Enhanced photoelectric conversion efficiency of dye-sensitized solar cells by the incorporation of flower-like Bi 2 S 3 : Eu 3 + sub-microspheres

In this paper, TiO2-Bi2S3 and TiO2-Bi2S3:Eu composite photoanodes were successfully designed, which can not only fully absorb visible light but also transfer the electron from Bi2S3 to TiO2 conduction band due to the narrow band gap and high conduction band of Bi2S3. Compared to pure TiO2 cell, the photoelectric conversion efficiencies of TiO2-Bi2S3 and TiO2-Bi2S3:Eu composite cells were increased significantly. In addition, the efficiency of TiO2-Bi2S3:Eu composite cells were higher than that of TiO2-Bi2S3 cell which could be attributed to the larger BET surface area of Bi2S3:Eu. The electron transport and interfacial recombination kinetics were investigated by the electrochemical impedance spectroscopy and intensity-modulated photocurrent/photovoltage spectroscopy. The results indicated that the interfacial resistance of the TiO2-dye|I3/I electrolyte interface of TiO2-Bi2S3:Eu composite cell was much bigger than that of pure TiO2 cell. In addition, the TiO2-Bi2S3:Eu cell has longer electron recombination time and longer electron transport time than pure TiO2 cell. The charge collection efficiency of TiO2-Bi2S3:Eu composite cell was higher than that of pure TiO2 cell.

Scientific RepoRts | 6:23395 | DOI: 10.1038/srep23395 As a result, if one can design down-conversion luminescent TiO 2 -Bi 2 S 3 :Eu 3+ composite photoanodes, not only the utilization of visible light can be improved but also the electron can transfer from Bi 2 S 3 to TiO 2 conduction band (see Fig. 1). And thus, the efficiency of the solar cells can be enhanced. In addition, metal ions doping semiconductor is also an effective strategy to improve the photocatalytic performance. Based on the consideration above, we report the synthesis of flower-like Bi 2 S 3 :Eu 3+ through a hydrothermal route, and introduce Bi 2 S 3 :Eu 3+ to the dye-sensitized solar photoanodes. The BET surface areas increased with increasing Eu 3+ concentration. The photoelectric conversion efficiencies of TiO 2 -Bi 2 S 3 and TiO 2 -Bi 2 S 3 :Eu 3+ composite cells were significantly increased compared to pure TiO 2 cell. The electron transport and interfacial recombination kinetics of cells were investigated in detail. Figure 2(a-d) represent the typical SEM images of the Bi 2 S 3 products with different Eu 3+ concentrations, which show that the products are composed of flower-like nanostructures. The average diameter of these superstructures is about 500 nm. The TEM and HRTEM images are presented in Fig. 2(e,f). The HRTEM image reveals that the interplanar spacing of 0.36 nm corresponds to the (130) plane of Bi 2 S 3 . Figure 3 shows the XRD patterns of Bi 2 S 3 nanocrystals (without annealing) with different reaction time, which are in good agreement with the standard data of orthorhombic phase Bi 2 S 3 (JCPDS 17-0320). No other impurity peaks were detected. Figure 4 shows the XRD patterns of Bi 2 S 3 nanocrystals after annealing at different temperatures. It can be seen crystalline size increases with increasing the annealing temperature. The peaks in Fig. 4 marked by asterisk (*) arise from cubic phase Bi particles (JCPDS 44-1256). The other diffraction peaks can be indexed to the orthorhombic phase Bi 2 S 3 . Figure 5 shows the XRD pattern of Bi 2 S 3 :Eu 3+ nanocrystals (without  annealing) with different Eu 3+ concentrations. Obviously, no other impurity peaks were detected with increasing Eu 3+ concentration. Figure 6 shows the Raman spectra of Bi 2 S 3 :Eu 3+ with different Eu 3+ concentrations. The typical features in Raman spectra were located at 129 cm −1 , 610 cm −1 and 965 cm −1 .The 610 and 965 cm −1 bands are assigned to the Bi-S stretching vibrations. The 129 cm −1 is attributed to the surface of the optical phonon modes 29 .    In order to investigate the effects of TiO 2 -Bi 2 S 3 :Eu 3+ on the photoelectric properties of DSSCs, the DSSC prototype devices were fabricated by using N719-sensitised TiO 2 -Bi 2 S 3 :Eu 3+ composite electrodes. Figure 8 Table 1. The result indicated that the photoelectric conversion efficiencies of the TiO 2 -Bi 2 S 3 and TiO 2 -Bi 2 S 3 :Eu 3+ composite cells were higher than that of pure TiO 2 cell. The best photoelectric conversion performance was observed when the mass concentration of Bi 2 S 3 :Eu 3+ was 3%. The high Voc of the TiO 2 -Bi 2 S 3 could be attributed to heavy doping effects. Heavy impurity doping makes the conduction and valence bands shift, and brings about the so-called Band Gap Narrowing that resulting in the decrease of open circuit voltage. Figure 8(b) shows the incident photon to current (IPCE) spectra of pure TiO 2 , TiO 2 -Bi 2 S 3 , and TiO 2 -Bi 2 S 3 :Eu 3+ composite cells. The results indicated that the photon-to-current conversion efficiency obviously increases by the incorporation of Bi 2 S 3 :Eu 3+ . With the increase of the proportion of Bi 2 S 3 :Eu 3+ in TiO 2 -Bi 2 S 3 :Eu 3+ cell, the efficiency increases first, and then decreases. At low concentrations of Bi 2 S 3 :Eu 3+ , the increase of the efficiency with the proportion of Bi 2 S 3 :Eu 3+ could be attributed to the narrow bandgap and higher conduction band of Bi 2 S 3 , which not only improve the utilization of visible light but also transfer the electron from Bi 2 S 3 to the conduction band of TiO 2 . However, the incorporation of Bi 2 S 3 :Eu 3+ can influence the electrical conductivity of TiO 2 and lead to a decrease in photocurrent. In addition, the effects of pure Bi 2 S 3 on the photoelectric properties of DSSC were also studied. The results indicated that the photoelectric conversion efficiency of TiO 2 -Bi 2 S 3 cell was lower than that of TiO 2 -Bi 2 S 3 :Eu 3+ cell.  It is well known that the photoelectric performance was closely related to the ratios of the surface areas of samples. N 2 adsorption-desorption isotherms and the corresponding BJH pore size distribution plots of the as-obtained Bi 2 S 3 :Eu 3+ with different Eu 3+ concentrations were performed to determine the surface area of the samples, as shown in Fig. 9. The BET surface areas are 4.8435, 5.4181, 6.6296, and 7.1739 m 2 /g for 0%, 10%, 15%, and 20% Eu 3+ , respectively.

