M. M. Aliyu

Prospects of Back Surface Field Effect in Ultra-Thin High-Efficiency CdS/CdTe Solar Cells from Numerical Modeling

Polycrystalline CdTe shows greater promises for the development of cost-effective, efficient, and reliable thin film solar cells. Results of numerical analysis using AMPS-1D simulator in exploring the possibility of ultrathin, high efficiency, and stable CdS/CdTe cells are presented. The conventional baseline case structure of CdS/CdTe cell has been explored with reduced CdTe absorber and CdS window layer thickness, where 1 μm thin CdTe and 50 nm CdS layers showed reasonable efficiencies over 15%. The viability of 1 μm CdTe absorber layer together with possible back surface field (BSF) layers to reduce minority carrier recombination loss at the back contact in ultra thin CdS/CdTe cells was investigated. Higher bandgap material like ZnTe and low bandgap materials like Sb2Te3 and As2Te3 as BSF were inserted to reduce the holes barrier height in the proposed ultra thin CdS/CdTe cells. The proposed structure of SnO2/Zn2SnO4/CdS/CdTe/As2Te3/Cu showed the highest conversion efficiency of 18.6% (Voc = 0.92 V, Jsc = 24.97 mA/cm2, and FF = 0.81). However, other proposed structures such as SnO2/Zn2SnO4/CdS/CdTe/Sb2Te3/Mo and SnO2/Zn2SnO4/CdS/CdTe/ZnTe/Al have also shown better stability at higher operating temperatures with acceptable efficiencies. Moreover, it was found that the cells normalized efficiency linearly decreased with the increased operating temperature with relatively lower gradient, which eventually indicates better stability of the proposed ultra thin CdS/CdTe cells.

Recent Developments of Flexible CdTe Solar Cells on Metallic Substrates: Issues and Prospects

This study investigates the key issues in the fabrication of CdTe solar cells on metallic substrates, their trends, and characteristics as well as effects on solar cell performance. Previous research works are reviewed while the successes, potentials, and problems of such technology are highlighted. Flexible solar cells offer several advantages in terms of production, cost, and application over glass-based types. Of all the metals studied as substrates for CdTe solar cells, molybdenum appears the most favorable candidate, while close spaced sublimation (CSS), electrodeposition (ED), magnetic sputtering (MS), and high vacuum thermal evaporation (HVE) have been found to be most common deposition technologies used for CdTe on metal foils. The advantages of these techniques include large grain size (CSS), ease of constituent control (ED), high material incorporation (MS), and low temperature process (MS, HVE, ED). These invert-structured thin film CdTe solar cells, like their superstrate counterparts, suffer from problems of poor ohmic contact at the back electrode. Thus similar strategies are applied to minimize this problem. Despite the challenges faced by flexible structures, efficiencies of up to 13.8% and 7.8% have been achieved in superstrate and substrate cell, respectively. Based on these analyses, new strategies have been proposed for obtaining cheaper, more efficient, and viable flexible CdTe solar cells of the future.

Comparative study of ZnS thin films grown by chemical bath deposition and magnetron sputtering

Zinc sulphide thin films have been deposited on FTO coated glass substrates using the sputtering and chemical bath deposition techniques. ZnS thin film is at first grown by CBD using a aqueous solution of thioria, ammonia, ammonium chloride and zinc chloride, respectively; whereas ZnS thin film is also grown from sputtering technique at a substrate temperature 300 °C. The grown films from the both processes were annealed in a vacuum furnace of nitrogen ambient with pressure 250-300 mTorr. A comparative study of structural and optical study of these films was carried out by means of XRD, AFM and UV-Vis spectrometry. It has been found that the films in both processes are polycrystalline in nature having the (200) preferential orientation. The crystallite grain size, lattice constant, microstrain and dislocation densities of the films are quite different in both processes as observed from XRD analysis. Surface structure and topography were observed from the AFM images. The higher r.m.s values of surface roughness as well as bigger grains are observed in CBD grown ZnS thin films. The band gap has been found 3.58 and 3.54eV for the films prepared from CBD and sputtering, respectively.

A numerical analysis on CdS:O window layer for higher efficiency CdTe solar cells

Polycrystalline thin film CdTe solar cell continues to be a leading candidate in the PV research and market because of its cost effectiveness and efficiency. In the conventional CdTe cell, polycrystalline cadmium sulfide (CdS) has been used as the best suited n-type heterojunction partner in the last few decades. This study demonstrates the use of novel CdS:O film as n-type heterojunction partner of CdTe cell, which has higher optical band gap (2.42–3.1eV), better lattice-match with CdTe and reduces the unwanted diffused layers than the poly-CdS layer. This novel CdS:O material is utilized in the baseline case of CdTe cell and a cell conversion efficiency as high as 18.5% (Jsc =26.56 mA/cm2, Voc = 0.95 V and FF=0.8) has been found by numerical analysis utilizing AMPS-1D software. The cell normalized efficiency and Voc are found to decrease linearly at the operating temperature gradient of −0.2%/°C, indicating higher stability of the material at higher operating temperatures.

