Elsevier

Physica B: Condensed Matter

Volume 599, 15 December 2020, 412499
Physica B: Condensed Matter

Characterization of CdS films and CdS/Si heterojunctions prepared by ultrasonic spray pyrolysis and their response to light

https://doi.org/10.1016/j.physb.2020.412499Get rights and content

Abstract

In this work CdS thin films were deposited onto glass and p-Si substrates by ultrasonically spraying of precursor solutions prepared in molarity ranging from 0.025 M, to 0.1 M. Structural investigations revealed that all films have hexagonal structure and mean crystallite size was found to be in the range of 18 nm–21 nm. On the other hand, CdS films exhibited 65–70% optical transmittance and band gap energy for all films was found to be about 2.42 eV. Electrical measurements of CdS/Si heterojunctions were carried out under both dark and illumination conditions. Calculated ideality factor and zero-bias barrier height ranged from 3.02 to 2.66 and 0.74 eV–0.78 eV according to TE theory whereas they ranged from 6.91 to 4.73 and 0.71 eV–0.74 eV according to Cheungs’ method. Increase in reverse saturation current when heterojunctions were illuminated indicated that they have good sensitivity to solar light.

Introduction

CdS, which is one of the members of metal chalcogenides, has been investigated for several decades due to its specific properties such as having direct band gap ~2.42 eV at 300 K and n-type conductivity [1,2]. These properties make them suitable material for the several applications such as thin film transistors [3], light emitting diodes (LEDs) [4], photodetectors [[5], [6], [7]], gas sensors [8]. Especially its large band gap and high optical transmittance in visible region makes them prominent partner for CdTe [9], Cu(In,Ga)Se2, Cu(In,Ga)(S,Se)2 [10], Cu2ZnSn(S,Se)4 [11] in thin film solar cells as well as emerging perovskite solar cells [12]. Moreover, CdS enable to achieve 12.29% efficient heterojunction solar by depositing it on p-type c-Si [13]. Therefore, it is considered as key material for solar energy harvesting devices.

CdS can be found in two different phases as to be meta-stable cubic (zinc blende) and stable hexagonal (wurtzite) phase or mixture of them [14,15]. Formation of these phases depend on several deposition parameters but deposition technique may be the most deterministic among the other parameters [16,17]. On the other hand, phase transformation may take place post-thermal treatment as reported by Ziabari and Ghodsi [18]. Deposition of CdS films can be realized by either physical deposition techniques such as thermal evaporation [12], DC magnetron sputtering [13], RF magnetron sputtering [17,19,20] or chemical deposition techniques such as spin coating [9] chemical bath deposition (CBD) [16,21], ultrasonic spray pyrolysis (USP) [2] and successive ionic layer adsorption and reaction (SILAR) [22]. Among these techniques, USP, in which ultrasonic atomizer is utilized to atomize precursor solution, steps forward among others to obtain thin or thick films due to several advantages including being of relatively low-cost deposition technique, does not requiring high quality substrates and enabling to coat substrates with complex geometries [23,24]. Physical properties of spray deposited thin films strongly depend on deposition parameters such as; substrate temperature, precursor solution molarity, solution flow rate, substrate to spray nozzle distance etc. [25,26]. Therefore, in this comprehensive work we aimed to investigate the effect of solution molarity on some physical properties of CdS thin films as well as the device characteristics of n-CdS/p-Si heterojunctions under dark and light conditions.

Section snippets

Experimental details

Prior to deposition of CdS thin films, glass and p-Si wafer used as substrates were cleaned as described in the following. Glass substrates were sonicated in soap water and distilled water (DW) for 15 min, respectively. After that, they were rinsed under running DW and left to dry. For the cleaning of p-Si wafers, RCA cleaning procedure was adopted to remove both organic and metallic impurities and wafers were dipped into HF solution at the final stage of RCA process to remove native oxide

Structural investigations

Structural properties of all films were evaluated by using x-ray diffraction (XRD) patterns given in Fig. 1(a). Diffraction peaks that appear on XRD patterns for all films were assigned to hexagonal structure of CdS (JCPDS card no:41–1049). It is known that crystalline structure of CdS is more stable compared to that of cubic phase under ambient conditions and this poses an advantage for some applications such as solar cells [27,28]. Fig. 1(a) also exhibits that intensity of XRD reflections

Conclusion

In this work, deposition CdS thin films onto both glass and p-Si substrates were realized in order to investigate the effect precursor solution molarity on both physical properties of CdS films and CdS/Si heterojunctions. Structural investigations demonstrated that mean crystallite size increased slightly with the increase in solution molarity. We may also say that the lattice strain values decreased as well. However, lowest strain value was noted for sample S2. Transmittance of CdS film were

CRediT authorship contribution statement

Vildan Bilgin: Project administration, Methodology, Formal analysis, Investigation, Funding acquisition. Emrah Sarica: Methodology, Formal analysis, Investigation, Writing - original draft. Barbaros Demirselcuk: Methodology, Formal analysis, Investigation, Writing - review & editing. Kadir Ertürk: Methodology, Formal analysis, Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by The Scientific and Technological Research Council of Turkey (TÜBİTAK) under the project number 111T057.

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