Solar cell Essay Example

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Preparation of Si Wafer — Photolithography

N-type Si wafer doped with phosphorous was rinsed using acetone and dried under a stream of nitrogen. The wafer was 525
Solar cell m thick, 1-5Solar cell  1m, with 100 nm thermal oxide, ABC GmbH, Germany. The Au grid structure with an active area of 0.087 defined by photolithography was performed in a clean room. Positive photoresist — the AZ1518 microresist technology, GmbH, Munich, Germany was applied on the Si wafer by spin coating at 3000 rpm for 30 s, and then softbaking on a hot plate at 100 oC for for 50 s. This was done using an AREC heating magnetic stirrer from Rowe Scientific. The coated wafer was cooled to room temperature before designing grid patterns using a mask aligner – the EVG 610. The wafer was then inserted in a developer solution – AZ 726 MIF, obtained from AZ Electronic Materials, GmbH, Munich, Germany for 15s for it to develop photoresist. It was then rinsed with water and dried under a stream of nitrogen gas. The Si wafer post-baking process was done on a hot plate with the pattern defined at 115oC for 50s. Using the Quorumtech Q300T-D sputter coater, the Au/Cr 90/5 nm, 90 nm metal electrode was applied with a quartz crystal microbalance to monitor the thickness of the metal electrode. The substrate was then immersed in acetone for about 90 minutes followed by a mild rub to dissolve the photoresist. Photovoltaic cell substrates were then prepared by cutting pieces of Si sized 1.5 cm2. A drop of buffered oxide etch was applied on the surface and the active area to remove the SiO2 layer on the surface. To establish the reaction rate, a controlled test was first performed on a piece of Si before applying on the cellsubstrates.

SiO2 + 6HF H2SiF6 + 2H2O

Fabricating the Pristine SWCNT solar cells

Nanotube films were prepared using vaccum filtration. This was completed by initially mixing an appropriate amount of nanotube suspension with milliQ water to make a solution of 250 mL.

The solution was then filtered using a vaccum through a series of two microporous filter papers. The filter paper at the bottom was VSWP Millipore, 0.025
Solar cell  2m pore size was patterned with holes similar to the size of the desired nanotube. The top filter paper, HAWP Millipore, 0.45
Solar cell  3m pore size remained unpatterned.

The difference in the rate of flow through the filter papers causes preferential flow of solution through the top film where the top film is patterned. Thus, the nanotubes are caught by the top film in a similar shape as that of the template film. After the solution passes through both films, it was passed through the filtration media two more times to allow enough nanotubes to be retained on the film. After this, the nanotubes were then passed in pure MilliQ water to remove Triton X-100 surfacant remaining in the nanotube film. The template used in these experiments produces 0.5 cm2 films in each filtration. One film is for attaching to glass for measurement of sheet resistance and optical transmittance, while the other one is for attaching to solar cells for measurement of cell efficiency (see figure 3). Nanotube films were then attached either the silicon substrate or glass prewashed in ethanol.

Figure 3: Fabrication of films [insert figure]

The films were then cut from the filter paper and placed on the substrate. Wetting was done using a small drop of water and the nanotube sandwiched between a piece of Teflon and a piece of glass clamped together.The substrate was then heated at 80oC for about 15 minutes, then cooled in darkness for 30 minutes. The substrates were then washed in acetone three times – 30 minutes with stirring for every wash to remove the filter paper.

To complete the preparation of cells, the reverse side of all Si pieces was manually removed by scratching to remove the layer of oxide. A gallium indium eutectic (eGaIn) was then applied on the surface of Si before attaching a piece of stainless steel on the back of each piece (see figure 4). The cells were then tested and further subjected to different post-fabrication treatment procedures. First, a 2% drop of HF was applied on the active area to etch a coat of SiO2 formed between the nanotube film and the Si formed during the attachment. This was then followed by treating the nanotube film with drops of thionyl chloride and left until to evaporate. Before testing, the residue was washed with ethanol. In the last step, the silicon pieces were treated with 2% HF which significantly improved performance. However, the reason for the improved performance is not clear.

Figure 4: Pristine SWCNT solar cell [insert figure]

The amount of SWCNTs was controlled in both the layered and mixture solar cells, to make devices with different concentration of dispersion. The concentration dispersion can be determined using the Beer-Lambert law:

Solar cell  4

A – absorbance

Solar cell  5 – extinction coefficient

Solar cell  6 – Concentration of the CNT

Solar cell  7 – length of the cell path

The extinction coefficient varies for metallic and semiconducting nanotubes, and also varies for different species. Quantification of the concentration of CNT was accomplished by calculating the extinction coefficient and the concentration. These calculations were made at three wavelengths in order to compensate for ambiguties in estimation of the concentration of CNT; 689 (M11,
Solar cell  8). The concentration of the CNT solution was 0.009 mg/mL and 0.045 mg/mL.