Solar Cell Basics for Teachers

Introduction

Fabrication Processes and Theory

Fabrication Sequence

Solar Cell Theory of Operation

Experimental Results

IV curve

Fill factor

Power Efficiency

Quantum Efficiency

Recommendations

Lecture Modifications

Laboratory Modifications

Summary & implementation into your curriculum

Microfabrication can be summarized as five basic methods:

Doping

The purpose of doping is to change the conductivity of a semiconductor material, such as silicon. Dopant atoms are delivered by ion implantation or sometimes by diffusion in a gas, liquid, or solid state. Small amounts of dopant atoms can change the ability of a semiconductor to conduct electricity. In p-type doping, boron or gallium are used as dopants. These elements each have three electrons in their outer orbitals. When they are mixed into the silicon lattice, they create "holes" in the valence band of silicon atoms. Doping silicon with boron creates a p-type semiconductor. In n-type doping, by doping pure silicon with a Group V element, such as phosphorus, extra valence electrons that are not bound to individual atoms can be added, making the compound an n-type semiconductor.

Oxidation

Thermal oxidation is a method of forming a thin oxide layer (usually silicon dioxide) on the surface of a wafer. The technique forces an oxidant to diffuse into the wafer at high temperatures and react with it. As more and more silicon atoms combine with oxygen, the silicon dioxide layer gradually and evenly grows. Dry oxidation (using oxygen molecules) is slower, but the oxide quality is better. Wet oxidation (using water molecules) is faster, but the quality suffers because hydrogen diffuses out of the film. This creates a path that electrons can follow. The thickness of the oxide layer is controlled by the oxidation time and temperature. By adjusting these, manufacturers can achieve the desired thickness. The silicon dioxide layer acts as an excellent insulator, blocking the flow of current across the wafer surface. This insulation is essential for the following processes, such as metal deposition.

Photolithography

Photolithography uses ultraviolet light through a mask onto a photosensitive polymer (photoresist, PR) to create the desired pattern. There are two methods to choose from, one using positive photoresist and the other using negative photoresist. Positive photoresist - the exposed photoresist is removed in the developer. Negative photoresist - UV light cross-links polymer and the developer rinses away non-cross-linked chains. PR protects the area for the next step which is etching.

Etching

The etching process involves eliminating unwanted material from the surface of the wafer by removing one or more layers. Semiconductor manufacturers typically use wet etching and dry etching to accomplish this. Dry etching uses excited ions in the plasma to collide with the material and remove it without any chemicals. Wet etching uses liquid chemicals or etchants to remove layers of base material. We used a 6:1 Buffered Oxide Etch to remove the silicon dioxide.

Deposition

Deposition is a process that involves adding thin layers of material (such as aluminum) onto a wafer. The materials used are usually selected based on their chemical and physical properties. Metal layers are used to create electrical connections between circuits. Dielectric layers are used to isolate the layers and protect them from impurities. Physical vapor deposition is a process in which a substance is evaporated from a solid or liquid source and transported as vapor through a vacuum or low-pressure gas environment and condensed on a wafer.

Google Sites

Report abuse