Solar Cell Technology Essay, Research Paper

Single junction solar cells:

The most common structure is a semiconductor material into which a large-area diode, or p-n junction, has been formed. The fabrication processes tend to be traditional semiconductor approaches diffusion, ion implantation and so on. Electrical current is taken from the device through a grid contact structure on the front that allows the sunlight to enter the solar cell, a contact on the back that completes the circuit, and an anti-reflection coating that minimises the amount of sunlight reflecting from the device. Figure 2 is a schematic depiction of a rudimentary solar cell that shows the important features.

The fabrication of the p-n junction is key to successful operation of the photovoltaic device (as well as other important semiconductor devices). We will assume that the semiconductor material is single-crystal silicon. Although photovoltaic technologists today use many other varieties of semiconductors, crystalline-silicon concepts represent a reasonable compromise for this discussion because they are well known and understood by physics students.

Silicon is representative of the diamond crystal structure. Each atom is covalently bonded to each of its four nearest neighbors; that is, each silicon atom shares its four valence electronic with the four neighboring atoms, forming four covalent bonds. Silicon has atomic number 14, and the configuration of its 14 electrons is 1s22s22p63s23p2. The core electrons, 1s2, 2s2 and 2p6, are very tightly bound to the nucleus and, at real-world temperatures, do not contribute to the electrical conductivity. At absolute zero, as N silicon atoms are brought together to form the solid, two distinct energy bands are formed-the lower, “valence” band and the upper, “conduction” band. The valence band has 4N availability energy states and 4N valence electrons and is therefore filled. Conversely, the conduction band is completely empty at absolute zero. Thus the semiconductor is a perfect insulator at absolute zero. As the temperature of the solid is raised above absolute zero, energy is transferred to the valence electrons, making it statistically probable that a certain number of the electrons will be raised in energy to such an extent that they are free to conduct electrical charge in the conduction band. These electrons are called intrinsic carriers. The amount of energy necessary to bridge the valence and conduction bands is referred to as the forbidden gap or energy gap Eg, which is 1.12 eV at room temperature for silicon. Even at room temperature, however, the amount of conductivity is still quite small. At 300 K there are 1.6 x 1010 intrinsic carriers per cubic centimeter; thus the material is still a very good insulator compared with a metal, which has approximately 1022 carriers per cubic centimeter.

To modify the conductivity to more useful values, one must introduce small controlled amounts of impurities into the host materials. By substituting, or “doping,” the silicon, which is in group IV of the periodic table, with either group III materials (boron, aluminum, gallium or indium) or group V materials (phosphorous, arsenic or antimony), the number of conduction band electrons or valence band holes can be precisely controlled.

A group V dopant completes the covalent bond and leaves an additional, loosely bound electron that can be transferred to the conduction band by an energy of about 40-50 meV, termed the ionization energy. Group III impurities leave the covalent bond deficient of one electron (that is, with a hole). An electron from the valence band can transfer to the empty site and satisfy the bond requirement. In effect the hole moves, because the transferred electron leaves behind a hole. The amount of energy required to thus place the hole in the valence band ranges from 45 to 160 meV.

By varying the density of the doping impurities, one can design the silicon to range from a poor conductor of electricity to a near-metallic conductor. Silicon that has been doped with group III elements is called a p-type semiconductor; that doped with group V elements is called an n-type semiconductor.

The p-n junction diode

When a uniform p-type sample is metallurgically joined to a uniform n-type sample, the configuration produces the all-important p-n junction. Instantaneously the positive and negative electrical charges redistribute establishing internal electric fields that determine, in part, the properties of the semiconductor diode. At the instant of formation, there exist on the n side, extending to the junction, uniform concentrations nn0 of mobile free electrons and pn0 of mobile free holes. On the p side there exist uniform concentrations of pp0 of mobile holes and np0 of free electrons, also extending to the junction. The concentrations satisfy the relation:

Here ni is the intrinsic-carrier concentration at the given temperature of the material. At the instant of junction formation, the concentration of electrons is much larger on the n side than on the p side. An analogous condition applies to the hole concentrations, which are larger on the p side than the n. The large difference in carrier concentrations sets up an initial diffusion current: Electrons flow from the n region into the p region, and holes flow from the p region into the n region. This flow of charge results in a region near the junction that is depleted of majority carriers-that is, of electrons on the n side and of holes on the p side. The fixed donor and acceptor impurity ions in this depletion region are no longer balanced by the free charges that were there. As a result, an internal electric field builds up with a direction that opposes further flow of electrons from the n region and holes from the p region. The magnitude of the field is such that it exactly balances the further flow of majority carriers by diffusion. The region around the junction is depleted of majority carriers, and a space-charge layer forms in the region of high electric fields, as shown in figure 3. Thus in the absence of externally applied potentials, no current will flow.

