More and more researchers have paid attention to dye-sensitized solar cells since Grtzel introduced the nanoporous semiconductor electrode with large band gap as dye supporter in dye-sensitized solar cell^[1]. The materials,which can be used as the nanoporous electrode for electron transmission,include TiO₂,ZnO,SnO₂,Nb₂O₅ etc. Most attention of researchers was focused on nanoporous TiO₂ thin film. Dye-sensitized TiO₂ nanocrystalline thin film solar cell was regarded as a low-cost and high efficiency solar cell showing a potential application in future^[1]. The studies on choice of dye,liquid electrolyte and various doping in TiO₂ have tended to be mature. ZnO was considered to be a good replacement of TiO₂ in dye-sensitized solar cells on account of its similar band energy (3.3 eV) to that of TiO₂ (3.2 eV) and their analogous structure. Furthermore,electron can be faster transported and more efficiently collected in nanoporous ZnO thin film than in nanoporous TiO₂ thin film. Up to now,nanoporous ZnO photoanode can be prepared by blade coating^[2],mechanical extruding^[3],chemical bath deposition^[4],chemical vapor deposition^[5],low temperature hydrothermal^[6],sputtering^[7],molecular beam epitaxy^[8] and electrochemical deposition method. Compared to the other methods,electrochemical deposition showed economical advantages and versatility in application to various materials in complex shape. The solution phase techniques often allow highly ordered crystal growth when they are properly controlled. The electrochemical deposition can be better controlled in many factors such as temperature,time,electrolyte,stirring,and so on. Especially,one-step electrochemical deposition developed by Yoshida can prepare ZnO/dye hybrid thin film with different microstructures as different dyes were added in the bath solution^{[9,10,11,12,13,14]}.

β-CuSCN is a candidate as a hole transport material because of its hexagonal structure^[15]. Similarly,CuSCN can also be processed by the electrochemical deposition method. The factors to affect the morphology of CuSCN in electrochemical deposition were investigated by several researchers^[16,17].

Here,one-step electrochemical deposition was employed to serially deposit a ZnO/dye hybrid thin film and a CuSCN thin film. Prior to deposition of CuSCN,eosin Y or rhodanine B was sensitized on the nanoporous ZnO thin film prepared by extracting dye from the as-deposited ZnO/dye hybrid thin film. The as-deposited ZnO/dye/CuSCN thin film was covered by a carbon counter electrode to form a solid-state dye-sensitized solar cell with the ZnO thin film as an electron transmission layer,the CuSCN thin layer as a hole transport material and dye as a photosensitizer. 1 Experimental 1.1 Chemicals

ZnCl₂,KCl and LiClO₄ were purchased from Sinopharm chemical reagent,and NaOH,rhodanine B (RB),eosin Y (EY),Cu(ClO₄)₂ and LiSCN were purchased from Sigma-aldrich. Fluorine-doped tin oxide conducting glass (FTO,10 Ω·sq^-1,Nippon Sheet Glass Co.,Ltd. Japan) was ultrasonically cleaned sequentially in deionized water,acetone and isopropanol and finally soaked in isopropanol. The well-cleaned FTO was used as a substrate for deposition of the ZnO/dye hybrid thin film. 1.2 Preparation of nanoporous ZnO thin film

FTO substrates with size of 2 cm×2 cm were activated in the bath by the electrochemical method,which was operated in a three-electrode system with a FTO glass fixed to the rotating disk electrode (RDE) as a working electrode,a Pt wire as a counter electrode and saturated calomel electrode (SCE) as a reference electrode at potential of -1.2 V for 10 min. The rotating speed was controlled at 500 rpm and the temperature was maintained at 70 ℃. The electrolyte was 200 mL KCl (0.1 mol·L^-1) aqueous solution. Then,ZnCl₂ was added into the bath and its concentration was kept at 5 mmol·L^-1. The Pt wire was replaced by a Zn wire to deposit a layer of pure ZnO on the activated FTO sheet at -1.1 V (vs. SCE) for 5 min. At last,EY was poured into the bath and the concentration of EY in the bath was 5 mmol·L^-1. Electrochemical deposition of ZnO/dye hybrid thin film was done for 20 min at the same potential,temperature and rotating speed. O₂ was piped into the electrolyte at a sustainable speed of 100 mL·min^-1 during the whole electrodepositing procedure. The as-deposited ZnO/EY hybrid thin films were rinsed with deionized water and dried in air for 30 min at room temperature. The EY can be completely extracted by immersing the hybrid thin film in 30 mL NaOH aqueous solution (pH=11) for 12 h. The as-desorbed ZnO nanoporous thin film was dipped into 5 mmol·L^-1 EY (or RB) alcoholic solution over night at room temperature for monolayer adsorption of dye. 1.3 Electrochemical deposition of CuSCN on the ZnO nanoporous thin film

