Effect of Zinc Oxide on the E ciency Enhancement of Al/Li2O/PSi/Si/Al solar cell


 In this paper, electrochemical etching of the p-type silicon wafer is used to prepare p-type porous silicon with current density of 10 mA.cm− 2 for 10 minutes. Field Emission Scanning Electron Microscopy (FESEM) has been used to study porous silicon layer surface morphology. Zinc oxide and lithium oxide nanoparticles are prepared separately by chemical precipitation method and simple precipitation method, respectively and deposited on glass substrates by drop casting method. Moreover,, the structural properties of the films were analyzed by using XRD and SEM. The XRD results showed that the ZnO and Li2O films are polycrystalline with hexagonal wurtzite structure and cubic structure, and preferred orientation along (101) and (003) planes, respectively. Using Scherrer's formula, the crystallite size was measured and it was found that ZnO and Li2O thin films have a crystallite size of 22.04 and 45.6 nm respectively. Surface topography of the prepared thin films is studied by using Scanning Electron Microscopy (SEM). Later, certain proportions of both materials were mixed and deposited on porous silicon using drop casting method at thickness of 1.4 µm. After that, the characteristics of the solar cell were investigated. Mixing zinc oxide nanoparticles in particular proportions with lithium oxide played a major role in increasing the solar cell's performance. The highest prepared film efficiency was obtained at mixing ratio (0.5: 0.5) for (ZnO: Li2O) and its value was (11.09 %).


Introduction
Since the rst practical solar panels were demonstrated more than forty years ago, single or multicrystalline silicon solar cells have been the mainstay of the photovoltaic (PV) industry. About 85% of the 90 MWP of worldwide PV module shipments in 1996 were based on crystalline silicon [I]. Visible light emission measurement of porous silicon (PS) has shown that quantum size effects can in uence the optical properties of silicon dramatically [2]. Recently, the exponential increase in the energy absorption coe cient in porous silicon samples has been due to the crystallite size distribution and the change in the optical transitions oscillator strength with the con nement of the crystallites [3].
Because porous silicon lms can be formed by relatively simple procedures, new solar-cell structures incorporating porous silicon can conceivably be developed at low cost. An early application of porous silicon, which focused on minimizing the optical losses in single-and multi-crystalline Si solar cells, was reported fteen years ago [4]. After the formation of the diffused p +n junction, the anodic dissolution process for the processing of PS was carried out. By preparing a very thin (~100 nm) and highly porous layer, which allowed the deposited contact metal to reach the base silicon, the re ectance reduced from 37% to 8% [5].
The current-voltage characteristic of multi-crystalline solar cells indicate that the PS coating, short-circuit current, and open-circuit voltage improved, whereas the series resistance, ll factor, and shunt resistance were slightly in uenced. A recent study has demonstrated that porous Si increases both the performance of conversion and the stability of non-aqueous solution Si photocathodes [6]. The increased conversion e ciency has been due to the role played by the PS layer at the Si substrate/solution interface in decreasing charge recombination. Furthermore, by blocking the creation of an oxide layer at the interface, the porous layer stabilizes the Si photocathode from photo-oxidative corrosion. In another study, optical losses were minimized by electrochemical etching a 10 lj, m-thick porous layer on single-crystal Si substrates and treating this cell in an HF-based solution [7,8]. Investigations into the use of porous silicon in silicon solar cells have shown that PS/Si solar cells exhibit an improvement in conversion e ciency (about 25-30) % compared to a cell without a PS layer. At the same time, the e ciency of PS layered silicon solar cells is greater than that of traditional ARC silicon solar cells [9]. Ag-induced chemical etching with low surface re ectance (<5)% of multicrystalline silicon solar cells containing nanoporous black silicon, results in a major e ciency improvement of (26) % [10]. The manufacture of nanoporous structures on screen-printed silicon solar cells using wet chemical etching with sizecontrolled silver nanoparticles shows less than (5)% re ectance and (15.7)% e ciency [11]. One of the main reasons for improving PS/Si solar cell e ciency is the low value of effective re ectance (about 1-3) % for nanoporous silicon layer that signi cantly reduces optical losses. A wide-band gap of nanoporous silicon (up to 1.9 eV) leads to the widening of the cell's spectral photosensitivity area to the ultraviolet portion of the solar spectrum and promotes the performance of PS-layered silicon solar cells to increase.
In addition, the PS layer acts as a luminescence down converter that transforms blue solar light into redorange light, producing additional pairs of electron-hole pairs [12]. The use of porous silicon Bragg mirrors on the back of silicon solar cells will improve performance [13]. The passivation and properties of Si-H and Si-O bonds on pore surfaces are also of high signi cance since they can increase the lifetime of minority carriers [14,15]. In the present work, the mix (ZnO:Li 2 O) lm was synthesized by the use of dropcasting method. The properties of prepared (ZnO, Li 2 O) lms were each studied separately. In addition, the solar cell was prepared by using different mix ratio of (ZnO:Li 2 O) nanoparticle on the porous silicon.
This study focuses on the effect of mixing (ZnO:Li 2 O) nanoparticles when deposited on porous silicon to improve the conversion e ciency of porous silicon(p-type) solar cells.