Discussion
EIS is a powerful method to investigate internal resistances for the charge-transfer process of DSSCs. The wide frequency range of EIS means that it can measure wide-scale internal resistances of each electrochemical step at the same time 30,31 . DSSCs are complex systems which are composed of several interfaces. A high level of electron accumulation must occur because photogenerated electrons are not extracted immediately at the electrode contact under illumination. Generally, the impedance at low frequency (0.05-1 Hz) refers to the Nernst diffusion of I 3 − /I − within the electrolyte. The impedance at high frequency (1-100 kHz) corresponds to the capacitance and charge-transfer resistance at the Pt | I 3 − /I − electrolyte interface. The medium-frequency response at 1 Hz-100 Hz is related to the photoelectrode-dye|I 3 − /I − electrolyte interface, where the accumulation of photoelectrons and redox shuttles is expected 32,33 . Figure 10 shows the EIS of pure TiO 2 cell and TiO 2 -Bi 2 S 3 :Eu 3+ cell. It can be seen   that the interfacial resistance of the TiO 2 -dye|I 3 − /I − electrolyte interface of TiO 2 -Bi 2 S 3 :Eu 3+ cell is much bigger than that of pure TiO 2 cell.
The inset in Fig. 10 shows the equivalent circuit fitting of the impedance spectra, R s [C 1 (R 1 O 1 )](R 2 CPE), which was used for all the DSSCs. R s is the series resistance, corresponding to the sheet resistance of the FTO glass, the contact resistance and the wire resistance. R 2 represents the charge transfer resistance between the photoelectrode-dye | I 3 According to the equivalent circuit, the EIS data obtained by fitting the impedance spectra of composite DSSCs are listed in Table 2. It can be seen that R 2 , representing the interfacial resistance of the TiO 2 -dye | I 3 − /I − electrolyte interface, is 30.31 Ω for pure TiO 2 cell and 41.30 Ω for TiO 2 -Bi 2 S 3 :Eu 3+ composite cell. It is noted that the lower interfacial resistance can result in higher interfacial electron transfer, which is a beneficial factor for enhanced photoelectric conversion efficiency. In addition, the series resistance (R S ) for pure TiO 2 cell and TiO 2 -Bi 2 S 3 :Eu 3+ cell are separately 31.53 Ω and 41.26 Ω, indicating that the incorporation of Bi 2 S 3 :Eu 3+ is not beneficial for the interfacial electron transfer of FTO|TiO 2 .
In DSSCs, the electron recombination time (τ n ), the electron transport time (τ d ), and the charge collection efficiency (η cc ) are important factors for the performance of DSSCs. Time-resolved photoluminescence spectrum can be used as an effective method to characterize the interface electron transport and electron recombination of the solar cell 34,35 . But restricted by the conditions, we can not test Time-resolved photoluminescence spectrum. However, the IMVS and IMPS are also a kind of effective characterization methods, which can be used to characterize the transmission life, charge separation and recombination of interface electrons. The IMPS response plots and IMVS response plots of pure TiO 2 cell and TiO 2 -Bi 2 S 3 :Eu 3+ composite cell are shown in Fig. 11. Compared with pure TiO 2 cell, the TiO 2 -Bi 2 S 3 :Eu 3+ composite cell has longer electron recombination time and longer electron transport time. It noted that longer transport time can result in poorer photoelectric properties, while longer recombination time is beneficial for enhancing photoelectric properties.
The charge collection efficiencies (η cc ) of DSSCs are determined by the relation: η cc = 1 − τ d /τ n . Where, τ d is a charge transport time and τ n is a charge recombination lifetime. Figure 12 shows the charge collection efficiencies of pure TiO 2 cell and TiO 2 -Bi 2 S 3 :Eu 3+ cell. TiO 2 -Bi 2 S 3 :Eu 3+ composite cell has a higher charge collection efficiency than pure TiO 2 cell. All these results indicated that the performance of the solar cells can be improved by adding Bi 2 S 3 :Eu 3+ .
In summary, flower-like Bi 2 S 3 :Eu 3+ was successfully prepared by a facial solvothermal method. The obtained Bi 2 S 3 :Eu 3+ was chosen to design TiO 2 -Bi 2 S 3 :Eu 3+ composite photoanodes. The result indicated that the photoelectric conversion efficiency were enhanced greatly by the incorporation of Bi 2 S 3 :Eu 3+ . The best photoelectric conversion performance was observed when the mass concentration of Bi 2 S 3 :Eu 3+ was 3 wt%. The result of EIS analysis revealed that the interfacial resistance of the TiO 2 -dye | I 3 − /I − electrolyte interface of TiO 2 -Bi 2 S 3 :Eu 3+ composite cell was much bigger than that of pure TiO 2 cell. In addition, the TiO 2 -Bi 2 S 3 :Eu 3+ composite cell exhibited longer electron recombination time, longer electron transport time, and higher charge collection efficiency than   Table 2. Parameters obtained by fitting the impedance spectra of composite solar cells using the equivalent circuit in the inset of Fig. 9.
those of pure TiO 2 cell. Of course, the enhancement of the efficiency of the TiO 2 -Bi 2 S 3 :Eu 3+ composite cells was also related to the larger BET surface areas of Bi 2 S 3 :Eu 3+ .

