The effect of CdCl2 treatment on the structural properties and electrical conductivity types of ZnTe thin films

This paper reports the successful surface treatments of p- and n-ZnTe thin film semiconducting layers prepared by electrodeposition technique. The surface treatment was carried out by using a chlorine precursor which is CdCl2. The initial experiment carried out by applying CdCl2 treatment to ZnTe layers of approximately 1200 nm showed that the CdCl2 treatment did not introduce additional Cdrelated phase to the ZnTe layers as revealed by the X-ray diffraction (XRD) measurements carried out on glass/FTO/p-ZnTe and glass/FTO/n-ZnTe mono-layers. The electrical conductivity type as determined from photo-electro-chemical (PEC) cell measurement also showed that after applying CdCl2 aqueous solution for the treatment of p- and n-ZnTe layers grown on conducting glass substrates, the type remains unchanged.Keywords: p-ZnTe, n-ZnTe, mono-layers, CdCl2 surface treatment, structural, electrica

Growth and characterisation of n- and p-type ZnTe thin films for applications in electronic devices

Growth and characterisation of n- and p-type ZnTe thin films forapplications in electronic devicesO.I. Olusolaa,b,*, M.L. Madugua, N.A. Abdul-Manafa, I.M. DharmadasaaaElectronic Materials and Sensors Group, Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, United KingdombDepartment of Physics, School of Science, The Federal University of Technology, FUTA, Akure P.M.B. 704, Nigeriaarticle infoArticle history:Received 8 July 2015Received in revised form16 October 2015Accepted 4 November 2015Available online 7 November 2015Keywords:Electrodepositionn-type ZnTep-type ZnTeIntrinsic dopingZnTe homo-junction diodeabstractThe growth of n- and p-type ZnTe thin films have been achieved intrinsically by potentiostatic elec-trodeposition method using a 2-electrode system. Cyclic voltammogram have been used to obtain rangeof growth voltages required to form stoichiometric thin films of ZnTe. The ZnTe thin films have beenelectrodeposited (ED) on glass/fluorine-doped tin oxide (FTO) conducting substrates in aqueous solutionsof ZnSO4$7H2O and TeO2. The films have been characterised for their structural, electrical, morphological,compositional and optical properties by using X-ray diffraction (XRD), Raman spectroscopy, Photo-electrochemical (PEC) cell measurements, DC conductivity measurements, Scanning electron microscopy(SEM), Atomic force microscopy (AFM), energy-dispersive X-ray analysis (EDX) and Optical absorptiontechniques. The XRD results reveal that the electroplated films are polycrystalline and have hexagonalcrystal structure with the preferred orientation along (002) plane. UVeVisible spectrophotometer hasbeen used for the bandgap determination of as-deposited and heat-treated ZnTe layers. The bandgap ofthe heat-treated ZnTe films are in the range (1.90e2.60) eV depending on the deposition potential. PECcell measurements show that the ED-ZnTe films have both n- and p-type electrical conductivity. The DCconductivity measurements revealed that the average resistivity of n-ZnTe and p-ZnTe layers of equalthickness is of the order of 104Ucm; the magnitude of the electrical resistivity of p-ZnTe is almost fivetimes greater than that of the n-ZnTe layer. Using the n- and p-type ZnTe layers, p-n homo-junctiondiodes with device structure of glass/FTO/n-ZnTe/p-ZnTe/Au were fabricated. The fabricated diodesshowed rectification factor of 102, reverse saturation current of ~10.0 nA and potential barrier heightgreater than 0.77 eV indicating electronic device quality of these layer

Effect of thickness: as case study of electrodeposited CdS in Cds/CdTe based photovoltaic devices

