ABSTRACT. Keywords: Solar simulator, spectral irradiance, measurement, traceability chain, uncertainty analysis 1. INTRODUCTION

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1 Uncertainty Analysis of Solar Simulator s Spectral Irradiance Measurement Meng Haifeng*, Xiong Limin, He Yingwei, Liu Dingpu, Zhang Jieyu, Li Wenxin Division of Metrology in Optics and Laser, National Institute of Metrology, Beijing, P.R. China, ABSTRACT Solar simulator is a key instrument for photovoltaic field, which aims to act the role of natural sunlight irradiance indoor, and we should identify how similar they are in quantity. The critical factor of similarity lies in its spectral irradiance, because of solar cells wavelength-dependent spectral responsivity, spectral mismatch of solar simulator and sunlight can induce large errors during characteristic parameters measurement. In this article, a method for measuring solar simulator s spectral irradiance was proposed along with its uncertainty analysis. A calibrated fiber optic spectrometer was employed here for spectral measurement, which was used for calibrating various kinds of solar simulators manufactured with different mechanisms. Considering three main sources of measurement uncertainty, that is, the declared uncertainty of the calibrated spectrometer (u 1 ), cosine correction (u 2 ) and repeatability of measurement (u 3 ), we estimated its combined expanded uncertainty is U = 6.2% (with coverage factor k = 2). Also, we have made a comparison of our spectral measurement results with methods traceable to other country s national institute of metrology, such as NIST traceable. This work is significant for the performance calibration and classification of solar simulators, so that plays a great role in solar energy industry. Keywords: Solar simulator, spectral irradiance, measurement, traceability chain, uncertainty analysis 1. INTRODUCTION Solar simulator, the best candidate for simulating natural sunlight indoor, is one of the most pivotal instruments in photovoltaic industry. It is widely used in indoor current-voltage (I-V) characteristics as well as irradiance exposure of photovoltaic devices, so that critical parameters such as short-circuit current (Isc), open-circuit voltage (Voc), maximum power (Pmax), conversion efficiency (η), fill factor (FF) and so on, can be obtained [1-4]. Photovoltaic (PV) devices always work under the natural sunlight, so we need solar simulator with its performance very close to natural sunlight to reflect their characteristics in real work conditions. However, solar simulator is not equivalent to natural sunlight, especially spectral irradiance. Natural sunlight is broadband, as illustrated in ASTM G173 spectra (280 nm 4000 nm) [5, 6] and IEC [7], while simulator s light source commonly is artificial Xenon arc lamp. Artificial lamps cannot simulate the sunlight spectrum exactly, even with filters. The inevitable deviations of the spectrum of artificial light sources from the standard spectrum have to be taken into account by a spectral mismatch factor. Mismatch in spectral irradiance between solar simulator and natural solar light can induce large errors and uncertainty in PV devices I-V characteristic, because solar cell s spectral responsivity is dependent on wavelength [8-11]. So it s urgent to identify the spectral irradiance of solar simulators. To define it, international standard IEC proposed corresponding criterion and classification, that is make a comparison between the spectral irradiance of solar simulator and AM1.5G solar light [12]. It prescribes the requirements for solar simulators used in PV measurements and provides definitions of solar simulators classifications. Finding a practical method for solar simulators' spectral irradiance measurement is of great importance. To date, many related institutes or laboratories were endeavored in that, and various kinds of systems and equipments were emerged [13-15]. NREL designed a dual detector system to capture the waveform of flash at individual wavelength of light, which has a wavelength range of 280 nm to 1720 nm at 5 nm intervals [14]. NIST reported spectral irradiance measurements of its own pulsed solar simulator by using a high-speed, diode-array spectroradiometer that was calibrated against NIST standards [15]. In our previous article, we have proposed a versatile and portable integrated system to satisfy the requirements of on-site calibration for various kinds of solar simulators, so that service for hundreds of PV manufacturers and related authorities [16]. *menghf@nim.ac.cn; phone ; fax ; 6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Optoelectronic Materials and Devices for Sensing, Imaging, and Solar Energy, edited by Yadong Jiang, Junsheng Yu, Zhifeng Wang, Proc. of SPIE Vol. 8419, 84193A 2012 SPIE CCC code: /12/$18 doi: / Proc. of SPIE Vol A-1