Effects of thermal annealing on structural and optical properties of sputtered CdS thin films for photovoltaic application

The structural and optical properties of annealed CdS are studied in this work. The CdS films are deposited on ITO coated glasses by sputtering at different substrate temperatures and subsequently annealed in an O2/N2 ambient. It has been observed from XRD diffraction that the films show a trend of conversion from poly crystalline to amorphous or mixed phases after annealing. The films fabricated at room temperature (RT) have been found in complete amorphous form. The surface roughness of the films drastically increased due to thermal annealing observed from AFM images. Optical properties of the films were observed using UV-Vis spectrometer and band gaps of the films were found in the range of 2.80 to 3.08 eV. The annealed films exhibited the blue shift in the direct allowed transition energy band gaps, possibly due to the oxygen incorporation during annealing suggesting the transformation to CdS:O films.

An analysis on structural and optical properties of Zn x Cd 1−X S thin film deposited by RF magnetron sputtering

The compositional, crystal and microstructural, optical transmission and absorption properties of ZnxCd1-xS thin films at low zinc (Zn) content (x=0.2) fabricated by RF co-sputtering using CdS and ZnS were investigated. These ternary compounds were characterized by energy dispersive X-ray analysis (EDX) for composition, X-ray diffraction (XRD) for crystal structure, UV-vis for optical study, forced emission scanning electron microscopy (FESEM) for surface morphology and atomic force microscopy (AFM) for surface topology. XRD patterns of ZnxCd1-xS thin films showed the hexagonal structure with a strong reflection peak of (002) plane. From FESEM, the average grain size smaller than 40 nm was observed. Band gap of the films was determined to be 2.6 eV. The average and root mean square roughness of ZnxCd1-xS (for x= 0.2) were checked by atomic force microscopy (AFM). The surface of the thin films was found to be smoother, homogenous and densely packed with uniform growth.

Influence of RF Power in the Growth of Aluminium Zinc Oxide (AZO) Thin Films by RF Sputtering

Aluminium doped zinc oxide (AZO) is fast becoming an important thin film material for applications as transparent conducting oxide (TCO) in several thin film solar cells, smart windows and many devices using touch screen displays. This is due to its good electrical and optical characteristics as well as lower cost and good abundance. Although sputtering is the general method for industrial fabrication of this material, but film characteristics depend strongly on fabrication processes. Thus, optimal films are obtained by optimization of the deposition conditions. In this work, we investigated the effects of RF deposition power on AZO thin films. Samples of similar thicknesses were grown under similar conditions in an RF sputtering chamber at different RF powers. The samples were then characterized using FESEM, AFM, UV-Vis, XRD and Hall effect measurement tools. Results indicate that the surface morphology is slightly affected with larger grain sizes obtained at higher RF powers. Also the surface roughness, average transmittance, conductivity and deposition rate all increase with the RF power. The lowest as-deposited resistivity of 15.3x10-3 Ω/cm was obtained, at the highest RF power of 100 W. This film also have the highest values of carrier concentration, mobility and figure of merit of 4.24x1020 cm-3, 0.96 cm2/V and 0.27x10-3 Ω respectively. This work highlights the significance of RF power in the fabrication of good quality AZO thin films.

Zn x Cd 1−x S as prospective window layer in CdTe thin film solar cells from numerical analysis

In this work, a conventional structure of CdS/CdTe cells was investigated, where Zn<inf>x</inf>Cd<inf>1−x</inf>S window material that possesses wider bandgap replaced CdS window layer. The effect of composition variation in Zn<inf>x</inf>Cd<inf>1−x</inf>S was analysed for high efficiency cell using AMPS 1D Simulator and the optimal value of x was found to be around 8%. Moreover, to explore the possibility of high efficiency, ultra thin and stable Zn<inf>x</inf>Cd<inf>1−x</inf>S/CdTe cells the CdTe absorber layer thickness was decreased to the extreme limit and 1μm CdTe layer showed reasonable range of efficiency with stability. The thickness of Zn<inf>x</inf>Cd<inf>1−x</inf>S window layer was reduced to 80 nm to increase the overall cell performance with the insertion of ZnO/Zn<inf>2</inf>SnO<inf>4</inf> as the buffer layer. The proposed cell with Zn<inf>x</inf>Cd<inf>1−x</inf>S as window layer showed promising result with an efficiency of 22.42% (Voc = 0.98 V, Jsc = 29.35 mA/cm², FF = 0.85) using As<inf>2</inf>Te<inf>3</inf> as back surface field (BSF) and copper (Cu) as final back contact. The cell normalized efficiency was found to be decreased linearly at the gradient −0.24/C with the operating temperature that indicated better stability of Zn<inf>x</inf>Cd<inf>1−x</inf>S/CdTe solar cells.

