Enhanced efficiency of dye-sensitized solar cells doped with green phosphors LaPO4:Ce, Tb or (Mg, Zn)Al11O19:Eu

We have successfully introduced green phosphors LaPO4:Ce, Tb (G4) or (Mg, Zn)Al11O19:Eu (G2) into TiO2 photoelectrode of dye-sensitized solar cells. The conversion efficiency of the G4-doped device was enhanced by 30% compared with the pristine TiO2 photoelectrode. The green phosphor doped at 5-wt.% ratio contributed to the reduction of resistances of the surface and interface of the photoelectrode and to the great enhancement of the absorption spectrum in UV-visible and near-infrared regions. The internal resistances and absorbance of the photoelectrode directly affect the power conversion efficiency. Green phosphor plays an important role towards the realization of high-efficiency dye-sensitized solar cells.


Background
The uses of different types of nanostructured materials in dye-sensitized solar cells (DSSC) have attracted worldwide attention as a low-cost alternative to traditional photovoltaic device [1][2][3][4][5]. This is because nanostructures of materials enhance the surface area to allow a higher amount of dye molecules to be adsorbed, and the nature of electron transport in oxide nanoparticle films is fairly well understood. The scientific community is still struggling to find optimum nanostructures and materials for the best solution to overcome issues associated with stability, efficiency, and cost-effective mass production [6,7].
Normally, in DSSCs, photons interact with dye molecules to create excitons. These excitons come into contact with nanoparticles/nanostructures at the surface of the photoelectrode and are rapidly split into electrons and holes. Electrons are injected into the photoelectrode, and holes leave the opposite side of the device by means of redox species (traditionally the I − /I 3 − couple) in the liquid or solid-state electrolyte used in DSSCs to ensure efficient electron transfer to the redox couple [8][9][10][11]. It is important to apply different materials and structures to enhance light photon interaction with dye molecules to achieve a higher proportion of excitons.
Recent research shows that the implementation of a UV-absorbing luminescent wavelength converter (which is emitted at longer wavelengths) not only remarkably improves the photochemical stability of the DSSCs but also enhances the efficiency of the cell [3,[11][12][13][14].
LaPO 4 :Ce, Tb (G4) and (Mg, Zn)Al 11 O 19 :Eu (G2) have been widely used in tricolor phosphor lamps and PDP displays as highly effective green phosphor additives [15][16][17][18]. YVO 4 :Bi 3+ , Ln 3+ (Ln = Dy, Er, Ho, Eu, and Sm) phosphors are proposed to be promising UV-absorbing spectral converters for DSSCs as they possess broad absorption band in the whole UV region of 250 to 400 nm and could emit intense visible lights. When excited by ultraviolet light, G4 emits 550 nm of light in the green region. Considering this point, the doping of green phosphors LaPO 4 :Ce, Tb or (Mg, Zn)Al 11 O 19 :Eu into TiO 2 photoelectrodes could lead to higher efficiency in dyesensitized solar cells. Field emission-scanning electron microscopy (FE-SEM) was used to determine the morphology of this hybrid photoelectrode. The absorption and luminescence properties of dye and green phosphor ceramics were investigated using UV spectrophotometry and photoluminescence spectrometry. Electrochemical measurements were used to optimize the weight percentage of fluorescent materials doped in TiO 2 photoelectrode, which had higher conversion efficiency (η), fill factor (FF), open-circuit voltage (V oc ), and shortcircuit current density (J sc ) as a result.

