Studies on the characteristics of \(\hbox {TiO}_2\) photoanode and flavanol pigment as a sensitiser for DSSC applications


In the present work, dye-sensitised solar cell (DSSC) is fabricated using natural dye (Tecoma Stans)-sensitised \(\hbox {TiO}_{\mathrm {2\, }}\) as the photoanode, platinum as the counter electrode and lithium iodide and iodine as the electrolyte. Initially, \(\hbox {TiO}_{\mathrm {2\, }}\) nanoparticles are synthesised by the sol–gel technique using glacial acetic acid as the hydrolysing agent. The photoanodes are prepared by the Doctor Blade technique using the synthesised nanoparticles coated onto the fluorine-doped tin oxide (FTO) substrate and annealed at 400, 500 and \(600^{\circ }\hbox {C}\) for 30 min. Structural analysis shows the anatase phase of the \(\hbox {TiO}_{\mathrm {2}}\) thin film with a tetragonal crystal structure. The optical transmittance is found to be around 95% with an optical band gap close to 3.1 eV and increases with an increase in annealing temperature. From the PL emission spectra, the excitation of the titania band is observed. The pigment flavan-4-ol which acts as a sensitiser is extracted from Tecoma Stans flower. From the absorption study of the pigment, the active region of radiation and the band gap are found to be around 550 nm and 2 eV respectively. FTIR analysis of the pigment shows a stable structure similar to that of the prepared photoanodes. CV analysis of the sensitiser shows maximum oxidation within the active region. Field emission scanning electron microscope (FE-SEM) analysis shows that the prepared photoanode (\(600^{\circ }\hbox {C}\)) has spherical shape. The overall photoconversion efficiency of Tecoma Stans-sensitised \(\hbox {TiO}_{\mathrm {2}}\) photoanode is 0.4491%.


The recent phenomenal progress achieved in the characterisation of nanocrystalline materials and fabrication of devices has shown a new alternative concept to the present-day p–n junction photovoltaic devices. Dye-sensitised solar cell (DSSC) technology, the third-generation solar cell technology, is a promising solar energy technology due to its high transfer efficiency, low cost, and easy preparation process as reported by O’Regan and Gratzel in 1991 [1]. In DSSC, the optical absorption process is by a sensitiser and charge separation process is by a wide band gap \(\hbox {TiO}_{\mathrm {2}}\) with nanocrystalline morphology [1, 2]. \(\hbox {TiO}_{\mathrm {2}}\), the most promising semiconductor nanomaterial, owing to the advantages of low cost, high reactivity and photochemical stability found applications in photocatalysis, supercapacitors, batteries and photovoltaics [3]. The other applications are in photoelectrochemical solar cells [1, 4], photocatalytic water splitting, cancer cell treatments [5, 6], electrochromic devices [7], electrochemical gas sensors [8] and lithium-ion batteries [9]. Also, it has good chemical stability, better dye adsorption ability, high refractive index, excellent charge transport property and low toxicity [1, 10]. DSSCs can be easily to fabricated using conventional techniques, they are semi-flexible and semi-transparent and they have a wide variety of applications [11]. The essential conditions to prepare controlled \(\hbox {TiO}_{\mathrm {2}}\) morphologies include the synthesis method and experimental factors such as the precursor of \(\hbox {TiO}_{\mathrm {2}}\), solvent effect, temperature, chelating agent and time taken for the synthesis process [12]. The synthesis of \(\hbox {TiO}_{\mathrm {2}}\) can be done using sol–gel technique [13], hydrothermal treatment [14], spray pyrolysis [15] and chemical vapour deposition [16]. In the present work, \(\hbox {TiO}_{\mathrm {2\, }}\) NPs are synthesised by the sol–gel technique and the electrodes are prepared by the Doctor Blade technique. The flavan-4-ol pigment is analysed to replace the conventional sensitiser used so far in DSSC. A common feature of all flavonoids is their ability to absorb appreciable visible spectrum (500–550 nm). Also, the absorbance can be made to shift to longer wavelengths by increasing the conjugation of the pigment and by decreasing the three planar ring saturation. However, the intensity of absorption depends on the extent of acylation, vacuolar pH and co-pigmentation with other flavonoids and chelation to metals.

