Spinel zinc ferrite nanostructured thin-films for enhanced light-harvesting in polycrystalline solar cells

Global energy consumption is increasing rapidly day by day and the energy produced from fossil fuels may not be sufficient for future requirements [1]. Solar energy is a significant source of renewable energy and an extremely successful renewable resource due to its economic efficiency, enhanced energy performance and ability to be used in a variety of places. Polycrystalline silicon solar cells show less power conversion efficiency (PCE) due to reflection loss and light to heat conversion. One of the main aspects involved in order to achieve high-performance silicon solar cells is to reduce optical losses and increase light transmittance and trapping characteristics. Several techniques were used to increase the PCE and one such effective technique is the use of anti-reflection coating (ARC) [2, 3].

Recent studies reveal that magnetic nanoparticles can significantly improve the PCE of solar cells [4]. Magnetic oxide nanoparticles [M(II) Fe(III)₂O₄ where M represents Co, Mn, Ni, Zn or Fe] have attracted considerable interest due to its wide range of applications [5]. Spinel ferrites possessing the cubic structure are described by the formula AB₂X₄, where A and B indicate tetrahedral and octahedral cation sites, respectively, in an face centred cubic (FCC) anion (oxygen) sub-lattice. The zinc ferrite (ZnFe₂O₄) has a regular spinel structure with Zn²⁺ ions in the A-site and Fe³⁺ ions in the B-sites [6] Among the spinel ferrites, ZnFe₂O₄ attracts great attention as it is a promising n-type semiconductor photocatalyst with 1.9 eV bandgap [7]. Spinel ferrites are widely used in different applications such as magnetic storage media [7], microwave devices [8], gas sensors [9], wastewater treatment [10], biomedical applications [11, 12], nanoelectronics [13], ferrofluids [14], catalysts [15], batteries [16], but only a few works have been carried out for PCSSC solar cells [17].

Properties of the ferrites are affected by particle size, cation distribution, morphology, synthesis and fabrication methods. ZnFe₂O₄ can be synthesized using different methods such as sol-gel technique [18,19,20] co-precipitation [21], hydrothermal [22], solvothermal [23], ball milling [24], and ceramic route [25]. Among them, co-precipitation is a promising technique for ferrite processing due to low production cost, controllable particle size, reactivity, enhanced uniformity and purity [26]. Spin-coating is a commonly used technique for optical coatings to manufacture extremely reproducible film thickness [27]. Single-layer coating on solar cells would not be sufficient to reduce reflection. Therefore, multilayer coating of ARC is often preferred for further reduction of reflection loss [28,29,30].

This present research work focuses on using the spinel ZnFe₂O₄ nanocrystallites as a light harvesting ARC material for enhancing the PCE of polycrystalline silicon solar cells (PCSSC). The ZnFe₂O₄ spinel nanocrystallites were synthesized using co-precipitation method, which are deposited on the PCSSC by using spin coating technique. The structural, optical, electrical behaviour of spinel ZnFe₂O₄ NTF covered and uncovered PCSSC were investigated under controlled atmospheric condition.

The XRD pattern of the as synthesized ZnFe₂O₄ sample calcinated at 510 °C is shown in Fig. 2. From the XRD pattern, it is observed that the obtained diffraction peaks can be clearly indexed with the spinel face centred cubic symmetry and the crystallographic peaks are well matched to the standard powder diffraction data, JCPDS Card No. 82-1042. The obtained Miller indices (220), (311), (400), (422), (511), (440) are substantially indexed with the spinel ZnFe₂O₄ structure. The most significant peak at the plane (311) was used to measure the approximate size of crystallite using Scherrer formula. The Miller indices and position of diffraction peaks of as synthesized spinel ZnFe₂O₄ are in accordance with the earlier report [31].

