Enhancing Performance of CdS Quantum Dot-Sensitized Solar Cells by Two-Dimensional g-C3N4 Modified TiO2 Nanorods

In present work, two-dimensional g-C3N4 was used to modify TiO2 nanorod array photoanodes for CdS quantum dot-sensitized solar cells (QDSSCs), and the improved cell performances were reported. Single crystal TiO2 nanorods are prepared by hydrothermal method on transparent conductive glass and spin-coated with g-C3N4. CdS quantum dots were deposited on the g-C3N4 modified TiO2 photoanodes via successive ionic layer adsorption and reaction method. Compared with pure TiO2 nanorod array photoanodes, the g-C3N4 modified photoanodes showed an obvious improvement in cell performances, and a champion efficiency of 2.31 % with open circuit voltage of 0.66 V, short circuit current density of 7.13 mA/cm2, and fill factor (FF) of 0.49 was achieved, giving 23 % enhancement in cell efficiency. The improved performances were due to the matching conduction bands and valence bands of g-C3N4 and TiO2, which greatly enhanced the separation and transfer of the photogenerated electrons and holes and effectively suppressed interfacial recombination. Present work provides a new direction for improving performance of QDSSCs.


Background
As one kind of novel solar cells, quantum dot-sensitized solar cells (QDSSCs) have attracted worldwide scientific and technological interest [1]. Basically, the structure of a QDSSC includes photoanode (a layer of porous oxide semiconductor with wide bandgap covered by semiconductor QDs as sensitizers), liquid electrolyte, and counter electrode. Many factors such as morphologies of oxide semiconductors, selection of sensitizers, and counter electrodes et al. could greatly affect the photoelectric conversion efficiency (PCE) of QDSSCs. Therefore, many efforts have been devoted to investigate these factors. Recently, a PCE of 9.01 % was achieved using CdSe 0.65 Te 0.35 quantum dot (QD) as sensitizers [2]. However, the PCE of QDSSC is still far behind its theoretical efficiency, and further researches from different aspects are still required to improve the efficiencies of QDSSCs.
TiO 2 is one of the most important semiconductors as the photoanode material which is the key components in the configuration of QDSSCs. Since the breakthrough work on colloidal TiO 2 based DSSCs by O'Regan and Grätzel in 1991, various TiO 2 nanostructures have been used in QDSSC, like nanoparticles, nanosheet, and nanorod [3][4][5][6][7][8]. Among them, single-crystalline TiO 2 nanorod array would be one of the most desirable nanostructures for preparing photoanode of QDSSC due to its effective charge transfer property as well as excellent light harvesting ability. Inorganic semiconductors QDs such as CdS, PbS, PbSe, CdTe, CdSe, and Bi 2 S 3 have been used to assist as a sensitizer for solar devices [9]. Among them, CdS is considered to be one of the potential photovoltaic semiconductive materials for its broadly tunable bandgap. The combination of wide bandgap semiconductors and CdS QDs can preferably collect the visible light used in photoelectrochemical applications. Scheme 1 Schematic illustration of CdS/g-C 3 N 4 /TiO 2 nanorod photoanode structure Fig. 1 Morphologies of TiO 2 /FTO and g-C 3 N 4 /TiO 2 /FTO photoelectrodes: typical top view SEM images of TiO 2 /FTO photoelectrode (a) and g-C 3 N 4 /TiO 2 /FTO photoelectrode (b); typical cross-sectional view of the well-aligned TiO 2 nanorod array (c) and g-C 3 N 4 /TiO 2 photoelectrode (d); typical TEM image of single TiO 2 nanorod deposited with CdS QDs (10 cycles) (e), and HRTEM of CdS QD decorated TiO 2 nanorod (f) One of obstacles which limit the performance of QDSSCs is the photogenerated carrier recombination. In order to restrain such recombination, introducing a passivation layer such as Al 2 O 3 and ZnS between photoanode and electrolyte would be an effective method [10], which can retard the recombination by partially separating the electrons and electrolyte. Recently, graphitic carbon nitride (g-C 3 N 4 ) has drawn much attention as a metal-free photocatalyst due to high photocatalytic efficiency. [11][12][13] Due to the band structure of g-C 3 N 4 , type II band alignment could be formed between g-C 3 N 4 and TiO 2 , which can significantly prevent the migration of photogenerated electrons from TiO 2 and QDs to the electrolyte [14]. Moreover, introducing g-C 3 N 4 could expand the absorption range of sunlight. Therefore, introducing g-C 3 N 4 into TiO 2 -based photoanodes should improve the performance of QDSSCs.
However, most reports about applications of g-C 3 N 4 are for photocatalysts, and few works for solar cells could be found. Very recently, Wu et al. reported the improved short circuit current of ZnO-based dyesensitized solar cells (DSSCs) using g-C 3 N 4 as multifunctional protecting layer of ZnO particles [15]. Xu et al. reported enhanced PCE of DSSCs using g-C 3 N 4 modified TiO 2 nanosheets [16]. In present work, we investigated the effect of g-C 3 N 4 as both recombination retarding layer and sensitizer on the performance of QDSSC. Single crystal TiO 2 nanorod array was prepared by hydrothermal method on a transparent conducting glass and spin-coated with g-C 3 N 4 , leading to the formation of g-C 3 N 4 /TiO 2 heterostructure. Compared with pure TiO 2 nanorod array photoanodes, the g-C 3 N 4 modified photoanodes showed an obvious improvement in cell performances. The results of I-V characteristic exhibited that introducing g-C 3 N 4 increased both the open circuit voltage and short circuit photocurrent density, and the possible mechanism is discussed.

