lh

of the nanocomposites, especially for G-AR. This indicates thatthe covalent bonding of GQD can enhance the hole transferefficiency. Moreover, the electron affinity (EA) and ionizationpotential (IP) were calculated to describe the energy barrier forhole and electron injection.55 As shown in Fig. 5, covalentbonding of GQD has no effect on the EA value. G-CR exhibitsthe lowest IP value, followed by G-FRA, G-AR and DM, respec-tively, meaning that these nanocomposites would show thebetter hole injection capacity.

Nonlinear optical (NLO) properties, such as isotropic polar-izability (a) and total hyperpolarizability (btot), are important forunderstanding the behavior of organic photovoltaic devices,which are determined via the delocalization of intramolecularcharge. Hence, the NLO parameters can give the tendency ofthe electron charge transfer efficiency. Patil et al. found that the

Fig. 4 Electron density difference, centroids of charges and CT parameters of DM and nanocomposites.

Fig. 5 Calculated ionization potential (IP), electron affinity (EA), andreorganization energies (le and lh) of DM and nanocomposites.



NLO properties of sensitizers are directly related to the effi-ciency of DSSCs.56 Thus, systematically investigating the NLOproperties is beneficial to understand the relationship betweenthe electronic structure and photovoltaic performance. The aand btot of DM and the nanocomposites were calculated, whichcan be expressed as:

a ¼ 1

3ðaxx þ ayy þ azzÞ (3)

btot

¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðbxxxþbxyyþbxzzÞ2þðbyyyþbxxyþbyzzÞ2þðbzzzþbxxzþbyyzÞ2

q(4)

As shown in Table 1, the highest a was observed for G-AR,followed by G-CR, G-FRA and DM, respectively. Similar trendscan be observed for btot in Table 2, implying that the covalentbonding of GQD is beneficial to enhance the NLO response andelectron charge transfer. Therefore, these nanocomposites willexhibit a higher PCE compared to MD. Among the individualcomponents of btot, bxxx exhibits the maximum values for DMand the nanocomposites, which suggests that the electrons willundergo a unidirectional transfer process.

DSSCs as light-to-electricity devices require a large photo-current response in the visible-near-IR range. Hence, theabsorption spectrum of the dye should overlap with the solarspectrum as much as possible to enhance the harvesting ofsunlight. The simulated absorption spectra are depicted inFig. 6 and the main characteristics upon photo-excitation aresummarized in Table 3. As seen in Fig. 6(a), the nanocompo-sites exhibit an intense and broad spectral response in thevisible region compared to DM, which is beneficial to enhancethe efficiency of DSSCs. It is worth noting that the molarabsorption coefficient (e) increases greatly around the regionof 300–450 nm after the covalent bonding of GQD, indicatingthat the covalent bonding of GQD can effectively facilitate theharvesting of solar radiation at a lower wavelength of sunlightand further improve the utilization of light. Especially G-ARexhibits more intense and broad absorption, implying that theamidation reaction is the best way to enhance the light

absorption property. In addition, the oscillator strength is animportant parameter related to the light harvesting efficiency.According to Table 3, it can be seen that the oscillator strengthincreases after the covalent bonding of GQD. Based on theresults of the absorption spectra, the important frontier mole-cular orbitals are plotted in Fig. 7, which may be involved in theelectronic excitation process. The LUMOs for DM and thenanocomposites are localized in the p-spacer and acceptormoiety. The HOMOs for DM and HOMO�2 for the nanocom-posites show similar FMO spatial distributions, which aremainly distributed on the donor and 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c] pyrrole-1,4-dione units. This distribu-tive character can generate effective spatial separation forintra-molecular charge transfer upon light excitation. Thefluorescence properties of DM and the nanocomposites werecalculated and listed in Table S4 (ESI†), and the simulatedfluorescence spectra are shown in Fig. S2 (ESI†). The fluores-cence peaks of DM, G-AR, G-CR and G-FRA are 598, 500, 597

Table 1 Isotropic polarizabilities of the dye DM and nanocomposites(1 � 10�24 esu)

axx axy ayy axz ayz azz a

DM 237.53 4.95 95.91 �1.33 �3.00 71.76 135.07G-AR 471.78 18.27 324.97 �48.32 72.6 201.59 332.78G-CR 470.44 �29.33 344.64 �82.38 �29.58 152.17 322.42G-FRA 339.24 29.27 317.74 88.95 �12.82 280.53 312.50

Fig. 6 Simulated absorption spectra of DM and nanocomposites.

