Multijunction Solar Cells

Transcription

1 Multijunction Solar Cells George Cherucheril Stephen March Avinav Verma EE 332: Fall 2011 Professor Dalal Iowa State University Department of Electrical Engineering

2 1 Abstract This paper gives an overview of solar cells and multijunction solar cells. The first part of the document describes the basic physics and design of single junction solar cells. It also highlights the history of solar cells and the advantages of solar technology. The second part of this paper discusses the physics, design, and fabrication process of multijunction solar cells. This section also describes concentrators and some potential problems with multijunction solar cells. The final part of the paper discusses practical uses of multijunction solar cells and i prospects for future design advancement. The section talks about where multijunction solar cells are currently found and some possible future advancements of the device. Introduction to Multijunction Technology Current world power consumption is between 12 and 13 terawatts per year and is only going to increase as technology changes and the population grows. The primary sources for energy consumption are fossil fuels, which are known to be detrimental to the environment due to the excess emission of greenhouse gases from their use. An alternative that has been slowly gaining momentum as a viable alternative to the fossil fuel dependence is the use of Photovoltaic (PV) or solar cells. Enough solar energy hits the atmosphere in one hour to support society s energy demands for an entire year; however, the approximate solar energy incidence is only 1 kw/m 2, and is very dilute [1]. Standard solar cells seen for consumer and industrial use employ single junction techniques with amorphous Silicon (a-si) based technology. Such technology is expensive considering their efficiency (or yield) is only 10.1% (see Table 1) [2]. Better efficiencies are needed if solar technology is to become a larger contender in the movement toward alternative energy sources. Such mandated increases in efficiency have been seen in a class of PV known as multijunction solar cells. The main principle being the use of multiple semiconductors arranged in a stack to more effectively capture electromagnetic radiation than the standard single junction cells. Current records in multijunction efficiencies are over 40% and show the promise for values above 50% in the coming years. Multijunctions are arguably the most promising area of PV technologies and are critical in the development of a green energy infrastructure.

3 2 I. Solar Cells Semiconductor solar cells are fundamentally simple devices. Semiconductors are able to absorb light and convert a percentage of the energy of the absorbed photons to electrical current. A solar cell can be thought of as a semiconductor diode which separates and collects electrons and holes and conducts the generated current in specific direction [3]. The solar cell is delicately designed to efficiently absorb and convert light energy from the sun into electrical energy. The first modern (PV) cell was created in 1954 by Bell Laboratories. Three researchers, Gerald Pearson, Calvin Fuller and Daryl Chapin, designed a silicon solar cell that was capable of 6% efficiency with direct sunlight. Since then the solar cell industry has grown to become a major player in the renewable energy market. Demand for more power is growing in the United States and around the world. As the supply of oil and other non-renewable energy decreases, new and efficient ways of generating power are needed to reduce the world s dependence on fossil fuels. Solar power is a possible solution to this problem because it does not require fossil fuels for operation and it uses a renewable source (i.e. the sun) for fuel. Solar Power also has a limited effect on the environment and does not pollute the environment like fossil fuel emission. The most power consumed is in the summer and conveniently that is the time solar energy can be utilized most effectively. The largest deterrent for using solar cells is the initial price, but in the long run the money invested can be recovered because of the durability of solar cells, as many can last for more than 20 years [4]. Solar cell efficiency has grown dramatically in recent years as some labs have increased efficiency to over 40% using multijunction solar cells [5]. This increase and the aforementioned advantages make solar power an extremely realistic option in the future. Band Theory Before the evaluation and an ultimate understanding of solar cells may be reached, a brief introduction to Band Theory is required. The classical Bohr model of the atom is insufficient when analyzing semiconductors, so a better model is needed; this leads to Band Theory. In Band Theory there are three classifications of material: insulator, semiconductor, and conductor. The difference between each material is the size of the energy needed to move from one energy state or band to another. This difference in energy is called the Energy Band Gap or simply Band