2 Herein, combining with IEC s classification on spectral irradiance, we ll emphasis the process of solar simulator s spectral irradiance measurement with examples. A calibrated fiber optic spectrometer was employed here, which has its declared uncertainty of u 1 : 6% (k = 2) (300 nm nm). Furthermore, we analyzed the measurement uncertainty along with uncertainty caused by spectrometer probe s cosine correction (u 2 ) and measurement repeatability (u 3 ). The errors introduced by cosine factor and measurement repeatability were minor in the process, a combined expanded uncertainty of U = 6.2% (k = 2) was summarized. 2. EXPERIMENTAL SECTION 2.1 Instruments For the solar simulators spectral irradiance measurement, a fiber optic spectrometer, AvaSpec-2048-USB2 spectrometer which has a USB2 interface with ultrafast data-sampling of 900 spectra per second and data-transfer in 1.8ms was employed here [16]. Firstly, we calibrate the optical fiber spectrometer against a standard lamp which is traceable to the National Institute of Metrology (NIM) blackbody. Then, with the calibration data, we use the spectrometer to measure the spectra of solar simulators. Each spectrometer has its particular calibration data, and we should load it before deducting the background of environment. 2.2 Measuring method In principle, our aim is to reflect the real state of solar simulator when characterize PV devices performance, so we should firstly conduct the spectral measurement in the same plane of test area with spectrometer s probe aligning to the light beam. To decrease measurement errors, mechanisms of solar simulator s light source should be considered. Though there are various kinds of solar simulators, light sources are always in two types, direct light and diffuse reflection light; particular treatments should be taken for each. For direct light, spectrometer s probe should be aligned with the normal of light source. While for diffuse reflection light, because of its complicated influencing factors, probe should vertical to the test plane and measure at least five times in different positions. Moreover, for the pulsed solar simulator, a trigger is needed to collect the spectral signal. 2.1 Examples During these years, we have measured several hundreds of solar simulators as well as outdoor sunlight by this method. Comparing the obtained real spectra of simulators with AM1.5G reference spectrum, classifications of spectral match are clear according to IEC [12]. The measured results are widely recognized by PV related industry and authority. Figure 1 demonstrated typical spectra of a solar simulator with different irradiance intensities (1000 W/m 2, 800 W/m 2 and 200 W/m 2 ). In comparison to AM1.5G spectrum, percent deviation and spectral match of all intervals were illustrated in Table 1. That is a solar simulator with spectral irradiance in class A. Irradiance (pw/cm2/nm) uv 50 o Wavelength Figure 1: Spectral curves collected with the fiber optic spectrometer for one solar simulator with different irradiance intensities. Green: 1000 w/m 2 ; Red: 800W/m 2 ; Blue: 200W/m 2. Proc. of SPIE Vol A-2

3 Ratio ( %) Match A A A A A A Ratio ( %) Match A A A A A A Ratio ( %) Match A A A A A A Table 1: Percent deviation in each interval of solar simulator s spectra in compassion to that of AM1.5G spectrum, from the upper to bottom, corresponding with irradiance intensity of 1000 w/m 2, 800W/m 2 and 200W/m 2, respectively. 3. UNCERTAINTY ANALYSIS During the measurement, main influencing factors for errors come from spectrometer itself, cosine correction and measurement repeatability. Hence, considering the three main typical uncertainty components, that is, the declared uncertainty of the calibrated spectrometer (u 1 ), uncertainty of cosine correction (u 2 ) and repeatability of measurement (u 3 ), combined uncertainty can be calculated by the following equation: U = u u2 u3 3.1 The declared uncertainty of the calibrated spectrometer (u 1 ) After calibrated by National Institute of Metrology (NIM) standard lamp which is traceable to NIM blackbody, a declared uncertainty of 6% (k = 2) was given in the wavelength range of 300 nm to 1100 nm. In our experiment, range of 400 nm to 1100 nm is enough according to IEC s definition [12]. So the uncertainty brought by the calibrated spectrometer itself is u 1 = 6% (k = 2) (400 nm nm). 3.2 Uncertainty of cosine correction (u 2 ) To identify the uncertainty of cosine correction, we conducted many experiments under solar simulators and natural sunlight. As illustrated in Figure 2 and Table 2, spectral curves collected with the spectrometer probe 0, 15, 30, 45, 60, 75 to normal incidence of the natural sunlight were shown. When the probe tilted 75, the maximum error is 4.0%. However, if we control the spectrometer probe align to the normal of light beam, even with a tile angle of 30 ; the maximum error of the six intervals is 1.0%, when the tilt angle smaller than 15, the maximum error is 0.7%. Proc. of SPIE Vol A-3

