Hybrid Solar Cell with TiO 2 Film : BBOT Polymer and Copper Phthalocyanine as Sensitizer

An organic-inorganic hybrid solar cell was fabricated using Titanium dioxide (TiO2): 2,5-bis(5tert-butyl-2-benzoxazolyl) thiophene (BBOT) film and Copper Phthalocyanine (CuPc) as a sensitizer. BBOT was used in photodetector in other reported research works, but as per best of our knowledge, it was not implemented in solar cells till date. The blend of TiO2: BBOT blend was used to fabricate the film on ITOcoated glass and further a thin layer of CuPc was coated on the film. This was acted as photoanode and another ITO coated glass with a platinum coating was used as a counter electrode (cathode). An optimal blend of acetonitrile (solvent) (50–100 %), 1,3dimethylimidazolium iodide (10–25 %), iodine (2.5– 10 %) and lithium iodide, pyridine derivative and thiocyanate was used as electrolytes in the hybrid solar cell. The different structural, optical and electrical characteristics were measured. The Hybrid solar cell showed a maximum conversion efficiency of 6.51 %.


Introduction
Earth contains a finite amount of fossil fuels and continuous use of these fossil fuels in huge amount is resulting in the depletion and high cost of these resources.Fossil fuels are a non-renewable source of energy, which means that these resources once depleted cannot be replenished at a sufficient rate for sustainable economic extraction in meaningful human time-frames.In con-trast, the many types of renewable energy resources, such as the wind and solar energy, are constantly replenished.
Solar energy in the direct or indirect form, is the source of most of the renewable energy, which can be used for heating homes, generating electricity and a variety of commercial and industrial uses.Therefore, to shift the dependency for energy from non-renewable to renewable resources, renewable energy has become an important topic for research [14].Out of all the renewable energy sources, solar energy has the most advantages because it cuts down the need for a distribution network since it is possible to place the supply at or near the consumption area [11].Photovoltaic effect was first observed by Becquerel [15].The conventional solar cell or first generation based solar cell is made of crystalline silicon.With further research and modification of the first generation solar cell, the thin film solar cell came into picture.This was based on thin films of silicon and other materials which reduced the costs, normally associated with conventional semiconductor wafer production.Presently, the third generation solar is still a newly emerging field of research which is based on solar cell made of organic materials.This organic based solar cell has the advantage over the previous generation solar cell as far as the cost of materials and manufacturing is concerned [1] and [2].The downside of the organic solar cell is the low efficiency.To overcome the major disadvantages of the pure organic solar cell, many significant changes have been made in the device structure, and also a new choice of materials consisting of both organic and inorganic materials was incorporated in the device.A solar cell based on organic and inorganic known as Hybrid solar cell has attracted attention due to its potential of reaching an efficiency of about 10 % [2], [3], [4], [5], [6] and [7].
In recent research investigation of polymer solar cell, a power conversion efficiency of ∼5 % was obtained [8] and [9].Copper phthalocyanine is chosen as sensitizer and 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT) for the organic blend of BBOT and TiO 2 in the active region of hybrid solar cell in this research because they have high optical stability, chemical stability and photovoltaic property [13].

Materials and Methods
The fabrication of hybrid solar cell involved following steps.Two pieces of ITO coated glass of the dimension of 2 cm × 1 cm were cleaned with Ethanol (C 2 H 5 OH) and de-ionized (DI) water (Fig. 1(a)).The resistance of the conducting side of the glass was measured with a multimeter to be 19-25 Ω on average.Then the glass slides were allowed for natural drying.

Working Principle
CuPc, which is a photo sensitizer, was fabricated on the top layer of the solar cell.CuPc absorbs photon from incident light.The BBOT particles, which were present next to the CuPc layer, are fluorescent in nature [17].BBOT particles receive the photons donated by the CuPc layer and radiate the photons to the active material [18].These radiated photons are absorbed and are trapped by the nanoporous TiO 2 particles, which are present in the blend.Some researchers have been reported regarding fabrication of photodetector using BBOT [16].The following mechanism was involved in conversion of electrical energy from the light energy [19], [20] and [21].
• Absorption: The incident photon is absorbed by the CuPc on the TiO 2 surface.After absorbing photon, the photosensitizer is excited from the ground state (S) to the excited state (S * ).
• Electron injection: The excited electrons are injected into the conduction band of the TiO 2 and BBOT blend.This oxidizes the photosensitizer (S + ).S * → S + + e − (TiO 2 ). (2) • Regeneration: The I − ion redox mediator, present in the electrolyte, donates electrons to the oxidized photosensitizer (S + ) and again regenerates the ground state (S), and the I − is oxidized to I − 3 .
• Collection: The oxidized redox mediator (I − 3 ), diffuses toward the counter electrode and electron is collected by the counter electrode.Iodide (I − ) is regenerated by reduction of triiodide (I − 3 ) on the counter electrode.

Structural Characterizations
Atomic Force Microscopy (AFM) study was carried out to analyse the surface morphology of the TiO 2 : BBOT.The crystalline structures of the nanoporous TiO 2 film (Fig. 6  firmed by the High Resolution X-Ray Diffraction analysis (HRXRD) (Panalytical, X'Pert Pro X-ray diffractometer).The TiO 2 : BBOT film showed new peaks due to the presence of the BBOT (Fig. 6(b)).These peaks were not present in the pattern of pure TiO 2 (Fig. 6(a)).All the diffraction peaks refer to anatase and rutile phases (JCPDS No. 84-1285 for anatase and 87-0920 for rutile) [7] and [12].The pattern shows an average size for both the phases.