Experimental methods

Synthesis and characterisation of \(\hbox {TiO}_{2}\) nanoparticles

In the present work, \(\hbox {TiO}_{\mathrm {2}}\) nanoparticles were synthesised by the sol–gel technique using titanium tetraisopropoxide (TTIP) (Sigma-Aldrich) as the precursor and glacial acetic acid (NICE) as the hydrolysing agent. The synthesised powder was coated onto the FTO substrate using Triton-X-100 (Hi-Media) as the binding agent and annealed at 400, 500 and \(600^{\circ }\hbox {C}\) for 30 min. The crystalline structure was identified using powder X-ray diffraction analysis using Shimadzu XRD 6000 diffractometer with CuK\(\upalpha \) radiation (\(\lambda =1.541\) Å). The functional group analysis was carried out by Perkin Elmer Spectrophotometer using the KBr pellet technique. The surface morphology was obtained from the field emission scanning electron microscope (FE-SEM) using FEGQUANTA 250. The luminescence studies were carried out by photoluminescence spectrophotometer (RF 6000) and reflectance UV-DRS (UV ISR 2600\(+\)) study was carried out in the region of 200 to 800 nm. The cyclic voltammetry studies were analysed by CH instruments. The IV characteristics of DSSC were measured by the Keithley instrument (2450 source meter).

Extraction of flavan-4-ol pigment

The alcoholic extraction of Tecoma Stans was done at room temperature by soaking the flowers in ethanol. The extract was then taken for phytochemical screening to identify the active functional structure present using the standard method [17]. After confirming the presence of the expected pigment, it was filtered out by chromatography. Then the pigment was subjected to absorption studies, FTIR and cyclic voltammetry analysis.

Fig. 1

Fabrication process of DSSC using Tecoma Stans-sensitised photoanode (\(600^{\circ }\hbox {C}\)).

Fabrication of the cell

The synthesised \(\hbox {TiO}_{\mathrm {2\, }}\) nanoparticles were coated onto the FTO substrate which acts as the photoanode. It was immersed in the natural sensitiser (Tecoma Stans) for 24 h in a dark place at ambient conditions. The platinum counter electrode was prepared by the electrodeposition method using chloroplatinic acid (5 mM) (Sigma Aldrich) with an applied potential of −0.4 V for 120 s [13]. The redox electrolyte was prepared using 0.5 M lithium iodide (Sigma Aldrich) and 0.05 M iodine (NICE) in acetonitrile (Hi-Media) solvent in the ratio of 1:9. The photoanode and the counter electrode were sandwiched with lithium iodide/iodine electrolyte. The cell area was kept at 0.5 cm \(\times \) 0.5 cm. The fabrication process of DSSC using Tecoma Stans-sensitised photoanode (\(600^{\circ }\hbox {C}\)) is shown in figure 1.

Results and discussion

Structural analysis

Figure 2 shows that the observed planes (101), (004), (200) and (105) match with the tetragonal \(\hbox {TiO}_{\mathrm {2}}\) anatase phase (JCPDS Card No. 21-1272) and is in good agreement with many other studies [18,19,20,21]. An improved crystallinity and slight shift in the diffraction peaks towards higher angle observed with an increase in the annealing temperature is due to the strain (tensile and compressive) developed in \(\hbox {TiO}_{\mathrm {2}}\) lattice and similar results are obtained by Moges Tsegaa and Dejene [22]. Microstructural parameters calculated for the most prominent peak (101) showed a decrease in FWHM from 1.13 to 0.6 with an increase in the annealing temperature (table 1), proving the improved crystallinity in the lattice of the semiconductor anode. The lattice constants (a and c) are calculated for planes of tetragonal structure using the following equation:

$$\begin{aligned} \frac{1}{d^{2}}=\, \frac{(h^{2}+\, k^{2}\, )}{a^{2}}+\, \frac{l^{2}}{c^{2}}, \end{aligned}$$

where \(d_{hkl}\) is the spacing between the planes corresponding to Miller indices (hkl)). It is noted that (table 1) the lattice constants a and c decrease from 3.796 to 3.762 Å and 9.496 to 9.368 Å respectively when the annealing temperature increases from 400 to \(600^{\circ }\hbox {C}\). The crystallite size (D) (table 1) of the most prominent anatase (101) is calculated using eq. (2) [23].