Numerical analysis on Zn x Cd 1−x S/CdTe solar cells with different buffer layers, front and back contacts

In this work, a numerical analysis on cadmium stannate (Cd<inf>2</inf>SnO<inf>4</inf>) as front contact, zinc stannate (Zn<inf>2</inf>SnO<inf>4</inf>) as buffer layer and antimony telluride (Sb<inf>2</inf>Te<inf>3</inf>) with molybdenum (Mo) as back contact has been conducted in the conventional (SnO<inf>2</inf>/CdS/CdTe/Ag) CdTe cell structures. Here, CdS window layer is replaced by zinc cadmium sulphide (Zn<inf>x</inf>Cd<inf>1−x</inf>S) aiming to improve efficiency and stability utilizing Analysis of Microelectronic and Photonic Structures (AMPS 1D) simulator. Efficiency as high as 17.0% has been found with 80 nm of Zn<inf>x</inf>Cd<inf>1−x</inf>S window layer for x=0.1, 1 μm of CdTe layer and 100 nm Zn<inf>2</inf>SnO<inf>4</inf> buffer layer without Sb2Te<inf>3</inf> back contact. However, ZnO insertion shows lower conversion efficiencies of 11.84% and 14.26%, respectively with and without Sb<inf>2</inf>Te<inf>3</inf> back contact. It has been found that 1 μm of CdTe absorber layer, 70 nm of Zn<inf>x</inf>Cd<inf>1−x</inf>S (x=0.1) window layer, 100 nm of Zn<inf>2</inf>SnO<inf>4</inf> buffer layer and 100 nm Sb<inf>2</inf>Te<inf>3</inf> back contact layer are sufficient for high efficiency (>18.5%) Zn<inf>x</inf>Cd<inf>1−x</inf>S/CdTe cells. Moreover, it has been found that the cell normalized efficiency linearly decreases with the increasing operating temperature at the temperature gradient of −0.3%/°C proving its stability as others.

Investigation of buffer layers, front and back contacts for Zn x Cd 1−x S/CdTe photovoltaic

A numerical analysis has been performed utilizing Analysis of Microelectronic and Photonic Structures (AMPS 1D) simulator to explore the possibility of higher efficiency and stable Zn<inf>x</inf>Cd<inf>1−x</inf>S/CdTe cells. Several cell structures with indium tin oxide (ITO) and cadmium stannate (Cd<inf>2</inf>SnO<inf>4</inf>) as front contact, zinc stannate (Zn<inf>2</inf>SnO<inf>4</inf>) and zinc oxide (ZnO) as buffer layer and antimony telluride (Sb<inf>2</inf>Te<inf>3</inf>) insertion with Nickle (Ni) as back contact has been investigated in the conventional (SnO<inf>2</inf>/CdS/CdTe/Ag) CdTe cell structures in which CdS is replaced by zinc cadmium sulphide (Zn<inf>x</inf>Cd<inf>1−x</inf>S) as window layer. Efficiency as high as 18.0% has been found with 80 nm of Zn<inf>x</inf>Cd<inf>1−x</inf>S window layer for x=0.05, 1 µm of CdTe layer and 100 nm Zn<inf>2</inf>SnO<inf>4</inf> buffer layer without Sb<inf>2</inf>Te<inf>3</inf> back contact. However, ZnO insertion shows low conversion efficiency of 7.84% and 12.26%, respectively with and without Sb<inf>2</inf>Te<inf>3</inf> back contact. It has been found that 1 µm of CdTe absorber layer, 70 nm of Zn<inf>x</inf>Cd<inf>1−x</inf>S (x=0.05) window layer, 100 nm of Zn<inf>2</inf>SnO<inf>4</inf> buffer layer and 100 nm Sb<inf>2</inf>Te<inf>3</inf> back contact layer are sufficient for high efficiency (>17.5%) Zn<inf>x</inf>Cd<inf>1−x</inf>S/CdTe cells. Moreover, it was found that the cell normalized efficiency linearly decreases with the increasing operating temperature at the temperature gradient of −0.25%/°C.