The electrical properties of n-CdS/p-CdTe and n-ZnS/p-CdTe heterojunctions fabricated by a combination of SILAR and vacuum deposition techniques

Abstract

The paper reports successful fabrication of CdS/CdTe and ZnS/CdTe heterojunctions by a combination of two different deposition techniques viz. successive ionic layer adsorption and reaction (SILAR) and conventional vacuum deposition. The SILAR deposited CdS and ZnS layers formed well adherent and stable heterojunctions with the vacuum deposited CdTe. The electrical characteristics of the heterojunctions were analyzed by thermionic emission model and Cheung's model. The ideality factor, series resistance, barrier height, space charge density and thickness of the depletion region were determined by rigorous I–V, J-V and C–V characterization. The barrier heights of the devices were found to be in the range of 0.8 eV–0.9 eV. The thickness of the depletion region was found to be 5.2 nm and 7.1 nm while the space charge density was found to be 3.6 × 10²² m⁻³ and 1.59 × 10²¹ m^-3, respectively, for CdS/CdTe and ZnS/CdTe heterojunctions.

Introduction

At present, the single crystal and polycrystalline silicon solar cells have dominant market share in the photovoltaics industry [1]. In reality, silicon is not an ideal material for solar cells. The quantum efficiency of silicon is low due to its indirect bandgap. Besides, due to the relatively high manufacturing costs of single crystal silicon, the Si based solar cells have remained largely beyond the reach of consumers in developing countries. In the last few decades, CdTe based thin film solar cells have gained significant market share due to their relatively lower cost [2]. The bandgap of CdTe is optimum for absorption in infrared and visible region of the spectrum with relatively higher quantum efficiency. In a typical CdTe thin film solar cell, a p-type CdTe layer is paired with an n-type window layer (such as CdS, ZnS, ZnSe, CdSe etc.) to form a heterojunction [[2], [3], [4]]. The polycrystalline CdTe thin films are relatively less expensive than their single crystal counterparts. Since a typical thin film layer has a thickness of less than a micron, the material cost per cell is also relatively low. In addition to these inherent advantages, the cost of the CdTe thin film solar cells can be further reduced if the window layers (CdS or ZnS) are deposited by chemical methods instead of the high-end, vacuum-assisted deposition techniques [[4], [5], [6], [7], [8]]. Besides, the CdTe thin film solar cells have been found to be suitable for both terrestrial and space applications as they have higher stability against the radiation damage which may be caused by the cosmic rays [[5], [6], [7]]. The chemical techniques such as chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) methods need very simple instrumentation and are ideal for large scale device fabrication at relatively low cost. The CBD technique has been employed previously to deposit window layer of the solar cells [[9], [10], [11], [12], [13]]. However, not enough attempts seem to have been made to use the SILAR technique even though it is much more refined and efficient than the conventional CBD. In CBD, all the chemical precursors are added to a single chemical bath where the reactions take place. This often leads to significant material wastage as the material precipitates on the walls of the container. However, in SILAR, the precursors containing the positive and negative ionic species are taken in separate vessels to minimize the precipitation and material wastage [14,15]. In the present research work, the suitability of SILAR technique to form CdTe/CdS and CdTe/ZnS heterojunctions has been investigated.