Table 2 Total hyperpolarizability of dye DM and nanocomposites (1 � 10�30 esu)

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btot

DM �735.79 �30.92 �5.98 �3.34 �4.90 1.52 0.81 2.29 �0.94 2.65 740.32G-AR �702.98 210.19 �64.07 9.54 �162.78 43.52 �6.37 �44.22 12.05 1.46 860.23G-CR 735.25 �115.98 10.21 �8.61 �79.90 7.15 �9.54 3.32 �0.44 �8.77 765.47G-FRA �731.66 �76.83 �3.40 12.28 �12.28 �0.28 �7.63 �3.18 0.28 �3.14 741.39

Table 3 The absorption characteristics of DM and nanocomposites

Dye E (eV) lmax (nm) f Main configuration (%)

DM 2.69 461 1.3482 H - L/61.74G-AR 2.68 463 1.4994 H�2 - L/50.33

3.27 379 0.9103 H�1 - L+2/30.123.28 378 2.2320 H�8 - L/36.623.29 377 2.0189 H - L+2/43.89

G-CR 2.70 459 1.4343 H�2 - L/56.793.16 393 0.8026 H - L+3/36.40

G-FRA 2.69 460 1.4245 H�2 - L/60.51



and 587 nm, respectively, which are significantly red-shiftedcompared to their absorption peak. It is worth noting that G-AR, G-CR and G-FRA exhibit a lower fluorescence intensitycompared to DM, especially for G-AR.

To enhance the photoelectric conversion efficiency ofDSSCs, the dye should absorb energy from sunlight as muchas possible. ZLHE(l) represents the capability to harvest light ata given wavelength, which can be calculated as:57,58

ZLHE(l) = 1 � 10�Gs(l) (5)

where G is the surface loading of dye (mol cm�2) adsorbed onthe semiconductor, which is calculated by the molecular sizeand s(l) is the molecular absorption cross-section (cm2 mol�1),which can be obtained from the molar absorption coefficiente(l) by multiplication with 1000.59,60 However, the photonnumber will change at different wavelengths, and thus forgreater precision, the average light harvesting efficiency,ZLHEðlÞ, was calculated, which can be expressed as follows

ZLHEðlÞ ¼Ð lbla1� 10�GsðlÞ

lb � ladl (6)

As shown in Fig. 8(a), G-AR exhibits the largest ZLHE(l) due toits highest molar absorption coefficient, followed by G-CR, G-FAand DM, respectively. For ZLHEðlÞ, the nanocomposites exhibitlarger values than that of DM. This result reveals that thecovalent bonding of GQD can enhance the utilization of light,further injecting more electrons into the semiconductor.Hence, the nanocomposites will exhibit a superior photocur-rent response.

Electron injection (Finj) and collection (Zcoll) efficiency areimportant characteristics determining the performance ofDSSC devices, which can be defined using the following

expressions:

Finj ¼ 1= 1þ tinjtrelax

� �(7)

Zcoll ¼ 1= 1þ ttranstrec

� �(8)

where trelax represents the relaxation lifetime for the excitedstate of the dye in solution, which is assumed to be 10 psaccording to experimental measurements.61 ttrans is the trans-port time for electrons in the TiO2 film toward the electrode. tinj

and trec denote the electron injection lifetime from the exciteddye toward TiO2 and the electron recombination lifetime,respectively, which are equal to the inverse of electron injectionrate (kinj) and electron recombination rate (krec) obtained fromthe Marcus theory.62,63

kinj=rec ¼ A

ffiffiffiffiffiffiffiffiffiffiffiffiffiffip

�hlkBT

rexpð�brÞ exp �ðDG

0 þ lÞ24lkBT

� �(9)

where A is a constant (0.025), h� is the reduced Planck constant,kB is the Boltzmann constant, l is the reorganization energy, Tdenotes the temperature, b is the attenuation factor (0.5 Å�1),and DG0 is the energy difference between the donor moiety ofthe dye and TiO2. As shown in Table 4, it seems that Finj andZcoll of the nanocomposites exhibit similar values to that of DM,i.e., close to 1. In addition, the lower krec of G-AR compared withDM indicates that the covalent bonding of GQD via the amida-tion reaction can reduce the electron loss.

JSC is an important parameter affecting the overall efficiencyof DSSCs, which is closely related to the integral photon fluxdensity, F(l), in the whole absorption region. The exact part ofF(l) generated by dyes per second per unit area at the given

Fig. 7 Selected frontier molecular orbitals (FMOs) of DM andnanocomposites.

Fig. 8 (a) Calculated ZLHE(lstrong) and ZLHEðlÞ for all dyes and (b) gener-ated Jph (mA cm�2) of DM and nanocomposites.



wavelength (l) can be obtained as follows:

FðlÞ ¼ ðR=DÞ2ZLHEðlÞ2pc

l4ðhc=eklT � 1Þ

¼ 40682:5� 1� 10�GsðlÞ

l4ðhc=eklT � 1Þ

(10)

For more details, please refer to these publications.57,64 Themaximal photon generated current ( Jph) can be described bythe following equation:

Jph ¼ q

ð40682:5� 1� 10�GsðlÞ

l4ðhc=eklT � 1Þ � dl (11)

As shown in Fig. 8(b), the values of Jph follow the order of G-AR 4 G-CR 4 G-FRA 4 DM. It is clear that the covalentbonding of GQD has a positive influence on the performanceof the DSSC due to the larger Jph. Especially for G-AR, whichexhibits the largest value among the nanocomposites. The flowof electric charge across a surface per second per unit area inthe whole spectrum equals the short circuit density ( Jmax

sc ).Therefore, we can infer that the covalent bonding of GQD viathe amidation reaction is the best way to enhance the photo-electric conversion efficiency of DSSCs.

