Construction of NiMoS3/BC heterostructure photoanodes and optimization of light scattering to improve the photovoltaic performance of dye sensitized solar cells (DSSCs)

In the last two decades, dye sensitized solar cells (DSSCs) have gotten a lot of attention from researchers and have progressed quickly. To promote commercialization and large-scale application of DSSCs, their eciency should be increased. This paper details signicant advancements in advanced NiMoS 3 /BC nanocomposites for improving photoanodes and DSSC conversion eciencies. The fabricated electrode samples were characterized by XRD, SEM, TEM, Raman, UV, PL and BET to explore the structural, morphological and optical properties. A signicant reduction band gap with enhanced light absorption and rapid prevention of electron hole pair was explored by UV-DRS and PL studies. The photocurrent density-voltage (J-V) and IPCE characteristics were analyzed for assembled solar cell. The NiMoS 3 /BC (NMSC5) nanocomposite DSSC showed a PCE of 8.85%, far higher than that of the NiMoS 3 (2.45%) and a PCE value equivalent to Pt CE (4.79 %). The enhanced PCE of the proposed electrodes are also discussed in scientically.


Introduction
Dye-sensitized solar cells have received signi cant attention in their potential applications although it is a challenge to achieve high power conversion e ciency (PCE). The development of highly active non-noble metal electro catalysts is essential for the successful renewable energy system [1]. In order to achieve a strong current density and to catalyze cyclic regeneration in the electrolytes, the counter electrode should simultaneously have outstanding electrocatalytic activity and good electrical conductivity as a critical component of the DSSCs. The counter electrode in DSSCs was used widely in platinum excellent electrocatalytic performance toward redox electrolytes [2]. However, low reserves and high platinum costs limit DSSCs' large-scale business use and huge opportunity [3]. Therefore, low cost electrode materials are important to improve the electrochemical performance of DSSCs in order to develop effectively [4][5][6].
For example, different types of carbons [7][8][9][10], inorganic compounds [11][12][13], composites [14] and metals [15], etc. Because of the adequate electrical conductivity, the strong stability and the low cost of carbon material, the e cient counter electrode DSSCs is considered to be a strong competitor [16]. Among many carbon material, BC has the unique one-dimension dendritic structure that could represent as a conducting electron transportation channel has attracted attention [17][18][19].
Transition metal sul des (TMSs) are regarded as an excellent alternative to precious metals with great optic, electric, magnetic and catalytic characteristics [20][21][22][23]. The TMSs have been attracted much attention as a counter electrode material because of its excellent redox properties and high conductivity [24][25][26][27]. It is worth nothing that some TMSs have demonstrated excellent electric properties with large speci c surface areas and unique structures, such as nanosphere [28], ower structures [29], nanorods [30] and nanotubes [31]. Metal sul de has a high performance catalytic but low conductivity and structural instability, as most reported metals-based materials [32]. As a result, in the sense of developing low-cost and high-e ciency hybrid materials photocatalysts, strengthening the support for TMSs on BC by creating an e cient electronic transfer network merits special attention in both the basic and technical elds. However, carbon can increase the electronic conductivity of metal sul des signi cantly [33][34][35].
The covering of a carbon layer on the NiMoS 3 surface with BC as a carbon source has been reported to signi cantly improve NiMoS 3 electrical conductivity. The carbon-coated nickel molebdinum sul de NiMoS 3 /BC combines the bene ts of two material, namely the high catalytic activity of the TMSs and the good carbon conductivity, simultaneously with the structure of the shell. In the meantime, it can signi cantly improve its cycle stability by introducing a thin carbon layer in the NiMoS 3 surface [36]. NiMoS 3 /BC has been a promising material for solar energy collection devices like thin-lm photovoltaics for the past decade. The increased absorption and wide surface area of NiMoS 3

Synthesis of NiMoS 3 /BC nanocomposites
The bio carbon was synthesized from aloe vera extract and dried at 80 o C for 12 h. The impurities in the aloe peels were washed away with water and ethanol before being dried at 105°C for 24 hours. After that, obtained substance was crushed into a powder with a particle size of 2.5 m. The resultant solution was transferred to an autoclave and kept at 120 o C for 24 hours after 5 g of aloe peel powder was mixed with 50 ml of DI water and vigorously stirred for about 30 minutes. The nal product was annealed at 105 o C for 24 hours after the hydrothermal reaction was completed. It was then annealed at 800°C for 5 hours in a nitrogen atmosphere before being used. Hydrothermal method has been used to prepare the NiMoS 3 /BC nanocomposites. Initially, PAN (2 g), 1 mmol NH 4 ) 6

DSSC fabrication setup
The present solar cell device is sandwich type, which is consists of photoanode, cathode and electrolyte solution, respectively. The photoanode and cathode of DSSC is fabricated using NiMoS 3 /BC and commercial platinum (Pt) paste deposition on FTO substrate using doctor blade method [19].

Characterization techniques
X-ray diffraction (XRD) (Japan, XD-3A) using CuKα radition and a scintillation counter detector were used to identify the crystalline structure of the samples. The morphological and optical properties of the samples were analyzed by Hitachi SU8010 type SEM instrument, H-600-II, Hitachi type TEM analytical instrument, UV-Vis spectrophotometer (UV-2550, Shimadzu, Japan). The electronic structure and chemical composition of the sample were determined by XPS (AXIS-165 Shimadzu, Japan). Autolab Potentiostate ECO CHEMIE was used to monitor the electrochemical performance.

