**4. The reliability of the multi-junction solar cell**

#### **4.1. Accelerated aging tests**

Under high concentration of several hundreds or even thousands suns, multi-junction solar cells will suffer high temperature and high current density, which are challenging the relia‐ bility of these devices [16]. To obtain the approval from CPV customers, it is necessary to demonstrate the reliability of multi-junction solar cells operating under high concentration.

A new certification standard, namely the IEC62108, has been developed in which the proce‐ dure for qualifying CPV systems and assemblies is described. The IEC62108 is currently the only international standard on assessment of high concentration solar receivers and mod‐ ules, which specifies the minimum requirements for the design qualification and the type approval of concentrator solar cells, and which gives the corresponding test procedures for each test sample, such as outdoor exposure test, electrical performance measurement, elec‐ trical test, irradiation test, and mechanical load test. After passing the IEC62108 certification, both the modules and assemblies can be suitable for long-term (25 years) operation in gener‐ al open-air climates.

The purpose of the thermal cycling test is to determine the ability of the receivers to with‐ stand thermal mismatch, fatigue, and other stresses caused by rapid, non-uniform or repeat‐ ed changes of temperature. This test is vital to the reliability of concentrator solar cells, since generally these devices have to operate at high concentration of more than 1000 suns, high operation current density of more than 10A/cm2 , high operation temperature of more than 60 °C and large temperature difference between day and night.

In order to simulate the real operating conditions, IEC 62108 requires that during the proc‐ ess of thermal cycling test for concentrator solar cells carried out in the oven, a current should be flowing through the chips. Table 6 shows the three optional conditions. In princi‐ ple, the temperature and the current injection time of cells are required to be accurately monitored during thermal cycling test. However, it is very difficult to monitor the real tem‐ perature of cells in real operating conditions, because a high electric current passing through the cells can lead to differences in temperature among the cells, the heat sink and the oven.


**Table 6.** The options of thermal cycling test from IEC 62108.

**Figure 16.** The optical and electrical properties of the cells with and without ARC.

**4. The reliability of the multi-junction solar cell**

Under high concentration of several hundreds or even thousands suns, multi-junction solar cells will suffer high temperature and high current density, which are challenging the relia‐ bility of these devices [16]. To obtain the approval from CPV customers, it is necessary to demonstrate the reliability of multi-junction solar cells operating under high concentration.

A new certification standard, namely the IEC62108, has been developed in which the proce‐ dure for qualifying CPV systems and assemblies is described. The IEC62108 is currently the only international standard on assessment of high concentration solar receivers and mod‐ ules, which specifies the minimum requirements for the design qualification and the type approval of concentrator solar cells, and which gives the corresponding test procedures for each test sample, such as outdoor exposure test, electrical performance measurement, elec‐ trical test, irradiation test, and mechanical load test. After passing the IEC62108 certification, both the modules and assemblies can be suitable for long-term (25 years) operation in gener‐

The purpose of the thermal cycling test is to determine the ability of the receivers to with‐ stand thermal mismatch, fatigue, and other stresses caused by rapid, non-uniform or repeat‐ ed changes of temperature. This test is vital to the reliability of concentrator solar cells, since generally these devices have to operate at high concentration of more than 1000 suns, high

, high operation temperature of more than

**4.1. Accelerated aging tests**

462 Optoelectronics - Advanced Materials and Devices

al open-air climates.

operation current density of more than 10A/cm2

60 °C and large temperature difference between day and night.

Using the thermal cycling test condition of TCA-1 from table 6, the cell temperature is con‐ trolled between -40 °C and 85 °C. A dwell time of 10 min of the high and low temperatures is required. The cycling period and frequency are 120 minutes and 12 cycles per day, respec‐ tively. In one thermal cycle, a specific current level of 7A is periodically on and off for 10 cycles, when the cell temperature is above 25 °C. In order to illustrate the changes of electri‐ cal performance of test samples, control samples are chosen and measured under the similar test condition. By this method, test condition variables are self-corrected, and the complex translation procedures are eliminated. Finally, the relative power Pr and relative power deg‐ radation Prd are defined as follows:

$$P\_r = \frac{P\_m}{P\_{mc}} \times 100\% \tag{25}$$

$$P\_{rd} = \frac{P\_{ri} - P\_{rf}}{P\_{ri}} \times 100\% \tag{26}$$

where Pm is the test sample's maximum power, Pmc is the control sample's maximum pow‐ er measured at the similar condition as Pm, and Prf and Pri are the relative powers meas‐ ured after and before the given test, respectively.