After photoinduced excitation, electron injection phenom-enon will occur from the excited dye to the semiconductor. Thisprocess is associated with the electron injection rate, which canbe quantified by the driving force of electron injection (DGinj):

DGinj = Edye* � ECB (12)

where Edye* is the excited state oxidation potential of the dye,which can be computed by subtracting the vertical excitationenergy (E0) from the redox potential of the dye in the groundstate (Edye), and ECB is the reduction potential of the conductionband energy of the semiconductor. A negative DGinj valuemeans that a dye has sufficient driving force to ensure thesmooth injection of electrons into TiO2. As shown in Fig. 9, thenanocomposites have a more negative DGinj compared to DM,implying that the covalent bonding of GQD will increase thedriving force. After electron injection process, the oxidized dyeneeds to get electrons from the electrolyte to regenerate. Thisinterfacial process is important for the continuous operation ofDSSCs. To achieve efficient regeneration, sufficient drivingforce of regeneration (DGreg) is required, which can beexpressed as: DGreg = Edye � Eredox, where Eredox represents theredox potential of the electrolyte. The DGreg of nanocompositesis in the range of 0.25–0.53 eV, which is smaller than that of DM(0.76 eV), indicating that the covalent bonding of GQD can

enhance the regeneration efficiency. Therefore, the covalentbonding of GQD is a promising way to improve the photovoltaicperformance of DSSC.

The molecular electrostatic potential (MEP) is usually usedto predict nucleophilic and electrophilic attack in a chemicalreaction. The values of the electrostatic potential on the surfaceincrease in the order of red o orange o yellow o green oblue. The color code of the MEP maps ranges from �0.05 a.u.(deepest red) to 0.05 a.u. (deepest blue). The red and blue zonesdescribe the reactive region for electrophilic and nucleophilicattack, respectively. As shown in Fig. 10(a), the negative chargesare associated with the nitrogen atom of the cyano group andoxygen atom of the 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyr-role-1,4-dione group. Previous studies have proposed that thereelectrostatic attraction between the negatively charged atomsand iodine electrolyte.65,66 Therefore, those interaction sitescan be used to analyze the charge recombination between thedye and electrolyte. As is known, the collected electrons in theTiO2 conduction band can recombine with the electrolyte,

Table 4 Estimated electrochemical parameters for DM andnanocomposites

kinj/s�1 krec/s�1 Finj Z

coll

DM 4.99 � 1012 1.85 � 109 0.980 0.996G-AR 3.87 � 1012 1.67 � 109 0.975 0.997G-CR 8.43 � 1012 3.82 � 109 0.988 0.992G-FRA 7.84 � 1012 3.38 � 109 0.987 0.993

Fig. 9 Driving force for electron injection (DGinj) and regeneration (DGreg)in DM and nanocomposites.

Fig. 10 (a) MEP maps and (b) binding energies of the dye–I2 complex (unitin kcal mol�1).



which is the main cause of electronic losses. This process has anegative effect on DSSCs. The high concentration of electrolytearound the TiO2 surface will reduce the electron lifetime in thesemiconductor and accelerate the charge recombination. Thebinding site of I2 should be far away from the TiO2 surface tominimize the electron recombination rate. According to theanalysis of MEP, higher charge recombination will occuraround the nitrogen atom of the cyano group. Hence, thisbinding site was considered to analyze the reorganizationprocess. As shown in Fig. 10(b), they exhibit similar bindingenergies. It is remarkable that the oxygen atom of the amidelinkage between DM and GQD show a negative charge. Thisposition will attract some electrolyte, which can decrease theiodine concentration around the acceptor part. Therefore, thecovalent bonding of GQD via the amidation reaction cansuppress charge recombination, which possibly increases theopen circuit photovoltage.