4 3 Gap as shown in Figure 1. The smaller the band gap, the better a material is able to conduct the flow of charge carriers (i.e. electrons) from energy state to energy state giving rise to electric current. The lower energy band is referred to as the Conduction band and the upper energy band is referred to as the Valence band [6]. Figure 1: Conduction and Valence Band with Empty Middle Region or Band Gap, E g [6] As with most natural processes, carriers want to minimize their energy. For this reason, carriers are found in mass quantity in the valence band and in small amounts in the conduction band during thermal equilibrium conditions, thus many available high energy states are available to carriers, but these states are mostly empty because the carriers do not have enough energy to move from a low energy to a high energy state under normal thermal equilibrium situations. As shown in Figure 2, insulators have a large band gap and the band gap of conductors is thought to be zero. [6] Figure 2: Comparison of Band Gaps for Different Material Types [6] A great level of control may be attained by controlling the ability to have a material conduct at will. Such a level of control is available in semiconductors because the band gaps are small enough such that natural and easily attainable man-made processes can provide enough energy to charge carriers to traverse the band gap, going from the lower energy state in the valence band to

5 4 the higher energy state in the conduction band. When considering solar cells, the band gaps in the semiconductors used need to be comparable to the energy of incoming photons. Figure 3 provides a graphical example of the band gaps and relationship to common parts of the Electromagnetic Spectrum. [7] Figure 3: Comparison of Band Gap of Common Semiconductors and the Electromagnetic Spectrum [7] Solar cells are devices composed of semiconductors and are used to generate electric power when exposed to electromagnetic radiation. Based on the band gap of the materials used in creating the cells, not all the electromagnetic radiation may be used. That is, only certain parts of the electromagnetic spectrum may be used for solar cell technology due to the physical limitations of the semiconductor materials used. As the name of implies, solar indicates that the sun is the primary source for radiation is the sun. Figure 9 shows the radiation that is seen on Earth and how different types of semiconductors absorb light. It can also be seen through the figure that the peak in the available radiation of the distribution occurs on the range of visible light (i.e nm). This for this reason, solar cells are designed such that the material properties of the semiconductors are best this region of the electromagnetic spectrum. [7] Physical Representation of a Solar cell A simple solar cell uses a metallic grid to form one of the electrical contacts of the diode part of the solar cell. The grid allows light to hit the semiconductor between the grid lines and be absorbed and converted into electrical energy. An antireflective layer is fabricated between the

6 5 grid line to limit reflection and increase the amount of light absorbed by the cell [3]. Under the metal grid and the antireflective coat is a p-n junction. A p-n junction is the most fundamental part of any solar cell. A p-n junction is formed at the boundary between an n-type semiconductor and a p-type semiconductor. The cell is fabricated through diffusion, ion implantation, or epitaxial methods. Figure 4 shows the physical structure of a basic solar cell. Figure 4: A Basic Schematic of a Solar Cell [3] Solar cells can be created with a number of semiconductor materials. The most commonly used are the forms of silicon: monocrystalline, polycrystalline, and amorphous. Monocrystalline- The crystal lattice of the solid is continuous and unbroken to its edges. The monocyrstalline solar cell can have up to 24% efficiency. Though they are more expensive than polycrystalline and amorphous solar cells. [8] Polycrystaline- Composed of many small regions of single crystal materials. Polycrystalline Solar Cells have lower efficiency and costs than monocrystalline solar cells. They have an efficiency of up to 20% percent. The material is commonly found in solar panel systems[2].

