interconnection losses and a Light Reflective Film (LRF) on the top of ribbons, ... electrical loss, thanks to low resistance connection technology, is also analyzed.
19.86% Aperture Efficient World Record P-type Multi-crystalline Module with 20.59% Efficient PERC Solar Cells Shu Zhang, Yang Yang, Daming Chen, Weiwei Deng, Hongwei Huang, Pietro P. Altermatt, Jianmei Xu, Zhiqiang Feng and Pierre J. Verlinden State Key Laboratory of PV Science and Technology, Changzhou Trina Solar Energy Co., Ltd., Changzhou, Jiangsu 213031, China Abstract —Using p-type multi-crystalline silicon wafers, 156 × 156 mm2 passivated emitter and rear (PERC) cells with 5 busbars were fabricated with an average efficiency of 20.59%. The module uses two technical improvements to increase the conversion efficiency: half-cell technology to reduce interconnection losses and a Light Reflective Film (LRF) on the top of ribbons, as well as between cells, to improve lighttrapping. The module composed of 120 half-cells achieved a new world record with an aperture efficiency of 19.86% on an area of 15,143 cm2. The Cell-to-Module (CTM) factor of the champion module reaches 100%. The optical loss at both cell and module levels is analyzed with the aid of the Module Ray Tracer (MRT) from PV Lighthouse. The simulation of the optical yield from cell to module is presented and is shown to be in agreement with the measured values. The reduction in electrical loss, thanks to low resistance connection technology, is also analyzed. Index Terms —half-cell technology, Light-trapping, PERC cell, optical loss, electrical loss, aperture efficiency
I. INTRODUCTION Although the market share of mono-crystalline silicon recently showed some increase, multi-crystalline silicon technology is still dominating in the PV industry [1]. IHS marketing data shows that the market share of multicrystalline silicon in global PV shipments is accounted for more than 65% in 2016 [2], while mono-crystalline Si represents about 25%. To improve the efficiency of solar cells or modules using multi-crystalline silicon technology, one of the most promising technologies is the Passivated Emitter and Rear (PERC) cell design. In 2015, Trina Solar reported a PV module with a world record aperture efficiency of 19.2% on an aperture area of 15,127 cm2 [3]. In March 2016, Hanwha-Q-cells announced a new world record efficiency of 19.5% on multi-crystalline silicon solar module with an aperture area of 15,349 cm2 [4]. In July 2016, Trina Solar announced that the average efficiency of its p-type industrially-produced multi-Si PERC cells had reached 20.16%. The average power output of such PV modules, consisting of 60 pieces, is up to 286 W [5]. In this paper, we describe the technical details of the recently achieved aperture record efficiency module, including its advanced module technologies and the PERC cell technology. We analyze the optical loss and electrical loss of the champion module in detail.
II. EXPERIMENTAL A cross section of the PERC cell structure is shown in Fig. 1. These cells were fabricated on p-type 156 × 156 mm2 multi-crystalline Si wafers with a resistivity between 1.0 and 3.0 ∙cm and a thickness of 190 ±20 µm.
Fig. 1. Schematic cross-section of the PERC cell.
At the module level, in addition to the high-efficiency multi-Si PERC cells, we apply a series of advanced module technologies including: (i) a front glass with a very low iron concentration and with anti-reflective coating (ARC), (ii) a UV-transparent EVA, (iii) a structured light-reflective film (LRF) between cells and on the top of the ribbons, (iv) halfcell connection and (vi) split junction-boxes. The process flow chart of the module assembly is shown in Fig. 2.
Fig. 2. Process flow of this advanced multi-crystalline PV module.