For comparison, eight San'an company's cells and eight B-company's cells were tested to‐ gether. Tables 7 and 8 show the relative power degradation of San'an Company's and Bcompany's receiver samples after different numbers of thermal cycles, respectively. The output powers gradually decrease with the increasing thermal cycles due to the samples' degradation. It is found that the relative power degradations of tested samples are within 10%. The degradation is believed to be responsible for the perimeter degradation [16, 17]. According to González et al., the arbitrary definition of device failure is a 10% of power loss, so the majority of test samples do not have failure, except the c B-company's receiver #56, the relative power degradation of which is from 12.85% after 560 thermal cycles to 14.89% after 1000 thermal cycles. Besides, from visual inspection on these samples, the DBCs sol‐ dered on alumina substrates are not peeled off after the 1000 thermal cycles, which indicates that it is suitable for long-term (~25 years) operation in general open-air climates.

no less than 25 years. According to the requirements from IEC 62108, this paper presents the reliability results from thermal cycling tests performed on San'an company's high concentra‐ tion solar cells. We find that the light emitting intensity and the relative power degradation

III-V Multi-Junction Solar Cells http://dx.doi.org/10.5772/50965 465

Concentrated photovoltaic (CPV) system is usually located in sunny places for large-scale photovoltaic (PV) power station with installation capacity of 1~1000 megawatt (MW). It is composed of Fresnel lenses to concentrate, III-V multi-junction solar cells, polar axis type or pedestal type tracking system and integrated control method. By focusing sunlight onto high-efficiency solar cells, CPV is able to use fewer solar cells than traditional photovoltaic power. Since CPV has a high power-generating capacity with movable parts, easy to manu‐

According to the CPV Consortium, "CPV, with its higher efficiency delivers higher energy production per megawatt installed, provides the lowest cost of solar energy in high solar re‐ gions of the world. The technology is in its early stage with significant headroom for future innovation, and it has the ability to ramp to gigawatts of production very rapidly. Many of the limitations for PV in the past are overcome by advances in CPV technology."As of 2011, the global bases of installed CPV produced totally just 60 megawatts, according to the CPV Consortium. The organization predicts that capacity will rise to 275 megawatts by the end of 2012, 650 megawatts by the end of 2013, 1,100 megawatts by end of 2014 and 1,500 mega‐

World-widely, 40 MW Amonix power plants will be installed from 2012 on, at the same time 32.7 MW power plant located at Alamosa Colorado was measured during the week of March 2012. ISFOC (Institute of Concentration Photovoltaic Systems) main goal is to pro‐ mote the CPV industrialization. For this purpose, ISFOC has made the installation of CPV Plants, up to 2.7 MW, all over the region of Castilla la Mancha. A lot of CPV power plants will be installed in near future without being introduced more. However, focusing on Chi‐ na, the relative long history of advanced CPV technology development, the years' experi‐ ence of power plant operation, mature systems with high performance and reliability, the leading position of the western participants will set up a benchmark in the field and gain more attention and more shares from Chinese CPV market. For a few domestic CPV compa‐ nies with installation records, further efforts are required to improve the performance and reliability of CPV products, to lower the cost by setting up complete supply chains in CPV industry, to facilitate the utilization of abundant solar resources from the north and west to the south and east via setting up transmission networks, so that a Chinese CPV market can

The largest CPV power plant project in China was assembled at Golmud, Qinghai province by Suncore Photovoltaic Technology Co., Ltd, with the capacity of more than 50 MW. Sun‐ core is a Sino-US joint venture established by San'an and Emcore. 1 MW of the project using 500 suns terrestrial system and 2MW using 1000 suns terrestrial system has been finished, as

of San'an company's receivers are similar to that of B-company's receivers.

facture and to maintain, it is very suitable for a large scale PV power station.

