Enhanced Performance of Dye-Sensitized Solar Cells with Graphene/ZnO Nanoparticles Bilayer Structure

This study reports characteristics of dye-sensitized solar cells (DSSCs) with graphene/ZnO nanoparticle bilayer structure. The enhancement of the performance of DSSCs achieved using graphene/ZnO nanoparticle films is attributable to the introduction of an electron-extraction layer and absorption of light in the visible range and especially in the range 300–420 nm. DSSC that was fabricated with graphene/ZnO nanoparticle film composite photoanodes exhibited a Voc of 0.5 V, a Jsc of 17.5mA/cm , an FF of 0.456, and a calculated η of 3.98%.

In this work, a graphene film with high electron mobility is incorporated into a ZnO nanoparticle film to form a compact layer for use in dye-sensitized solar cells.This investigation studies the optical, structural, and surface properties of a graphene film that is grown on ITO electrodes by radiofrequency magnetron sputtering, as functions of thickness, in high-performance DSSCs.The introduction of the sputtered graphene film (electron-extraction layer) with optimal thickness enhances the efficiency of conversion of the DSSCs.

Experimental Setup
In this study, a graphene film was prepared on ITO glass using a radio frequency magnetron sputtering system.Table 1 lists the typical deposition conditions for the graphene film.The resistivity of the graphene film is around 2.2 × 10 3 ohm-cm.The ZnO nanoparticle film was deposited by ultrasonic spray pyrolysis at atmospheric pressure on ITO/graphene glass.Three aqueous solutions, Zn(CH 3 COO) 2 ⋅2H 2 O (0.5 mol/l), CH 3 COONH 4 (2.5 mol/l), and In (NO 3 ) 3 (0.5 mol/l), were used as sources of zinc, nitrogen, and indium, respectively.The atomic ratio of Zn/N in the N-doped film was 1 : 2 and that of Zn/N/In in the N-In codoped film was 1 : 2 : 0.15 [20].An aerosol of the precursor solution was produced using a commercial ultrasonic nebulizer.Colloidal TiO 2 was prepared from 6 g nanocrystalline powder (Degussa, P25 titanium oxide, Japan) and both 0.1 mL of Triton X-100 and 0.2 mL of acetylacetone, which were stirred together for 24 hrs.Subsequently, the mixture was spin-coated on ITO glass and ITO/graphene/ZnO substrate to a thickness of approximately 15 m, and a 3 × 3 mm 2 active area was defined.Thereafter, the prepared thin film photoelectrode was immersed in a 3 × 10 −4 M Ru-metal complex dye, D719 ([RuL2(NCS)2] : 2 TBA), at room temperature for 24 hrs, before it was sintered at 450 or 500 ∘ C for 30 min, to increase its anatase content (anatase : rutile = 85 : 15) [32].The electrolyte included 0.05 M iodide, 0.5 M lithium iodide, and 0.5 M 4-tert-butylpyridine (TBP) in propylene carbonate.A 100 nm thick layer of platinum was sputtered onto the ITO substrate to form a counter electrode.Cells were fabricated by placing sealing films between the two electrodes, leaving only two via-holes for injection of the electrolyte.The sealing process was performed on a hot plate at 100 ∘ C for 3 min.Then, the electrolyte was injected into the space between the two electrodes through the via-holes.Finally, the via-holes were sealed using epoxy at a low vapor transmission rate.Figure 1 schematically depicts the complete structure.
A field emission scanning electron microscope (FESEM) (LEO 1530) was adopted to examine the cross-section and surface morphology of the cells.Additionally, the current density-voltage (J-V) characteristics were measured using a Keithley 2420 programmable source meter under irradiation by a 1000 W xenon lamp.Finally, the irradiation power density on the surface of the sample was calibrated as 1000 W/m 2 .Figure 3 presents the absorption spectra of the TiO 2 , ZnO nanoparticle, and graphene/ZnO nanoparticle/TiO 2 films.The ZnO nanoparticle thin film yields a strong absorption peak at ∼380 nm, revealing the existence of crystalline wurtzite hexagonal ZnO.The DSSC with the graphene film clearly has higher absorption intensity than the DSSC without the graphene film in the visible range, and especially in the range 300-420 nm.

