Photovoltaic Performance of Spray-Coated Zinc Oxide Nanoparticles Sensitized With Metal-Free Indoline Dyes

Photovoltaic properties of nano-sized zinc oxide (ZnO) films sensitized with a conventional ruthenium complex (N719) and two metal-free organic indoline dyes (D-149 and D-205) were compared. The ZnO nanoparticles were deposited on transparent conductive aluminum-doped ZnO coated glass substrates (AZO) by spray-coating deposition method and then annealed in air at 500 C. Using the ZnO-coated AZO as transparent conductive substrates, dye-sensitized solar cells (DSCs) were prepared with the N719, D149 and D205 dyes as the sensitizers. The photoaction spectra of the incident photon-to-current conversion efficiency (IPCE) of the DSCs revealed that the indoline-sensitized solar cells were higher and broader than the ruthenium-sensitized solar cell in the photo-absorption behavior. Under AM 1.5 simulated sunlight (1000 W m), the indoline-sensitized ZnO solar cells yielded solar-to-electric energy conversion efficiency of 3.02 and 2.26% for the D-205 and D-149 respectively, while the N719 sensitized ZnO recorded only 0.97%. The superior performance of the indoline-sensitized solar cells was attributed to mainly higher sunlight harvesting efficiency of these metal-free organic dyes.


Introduction
Commercially available solar cells are currently based on inorganic semiconductor (silicon, cadmium-terullide CdTe and copper-indium-gallium-selenide CIGS) materials.These inorganic semiconductor materials are expensive and besides their device fabrication processes are tedious and complex.Therefore, solar cells based on organic materials appear to be highly promising and low cost alternative for the photovoltaic energy sector.Dye-sensitized solar cell (DSC) is a photovoltaic device that relies on a dye as the sunlight-absorber material, a porous oxide semiconductor film coated on a transparent conducting film, an electrolyte containing a redox couple and a counter electrode.The dye first absorbs light, producing excitons which dissociate at the dye-semiconductor interface, resulting in the injection of photoelectrons into the conduction band of the porous semiconductor.While the electron is transported through the porous semiconductor to the external circuit, the hole migrates through the electrolyte solution to the counter electrode where it recombines with an electron.Therefore, the dye plays a critical role in the operation of the DSC.The development efforts in the synthesis of dye sensitizers can be grouped into two areas, namely the synthesis of ruthenium complex dyes such as N3 (Grätzel, 2004;Nazeerudin et al., 1993), N719 (Nazeerudin et al., 2002), Z907 (Wang et al., 2002;Wang et al., 2003) and black dye (Nazeerudin et al., 2001;Chiba et al., 2006) and the synthesis of metal-free organic donor dyes (Chen et al., 2010;Guerin et al., 2010;Tefashe et al., 2010;Chen et al., 2011).The former class of compounds contains expensive ruthenium metal and requires careful synthesis steps, while the latter can be prepared rather inexpensively by following designed rules.At present, the state-of-the-art DSCs based on ruthenium complex dye as the sensitizer and TiO 2 semiconductor have an overall power conversion efficiency of more than 11% under standard Air Mass 1.5 (AM 1.5) illumination (Nazeerudin et al., 2005;Chiba et al., 2006).However, the molar extinction coefficients of these dyes are low compared with most metal-free organic dyes (Wang et al., 2005).In contrast to the ruthenium complex dyes, different light absorbing groups can be introduced into the organic framework of the metal-free dye in order to tune the spectral absorption over wide wavelength and also to achieve high extinction coefficients.Major progress has been made in the use of metal-free organic dyes as sensitizers in DSCs with the highest solar-to-electric power conversion efficiency exceeding 8% (Ito et al., 2006;Ito et al., 2008).In our previous work, we have reported the preparation and characterization of sputtered aluminum and gallium co-doped zinc oxide (ZnO) films as conductive substrates in dye-sensitized solar cells (Onwona-Agyeman et al., 2013).In this work, we have compared two metal-free indoline dyes (D149 and D205) and a ruthenium complex dye (N719) to sensitize the film composites on which ZnO nanoparticles were deposited by spray-coating method.The ZnO photoelectrodes were formed on transparent and conductive aluminum-doped zinc oxide films (AZO) instead of the usual fluorine-doped tin oxide (FTO) to eliminate lattice mismatch and thermal expansion differences during heat treatments.The photovoltaic properties of the sensitized ZnO DSCs were evaluated under standard AM 1.5 simulated sunlight (1000 W m -2 ) illumination.

Experimental Procedure
The AZO films were prepared by radio frequency (rf) magnetron sputtering using a mixed ceramic target consisting of ZnO (97.5 wt.%) and Al 2 O 3 (2.5 wt.%) (Hirahara et al., 2012).During the deposition of the AZO film, rf power was kept at 100 W, substrate temperature at 300 C and sputter pressure at 3 Pa.The resultant AZO films prepared under these conditions yielded films with sheet resistance of 8 /sq and the average transmittance of 82% within the wavelength range of 400-800 nm.ZnO powder (20 nm particle size, Wako Chemicals, Japan), few drops of glacial acetic acid and 40 ml of ethanol were mixed and ultrasonically dispersed for 10 min.The mixture was then sprayed onto AZO substrates heated at 150 C and subsequently annealed in air at 500 C for 30 min.The resultant ZnO photoelectrodes (active cell area ~ 0.25 cm 2 ) were then immersed separately in a mixture of acetonitrile/tert-butanol (volume 1:1) containing either 5  10 -4 M indoline (D149 or D205) or ruthenium dyes for 12 h.The dye-coated ZnO photoelectrodes were removed, rinsed with acetonitrile and allowed to dry.Finally, the ZnO photoelectrodes were sandwiched with a platinum-coated counter electrode and the intervening space filled with an electrolyte solution (0.1 M LiI, 0.05 M I 2 , 0.5 M tert-butyl pyridine, 0.6 M dimethylpropylimidazolium iodide in methoxyacetonitrile).The photocurrent action spectra (50 W cm -2 ) and the current-voltage (I-V) characteristics of the solar cells at AM 1.5 (1000 W m -2 ) simulated sunlight irradiation were recorded with a calibrated solar cell evaluation system (JASCO, CEP-25BX).

Wavelength / nm
Figure 1 shows the X-ray diffraction (XRD) pattern of the as-grown AZO film prepared by rf magnetron sputtering.The XRD pattern revealed that, the AZO film orientation is mainly along the (002) direction.The inset in Figure 1 is the transmittance spectrum of the same AZO film with an average transmittance of 82% within the wavelength range of 400-800 nm.
The chemical structures of the indoline dyes (D149 and D205) used in the sensitization of ZnO films in this work are shown in Figure 2. The D205 dye is designed by introducing an octyl substitute, in place of ethyl group, into the rhodanine ring of the D149.Indoline dyes have also been previously used to sensitized oxide semiconductors such as TiO 2 (Ito et al., 2008) and SnO 2 (Onwona-Agyeman et al., 2006;Ariyasinghe et al., 2011).

Figure 1 .
Figure 1.XRD pattern of the AZO film used as a transparent conductive substrate in ZnO dye-sensitized solar cells.Inset is the transmittance spectrum of the AZO film measured at room temperature

Figure 2 .
Figure 2. Chemical structures of the indoline dyes used in the sensitization of ZnO electrodes: a) D149 and b) D205