Excimer laser processing of Dip coated SnO 2 :Sb

Previous work by the display research group, Nottingham Trent University [49, 101] has demonstrated that Excimer laser processing is a post processing technique that has potential to produce functionalised SnO2:Sb layers. As an initial evaluation for this

research project, the dip coated samples were processed using the 248 nm excimer laser system described in section 3.3.1. The laser parameters varied were laser fluence and number of pulses, with the samples laser processed in air at atmospheric pressure. A variable attenuator was used to change the laser fluence. Due to the limited number of dip coated SnO2:Sb samples, KWATO2 and KWATO6 were used for the laser

processing experiment and both samples were laser processed with the same conditions for comparison. The position of the laser processed area on the samples was varied to address the expected fluctuations due to the thickness/sheet resistance variations across the samples. The processing areas were marked on the rear glass side of the sample and labelled with the processing fluence. Then by varying the number of irradiated laser

___________________________________________________________________________ 4-6 pulses, the samples were laser processed and the sheet resistance of the processed area was subsequently measured using the four point probes.

Figure 4.3 shows the change in sheet resistance of the dip coated SnO2:Sb samples due

to the laser processing. The as-deposited sheet resistance of the samples are denoted by a zero number of pulses.

For all three laser fluences tested, a reduction of sheet resistance was observed with increase of number of laser pulses. The greatest reduction in sheet resistance occurred within the first two laser pulses. The variation of the sheet resistance with respect to the increase of laser fluence was difficult to explain due to variation of the initial sheet resistances within the processing areas. When the laser fluence was at 70 mJ.cm-2 the films were typically ablated at the higher number of pulses tested (> 10).

Figure 4.3 : The variation in sheet resistance pre and post laser processing as a function of number of pulses and laser fluence.

The resultant films were further analysed for transmission and microstructure. The crystallinity of the as-deposited and laser processed SnO2:Sb films were investigated

___________________________________________________________________________ 4-7 using X-ray diffraction using the PANalytical X’Pert PRO diffraction system with CuKα1 source (wavelength = 1.54056 Å). For the XRD analysis, a dip coated SnO2:Sb

sample was KrF laser processed with 10, 100, and 1000 pulses at 100 mJ.cm-2. The resultant XRD patterns of the as-deposited and KrF laser processed SnO2:Sb are shown

in figure 4.4, indicating that the films are of polycrystalline nature. The broad main diffraction peaks are attributed to the (110), (101), and (211) peaks of SnO2 (SnO2:Sb

ICSD PDF file 01-088-2348) at 26o, 33o, and 51o and corresponds to the tetragonal cassiterite structure. Qualitatively, it can be observed that with the increase of number of laser pulses, the diffraction peaks become sharper. By using the Equation 3.2, the Scherrer equation, the corresponding crystallite size of the as deposited and laser processed material can be estimated.

Figure 4.4 : XRD of as–deposited and laser processed dip coated SnO2:Sb

Table 4.2 lists the crystallite size (nm) as estimated by the Scherrer equation, with the data from the (110) and (101) diffraction peaks being used in the calculation. It is observed from these results that there is a significant change in the estimated crystalline size as a result of the KrF laser processing, but this change is not clearly correlated to

___________________________________________________________________________ 4-8 number of laser pulses, and is more likely to be indicative of an initial change due to the effect of the first few pulses.

Preparation Orientation 2 Theta (2 ) FWHM Grain Size (nm) As- deposited 110 26.6 1.01 8 101 33.9 1.10 7 KrF laser processed 10 pulses 110 26.6 0.6 14 101 33.9 0.5 17 KrF laser processed 100 pulses 110 26.6 0.49 16 101 33.9 0.64 13 KrF laser processed 1000 pulses 110 26.6 0.55 15 101 33.9 0.57 15

Table 4.2 : Crystalline size (nm) of SnO2:Sb calculated using Scherrer equation.