The CuSCN thin layer was successively electrodeposited on the dye-sensitized ZnO nanoporous thin film in a three-electrode system with the Pt wire as a counter electrode as mentioned above. The electrolyte contained 10 mmol·L^-1 Cu(ClO₄)₂，5 mmol L^-1 LiSCN and 0.1 mol L^-1 LiCLO₄. The mixed solution of water and ethanol with a volume ratio of 3∶1 was used as solvent of the electrolyte^[18]. Temperature and rotating speed during the electrodeposition was varied in this procedure to change the microstructure of the CuSCN layer and to increase the light-to-electric conversion efficiency of the solid-state dye-sensitized solar cells based on the deposited thin films. 1.4 Characterizations of the deposited thin films

The morphology of the deposited thin film was observed under a field emission scanning electron microscope (SEM,S-4800 Hitachi). The X-ray diffraction (XRD) patterns were collected with a 2θ range from 10 ° to 80 ° at a scanning rate of 8 °·min^-1 (Rigaku D/max-2500,Cu Kα). The photoelectric conversion efficiency was measured with a graphite sheet (1 cm×2 cm) as a counter electrode and the FTO/ZnO/dye/CuSCN as a photo-electrode. The active area of solar cells was fixed at 0.2 cm². The photocurrent-voltage (J-V) curves were measured under illumination with a solar simulator at 1 sun (AM1.5,100 mW·cm^-2). A potentiostat (EG&G Model 273) was used to control the potential and record the current values. 2 Results and discussion

The cathodic electrodeposition of ZnO in the O₂ saturated bath takes place via the following sequence^[19],

The primary reaction is the reduction of O₂ to form OH^- ions which react with Zn²⁺ ions to precipitate ZnO. The reduction of O₂ is limited by the diffusion of O₂ in the bath solution. Therefore,A FTO rotating disk electrode was employed to electrodeposit porous ZnO/dye hybrid thin film in the O₂ saturated solution with addition of EY. To block contact of electrolyte with the FTO substrate as the deposited porous thin film was used as a photo-electrode to fabricate dye-sensitized solar cells,a thin layer of pure ZnO was first deposited in the bath solution without EY prior to deposition of the porous hybrid thin film. An obvious boundary can be observed on the cross section SEM morphology as shown in Figure 1a. The pure ZnO was electrodeposited to be a compact layer with thickness of about 500 nm. After adding EY in the bath,ZnO nanowire began to grow steadily. And orderly c-axis oriented nanoporous ZnO thin film was acquired on the compact ZnO layer^[11]. Figure 2a showed the chronoamperogram measured during the cathodic electrolyses at the active FTO electrode covered with a compact ZnO thin layer in aqueous solution of ZnCl₂ with addition of EY saturated with O₂. At the deposition potential of -1.1 V vs. SCE,The current density for ZnO deposition was limited and a stable current density value of about 1.2 mA·cm^-2 was kept. The red thin film was obtained due to the formation of ZnO/EY hybrid thin film. The COOH in EY molecule as an adsorption group attaches the surface of ZnO particles. The adsorption force depends on the solution properties. In the alkaline solution,the adsorption force of the COO^- group will become weak,and EY will prefer to extract from the film to the solution without destroying the ZnO nanoporous structure. This phenomenon demonstrates EY was deposited on the surface of ZnO crystals. Therefore,the ZnO/EY hybrid thin film can be used as a dye-sensitized thin film electrode to fabricate dye-sensitized solar cells. But,multilayer EY molecules exist in the as-deposited hybrid thin film,which will lead to a very low light-to-electric conversion efficiency. Further,the ZnO porous thin film after extraction of EY can be readsorbed by EY or other kinds of dyes to form the dye-sensitized thin film with an ideal monolayer of dye^[9]. Re-adsorption of dye was done by dipping the porous thin film in dye alcoholic solution. The adsorption force of the COO^- group on ZnO particle surface will be strong in the alcoholic solution,which benefits the adsorption of dye on the ZnO porous thin film. In this study,EY and RB were readsorbed on the ZnO nanoporous thin film for further electrodeposition of CuSCN,respectively.