Structural analysis
XRD patterns were analyzed in depth to clarify the crystal structure of NiMoS 3 /BC composite samples. Figure 1 shows the powder XRD pattern of pure BC, NiMoS 3 and NiMoS 3 /BC samples, respectively. Since the NiMoS 3 /BC pattern reveals a prevalent diffraction peak at 26 ° that can be indexed to the (002) carbon plane (see in Fig. 1). The well crystalline cubic structure of NiMoS 3 was con rmed with respective pattern and the ndings are well coincide with standard pattern (Joint Committee on Powder Diffraction Standards card no. 73-1508). Further, Raman spectra were taken to identify the crystalline phase and the corresponding spectra is shown in Fig. 2. BC pattern clearly expose the two band like D and G with equivalent wave number of 1356 and 1588 cm − 1 , respectively. The E 2g and A 1g Raman mode of NiMoS 3 was positioned at 365 and 410 cm − 1 is due to the crystalline phase of cubic structure [37,38]. Both BC and NiMoS 3 Raman modes are presented in the composite samples is con rming the successful incorporation BC in the NiMoS 3 . Figure 3 shows the SEM and TEM images of the samples which were taken to identify the morphological characteristics of the samples. Both SEM and TEM images clearly expose the sheets like BC and spherical shaped NiMoS 3 . In the composite samples the uniform spherical of NiMoS 3 was disported uniformly of the surface of BC nanosheets. In the high-resolution TEM (HRTEM) images, lattice with a spacing of 0.29 nm, which can be assigned to the (002) plane of NiMoS 3 .

Optical properties
UV-Vis absorption spectra and room temperature photoluminescence a spectrum was carried out to know the ability of absorption and recombination of charge carries. Figures 4 and 5 shows the UV and PL spectra of the samples. The absorption clearly falls in the range of 380-440 nm. Using K-M model [39,4 0] the estimated band gap energies are 3.26-2.81 eV. The broad emissions were found at 440-480 nm in the PL spectra with the excitation wavelength of 350 nm using He-Ne laser source. The effective recombination of electron-hole pair and chare carriers were found in the spectra due to the reduced PL emission intensity in the composite samples.

Surface area analysis
In order to analyse the porosity of the samples, the N 2 -adsorption/desorption tests were performed as shown in Fig. 6. The curve indicates standard type IV isotherm hysteresis as per the description of the Global Alliance of Pure and Applied Chemistry [41,42]. Much of the data points come under the 2-50 nm range, suggesting that the composites of the NiMoS 3 /BC have a good number of mesoporous.

Solar cell charectertics
As described before, the DSSCs of the sandwich type were manufactured and studies of J-V characteristics were assessed. Figure 7 displays the graphic image of the assembled unit. For DSSCs possessing Pt, NiMoS 3 and NiMoS 3 /BC nanocomposites, Fig. 8 elucidates the J-V curves. The NiMoS 3 /BC (NMSC5) nanocomposite DSSC showed a PCE of 8.85%, far higher than that of the NiMoS 3 (2.45%) and a PCE value equivalent to Pt CE (4.79 %). The corresponding incident photoelectron current emission (IPCE) spectra also illustrate that NMSC5 electrode shows high IPCE value than compared with other photoanode materials (Fig. 9). The overall photovoltaic parameter values are estimated for J-V curves and the values are summarized in Table 1. Figure 10 demonstrates the graphic illustration of the electrode photo-conversion mechanism. The improved BC photo anode level makes it much easier to supply the charged particles within the NiMoS 3 and lessens recombination as well. According to our analysis, the deterioration of photovoltaic productivity demonstrated signi cantly greater diffusion rate and higher recombination effect in the electron transport assessment. Good NiMoS 3 /BCNT performance compared to the bare NiMoS 3 provides better electron conductivity among NiMoS 3 and BC, however. The photo-generated electrons e ciencies are also affected by better dispersion of N719 zirconium dye with carbonyl group.

Conclusions
In this report, pure NiMoS 3 and composite with various amounts of bio carbon hybrid materials were synthesized by facile hydrothermal method. The well crystalline cubic structure of NiMoS 3 was con rmed with respective pattern and the ndings are well coincide with standard pattern (Joint Committee on Powder Diffraction Standards card no. 73-1508). Both SEM and TEM images clearly expose the sheets like BC and spherical shaped NiMoS 3 . In the composite samples the uniforma spherical of NiMoS 3 was desperted uniformly of the surface of BC nanosheets. NiMoS 3 /BC electrode shows high surface area (98.3 m 2 /g) and pores (12.9 nm) than compared with bare NiMoS 3 (39.4 m 2 /g and 23.4 nm). The NiMoS 3 /BC (NMSC5) nanocomposite DSSC showed a PCE of 8.85%, far higher than that of the NiMoS 3 (2.45%) and a PCE value equivalent to Pt CE (4.79 %). The improved BC photo anode level makes it much easier to supply the charged particles within the NiMoS 3 and lessens recombination as well.