Solar cell is a key device that converts the light energy into the electrical energy in photovoltaic energy conversion. In most cases, semiconductor is used for solar cell material. The energy conversion consists of absorption of light (photon) energy producing electron–hole pairs in a semiconductor and charge carrier separation. A p–n junction is used for charge carrier separation in most cases. It is important to learn the basic properties of semiconductor and the principle of conventional p–n junction solar cell to understand not only the conventional solar cell but also the new type of solar cell. The comprehension of the p–n junction solar cell will give you hints to improve solar cells regarding efficiency, manufacturing cost, consuming energy for the fabrication, etc. This chapter begins with the basic semiconductor physics, which is necessary to understand the operation of p–n junction solar cell, and then describes the basic principles of p–n junction solar cell. It ends with the concepts of solar cell using nanocrystalline materials. Because the solar cells based on nanocrystalline materials are complicated compared with the conventional p–n junction solar cell, the fundamental phenomena are reviewed.

Solar cells are semi-conductor devices which use sunlight to produce electricity. They are manufactured and processed in a similar fashion as computer memory chips. Solar cells are primarily made up of silicon which absorbs the photons emitted by sun’s rays. The process was discovered as early as 1839. Silicon wafers are doped and the electrical contacts are put in place to connect each solar cell to another. The resulting silicon disks are given an anti-reflective coating. This coating protects sunlight loss. The solar cells are then encapsulated and placed in an aluminium frame. The process requires continuous monitoring to ensure quality control over a period of time. After the manufacturing process is complete they undergo final test to check their efficiency under normal conditions

Nowadays, solar cell technologies play an import role in electrical power production due to greater power consumption and large population. The efficiency of solar cells is one of the most important parameters in directly converting light to electricity. To improve the conversion efficiency of solar cells, graphene has been extensively researched to use as counter electrodes, electron donors, or acceptors. For example, Shin et al.fabricated porous silicon heterojunction solar cells by using graphene doped with silver nanowires (AgNWs). The fabricated solar cells showed a maximum efficiency of 4.03% with AgNW concentration of 0.1 wt% at 3 s of deposition time. Shi et al. improved the solar cell efficiency by fabricating graphene–silicon solar cells by adding carbon nanotubes into graphene. The carbon nanotube-embedded graphene was the electrode for charge collection. When photo absorption occurred in silicon, minority carriers (holes) transferred to electrodes along CNTs resulted in an increased solar cell efficiency of 15.2%.

A solar cell is made of two types of semiconductors, called p-type and n-type silicon. The p-type silicon is produced by addingatoms—such as boron or gallium—that have one less electron in their outer energy level than does silicon.  Because boron has one less electron than is required to form the bonds with the surrounding silicon atoms, an electron vacancy or “hole” is created.

The n-type silicon is made by including atoms that have one more electron in their outer level than does silicon, such as phosphorus. Phosphorus has five electrons in its outer energy level, not four. It bonds with its silicon neighbor atoms, but one electron is not involved in bonding. Instead, it is free to move inside the silicon structure.

A solar cell consists of a layer of p-type silicon placed next to a layer of n-type silicon (Fig. 1). In the n-type layer, there is an excess of electrons, and in the p-type layer, there is an excess of positively charged holes (which are vacancies due to the lack of valence electrons). Near the junction of the two layers, the electrons on one side of the junction (n-type layer) move into the holes on the other side of the junction (p-type layer). This creates an area around the junction, called the depletion zone, in which the electrons fill the holes.

When all the holes are filled with electrons in the depletion zone, the p-type side of the depletion zone (where holes were initially present) now contains negatively charged ions, and the n-type side of the depletion zone (where electrons were present) now contains positively charged ions. The presence of these oppositely charged ions creates an internal electric field that prevents electrons in the n-type layer to fill holes in the p-type layer.

When sunlight strikes a solar cell, electrons in the silicon are ejected, which results in the formation of “holes”—the vacancies left behind by the escaping electrons. If this happens in the electric field, the field will move electrons to the n-type layer and holes to the p-type layer. If you connect the n-type and p-type layers with a metallic wire, the electrons will travel from the n-type layer to the p-type layer by crossing the depletion zone and then go through the external wire back of the n-type layer, creating a flow of electricity.

A solar cell is an electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electric power. This process requires firstly, a material in which the absorption of light raises an electron to a higher energy state, and secondly, the movement of this higher energy electron from the solar cell into an external circuit. The electron then dissipates its energy in the external circuit and returns to the solar cell. A variety of materials and processes can potentially satisfy the requirements for photovoltaic energy conversion, but in practice nearly all photovoltaic energy conversion uses semiconductor materials in the form of a p-n junction.

Cross section of a solar cell. Note: Emitter and Base are historical terms that don't have meaning in a modern solar cells. We still use them because there aren't any concise alternatives. Emitter and Base are very embedded in the literature and they are useful terms to show the function of the layers in a p-n junction. The light enters the emitter first. The emitter is usually thin to keep the depletion region near where the light is strongly absorbed and the base is usually made thick enough to absorb most of the light. 

The basic steps in the operation of a solar cell are:

• the generation of light-generated carriers;
• the collection of the light-generated carries to generate a current;
• the generation of a large voltage across the solar cell; and
• the dissipation of power in the load and in parasitic resistances.