Enhanced Performance of Dye-Sensitized Solar Cells with Nanostructure Graphene Electron Transfer Layer

The utilization of nanostructure graphene thin films as electron transfer layer in dye-sensitized solar cells (DSSCs) was demonstrated. The effect of a nanostructure graphene thin film in DSSC structure was examined. The nanostructure graphene thin films provides a great electron transfer channel for the photogenerated electrons from TiO 2 to indium tin oxide (ITO) glass. Obvious improvements in short-circuit current density of the DSSCs were observed by using the graphene electron transport layer modified photoelectrode. The graphene electron transport layer reduces effectively the back reaction in the interface between the ITO transparent conductive film and the electrolyte in the DSSC.


Introduction
Dye-sensitized solar cells (DSSCs), also known as Grätzel's cell, are of particular interest in the field of solar energy owing to their low cost and simplicity of fabrication [1][2][3][4][5].The DSSCs have a basic structure that comprises two conductive substrates, an absorbing layer of semiconductor materials with wide band gap, dye molecules, and a redox electrolyte.
The basic principle of operation of DSSCs is that electrons are injected from the photoexcited dye into the conductive band of the semiconductor under illumination; meanwhile, the electrolyte reduces the oxidized dye and transports the positive charges to the counter electrode.However, one of the major issues hindering the rapid commercialization of DSSCs is their lower conversion efficiency compared to conventional p-n junction solar cells [6].That may be attributed to poor charge separation in DSSC structure.Therefore, charge transfer structure, such as Au nanoparticles and quantum dots, has been employed in a DSSC to improve the device performance through charge separation in the photoelectrodes [7][8][9][10].Graphene is a potential material for many applications due to their high electron mobility, outstanding optical properties, and thermal, chemical, and mechanical stability [11][12][13][14][15]. Therefore, this study investigates the effect on the graphene layer as electron transport layer in the DSSC structure deposited by the magnetron sputtering method; in particular, it examines the performance of the DSSCs with the graphene electron transport layer.

Experimental
A 60 nm thick graphene layer was sputtered on indium tin oxide (ITO) conductive glass substrate by radio-frequency magnetron sputtering using a graphite target as an electron transport layer to improve the electron transfer in the DSSC structure.Next, the solution consisting of 1 g TiO 2 nanocrystalline powder with diameter ∼25 nm, 1 mL of triton X-100, acetic acid, and deionized water were mixture as colloidal solution, and the colloidal solutions were daubed uniformly onto the graphene electron transfer layer to form a thick film.The films were annealed at 450 ∘ C for 10 min.Thereafter, the photoelectrode with the graphene layer was immersed in a 3 × 10 −4 M solution of N719 dye adsorption ((Bu 4 N) 2 -[Ru(dcbpyH) 2 (NCS) 2 ] complex) in ethanol for 24 hr, before being sintered at 450 ∘ C for 30 min, to increase its anatase content.The electrolyte was composed of 0.05 M iodide and 0.5 M lithium iodide with and without 0.5 M 4-tertbutylpyridine (TBP) in propylene carbonate.Then, a 100 nm thick layer of platinum was sputtered onto ITO substrate as a counter electrode.Cells were fabricated by placing sealing  films (SX1170-60, SOLARONIX) between the two electrodes and leaving just two via-holes for injection electrolyte.The sealing process was carried out 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 the epoxy with low vapor transmission rate.Figure 1 schematically depicts the complete structure.Figure 1 shows the cross-section of the completed structure.The current density-voltage (-) 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 at 100 mW/cm 2 .

Results and Discussion
Figure 2 shows the absorption of TiO 2 DSSCs with and without the graphene electron transfer layer.Absorption of the DSSC with the 60 nm thick graphene electron transport layer has obviously higher absorption intensity than the DSSC with the 100 nm thick graphene electron transport layer and without the graphene electron transport layer in visible range, especially in the range of 310-400 nm.That means the graphene electron transport layer has an increased absorption coefficient.Therefore, the graphene electron transport layer is also as an absorption layer to improve the absorption of the solar cells.Figure 3 shows the - characteristics of the DSSCs.The cell performance was measured under AM 1.5 illumination with a solar intensity of 100 mW/cm 2 at 25 ∘ C. The cell has an active area of 3 × 3 mm 2 and no antireflective coating.The measured cell parameters, open-circuit voltage ( oc ), shortcircuit current ( sc ), fill factor (FF), and energy conversion efficiency ( ff ) are shown in Table 1.As shown in Figure 3, TiO 2 DSSCs with graphene electron transfer layer exhibited the following static parameters:  oc of 0.5 V and  sc of 17.5 mA/cm 2 .The fill factor (FF) can be estimated by [16] where   is the maximum output current density and   is the maximum output voltage.Therefore, the value of FF results is equal to 0.456.Similarly, the energy conversion efficiency ( ff ) can be calculated by [16]  ff =      inc (2) with  inc as the incident power and  ff results to be 3.98%, respectively.The improvement may be attributed to  the incorporation of the graphene electron transport layer.Figure 4 shows the energy level diagram and mechanism of photocurrent generation in TiO 2 DSSCs with the graphene layer.The work function of the graphene layer is around 4.5 eV [17,18].Graphene has a work function similar to that of the ITO (4.8 eV) electrode.The graphene layer does not prevent the flow of injected electrons down to the ITO electrode because its work function exceeds that of the ITO electrode [19][20][21].The CB and VB are the conduction band and valence band, respectively.The LUMO and HOMO are the lowest unoccupied molecular orbit and highest occupied molecular orbit, respectively.Therefore, the brief operating process is as follows.Dye N719 was excited by incident light, and electrons transit from HOMO to LUMO.Electrons are injected into the graphene electron transport layer via the TiO 2 photoelectrode.The electrons transferred to the graphene electron transport layer were collected at the back contact to generate a photocurrent.Therefore, the inserted graphene layer collects electrons and acts as a transporter in the effective separation of charge and rapid transport of the photogenerated electrons.According to Figures 2 and 3, the enhanced performance of DSSCs with a graphene was attributed to the increase in electron transport efficiency and light absorption in visible range.

Conclusions
DSSCs with a graphene electron transport layer were prepared on an indium tin oxide glass substrate by radiofrequency magnetron sputtering.This work discusses the improvement associated with the introduction of a graphene layer in DSSCs.The enhanced performance of DSSCs with a graphene may be attributed to the increase in electron transport efficiency and light absorption in visible range, especially in the range of 310-400 nm.Therefore, the shortcircuit current density and efficiency of conversion of solar energy to electricity were increased from 6.9 mA/cm 2 and 1.45% to 17.5 mA/cm 2 and 3.98%, respectively, under simulated full sun illumination.By incorporation of the graphene electron transport layer, the device efficiency can be increased by over 170%.

Figure 1 :
Figure 1: Schematic cross-section of the completed structure.

Figure 2 :
Figure 2: Absorbance spectra of the DSSCs with and without the graphene electron transfer layer.

Figure 3 :
Figure 3: - curves of the DSSCs with and without the graphene electron transfer layer under illumination.

1 Figure 4 :
Figure 4: Energy level diagram and mechanism of photocurrent generation in the DSSCs with the graphene electron transfer layer.

Table 1 :
The parameters for TiO 2 DSSCs with and without graphene electron transport layer.