Doped graphene quantum dots

Doping is promising approach to regulate the photoelectricperformance of the object materials. Thus, to study how dopedGQD affects the electron transport mechanisms, six dopingmodels are considered here, and the front and side view ofpristine and doped GQO are illustrated in Fig. S3 (ESI†). Whenthe C atom was replaced by a B and N atom, the average bondlength of B–C and N–C is 1.50 and 1.41 Å, respectively, which issimilar to C–C bond length of 1.42 Å. P atom doping results inan increase in the bond length of P–C to approximately 1.75 Åand deformation of the six-membered ring near the doping site.It can be seen from the side view that the P atom protrudesfrom the graphene plane to release this stress. Based on theanalysis in the previous section, the amidation reaction may bethe best method to enhance the photoelectric conversionefficiency of DSSCs. Therefore, all the nanocomposites wereconstructed via the amidation reaction. For the type I nano-composites, it is obvious from Fig. 3(b) that the dual-doping ofB atoms can significantly cause the LUMO energy to decrease,while the dual-doping of N or P atoms can increase the HOMOenergy. Among the type II nanocomposites, NPG-AR exhibitsthe highest HOMO energy. Therefore, it can be deduced that Nor/and P-doped GQO can lead to an increase in the HOMOenergy. It is obvious from Fig. 6(b) that the molar absorptioncoefficient (e) decreases after covalent bonding of doped GQDs.It is worth noting that type II nanocomposites exhibit a largermolar absorption coefficient (e) than that of type I nanocom-posites, indicating that the co-doping of two different atoms ismore beneficial to improving the light-absorption properties.Compared to G-AR, the curves of BBG-AR, NNG-AR and NBG-ARare broadened in the range of 450–650 nm, implying that N- or/and B-doped GQD can enhance the light harvesting abilities ata higher wavelength of sunlight. As shown in Table S5 (ESI†),BBG-AR, PPG-AR and NPG-AR exhibit a smaller oscillatorstrength than that of G-AR. The HOMO�12 for BBG-AR ismainly located on the B/B-doped GQD with a little contributionfrom the donor part, whereas the LUMO is mainly distributedon B/B-doped GQD (see Fig. S4, ESI†). This distributive

character cannot generate an efficient charge-separated state.Compared with G-AR, NBG-AR and BPG-AR exhibit similarelectron distribution shapes. In contrast, for PPG-AR andNPG-AR, the HOMO-3 is primarily localized on the dopedGQD, and the LUMO is mainly localized on the p-spacer andacceptor moiety. They exhibit better charge-separated statesthan that of G-AR. Among the nanocomposites with dopedGQDs, NBG-AR and BPG-AR exhibit more negative DGinj andsmaller DGreg than that of G-AR. As shown in Fig. 8(a), thenanocomposites with doped GQD exhibit higher ZLHEðlÞthan that of pure GQD due to their broad spectral response.However, the molar absorption coefficient decreases aftercovalent bonding of doped GQDs, which leads to a decreasein ZLHE(lstrong). It can be seen from Fig. S5 (ESI†) that NBG-ARand BPG-AR exhibit a lower binding energy than that of G-AR,which means that the covalent bonding of N/B and B/P co-doped GQD can reduce the electron recombination rate. Inaddition, the B and P atoms of doped GQD show a negativecharge (see Fig. S6, ESI†), which can attract some electrolyte faraway from the acceptor part. Therefore, B or P atom-doped GQDis beneficial to suppress charge recombination, thus improvingthe efficiency of DSSCs.

Graphene oxide quantum dots

GO is decorated by oxygenated functional groups (O or OH) onsp2 and sp3-hybridized carbon. Furthermore, hybridized GOmaterials show better nonlinear optical properties.67 Herein,the nanocomposites composed of GOQD with DM via amida-tion reaction are labeled as GO1-AR, GO2-AR and GO3-AR. Asshown in Fig. 3(c), there is no change in the LUMO energy ofthe nanocomposites. The HOMO energy of GO1-AR, GO2-ARand GO3-AR are slightly up-shifted compared to that of G-AR.The energy gaps follow the order of G-AR 4 GO3-AR 4 GO1-AR 4GO2-AR, implying that the covalent bonding of doped GOQDwill reduce the energy gap, which is beneficial for the excitationprocess. It can be seen from Fig. 6(c) that GO3-AR and G-ARhave similar absorption curve shapes. This indicates that thepresence of additional OH radicals on the edges of the thin filmsheet has no effect on the spectrum. For GO1-AR and GO2-AR,they exhibit smaller molar absorption coefficients than that ofG-AR. It can be noted that the presence of additional OH and Oradicals on the basal plane can further facilitate the absorbanceof the unused lower wavelength of sunlight than that of GQD.The oscillator strengths corresponding to the lowest-lyingexcitation energy are about 1.4838, 1.4850 and 1.4949 forGO1-AR, GO2-AR and GO3-AR, respectively, exhibiting adecrease compared to that of G-AR. G-AR and GO3-AR exhibitsimilar FMO spatial distributions (see Fig. S4, ESI†), the elec-tron densities of HOMO�2 are mainly distributed on the donorand 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dioneunits, and the LUMO is localized on the p-spacer and acceptormoiety. It can be found that the presence of additional OHradicals on the edges of the thin film sheet has no effect on theFMO. For GO1-AR and GO2-AR, the electron densities ofHOMO�2 are mainly distributed on GO and LUMO is localizedon the p-spacer and acceptor moiety, implying that the



presence of oxygen-containing functional groups on the basalplane is beneficial to facilitate spatial separation. It can be seenfrom DGinj and DGreg that the covalent bonding of GOQD isbeneficial for electron injection and dye regeneration, whichwill enhance the efficiency of DSSCs. In addition, GO1-AR andGO2-AR exhibit larger ZLHEðlÞ than that of G-AR, implying thatthe presence of additional O and OH radicals on the basalplane can improve the utilization of light. According to Fig. S6(ESI†), the presence of O and OH radical on the basal planeexhibit a negative charge, which can decrease charge recombi-nation. GO3-AR has a lower binding energy than that of G-AR.Hence, covalent bonding of GOQD is beneficial to enhance theefficiency due to the lower electron recombination rate.