$$\begin{aligned} D=\, \frac{k\lambda }{\beta \cos \theta } \end{aligned}$$

where \(\lambda \) is the X-ray wavelength of the CuK\(\upalpha \) radiation (1.541 Å), \(\beta \) is the full-width at half-maximum (FWHM) in radian and \(\theta \) is the Bragg’s diffraction angle. The increase in the crystallite size from 7 to 14 nm with the increase in the annealing temperature is due to the diffusion of titania, causing the merging of adjacent mesopores. Also, the spatial confinement of mesopores array controls the formation and growth of the anatase phase, thereby leading to a more or less uniform distribution of titania nanocrystals [23], which enhances the conversion of the amorphous film into anatase phase [24]. Furthermore, when the grain size increases, an inverse Hall–Petch effect has been observed [25]. During the annealing process, grain boundaries are relaxed leading to a change in the interfacial properties. These observations show that the contribution by lattice distortion is gradually decreased. When the grain size is small, different mechanism accommodated by grain boundaries may dominate plastic deformation [26, 27].

Table 1 FWHM, grain size, lattice strain and lattice constants of \(\hbox {TiO}_{\mathrm {2}}\) nanoparticle and photoanodes annealed at 400, 500 and \(600^{\circ }\hbox {C}\).
Table 2 Photoconversion efficiency of the prepared sample at \(600^{\circ }\hbox {C}\).
Fig. 2

XRD patterns of the synthesised \(\hbox {TiO}_{\mathrm {2}}\) nanoparticle and photoanodes annealed at 400, 500 and \(600^{\circ }\hbox {C}\).

Fig. 3

(a) % Transmittance and (b) band-gap plots of \(\hbox {TiO}_{\mathrm {2\, }}\) nanoparticle and photoanodes annealed at 400, 500 and \(600^{\mathrm {o}}\)C.

In general, the influence of temperature will enhance the crystallinity of the material. This process takes place during the transformation from liquid to solid phases which leads to nucleation and growth. Due to the influence of temperature, nucleation and growth of the particles will increase. One important parameter in nucleation is supersaturation. The effect of variation in temperature leads to lowering of the maximum supersaturation. At higher temperatures, many collapsing events occur leading to the higher density of the formed particles. According to the collision theory, the collision frequency is proportional to the temperature and hence increases the rate of reaction. It is noted that the crystallinity of the prepared samples increases with increasing thermal treatment. In general, the grain size increases with a decrease in volume. According to the Anderson Gruneisen parameter (3) for spherical nanosolids, the volume is calculated using the following equation [28]:

$$\begin{aligned} V=V_{\mathrm {0}}{\left[ \frac{1}{1-\, {\delta }_{0}{\alpha }_{T}\left( 1-\frac{2d}{D} \right) ^{-1}\left( T-T_{0} \right) }\right] }^{\frac{1}{{\delta }_{0}}}, \end{aligned}$$

where \(V_{\mathrm {0}}\) and V are the initial and final volume of the particles, \(\delta _{\mathrm {0}}\) is the bulk modulus [29], \(\alpha _{\mathrm {T}}\) is the thermal expansion coefficient [30], d is the diameter of the atom, D is the grain size, \(T_{\mathrm {0\, }}\) and T are the initial and final temperature. It is noted that the calculated volume from 137.16 to \(136.09\, \hbox {m}^{\mathrm {3}}\) increases the crystallite size of the particles (table 1). Hence, this theory satisfies the decrease in the volume of the prepared particles with increase in temperature.

Optical analysis

From figure 3a, the % transmittance is found to be around 95% and shows good homogeneity and low surface roughness of the prepared thin films [31]. From the transmittance spectra, the transmittance at about 390, 400 and 320 nm for 400, 500 and \(600^{\circ }\hbox {C}\, \hbox {TiO}_{\mathrm {2\, }}\) thin films begin to decrease rapidly and at 310 nm it approaches zero. This is due to the absorption of light by the valence band (occupied band) electron to the conduction band (unoccupied band) electron of \(\hbox {TiO}_{\mathrm {2}}\).