7 6 Amorphous- Contains no periodic structure unlike the other types of solids. It is commonly used to build thin film solar cells (a solar cell that is made by depositing one or more thin layers of PV materials on a substrate). Amorphous is the cheapest of these three silicon based options but it is also the least efficient. Silicon is used because its absorption characteristics are a good match to the solar spectrum, and silicon is extensively used in the semiconductor industry so its fabrication technology is well developed (ergo cheaper to use). Solar cells can also be constructed using GaAs, GaInP, Cu(InGa)Se2, and CdTe. Physics of Solar Cells Semiconductors elements in Group IV like silicon or germanium usually form four covalent bonds, by using every available electron in the element. Free electrons can be generated in the material by adding impurities to the basic crystal structure usually from Group V (like As or P) to make an n-type material or Group III elements (like B) to make a p-type material. Impurities from Group V are called donors because the free electrons in the structure jump to the conduction band. Conversely, impurities from Group III are called acceptors because extra holes are added to the material by doping causing electrons from the valence band and leave a hole in the absence of the displaced electron. When n-type and p-type semiconductors are next to each other the excess electrons diffuse to the p-side and the excess holes diffuse to the n-side. This is called a p-n junction [3], which is illustrated in Figure 5. Both sides are likely to recombine with their opposite counterpart and produce a positive or negative ion. The area near the junction boundary where mobile charges are not present but ions are is called the depletion region (see Figure 6). The presence of the oppositely charged particles in the depletion region creates an electric field that goes from the n-side to the p-side. Eventually the generated electrical field will grow strong enough that it offsets diffusion [7].

8 7 Figure 5: Band Diagram of a p-n Junction in Thermal Equilibrium [3] Figure 6: Depletion Region of a p-n Junction [7] When a photon hits a p-n junction an electron hole pair (EHP) is created. Majority carriers (conduction band electrons on the n-side and holes on the p-side) are relatively unaffected by the additional photons because the EHPs created are irrelevant compared to the majority-carrier concentrations. The minority carriers (the number of conduction-band electrons in the p-side or the number of holes in the n-side) increases drastically. This cancels out the balance between diffusion and electrostatic forces. Minority carriers start to diffuse to the oppositely doped side, thus giving rise to electric current, potential, and power generation. All solar cells are designed with this basic principle to enable the movement of charge [8].

9 8 Circuit representation of PV cell In assessing the quality of conversion from solar radiation to electrical power, quantitative measurements are required to properly understand the amount of power produced by a solar cell. An equivalent and simplified circuit model of a solar cell is made up of two primary components: a current source and diode. An ideal solar cell may be modeled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component need to be added to the replacement circuit. The current source represents the photocurrent I ph generated within the illuminated cell. The series resistance R s includes the ohmic contributions of the electrodes, the contact between the semiconductor and the metal, and the resistivity of the active materials. This resistance has to be minimized for maximum solar cell efficiency. The shunt resistance R p illustrates the potential leakage current through the device. R p has to be maximized to reach high efficient cells.[9] Such a circuit is shown in Figure 7. Figure 7: Single Junction Solar Cell Equivalent Circuit [9] The diode is essentially a j p-n junction with a current voltage relationship for the whole circuit given as follows: ( ( ) ) (1) [9] Where I is the current through the diode, I o is the reverse saturation current of the diode, I s is the current through the series resistance (R s ), and I ph is the current generated by light. Also, the

10 9 quality factor of the diode is n (often n = 1). For large values of R p and small values of R s, the expression for the open circuit voltage, V oc, becomes: ( ) ( ) (2) [9] The quantities of the open circuit voltage, V oc, and the short circuit current, I sc (= I ph ), are used when calculating the power produced by the solar cell because V oc and I sc represent the largest forward voltage and current to be seen by the solar cell. Using the basic relationship between power, DC voltage, and DC current,, the maximum power of the solar cell may be given as P max, where P max < I sc V oc. The relationship between these quantities is given in Figure 8. Figure 8: Power Curve with I sc and V oc as Functions of Applied Solar Intensity [4] The fill factor (FF) indicates what percentage of the area produced by Isc and Voc and is thus related to P max as cell may be found as From the maximum power expression, the efficiency, η, of a solar, where P sun 100mW/cm 2. For modern day solar cells, the efficiency is dependent on the type of material used as a semiconductor in the p-n junction, the number of junctions used, the crystal structure of the semiconductor used, and the thickness of the solar cell [4]. Over the past 50 years, great advancements in the understanding of semiconductors has led to the use of better materials and the implementation of innovative solar cell designs have allowed for increases in efficiency. Present efficiency values for different solar cells are given in Table 1, and it can be seen that more complicated solar cell geometries