The effect of electrodeposition technique on CdS thickness incorporated in CdS/CdTe-based solar cell has been investigated using all-electrodeposited g/FTO/n-CdS/n-CdTe/p-CdTe multilayer device configuration. The optical, morphological and structural properties of the electroplated CdS were investigated for CdS thicknesses between 50nm and 200 nm. The observed CdS bandgap ranges between 2.42 and 2.46 eV. The morphological analysis shows full coverage of underlying g/FTO substrate for all CdS thicknesses except for the 50 nm which shows the presence of gap in-between grains. The structural analysis shows a preferred orientation of H(101) for all the CdS thicknesses except the 50 nm thick CdS which shows either a weak crystallinity or an amorphous nature. The fabricated solar cell shows a maximum conversion efficiency of ~11% using CdS thickness ranging between 100 and 150 nm. These results show that although low CdS thickness is desirable for photovoltaic application, the effect of nucleation mechanism of deposition technique should be taken into consideration

Investigating the electronic properties of multi-junction ZnS/CdS/CdTe graded bandgap solar cells

The fabrication of multi-junction graded bandgap solar cells have been successfully implemented by electroplating three binary compound semiconductors from II-VI family. The three semiconductor materials grown by electroplating techniques are ZnS, CdS and CdTe thin films. The electrical conductivity type and energy bandgap of each of the three semiconductors were determined using photoelectrochemical (PEC) cell measurement and UV-Vis spectrophotometry techniques respectively. The PEC cell results show that all the three semiconductor materials have n-type electrical conductivity. These two material characterisation techniques were considered in this paper in order to establish the relevant energy band diagram for device results, analysis and interpretation. Solar cells with the device structure glass/FTO/n-ZnS/n-CdS/n-CdTe/Au were then fabricated and characterised using current-voltage (I-V) and capacitance-voltage (C-V) techniques. From the I-V characteristics measurement, the fabricated device structures yielded an open circuit voltage (Voc) of 670 mV, short circuit current density (Jsc) of 41.5 mAcm-2 and fill-factor (FF) of 0.46 resulting in ∼12.8% efficiency when measured at room temperature under AM1.5 illumination conditions. The device structure showed an excellent rectification factor (RF) of 104.3 and ideality factor (n) of 1.88. The results obtained from the C-V measurement also showed that the device structures have a moderate doping level of 5.2×1015 cm-3

Investigating the effect of GaCl3 incorporation into the usual CdCl2 treatment on CdTe-based solar cell device structures

The incorporation of GaCl3 into the usual CdCl2 post-deposition treatment solution of CdTe-based solar cells has been investigated. Both CdS and CdTe layers used in this work were prepared by electroplating technique and they are n-type in electrical conduction as observed from photoelectrochemical cell measurement technique. Before applying the chemical treatments to the device structures, the effect of GaCl3 incorporation into the usual CdCl2 treatment was first studied on the structural and optical properties of CdTe thin films. The results of the optical properties show that the bandgap of CdTe thin films treated with a mixture of GaCl3 + CdCl2 is closer to that of bulk CdTe layers than the ones treated only with CdCl2 solution. The structural properties also showed that CdTe thin films treated with GaCl3 + CdCl2 is more crystalline than CdTe thin films treated only with CdCl2 solution. The addition of GaCl3 into the CdCl2 solution have been seen to drastically enhance the solar-to-electric conversion efficiency of CdS/CdTe based solar cells. One of the effects of Ga incorporation into the usual CdCl2 treatment was seen in the series resistance reduction which ultimately leads to enhancement in the observed short-circuit current density, fill factor and overall solar cell efficiency. For the glass/FTO/n-CdS/n-CdTe device structures, the cell efficiencies were observed in the range 1.9–2.1% after being treated with CdCl2 solution only. When treated with CdCl2 + GaCl3, the efficiency increased to 6.1–6.4%. Subsequent study on multi-junction graded bandgap solar cells using the GaCl3 + CdCl2 chemical solution for the surface treatment of glass/FTO/n-ZnS/n-CdS/n-CdTe device structures results in solar cell efficiency >10%