A review of concentrator silicon solar cells

Renewable and Sustainable Energy Reviews 51 (2015) 1697–1708

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A review of concentrator silicon solar cells Yupeng Xing a,n, Peide Han b, Shuai Wang b, Peng Liang b, Shishu Lou b, Yuanbo Zhang b, Shaoxu Hu b, Huishi Zhu b, Chunhua Zhao b, Yanhong Mi b a Tianjin Key Laboratory of Film Electronic and Communication Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, China b State Key Lab on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

art ic l e i nf o

a b s t r a c t

Article history: Received 27 November 2013 Received in revised form 1 July 2015 Accepted 8 July 2015

The problems of worldwide energy shortage and environment pollution are becoming more and more serious, thus lots of attention has been paid to renewable and sustainable energy. The development of photovoltaic technology was rapid in recent years as one of the promising renewable energy, the worldwide total amount of photovoltaic power plants had reached nearly 100 GW in 2012, and about 90% of the worldwide solar cells are crystalline silicon solar cells. But there is still a large gap between the electricity costs of photovoltaic and traditional fossil energy, lots of methods have been tried to decrease the costs. The efficiency of the cell could be increased by concentration, and parts of the solar cells are replaced by cheaper optical elements in concentration photovoltaic, thus the costs of photovoltaic could be decreased by concentration. The traditional solar cells used for concentration were III–V multi-junction solar cells, their costs were high although they had high efficiency, thus people tried to use cheaper silicon solar cells for concentration to decrease the costs further. In this work, six kinds of silicon solar cells with different structures used for concentration were summarized; the device structures, manufacturing processes and efficiencies of the cells were compared. The prospects of concentrator silicon solar cells were predicted, the Si HIT cell using back contact structure, the multi-junction cell containing Si back contact cell and the Si VMJ cell used with Ge and GaP VMJ cells were considered to have good potential in low, middle, high and very high concentration photovoltaic. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Concentrator Silicon solar cell Series resistance Device structure Efficiency Concentration ratio

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concentration systems using silicon solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Requirements for concentrator silicon solar cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concentrator silicon solar cells with different structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Front and back contact cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Back contact cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Vertical junction cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Micro cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Silicon-based hetero-junction and thin film cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Silicon-based multi-junction cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Summarization and prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Corresponding author. Tel.: þ 86 22 6021 4221. E-mail address: [email protected] (Y. Xing).

http://dx.doi.org/10.1016/j.rser.2015.07.035 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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1. Introduction The problems of worldwide energy shortage and environment pollution are becoming more and more serious, thus lots of attention has been paid to renewable and sustainable energy. According to the statistics made by International Energy Agency (IEA), 13.6% of global energy came from renewable and sustainable energy in 2012 (hydro was 2.4%, biofuel and waste were 10%, others including geothermal, heat, solar and wind were 1.1%), and the data was 12.4% in 1973 [1], the increase mainly came from the increase of geothermal, heat, solar and wind energy, the emission of global carbon dioxide did not increase in 2014 because of the increase of renewable and sustainable energy, which was the first time in recent years [2]. Although Solar Energy only occupies a small proportion in current renewable and sustainable energy, its development was rapid in recent years [3–5], photovoltaic (PV) technology can convert the sunlight into DC (direct current) electricity directly, solar cells could be used in off-grid and grid electric power systems, the size and number of solar cells and modules could be chosen according to the electric power demand of the systems easily, thus PV technology was used more conveniently than other kinds of renewable and sustainable energy [6]. The scale of worldwide PV industry is becoming larger and larger [7,8], the worldwide total production of solar cells and modules had reached 38.5 GW in 2012, the worldwide total cumulative amount of PV power plants had reached nearly 100 GW in 2012 [9], the total production of solar cell modules was about 24 GW in China in 2013 and the total amount of PV power plants was about 7.9 GW in China in 2012 [10], it was estimated that the worldwide electricity generated by solar cells will reach 280 TW h in 2030 [4,11]. There are many kinds of solar cells, and about 90% of the worldwide solar cells are crystalline silicon solar cells, which are made of monocrystalline and multicrystalline silicon wafers [3], others include III–V, thin film, dyesensitized, organic solar cells, and so on [6]. The crystalline silicon solar cell has many advantages compared to other kinds of solar cells; it has non-toxic, high stability, relative low costs and high efficiency. The manufacturing costs of crystalline silicon solar cell modules have been far below US$ 1/W now [12], the lowest costs of a Kilowatt hour generated by PV have been decreased to 9 Eurocents in Germany, it will be decreased to that of the electricity generated by coil and natural gas in abundant sunshine region in 2025 according to the report made by Fraunhofer Institute for Solar Energy Systems, and PV will be the cheapest electricity technology [13]. Lots of methods have been tried to decrease the costs of PV further [14], an effective method is to increase the efficiency of solar cell to increase its output power, the efficiency of the commercial P-type crystalline silicon solar cell has reached 20.3% [15]. The efficiencies of all kinds of cells have been increased by many companies and institutes for several years, the highest efficiencies of III–V, CIGS (Cu(In,Ga)(S,Se)) and silicon solar cells were all got under concentration, the highest efficiency of III–V cell was 46% under 508 suns [16], which was the highest efficiency among all kinds of cells [17], the highest efficiency of CIGS cell was 23.3% under 15 suns [18], and the highest efficiency of silicon cell was 27.6% under 92 suns [17,19], because the open circuit voltage of solar cell increases with concentration ratio logarithmically, the efficiency of solar cell will increase with concentration ratio when it was smaller than a certain value [20]. The solar cells which are the most expensive parts of PV system were replaced by much cheaper optical elements when using concentration technology, thus the costs could be decreased further, and the concentrator solar cell was considered to be one of the third generation solar cells [21]. More than 100 MW concentration

photovoltaic (CPV) systems have been built around the world [22], and most of the systems used the III–V multi-junction solar cells based on Ge or GaAs substrates. However, the costs of Ge and GaAs substrates were high, the costs of CPV systems were about US$ 3/W in 2012 [23], it is still higher than that of silicon PV systems now. So people try to use silicon solar cells which were much cheaper in CPV systems, and the concentration ratio can be set either high [24] or low [25] corresponding to the structure of the cell. Schwartz et al. [26–28] and Blakers [29] had ever given reviews on silicon solar cells used for concentration in 1970s, 1980s and 2000s, but there was lots of progress in recent years, thus we gave a latest review in this work. The purpose of this work was to compare different kinds of concentrator silicon solar cells and to give the most promising cells. The following content will be discussed in this work: the concentration systems using silicon solar cells were reviewed simply in the first part; the basic requirements for concentrator silicon solar cell to realize high efficiency were discussed simply in the second part; six kinds of silicon solar cells with different structures used for concentration were summarized, the device structures, manufacturing processes and efficiencies of the cells were compared in the third part; the prospects of concentrator silicon solar cells were predicted in the last part.