11 10 produce higher efficiencies. Newer technologies (such as organics) have efficiencies below the simple amorphous silicon. Even though amorphous silicon is not the highest in efficiency, it is the most common material for consumer solar cells because production and fabrication techniques with Si have been streamlined over the last 50 years. This leads to lower prices of production and more common application in solar cells. Table 1: May 2011 Efficiency Values for Common PV Materials [2] Classification Type Efficiency (%) Si crystalline 25 multicrystalline 20.4 thin film transfer 19.1 thin film submodule 10.5 Si (amorphous) 10.1 Si (nanocrystalline) 10.1 III-V cells GaAs (thin film) 28.1 Organic polymer 2.8 submodule 3.5 Multijunction GaInP/GaAs/Ge 32 GaAS/CIS (thin film) 25.8

12 11 II. Multijunction Solar Cells In order for a solar cell to work, the energy from incoming photons must have enough energy to give electrons in the valence band sufficient energy to move to the conduction band. Electrons are able to conduct to the conduction band without difficulty but are often conducted so far beyond, or overshoot, the conduction band that a large portion of the energy is lost to heat the in the crystal lattice as the electron moves to a lower energy state at the bottom of the conduction band. If the energy of incoming photons is smaller than the band gap, electrons are not given sufficient energy to move from the lower energy valence band to the higher energy conduction band, and the incoming photons are not absorbed by the solar cell thus passing through the material. This leaves a challenge to efficiently move electrons to the conduction band and lose as little energy as possible to the lattice from overshooting. The solar spectrum incorporates photons of different energy based on different wavelengths, so a single p-n junction solar cell will only have limited range for conversion efficiency where electrons move just beyond the conduction band and lattice heat loss is limited. The solution to this efficiency challenge is to use multiple p-n junctions stacked on top of each other with differing band gap values such that together they are able to absorb a wider range of the solar spectrum. This idea is referred to as Multijunction Solar Cells. [7] An example of the increased absorption can be seen in Figure 9. Figure 9: Comparison of Single Junction-Si Solar Cell to Common Triple Junction Solar Cell [4]

13 12 Multijunction or Tandum solar cells use a combination of different semiconductor materials to form junctions to optimize the conversion of photons into electricity. Multijunction cells were first studied in the 1960s and projected theoretical maximums in efficiency were between 38.2% and 51% depending on manufacturing technique [10]. Advancements in semiconductor technology have elevated the theoretical maximum efficiency of multijunction cells to 86.8% from those first projected values in the 1960s. Progress in the development of multijunction efficiency has shown promising results [4]. From some of the initial demonstrations in the 1980s of 16% to the current record of 43.7% set by Solar Junction in the middle of 2011 by using Concentrated PVs with a dilute nitride cell architecture, it is expected that within the coming years concentrated multijunctions for commercial use will reach efficiencies of 50%.[11] Quantum Efficiency When understanding the advantages to multijunctions, Quantum efficiency (QE) must be held in high regard. The QE of the solar cell is the number of charged carriers absorbed by the cell to the number of photons of a given energy that shines of the solar cell. The QE is 100% if the all possible photons of a certain wavelength are absorbed and all resulting minority carriers are collected. This overall efficiency depends on many factors including the temperature, amount of incident radiation and the surface area of the solar cell. An example of the QE for a GaInP/Ga/InAs/Ge triple junction is given in Figures 10. Figure 10: QE of Triple Junction System (right) and Corresponding Cell Design (left) [12]