**4.2. Discussion on outdoor power plant performance**

watts by the end of 2015.

be actually initiated, developed and matured.


**Table 7.** Relative power degradation of San'an Company's receiver samples after different numbers of thermal cycles.


**Table 8.** Relative power degradation of B-company's receiver samples with different numbers of thermal cycles.

In conclusion, high concentration multi-junction solar cells are still at an early stage of tech‐ nological development, and thus it is necessary to demonstrate the reliability of these solar cells before their industrialization. Accelerated aging test is a necessary tool to demonstrate the reliability of concentration photovoltaic solar cells, which is expected to be working for no less than 25 years. According to the requirements from IEC 62108, this paper presents the reliability results from thermal cycling tests performed on San'an company's high concentra‐ tion solar cells. We find that the light emitting intensity and the relative power degradation of San'an company's receivers are similar to that of B-company's receivers.

#### **4.2. Discussion on outdoor power plant performance**

output powers gradually decrease with the increasing thermal cycles due to the samples' degradation. It is found that the relative power degradations of tested samples are within 10%. The degradation is believed to be responsible for the perimeter degradation [16, 17]. According to González et al., the arbitrary definition of device failure is a 10% of power loss, so the majority of test samples do not have failure, except the c B-company's receiver #56, the relative power degradation of which is from 12.85% after 560 thermal cycles to 14.89% after 1000 thermal cycles. Besides, from visual inspection on these samples, the DBCs sol‐ dered on alumina substrates are not peeled off after the 1000 thermal cycles, which indicates

that it is suitable for long-term (~25 years) operation in general open-air climates.

464 Optoelectronics - Advanced Materials and Devices

**Serial sample 0 cycle 360 cycles 560 cycles 760 cycles 1000 cycles** #182B5 0.00% -2.47% -4.00% -5.56% -8.02% #183D1 0.00% -0.14% -3.19% -3.88% -5.44% #182D5 0.00% -3.66% -4.78% -5.82% -8.21% #183B1 0.00% -2.48% -4.83% -6.33% -8.25% #182D1 0.00% -4.62% -5.38% -6.21% -7.95% #183B6 0.00% -5.85% -6.08% -5.84% -7.09% #183A4 0.00% -5.02% -5.70% -6.97% -7.96% #183D5 0.00% -5.47% -5.83% -6.06% -7.37%

**Table 7.** Relative power degradation of San'an Company's receiver samples after different numbers of thermal cycles.

**Serial sample 0cycle 360cycles 560 cycles 760 cycles 1000 cycles** #112 0.00% -5.93% -6.83% -6.84% -7.72% #44 0.00% -4.10% -5.60% -6.12% -8.11% #56 0.00% -8.02% -12.85% -13.05% -14.89% #94 0.00% -2.76% -4.94% -5.19% -6.78% #78 0.00% -5.17% -6.39% -6.76% -7.36% #90 0.00% -7.14% -7.75% -8.17% -8.32% #97 0.00% -3.19% -3.38% -4.38% -7.42% #136 0.00% -6.30% -6.69% -7.05% -9.34%

**Table 8.** Relative power degradation of B-company's receiver samples with different numbers of thermal cycles.

In conclusion, high concentration multi-junction solar cells are still at an early stage of tech‐ nological development, and thus it is necessary to demonstrate the reliability of these solar cells before their industrialization. Accelerated aging test is a necessary tool to demonstrate the reliability of concentration photovoltaic solar cells, which is expected to be working for Concentrated photovoltaic (CPV) system is usually located in sunny places for large-scale photovoltaic (PV) power station with installation capacity of 1~1000 megawatt (MW). It is composed of Fresnel lenses to concentrate, III-V multi-junction solar cells, polar axis type or pedestal type tracking system and integrated control method. By focusing sunlight onto high-efficiency solar cells, CPV is able to use fewer solar cells than traditional photovoltaic power. Since CPV has a high power-generating capacity with movable parts, easy to manu‐ facture and to maintain, it is very suitable for a large scale PV power station.