2. Concentration systems using silicon solar cells The concentration systems could be divided into low (1–10 suns), middle (10–100 suns), high (100–1000 suns), and very high (larger than 1000 suns) concentration systems according to their concentration ratio [30], the difficulties in tracking and cooling increase with concentration ratio, the silicon solar cells could be used in above four kinds of concentration systems. The concentration could be realized by using optical elements such as reflectors and Fresnel lens, the incident light could be concentrated to the cell by reflection and transmission [31], as shown in Figs. 1 and 2, which were drew by Tsai and Zhuang et al. [32,33]. The downconverter materials also could be used to concentrate the incident light to the cell, the incident light was absorbed by the downconverter materials first, and then the light with longer wavelength whose photon energy was close to the bandgap of silicon was emitted by the downconverter materials to the cell. The downconverter materials were always made into waveguides to guide the light to the cells and they were always called luminescent concentrator, the cells could be placed at the top, bottom or edge of the waveguides, as shown in Figs. 3 and 4, the systems were made by Yoon and Zhang et al. [34,35], the light intensity reaching the cell was always larger than 1000 W/cm2 (1 sun) [35,36]. Using the downconverter materials to realize concentration could decrease the demand of tracking, because the effect of non-uniform illumination on the luminescent concentrator was little compared to above optical elements [34,35]. But the efficiency of the luminescent concentrator system was still low compared to traditional optical concentrator system, the highest efficiency was just 7.1% under 2.5 suns using GaAs cells [17,37]. The upconverter materials could be placed at the bottom of the bifacial illuminated cell under concentration, these materials can absorb the light which could not be absorbed by silicon and emit the light with shorter wavelength which could be absorbed by silicon to the back surface of the cell [38,39]. Above optical concentrator and luminescent concentrator elements could be used together, Arnaoutakis et al. added compound parabolic concentrator to the upconverter materials which was placed at the bottom of the cell, it increased the efficiency of the cell from 0.123% to 0.163% under 0.024 W/cm2 illumination of 1523 nm light [38].

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3. Requirements for concentrator silicon solar cell Similar to the common solar cells used for non-concentration, the concentrator silicon solar cell also must have high efficiency to decrease the costs, thus it must meet the following five requirements [29]:

Fig. 1. The schematic of reflector concentrator drawn by Tsai et al. [32]. Reprinted from Solar Energy, 115, Tsai CY, Improved irradiance distribution on high concentration solar cell using free-form concentrator, 694–707, Copyright (2015), with permission from Elsevier.

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1. Best substrates with high minority carrier lifetime; 2. Best edge, front surface and back surface passivation; 3. Best design of substrate and emitter dopants to minimize the electrical losses caused by series resistance and recombination; 4. Best design of front surface antireflection layer and light trapping in the cell; 5. Best design of metal electrode structure to minimize the optical and electrical losses caused by metal electrodes. Above first to fourth requirements were very similar to those for common silicon solar cells, and lots of works have been done [40], thus these will not be discussed in detail in this work. The fifth requirement was very important for the concentrator solar cell, because its current increases with concentration ratio linearly, the power losses caused by series resistance are equal to the square of current multiplied by series resistance, it will increase with concentration ratio quadratically, and the efficiency losses caused by series resistance will increase with concentration ratio linearly assuming that the series resistance is constant. Thus, the series resistance of the concentrator silicon solar cell must be made lower than that of common silicon solar cells to decrease the power losses under concentration to keep the high efficiency, and the series resistance could be decreased by changing the structure of the cell.

4. Concentrator silicon solar cells with different structures Six kinds of silicon solar cells used for concentration will be discussed below: front and back contact cell, back contact cell, vertical junction cell, micro cell, silicon based hetero-junction cell and silicon based multi-junction cell. These cells are mainly made of CZ (Czochralski) and FZ (Float Zone) monocrystalline silicon wafers, multicrystalline silicon wafers and silicon based thin film materials are rarely used because of their low minority carrier lifetime and light-induced recession under concentration. 4.1. Front and back contact cell

Fig. 2. The schematic of Fresnel lens concentrator drew by Zhuang et al. [33]. Reprinted from Optics and Laser Technology, 60, Zhenfeng Z, Feihong Y, Optimization design of hybrid Fresnel-based concentrator for generating uniformity irradiance with the broad solar spectrum, 27–33, Copyright (2014), with permission from Elsevier.

The front and back contact cell, which also could be called bifacial contact cell, is the most basic structure of silicon solar cell, the structure of the bifacial contact cell made by Paternoster et al. is shown in Fig. 5 [41]. The front surface of the cell is usually consisted of upright or inverted pyramids in the order of micrometers and coated with passivation and antireflection layers to decrease light reflection, finger metal electrode is made on the front surface, the emitter is made below the front surface, the back

Fig. 3. The schematics of the solar cells placed at the top of the waveguide made of downconverter material to realize concentration, the system was made by Yoon et al. [34]. Reprinted by permission from Macmillan Publishers Ltd.: [NATURE COMMUNICATIONS] (Yoon J, Li LF, Semichaevsky AV, Ryu JH, Johnson HT, Nuzzo RG, et al. Flexible concentrator photovoltaics based on microscale silicon solar cells embedded in luminescent waveguides. Nat Commun. 2011;2; 343), copyright (2011) at http://www.nature. com/ncomms/index.html.