14 13 Physics of Multijunctions The most common materials used in multijunction solar cells are III-IV semiconductors. III-IV compounds have become the basic materials for modern opto-elctronic devices. These III- V compounds such as gallium arsenide (GaAs), indium phosphide (InP) and gallium antimonide (GaSb) have some excellent characteristics that allow for fabrication high efficiency solar cells. Some of the characteristics of these materials are direct energy band gaps, high optical absorption coefficients, and good values of minority carrier lifetimes and mobilities [13]. When designing a multijunction solar cell there are three basic design considerations: band gap differentiation, lattice constant matching, and current matching. Band gap Differentiation Band gaps determine what wavelength of photons can be absorbed by a certain material and how much energy can be obtained by each photon. The difference between band gaps in multijunctions should be made as small as possible. This is because the amount of excess energy converted to light equalizes the photon energy and the band gap of the absorbing material [14]. One of the main goals and the most important mechanism of multijunction solar cells is to absorb as much of the light spectrum as possible. To do this, the top layer of the cell will have the largest band gap and the bottom layer will have the smallest band gap. This design allows the top layer to absorb the high energy photons while the lower layers absorb the lower energy photons that were not able to be absorbed by the top layers. When a photon strikes the multijunction solar cell it will be absorbed by the first layer if the photon energy is greater than the respective first material s band gap energy. If the photon energy is not greater than the band gap energy, it will move down to the next layer and will be absorbed if its energy is greater than the new layer s band gap energy. This process will continue itself until the photon gets to the last layer of the solar cell with the lowest band gap energy. An example of this is seen in a triple junction solar cell which uses GaInP, GaAs, and Ge, which have band gaps of 1.8 ev, 1.4 ev, and 0.7 ev, respectively, and the spectrum absorption is seen in Figure 9 and structure in Figure 10.

15 14 Lattice Constant Matching The lattice constant of semiconductor materials is the distance between atoms locations in a crystal. To make an effective multijunction solar cell it is important that the materials used have a similar lattice constant. If there is a mismatch in the lattice structures of the material defects or dislocations can occur. These defects can cause recombination which is when electrons in the conduction band drop back down into the valence band. This will decrease the device s V oc, I sc, and its fill factor, which means the maximum power of the device will go down [7]. A triple junction solar cell which uses GaInP, GaAs, and Ge conveys this concept with each material have the following lattice constants: Å, 5.65 Å, and 5.65 Å. Even though the lattice constants of different semiconductor materials are close enough to match, they may still have differences in band gaps (see Figure 11). This idea is used extensively when designing multijunctions because materials are compatible while still having a difference in band gaps to allow for different levels of photonic absorption and high efficiency devices. Figure 11: Relationship Between Band gap and Lattice Constants [15] Current Matching Most multijunction solar cells have a monolithic structure, meaning that all the materials are grown on one substrate and on top of each other. This means all the materials are connected together like a series circuit. The main fundamental of a series circuit is that all devices have the

16 15 same current running through them and current is limited by the smallest current produced by a particular device or junction. All of the junctions of a multijunction solar cell should have the same current to maximize efficiency, which means that the junctions absorb photons at the same rate. To ensure that different materials absorb photons at the same rate, designers change the thickness of each material [16]. To determine the thickness, one must determine which photons in the solar spectrum the material will absorb and the absorption constant of the material. The thickness of the material can be minimal if there are a lot of photons that exceed the band gap of the material, or if the absorption constant of the material is high because the photon can pass through less of the material before being absorbed. Common absorption coefficient values are shown is Figure 12. For example in a GaInP/GaAs/Ge multijunction solar cell, the Ge layer is thick because of its lower absorption rate. For every layer added to the multijunction solar cell the overall current of the device is lowered. This occurs because a fixed total number of photons are distributed over increasing numbers of cell layers, which decreases the amount available for electron promotion in any one cell layer [7]. The electrons that are promoted, however, have a greater electric potential; therefore, the addition of more layers increases the voltage of the device. The increase in voltage more than compensates for lost current, so the overall total power of the device increases. Figure 12: Absorption Coefficient, α, vs. Wavelength for Various Semiconductor Materials [14]