Results and Discussion
Figure 4 shows the XRD plots of the TiO 2 film electrodes before and after annealing.The TiO 2 films were dried in air at room temperature for 10 min and then annealing at 450 ∘ C for 30 min.Two dominant anatase diffraction peaks, (101) (2 = 25.28 ∘ ) and (004) (2 = 37.73 ∘ ), are observed.Following annealing, the sample was highly crystalline and all of the diffraction peaks could be indexed to anatase TiO 2 .
In optoelectronic devices, proper contact between the electrode and the transporter (recombination and back transfer) is crucial for charge collection.Figure 5 presents the schematic energy level diagram of the DSSCs with the graphene and ZnO nanoparticle film.Graphene has a work function (−4.42 eV versus vacuum) similar to that of the ITO (−4.8 eV versus vacuum) electrode.The graphene does not prevent the flow of injected electrons down to the ITO electrode because its work function exceeds that of the ITO electrode [33][34][35].Therefore, the implanted graphene collects electrons and acts as a transporter in the effective separation of charge and rapid transport of the photogenerated electrons.
Based on the above discussion, the incorporation of graphene into ZnO nanoparticle film enables DSSC devices to operate more efficiently.Figure 6 plots photocurrent J-V curves of the DSSCs obtained under 100 mW/cm 2 illumination and the AM 1.5 G condition without and with the graphene and ZnO nanoparticle film, fabricated on an ITO glass substrate.The cell has an active area of 3 × 3 mm 2 and no antireflective coating.Table 2 presents the measured cell parameters-open-circuit voltage ( oc ), short-circuit current ( sc ), fill factor (FF), and energy conversion efficiency ().The DSSC that was fabricated with graphene/ZnO nanoparticle film composite photoanodes exhibited a  oc of 0.5 V, a  sc of 17.5 mA/cm −2 , an FF of 0.456, and a calculated  of 3.98%.Incorporating graphene oxide into the graphene film effectively decreases the internal resistance within the photoanodes and prolonged the electron lifetime.Therefore, the improved photovoltaic properties of DSSC with the graphene/ZnO nanoparticle film photoanode are attributable to the strong absorption of dye and the high light harvesting efficiency, which reduce electron recombination loss.

Conclusion
This work discusses the improvement that is made by the introduction of a sputtered graphene/ZnO nanoparticle film into DSSCs.The enhancement of the performance of DSSCs by the introduction of graphene/ZnO nanoparticle films may be attributed to the introduction of an electron-extraction   layer and the absorption of light in the visible range, especially in the range 300-420 nm.A DSSC that was fabricated with graphene/ZnO nanoparticle film composite photoanodes had a  oc of 0.5 V, a  sc of 17.5 mA/cm −2 , an FF of 0.456, and a calculated  of 3.98%.Accordingly, the improvement of photovoltaic properties of DSSC by the introduction of the graphene/ZnO nanoparticle film photoanode is attributable to the strong absorption of dye and the high light harvesting efficiency, which can reduce the electron recombination loss.
The above results demonstrate the potential application of  graphene oxide to improve for enhancing the performance of ZnO nanoparticle-based DSSCs, which can be produced on a large scale at low cost.

Figure 5 :
Figure 5: Energy level diagram and mechanism of photocurrent generation in DSSCs with GO and ZnO nanoparticles.

Table 1 :
Typical deposition conditions for graphene film.

Table 2 :
Parameters of TiO 2 DSSCs with and without the graphene and ZnO nanoparticle film, fabricated on bare ITO glass substrate.Figure 3: Absorbance spectra of TiO 2 , ZnO nanoparticle, and graphene/ZnO nanoparticle/TiO 2 film.