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Fig. 4. The schematics of the solar cells placed at the edge (a) and bottom (b) of the waveguide made of downconverter material to realize concentration, the systems were made by Zhang et al. [35]. Reprinted from Solar Energy, 117, Zhang J, Wang M, Zhang Y, He H, Xie W, Yang M, Ding J,Bao J, Sun S, Gao C, Optimization of large-size glass laminated luminescent solar concentrators, 260–267, Copyright (2015), with permission from Elsevier.

Fig. 5. The structure of the bifacial contact cell made by Paternoster et al. [41]. Reprinted from Solar Energy Materials and Solar Cells, 134, Paternoster G, Zanuccoli M, Bellutti P, Ferrario L, Ficorella F, Fiegna C, Magnone P, Mattedi F, Sangiorgi E, Fabrication, characterization and modeling of a silicon solar cell optimized for concentrated photovoltaic applications, 407–416, Copyright (2015), with permission from Elsevier.

surface field (BSF) is always made above the back surface, and the back surface is covered by metal electrode wholly. The PERL (passivated emitter rear locally diffused) silicon solar cell made by Zhao et al. based on this structure had ever maintained the highest efficiency of silicon solar cells for 15 years [42]. A balance between the electrical and optical losses caused by front metal grids must be made [43], the series resistance must be made low to decrease the electrical losses as mentioned above, thus the grids must be made wide and thick. However, the shading losses of grids increase with width, thus people tried to reflect the light illuminated on the grids into the cell by shaping the grids in special structures [27]. Green et al. made grooves on the front surface of the cell by etching and made the direction of grids oblique to that of grooves, the efficiency of the cell was 25% under 50–100 suns [44], Cuevas et al. made triangular ridges on front surface by etching and made grids on the ridges, the efficiency was 26% under 88 suns [45]. Zhao et al. added prismatic covers to the front surface of the cell to reflect the light reflected by grids to the cell, the efficiency was 25% under 200 suns [46], Chiang et al. made a module using these cells, its efficiency was 20.5% under 79 suns

[47], which was the highest efficiency of silicon solar cell modules under concentration [17]. Above cells had high efficiency, but their manufacturing processes were complex, people tried to simplify the process to decrease the costs. Ruby et al. decreased the steps of photolithography to three to make the cell; the efficiency was 22.6% under 40 suns and 22.3% under 100 suns [48]. Paternoster et al. also used three steps of photolithography to make the cell; the structure of the cell is shown in Fig. 5 [41], they made four bus-bars surrounding the active region of the cell instead of common two bus-bars to reduce the series resistance, the efficiency was 22.1% under 80 suns and higher than 18% under 300 suns. They found that the short circuit current of the cell increased with concentration ratio super-linearly, and the excess was about 8% under 300 suns, the reason was that the diffusion length of photo-generated carriers increased under concentration. Ruby et al. and Paternoster et al. used one step of photolithography to define the active region of the cell, and used two steps to define the structure of front metal grids, they made T-shaped grids to decrease metal contact area to decrease surface recombination [41]. Morvillo et al. used two steps of photolithography to make the cell, they also made four bus-bars; the efficiency was 22% under 100 suns and more than 20% under 200 suns [49]. Castro et al. made pilot production of the cells, they also used two steps of photolithography, the highest efficiency was 21% under 100 suns and nearly 20% under 250 suns, Morvillo et al. and Castro et al. used one step of photolithography to define the active region and another step to shape the front metal grids [50]. Above cells were made by the CMOS-like semiconductor manufacturing process, the front metal grids were made by photolithography, metal evaporation and lift-off processes, the costs were high even the steps of photolithography were decreased to two, thus people tried to use cheaper process to make the grids. Wenham et al. made the laser grooved buried contact (LGBC) cell [51], the trenches were grooved by laser on the front surface of wafer first, and the grids were made in the trenches by plating, this process was much cheaper, the structure of the LGBC cell made by Vivar and the researchers from BP Solar company is shown in Fig. 6 [52]. The grids could be made narrow and high by this process, thus the electrical and optical losses could be made small at the same time. Zhang et al. redesigned the trench structure and made the grids filled in the bottom of the trenches, thus part of the light illuminated on the grids was reflected into the cell and the shading loss was decreased further, the efficiency of the cell was in the range of 21–22% under 20 suns [53]. Most of above cells were made using FZ wafers in laboratories or pilot production lines, the BP Solar company started to make the LGBC cells in industrial production line using CZ wafers

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in 1993, the cells were used in the EUCLIDES project, and the costs decreased significantly [52], the highest efficiency was 20.1% under 10 suns, 19.8% under 20 suns and 18% under 40 suns [54]. The BP Solar company had been producing the LGBC cells for several years, Vivar and the researchers from BP Solar company increased the efficiency to about 20.2% under 25 suns [52]. The national renewable energy center (NREC) of British also made LGBC cells in industrial production line using CZ wafers, the front dicing technique was used to decrease the edge recombination, the highest efficiency was 20.4% under 26 suns, 19.4% under 50 suns and about 18.9% under 100 suns, the cells were used in the European LAB2LINE, APOLLON and ASPIS projects [55]. People tried to use common industrial crystalline silicon solar cell manufacturing process to make the concentrator silicon solar cells to decrease the costs further, screen-printing technology was used to make the front metal grids, the common solar grade multicrystalline and monocrystalline silicon wafers were used. The structure of the cell was similar to that of the common industrial crystalline silicon solar cell, but its size was made much smaller to decrease its operating current and to suit the concentrator. Coello et al. made the cells using industrial crystalline silicon solar cell manufacturing process, they used plating technology to increase

Fig. 6. The structure of laser grooved buried contact cell made by Vivar et al. [52]. Reprinted from Solar Energy Materials and Solar Cells, 94, Vivar M, Morilla C, Anton I, Fernandez JM, Sala G, Laser grooved buried contact cells optimised for linear concentration systems, 187–193, Copyright (2010), with permission from Elsevier.

Fig. 7. The structure of the PERC cell.