At higher temperatures, the prepared films show decreased transmittance which is due to the increase in grain sizes and diffusion of light scattering in prepared films [31]. Tauc’s plot (figure 2b) of (\(\alpha h\nu )^{\mathrm {1/2\, }}\) vs. \(h\nu \) show the indirect allowed transitions and using the following relation [32]

$$\begin{aligned} (\alpha h\nu )^{\mathrm {2}} = A(h\nu - \quad E_{g}) , \end{aligned}$$

where \(\alpha \) is the absorption coefficient and A and \(E_{g} \) are the constant and the indirect band-gap energy of the material, respectively. The decrease in optical band gap from 3.91 to 3.4 eV with an increase in annealing temperature from 400 to \(600^{\circ }\hbox {C}\) is due to an increase in crystallite size. The quantum size effects weaken when the grain sizes increase and hence the band-gap energy decreases. Similar results are observed by many other researchers [31, 33].

Photoluminescence analysis

Photoluminescence (PL) is a surface phenomenon as the defects and the surface environment affect the emission spectrum. Hence, it is used to analyse the charge carrier trapping, migration and charge transfer, and the existence of defects on the surface. It is considered to be a macroscopic manifestation of the recombination of charge carrier/photogenerated carrier occupying the tiny regions below 10 nm of the semiconducting surface, i.e. surface and subsurface oxygen vacancies [32]. In nanorange materials, the PL process is closely related to the surface stoichiometry and the kind of surface states could change by the annealing process [33]. The origin of PL emission in anatase \(\hbox {TiO}_{\mathrm {2\, }}\) is due to (a) self-trapped excitons (STE), (b) oxygen vacancies and (c) surface states (defects) [34,35,36,37]. Also, the PL intensity quantifies the recombination efficiency of photogenerated charges. \(\hbox {TiO}_{\mathrm {2}}\) is a strong metal oxide in which the valence band is composed of 2p orbitals of oxygen atoms and the conduction band is due to the 3d orbitals of titanium. They get oxidised to \(\hbox {Ti}^{\mathrm {3+}}\), \(\hbox {Ti}^{\mathrm {2+}}\) or \(\hbox {Ti}^{\mathrm {+\, }}\) oxidation states when it is exposed to the surface and as a result, some localised energy levels are introduced in the forbidden gap [38]. As an indirect band-gap semiconductor, it is difficult to observe band-gap luminescence from \(\hbox {TiO}_{\mathrm {2}}\) nanocrystals. The emission probabilities of indirect transitions are lower than that of direct transitions. The peak position and spectral shape observed in the PL study strongly depend on the surrounding gaseous environment and the excitation conditions. In the present study, the observed PL spectrum extended in the visible region is due to the presence of the oxygen vacancy state associated with \(\hbox {Ti}^{\mathrm {3+}}\) in between the valance and conduction bands in anatase \(\hbox {TiO}_{\mathrm {2\, }}\) [39].

Fig. 4

Photoluminescence spectra of the \(\hbox {TiO}_{\mathrm {2}}\) films annealed at (a) 400, (b) 500 and (c) \(600^{\mathrm {o}}\hbox {C}\).

Fig. 5

UV\(\hbox {--}\)visible spectra of flavon-4-ol pigment.

Fig. 6

FTIR spectra of Tecoma Stans.

Photoluminescence (PL) emission spectra of photoanodes (figure 4) recorded with excitation wavelength at 400 nm showed strong bands around 395, 412, 454 and 467 nm for films annealed at 400, 500 and \(600^{\circ }\hbox {C}\), due to the transition of electrons from the conduction band to the valence band of \(\hbox {TiO}_{\mathrm {2}}\) [40]. The high surface area of the \(\hbox {TiO}_{\mathrm {2}}\) nanoparticles is influenced by surface defects during their performance in photocatalysis and solar energy conversion. These defects result in deep intra-band-gap states which are considered to be bad traps. But the shallow trap may contribute to the carrier transport which occurs by diffusion and is considered to be good traps. A broad blue emission found around 400–500 nm is due to the oxygen vacancy in \(\hbox {TiO}_{\mathrm {2}}\) film [41, 42]. The emission around 395 nm corresponds to the band-gap energy of the crystalline bulk anatase \(\hbox {TiO}_{\mathrm {2}}\) and it is relatively week [40]. The emission around 467 nm (2.65 eV) is due to the de-excitation from lower levels in \(\hbox {Ti}^{\mathrm {3+}}\) 3d states of \(\hbox {TiO}_{\mathrm {2}}\) to the acceptor level (deep) created by \(\hbox {OH}^{\mathrm {-\, }}\) [43].