Materials
FTO glasses were purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd. Acetone, ethanol, hydrochloric acid, and cadmium acetate and methanol were purchased from Beijing Chemical Works. Titanium butoxide was purchased from Shanghai Chemicals. Melamine, Na 2 S, S, urea, and acetic acid were purchased from Aladdin. CuSO 4 was acquired from Tianjin Guangfu Technology Development Co., Ltd. Na 2 S 2 O 3 was purchased from Damao Chemical Reagent, Tianjin.

Preparation of TiO 2 Nanorod Arrays
TiO 2 nanorod array was fabricated according to the previous report [17]. Typically, 15 mL of deionized water was mixed with 15 mL hydrochloric acid. The mixture was then stirred for 15 min followed by the addition of 0.5 mL of titanium butoxide. The mixture was transferred into a 45-mL autoclave. Then, cleaned FTO substrates were put into the autoclave, and the hydrothermal process was conducted at 150°C for 12 h.

Preparation of g-C 3 N 4 Paste
The g-C 3 N 4 was prepared using the method reported previously [18][19][20]. Briefly, 3 g melamine and 4 g urea were mixed in a 20-mL crucible, transferred into a muffle furnace, and heated to and kept at 550°C for 2 h. The yellow crystalline g-C 3 N 4 bulk was obtained and then fully grinded into pale yellow powders. The g-C 3 N 4 paste was prepared by mixing g-C 3 N 4 powders (0.8 g), ethyl cellulose (0.4 g), and α-terpinol (3.245 g) in anhydrous ethanol (8.5 mL) and stirring the mixture for 24 h.

Preparation of CdS/g-C 3 N 4 /TiO 2 Photoanodes
The g-C 3 N 4 paste was spin-coated on the as-prepared TiO 2 nanorod. The as-received g-C 3 N 4 /TiO 2 nanorod photoanodes were subjected to a sintering process in air at 450°C for 30 min. After cooling to room temperature, the photoanodes were decorated with CdS QDs by successive ionic layer adsorption and reaction (SILAR) method [21]. The g-C 3 N 4 /TiO 2 nanorod photoanode was successively dipped in a 0.05 M cadmium acetate methanol solution and a 0.05 M Na 2 S methanol solution each for 30 s. The two-step dipping procedure was termed as one cycle. The illustration of photoanode structure is shown in Scheme 1.

Preparation of CuS Counter Electrodes
CuS counter electrodes were made by chemical bath deposition (CBD) method. One molar CuSO 4 aqueous solution and 1 M Na 2 S 2 O 3 aqueous solution were mixed with the volume ratio of 1:4. The pH of the mixed solution was adjusted to 2 with acetic acid. Then, the FTO glasses were immersed into 100 mL as-prepared mixed solution. The above solution was heated to 70°C and kept for 4 h. After cooling down to the room temperature, the substrates were washed and dried in air and then heated to 130°C and kept for 30 min.