In the aspect of module optics, we used a type of glass with a uniform SiO2 nano-structure forming the ARC to minimize the reflection losses. A UV-transparent EVA was used as encapsulant to increase the incident irradiance on
the cell surface by about 1%. The optical principle of LRF between the cells and on the top of ribbon is relatively easy to understand. Most of incidence light falling onto the busbar, ribbons or between cells is usually reflected and is partially lost for the photovoltaic conversion. Placing an optically engineered reflective film in those areas allows for re-directing the reflected light toward the cells via total internal reflection at the front glass/air interface. An experiment was performed to study this effect of LRF. The baseline is a conventional module without LRF between cells and with a cell gap of 3 mm. We compare this baseline module with several test modules containing LRF between cells and with various gaps between cells. As shown in Fig. 3, larger gap between cells results in a larger power output (Pmpp), up to a very large gap. However, the aperture efficiency decreases as the gap between cells increases because the optical efficiency of this light path is not as great as if the light is directly falling onto the cell surface. In most cases, in order to reduce the levelized cost of electricity (LCOE), it is more important to increase the module efficiency instead of the module power. To achieve the highest aperture efficiency and also avoid cell breakage, both the cell gap within one string and the cell gap between adjacent strings are chosen to be 2 mm.
domain of the optical environment (unit cell) within the module is marked by the green dash line.
Fig. 4. Schematic cross-section of the advanced module.
III. RESULTS AND DISCUSSION A. I-V Characteristics of PERC Cells 500 cells were fabricated on a PERC industrial pilot line. The distribution of electrical parameters of the cells is shown in Fig. 5 and Table I.
Fig. 5. I-V parameters of 500 PERC cells fabricated on a pilot line: Median (line), percentile 25% (Q1) - 75% (Q3) (box area), the whiskers span from maximum to minimum.
Fig. 3. Multi-Si Module Pmpp relative change and Aperture Efficiency relative change under different cell gaps and with LRF between cell gap, compared to a module without LRF and with 3mm gap.
In regard of the electrical characteristics of the module, we implemented a half-cell technology to reduce the interconnection losses by a factor of 4 and to reduce the total series resistance of the modules by about 20%. A split junction-box is incorporated to further decrease the series resistance of the module. The cross section of the module is shown in Fig. 4. There are 3 typical optical environments, which are marked with A, B and C within the module. The light path noted A is for the light falling directly onto the cell area containing only AR-coated surface and narrow metal fingers; light path B is for light falling onto the LRF located on top of the busbars and ribbons; Light path C is for light falling between cells onto the LRF area. The irreducible simulation
The median cell efficiency of the whole batch, measured on internal production line, is 20.59%. While the maximum efficiency recorded for this batch was 20.78%, the best PERC cell fabricated in laboratory has reached 21.25%. The process of these experimental cells is similar to that of industrial PERC solar cells, with only one difference: they received a texturing step by reactive ion etching (RIE). Combined with excellent passivation and advanced selective emitter (SE) technology, the Voc of these PERC cells is as high as 660 mV, which is about 20 mV higher than that of typical conventional (i.e. not PERC) multicrystalline cells. Due to excellent light trapping structure of the solar cells, the median Jsc is as high as 38.95 mA/cm2, which is around 5% higher than for conventional multicrystalline Si solar cells.