Phytochemical analysis

The phytochemical analysis of the alcoholic flower extract confirmed the presence of compounds such as phenols, anthocyanin and flavonoids [17]. Using standard chromatography techniques, 10 mg of the flavon-4-ol pigment is filtered and collected.

Pigment stability

The filtered pigment, when subjected to UV–visible irradiation (figure 5), showed absorption around 650 nm and band gap around 2 eV. The FTIR analysis (figure 6) has also got similar peaks, as that of the synthesised photoanode owing to better lattice matching and less degradation by \(\hbox {TiO}_{\mathrm {2}}\) over a period of time due to the consistent UV irradiance.

Fig. 7

CV Analysis spectrum of flavon-4-ol pigment.

Fig. 8

FE-SEM image and relevant normal distribution of the \(\hbox {TiO}_{\mathrm {2}}\) photoanode at \(600^{\circ }\hbox {C}\).

Cyclic voltammetry

Figure 7 shows the cyclic voltammetric (CV) spectrum of flavon-4-ol pigment. The ability to donate electrons by the dye is the measure of total antioxidant concentration. Though such measurements are limited by the fact that we have limited knowledge about how far this antioxidant is active at the surface of the electrode, the HOMO–LUMO levels of the pigment helps in understanding the same. From figure 6, it is seen that the HOMO level (2.3 eV) is almost close to the absorption energy (1.9 eV) obtained from the UV–visible spectrogram. Hence, the radiation absorbed is enough for electron oxidation. The analysis is carried out using platinum as both photo and counter electrodes whereas calomel is kept as a reference and the scan rate is 50 mV/s.

Fig. 9

\(J{-}V\) characteristics of the Tecoma Stans-sensitised \(\hbox {TiO}_{\mathrm {2}}\) photoanode at \(600^{\circ }\hbox {C}\).

Morphological analysis

Figure 8 shows the FE-SEM image and the relevant diameter distribution of the \(\hbox {TiO}_{\mathrm {2}}\) photoanode at \(600^{\circ }\hbox {C}\). It is noted that the prepared \(\hbox {TiO}_{\mathrm {2}}\) photoanode shows spherical morphology of the particles and it is agglomerated in some places. When the temperature is set at \(300^{\circ }\hbox {C}\), due to the rapid nucleation of reactive intermediates, the \(\hbox {TiO}_{\mathrm {2}}\) nanodots will grow in a diffusion-control pattern, leading to the formation of spherical \(\hbox {TiO}_{\mathrm {2}}\) [44]. The average particle size is calculated using ImageJ software with the relevant diameter distribution and it is found to be about 21.39 nm which is close to the results obtained in XRD. In view of El-Naschie’s E-infinite theory [45], systems in the nanoscale may possess entirely new physical and chemical characteristics that result in properties that were neither well described by those of a single molecule of the substance, nor by those of the bulk material [46]. In the nanoscale, nanoeffect arises similar to that in the quantum world. This will result in quantum-like properties of unusually high surface energy, surface reactivity, high thermal and electric conductivity [47].

\({\textit{\textbf{J}}}{-}{\textit{\textbf{V}}}\) Characteristics

The photoanode annealed at \(600^{\circ }\hbox {C}\) is used to fabricate the cell as the band gap of the photoanode is found to be close to the \(\hbox {TiO}_{\mathrm {2}}\). The electrical measurement of DSSC is taken under a halogen lamp. The \(J{-}V\) characteristics of the natural dye-sensitised \(\hbox {TiO}_{\mathrm {2}}\) DSSC is shown in figure 9. The fill factor (FF) and efficiency (\(\eta )\) of the DSSC are calculated by the formulae

$$\begin{aligned}&\mathrm {FF}=\, \frac{V_{m}J_{m}}{V_{\mathrm {OC}}J_{\mathrm {SC}}}, \end{aligned}$$
$$\begin{aligned}&\eta =\, \frac{V_{\mathrm {OC}}J_{\mathrm {SC}}\mathrm {FF}}{P_{\mathrm {in}}}, \end{aligned}$$

