Silicon and Photovoltaic cells

Nibras Nassar

C 10.1Describe the doping of silicon to produce p-type and n-type semiconductors

C10.2 Describe how sunlight interacts with semiconductors

Re cap: Metals conduct electricity because of delocalized electrons. Non metal do not conduct electricity well except carbon. Silicon and Germanium are semiconductors.

-silicon crystal has a lattice structure which makes the electrons fixed under normal conditions, thus making it a poor conductor of electricity

In order to excite the electron you need to use light with a wavelength of 1.1 x 10-6 m.

Sunlight is able to excite an electron in silicon because it has a wavelength less than 1.1 x 10-6 m which means it has greater energy, when the electron is excited it is then free to move around in the silicon

This is known as the photo electric effect

The photoelectric effect theory is also used in photoelectric cells, the benefit of using these cells is that:

They do not pollute the environment They do not use up valuable fossil fuels

However the disadvantage is that: The cost of purifying silicon is high

To improve the efficiency of the photoelectric effect we should use doping.

Doping is adding small amounts of atoms from group three(Al, Ga, or Ln) or form group five (P or As)

For doping silicon with group five atoms you get a n type semiconductor because it conducts electricity due to the delocalized negative electrons.

However when doping silicon with a group three atom you get a p type semi conductor this is because the new silicon will have less electrons and therefore have gaps where the electrons need to be filled the electrons will keep moving to fill the gap

Liquid Crystals

C11.1 identity molecules that are likey to show liquid crystal properties and explain their liquid crystal behavior on a molecular level

Only biphenly nitriles will be assessed, the polar intermolecular forces are strong enough to align in a common direction

The biphenyl group makes the molecules more rod shaped and rigid

The long alkane chain ensures that they will not pack closely together allowing it to keep its liquid properties.

Each pixel contains a liquid crystal sandwiched between two glass plates. The plates have scratches at 90 degrees to each other

The molecules in contact with the glass line up with the scratches and molecules form a twisted arrangement between the plates due to the intermolecular bonds.

Plane polarized light is rotated with the molecules and so is rotated through the 90 degrees as it passes through the film. When the polarizers are aligned with the scratches the light will be seen.

As a voltage is applied across the film the polar molecules will align with the field and so the twisted structure is lost. Plane polarized light is no longer rotated and so the pixel appears dark

Describe and explain in molecular terms the working of a twisted nematic liquid crystal.

Kevlar is a Lyotropic liquid crystal. Lyotropic liquid:consists of two or more components that exhibit

liquid-crystalline properties in certain concentration ranges. It has rigid rod shaped molecules due to the benzene rings The concentration of the solution determines the alignment of the

molecules Kevlar has strong bonds due to hydrogen bonding, but in strong

acid solution the oxygen and nitrogen atoms in the amide linage become protonated and this breaks the strong bonds leading to Kevlar loosing its liquid crystal and protective properties.

Describe the liquid crystal properties of Kevlar, and explain its strength and its solubility in concentrated sulfuric acid