4 Irradiance (NW /cm2 /nm) 100 ' '. LU Wavelength Figure 2: Spectral curves collected with the spectrometer probe 0, 15, 30, 45, 60, 75 degree to normal incidence of the sunlight, from upper to bottom. %Z'T %0'1' %t't %5'Z %6'1 %1'T 1'66 8'1-0 T T'SO T L'66 t'66 't6 NO.iO.I.ia ullllllltp.1v O%) ()pull '001 5' 0 T Z't01 L'66 9'66 L't 6 04)) ) opi'21 09 L '66 5'ZO1 L' OT 1-'001 1-'001 8't6 L'66 L'TOT 9' 0T Z'TOT 8'001 Z't6 1-'66 1-' TOT 9' CO 1 1;101 0'101 8' 6 1'66 L'00T 9' O1 Z'ZO1 '101 8' L (um) 00L-009 (um) )) op LIN 21 Ḻ - t OIaIg opt'21-1 (%) Opl'. 0 Table 2: Percent deviations and maximum errors of spectral curves collected with the spectrometer probe 0, 15, 30, 45, 60, 75 to normal incidence of the sunlight, compared with AM1.5G spectrum. When we measure the spectral irradiance for solar simulators, we ensure that the probe s maximum title angle is smaller than 15 at least. An exemplified case as displayed in Figure 3 and Table 3, small difference shown the maximum error was about 0.7%. That was tiny compare to u 1 = 6% (k = 2). Proc. of SPIE Vol A-4

5 Irradiance (NW /cm2/nm) n ' Wavelength Figure 3: Spectral curves collected with the spectrometer probe 0, 5, 15 to normal incidence of the solar simulator s beam r/) Maximum Error (%) 0.1% 0.2% 0.2% 0.2% 0.2% 0.7% Table 3: Percent deviations and maximum errors of spectral curves collected with the spectrometer probe 0, 5, 15 to normal incidence of the solar simulator, compared with AM1.5G spectrum. 3.3 Repeatability of measurement (u 3 ) Random error caused by measurement repeatability should also be considered in the process. Five times of spectral irradiance measurement with the same conditions exactly were conducted, such as the same solar simulator, the same spectrometer and probe angle, the same testing position and testing person, etc. Figure 4 shown very close spectral curves, while Table 4 demonstrated percent deviations of 6 intervals compared to AM1.5G spectrum and maximum error (0.6%) of the repeating experiments. Irradiance (NW/cm2/nm) Wavelength Figure 4: Results of repeating experiments in the same conditions, five spectral curves were very similar to each other. Proc. of SPIE Vol A-5

6 Ratio ( %) Ratio ( %) Ratio ( %) Ratio ( %) Ratio ( %) Maximum Error ( %) 0.6% 0.4% 0 0.1% 0.5% 0.6% Table 4: Data of percent deviations and maximum errors collected in repeating experiments. In summary, the combined uncertainty of solar simulators spectral irradiance measurement is: U = u1 2 + u 2 + u3 = (3%) + (0.7%) + (0.6%) 3.1% And with a coverage factor of k = 2, the combined expanded uncertainty is U = 6.2% (k = 2). 3.4 Compare to other systems Comparisons of our results and reports launched by the other countries were made, such as RaySphere spectrometer by Rhineland TÜV and a NIST traceable system. We got the same classification of the same solar simulators with acceptable errors in percent deviations from AM1.5G reference spectrum. As shown in Table 5, compared with a NIST traceable integral sphere spectrometer, the largest error is only 3.8%. Moreover, the results we collected with optic fiber spectrometer are more stable NIST traceable (Using integral sphere) Ratio ( %) Ratio ( %) NIAI traceable (Using optic fiber spectrometer) Ratio ( %) Ratio ( %) Maximum Error ( %) 3.8% 1.0% 2.4% 1.6% 3.7% 2.2% Table 5: Comparison with a NIST traceable measurement system by measuring the spectral irradiance of the same simulator (light source: diffuse reflection). The largest error is 3.8%, and cosine angle is the main factor. The stability of our results is better as the data shown. Proc. of SPIE Vol A-6