where \(P_{\mathrm {in}}\), the power input, is the intensity of the incident light (100 mW\(/\hbox {cm}^{\mathrm {2}})\). From figure 9, it is noted that the open-circuit voltage (\(V_{\mathrm {OC}})\) is found to be 0.519 V and the short-circuit current (\(J_{\mathrm {SC}})\) is found to be 2.335 mA\(/\hbox {cm}^{\mathrm {2}}\). A fill factor of 0.37 and an efficiency of 0.4491% are observed (table 2) for DSSC with flavon-4-ol pigment extracted from Tecoma Stans. The lower value of the cell efficiency may be due to the energy mismatch between the photoanode and the dyes or their intrinsic low diffusion coefficients in electrolyte [48]. Also, it may be due to the interface resistance of \(\hbox {TiO}_{\mathrm {2}}/\)dye/electrolyte. Thus, introducing functional group such as carboxyl group and optimising the structure of the natural dye may result in the improved efficiency [49]. Also, dye aggregation on \(\hbox {TiO}_{\mathrm {2}}\) film may reduce the absorptivity and hence hinder the charge injection [50]. Generally, \(J_{\mathrm {SC}}\) resulted from the collective measure of light-harvesting efficiency, charge separation efficiency and charge collection efficiency, which depends on materials and nanostructures comprising DSSC. From the results, low \(J_{\mathrm {SC}}\) may be due to charge separation efficiency [51].


\(\hbox {TiO}_{\mathrm {2}}\) photoanodes are prepared using synthesised \(\hbox {TiO}_{\mathrm {2}}\) nanoparticles by the Doctor Blade technique. X-ray diffraction shows tetragonal crystal structure with the anatase phase and the peaks matche with the standard JCPDS data. The crystallite size increases from 7 to 14 nm when the annealing temperature increases from 400 to \(600^{\circ }\hbox {C}\), due to the agglomeration of crystallites at a higher temperature. The morphological analysis shows the spherical shape of the nanoparticles. The optical analysis shows that the % transmission is around 95% and the calculated energy gap decreases with the annealing temperature. The observed blue emission around 400–500 nm in the PL spectrum is due to the oxygen vacancy and the band observed around 395 nm is due to the transition of electrons from the conduction band to the valence band. The study of flavan-4-ol extract is done from alcoholic extracts of Tecoma Stans flower. The pigment filtered using chromatography showed absorption around 550 nm with maximum oxidation around 1.4 V. Also, from the FTIR analysis, similarity of the pigment lattice with that of the photoanode prepared is observed. Thus, this pigment could be more stable than any other conventional natural extract sensitiser used so far. From the results, it is concluded that photoanode annealed at \(600^{\circ }\hbox {C}\) exhibited the optimal structural properties with pure anatase phase, crystallite size, spherical shape and PL intensity. In addition, using the flavan-4-ol pigment as the sensitiser is expected to improve the active region of the synthesised \(\hbox {TiO}_{\mathrm {2\, }}\) photoanode probing to better efficiency for DSSC if commercialised. The \(J{-}V\) characteristics show a conversion efficiency of 0.4491%. The lower value of the efficiency may be due to their intrinsic low diffusion coefficients in an electrolyte.


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The authors would like to thank Dr K Murugadass, Assistant Professor, Department of Sciences, Dr M Karthega, Assistant Professor, Amrita Materials Science Lab, Dr T G Sathish Babu, Associate Professor, Bio-Sensor Research Lab and Dr Sudip Kumar Batabyal, Research Scientist, Centre for Industrial Research and Innovation (ACIRI), Amrita Vishwa Vidyapeetham, Coimbatore, India, for providing lab facilities and their constant support.

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Correspondence to K S Rajni.

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Vedavarshni, S., Raguram, T. & Rajni, K.S. Studies on the characteristics of \(\hbox {TiO}_2\) photoanode and flavanol pigment as a sensitiser for DSSC applications. Pramana - J Phys 94, 112 (2020).

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  • \(\hbox {TiO}_{\mathrm {2}}\) photoanode
  • natural dye
  • optical analysis
  • dye-sensitised solar cell


  • 84.60.h
  • 88.40H