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the density and thickness of grids to decrease the series resistance, the efficiency was close to 14% under 15 suns [56], Fellmeth et al. made the cells with efficiencies of 20.4% under 15 suns and 19.1% under 50 suns [57]. Chen et al. also made the cells, they decreased the width of grids to 50 μm by using new screen printing technology, the efficiency was 20% under 3–16 suns [58], they predicted that the efficiency would be larger than 21% after using PERC (passivated emitter rear contact ) structure, which used a back passivation layer with local BSF and metal contact [15,59], the structure of the PERC cell is shown in Fig. 7, but extra efforts must be made to decrease the added series resistance caused by local metal contact. Yadav et al. used the common commercially monocrystalline and multicrystalline silicon solar cells made by an Indian company for concentration, they did electro-analytical research to the DC and AC (alternating current) characterizations of the cells using impedance spectroscopy technology, they found that the efficiency of the monocrystalline silicon solar cell increased from 13.2% under 1 sun to 16.3% under 10.227 suns [60], and the efficiency of multicrystalline silicon solar cell increased from 14% under 1 sun to 14.8% under 3.27 suns [61]. Varieras et al. made a concentration module of 3 suns using the commercial crystalline silicon solar cells and related module manufacturing process; they just used the patterned glasses to cover the cells which were made strips instead of common glasses [62]. Slooff et al. placed a commercial multicrystalline silicon solar cell to the edge of downconverter materials, the short circuit current and maximum output power of the cell increased about 1.7 times [37], Zhang placed a commercial monocrystalline silicon solar cell to the bottom of downconverter materials, the maximum output power increased about 1.38 times [35]. The back surface of above cells was covered by metal electrode fully, and the light could only illuminate into the cell through front surface. In fact, the back electrode also could be made finger shape just like the front metal grids, and the light could also illuminate into the cell through back surface, this cell was called bifacial illuminated cell. Rüdiger et al. made the bifacial illuminated cell using the CMOS-like semiconductor manufacturing process based on N-type wafer, the upconverter materials were placed to the bottom of the cell [63]. The structure of the cell is shown in Fig. 8, local contact structure was used, the front and back surfaces of the cell were made planar, the front and back antireflection layers were optimized to transmit the sub-bandgap photons to the bottom upconverter materials and to decrease the back surface reflection of the light emitted by upconverter materials around 980 nm [63]. Fischer et al. placed BaY2F8 and NaYF4 upconverter materials to the bottom of the bifacial illuminated cell under concentration, these materials absorbed the light around 1500 nm and emitted the light around 980 nm, the short circuit current of the cell increased 17.273.0 mA/cm2 under 94 717 suns using BaY2F8 upconverter materials [64], and it increased 13.1 mA/cm2 under 210 suns using NaYF4 upconverter materials [39].

Fig. 8. The structures of the bifacial illuminated cells made by Rüdiger et al. [63]. Reprinted from Solar Energy Materials and Solar Cells, 128, Rüdiger M, Fischer S, Frank J, Ivaturi A, Richards BS, Kramer KW, Hermle M, Goldschmidt JC, Bifacial n-type silicon solar cells for upconversion applications, 57–68, Copyright (2014), with permission from Elsevier.

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Above cells were made by CMOS-like semiconductor process or industrial crystalline silicon solar cell manufacturing process, the PN junctions of above cells were mainly made by diffusion, the front surface was covered by passivation and antireflection layer, and the grids were made by evaporation, plating or screen printing technology. Untila et al. made a bifacial illuminated cell using an innovative method, three changes were made: (1) the emitter and BSF were made by spinning on boron and phosphorus silicate glasses on the two surfaces of wafers first and then annealed, (2) transparent conducting oxide (TCO) films were deposited on the two surfaces by ultrasonic spray pyrolysis technology instead of common passivation layer, an indium fluorine oxide (IFO) film was deposited on the N þ -type layer and an indium tin oxide (ITO) film was deposited on the P þ -type layer, (3) 60 μm wide soldered copper wires were laminated to the two TCO films instead of silver grids, and the wires were connected to the interconnecting ribbons which were located outside the cell. Above electrical connecting method could decrease the optical losses of electrodes, because the bus bars were quitted from the front surface of the cell, besides the copper wires were circular and could reflect more light illuminated on them to the cell than silver grids which were rectangular [65], and the costs were decreased by using cheaper Cu materials. The structure of the cell is shown in Fig. 9 [66], the highest front and rear illuminated efficiencies of the cell which was made using N-type wafer were 17.6–17.9% and 16.7–17% under 1–3 suns [67], the front and rear illuminated efficiencies of the cell which was made using P-type wafer were 18.6–19% and 14.9–15.3% under 1–5 suns [68].

4.2. Back contact cell To solve the problems of series resistance and shading caused by front metal grids, the back contact cell was proposed and made by Schwartz et al. in 1970s, the efficiency was close to 17% under 50 suns [26], the structure of back contact cell simulated by Kluska et al. in Fraunhofer Institute for Solar Energy Systems is shown in Fig. 10 [69]. The positive and negative metal electrodes were both made on the back surface of the cell, thus there was no shading on front surface. The electrodes were interdigitated, the series resistance caused by electrodes could be decreased by optimizing the structure parameters and thickening the thickness of the electrodes. The front surface was also always consisted of upright or inverted pyramids in the order of micrometers and coated with passivation and antireflection layer; there was always a front surface field (FSF) below the front surface to decrease the series resistance and to shield the adverse effect of front surface recombination. After that, Sinton et al. made the back pointcontact cell with the efficiency of 27.5% under 100 suns, they used point-contact structure in metal contact region to decrease surface recombination [70]. Slade et al. (from Amonix Company) made

Fig. 9. The structure of the bifacial illuminated cell made by Untila et al. [66]. Reprinted from Solar Energy, 106, Untila GG, Kost TN, Chebotareva AB, Zaks MB, Sitnikov АМ, Solodukha OI, Shvarts МZ, Concentrator bifacial Ag-free LGCells, 88– 94, Copyright (2014), with permission from Elsevier.