Fig. 2

The crystallite size D was calculated using the Debye-Scherrer equation as follows: (1) D=0.9λβcosθ D = {{0.9\lambda } \over {\beta \cos \theta }} where, λ is the wavelength of X-ray, β is the Full Width at Half Maximum (FWHM) value of diffraction peaks and θ is the Bragg diffraction angle. It can be seen from Table 1 that all the diffraction peaks typically imitates the presence of nanocrystallites with size range of 40 to 104 nm. The efficiency of the PCSSC were increased for the nanocrystallites having higher size. The increase in output was followed by a major shift in short circuit current density (Jsc), with the difference in size.

Table 2 displays the XRF analysis of as synthesized spinel ZnFe₂O₄ nanocrystallites and shows their chemical compositions. As seen in Table 2, the chemical composition in the sample is well agreed with the elemental compositions of spinel ZnFe₂O₄. The atomic and molecular percentage of zinc and iron oxide confirms the formation of spinel zinc ferrite with chemical composition of Zn₁Fe₂O₄. Further, it can be seen from XRF analysis that the obtained spinel ZnFe₂O₄ nanocrystallites has 99.5% purity with very low quantity of other metal oxides including MnO, CaO, CuO, NiO, Sc₂O₃ and Cr₂O₃.

FTIR spectra is used to analyse the presence of metal oxides lattice vibrations in high energy fingerprint region as well as the organic functional bonding vibrations at low energy region. FTIR spectra as a function of transmittance (%) versus wavenumber (ν⁻) is displayed in Fig. 3. The transmittance bands at around 559 and 696 cm⁻¹ indicates the lattice vibrations of the metal oxide ion composed of tetrahedrally coordinated Fe-O bond of the regular spinel ferrites whereas the band at around 400 cm⁻¹ is associated with the octahedrally coordinated Fe-O bond lattice vibrations [32].

Fig. 3

The characteristic FTIR band at 3385 cm⁻¹ indicates the presence of water (O-H stretching frequency) molecules adsorbed on the surface of ZnFe₂O₄ nanocrystallites. The band at 2917 cm⁻¹ is due to the stretching vibration of C-H bond and the characteristic bands present at 1631 cm⁻¹ is due to the stretching frequency of C=C bonds. It can be revealed from FTIR analysis that the synthesized spinel ZnFe₂O₄ nanocrystallites has adsorbed carbon residue after calcinations and it is due to the usage of precursor ethanolic medium.

The surface topography, morphology and coating thickness of multilayer spinel ZnFe₂O₄ nanocrystallites coated PCSSCs were demonstrated using three dimensional (3D) AFM topography images. Fig. 4 exhibits the 3D AFM images of the layer by layer assembly of spinel ZnFe₂O₄ nanocrystallites modified PCSSC: a) single layer coating, b) double layers coating, c) triple layers coating and d) quarter layers coating. It can be seen from AFM analysis that the surface thickness of the L1, L2, L3 and L4 are found to be 103 nm, 205 nm, 260 nm and 431 nm respectively. The thickness and average particle size of the spinel ZnFe₂O₄ nanocrystallites assembly is increased due to the formation of multiple layer coating as well as the presence of Zn²⁺ ions in ferrite (Fe₂O₃) matrix influences the growth of particle size during the curing process [3]. AFM observation clearly indicates the formation of thin film growth processes with more aggregation of hard segments. Nano-micro phase separation can be observed in ZnFe₂O₄ nanocrystallites coated PCSSC with higher contents of ZnFe₂O₄ groups.

Fig. 4

The morphology and elemental composition of spinel ZnFe₂O₄ nanocrystallites coated (L3 Layer) PCSSC were measured using FE-SEM coupled with EDAX. The FE-SEM and EDAX spectra of the three layer (L3) coated PCSSC were displayed in Fig. 5(a) and Fig. 5(b). It can been observed from FE-SEM analysis that the spinel ZnFe₂O₄ nanocrystallites are uniformly assembled as cylindrical structure on PCSSC. The elemental composition of the layer three (L3) coated PCSSC was measured using EDAX at room temperature. Fig. 5(c) presents the elemental composition of spinel ZnFe₂O₄ nanocrystallites coated (L3 layer) PCSSC and confirms the presence of elemental Zn, Fe and O with trace amount of Ni. The percentage values obtained for Zn, Fe, Si, Ni and O elements are shown in Fig. 5(c). It is noted that the corresponding atomic mass ratio of the spinel ferrites are well aligned with the prepared stoichiometric ratio. The EDAX spectra and elemental composition (Fig. 5b & c) supports the presence of ZnFe₂O₄ nanocrystallites in surface coating on PCSSC.