4. Conclusion

In this work, the mechanism of three different covalent beha-viors between the sensitizer and graphene quantum dots wascomprehensively investigated via DFT and TD-DFT methods todetermine which one is the best approach to enhance thephotoelectric conversion efficiency of DSSCs. The theoreticalinvestigation showed that the nanocomposites exhibit betteroptical and electrical properties due to their lower energy gap,ionization potential, hole reorganization energy and sufficientdriving force for regeneration and electron injection, indicatingthat the covalent bonding of GQD is beneficial to enhance thephotoelectric conversion efficiency. Covalent bonding of GQDvia the amidation reaction greatly enhances the absorptionintensity, especially for a lower wavelength of sunlight, whichfurther improves the utilization of light and leads to a betterphotocurrent response. According to the analysis of the chargetransfer parameters and frontier molecular orbitals, the cova-lent bonding of GQD via the amidation reaction is beneficial topromote intermolecular charge transfer. Thus, using GQD todecorate the dye DM via the amidation reaction can improvemaximum photon generated current compared to other cova-lent behaviors. The covalent bonding of GQD via the amidationreaction can decrease the iodine concentration around theacceptor part, which will decrease the electron recombinationrate. Hence, it can be concluded that the covalent bonding ofGQD via the amidation reaction is the best way to enhance thephotoelectric conversion efficiency of DSSCs. In addition, thecovalent bonding of graphene oxide and doped graphene QDcan effectively suppress charge recombination.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support of this research fromthe National Natural Science Foundation of China (51779065),and the State Key Laboratory of Urban Water Resource andEnvironment, Harbin Institute of Technology (2019DX11).

Notes and references

1 J. Potocnik, Renewable Energy Sources and the Realities ofSetting an Energy Agenda, Science, 2007, 315, 810.

2 R. F. Service and Is It, Time to Shoot for the Sun?, Science,2005, 309, 548.

3 Z. Ning, Y. Fu and H. Tian, Improvement of dye-sensitizedsolar cells: what we know and what we need to know, EnergyEnviron. Sci., 2010, 3, 1170–1181.

4 M. Kar, R. Sarkar, S. Pal and P. Sarkar, Pathways forImproving the Photovoltaic Efficiency of Porphyrin andPhosphorene Antidot Lattice Nanocomposites: An Insightfrom a Theoretical Study, J. Phys. Chem. C, 2019, 123,5303–5311.

5 B. O’Regan and M. Gratzel, A low-cost, high-efficiency solarcell based on dye-sensitized colloidal TiO2 films, Nature,1991, 353, 737–740.

6 D. P. Hagberg, J.-H. Yum, H. Lee, F. De Angelis,T. Marinado, K. M. Karlsson, R. Humphry-Baker, L. Sun,A. Hagfeldt, M. Gratzel and M. K. Nazeeruddin, MolecularEngineering of Organic Sensitizers for Dye-Sensitized SolarCell Applications, J. Am. Chem. Soc., 2008, 130, 6259–6266.

7 T. W. Hamann, R. A. Jensen, A. B. F. Martinson, H. VanRyswyk and J. T. Hupp, Advancing beyond current genera-tion dye-sensitized solar cells, Energy Environ. Sci., 2008, 1,66–78.

8 S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E.Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger,M. K. Nazeeruddin and M. Gratzel, Dye-sensitized solar cellswith 13% efficiency achieved through the molecular engineer-ing of porphyrin sensitizers, Nat. Chem., 2014, 6, 242–247.

9 C. Dong, W. Xiang, F. Huang, D. Fu, W. Huang, U. Bach,Y.-B. Cheng, X. Li and L. Spiccia, Titania nanobundlenetworks as dye-sensitized solar cell photoanodes, Nano-scale, 2014, 6, 3704–3711.

10 J. Gong, K. Sumathy, Q. Qiao and Z. Zhou, Review on dye-sensitized solar cells (DSSCs): Advanced techniques andresearch trends, Renewable Sustainable Energy Rev., 2017,68, 234–246.

11 R. Kesavan, I. M. Abdellah, S. P. Singh, A. El-Shafei andA. V. Adhikari, Simple diphenylamine based D–p–A typesensitizers/co-sensitizers for DSSCs: a comprehensive studyon the impact of anchoring groups, Phys. Chem. Chem.Phys., 2019, 21, 10603–10613.

12 P. Ren, C. Sun, Y. Shi, P. Song, Y. Yang and Y. Li, Globalperformance evaluation of solar cells using two models:from charge-transfer and recombination mechanisms tophotoelectric properties, J. Mater. Chem. C, 2019, 7,1934–1947.

13 Y. Li, J. Liu, D. Liu, X. Li and Y. Xu, D-A-p-A based organicdyes for efficient DSSCs: A theoretical study on the role of p-spacer, Comput. Mater. Sci., 2019, 161, 163–176.

