Measurement of absorption coefficient

Unlike previous samples, the FSO coating is opaque and so prevents measurement of transmission. The detector was therefore repositioned to record the reflected intensity from the sample. These results are then used to ascertain the absorption coefficient of the material. However, the surface of the FSO was highly scattering and so prevented measurement of any reflected power. The high level of scattering observed is produced by the rough surface of the deposited FSO layer (figure 94).

Figure 94 shows the extent of the surface scattering produced when irradiating the FSO material using a green HeNe laser (532 nm). The high surface roughness, attributed to the deposition method, was analysed through the use of OM and SEM imaging techniques.

When utilising flame spraying the powder is melted before being deposited on the surface; however the use of a flame causes inhomogeneous heating of the powder. As a direct consequence, cooling of the deposited layer varies across the sample; this makes it

Figure 94: Left: Imaging of the scattering effect of the FSO layer produced using a HeNe laser at 532nm. Right: SEM image of the surface roughness of the FSO layer.

As measurement of the absorption properties of the FSO material was not possible, the next stage of testing was to determine the ablation threshold. However, the extremely high surface roughness (figures 93 and 94) coupled with the small focal spot (≈25 μm) prevent the characterisation of individual spots. In order to overcome this, a series of scans with an overlap of approximately 3 pulses per spot were used in conjunction with decreasing fluence. The ablation threshold was said to be equivalent to the point at which a plasma was no longer visible at the surface. Table 22 shows the results obtained for picosecond (ps) and femtosecond (fs) lasers, using wavelengths in the IR and visible regions.

Table 22: Approximate ablation thresholds of the FSO layer deposited on the PV cell.

Laser system Wavelength (nm) Ablation threshold (Jcm-2)

HighQ 10 ps 1064 ≈2.5

Talisker 10 ps 532 ≈2.4

Clarke MXR 180 fs 775 ≈2.0

Once the approximate ablation threshold was determined for all three lasers systems (table 22) a series of small hatches was made to ascertain whether efficient material removal was achievable with a ps laser system. IR (1064 nm) light from the HighQ ps laser when impinging on the FSO layer had minimal effect on the surface as no material ablation was

Using visible light enabled some material removal, however despite the approximate ablation threshold being determined as 2.40 Jcm-2, to achieve any significant removal of the FSO layer high fluences were required.

It was considered that selective processing would not be viable as the fluence required to remove a significant amount of the FSO was well in excess of the ablation threshold of the TCO underneath. This indicates that when the boundary interface between the FSO and TCO was reached, the subsequent laser pulses would also ablate the TCO layer. The effect of using high fluence can be observed in figure 95 where before achieving complete removal the use of several scans at high fluence caused a thermal build up within the substrate leading to fracture. The red circle indicates the point where the fracture began; from here it propagated in both directions splitting the sample into two pieces.

Figure 95: Image of fractured sample, the red circle indicates where the failure initiated. This damage propagated across the sample. The average power used here was 5 W with greater than 50 surface scans being used, leading to thermal build up and subsequent failure.

After testing the samples using a ps laser, the samples were then tested using the Clarke MXR fs system. Testing using fs pulses was carried out to investigate whether the extremely high pulse intensities and significantly smaller thermal effects obtainable with these pulse durations could enable efficient material removal.

Testing was carried out by scribing two small hatches on the surface of the cell using pulse energies of 15 and 30 μJ. As the intensity of the fs pulses are increased compared to ps pulses, this improved processing efficiency and enables material removal, exposing the FSO/TCO interface. To examine the effects of processing these areas were illuminated from

Whilst transmission of some light was observed, there were several dark regions where no transmission occurred. These dark regions were attributed to FSO that remained adhered to the surface. The residual material indicates that it is not possible to uniformly remove FSO from the surface of the cell. Consequently, more pulses are required.

Non-uniform removal of the FSO layer would require further irradiation in order to fully expose the FSO/TCO interface. Therefore, inefficient removal of this material increases the likelihood of damaging the TCO layer beneath. The residual FSO on the surface was attributed to the combination of high surface roughness and low material removal rate.

Figure 96: Optical microscopic image showing the transmission of light through FSO layer after processing with the Clarke MXR fs laser system. Fluence of 6.11 Jcm-2 (left) and 12.22 Jcm-2 (right) were used in processing. The dark regions signify areas of no transmittance, which are attributed to FSO remaining on the surface of the cell.

5.3.3 Summary

In this case study, the SUSP processing of a low cost solar cell was undertaken with the aim of creating an integrated solution for the manufacture of individual photovoltaic cells. The cell structure was a four-layer system consisting of a glass substrate, transparent conductive oxide (TCO), flame sprayed oxide (FSO) and a metal contact layer. Laser scribing was used to remove areas of both the TCO and FSO layers so as to enable efficient charge extraction from the PN (positive/negative) junction.

TCO was processed using a wavelength of 1064 nm with a traverse speed of 32 mm/s and a fluence of 4.09 Jcm-2, similar to ITO. This produced a series of non- conducting tracks across the sample.

After processing the TCO layer, scribing was performed on the FSO layer. Measurement of the absorption coefficient in both transmission and reflection was not possible due to the layer being opaque and having a high surface roughness. The ablation threshold was calculated to be approximately 2.5, 2.4 and 2.0 Jcm-2 using the HighQ, Coherent Talisker and Clarke MXR fs laser respectively. Processing at 1064 and 532 nm with ps pulses showed no significant FSO removal from the surface. One reason for this may be low absorption of the incident pulses by the FSO layer, this layer was predominantly silicon based, which has low absorption coefficients in both the visible and IR parts of the spectrum.

Fs processing at 775 nm resulted in removal of the FSO layer exposing the FSO/TCO boundary interface. This is due to the ultra-high intensities provided by focussed fs pulses (≈TWcm-2_{), at these intensities it is possible for non-linear absorption to occur.} This could account for why surface processing was only possible with fs pulses; when non- linear processes occur the absorption coefficient is a function of the intensity and becomes almost completely independent of the material properties. However, the high surface roughness prevented uniform removal (figure 96). The inconsistent removal combined with the high fluence required to process the FSO layer negated the use of selective processing, as the powers required to remove this layer would result in damage to the TCO and glass. From this it was determined that selective laser processing was not a suitable fabrication technique for this photovoltaic cell.

A limitation of selective processing was identified in this study. The structure of the cell prevented selective processing as the material with the lower ablation threshold (TCO) was deposited beneath a material with a higher ablation threshold (FSO).