TABLE I ELECTRICAL PARAMETERS OF THE PILOT RUN CELLS FOR THE CHAMPION MODULE, AS MEASURED IN-HOUSE. ELECTRICAL PARAMETERS OF THE BEST LAB CELL, INDEPENDENTLY CONFIRMED BY FRAUNHOFER ISE. Groups Best Lab Cell
a
FF (%)
667.8 ± 2.3
39.78 ± 0.76
79.97 ± 0.52
a,b
b
η (%)
Jsc (mA/cm )
Pilot Cells a
2
Voc (mV)
659.8
38.95
21.25±0.43
80.16
20.59
All measurements are under standard conditions (1000W/m2, AM 1.5G and 25 ℃, total area 243 cm2 ). The Pilot Cells refer to the median value of the pilot cells TABLE II ELECTRICAL PARAMETERS AND DIMENSION OF THE CHAMPION MODULE, AS MEASURED BY FRAUNHOFER ISE CALLAB. Isc [A]
Uoc [V]
Pmpp [W]
FF [%]
Aperture Eff. [%]
Aperture 2 Area [cm ]
Average
4.80
78.87
300.73
79.52
19.86
15,142.5
Uncertainty[%]
± 1.5
± 0.6
± 1.8
± 1.5
± 1.8
--
B. New Multi-crystalline Silicon World Record Module
𝑃𝑚𝑝𝑝 𝑜𝑓 𝑚𝑜𝑑𝑢𝑙𝑒 𝐶𝑇𝑀 = ∑ 𝑃𝑚𝑝𝑝 𝑜𝑓 𝑐𝑒𝑙𝑙
𝑃𝑚𝑝𝑝 𝑜𝑓 𝑚𝑜𝑑𝑢𝑙𝑒
= ∑ 𝐶𝑒𝑙𝑙 𝑒𝑓𝑓.×𝑐𝑒𝑙𝑙 𝑎𝑟𝑒𝑎×1000𝑊/𝑚2
300
5
Module Current (A)
250
Module Current Module Power
4
200
3
150
2
100
1
50
0
0
0
(1)
Module Power (W)
Fig. 6 and Fig. 7 show a photograph and the I-V characteristics of the champion module that reached an aperture efficiency of 19.86% and its IV measurement result shown in Table II, confirmed by Fraunhofer ISE CalLab. As far as we know, this is the highest aperture efficiency ever reported for a multi-crystalline silicon based PV module. The CTM of the champion module reaches 100%. CTM is defined as:
20
40
60
80
Module Voltage (V)
Fig. 7. I-V Characteristics and the power curve of the champion module.
C. Optical Loss at the Cell Level and the Module Level
Fig. 6. Champion module with 19.86% aperture efficiency using 60 multi-Si PERC cells (cut in halves).
The optical loss analysis presented here is a further development of the method we introduced in a previous paper [7]. We selected two cells from our latest experiment. The cell fabrication process is the same as the 20.59% efficient cells. The difficulties to simulate the reflectance of these solar cells and to compare the result with measured data mainly are: (a) the roughness of the textured surface due to the RIE process (shown in Fig. 8) is difficult to characterize; and (b) the thickness of ARC on such roughed surface is not uniform. In MRT optical simulation, we assume a surface formed of pyramids, for which we fine tune the angle of the pyramids and thickness of ARC of the cell. For fitting in the long wavelength range, it is not necessary to do an adjustment of the thickness of the rear
passivation layer to obtain a good agreement because the thickness is well known and well characterized since the structure is planar. Other parameters, such as silicon wafer thickness and rear side metal contact coverage, are well known. By adjusting the remaining factors including (i) the rear side metal reflectivity, (ii) Lambertian factor at the aluminum interface, a good fitting of the cell reflectance has been achieved as shown in Fig. 9.
Fig. 8.Textured surface of cell due to the RIE process.
32.61 mA/cm2. The simulation demonstrated that the amount of light escaping from the module is remarkably reduced. The optical losses at the cell level and module level are shown in Fig. 10.
Fig. 10. Simulated optical loss of minimum cell and module unit.
The J_gen from a minimum cell unit to minimum module unit was simulated and compared with measured Jsc on the mini module, as shown in Fig.11, which also indicates the optical yield value, i.e. the ratio of the current density of the module to the current density of the non-encapsulated cell. For the module with LRF designed as the same as the champion module, the measured optical yield is 97.7% whereas the simulated optical yield is 97.4% with LRF and only 94% without LRF. The simulated value of the optical yield is in good agreement with the measured value. Compared with the module without LRF, the simulated optical yield increased by absolute 3.4%. Fig. 9. Simulated and measured reflectance of a cell and mini module (part A).