Fig. 10. The structure of the back contact cell simulated by Kluska et al. [69]. Reprinted from Solar Energy Materials and Solar Cells, 94, Kluska S, Granek F, Rüdiger M, Hermle M, Glunz SW, Modeling and optimization study of industrial n-type high-efficiency back-contact back-junction silicon solar cells, 568–577, Copyright (2010), with permission from Elsevier.

mass production of back contact cells using CMOS-like manufacturing process [71], they increased the efficiency to 27.6% under 92 suns [71], which is the highest efficiency of silicon solar cell under concentration [17], the production scale was over 10 MW/year, the cells were used in many CPV projects [72]. Above back contact cells with high efficiency were made by CMOS-like semiconductor manufacturing process using FZ wafers, thus the costs were high. People tried to use cheaper process like screen-printing technology and cheaper CZ wafers to make the back contact cells, the SunPower company has been the leader in this field for many years. Bunea et al. made the back contact cell for concentration using the standard production processes of SunPower company, the size of the cell was also smaller than that of the common back contact cells made by SunPower company, the efficiency was 23% under 9 suns and 22.5% under 20 suns [73], Sunpower company developed a concentration system of 7 suns based on these cells [74]. Smith et al. reported the latest generation 3 back contact cell made by Sunpower company using 121 cm2 N-type CZ wafers, the highest efficiency reached 25% under 1 sun, which was equal to that of the PERL cell made by Zhao et al. [75], the efficiency would reach 26% by decreasing the recombination of carriers further according to Swanson's prediction [76], and the efficiency would be higher under a certain concentration ratio. Luque established an analytical model to calculate the efficiency of the back point-contact cell under concentration and found that it would reach 33.1% under 500 suns after optimizing the device parameters and minimizing the recombination of carriers [77]. Fellmeth et al. made the metal wrap through (MWT) cell for concentration using industrial crystalline silicon solar cell manufacturing process, the efficiency was larger than 20% under 5–12 suns [78]. The MWT cell was a combination of bifacial contact cell and back contact cell, the grids were made on the front surface of the cell, but the bus-bars were made on the back surface and connected to grids through the roles in the cell to decrease the shading losses [57,78–80], the structure of the MWT cell drew by Lohmüller et al. is shown in Fig. 11 [80]. Ebert et al. made a concentrating module based on these MWT cells; its efficiency reached 19.2% under 9.9 suns [81]. Fellmeth et al. also tried to use PERC structure in MWT cell, the highest efficiency was 19.8% under 3 suns, the additional series resistance caused by local metal contact constrained its use under higher concentration ratio [57]. 4.3. Vertical junction cell The electrodes of above bifacial contact cells and back contact cells were always made into narrow strips, the photo-generated

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Fig. 11. The structure of the MWT cell drew by Lohmüller et al. [80]. Reprinted from Solar Energy Materials and Solar Cells, 137, Lohmüller E, Thanasa M, Thaidigsmann B, Clement F, Biro D, Electrical properties of the rear contact structure of MWT silicon solar cells, 293–302, Copyright (2015), with permission from Elsevier.

Fig. 12. The schematics of vertical junction sub-cell (a) and vertical multi-junction cell (b) consisted of vertical junction sub-cells [84]. Reprinted from Solar Energy, 94, Xing Y, Han P, Wang S, Fan Y, Liang P, Ye Z, Li X, Hu S, Lou S, Zhao C,Mi Y, Performance analysis of vertical multi-junction solar cell with front surface diffusion for high concentration, 8–18, Copyright (2013), with permission from Elsevier.

carriers in the two cells were collected through these electrodes, thus the series resistances of the two cells could not be made very low and the two cells were always used in the concentration systems whose concentration ratio was lower than 400 suns. The vertical junction (VJ) cell also called edge illuminated cell was proposed in the 1970s to reduce the series resistance of the cell further [82], the structure of the VJ cell is shown in Fig. 12(a). The metal electrodes are made on the lateral sides of the cell and have large contact areas, the direction of current flow in the cell is lateral, thus the cross section of carriers transportation is large and the series resistance is very low [24,83–85]. Besides, the width of the metal electrodes could be made very small, thus the shading losses caused by them are low, too [24,85]. The vertical junctions were often series-connected together to form the vertical multijunction (VMJ) cell, as shown in Fig. 12(b) [84], it could provide high voltage and low current, the low current and series resistance make the VMJ cell very suitable for high and ultra-high concentrations. The efficiency of the silicon VMJ cell made by Sater et al. using a simple process was 19.48% under 1200 suns and 19.19% under 2480 suns [24], Sater et al. built a concentration system whose concentration ratio was larger than 500 suns using these VMJ cells [86]. Franklin et al. made the “sliver” cell using the chemical etching method [87], the efficiency was 18.2% under 50

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suns. Its structure was similar to that of VMJ cell, but there were extra PN junctions below the front and back surfaces of the cell, it can only be used for middle concentration because of the series resistance caused by front surface emitter [87]. Goodrich et al. made three kinds of multiple vertical junction (MVJ) cells using the chemical etching method [88], deep trenches were etched in front or back surface of the cell, and the PN junctions were made around the trenches, thus the direction of current flow in the cell was also horizontal similar to the VMJ cell. The metal electrodes were made around the trenches, they also had large contact area and small width, thus the electrical and optical losses caused by electrodes was also very small. The first cell was the front-grooved cell, which was similar to the LGBC cell, but the trenches were much deeper, its efficiency was 18.5% under 500 suns. The second cell was the front-back-grooved cell, the added back surface grooves reduced the series resistance further and improved the FF of the cell, thus it could be used under higher concentration ratio and its efficiency was 18.5% under 1000 suns. The third cell was the back-grooved cell, which was similar to the back contact cell, its efficiency was not reported but it should be larger than 25% under high concentration ratio according to the numerical simulation results [88]. Pozner et al. calculated the efficiency of VMJ cell under high concentration using a TCAD (Technology Computer Aided Design) software, they found that the efficiency could be close to 30% under 1000 suns and higher concentration ratio after optimizing the device parameters [85]. Braun et al. also calculated the efficiency and found that it was close to 27.5% under 10,000 suns, which was not an optimized result [89]. However, front and back surfaces passivation layers with high quality were required for the VMJ cell to get high efficiency [90], we found that this requirement could be released significantly by adding either P þ -type or N-type dopant to the front and back surfaces of the P-type VMJ cell, as shown in Fig. 13 [83,84], the series resistance increased little when adding N-type dopant to front surface, because the width of this cell was much smaller than that of the “sliver” cell. 4.4. Micro cell The costs of the cells could be decreased by decreasing their sizes, thus people tried to make micro silicon solar cells, whose length or width was in the order of millimeters. Yoon et al. made micro silicon solar cells on P-type CZ wafers using the CMOS-like semiconductor manufacturing process, the structure of the cell was similar to that of bifacial contact cell, as shown in Fig. 14 [91]. The cells were lifted off from the wafer by a soft stamp when their basic structure was made, then they were printed to a substrate, the positive and negative electrodes were evaporated to the front surface of the cells and were connected to make a module, the process is shown in Fig. 15 [91]. The efficiency of the cell could reach 11.6% under 1 sun after adding backside reflector (BSR), the length, width and thickness of the cell were about 1.55 mm, 50 mm