14 W. Zhang, L. Wang, L. Mao, J. Jiang, H. Ren, P. Heng,H. Ågren and J. Zhang, Computational Protocol for PrecisePrediction of Dye-Sensitized Solar Cell Performance, J. Phys.Chem. C, 2020, 124, 3980–3987.



15 N. Wazzan and A. Irfan, Theoretical study oftriphenylamine-based organic dyes with mono-, di-, andtri-anchoring groups for dye-sensitized solar cells, Org.Electron., 2018, 63, 328–342.

16 Y. Gao, W. Guan, L.-K. Yan, R. Jia and Z.-M. Su, Theoreticalinvestigation of the influence of different electric fielddirections and strengths on a POM-based dye for dye-sensitized solar cells, Mater. Chem. Front., 2021, 5, 929–936.

17 A. R. Tapa, W. Xiang, A. S. A. Ahmed, S. Wu, B. Li, Q. Liu,C. M. J. Chuti and X. Zhao, Porous rGO/ZnSe/CoSe2 dis-persed in PEDOT:PSS as an efficient counter electrode fordye-sensitized solar cells, Mater. Chem. Front., 2021, 5,2702–2714.

18 K. Kakiage, Y. Aoyama, T. Yano and K. Oya, J.-i. Fujisawaand M. Hanaya, Highly-efficient dye-sensitized solar cellswith collaborative sensitization by silyl-anchor and carboxy-anchor dyes, Chem. Commun., 2015, 51, 15894–15897.

19 M. R. Elmorsy, L. Lyu, R. Su, E. Abdel-Latif, S. A. Badawy,A. El-Shafei and A. A. Fadda, Co-sensitization of the HD-2complex with low-cost cyanoacetanilides for highly efficientDSSCs, Photochem. Photobiol. Sci., 2020, 19, 281–288.

20 M. Younas, M. A. Gondal, U. Mehmood, K. Harrabi,Z. H. Yamani and F. A. Al-Sulaiman, Performance enhance-ment of dye-sensitized solar cells via cosensitization ofruthenizer Z907 and organic sensitizer SQ2, Int. J. EnergyRes., 2018, 42, 3957–3965.

21 M. Younas and K. Harrabi, Performance enhancement ofdye-sensitized solar cells via co-sensitization ofruthenium(II) based N749 dye and organic sensitizer RK1,Sol. Energy, 2020, 203, 260–266.

22 J.-M. Ji, H. Zhou, Y. K. Eom, C. H. Kim and H. K. Kim, 14.2%Efficiency Dye-Sensitized Solar Cells by Co-sensitizing NovelThieno[3,2-b]indole-Based Organic Dyes with a PromisingPorphyrin Sensitizer, Adv. Energy Mater., 2020, 10, 2000124.

23 F. Gao, C.-L. Yang and G. Jiang, Effects of the couplingbetween electrode and GQD-anthoxanthin nanocompositesfor dye-sensitized solar cell: DFT and TD-DFT investiga-tions, J. Photochem. Photobiol., A, 2021, 407, 113080.

24 B. Bozkurt Çırak, Ç. Eden, Y. Erdogan, Z. Demir,K. V. Ozdokur, B. Caglar, S. Morkoç Karadeniz, T. Kılınç,A. Ercan Ekinci and Ç. Çırak, The enhanced light harvestingperformance of dye-sensitized solar cells based on ZnOnanorod-TiO2 nanotube hybrid photoanodes, Optik, 2020,203, 163963.

25 S. Padmanathan and A. Prakasam, Design and fabricationof hybrid carbon dots/titanium dioxide (CDs/TiO2) photo-electrodes for highly efficient dye-sensitized solar cells,J. Mater. Sci.: Mater. Electron., 2020, 31, 3492–3499.

26 S. Vijaya, G. Landi, J. J. Wu and S. Anandan, Ni3S4/CoS2 mixed-phase nanocomposite as counter electrode for Pt-free dye-sensitized solar cells, J. Power Sources, 2020, 478, 229068.

27 D. Zhou, Z. Xia, H. Shang, H. Xiao, Z. Jiang, H. Li, L. Zheng,J. Dong and W. Chen, A rational design of an efficientcounter electrode with the Co/Co1P1N3 atomic interfacefor promoting catalytic performance, Mater. Chem. Front.,2021, 5, 3085–3092.

28 J.-Y. Xi, R. Jia, W. Li, J. Wang, F.-Q. Bai, R. I. Eglitis and H.-X. Zhang, How does graphene enhance the photoelectricconversion efficiency of dye sensitized solar cells? Aninsight from a theoretical perspective, J. Mater. Chem. A,2019, 7, 2730–2740.

29 J. Liu, X. Kang, X. He, P. Wei, Y. Wen and X. Li,Temperature-directed synthesis of N-doped carbon-basednanotubes and nanosheets decorated with Fe (Fe3O4,Fe3C) nanomaterials, Nanoscale, 2019, 11, 9155–9162.

30 F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake,M. I. Katsnelson and K. S. Novoselov, Detection of indivi-dual gas molecules adsorbed on graphene, Nat. Mater.,2007, 6, 652–655.

