Energy Yield Prediction for II-VI-Based Tandem Solar Cells

In the quest for the Holy Grail of power generation, many are tempted to try and harness the power of water. After all, nature provides us with the perfect source of renewable energy; the sun shines down on the water, which heats up and becomes steam that powers turbines and generates electricity.

While it is true that the water cycle is an inexhaustible source of energy, we must consider the limitations of this approach. For a start, the water needs to be clean, fresh and available to ensure that it can be used without becoming polluted. Moreover, we must take into account the fact that the water is needed to sustain life; the more demand there is for power generation, the more chances there are for pollutants to be released into the environment. This is why many opt for alternative means of energy generation; after all, it is not always practical or advisable to harness the power of water.

As a result of this desire to avoid or reduce our dependence on environmentally harmful energy sources, there has been increased research into novel and more sustainable approaches. One such area of research is the investigation of tandem solar cells, which are effectively two or more solar cells connected in series to create a higher voltage and therefore more power. Solar cells made from the II-VI group of semiconductors, in particular, offer great potential in this regard as they can be operated at relatively high temperatures and high irradiances, which enables the creation of a greater power output compared to cells made from more traditional semiconductors such as silicon or cadmium telluride.

Creating High-Quality Tandem Solar Cells

A tandem solar cell is one in which two or more solar cells are electrically connected in series, with the current flowing through both cells, hence the name. The aim is to increase the voltage and therefore the power generated by the device. Because the current flows through both cells, the total voltage across the device will be the sum of the voltages of each cell. In this way, connecting two or more solar cells into a series becomes a simple matter of connecting an external load across the cells and measuring the voltage across the load (Figure 1). This method of creating high-quality tandem solar cells is more commonly known as “voltage addition”. In technical terms, the voltage addition technique allows the creation of electrical energy from light using low-cost, widely available materials that are environmentally friendly and allow for large-scale manufacturing.

There are various ways in which the voltage can be added, for example, using a diode, a field-effect transistor (FET), or a metal oxide semiconductor (MOS) photocell. The most suitable voltage addition circuitry will be determined by specific device parameters, operating requirements and costs. In general, however, the diode connection is most frequently used because it is simple to implement and does not require additional circuitry. The diodes can simply be connected in series across the solar cells. Likewise, an n-p-n junction diode can be used, with the p-type semiconductors acting as voltage sources, with the n-type semiconductor connected in parallel to the p-type semiconductors, and the n-type and p-type semiconductors being connected in series (Figure 2). A junction diode is basically a two-terminal device, with one terminal being connected to the n-type semiconductor and the other terminal being connected to the p-type semiconductor, and one side of the junction being reverse-biased. This causes the depletion region to extend into the p-type semiconductor, thus ensuring that a potential difference exists between the two terminals of the diode (Figure 3).

Using a junction diode across two solar cells requires that both cells must be of the same type as determined by the diode. For example, if one cell is p-type and the other cell is n-type, then only an n-p-n junction diode can be used for voltage addition. Another important factor to consider is the polarity of each individual cell. The polarity of a cell is determined by the voltages of each terminal with respect to ground, for example, the positive terminal of a cell is relative to ground while the negative terminal is not. Cells with the same polarity can be easily connected in series to create a high voltage. For example, in the case of two solar cells each having a +5V terminal, the positive terminals of the two cells can be connected together to create a +10V terminal and the negative terminals can be connected to ground to create a 0V terminal (Figure 4).

Potential Applications Of Tandem Solar Cells

Tandem solar cells have numerous applications, with the most common being to provide power to electronic devices such as cell phones, torches, radios, and so forth. In space research, UV photodetectors made from II-VI semiconductors can be used to detect UV light that is generated by the Sun, which may be used, for example, to create propellant for propulsion systems or to trigger chemical reactions.

II-VI semiconductors are also used in the manufacture of flat-panel displays. Because they enable the generation of high voltages, it is possible to fabricate small displays that are extremely thin because they do not require bulky circuit boards to connect the individual cells. Instead, individual diodes are used as terminals and are therefore only a few microns thick. This type of display, which is also known as an “integrated diode” or “diode-stack array”, is much more efficient than traditional liquid crystal display (LCD) panels because it does not require backlighting and can generate its own light, which significantly reduces energy consumption.

The Rise Of Tandem Solar Cells

The use of tandem solar cells dates back to at least the 1960s, with U.S. patent #3,043,852 describing a system in which two or more solar cells are used in combination to generate electricity. In the intervening years, little advancement has been made in terms of voltage addition and most tandem solar cell research has been in the development of devices that are more efficient at generating electricity. Today, however, the use of tandem solar cells is becoming more commonplace as it is possible to achieve high voltages using materials and processes that are relatively inexpensive. Moreover, with the increasing demand for clean energy and the decreasing cost of solar cells, this approach has become more attractive. Indeed, as the cost of solar cells continues to fall and the ability to create greater voltages using inexpensive materials becomes more prevalent, more and more applications for tandem solar cells will be found.

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