17 16 Design Structure of a Multijunction Solar Cell The top of structure of a multijunction solar cell is very similar to the top of a single junction solar cell. On top are metal contacts, usually made of Al, that touches the two sides of the structure as shown by Figure 13. Under the top lies a p- or n-type doped semiconductor, in Figure 13 an n-doped GaAs layer is shown. Also on the stop of the device is an anti-reflective coating. The coat is generally a dual-layer dielectric stack. Common materials used to make the coating are TiO2/Al2O3, Ta2O5/SiO2, or ZnS/MgF2 [4]. The reflective coating is designed to reduce the large reflectance from around 30% to less than 1%. After the reflective coating is the top cell, which is a p-n having the largest band gap of any other cell. The cells are connected via tunnel junction. A tunnel junction is junction of two highly doped p- and n-type semiconductors. The high doping creates an extremely thin depletion region that allows tunneling to occur across the junction. The junction creates an effective potential barrier for minority carriers and minimizes optical loss. In order for the multijunction to preform properly the band gap of the tunnel junction must be greater than the next cell [17]. The tunnel junction helps to separate the p-type first material from the n-type second material and it also connects the two p-n junctions without have a large voltage drop. Following the tunnel junction is the n-type junction of the middle cell. The middle cell s bandgap is less than the first cell s but larger than next cell s if one exists. If another layer exists in the solar cell it is separated from the previous cell with a tunnel junction. As seen in the Figure 13, between each cell s p-n junctions are two layers: a window layer and a back-surface field layer. Both layers create a similar heterojunction, with the window layer being n-type and the back-surface field layer being p-type. The purpose of the window later is to reduce surface combination while the back-surface field layer decreases the scattering of carriers towards the tunnel junction [7]. The bandgap of the window must be greater than the back-surface field layer and both layers lattice constant must be similar to the cell s main material in order for the multijunction be effective. Before the last layer of the multijunction solar cell are a buffer layer and a nucleation layer. These layers are there to control the recombination of minority carriers and to create a diffusion barrier for minority carriers. The final cell of the device is also the substrate of the device [18]. The material also has the smallest band gap and should theoretically absorb most of the final photons available.

18 17 Figure 13: Structure of a Triple-Junction PV Cell [4] Circuit equivalent of triple Multijunction solar cell The equivalent circuit for a 3-junction multijunction cell is given in Figure 14. The top layer corresponds to the material with the largest band gap, and the bot is the bottom layer with the smallest band gap. The expression for the current density of each layer of the junction may be given as: ( ) ( ( ) ) ( ( ) ) (3) [12] The addition of the J i 02 and R i p values are included to incorporate a more realistic current-voltage relationship through the 2 diode model. Such a model is used to account for temperature variations and improved accuracy at low irradiance values in the vicinity of V oc [19]. This model is not limited to a 3-junction system and may be extended to a junction system of any size. [12]

19 18 Figure 14: Triple Junction Solar Cell Equivalent Circuit [12] Fabrication of Multijunction Solar cell The most common way to fabricate multi-junction solar cells is to monolithically grow the layers of the device on top of each other. The two most common ways to do this are by molecular organic chemical vapor deposition (MOCVD) (example in Table 2) or molecular beam epitaxy (MBE) [4]. The difference between these two epitaxial methods is that the growth of the crystals is by chemical reaction instead of physical deposition. MOCVD is the preferred method because it produces a high quality crystal and it can also be scaled to produce in large capacities. Another method is mechanical stacking, which grows each layer of the device independently instead of on top of each other bringing all the cells together. The mechanical method is not preferred because of expense, heat-sinking, and bulkiness [15]. Table 2: GaIn Solar Cell Grown on Ga-As Substrate Using MOCVD Technology [15]