31 G. Jiao, X. He, X. Li, J. Qiu, H. Xu, N. Zhang and S. Liu,Limitations of MTT and CCK-8 assay for evaluation ofgraphene cytotoxicity, RSC Adv., 2015, 5, 53240–53244.

32 C. Wan, X. Duan and Y. Huang, Molecular Design of Single-Atom Catalysts for Oxygen Reduction Reaction, Adv. EnergyMater., 2020, 10, 1903815.

33 Z.-B. Liu, Y.-F. Xu, X.-Y. Zhang, X.-L. Zhang, Y.-S. Chen andJ.-G. Tian, Porphyrin and Fullerene Covalently Functiona-lized Graphene Hybrid Materials with Large NonlinearOptical Properties, J. Phys. Chem. B, 2009, 113, 9681–9686.

34 D. Kiessling, R. D. Costa, G. Katsukis, J. Malig,F. Lodermeyer, S. Feihl, A. Roth, L. Wibmer, M. Kehrer,M. Volland, P. Wagner, G. G. Wallace, D. L. Officer andD. M. Guldi, Novel nanographene/porphyrin hybrids – pre-paration, characterization, and application in solar energyconversion schemes, Chem. Sci., 2013, 4, 3085–3098.

35 P. Guarracino, T. Gatti, N. Canever, M. Abdu-Aguye,M. A. Loi, E. Menna and L. Franco, Probing photoinducedelectron-transfer in graphene–dye hybrid materials forDSSC, Phys. Chem. Chem. Phys., 2017, 19, 27716–27724.

36 B. Mandal, S. Sarkar and P. Sarkar, Theoretical Studies onUnderstanding the Feasibility of Porphyrin-Sensitized Gra-phene Quantum Dot Solar Cell, J. Phys. Chem. C, 2015, 119,3400–3407.

37 F. Gao, C.-L. Yang, M.-S. Wang and X.-G. Ma, Computationalstudies on the absorption enhancement of nanocompositesof tetraphenylporphyrin and graphene quantum dot assensitizers in solar cell, J. Mater. Sci., 2018, 53, 5140–5150.

38 A. Dhar, N. Siva Kumar, M. Asif and R. L. Vekariya, Fabrica-tion of D–p–A sensitizers based on different donors sub-stituted with a dihydropyrrolo[3,4-c]pyrrole-1,4-dione bridgefor DSSCs: influence of the CDCA co-absorbent, NewJ. Chem., 2018, 42, 12024–12031.

39 S. Chen, N. Ullah, T. Wang and R. Zhang, Tuning the opticalproperties of graphene quantum dots by selective oxidation:a theoretical perspective, J. Mater. Chem. C, 2018, 6,6875–6883.

40 K. Song, Y. Long, X. Wang and G. Zhou, Theoretical inves-tigations of transport properties of organic solvents incation-functionalized graphene oxide membranes: Implica-tions for drug delivery, Nano Res., 2018, 11, 254–263.

41 X. Wang, Y. Li, P. Song, F. Ma and Y. Yang, Second-OrderNonlinear Optical Switch Manipulation of Photosensitive



Layer by an External Electric Field Coupled with GrapheneQuantum Dots, J. Phys. Chem. A, 2019, 123, 7401–7407.

42 M. Hu, H. Zhang, L. Yang and R. Lv, Ultrahigh rate sodium-ion storage of SnS/SnS2 heterostructures anchored onS-doped reduced graphene oxide by ion-assisted growth,Carbon, 2019, 143, 21–29.

43 M. Sun, Q. Ren, Y. Zhao, J.-P. Chou, J. Yu and W. Tang,Electronic and magnetic properties of 4d series transitionmetal substituted graphene: A first-principles study, Car-bon, 2017, 120, 265–273.

44 J. Tomasi, B. Mennucci and R. Cammi, Quantum Mechan-ical Continuum Solvation Models, Chem. Rev., 2005, 105,2999–3094.

45 Y. Kurumisawa, T. Higashino, S. Nimura, Y. Tsuji, H. Iiyamaand H. Imahori, Renaissance of Fused Porphyrins: Substi-tuted Methylene-Bridged Thiophene-Fused Strategy forHigh-Performance Dye-Sensitized Solar Cells, J. Am. Chem.Soc., 2019, 141, 9910–9919.

46 A. Pradhan, T. Morimoto, M. Saikiran, G. Kapil, S. Hayaseand S. S. Pandey, Investigation of the minimum drivingforce for dye regeneration utilizing model squaraine dyesfor dye-sensitized solar cells, J. Mater. Chem. A, 2017, 5,22672–22682.

47 Y. Li, X. Li and Y. Xu, Theoretical screening of high-efficiency sensitizers with D-p-A framework for DSSCs byaltering promising donor group, Sol. Energy, 2020, 196,146–156.