Methods
Synthesis of flower-like Bi 2 S 3 nanocrystals. In a typical experiment, Bi(NO 3 ) 3 , CH 4 N 2 S and Eu(NO 3 ) 3 were separated add to ethylene glycol (10 ml), and the solution was thoroughly stirred. Subsequently, the solution was transferred to a 50 ml Teflon-lined autoclave for 12 h at 180 °C. After cooling to room temperature, the final products were collected by means of centrifugation, washed with ethanol, dried at 80 °C in air and then annealed at different temperature.  37 . The dye-sensitized photoanode was assembled with a Pt counter electrode into a sandwichtype cell. The sandwich-type cell was further fixed together with epoxy resin.The space between the electrodes was filled with the electrolyte, which comprised 0.   (γ = 1.541874 Å), keeping the operating voltage and current at 40 kV and 40 mA. The size and morphology of the final products were investigated by scanning electron microscopy (SEM, Hitachi, S-4800) and transmission electron microscopy (TEM, JEOL, JEM-3010). UV-Vis absorption spectrum were determined by a UV-Vis spectrophotometer (Shimadzu UV-2550, Tokyo, Japan). The Raman spectra were measured by a HORIBA JOBIN YVON LabRam-HR 800 micro-Raman spectrometer.

Fabrication of photoelectrodes.
Photovoltaic properties study. Photovoltaic measurements were carried out with a solar simulator (Oriel, USA) equipped with an AM 1.5G radiation (1 sun conditions, 100 mW cm −2 ) filter was used as the light source. Current-voltage (J-V) curves were measured with a BAS100B electrochemical analyzer (Zahner Elektrik, Germany). The area of DSSCs is 1.5 cm 2 and the irradiation area is 0.09 cm 2 with a light intensity meter. The photoanode of Bi 2 S 3 :Eu 3+ films were fabricated in the same condition. To make the data strictly and scientifically, all the cells was test for at least 5 times then obtained an average value. The EIS were performed with a computer-controlled IM6e impedance measurement unit (Zahner Elektrik, Germany) and carried out by applying sinusoidal perturbations of 10 mV with a bias of − 0.8 V at a frequency ranges from 10 mHz to 1 MHz. The obtained spectra were fitted with ZsimpWin software in terms of appropriate equivalent circuits. The electron transport and recombination properties were measured by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) (Zahner Elektrik, Germany). The DSSCs were probed through the photoanode side by a frequency response analyzer using a white lightemitting diode (wlr-01) as the light source. The frequency range was 0.1-1000 Hz. The irradiated intensity was varied from 30 to 150 mW cm −2 .