Experimental Work
Fabrication of porous silicon p+ silicon wafer (5 Ω/cm 2 , 500 μm), from (Bioanalyse, Turkey) with orientation of (100) was used as a substrate. It was rst washed by ultrasound bath twice with acetone and methanol. HF (40%) was then diluted at a 1:10 ratio to distilled water (DI), and the surface was engraved to eliminate any remaining particles on the Si-surface. Porous silicone was prepared by electrochemical etching of Si wafer surface.
Instead of a mixture of HF (45%) and absolute ethanol, cleaned Si was put on the bottom of the Te on cells with ratio 1:1. A gold ring was used within 10 min as an electrode with a current density of 10 mA.cm -2 . After that, the porous silicon was soaked in distilled water before it dries by nitrogen gas [16], as seen in Figure1.
Preparation of nanoparticles a. ZnO nanoparticles ZnO NPs were prepared by chemical precipitation method. Zinc chloride (ZnCl 2 ) and sodium hydroxide (NaOH) were used as precursors and Poly (vinyl chloride) (PVC) as a stabiliser. Using a standard procedure, 13.6 g of ZnCl 2 (Central Drug House (P) India, 97.0%) solution was prepared in 100 ml of DI water and kept under constant stirring at 75 °C for complete dissolution, which shows transparency.
Then, the desired amount (25 ml) of NaOH (1 M) and 0.5 g of PVC (Sigma Aldrich USA, 99.9%) was used during a typical transaction. At the end of the reaction, the solution was allowed to settle, and the supernatant layer was poured off and washed with double distilled water and ethyl alcohol. The washing procedure was done several times in order to remove the residual impurities present in the sample. The nal white products were dried at 500°C in a hot air oven for 1 hour to obtain nanosized ZnO powder particles.  reports [19]. The favorite direction is presented in Table (I). The diffraction peaks of the prepared thin lm show hexagonal wurtzite structure. Accordingly, the XRD results revealed that the lm obtained in this study consisted of a pure (ZnO) phase without any secondary phases. Figure 3( Figure 4(a) shows semi-spherical shapes of zinc oxide whose dimensions do not exceed 253 nm whileFigure 4(b) consists of sheetsof lithium oxide whose dimensions do not exceed 1 μm.
Field Emission Scanning Electron Microscopy (FESEM) Figure 5 represents FESEM images of p-type porous silicon. The image reveals that electrochemical etching has been effective in preparing the porous surface of the silicon wafer. Also, it can be observed that the pores distribution is irregular. The pores indicate that silicon's surface area is increased [27].  and reverse direction. The forward current of solar cells is very weak at voltages lower than 1.8 V. This current is referred to as a recombination current which exists only at low voltages. This is created when the conductive band is excited by each electron to form the valence band. The second high voltage region represents the diffusion or recombination region, which depends on the resistance of the series. The bias voltage will deliver electrons with su cient energy in this eld to penetrate the barrier between the two sides of the junction. The measured open-circuit voltage (V oc ), short-circuit current density (J sc ), ll factor (FF %) and e ciency ( ) were calculated and depicted in Table (II). The ndings suggest that the mixture of unique proportions of zinc oxide and lithium oxide improves the performance of the ZnO solar cell as well as the absorption e ciency over a greater fraction of the solar spectrum. A catalyst's photo activity is driven by its ability to create electron-hole pairs that are photo generated. It has been noted that ZnO's solar energy conversion e ciency is in uenced by its optical absorption ability, which is related to its wide band energy difference. As a photo catalyst for the solar-driven photo degradation process of persistent organic pollutants, ZnO nanostructures have been shown to be a possible candidate. This is due to its low cost of production, non-toxicity and ability to absorb larger fractions of the solar spectrum [31]. It was observed that Li generates additional holes at the Zn substitution site, while it generates additional electrons at the interstitial site. It can be hypothesized that Li serves as a defect mediator in ZnO NPs [32,33].
Li has the smallest group-I element ion radius, which is very similar to that of Zn (0.74 Å) and is a signi cant factor in obtaining high p-type ZnO optical e ciency. Li ions also act in signi cant types of Zn sites as shallow acceptors. However, it can conveniently hold an interstitial position (Li) due to the limited ionic radius of the Li-ion, which can serve as a donor. In general, the combination of ZnO with Li-ions boosts Egopt's value over that of pure ZnO lms [33]. Lithium itself does not induce any visible emission of luminescence, although the relative concentration of inherent defects varies with increasing doping percentage. This will assist in tuning the emission linked to the intrinsic defect. These types of defect in ZnO NPs with the Li provide e cient use of materials in the visible light. New oxide-based catalytic materials are particularly suitable for environmentally sustainable processes and for the reduction in manufacturing costs of modern catalysis [34]. This study represents an excellent premise to obtain less expensive photo catalysts using ZnO and Li 2 O to build heterogeneous structures to increase charge separation e ciency by creating photo catalysts with high e ciency and improved response capacity to visible light [35]. All results indicate that the sandwich structure of the ZnO:Li 2 O/p-type PSi could be used as a solar cell.
[36].  39], the reason is to achieve good lateral conduction of electrons and holes through the material. Also, the electrochemical environment, the physical location (depth below surface) of the pn junction, and the thickness of the porous silicon lm proper were suitable for accomplishing different role of electrochemical etching conditions in the growth of (PSi) layers. In this study, higher e ciency was recorded compared to the references [37,40] because mixing has been shown to be very effective in improving the device's working function and electrical conductivity, resulting in e cient separation and collection of electron-hole pairs in solar cells that are photo-induced.   Figure 1 Electrochemical etching set-up schematic diagram.