Solar Cell Physics

Solar, or photovoltaic (PV), cells are electronic devices that essentially convert the solar energy of sunlight into electric energy or electricity. The physics of solar cells is based on the same semiconductor principles as diodes and transistors, which form the building blocks of the entire world of electronics.

In the later part of the century, physicists discovered a phenomenon: when light is incident on liquids or metal cell surfaces, electrons are released. However, no one had an explanation for this bizarre occurrence. At the turn of the century, Albert Einstein provided a theory for this which won him the Nobel Prize in physics and laid the groundwork for the theory of the photoelectric effect. Figure 1 shows the photoelectric effect experiment. When light is shone on metal, electrons are released. These electrons are attracted toward a positively charged plate, thereby giving rise to a photoelectric current. Einstein explained the observed phenomenon by a contemporary theory of quantized energy levels, which was previously developed by Max Planck. The theory described light as being made up of miniscule bundles of energy called photons. Photons impinging on metals or semiconductors knock electrons off atoms. In the 1930s, these theorems led to a new discipline in physics called quantum mechanics, which consequently led to the discovery of transistors in the 1950s and to the development of semiconductor electronics.

                                                                                fig 1

Most solar cells are constructed from semiconductor material, such as silicon (the fourteenth element in the Mendeleyev table of elements). Silicon is a semiconductor that has the combined properties of a conductor and an insulator. Metals such as gold, copper, and iron are conductors; they have loosely bound electrons in the outer shell or orbit of their atomic configuration. These electrons can be detached when subjected to an electric voltage or current. On the contrary, atoms of insulators, such as glass, have very strongly bonded electrons in the atomic configuration and do not allow the flow of electrons even under the severest application of voltage or current. Semiconductor materials, on the other hand, bind electrons midway between that of metals and insulators. Semiconductor elements used in electronics are constructed by fusing two adjacently doped silicon wafer elements. Doping implies impregnation of silicon by positive and negative agents, such as phosphor and boron. Phosphor creates a free electron that produces so-called N-type material. Boron creates a “hole,” or a shortage of an electron, which produces so-called P-type material. Impregnation is accomplished by depositing the previously referenced dopants on the surface of silicon using a certain heating or chemical process. The N-type material has a propensity to lose electrons and gain holes, so it acquires a positive charge. The P-type material has a propensity to lose holes and gain electrons, so it acquires a negative charge. When N-type and P-type doped silicon wafers are fused together, they form a PN junction. The negative charge on P-type material prevents electrons from crossing the junction, and the positive charge on the N-type material prevents holes from crossing the junction. A space created by the P and N, or PN, wafers creates a potential barrier across the junction. This PN junction, which forms the basic block of most electronic components, such as diodes and transistors, has the following specific operational uses when applied in electronics:

In diodes, a PN device allows for the flow of electrons and, therefore, current in one direction. For example, a battery, with direct current, connected across a diode allows the flow of current from positive to negative leads. When an alternating sinusoidal current is connected across the device, only the positive portion of the waveform is allowed to pass through. The negative portion of the waveform is blocked. In transistors, a wire secured in a sandwich of a PNP-junction device (formed by three doped junctions), when properly polarized or biased, controls the amount of direct current from the positive to the negative lead, thus forming the basis for current control, switching, and amplification, as shown in Fig 2

fig 2

 

In light-emitting diodes (LEDs), a controlled amount and type of doping material in a PN-type device connected across a dc voltage source converts the electric energy to visible light with differing frequencies and colors, such as white, red, blue, amber, and green. In solar cells, when a PN junction is exposed to sunshine, the device converts the stream of photons (packets of quanta) that form the visible light into electrons (the reverse of the LED function), making the device behave like a minute battery with a unique characteristic voltage and current, which is dependent on the material dopants and PN-junction physics. This is shown in Fig 3

The bundles of photons that penetrate the PN junction randomly strike silicon atoms and give energy to the outer electrons. The acquired energy allows the outer electrons to break free from the atom. Thus, the photons in the process are converted to electron movement or electric energy as shown in Figure 1.4. It should be noted that the photovoltaic energy conversion efficiency is dependent on the wavelength of the impinging light. Red light, which has a lower frequency, produces insufficient energy, whereas blue light, which has more energy than needed to break the electrons, is wasted and dissipates as heat.

 

fig 3