Exercise 8.4-1

All Quick Questions to

8. Solar Cells

General Concerns

Give some rough numbers (with some reasoning whereever applicable), always per m², for

Maximum solar power.
Maximum and practical efficiency of "standard" Si solar cells.
Average power for "standard" Si solar cell.
Average energy harvest of "standard" Si solar cells per year.

Compare indirect and direct semiconductors with respect to light absorption at the "band edge", i.e. for light energies around bandgap energy. What follows for solar cells?

What is your first priority with respect to the coupling of light and semiconductor when you want to make a solar cell with a good efficiency?

Draw the current density (j) - voltage (U) characteristics of a pn-junction in the dark and under illumination in the interesting part of the j-U plot. Point out the important points of the illuminated diagram and give some approximate numbers for typical intense sun light.

The typical j-U equation for a Si pn-junction is

j =	j₁	·	æ ç è	exp	eU kT	– 1	ö ÷ ø	+	j₂	æ ç è	exp	eU 2kT	– 1	ö ÷ ø	– j_Ph

Discuss the origin of the j terms. Compare (qualitatively) the magnitude of j₁ and j₂. What kind of properties of Si influence the value of j_Ph?

Why is "dirty" Si not good for solar cells? Hint: Follow the fate of a photon-generated carrier.

Draw the j-U curve of an illuminated decent solar cell. Draw (qualitatively) the power curve into this diagram. Discuss the curve shortly with respect to real power applications

Your electrical energy bill shows that you, personally, consumed 2 000 kWh electrical energy per year in your home. How man square meters of solar cells (roughly) would you need on your roof to supply this much energy?

Give the eqivalent circuit diagram of a realistic Si solar cell. Discuss the components with the aid of schematic IV-characteristics.

Define the fill factor of a solar cell and discuss its dependence on solar cell parameters.

	Making Bulk Si Solar Cells
		Discuss the basic requirements for mass production of solar cells including technical constraints resulting from economical boundary conditions
		Describe the essential production steps of a mc-Si solar cell. Start with suitable poly-Si and discuss essential problems encountered with the solutions. Use schematic drawings.
		Describe "anti reflection" technologies.
		Compare screen-printing for the deposition of the metallic grid on a Si solar cell to other layer deposition methods. Hint: Consider that a good solar cell may deliver 5 A at 0.5 V and consider how that converts into thickness requirements of the metal grid layer. The specific resistivity rof a decent metal is about 2 µWcm, the resistance R for a cross sectional area of A cm² and a length l is R = r · l / A.

	Making Thin Film Solar Cells
		List basic requirements that semiconductors must meet if they are to be used for thin film solar cells.
		Give some examples of existing thin film solar cell technologies and list their strengths and weaknesses.
		What is the logic behind "concentrator cells"? Discuss the basic principle and necessities concerning application.
		Give a schematic cross section through a CIGS solar cell, indicating the major layers and the in-situ series connection.

With frame

8.4.1 Summary to: 8 Solar Cells