Fig. 13. The structure of the VMJ cell with front and back surface dopants.

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and 15 mm, respectively, and above device parameters could be changed easily by varying the manufacturing process parameters. Lens arrays were placed on the front surface of the cells to concentrate light to the cells, the concentration ratio was about 5.9 suns, and the maximum output power of the cell increased about 2.5 times [91]. After that, they placed the microcells in a kind of downconverter materials to realize concentration, the cells were separated by a certain distance, as shown in Fig. 3 [34], the downconverter materials absorbed the sunlight around the cell and emitted the light around 600 nm, part of the emitted light was absorbed by the cell, the maximum output power of the cell increased about 3.2 times [34]. Yao et al. made similar microcells, the positive and negative electrodes were made on the front and back surfaces of the cell, respectively, local contact structure was used on the front electrode, SiO2 passivation and antireflection layer were added to the front surface, the efficiency was 11.7% under 1 sun. The cells were placed in polymer waveguide and separated by a certain distance, BSR was added to the back surface

of the module, part of the incident light around the cells will be reflected to the cells to realize concentration, the structure was similar to which is shown in Fig. 3 [34], they found that the efficiency of the cell reached the highest under 8 suns [92]. Liu et al. made spherical microcells using the P-type silicon spheres with the diameter of about 1 mm, the silicon spheres were made from the scraps generated in the production of CZ wafers. The spherical emitter was made by diffusion, TiO2 film was deposited to the surface of the cell as the antireflection layer, and then the microcells were made into a module by a special process, the positive and negative electrodes were evaporated to the back hemispheres of the cells, resin lens were made to the front hemispheres of the cells, the concentration ratio was 4.4 suns, and the efficiency of a micro cell was 11.3% [93]. The efficiency of above microcells was still low because the manufacturing process was still immature, and the efficiency could be increased by optimizing the device and manufacturing process parameters. 4.5. Silicon-based hetero-junction and thin film cells

Fig. 14. The structure of the micro cell made by Yoon et al. [91]. Reprinted by permission from Macmillan Publishers Ltd: [NATURE MATERIALS] (Yoon J, Baca AJ, Park SI, Elvikis P, Geddes JB, Li LF, et al. Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator module designs. Nat Mater. 2008;7:907–15.), copyright (2008) at http://www.nature. com/ncomms/index.html.

The emitter of above cells was always made by diffusion and was heavy dopant, thus the lifetime of carriers was always low in the emitter. People tried to deposit an amorphous silicon (a-Si:H) layer on Si substrate to make the emitter, the recombination in the emitter would be decreased and the separation of quasi-Fermi level between the emitter and substrate would be increased, the cell was called SHJ (amorphous silicon/crystalline silicon heterojunction) cell [94]. Barnett et al. used a SHJ cell in a tandem cell for concentration [95], the efficiency of the SHJ cell was 5.4% filtered by GaAs cell under 8.7 suns [95]. People also tried to insert an intrinsic thin a-Si:H layer between the emitter and substrate to decrease the surface recombination further, and the cell was called HIT (hetero-junction intrinsic thin layer) cell. Jang et al. tried to use the HIT cell under 25 suns, but they did not report its efficiency

Fig. 15. The schematic of the micro cell module made by Yoon et al. [91]. Reprinted by permission from Macmillan Publishers Ltd.: [NATURE MATERIALS] (Yoon J, Baca AJ, Park SI, Elvikis P, Geddes JB, Li LF, et al. Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator module designs. Nat Mater. 2008;7:907–15.), copyright (2008) at http://www.nature.com/ncomms/index.html.