Fig. 5

Fig. 6 and 7 depicts the transmittance and reflectance of ZnFe₂O₄ samples with various film thicknesses. All the spinel ZnFe₂O₄ nanocrystallites coated samples rendered better transmittance, higher than 85%. The transparency gradually increases and reflection decreases from L1 to L3 deposition while reverse effect on reflection was observed in L4 layer deposition due to the aggregation of crystallites.

Fig. 6

Fig. 7

This also demonstrates the importance of thickness on reflection characteristics of spinel ZnFe₂O₄ nanocrystallites coating. It can be confirmed from Fig. 6 and 7 that the layer thickness influences the optical transparency as well as anti-reflection capacity of spinel ZnFe₂O₄ nanocrystallites functionalized silicon solar cells. It can be noted that L4 not only lost its transparency, but it also increased its reflection properties and reduced the anti-reflection behaviour due to the aggregation of spinel ZnFe₂O₄ nanocrystallites in coating microstructure.

Fig. 8 shows the resistivity of the spinel ZnFe₂O₄ nanocrystallites, which is measured by using Four Probe technique. The L3 layer coating on the PCSSC shows minimum resistivity of 3.0 × 10⁻³Ω cm compared to the reference (uncoated) PCSSC resistivity of 6.8 × 10⁻³Ω cm (Fig. 8). Further, the slight increase in the electrical resistivity of the L4 layer may be due to the change in crystalline size without any change in crystal orientation based on the thickness effect [3, 4].

Fig. 8

Fig. 9 presents the I–V characteristics of the spinel ZnFe₂O₄ nanocrystallites coated and uncoated PCSSC with the dimensions of 52 mm × 32 mm. The I–V characteristics were performed using a solar simulator under a closed-loop light stabilization system with the simulated 1.5 AM global spectrum. It clearly shows that the increased layers of spinel ZnFe₂O₄ nanocrystallites coating increases the open circuit voltage (Voc), short circuit current (Isc), fill factor and the efficiency η (%), as shown in the Table 3. Three layers of spinel ZnFe₂O₄ nanocrystallites deposited PCSSC demonstrates relatively higher Isc and Voc with enhanced power conversion efficiency (PCE) than the reference (uncoated) and other coated PCSSC.

Fig. 9

Nanocrystalline spinel ZnFe₂O₄ was prepared from precursors (ZnCl₂ and FeCl₃·6H₂O) through simple co-precipitation method. The prepared spinel ZnFe₂O₄ nanocrystallites were comprehensively characterized by XRD, XRF and FTIR spectroscopy. Spin coating technique was used to fabricate multilayer spinel ZnFe₂O₄ nanocrystallites coatings on PCSSC. The microstructure, morphology, composition of deposited thin films was characterized by AFM, FESEM and EDAX analysis. The transmittance, reflectance, electrical, light harvesting and solar energy conversion properties of developed thin film coatings on PCSSC was demonstrated using UV-visible spectroscopy, four probe technique and IV characterization respectively. The present research outputs confirm that the developed spinel ZZnFe₂O₄ nanocrystallites thin films accommodate the transmittance of 90.1% and the reduction in reflectivity of 20.3% with improved light trapping and harvesting efficiency of 17.5%. Remarkably, 20% enhanced PCE of PCSSC has been achieved by deposition of L3 layer coated (260 nm thickness) spinel ZnFe₂O₄ nanocrystallites thin films on PCSSC. Hence, it is evident that the spinel ZnFe₂O₄ nanocrystallites act as a suitable light harvesting and anti-reflection material for PCSSC.