Regarding the optical modeling of the module, we measured the reflectance R() of a mini-module at different locations with a 2 × 2 cm2 collimated beam and an Ulbricht integrating sphere (See Fig. 4): (i) pointing to the cell area with fingers (part A), (ii) pointing to the cell area without fingers (part A) and (iii) pointed to the LRF (parts B and C). All these measured reflectance spectra R() were compared with reflectance spectra simulated by MRT from PV Lighthouse using the geometry of the spectrometer opening and suitable material parameters. Fig. 9 shows that, in most of the wavelength range, the simulated and measured reflectance spectra of the mini module, for light path A, are comparable. With these material properties, we then extend the simulation domain from 2 × 2 cm2 to 15.6 × 7.8 cm2 to simulate the entire module. At the module level, the properties of the LRFs are considered in MRT as custom wavelength-dependent reflector with a V-groove morphology. We simulated two groove structures, in X and Y directions respectively. The calculated optical generation (J_gen) due to the LRF on the busbar top or between cells is
Fig. 11. Optical yield comparison from cell to module by measurement and simulation.
Fig. 12 shows that the difference in current density for modules with or without LRF on the top of the ribbons is not constant but varies with the angle of incidence. LRF is beneficial up to 40°from normal incidence, which is most of the direct sunlight energy in a stationary module over the year. However, LRF is not advantageous for incidence
angle greater than 40°, and therefore not advantageous for locations with a high proportion of diffuse sunlight.
Although the proportion of power loss in each part is almost the same, the actual power loss for the champion module is around 1/4 of the standard full cell module. The main contribution comes from the fact that the length of the ribbons to interconnect the cells in one string is cut in half. Since the photo-generated current is half and the length of the ribbon is also half, compared to the standard design, the interconnection series-resistance loss is divided by 4. Due to excellent electrical design at the champion module level, the power loss induced by module assembling is smaller than 3W. While for the standard 60 piece full cells module, this value would be higher than 10W. IV. CONCLUSION
Fig. 12. Current density of modules with conventional flat ribbon and with a LRF on the top of ribbon at different incidence angles.
D. Electrical Loss at the Module Level The total electrical losses of a PV module come from several elements, such as the cell resistance, cell-to-ribbon contact resistance, ribbon resistance, string connector resistance and junction box resistance (Fig.13). Herein, we neglect the cell-to-ribbon contact resistance [8]. We discuss the power loss due to module assembling, including cell connector ribbon, string connector and junction boxes.
500 multi-crystalline Si cells were fabricated on a PERC industrial pilot line using a similar process to that of industrial PERC solar cells. An average efficiency of 20.59% was achieved. 60 pieces of these 156 × 156 mm2 PERC cells were assembled as half-cells in the module, which reached an aperture efficiency of 19.86% on the aperture area of 1.514 m2. The optical loss of the champion module was analyzed with the aid of ray tracing simulation. V. ACKNOWLEDGEMENTS The authors would like to thank the entire R&D team and Golden Line team of Trina Solar for their contributions and team spirit to achieve these outstanding results. This work is supported by the Natural Science Foundation of Jiangsu Province under the Project Number of BK20151192. VI. APPENDIX A The main power loss from cells to module is the resistance loss from the cell connector. The diagram of the cell connectors are shown in Fig. 15. This part of power loss is defined as: 𝜌𝑙
2 Ploss−cell connector =2𝑁𝐼𝑚𝑝𝑝 + 3𝑛𝐴
Fig. 13. The components of module Rs.
The calculated result is shown in Fig. 14 and the detailed model of calculation is described in Appendix A.
Fig. 14. Power loss of cell connector, string connector and junction box for PV modules.
