Structure performance relationship

Figure 6.21: Cell parameters as function of growth voltages of CdTe layers (a) V_oc against Growth voltage, (b) Jsc against Growth voltage, (c) FF against Growth voltage and (d) Efficiency against Growth voltage.

6.6 Structure performance relationship

Table 6.5 shows the summary of the relationship between material properties and the device performance. The stoichiometric CdTe is grown at Vg = 1.576 V, and hence the material contains larger crystallites, correct bandgap for CdTe (1.45 eV) and produce best efficiency values. At lower growth voltages material becomes Te-rich and at higher growth voltages material becomes Cd-rich and hence the crystallinity deteriorates due to presence of two phases in the layer. Therefore the bandgaps increase due to quantum effects and the device efficiency reduces.

139

Table 6.5: Summary of the material structure and device performance relationship.

Growth voltage (V)

XRD parameters Bandgap (eV)

Cell efficiency,

η (%) FWHM Grain size,

D (nm)

1.563 0.134 11.4 1.55 1.16

1.576 0.100 15.8 1.45 3.70

1.587 0.112 13.7 1.55 1.20

140 6.7 Summary

CdTe has been deposited on glass/FTO/CdS substrates using a low-cost aqueous electrodeposition method using two-electrode system. PEC measurements confirmed that at lower and higher cathodic voltages from the stoichiometric voltage, the polarities of the PEC signal were p- and n- type indicating Te and Cd richness respectively. At a growth voltage of 1.576 V the PEC signal shows intrinsic implying the film has stoichiometric composition.

The XRD results for both as deposited and heat treated CdTe layers have cubic structure with the main diffraction peak at 2θ = 23.642^o which corresponds to preferred orientation along (111) plane of cubic phase. At voltages less than 1.576 V, the deposition rate of Te is greater than that of Cd, and hence the layers are rich in Te.

When the growth voltage is higher than 1.576 V, the deposition rate of Cd is greater than that of Te, and therefore the layers are rich in Cd. When the CdTe composition is stoichiometric, only the (111) peak is dominant in the XRD diffractogram indicating a high degree of texture. Away from stoichiometry, crystallinity is poor due to presence of two phases.

The optical absorption measurement shows the value of bandgap, Eg for as deposited samples away from stoichiometry ranges from 1.62 eV to 1.55 eV. Similarly, after heat treatment, the bandgap values decreased and the absorption edge of the curve sharpened, giving a bandgap of 1.45 eV for the stoichiometric film. The optical transmission spectrum for as deposited samples was found to be above 80% except for the one grown at 1.576 V. After heat treatment the transmission decreases and the absorption edge of all the CdTe films sharpened.

SEM studies showed that as deposited layers had pin holes between the crystallites.

These pin holes seem to be more common in the samples grown at lower deposition voltages. As the voltage increases, the pin holes diminish up to the stoichiometry voltage and then start appearing again. After heat treatment the grain size increase and the pin holes are reduced. The 3D-AFM image shows electrodeposited CdTe has highly-ordered and densely packed nano-rod arrays oriented perpendicular to the glass/FTO/CdS substrates. The nano-rod nature of CdTe could reduce the recombination of photo-generated charge carriers and allow their passage with high mobility. The 2D-AFM picture shows clusters of CdTe with large grains. The material clusters are in micro-scale range with an average size of ~1.2 μm.

141

Raman measurements show two peaks related to CdTe layers identified at 141 cm^-1 and 166 cm^-1 which correspond to the fundamental transverse and longitudinal optical phonons (1TO) and (1LO) respectively. The other peak at 123 cm^-1 is known to be the A1 phonon of pure tellurium. This result shows that the CdTe films are rich in Te by indicating a strong Te- peak. Raman spectroscopy has been shown to provide a good finger print for identifying CdTe layers. The XPS work reveals that electrodeposited CdTe has a similar spectrum to that of cleaved CdTe except for the additional presence of C and O peaks, as expected. The composition estimated for these layers by XPS is 53:47 (Cd:Te).

The linear I-V curve of glass/FTO/CdS/CdTe/Au contacts under AM 1.5 illumination at different growth voltages give the maximum efficiency of 3.70% with Voc ~540 mV and FF ~0.49 from the stoichiometric CdTe grown at 1.576 V. The efficiency decreases as it deviates from the stoichiometry. The low FF of most of the devices at higher and lower growth voltages may be due to high resistance created by small grains, large recombination process present or detrimental leakage paths present within the devices.

From the results above, the optimum CdTe voltage has been determined as 1.576 V.

Therefore, the objective has been achieved.

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