Contact can be made with the two ends of the p-n junction to form a two-terminal device. A positive voltage applied to the p side relative to the n side encourages current flow across the junction. Conversely, a negative voltage applied to the p side relative to the n side further discourages current flow relative to the zero-voltage case. The former condition is referred to as forward bias and the latter as reverse bias. These two conditions can best be described in terms of the ideal-diode equation:

Here I is the external current flow, I0 is the reverse saturation current, q is the fundamental electronic charge of 1.602 x 10-19coulombs, V is the applied voltage, k is the Boltzmann constant, and T is the absolute temperature. Under large negative applied voltage (reverse bias), the exponential term becomes negligible compared to 1.0, and I is approximately -I0. I0 is strongly dependent on the temperature of the junction and hence on the intrinsic-carrier concentration. I0 is larger for materials with smaller bandgaps than for those with larger bandgaps. The rectifier action of the diode-that is, its restriction of current flow

to only one direction-is key to the operation of the photovoltaic device.[3]

If light is allowed to impinge on a p-n junction device, the equilibrium conditions of the device are disturbed. Minority carriers-that is, electrons in the p material and holes in the n material are created in sufficient quantities to lower the potential energy barrier at the junction, allowing current to flow and establish a voltage at the external terminals. The availability of current and voltage produces usable power.

Solar cell materials

Single-crystal silicon has been the material of choice for high-performance, highly reliable solar cells since the successful deployment of silicon photovoltaic systems for space power. Most of the terrestrial photovoltaic power systems sold today are also crystalline silicon. The need to lower the cost of terrestrial photovoltaic power has focused research efforts on alternative materials as well as on less expensive means of producing solar-grade silicon.

Crystalline silicon is made by growing large cylindrical single crystals, called boules. The boules are sliced into thin wafers, from which photovoltaic devices are made. Slicing is an expensive and material-wasteful process. Several approaches have been investigated to minimize the cost of the original silicon material and to eliminate the slicing step.

A less expensive material, polycrystalline silicon, bypasses the expensive and energy-intensive crystal growth process. The molten silicon is instead cast directly into either cylindrical or rectangular ingots. The polycrystalline material has a large number of crystallites separated by grain boundaries. The material has poorer crystalline quality, and light-induced electron-hole pairs can recombine at the grain boundaries without producing current in the external circuit. Although polycrystalline materials result in less efficient solar cells than crystalline silicon, they are sufficiently cheaper that they are commercially viable. The cast material must still be sliced, however, leading to a loss of about half of the material. Improvements in sawing techniques such as multiple-wire saws continue to reduce the loss in producing thinner wafers.

Another approach to producing less costly materials is to avoid most of the sawing altogether. Several techniques that produce silicon in sheet form have been developed. The first commercial success was the edge-defined film-fed growth (EFG) ribbon process, in which polycrystalline silicon is grown by extracting the crystallizing silicon melt through a graphite die. By this technique, thin ribbons of polycrystalline silicon can be grown either as multiple separate ribbons or as polygons of material that can be separated into silicon blanks for fabrication into finished solar cells with minimal loss of material. An alternative approach has been to grow the ribbon from parallel supporting dendrites (like a soapy water film grown between two wires). By carefully controlling the thermal profiles, one can grow a film of nearly single-crystal material. Other techniques, such as horizontal ribbon growth and spin casting, have also been demonstrated. Regardless of the approach, ultimately the cost of silicon solar cells will depend on the starting material.

The lowest-cost approach would be to minimize the required amount of semiconductor material. Thin films have been developed that are only a few micrometers thick. Such films are produced by a number of vapor-deposition approaches carried out with in-line, highly automated systems. The techniques are adaptable to a number of semiconductor materials that are optimized for solar cell operation. It has been shown that silicon, with its bandgap of 1.12 eV, is not optimal. Materials with bandgaps nearer to 1.5 eV, such as GaAs and CdTe, have higher theoretical efficiencies. Thin films are cheaper than crystalline structures but typically have lower efficiencies. Ultimately, however, thin films will be necessary for producing low-cost electricity, because the bottom line-the cost per watt-is more important than efficiency.

Performance characteristics

The highest-efficiency single-junction solar cells are made from crystalline silicon and GaAs. Silicon cells of 23% efficiency and GaAs cells of 25% efficiency have been confirmed. When the same materials are used in concentrator applications, the efficiencies increase to 28% and 29%, respectively. The highest efficiency that has been confirmed is 34% for a GaAs-GaSb stacked cell operating at 100-Suns concentration-light concentrated to an intensity 100 times that of ordinary sunlight. For 1-Sun conditions, the efficiency of polycrystalline silicon is approximately 18%; that of cells made using the edge-defined film-fed growth-ribbon process, 14%; and that of dendritic web cells, 15.5%. (Dendritic web cells are built from single-crystal

films grown between two dendritic seed crystals.) The highest thin-film cell efficiency has been confirmed at 15.8%, for cadmium telluride. Thin films of silicon on ceramic substrates have yielded efficiencies of 15.7%; copper indium diselenide, 12-13% (almost 16% has been achieved by the addition of gallium); and amorphous silicon, 12% before light soaking (amorphous silicon efficiencies fall off for a time before stabilizing). Research continues, and efficiency increases are expected in all materials.

Future

The demonstration of higher performance, lower cost and better reliability in today’s photovoltaic systems is leading many different end users to assess the value of these systems for their particular applications. Aggregation of these applications will lead industry to commit to larger, more cost-effective production facilities, leading to yet lower costs. Public demand for environmentally benign sources of electrical energy will hasten adoption of photovoltaics. How quickly these adoptions occur will be dictated by the economic viability of photovoltaics with respect to the competing options. It is no longer a question of if, but when and in what quantity, photovoltaic systems will gain adoption. Clearly the energy system of the future has become today’s consideration.