The Filmetrics spectrometer was used to measure the transmission spectra of the as deposited and laser processed dip coated SnO2:Sb samples. Figure 4.5 (a) & (b) show

the transmission spectra for dip coated SnO2:Sb samples laser processed at 20 mJ.cm-2

and 40 mJ.cm-2 respectively. With the exception of the area laser processed at 40 mJ.cm-2 with 1000 pulses, the transmission at 550 nm of the as-deposited, and laser processed films was > 85%. The transmission of the laser processed at 40 mJ.cm-2 with 1000 pulses was reduced to 79%. This is consistent with an observed discolouration of the films when processing with higher number of pulses.

___________________________________________________________________________ 4-9 Figure 4.5 : Transmission spectra for dip coated SnO2:Sb on glass substrate laser

processed with (a) 20 mJ.cm-2 (b) 40 mJ.cm-2.

0.75 0.8 0.85 0.9 0.95 350 450 550 650 750 850 950 1050 Wavelength (nm) T ra n s m is s io n As deposited 10 pulses 100 pulses 1000 pulses 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 350 450 550 650 750 850 950 1050 wavelength (nm) T ra n s m is s io n As deposited 10 pulses 100 pulses 1000 pulses (a) (b)

___________________________________________________________________________ 4-10

4.3 Inkjet printing of SnO

:Sb

Following the initial processing and characterisation of dip coated SnO2:Sb samples; the

dip coated ink was then formulated to give stable jetting with good wetting characteristics. The ink formulation work was carried out by Sun Chemicals, UK and the samples were inkjet printed at PEL, UK. The author undertook training on the printing equipment with PEL and carried out the sample printing work.

The initial investigation was based around a batch of 40 SnO2:Sb samples inkjet printed

onto Eagle 2000 glass substrates (25x 25 mm2) and photo paper substrates. Sun chemical SnO2:Sb ink formulation, U5604 was used to inkjet print these samples. Table

4.3 shows a summary of the sample details. Before inkjet printing, the glass substrates were cleaned and corona treated for ~2 s at 24.5 kV, 20 W. A Xaar 126 drop on demand piezoelectric printer head, with nozzle pitch of 137 µm was used to inkjet print the SnO2:Sb samples. The typical firing frequency of these nozzles is 5.2 kHz. All the

samples were inkjet printed using micro step of 120 µ and the printing speed was set to 100 mm/s. The droplet height (distance between nozzles to substrate) was set to 1 mm. unless otherwise stated all the inkjet printed samples were printed at room temperature. Figure 4.6 shows examples of inkjet printed SnO2:Sb films on glass and photo paper

substrates.

Figure 4.6 : Inkjet printed (3 passes) SnO2:Sb onto Eagle XG glass substrate (top) and

___________________________________________________________________________ 4-11 Batch A

3 passes on glass substrate

Oven dried: ~130⁰C. ~ 15 min

PEL/001 to PEL/010

Batch B

6 passes on glass substrate

Oven dried: ~130⁰C. ~ 15 min

PEL/011 to PEL/020

Batch C

3 passes on photo paper

Oven dried: ~130⁰C. ~ 15 min

PEL/021 to PEL/030

Batch D

6 passes on photo paper

Oven dried: ~130⁰C. ~ 15 min

PEL/031 to PEL/040

Table 4.3 : Summary of the inkjet printed SnO2:Sb samples.

The initial sheet resistance of the inkjet printed SnO2:Sb samples were measured using

four point probes and the results were shown in figure 4.7. The As-deposited sheet resistance of Batch A were in the range of 4.8 MΩ/sq (± 10%) and Batch B was around 3.6 MΩ/sq (± 5%). In Batch B, some of the inkjet printed layers (PEL/015 – PEL/020) showed no measurable conductivity. These samples also exhibited poor surface quality of the films, due to the `coffee ring’ effect which is further discussed in section 4.3.3. The samples which were printed onto photo paper (batch C and Batch D) also showed no measurable conductivity.

___________________________________________________________________________ 4-12 Figure 4.7 : Initial sheet resistance of inkjet printed SnO2:Sb on glass substrates.