Fabrication of DSSC
TiO 2 powder was thoroughly dispersed for 10 h at 300 rpm using a ball mill (Planetary Mono Mill, FRITSCH, Oberstein, Germany), adding acetyl acetone, poly(ethylene glycol), and a Triton X-100 to obtain a viscous TiO 2 paste. The doped green phosphors were added to the TiO 2 paste and mixed in a ball mill for 2 h. The TiO 2 and green phosphor-doped TiO 2 pastes were coated onto fluorine-doped SnO 2 conducting glass plates (FTO, 8 Ω cm −2 , Pilkington, St. Helens, UK) using squeeze printing technique, followed by sintering at 450°C for 30 min. Approximately 8-to 10-μm-thick TiO 2 film was deposited onto a 0.25-cm 2 FTO glass substrate. Glass-FTO/TiO 2 and phosphor-doped TiO 2 electrodes were immersed overnight (ca. 24 h) in a 5 × 10 −4 mol/L ethanol solution of Ru(dcbpy) 2 (NCS) 2 (535-bis TBA, Solaronix), rinsed with anhydrous ethanol, and dried. A few drops of the liquid electrolyte were dispersed onto the surface, and a full cell assembly was constructed for electrochemical measurements. A Pt-coated FTO electrode was prepared as a counter electrode with an active area of 0.25 cm 2 . The Pt electrode was placed over the dye-adsorbed TiO 2 thin film electrode, and the edges of the cell were sealed with 5-mm wide strips of 60-μm-thick sealing sheet (SX 1170-60, Solaronix). Sealing was accomplished by hotpressing the two electrodes together at 110°C.

