Study of Fermi level position before and after CdCl2 treatment of CdTe thin films using ultraviolet photoelectron spectroscopy

J Mater Sci: Mater Electron (2016) 27:5039–5046 DOI 10.1007/s10854-016-4391-y Study of Fermi level position before and after CdCl2 treatment of CdTe thin films using ultraviolet photoelectron spectroscopy I. M. Dharmadasa1 • O. K. Echendu2 • F. Fauzi1 • H. I. Salim1 • N. A. Abdul-Manaf1 • J. B. Jasinski3 • A. Sherehiy3 • G. Sumanasekera3 Received: 26 November 2015 / Accepted: 20 January 2016 / Published online: 3 February 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com Abstract The CdCl2 treatment used in the development of high efficiency CdTe solar cells is an essential processing step but remains fully unexplored. What really happens during this treatment is not yet fully understood. The changes in doping concentrations during this processing step are a key parameter to investigate. Determination of the position of the Fermi level (FL) is a good method to explore these changes and therefore photoelectrochemical cell method and ultraviolet photoelectron spectroscopy method have been used to investigate these trends. Four different CdTe layers prepared by electroplating have been used for this investigation. The overall observations indicate the settling down of the FL in the upper half of the bandgap after CdCl2 treatment. 1 Introduction Heat treatment of CdTe thin films in the presence of CdCl2 is a key step in fabricating high efficiency solar cells. Recrystallisation to form large grains and defect passivation have been experimentally observed but the doping effects on the CdTe material have not been examined in detail [1]. The doping effects are key in electronic device performance, and therefore this is an important area needing & I. M. Dharmadasa 1 Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UK 2 Department of Physics, Federal University of Technology, Owerri, P. M. B. 1526, Owerri, Nigeria 3 Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY 40292, USA careful experimentation in order to improve understanding of this crucial processing step. The formation of a photovoltaic active rectifying junction in any solar cell mainly depends on the type (n-type or p-type) of the materials used, and the level of doping concentration of the main absorber layer, CdTe. The formation of a healthy depletion region in the junction mainly depends on the doping concentration. During the past few decades, the CdTe used in solar cell fabrication has been assumed to be p-type and therefore the junction was considered as a simple p–n junction. Since the CdTe material can exists in both n- and p-type electrical conduction forms, it is essential to know the type of the materials used and the doping concentrations, before critical analysis and interpretation of any experimental results. The photoelectrochemical (PEC) and ultraviolet photoelectron spectroscopy (UPS) measurements enable the scientists to find the electrical conductivity type of a semiconducting material. Furthermore, the position of the Fermi level (FL) is an indirect method to find the electrical conductivity type and estimate doping levels of the material. Electronic devices cannot be developed without knowing the electrical conductivity type and doping levels of the materials used. Our most recent work with PEC cell measurements on as-deposited and CdCl2-treated CdTe produced some illuminating results [1, 2]. This work showed that the n-CdTe layers gradually move towards p-type, and p-CdTe layers gradually move towards n-type during the CdCl2 treatment of electrodeposited CdTe layers. This indicates a drastic doping effect, causing the movement of FL across the CdTe bandgap. To confirm this FL movement across the forbidden bandgap, UPS has been used to determine the position of the FL as a function of CdCl2 treatment. The results from PEC and UPS studies are presented and discussed in this communication. 123 5040 2 Experimental 2.1 Growth of CdTe layers by electroplating The CdTe layers used in this work were electrodeposited on glass/FTO (Flourine doped Tin Oxide)/CdS surfaces using aqueous solutions containing Cd-precursor and TeO2 solution. pH value was maintained at 2.00 at the start of the growth and temperature was raised to 85 °C for 2-electrode system and to 70 °C for 3-electrode system. A dilute TeO2 solution was added to the electrolyte at regular intervals in order to maintain a low level of Te ions in the deposition bath. Four different CdTe layers used in this work were grown using CdSO4 [3], Cd(NO3)2 [4] and CdCl2 [5] as precursors for Cd ions. The structures of the layers investigated using PEC cell and UPS were glass/FTO/CdTe and glass/FTO/CdS/CdTe respectively with the thickness of CdTe layer varying in the range 1.50–1.80 lm. The main aim of the UPS work was to examine the position of the FL and hence to explore the doping pattern of CdTe layers during CdCl2 treatment. This information will then lead to finding the most suitable CdTe layers for fabricating CdS/ CdTe thin film solar cells. 2.2 Cadmium chloride treatment As-deposited CdTe layers were CdCl2-treated in two stages in this work, in order to investigate the trend of movement of FL in the bandgap. Saturated CdCl2 solution was diluted to 1 % using de-ionized water. In the first step, CdTe layers were dipped in this solution for 5 min, allowed to dry and then heat-treated at 440 °C for 8 min. In the second step, the same CdTe layers were treated with CdCl2 again in a similar way and heat-treated at 440 °C for 16 min. In this two-stage treatment, temperature was kept constant, but the heat treatment duration was doubled to induce changes within the material. UPS studies were carried out on as- Fig. 1 Typical PEC measurement results for CdTe grown using a sulphate precursor and b nitrate precursor. Note the possibility of growth of n-type and p-type CdTe layers simply by changing the stoichiometry of the materials. The CdTe layers used for UPS measurements were grown close to the stoichiometry points at Vi to obtain crystalline CdTe layers 123 J Mater Sci: Mater Electron (2016) 27:5039–5046 deposited samples and on the CdCl2-treated samples already treated in the two stages mentioned above. In device processing, the CdCl2 treatment has only one stage, but this work has been carried out in two stages in order to observe the trend in FL movement during this treatment. Although not ideal, this two stage treatment was purposely selected in order to observe gradual changes in the FL position. 2.3 Photoelectrochemical cell measurements The photoelectrochemical (PEC) cell measurements were carried out on all electrodeposited layers in order to determine their electrical conductivity type. The CdCl2 treatment was carried out in one stage, very similar to the process carried out prior to device fabrication. CdTe surfaces were treated with 1 % CdCl2 solution, dried and heat treated at 440 °C for 24 min. The PEC si (...truncated)