20 19 Problems and future improvements for multijunction. With efficiencies soaring over 40%, multijunction solar cells have proved to be better than any single junction device thus far. Of course, multijunction solar cells are not without their problems. The biggest issue with multijunction cells is the cost. The cost for a multijunction solar cell is quite expensive compared to a single junction silicon solar cell. The cost is so high because the materials used are more expensive and have a low rate of yield. Multijunctions are also more difficult to fabricate and use several materials to create a solar panel that is the same size of a single junction solar panel. Researchers are trying to create a cheaper multijunction by using less expensive materials with the goal of achieving similar, if not higher, efficiencies [20]. Table 3 shows weight and cost some semiconductors. Table 3 uses a cell cost per square centimeter of $1.25 for Si, $2.08 for thin Si, $20.83 for GaAs/Ge, as compared to $26.04 for a multijunction solar cell. a Table 3: Weight and Cost of Common Semiconductor Materials [20] Concentrators and how they apply to Multijunctions Compared to standard single junction solar cells, multijunction solar cells are more expensive per unit area of cell because of their increased complexity and the materials used in multijunctions have not seen the same reduction in manufacturing cost as the silicon used for most single junction units. One technology used to counter this issue is by using concentrators and is extensively examined in the field of Concentrated Photovoltaics (CPV). CPV was

21 20 introduced by Sandia labs in the 1970s and uses optical devices, such as a Fresnel lens, to focus solar light on to the solar cell to give a smaller area of solar cell the same amount of exposure as a large solar array [21]. Concentrators are measured in units of suns to express the level of intensity seen per unit area of concentrated solar cell. For example: 1000 suns means a concentrator increases intensity of light seen per unit area of a solar cell by 1000 times a solar cell without a concentrator. The application of concentrators is critical to multijunction technology because it allows multijunction solar cells to be manufactured at a much lower cost and making them more competitive with single junction solar cells. Cost is lowered because the relative value of a lens is much lower than semiconductor material. This enables multijunctions to be a viable alternative to single junctions when only small areas area available for illumination. Typical Concentrator values for common solar arrangements may be seen in Table 4. Note, multijunctions are MJ cells and have the highest concentration to optimize absorption. Table 4: Standard Concentration Ratios [Suns] for Different Solar Converter Systems [11] Current issues persist, however, with concentrators because the sun must be followed as it crosses the sky in order to optimize the concentration process, and such systems designed to track the sun are expensive. Also, the intense concentration of solar radiation on a single location can cause the semiconductor material to overheat, degrade, and become less efficient rather than more efficient as intended [22].

22 21 III. Future of Multijunctions Structure Development in the future The current standard for number of junctions in multijunction systems is 3 or 4 junctions. As semiconductor technology progresses, a higher number of junctions will be used. As mentioned with the lattice matching, these additional junction levels must maintain lattice matching. The best way to improve the number of junctions while maintaining lattice matching is through using semiconductors of progressively decreasing band gap values while progressing from the top layer of the device. This can be done by using alloys, such as Ga x In 1-x P, and adding something with a similar lattice constant but large band gap, like Al (from Figure 11), to get another layer with a different band gap. A good example of this can be seen in Figure 15, where the addition of Al to different layers enables more junctions to be constructed [23]. Figure 15: Increasing Number of Junctions by Adding Al and N to Adjust Band Gap values [23] The implementation of more junctions allows for greater efficiency because there is a smaller incremental change in the change in band gaps from the top layer to bottom layer in the solar cell. The incremental change means less thermal loss from high energy photons reaching small band gaps in the lower levels. The more cell junctions lead to greater cell s efficiency, as shown in the Figure 16 [24].