Fabrication of QDSSCs
The as-prepared CdS/g-C 3 N 4 /TiO 2 nanorod photoanode and CuS counter electrode were assembled to a sandwich-type cell and penetrated with a polysulde electrolyte that consisted of 1 M Na 2 S and 1 M S in methanol and H 2 O solution (v/v = 7:3).

Results and Discussion
The morphologies of the as-prepared TiO 2 nanorods and g-C 3 N 4 /TiO 2 nanorods on FTO substrate are shown in Fig. 1. As shown in Fig. 1a, TiO 2 nanorods with high density in the average diameter~100 nm are formed uniformly on FTO substrate. For these nanorods, while the side facets are smooth, the shape of top facets is square and composed of many step edges. These steps are responsible for further growth of the TiO 2 nanorod, and these results show the expected growth habit of the tetragonal crystal. From the cross-sectional image of the sample as shown in Fig. 1c, it is obvious that the wellaligned nanorods are nearly normal to the FTO substrate. The length of the nanorods is about 3 μm. For TiO 2 nanorods capped by g-C 3 N 4 , Fig. 1b shows that discontinuous g-C 3 N 4 layer was coated on the surface of TiO 2 nanorods. These remaining vacancies could assure that CdS quantum dots can be deposited on g-C 3 N 4 as well as TiO 2 nanorods. The cross-sectional view in Fig. 1d indicates that g-C 3 N 4 was successfully coated on the TiO 2 nanorods with the thickness of about 0.8 μm. Figure 1e shows the TEM image of TiO 2 nanorod decorated with CdS QDs for 10 cycles. Compared with bare TiO 2 nanorod, the rough surface could be observed after CdS QD deposition, indicating that large amounts of CdS QDs had been deposited on the TiO 2 nanorods. This is further confirmed by HRTEM image (Fig. 1f ). The lattice fringe space of 0.319 and 0.336 nm corresponds to the (110) plane of tetragonal rutile TiO 2 , and (111) planes of the cubic phase of CdS could be confirmed. Figure 2 shows the XRD curves of FTO substrate, TiO 2 /FTO, g-C 3 N 4 /TiO 2 /FTO, and CdS/g-C 3 N 4 /TiO 2 / FTO, respectively. The XRD result of TiO 2 /FTO exhibits a greatly increased (002) and (101) diffraction, suggesting the vertical growth of highly oriented titania nanorods on FTO, which is consistent with SEM observation. After coating with g-C 3 N 4 , a peak at 27.7°could be observed which is attributed to the typical (002) plane of the g-C 3 N 4 . After the deposition of CdS QDs, the XRD pattern of CdS/g-C 3 N 4 /TiO 2 /FTO shows diffraction peaks corresponding to the hexagonal wurtzite phase of CdS. Figure 3 shows the EDX mapping images of CdS/g-C 3 N 4 /TiO 2 photoanode. The Sn comes from FTO substrate, and O is originated from FTO substrate and TiO 2 nanorods. The same position of S and Cd indicates the CdS QD formation. The position distribution of C and N is similar, indicating the formation of g-C 3 N 4 after spin coating. The EDX results are further confirmed by XPS.
The XPS survey in Fig. 4a exhibits that the existence of C, N, Cd, S, Ti, and O in the CdS/g-C 3 N 4 /TiO 2 photoanode. The Ti 2p3/2 and 2p1/2 centered at 458.1 and 463.8 eV are in agreement with those of pure TiO 2 (Fig. 4b) [22][23][24][25]. The C 1s shown in Fig. 3c has three peaks situated at 284.5, 288.4, and 285.6 eV, which corresponds to sp2 C-C bonds, sp2-bonded carbon in N-C=N, and sp3-bonded carbon species, respectively. For N, three peak signals of N1s located at 398.5, 400.1, and 401.1 eV are present and attributed to sp2 bond N in triazine rings, tertiary N in N-(C)3 units, respectively [26]. These results indicate the presence of graphite-like C 3 N 4 . Moreover, the Cd 3d-related peaks at 404.65 and 411.4 eV are observed and attributed to Cd 3d5/2 and Cd 3d3/2, respectively. The S2p XPS spectra can be separated to two peaks at 161.1 and 162.3 eV which are ascribed to S 2− in CdS [27]. Figure 5 shows the comparison of the FTIR spectra of pure TiO 2 nanorod and TiO 2 nanorod/g-C 3 N 4 . The strong absorption between 500 to 800 cm −1 represents the bonds of Ti-O-Ti in both of the curves [28]. When g-C 3 N 4 sheets are coated on TiO 2 nanorods, several strong bands could be observed in the range of 1200-1700 cm −1 which are typical stretching modes of CN heterocycles [29]. Moreover, the peak at 813 cm −1 is due to variation of triazine units [30]. These absorption peaks once again confirm the existence of C 3 N 4 on the as-prepared TiO 2 nanorod photoanode.