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[96]. The efficiency of the HIT cell made by Panasonic Corporation using 154 cm2 N-type CZ wafer has reached 25.6% under 1 sun [97], which is currently the highest efficiency of silicon solar cells under 1 sun [17]. Back contact structure was used to decrease the optical losses caused by electrodes, the manufacturing process of the a-Si: H layer was optimized to realize both small series resistance and high quality passivation. The grids were made by plating technology, their thickness reached tens of micrometers and their width was optimized, thus the series resistance of the cell was decreased to a low value and the FF of the cell reached 0.827. Coletti et al. used heterojunction structure in large area MWT cell; its efficiency reached 20.3% under 1 sun [79], the efficiencies of above two cells will increase further under a certain concentration ratio. Above cells were all made of crystalline silicon materials, people also tried to make silicon based thin film cells which had low costs for concentration. Kasashima et al. used a heterojunction microcrystalline silicon (μc-Si:H) thin film cell which was made by plasma enhanced chemical vapor deposition (PECVD) for concentration, they deposited wide bandgap microcrystalline silicon oxide (μc-Si1  xOx:H) film as the top P-type layer to decrease the recombination of carriers, the efficiency of the cell reached 10.4% under 11.8 suns [98]. They also used two-junction and three-junction cells made of a-Si:H and μc-Si:H films for concentration, the current mismatch between the sub-cells increased with concentration ratio, the two-junction cell had the highest efficiency of 12.2% under 2.7 suns [99]. They also did numerical analysis to above cells using the AMPS-1D (Analysis of Microelectronic and Photonic Structures) software and found that the efficiencies of single-junction and three-junction cells could reach 13% and 23% under 50 suns after optimization [98,99]. However, the light-induced recession of the thin film cells will be more serious under concentration, the fill factor and efficiency of the cell decreased quickly with the increase of illumination time under 10 suns [100]. 4.6. Silicon-based multi-junction cell People tried to make silicon based multi-junction cell to make full use of the violet and infrared parts of sunlight, the traditional method was to grow III–V materials on silicon substrate to make the bifacial contact multi-junction cell. Lueck et al. grew compositionally graded SiGe buffer layer as the virtual substrate by ultrahigh vacuum chemical vapor deposition (UHV-CVD) on Si substrate first, and then they grew a two-junction GaInP/GaAs cell on the buffer layer using solid source molecular beam epitaxy (MBE). The efficiency of the cell was 19.8% under 1 sun, and the efficiency of the same cell grown on GaAs substrate was 23.6% [101]. Calabrese et al. made porous structure on Si substrate to improve the crystalline quality of Ge virtual substrate [102]. Grassman et al. grew a GaAsP cell on Si substrate directly using MBE [103], GarcíaTabarés et al. grew a GaP cell on Si substrate directly using metalorganic vapor phase epitaxy (MOVPE) [104] and Wilkins et al. grew a GaAs cell on Si substrate which was also made porous structure directly using chemical beam epitaxy [105], but above researchers did not report the efficiencies of the cells, thus it is still hard to grow high quality III–V multi-junction solar cell on Si substrate directly. To overcome the constraints of lattice mismatch between III–V materials and Si in above hetero-epitaxy processes, Derendorf et al. used wafer bonding technology to make the three junction GaInP/GaAs/Si cell [106], the efficiency was 23.6% under 71 suns [107]. Yang et al. used direct metal interconnecting technology to make a three-junction GaInP/InGaAs/Si cell, the back surface grids of a GaInP/InGaAs cell and the front surface grids of a Si cell were interconnected together under heat and pressure, the efficiency of the cell was 25.5% under 1 sun [108], Lin et al. made a three-junction GaInP/GaAs/GaInAs cell using this

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method recently [109]. The theoretical efficiencies of silicon based III–V multi-junction solar cells have been calculated by many people using analytical and numerical calculation methods, the efficiency of the InGaN/Si cell was 36% under 500 suns calculated by Hsu et al. [110], the efficiencies of the GaNPAs/Si and GaNPAs/ GaNPAs/Si cells were 44.2% and 47.4% under 500 suns calculated by Geisz et al. [111], and the efficiency of the GaInP/GaAs/Si cell was 53.8% under 500 suns calculated by Derendorf et al. [107,112]. The perovskite solar cell which mostly used CH3NH3PbI3 material to make the absorbing layer has attracted a lot of attention recently, its efficiency has reached 20.1% [113], the bandgap of CH3NH3PbI3 material was larger than that of silicon. Mailoa et al. used several methods of film growth to make the two-junction perovskite/silicon cell, a N þ þ type a-Si:H layer was deposited on the P þ þ type emitter of the bottom Si cell to make the N þ þ /P þ þ tunnel junction to connect the two sub-cells, the tested efficiency was 13.7% under 1 sun and the ultimate efficiency was 35% [114]. Filipič et al. calculated the efficiencies of the two-junction perovskite/silicon cell based on the tested complex refractive parameters of all layers of the cell, they used a SHJ cell as the bottom cell; they found that the efficiency could reach 30.3% under 1 sun [115]. The efficiency of the perovskite/silicon tandem cell could be increased further under a certain concentration ratio, but its stability problem under high light intensity must be solved. The sub-cells of above silicon based multi-junction cells were in series; the multi-junction cells only had two terminal electrodes; the short circuit currents of all sub-cells must be made equal to each other to realize the high efficiency. The multi-junction cells could be divided into individual cells with individual terminal electrodes; the concentrated sunlight could be separated into different beams with different wavelengths; the beams are directed to corresponding cells with suitable bandgap by optical elements, it is called spectral beam splitting technology [116,117]. The individual cells could be mechanically stacked on top of each other in optical series to form the tandem cell just like above multi-junction cell, they also could be placed next to each other in optical parallel to receive the suitable light [118], as shown in Fig. 16, and we also called them silicon based multi-junction cell in this work. Gee et al. made a two-junction GaAs/Si tandem cell with the efficiency of 31% under 347 suns [119], DiNetta et al. made a two-junction AlxGa1-xAs/Si tandem cell with the efficiency of 29.2% under 100 suns [120]. Green et al. built a power tower concentrating system, they used a bandpass filter to split the concentrated sunlight into two parts, a three-junction GaInP/

Fig. 16. The schematics of two kinds of spectrum splitting concentration photovoltaic systems, the individual sub-cells of the multi-junction cell were placed in optical series (a) and optical parallel (b).

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GaInAs/Ge cell was used to absorb the reflected light, a Si back contact cell made by Sunpower company was used to absorb the transmitted light, the concentration ratio was 365 suns and the total efficiency reached 40.4%, which was the highest efficiency of the concentration systems of this kind [121]. Barnett et al. proposed a six-junction tandem cell, which was consisted of a sub-cell with the bandgap of 2.4 eV, a GaInP sub-cell, a GaAs sub-cell, a Si sub-cell, a sub-cell with the bandgap of 0.95 eV and a sub-cell with the bandgap of 0.7 eV from top to down [95], they calculated the overall system efficiency and found that it could reach 53.5% under 20 suns. The sub-cells of above multi-junction cells were front and back contact cells and used planar PN junctions, Braun et al. proposed a three-junction GaP/Si/Ge tandem cell, and the subcells were VMJ cells, they calculated the efficiency and found that it could reach 40% under 10,000 suns [89]. The top cells of above tandem cells were made of III–V materials, which have high costs or toxicity, White et al. proposed to replace the wide bandgap III–V materials with earth-abundant semiconductor materials, for example amorphous silicon, Cu(In,Ga)(S,Se), Cu2ZnSnS4 and Sb2S3, they calculated the efficiency of the tandem cell using an analytical method and found that it could reach 32% under 1 sun when the luminescence efficiency and diffusion length of the top cell were larger than 10  5 and 50 nm [122]. Lal et al. calculated the efficiency of the two-junction perovskite/silicon tandem cell using the method similar to White et al.'s; they found that its efficiency could reach 35% under 1 sun after optimizing the light management in the tandem cell [123].