𝑛′ (
𝐼𝑚𝑝𝑝 2 𝜌 𝑛
)
𝐼𝑚𝑝𝑝 2 𝜌
𝑙′+𝑛" ( 𝐴
𝑛
)
A
𝑙"
(2)
Where: ρ is electrical resistivity of the ribbons, typ. 2.0×10-8 Ω.m; l is the length of the ribbons for one cell, 78mm for a half cell; l’ is the length of the ribbon between the cells, 2mm for the champion module; l’’ is the length of the ribbon between the cells and string connector, 3 mm; A is cross sectional area of the ribbons, typ. 0.25 mm × 1.1 mm; N is the number of the cells in the module, 120 pcs for the half-cell module; n is the number of the busbar in the cells, 5;
n’ is the number of gap between the cells, 570 for this module; n’’ is the number of gap between the cells and strings, 60 for the module; Impp is the maximum-power-point operating current for the cells, about 4.45 A for this case； The total power loss caused by the series resistance of the cell connector in the “half-cell” module is 2.01 watts.
n1, n2, n3, n4, n5 are the number of string connector where the currents are 0.2Impp, 0.4Impp, 0.6Impp, 0.8Impp, Impp respectively; n6 is the number of terminal connector where the current is Impp. The power loss caused by the series resistance of the string connector in the “half-cell” module is 0.14 watt. The power loss caused by junction box is defined as:
𝑃𝑙𝑜𝑠𝑠,𝐽𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑏𝑜𝑥 = 𝐼𝑚𝑝𝑝 2 𝑅𝐶𝑎𝑏𝑙𝑒 𝑇𝑒𝑟𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛 + 𝐼𝑚𝑝𝑝 2 𝑅𝐶𝑎𝑏𝑙𝑒𝑠 + 𝐼𝑚𝑝𝑝 2 𝑅𝐶𝑜𝑛𝑛𝑒𝑐𝑡𝑜𝑟 (4) The typical Rs components of J-box are set as shown in Fig. 17. The power loss due to the Junction box, cables and connectors is less than 0.3watt.
Fig 15. Diagram of the cell connector.
Fig. 17. Resistance parts of junction box.
REFERENCES
Fig. 16. Diagram of current flowing in the string connector.
Similarly, according to the diagram of the current flowing in the string connector within the module shown in Fig. 16 and the power loss due to string connector is shown below:
𝑃𝑙𝑜𝑠𝑠,𝑠𝑡𝑟𝑖𝑛𝑔 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑜𝑟 = ∑𝑛𝑖=1 𝐼𝑖2 𝑅𝑖
(3)
Where: ρ is electrical resistivity of the ribbons, typ. 2.0×10-8 Ω﹒ m; l1, l2, l3, l4, l5 are the length of the string connector where the currents are 0.2Impp, 0.4Impp, 0.6Impp, 0.8Impp, Impp respectively; l6 is the length of the terminal connector which is connected with junction box, where the current is Impp; A is cross sectional area of the ribbons, 0.35 mm × 8 mm;
[1] “Personal communication with IHS PV Market subject expert”, 2017. [2] “International Technology Roadmap for Photovoltaic (ITRPV) 2016 Results Eighth Edition”, 2017. [3] S. Zhang, et al., “335-W world-record p-type monocrystalline module with 20.6% efficient PERC solar cells”, IEEE Journal of Photovoltaics, vol. 6(1), pp. 145-152, 2016 [4] Scherff MLD, et al, “P-type multi-crystalline silicon solar cell module with 19.5% efficiency on aperture area”, SiliconPV， 2016. [5] http://pv.energytrend.com/news/Trina_Solar_Multisi_PERC and_DP_Cells_Achieve_Efficiency_Records.html [6] Y. Yang et al, “Combining ray tracing with device modeling to evaluate experiments for an optical loss analysis of crystalline Si solar cells and modules”, SiliconPV, 2017. [7] W.W. Deng, F. Ye, et al, “Development of high-efficiency industrial p-type multi-crystalline PERC solar cells with efficiency greater than 21%”, SiliconPV, 2016. [8] L.J. Caballero, et al, “Series Resistance Modelling of Industrial Screen-Printed Monocrystalline Silicon Solar Cells and Modules Including the Effect of Spot Soldering, Photovoltaic Energy Conversion”, Conference Record of the 2006 IEEE 4th World Conference, pp. 1388, 2006.