23 22 Figure 16: Increasing Junctions From 1,2,3, to Alters Efficiency and Graphical Progression of Solar Efficiency Based on Number of Junction [4][24] Where Do We Find Multijunctions Multijunctions can be very beneficial in the future. Multijunctions are used in various technologies all over the world but due to the high they are mostly used in the satellites and aerospace industries. For example: NASA commonly uses multijunction solar panels projects. NASA Mars project using multijunction Photovoltaic cells have a reduced efficiency on Mars compared to Earth, this occurs because of the reduced sunlight intensity, dust particles, and colder temperatures. In order to solve this problem, NASA and Spectrolab modified the GaInP/GaAs/Ge multijunction solar cell. They slightly altered the epitaxial structure of the cell and also increased current generation by 5% to achieve better current matching. The modified solar cell was called an Ultra Triple Junction solar cell. The first step was to define the surface incident spectrum using the previous Mars mission data. One simulation occurred at 30 latitude solar spectrum (equator) and the other at 60 latitude solar spectrums (poles). When testing the Ultra Triple Junction solar cell in a Mars simulator, the device achieved an efficiency of 24.9% at 60 and an efficiency of 25.8% at 30. These results show that multijunction solar cells have potential to be effective in future explorations on Mars. [25][26]

24 23 Solar Blanket technology Another future technology is the multijunction solar blanket (shown in Figure 16). Designed for space applications, this technology uses flexible triple junction solar cell technology. Prototype blankets have demonstrated power levels of 500W/kg. The reason this technology is increasing in use is its application of alternative thin film technologies. Such technologies are of interest for space use because of lower manufacturing costs and higher radiation resistance. The thin PV films are preferred because they can be made on thin flexible substrates yielding a high specific power density. Advancements in thin film technology are making the triple junction film thinner and increasing the cell efficiency and performance. Current triple junction ranges from 75 to 100 µm thick and it achieves an efficiency of 28.6%. The solar blanket is made using 2-Mil Kapton sheets with space grade adhesive bonding connected to the GaInp/GaAs/Ge solar cell. Next, a space grade conformal coating is applied and cured. Fully active GaInP/GaAs/Ge thin multijunction cell in series have been tested with no electrical degradation even when wrapped about a 10 cm radius of curvature (thus proving their flexibility). Production time of the blanket takes about 6 hours. Groups are trying to enhance blanket technology by improving optical coatings, interconnect schemes, and substrate materials. They are also trying to evaluate the stiffness, environmental, and acoustic properties to successfully deploy the blanket in various conditions similar to those seen by satellites. The biggest challenge for the blanket technology is the fabrication of very thin triple junction solar cells with less than 100 µm thickness. However, the blanket is easy to break so improvements to the stiffness of the blanket are needed as well [27]. Figure 16: Sample of Flexible Solar Blanket Material with 12 Cell Modules [27]

25 24 DOE Solar Energy Technology Program The Department of Energy (DOE) is trying to create higher performing photovoltaic devices to enhance the energy infrastructure of the nation. The DOE has multiple projects that try to improve polycrystalline thinfilm PV and multijunction concentrator devices. The DOE is trying to develop multijunction solar cells that can be incorporated into structures that concentrate sunlight using mirror or lenses. The DOE believes that multijunction solar cells can be used to create low-cost electricity generated from concentrated sunlight. If the DOE s project is successful, solar energy can become more competitive in the energy markets and multijunction technology will serve as the leader in the PV market. [28] With the advent of efficiency levels greater than 40% in multijunction technology, the DOE s interest in using multijunction technology has increased as well. The DOE has been able to incorporate multijunction technology in space and aerospace systems, but the DOE is pushing to get multijunction technology into the consumer markets such as in the power plant market. Challenges still persist with system size, reliability, and cost. Despite these challenges, the DOE forecasts multijunction-concentrator use and technological advancements to dramatically increase, and enable multijunctions to serve as a viable means for reaching greater energy infrastructure. [28]

26 25 IV. Conclusion Photovoltaic technology has advanced enough to where it has become a realistic option to help lower the world s dependency on fossil fuels. More specifically, multijunction solar cells have shown the greatest potential in terms of efficiency. The recorded efficiencies have soared over 40% and are projected to reach 50% in the coming years. The fundamental principle behind multijunctions of incorporating multiple materials of different band gaps to effectively capture light make the technology the most viable and promising PV device. With future developments to multijunctions including the addition of more junctions, improved concentrator technology, and continued advancement in fabrication to cut costs, multijunction technology is projected to become the leader of the PV industry [28].

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