The cell performances are investigated as shown Fig. 6, and corresponding parameters are listed in Table 1. For both CdS/TiO 2 and CdS/g-C 3 N 4 /TiO 2 electrodes, the cell performances with different deposition cycles of CdS QDs are investigated. Both two kinds of electrodes exhibit the best performance with 10 cycles of CdS QD deposition, and the efficiency decreases with further increasing deposition cycles. This is probably due to the excessive deposition of QDs. If the deposition cycles of CdS QDs are more than 10, CdS QD with larger average size would be produced, and the aggregation and convergence among CdS QDs could happen at the surface of g-C 3 N 4 /TiO 2 . The larger CdS QDs would have poor ability to generate multiple excitons, originating from the disappearance of the quantum effect [31]. As shown in Table 1, the measurements of I-V characteristic indicate that the addition of g-C 3 N 4 increases both the open circuit voltage and short circuit photocurrent density. As shown in Fig. 7, the photon-to-current conversion efficiency (IPCE) value is improved after coating g-C 3 N 4 in the range of 300-600 nm. Compared with CdS/TiO 2 electrode, it is worth noticing that the IPCE of CdS/g-C 3 N 4 /TiO 2 electrodes is enhanced obviously between 400 and 500 nm. The maximum IPCE value occurs at 470 nm which is very close to the bandgap of g-C 3 N 4 used in this work. The improvement of IPCE could be due to the synergistic effect of g-C 3 N 4 and CdS QD for sensitizing TiO 2 nanorods.
The mechanism of the performance improvement of QDSSCs in this work is suggested as below. As illustrated in Fig. 8, a type II band alignment between TiO 2 and g-C 3 N 4 could be built due to suitable band structure of g-C 3 N 4 . Therefore, the immigration of photogenerated electrons from the conduction band (CB) of TiO 2 and CdS QDs to g-C 3 N 4 and electrolyte would be restrained. The g-C 3 N 4 layer on TiO 2 nanorods acted as both block layer and effective light absorption layer could effectively promote the electron transport by retarding the backward recombination of electrons from TiO 2 and electrolyte and also contribute additional electrons to increase the electron concentration in the photoanodes, thus to enhance the performance of QDSSCs. Moreover, the synergistic effect of g-C 3 N 4 and CdS QDs for sensitizing TiO 2 nanorods would be the other reason. As shown in IPCE measurement, introducing g-C 3 N 4 will further improve photoelectron injection to TiO 2 particularly in the range of 400-500 nm, which suggests that the existence of g-C 3 N 4 layer will supplement the adsorption of sunlight. The matching conduction bands and valence bands of g-C 3 N 4 and TiO 2 greatly enhanced the separation and transfer of the photogenerated electrons and holes in the composite; thus, the photoelectrochemical performance of the g-C 3 N 4 /TiO 2 electrode is improved.

Conclusions
In summary, we introduce two-dimensional g-C 3 N 4 layer in the single crystal TiO 2 nanorod array photoanode.  Compared with pure TiO 2 nanorod array photoanodes, the g-C 3 N 4 modified photoanodes showed an obvious improvement in cell performances, and a champion efficiency of 2.31 % was achieved, giving 23 % enhancement in cell efficiency. The improved performances were due to the matching conduction bands and valence bands of g-C 3 N 4 and TiO 2 , which greatly enhanced the separation and transfer of the photogenerated electrons and holes and effectively suppressed interfacial recombination. Present work provides a new direction for improving the performance of QDSSCs.