48 H.-C. Zhu, C.-F. Li, Z.-H. Fu, S.-S. Wei, X.-F. Zhu andJ. Zhang, Increasing the open-circuit voltage and adsorptionstability of squaraine dye binding onto the TiO2 anatase(101) surface via heterocyclic anchoring groups used forDSSC, Appl. Surf. Sci., 2018, 455, 1095–1105.

49 H. Wu, T. Ma, C. Wu, L. Yan and Z. Su, Effect of polyox-ometalate in organic-inorganic hybrids on charge transferand absorption spectra towards sensitizers, Dyes Pigm.,2017, 142, 379–386.

50 P. N. Samanta, D. Majumdar, S. Roszak and J. Leszczynski,First-Principles Approach for Assessing Cold Electron Injec-tion Efficiency of Dye-Sensitized Solar Cell: Elucidation ofMechanism of Charge Injection and Recombination, J. Phys.Chem. C, 2020, 124, 2817–2836.

51 T. Lu and F. Chen, Multiwfn: A multifunctional wavefunc-tion analyzer, J. Comput. Chem., 2012, 33, 580–592.

52 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N.Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,

R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT,2009.

53 M. Gratzel, Photoelectrochemical cells, Nature, 2001, 414,338–344.

54 G. Zhang, Y. Bai, R. Li, D. Shi, S. Wenger, S. M. Zakeeruddin,M. Gratzel and P. Wang, Employ a bisthienothiophenelinker to construct an organic chromophore for efficientand stable dye-sensitized solar cells, Energy Environ. Sci.,2009, 2, 92–95.

55 T.-F. Lu, W. Li, J. Chen, J. Tang, F.-Q. Bai and H.-X. Zhang,Promising pyridinium ylide based anchors towards high-efficiency dyes for dye-sensitized solar cells applications:Insights from theoretical investigations, Electrochim. Acta,2018, 283, 1798–1805.

56 D. S. Patil, K. C. Avhad and N. Sekar, Linear correlationbetween DSSC efficiency, intramolecular charge transfercharacteristics, and NLO properties – DFT approach, Com-put. Theor. Chem., 2018, 1138, 75–83.

57 L.-J. He, J. Wang, J. Chen, R. Jia and H.-X. Zhang, The effectof relative position of the p-spacer center between donorand acceptor on the overall performance of D-p-A dye: atheoretical study with organic dye, Electrochim. Acta, 2017,241, 440–448.

58 L.-J. He, W. Wei, J. Chen, R. Jia, J. Wang and H.-X. Zhang,The effect of D–[De–p–A]n (n = 1, 2, 3) type dyes on theoverall performance of DSSCs: a theoretical investigation,J. Mater. Chem. C, 2017, 5, 7510–7520.

59 M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,E. Mueller, P. Liska, N. Vlachopoulos and M. Graetzel,Conversion of light to electricity by cis-X2bis(2,20-bipyridyl-4,40-dicarboxylate)ruthenium(II) charge-transfer sensiti-zers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystallinetitanium dioxide electrodes, J. Am. Chem. Soc., 1993, 115,6382–6390.

60 M. Gratzel, Solar Energy Conversion by Dye-SensitizedPhotovoltaic Cells, Inorg. Chem., 2005, 44, 6841–6851.

61 N. J. Cherepy, G. P. Smestad, M. Gratzel and J. Z. Zhang,Ultrafast Electron Injection: Implications for a Photoelec-trochemical Cell Utilizing an Anthocyanin Dye-SensitizedTiO2 Nanocrystalline Electrode, J. Phys. Chem. B, 1997, 101,9342–9351.

62 R. A. Marcus, On the Theory of Oxidation-Reduction Reac-tions Involving Electron Transfer. I, J. Chem. Phys., 1956, 24,966–978.

63 T. J. Meade, H. B. Gray and J. R. Winkler, Driving-forceeffects on the rate of long-range electron transfer inruthenium-modified cytochrome c, J. Am. Chem. Soc.,1989, 111, 4353–4356.

64 Y. Li, X. Li and Y. Xu, A rational design of excellent light-absorbing dyes with different N-substituents at the phe-nothiazine for high efficiency solar cells, Spectrochim. Acta,Part A, 2020, 234, 118241.



65 M. Tuikka, P. Hirva, K. Rissanen, J. Korppi-Tommola andM. Haukka, Halogen bonding—a key step in charge recom-bination of the dye-sensitized solar cell, Chem. Commun.,2011, 47, 4499–4501.

66 M. Miyashita, K. Sunahara, T. Nishikawa, Y. Uemura,N. Koumura, K. Hara, A. Mori, T. Abe, E. Suzuki andS. Mori, Interfacial Electron-Transfer Kinetics in Metal-

Free Organic Dye-Sensitized Solar Cells: Combined Effectsof Molecular Structure of Dyes and Electrolytes, J. Am. Chem.Soc., 2008, 130, 17874–17881.

67 K. Garg, R. Shanmugam and P. C. Ramamurthy, New covalenthybrids of graphene oxide with core modified and -expandedporphyrins: Synthesis, characterisation and their non linearoptical properties, Carbon, 2017, 122, 307–318.


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