Optical Characterizations
The UV-VIS optical absorption measurement (250 − 1000 nm) was done on the nanoporous TiO 2 , BBOT, CuPc and CuPc: BBOT: TiO 2 sample at room temperature by a UV-visible near infrared spectrophotometer (Lambda 950, Perkin Elmer) (Fig. 7).The nanoporous TiO 2 film shows a peak in UV region (375 nm) (Fig. 7(a)).The BBOT showed large absorption spectra in the UV-visible region with a peak at 352 nm ((Fig.7(b)).CuPc also had absorption spectra in visible region with main peaks at 618 nm and 722 nm and a small hump at 575 nm (Fig. 7(c)).CuPc: BBOT: TiO 2 sample had absorption peaks at 562 nm, 643 nm and 726 nm.The (αhν) 2 versus eV curves of the nanoporous TiO 2 film and CuPc:BBOT:TiO 2 sample are shown by Fig. 8.In case of nanoporous TiO 2 , the main band gap is shown for ∼ 3.29 eV, which was also reported by other authors [20].However, TiO 2 film exhibits the band gap at ∼ 2.01 eV, which may correspond to the CuPc.Another major hump is obtained at ∼ 3.67 eV which may be due to sub band gap transition of the nanoporous TiO 2 .The band gap shifting was reported in other journals as well [20].BBOT: CuPc hybrid sample.Electrical Characteristics 9 shows the Illuminance vs Voltage (L-V) characteristics of the solar cell at room temperature.The illuminance of incident light was measured by lux meter (HTC LX-101A) and a multimeter (FLUKE 289 TRUE RMS) was used to measure the voltage.The voltage was increasing with incident light and was saturated after a certain limit (0.645 V).The I-V characteristics study (Fig. 11(a)) was done using an I-V meter (Agilent technologies).The following parameters are measured as: open circuit voltage V oc = 0.6047 V, short circuit current I sc = 16.9 mA, maximum power point voltage V mp = 0.44 V and maximum power point current I mp = 14.8 mA. Figure 11

Calculation
During the calculation of maximum conversion efficiency at room temperature, the irradiation G, which was served as input power, was measured as 1000 W•m −2 i.e. 0.1 W•cm −2 and Air Mass (AM) of 1.5.
Fill Factor:

Conclusion
In this paper, we have demonstrated the fabrication of nanoporous TiO 2 : BBOT: CuPc film based hybrid solar cell.A layer of Copper Pthalocyanine (CuPc) was used as sensitizer and optimal blend of electrolytes was used to maximize the cell efficiency.ITO coated glass was used for both -the front and counter electrodes and a platinum solution coating was used on the counter electrode to maximize the collection of electrons and to reduce the current loss.The AFM and HRXRD studies confirmed the nanoporous morphology of the cell and the crystalline structure of the TiO 2 film respectively.The optical absorption study revealed the absorption properties of the TiO 2 , CuPc and BBOT individually and their absorption contributions in the blend.The I-V characteristics graph defined the values V oc = 0.6047 V, I sc = 16.9 mA, V mp = 0.44 V and I mp = 14.8 mA.The maximum conversion efficiency of the cell was measured to be 6.51 %, which is much higher than in other research works reported earlier for this type of solar cell.
A fine solution of nanoporous TiO2 powder (Global NanoTech), 2,5-Bis(5-tert-butyl-benzoxazo-2-yl) thiophene (BBOT) (Sigma Aldrich) (2:1) and acetic acid (CH 3 COOH) (Nice chemicals) was prepared.A small portion of the conductive side of an ITO coated glass piece was covered with tape to avoid generation of TiO 2 film: BBOT blend during fabrication.A film of the blend was coated on the conductive side by using tape casting method, the colour of the film was light yellow (Fig.1(b)).The dimension of the film was 2 cm × 1 cm.When the liquid part of the solution was evaporated naturally, it left a film of nanoporous TiO2 and BBOT (Fig.1(c)).After the growth of the film the tape was removed.The sample was annealed for 25 minutes at 400 • C (Fig.1(d)).After annealing the film colour changed to dark yellow (Fig.1(e)).Now, a film of Copper Pthalocyanine (CuPc) was generated on the TiO 2 and BBOT film and was allowed for natural drying (Fig.1(f)).For the fabrication of counter electrode, another glass slide was coated with platinum solution (Platisol, Solaronix) on the conductive

Fig. 1 :
Fig. 1: (a) Cleaned ITO coated glass pieces, (b) fabricated film of TiO2 and BBOT blend on ITO coated glass by the tape casting method, (c) the film after drying (light yellow) (d) The sample is being annealed on a hot plate at 400?C, (e) The sample after annealing (color turned to bright yellow), (f) CuPc coated TiO2: BBOT film.

Fig. 2 :Fig. 3 :
Fig. 2: (a) The solar cell is clamped between two binding clips, (b) electrolyte is being injected, (c) electrical characteristics are being measured.

Figure 5 (
Figure 5(a) and Fig. 5(b) show the 2D and 3D AFM images of the film of TiO 2 : BBOT blend.An average of 10 ∼ 15 nm TiO 2 and BBOT nanoparticles were observed in the images.

Fig. 10 :
Fig. 10: (a) the solar cell is clamped between two binding clips, (b) electrolyte is being injected, (c) electrical characteristics are being measured.