According to the CPV Consortium, "CPV, with its higher efficiency delivers higher energy production per megawatt installed, provides the lowest cost of solar energy in high solar re‐ gions of the world. The technology is in its early stage with significant headroom for future innovation, and it has the ability to ramp to gigawatts of production very rapidly. Many of the limitations for PV in the past are overcome by advances in CPV technology."As of 2011, the global bases of installed CPV produced totally just 60 megawatts, according to the CPV Consortium. The organization predicts that capacity will rise to 275 megawatts by the end of 2012, 650 megawatts by the end of 2013, 1,100 megawatts by end of 2014 and 1,500 mega‐ watts by the end of 2015.

World-widely, 40 MW Amonix power plants will be installed from 2012 on, at the same time 32.7 MW power plant located at Alamosa Colorado was measured during the week of March 2012. ISFOC (Institute of Concentration Photovoltaic Systems) main goal is to pro‐ mote the CPV industrialization. For this purpose, ISFOC has made the installation of CPV Plants, up to 2.7 MW, all over the region of Castilla la Mancha. A lot of CPV power plants will be installed in near future without being introduced more. However, focusing on Chi‐ na, the relative long history of advanced CPV technology development, the years' experi‐ ence of power plant operation, mature systems with high performance and reliability, the leading position of the western participants will set up a benchmark in the field and gain more attention and more shares from Chinese CPV market. For a few domestic CPV compa‐ nies with installation records, further efforts are required to improve the performance and reliability of CPV products, to lower the cost by setting up complete supply chains in CPV industry, to facilitate the utilization of abundant solar resources from the north and west to the south and east via setting up transmission networks, so that a Chinese CPV market can be actually initiated, developed and matured.

The largest CPV power plant project in China was assembled at Golmud, Qinghai province by Suncore Photovoltaic Technology Co., Ltd, with the capacity of more than 50 MW. Sun‐ core is a Sino-US joint venture established by San'an and Emcore. 1 MW of the project using 500 suns terrestrial system and 2MW using 1000 suns terrestrial system has been finished, as shown in Figure 17. Conversion efficiency of 500 suns and 1000 suns terrestrial system can reach as high as 25% and 28.5%, respectively.

**Figure 17.** power plant installed at Golmud Qinghai province China.

The direct normal insolation (DNI) distribution of the local environment and the mapping of China were displayed Figure 18 (a) and (b). The I-V curves of 227 receivers using 500 suns terrestrial system module tested outdoor was shown in Figure 19. One can see that the effi‐ ciency could reach as high as 24.03% at the condition of much dust on the surface of the Fresnel lens, which affecting the light transmittance. Therefore, the actual efficiency should be high than this nominal value.

**Figure 19.** I-V curves of the 227 Receivers module tested outdoor.

This work was supported by a foundation from the National High Technology Research and

, Jianqing Liu1

, Weiping Xiong1

and

III-V Multi-Junction Solar Cells http://dx.doi.org/10.5772/50965 467

Development Program (863 program) of China. (No. 2012AA051402).

, Minghui Song1

[1] Green, M. A. (2005). *Third generation photovoltics*, Berlin, Springer.

\*Address all correspondence to: mchuang@xmu.edu.cn

1 Xiamen San'an Optoelectronics Co., Ltd., China

2 Department of Physics, Xiamen University, China

**Acknowledgements**

**Author details**

Meichun Huang2\*

, Jingfeng Bi1

Gui jiang Lin1

**References**

**Figure 18.** The DNI distribution of the whole day in Golmud (a), and the annual average direct normal insolation (DNI) GIS data at 40km resolution for China (b) from NREL.

**Figure 19.** I-V curves of the 227 Receivers module tested outdoor.