Fig. 1

Fig. 1

Fig.1 Cross-section SEM morphologies of ZnO nanoporous thin film that EY was extracted (a) and CuSCN thin film electrodeposited on the EY-sensitized ZnO thin film (b)

On the EY-sensitized (or RB-sensitized) ZnO nanoporous thin film,CuSCN was electrodeposited serially. The porous structure of the ZnO thin film can enlarge the contact area between CuSCN and ZnO. The electrodepositing process of CuSCN can be separated into two steps according to the chronoamperogram recorded during deposition of CuSCN as shown in Figure 2b. At the beginning 5 min of electrodeposition,the current density was increased gradually and then kept at a stable value during the next 15 min. CuSCN grain firstly formed and grew inside the microhole of the porous ZnO thin film in priority because of the larger contact area inside porous structure than on the surface. The diffusion of ions inside porous structure is limited leading to a low deposition current density. After filling-in all holes,the current density will become stable. It can be obviously found in Figure 1b that the thickness of the film with electrodeposition of CuSCN for 5 min was approximately equal to that of only ZnO nanoporous thin film. Therefore,CuSCN prefers to grow inside the ZnO hole,and the electrodeposition current density increased gradually at the first 5 min as shown in Figure 2b. Then,CuSCN nanorods appear and come into being close as the depositing time was increased from 5 min to 20 min. The CuSCN nanorod seemed to be thicker and disordered with increasing of the depositing time. It can be regarded as a constant growth along c-axis orientation of CuSCN nanorod.

Fig. 2

Fig. 2

Fig.2 Chronoamperograms measured during deposition of ZnO/EY hybrid thin film on FTO substrate (a) and CuSCN on the EY-sensitized ZnO nanoporous thin film (b). Deposition was carried out with a rotating disk electrode at 500 r/min, ZnO was electrodeposited at -1.1 V (vs. SCE) and 70 ℃,CuSCN was deposited at 0.2 V (vs. SCE) and 25 ℃.

The X-ray diffraction spectrum shown in Figure 3 perfectly matched with that of β-CuSCN crystals (JCPDS card No.29-0581) and the diffraction peaks were marked in the figure. The as-deposited ZnO/dye/CuSCN thin film was demonstrated with a good crystallinity. In β-CuSCN lattice,the (003) plane was vertical to the c-axis and (101) plane were near parallel with slight tilting of 11.48° with respect to the c-axis. The orientation of CuSCN crystal could be evaluated by the peak ratio of (003)/(101) calculating from the relative diffraction peak intensity. The calculated results for CuSCN deposited on the different thin films were collected in Table 1. It was obvious that the peak intensity corresponding to (003) was larger than the others. Thus,CuSCN nanowire grew to be perpendicular to the substrate according to the peak ratio of (003)/(101) in Table 1. It is also apparent that both RB and EY restricted the c-axis orientation growth of CuSCN nanorod.

Fig. 3

Fig. 3

Fig.3 XRD patterns collected for CuSCN deposited on the EY-sensitized ZnO thin film (a),the RB-sensitized ZnO thin film (b) and the ZnO thin film (c),respectively. The CuSCN was electrodeposited for 20 min

Table 1

Table 1

Table 1

Table 1 The diffraction peak ratio of (003)/(101) for CuSCN deposited on ZnO thin films sensitized by different dyes as shown in Figure 3 Dye (003)/(101) Non 16.21 RB 3.45 EY 7.66

Table 1 The diffraction peak ratio of (003)/(101) for CuSCN deposited on ZnO thin films sensitized by different dyes as shown in Figure 3

