Current-Voltage Measurements
Researchers measure the performance of a PV device to predict the power the cell will produce. Current-voltage (I-V) relationships that measure the electrical characteristics of PV devices are depicted by what we call "I-V curves." These I-V curves are obtained by exposing the cell to a constant level of light, while maintaining a constant cell temperature, varying the resistance of the load, and measuring the current that is produced.
On an I-V plot, the vertical axis refers to current and the horizontal axis refers to voltage. The actual I-V curve typically passes through two significant points:
The short-circuit current (Isc) is the current produced when the positive and negative terminals of the cell are short-circuited, and the voltage between the terminals is zero, which corresponds to a load resistance of zero.
- The open-circuit voltage (Voc) is the voltage across the positive and negative terminals under open-circuit conditions, and the current is zero, which corresponds to a load resistance of infinity.
The cell may be operated over a wide range of voltages and currents. By varying the load resistance from zero (a short circuit) to infinity (an open circuit), we can determine the highest efficiency as the point where the cell delivers maximum power. Remember that power is the product of voltage times current. Therefore, on the I-V curve, the maximum-power point (Pm) occurs where the product of current times voltage is a maximum. No power is produced at the short-circuit current with no voltage, or at open-circuit voltage with no current. So we expect to find maximum power generated somewhere between these two points. Maximum power is generated at only one place on the power curve, at about the "knee" of the curve. This point represents the maximum efficiency of the solar device in converting sunlight into electricity.
A parameter known as fill factor measures the "squareness" of the I-V curve and describes the degree to which the voltage at the maximum power point (Vmp) matches Voc and that the current at the maximum power point (Imp) matches Isc. The higher the fill factor's percentage or match, the "squarer" the curve.
The conversion efficiency of a solar cell is the percentage of the solar energy shining on a PV device that is converted into electrical energy, or electricity. Improving this conversion efficiency is a key goal of much research and helps to make PV technologies cost competitive with more traditional sources of energy. The efficiency of solar cells is affected by a variety of factors, which are discussed in the next section.
Factors Affecting Conversion Efficiency
Much of the energy from sunlight reaching a PV cell is lost before it can be converted into electricity. But certain characteristics of solar cell materials limit a cell's efficiency. Some characteristics are fixed, but others can be improved by selecting appropriate materials and carefully designing the cell.
Wavelength of Light
Light is composed of photons—or packets of energy—that range in wavelength. When light strikes the surface of a solar cell, some photons are reflected and do not enter the cell. Other photons pass through the material, some are absorbed but only have enough energy to generate heat, and some have enough energy to separate electrons from their atomic bonds to produce charge carriers—negative electrons and positive holes.
"Bandgap" is a term often mentioned in detailed descriptions of solar cells. The bandgap is the minimum amount of energy needed to free an electron from its bond, and this energy differs for different semiconductor materials. The primary reason why PV cells are not 100% efficient is because they cannot respond to entire spectrum of sunlight. Photons with energy less than the material's bandgap are not absorbed, which wastes about 25% of incoming energy. The energy content of photons above the bandgap will be wasted surplus—re-emitted as heat or light—and accounts for an additional loss of about 30%. Thus, the inefficient interactions of sunlight with the cell material waste about 55% of the energy from the original sunlight.
Recombination
Charge carriers—which are electrons and holes—in a solar cell may inadvertently recombine before they make it into the electrical circuit and contribute to the cell's current. Direct recombination is a major problem for some materials, where light-generated electrons and holes randomly encounter each other and recombine. In other materials, indirect recombination occurs, where electrons or holes encounter an impurity, defect in the crystal structure, or interface or surface that makes it easier for them to recombine.
Natural Resistance
The natural resistance to electron flow in a cell decreases cell efficiency. These losses predominantly occur in three places: in the bulk of the primary solar material, in the thin top layer typical of many devices, and at the interface between the cell and the electrical contacts leading to an external circuit.
Temperature
Solar cells work best at low temperatures, as determined by their material properties. All cell materials lose efficiency as the operating temperature rises. Much of the light energy shining on cells becomes heat, so it is good to either match the cell material to the operation temperature or continually cool the cell.
Reflection
A cell's efficiency can be increased by minimizing the amount of light reflected away from the cell's surface. For example, untreated silicon reflects more than 30% of incident light. Various antireflection (AR) technologies help to optimize light absorption. Most commonly, a special coating is applied to the top layer of the cell. A single AR layer will effectively reduce reflection only at one wavelength. Better results, over a wider range of wavelengths, are possible with multiple AR layers. Another way to reduce reflection is to texture the top surface of the cell, which causes reflected light to strike a second surface before it can escape, thus increasing the probability of absorption. If the front surface is textured into pyramid shapes for antireflection, all incident light is bent so that it strikes the polished—but otherwise untreated—back surface of the cell at an angle. This texturing causes light to be reflected back and forth within the cell until it is completely absorbed.
Electrical Resistance
Larger electrical contacts can minimize electrical resistance, but covering a cell with large, opaque metallic contacts would block too much incident light. Therefore, a trade off must be made between loss due to resistance and loss due to shading effects. Typically, top-surface contacts are designed as grids, with many thin, conductive fingers spread over the cell's surface. However, it is difficult to produce a grid that maintains good electrical contact with a cell while also resisting deterioration caused by changes in temperature or humidity. Generally, the back-surface contact of a cell is simpler, often being just a layer of metal. Other designs for electrical contacts include placing everything on the cell's back surface, or, as in some thin films, depositing a thin layer of a transparent conducting oxide across the entire cell.