Efficiency Enhancement of Porous Silicon Solar Cell by Embedding of Some Metal Oxides Nanoparticles

Abstract

In this study, firstly electrochemical etching of the (n and p) type silicon wafers are used to prepare of (n and p) type of porous silicon with a current density of 10 mA.cm-2 at 10 minutes. The structural and morphological of the PSi were studied by using XRD, AFM, FESEM, and FTIR spectroscopy. The XRD results showed that the (n and p) type of PSi are monocrystalline structure with strong, sharp, and narrow single peak, and preferred orientation along (400) plane. AFM has been used to investigate the Average grain size, porosity for porous silicon. Surface morphology of the (n and p) PSi layer are so smooth and homogeneous, and the film consists of matrix of random distribution nanocrystalline silicon pillars which have the same direction. FESEM has been used to study porous silicon layer surface morphology, while FTIR analysis showed that Si dangling bonds of the as-prepared PSi layer having large amount of hydrogen to form weak Si–H bonds. Zinc oxide and lithium oxide nanoparticles are prepared separately by chemical precipitation and simple precipitation methods, respectively and deposited on silicon, FTO, and glass substrates by drop casting method to prepare thin films ZnO (Z3) and Li2O (L1). Moreover, the structural, morphological and optical properties of the films were studied by using XRD, AFM, SEM, and (UV-Visble) spectrophotometer, respectively. 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. The crystallite size was measured using Scherrer's formula, and it was found that ZnO (Z3) and Li2O (L1) thin films have a crystallite size of 22.04 and 45.6 nm respectively. AFM results showed homogenous and smooth thin films. Surface topography of the prepared thin films is studied by SEM. The results of UV-Vis spectra showed that all films have a good absorbance in Visible and near IR region. Consequently, it was chosen as an active absorption layer in applications of solar cell. The optical energy gap for allowed direct electronic transition was calculated using Tauc’s model and it was found that the values of energy gap of the films are 4.90 eV and 5.5 eV, respectively. Finally, certain proportions (0.25:0.75), (0.5:0.5), and (0.75:0.25) of (ZnO:Li2O) were mixed and deposited on porous silicon using drop casting method at thickness of 1.4 μm, then (Al) was deposited as a conductive electrode on the back face of the silicon cell by using thermal evaporation technology. After that, the characteristics of the solar cell and photodetector were investigated. The photocurrent density-voltage (J-V) curves characteristics of fabricated solar cells have been measured under simulated solar light (10 mW/m2), and the parameters including VOC, JSC, FF% and 􀟟% of solar cells were calculated. The results showed that the solar cell fabricated of mixing ZnO (Z3) with Li2O (L1) nanoparticles played a major role in increasing the solar cell's performance. The highest efficiency was obtained at mixing ratio (0.5: 0.5) for (ZnO: Li2O) and its value was (11.09%) for heterojunction (ZnO:Li2O)/PSi/p-Si/Al, and multi-junction based on (FTO/ZnO/Li2O/PSi/p- Si/Al) layer as an active layer recorded higher efficiency of (24%) compared to the other fabricated solar cells. The spectral responsivety measurements for the prepared photodetectors showed that they worked within the range (350-1000) nm, the highest spectral responsivety value for heterojunctions ZnO/PSi/p-Si/Al can be used for photodetector applications within near-infrared and visible spectrum, where the spectral responsivity (Rλ) and specific detectivity (D*) of ZnO/PSi/p-Si/Al photodetectors are about (2.08A/W) and (2.9x1013cm Hz1/2W- 1) respectively, at λ~ 450 nm.