7 4. CONCLUSIONS In summary, we have proposed a method for solar simulator s spectral irradiance measurement with a NIM calibrated fiber optic spectrometer. Combine with large amount of data collected, we made a reasonable uncertainty analysis. Considering the declared uncertainty of u 1 : 6% (k = 2) (300 nm nm), along with uncertainty caused by spectrometer probe s cosine correction (u 2 ) and measurement repeatability (u 3 ), combined expanded uncertainty of U = 6.2% (k = 2) was summarized, it is essential for metrology of photovoltaic industry. Acknowledgments: The spectrometer calibrating work is technically supported by the Radiometry and Colorimetry Laboratory of NIM (National Institute of Metrology, P.R. China). We appreciate the funding support of NIM (BSH1007, etc.) and General Administration of Quality Supervision, Inspection and Quarantine, P.R. China (AQSIQ). REFERENCES 1. IEC (Ed. 2), Photovoltaic devices - Part 1: Measurement of PV current-voltage characteristics, K. A. Emery, Solar simulators and I-V measurement methods, solar cells, 18, , R. J. Matson, K. A. Emery, R. E. Bird, Terrestrial solar spectral, solar simulation and solar cell short-circuit current calibration: A review, Solar cells, 11, , H. B. Serreze, R. G. Little, Large-area solar simulator: critical tools for module manufacturing, Technical Papers in PV Modules, Edition 1 - Photovoltaics International, ISO : Solar energy Reference solar spectral irradiance at the ground at different receiving conditions part 1: Direct normal and hemispherical solar irradiance for air mass 1.5, IEC (Ed. 2), Photovoltaic devices - Part 3: Measurement principles for terrestrial photovoltaic (PV) solar devices with reference spectral irradiance data, D. Dominé, G. Friesen, S. Dittmann, D. Chinaese, The influence of measurement errors in spectral irradiance of flash solar simulators on the spectral mismatch factor of PV modules, the 25 th EU PVSEV/WCPEC-5, Valencia, Spain, H. Müllejans, W. Zaaiman, R. Galleano, Analysis and mitigation of measurement uncertainties in the traceability chain for the calibration of photovoltaic devices, Meas. Sci. Technol. 20, , J. Hohl-Ebinger, W. Warta, Uncertainty of the spectral mismatch correction factor in STC measurements of photovoltaic devices, Progress in Photovoltaics: Research and Applications, 19, , C. Monokroussos, M. Bliss, Y. N. Qiu, C. J. Hibberd, T. R. Betts, A. N. Tiwari, R. Gottschalg, Effects of spectrum on the power rating of amorphous silicon photovoltaic devices, Progress in Photovoltaics: Research and Applications, 19, , IEC (Ed. 2), Photovoltaic devices - Part 9: Solar simulator performance requirements, T. W. Cannon, Spectral measurement of pulse solar simulators, National Center for Photovoltaic Program Review Meeting, Denver, Colorado, September 8-11, A. M. Andreas, D. R. Myers, Pulse analysis spectroradiometer system for measuring the spectral distribution of flash solar simulators, SPIE Optics and Photonics Conference: Optical Modeling and Measurements for Solar Energy Systems II, San Diego, California, H. W. Yoon, B. P. Dougherty, V. B. Khromchenko, Spectroradiometric characterization of the NIST pulsed solar simulator, Proc. of SPIE, 7410, , H. Meng, L. Xiong, Y. He, D. Liu, Research on integrated system for solar simulator performance calibration according to IEC , Proc. SPIE 8201, 82012L, Proc. of SPIE Vol A-7