5. Summarization and prediction The silicon solar cells can be used for low, middle, high and ultra-high concentrations (1 suns o Co 10,000 suns) as discussed above, the limiting efficiency of single-junction silicon solar cell under concentration has been calculated by Campbell et al., they found that it could arrive 36% after optimizing the light trapping and thickness of the cell [124]. The Auger recombination, which had more serious effect on saturation current and open circuit voltage under concentration, was the main limiting factor of limiting efficiency [125]. The limiting efficiency could be increased by using multi-junction structure; the limiting efficiency of silicon based three-junction cell was 47.5% under 1 sun [126] and would be higher under concentration. As mentioned in the introduction, the manufacturing costs and efficiency are the two key factors of concentrator cells, thus the cells which have high efficiency and low manufacturing costs have good potential in future CPV. The bifacial contact cells and back contact cells made by CMOS-like process have high efficiency (larger than 25%) under high concentration (100–200 suns) [19,44], but their costs are high even the process is simplified. The bifacial contact cells made by common industrial crystalline silicon solar cell manufacturing process using CZ wafers have low costs, their highest efficiency is larger 20% under low concentration and larger than 19% under middle concentration (C o50 suns) [57,58]. If multi bus-bar, advanced manufacturing technology of front metal grids and advanced device structures are used, the efficiency may reach 21% under a certain concentration ratio. The back contact cell and HIT cell could be made by using the reformed industrial crystalline silicon solar cell manufacturing process, thus the costs of the two cells increased little compared to common industrial crystalline silicon solar cells, their efficiencies have reached 25% [75] and 25.6% [97] under 1 sun and could be increased to 26% after optimizing the manufacturing process and device parameters. The back contact cells have been used under 7 suns [74], the HIT cell with the highest efficiency of 25.6% also used back contact structure, and its efficiency will increase under a

certain concentration ratio. The microcells and silicon based thin film cells have low costs, but their efficiencies are low (η o12.2%) and could only be used for low concentration [93,99]. The VMJ cell has the smallest series resistance among all cells, its costs is low, its highest efficiency is 19.48% under very high concentration (1200 suns) [24] and will increase after optimizing the manufacturing process and device parameters. The silicon based multijunction cell is considered to be a possible “ultimate photovoltaic solution” [126], its highest efficiency is larger than 40% under high concentration (365 suns) although its costs is high [121]. We think that the Si HIT cell using back contact structure has good potential in low and middle CPV, the multi-junction cell containing Si back contact cell has good potential in high CPV and the Si VMJ cell used with Ge and GaP VMJ cells has good potential in very high CPV if the costs and efficiencies of above cells were considered together.

6. Conclusion Current development of concentrator silicon solar cells was reviewed in this work, the device structure, manufacturing processes and efficiencies of the cells with different structures were discussed and compared, and the prospects of concentrator silicon solar cells were predicted based on the comparison. The efficiency of concentrator silicon solar cell will continue to increase with the development of process and technology, and the costs will continue to decrease with the increase of production scale. We think that the Si HIT cell using back contact structure, the multi-junction cell containing Si back contact cell and the Si VMJ cell used with Ge and GaP VMJ cells have good potential in low, middle, high and very high CPV.

Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant nos. 61275040, 60976046, 60837001, and 61021003, the National Basic Research Program of China (973 Program) (No. 2012CB934204) and by Chinese Academy of Sciences (No. Y072051002). References [1] IEA. Key world energy statistics. The International Energy Agency; 2014 (http:// www.iea.org/publications/freepublications/publication/key-world-energy-statis tics-2014.html and http://www.iea.org/publications/freepublications/publica tion/KeyWorld2014.pdf.). [2] Global energy-related emissions of carbon dioxide stalled in 2014 (〈http:// www.iea.org/newsroomandevents/news/2015/march/global-energy-relate d-emissions-of-carbon-dioxide-stalled-in-2014.html〉). [3] Hosenuzzaman M, Rahim NA, Selvaraj J, Hasanuzzaman M, Malek A, Nahar A. Global prospects, progress, policies, and environmental impact of solar photovoltaic power generation. Renew Sustain Energy Rev 2015;41:284–97. [4] Kumar Sahu B. A study on global solar PV energy developments and policies with special focus on the top ten solar PV power producing countries. Renew Sustain Energy Rev 2015;43:621–34. [5] Ming-Zhi Gao A, Fan C-T, Kai J-J, Liao C-N. Sustainable photovoltaic technology development: step-by-step guidance for countries facing PV proliferation turmoil under the feed-in tariff scheme. Renew Sustain Energy Rev 2015;43:156–63. [6] Tyagi VV, Rahim NAA, Rahim NA, Selvaraj JAL. Progress in solar PV technology: research and achievement. Renew Sustain Energy Rev 2013;20:443–61. [7] Parida B, Iniyan S, Goic R. A review of solar photovoltaic technologies. Renew Sustain Energy Rev 2011;15:1625–36. [8] Baig H, Heasman KC, Mallick TK. Non-uniform illumination in concentrating solar cells. Renew Sustain Energy Rev 2012;16:5890–909. [9] PV production grows despite a crisis-driven decline in investment 〈http://iet. jrc.ec.europa.eu/remea/sites/remea/files/jrc-pvreport2013-web.pdf〉. [10] Ming Z, Shaojie O, Hui S, Yujian G. Is the Sun still hot in China? The study of the present situation, problems and trends of the photovoltaic industry in China Renew Sustain Energy Rev. 2015;43:1224–37.

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