The as-deposited ZnO/dye/CuSCN thin film was covered by a carbon counter electrode to form a solid-state dye-sensitized ZnO nanocrystalline thin film solar cell with CuSCN as hole transporting materials. The performance of the deposited solar cells was measured under illumination with light intensity of 100 mW cm^-2. Here,the electrodeposition conditions for the CuSCN layer such as the rotating speed of RDE,bath temperature and dye to be sensitized on the ZnO porous thin film were changed to study the effect of these conditions on the photovoltaic performance of the as-deposited solid-state dye-sensitized solar cells. The photovoltaic parameters for the solar cells deposited under different conditions were collected in Table 2. The lower rotating speed in the electrodeposition of CuSCN on RB-sensitized ZnO thin film resulted in a higher short circuit photocurrent (J_sc) and fill factor (ff). At the lower rotating speed,the CuSCN nanorods tend to be more ordered leading to rapid transporting and collecting of holes from dye,which suppresses the recombination of holes with electrons in the ZnO thin film. While the electrodeposited temperature was controlled at 0 ℃,J_sc was increased compared to that of the device based on the CuSCN deposited at room temperature. The increased J_sc contributed to a nearly 1.5 times enhancement of photoelectric conversion efficiency (η). The lower deposition temperature can slow the growth speed of CuSCN crystalline grain and the rods tend to be more compact. The contact area between the counter graphite electrode and the CuSCN layer was increased on this occasion. Both the lower rotating disk speed and the lower temperature can enhance the photovoltaic performance of the as-deposited solar cells. The c-axis orientation of

Table 2 Photovoltaic parameters of solid-state ZnO/dye/CuSCN solar cells with CuSCN deposited under different dyes,rotating speeds and temperatures CuSCN was not more obvious in the ZnO/RB/CuSCN solar cell compared with the ZnO/EY/CuSCN solar cell so that the ZnO/RB/CuSCN solar cell showed a conversion efficiency enhancement although its open circuit photovoltage (V_oc) is low. The difference of action spectrum ranges for EY and RB should be also considered to affect the photovoltaic performance of solar cells. Although the performance of the solid-state dye-sensitized solar cells prepared by the only electrochemical deposition is not high,it can be further increased by improving the interfacial contact between the counter electrode and the CuSCN layer,changing the counter electrode materials,optimizing the microstructure of each layer of the ZnO/dye/CuSCN thin film,as well as the more suitable dye to deposit and sensitize the ZnO porous thin film. These results demonstrated that the reported electrochemical deposition method is a facile and environment-friendly approach to fabricate flexible solid-state dye-sensitized nanocrystalline thin film solar cell. And this approach is also suitable to the future application of dye-sensitized solar cell.

Table 2

Table 2

Table 2

Table 2 Photovoltaic parameters of solid-state ZnO/dye/CuSCN solar cells with CuSCN deposited under different dyes, rotating speeds and temperatures Dye Rotating speed/ (r·min-1) Temperature/℃ Jsc /(mA·cm-2) Voc /mV ff η/ % RB 250 25 0.52 325 0.30 0.05 EY 250 25 0.23 375 0.31 0.03 RB 250 0 1.30 185 0.32 0.08 RB 200 25 1.20 275 0.33 0.10

Table 2 Photovoltaic parameters of solid-state ZnO/dye/CuSCN solar cells with CuSCN deposited under different dyes, rotating speeds and temperatures

Solid-state dye-sensitized solar cells with the structure of ZnO/dye/CuSCN were prepared by the electrodeposition approach. The each step to electrodeposit solar cells can affect the microstructure of each layer. CuSCN crystalline grains firstly grow inside the hole of ZnO nanoporous thin film leading to a good interfacial contact. After filling-in all holes,CuSCN grows along to c-axis orientation,which is beneficial for hole transporting. Factors to influence the microstructure of the as-deposited solar cells were discussed. The lower rotating disk speed and temperature brought to higher photoelectric conversion efficiency. Photoelectric conversion efficiency could be increased as RB was used to fabricate a ZnO/RB/CuSCN solar cell. Poor contact of the CuSCN layer with the counter graphite electrode of the solid-state ZnO/dye/CuSCN solar cells may be the major limitation to the conversion efficiency.

Jingbo Zhang and Lina Sun thank Prof. Yoshida in Yamagata University,Japan for his kind supervision in electrodeposition of ZnO/dye hybrid thin films.