Characterization of DSSC
The surface morphology of the film was observed by FE-SEM (S-4700, Hitachi High-Tech, Minato-ku, Tokyo, Japan). A 450-W xenon lamp was used as light source for generating a monochromatic beam. Calibration was performed using a silicon photodiode, which was calibrated using an NIST-calibrated photodiode G425 as a standard.  Results and discussion Figure 1 shows FE-SEM cross-sectional images of a TiO 2 electrode doped with 5 wt.% of G2 (Figure 1a), G2 powder (Figure 1b), and a TiO 2 electrode doped with 5 wt.% G4 (Figure 1c) and G4 powder (Figure 1d). The size of the two green phosphor powder particles varied from 3 to 7 μm without uniformity. These nonuniform microsized structures of the fluorescent powder could create porous and rough surface morphologies on the surface of and within the TiO 2 photoelectrode. However, the maximum doping ratio was 5 wt.%. This type of structure has advantages for the adsorption of a higher percentage of dye molecules and also supports deeper penetration of the I -/I 3 redox couple into the TiO 2 photoelectrode. Figure 2a shows the absorption spectra of a pristine TiO 2 photoelectrode (black curve), a TiO 2 photoelectrode doped with 5 wt.% G2 (blue curve), and a TiO 2 photoelectrode doped with 5 wt.% G4 (red curve). The electrodes listed in the order of active absorption area are G4-doped photoelectrode > G2-doped photoelectrode > pristine TiO 2 photoelectrode. The absorption spectra indicate that more photon energy could be harvested. The effective spectrum ranges from 375 to 900 nm. These spectra cover a UV-visible-IR region. The emission spectra of G2 and G4 are shown in Figure 2b, which was obtained by excitation at 254 nm with the emission line at 517 nm for G2 and excitation at 288 nm with the emission line at 544 nm for G4. To determine the optimal contents of the dopant, optoelectric and electrochemical technology were used. The optimal content of green phosphor was 5 wt.%. Figure 3 shows electrochemical impedance spectroscopy measurements for pristine, G2-doped, and G4doped TiO 2 photoelectrode. In these observations, the Nyquist plots of the impedance characteristics were obtained from the dependence of the real axis resistance (Z re ) and imaginary axis resistance (Z im ) along with the  Photovoltaic properties include open-circuit voltage (V), short-circuit current density (mA cm −2 ), fill factor, power conversion efficiency (%), excitation wavelength (nm), and emission wavelength (nm). angular frequency. The diameter of the first semicircle at middle frequency illustrated in the spectra shows the charge-transfer resistance (R ct ) between the TiO 2 (or doped TiO 2 with G2 and G4) and electrolyte. The bulk resistances (R s ) of the pristine, G2-doped, and G4-doped TiO 2 electrodes are 12.8, 13.7, and 13.4 Ω, respectively. The R ct values of the pristine, G2-doped, and G4-doped TiO 2 electrode devices are 26.3, 21.9, and 19.8 Ω, respectively. In the case of G4-doped TiO 2 devices, smaller R ct means a decrease in interfacial resistance and an increase of energy conversion efficiency. The results show a significant effect on the internal resistance of the solar cell and, consequently, can affect the fill factor and conversion efficiency.
The incident photon-to-current conversion efficiency (IPCE) spectra show the cell of a pristine TiO 2 photoelectrode doped with 5 wt.% G2 and 5 wt.% G4. The pristine TiO 2 photoanode exhibits a maximum IPCE value of 55% at 530 nm, while for the cell with TiO 2 photoanode doped with G2 and G4, the peaks reach 65% and 70%, respectively, as shown in Figure 4. Moreover, an increase of IPCE value in the range of 550 to 650 nm for the cells with doped G2 and G4 photoanodes are observed due to the scattering effect of the G2 and G4 materials, which favor the improvement of J sc for the cell [19]. The increase in J sc with the amount of luminescent powder like G2 and G4 are due to the longer wavelength absorbed which transfers to visible light (550 to 650 nm). Also, larger particle sizes in G2 and G4 powders can extend the light transmission distance, improving incident light harvest and increasing the photocurrent [20].
The photoelectrochemical performance factors such as the FF and overall η were calculated by the following equations: where J sc is the short-circuit current density (mA cm −2 ), V oc is the open-circuit voltage (V), P in is the incident light power, and J max (mA cm −2 ) and V max (V) are the current density and voltage in the J-V curve at the point of maximum power output, respectively. Figure 5 shows J sc versus V oc characteristics of the DSSCs. The photoelectrochemical performance was measured by calculating η. The best conversion efficiency was 7.98% for the G4-doped device with a J sc of 17.8 mA cm −2 , a V oc of 0.67 V, and an FF of 0.67. The pristine TiO 2 and G2-doped device efficiencies were 6.15% and 7.16%, respectively. The open-circuit voltage changed slightly with the insertion of green phosphor, from 0.67 to 0.68 V, while the fill factor changed with the insertion from 0.63 to 0.67, and the short-circuit current changed from 14.3 to 17.8 mA cm −2 . For pristine TiO 2 , η was 6.15%, which increased to 8.0% for 5 wt.% fluorescent powder added to TiO 2 ( Table 1). The effect of different weight percentage ratios of fluorescent powder added to the TiO 2 was also investigated, and 5 wt.% was the optimum ratio. The DSSC with only TiO 2 had lower J sc and V oc because it has a lower proportion of excitons. When the fluorescent powder was added, the number of photons increased and hence increased the probability of photon and dye molecule interactions. Our results suggest that the insertion of green phosphor provides optimal electron paths by reducing the surface and interface resistance, by changing the surface morphology of the electrode. Efficiency was increased by a factor of 2.

Conclusions
In summary, we have successfully introduced a 5-wt.% ratio of green phosphors G4 or G2 into the TiO 2 photoelectrodes of dye-sensitized solar cells. The enhanced percentage of conversion efficiencies of devices doped with G4 or G2 were 30% and 16% with the opencircuit voltages of 0.67 and 0.68 V and the short-circuit currents of 17.8 and 16.5 mA cm −2 , respectively. The fill factors were 0.67 and 0.64, respectively. The 5-wt.% doping ratio of green phosphor contributed to the reduction of the resistances of the surface and the interface of the photoelectrode and enhanced the absorption spectrum in the UV-vis and near-infrared regions. The internal resistances and absorbance of the photoelectrode directly affected the power conversion efficiency. Green phosphor plays an important role towards the realization of high-efficiency dye-sensitized solar cells.