HETEROJUNCTION TECHNOLOGY THE SOLAR CELL OF THE FUTURE

HETEROJUNCTION TECHNOLOGY THE SOLAR CELL OF THE FUTURE

G. Roters 1, J. Krause 1, S. Leu 2, A. Richter 2, B. Strahm 3 1 Roth & Rau AG, An der Baumschule 6-8, 09337 Hohenstein-Ernstthal, Germany 2 Meyer Burger Technology Ltd, Schorenstrasse 39, CH-3645 Gwatt (Thun), Switzerland 3 Roth & Rau Research AG, Rouges-Terres 61, CH-2068 Hauterive, Switzerland HETEROJUNCTION TECHNOLOGY ABSTRACT Standard process Sel. Emitter process MWT process MB-PERC HJT process Wafer-based silicon photovoltaic (PV) production has only changed slightly in the last forty years. The standard concept comprises p-type silicon wafers, fired contacts and encapsulation. Cost reduction is necessary if PV is to survive without feed-in tariffs and be competitive with grid electricity costs. Therefore levelised cost of electricity (LCOE) is one of the primary metrics for the cost of electricity produced by both utility scale and distributed power systems. The fastest path to lower LCOE is to introduce high efficiency solar cell concepts like the heterojunction technology (HJT). Photovoltaic systems using heterojunction technology (HJT) modules outperform any other PV systems and this paper will explain why. Edge Isolation Additional 4 Process Steps Edge Isolation Laser Drilling Edge Isolation SiNx Capping- & AlOx passivation-layer a-si Front / Rear Side TCO / Metal Rear Contact Curing Print Rear Side Print Front Grid and p-contact Introduction Laser contact opening Meyer Burger develops solar technology from wafers to complete PV systems with the aim of promoting the widespread use of photovoltaics and making solar power a first-choice source of renewable energy. To this end, the company focuses strongly on developing solar cell technologies which allows highest efficiencies and highest energy output at lowest production costs. Recently Panasonic demonstrated efficiencies of 24.7% on laboratory scale cells [1]. With the discontinuation of the basic technology patent, heterojunction technology was opened to the public in 2010. Meyer Burger is now offering this appealing technology as a high performance key technology in the photovoltaic value chain. Print Rear Side Edge Isolation (Front) Edge Isolation (Rear) Print Front- & Rearside Significant potential for cost savings! Heterojunction cell technology combines the advantages of mono crystalline silicon (c-si) solar cells with the good absorption and the superior passivation characteristics of amorphous silicon (a-si) known from a-si thin film technology using readily available materials. The HJT design is not new. Sanyo (now Panasonic) first pushed this technology into mass production achieving around 20% cell efficiency. The simple structure of a HJT cell is shown in Figure 1. The thin intrinsic a-si:h layers deposited between c-si wafer and doped layers are key to achieving maximum performance from the cell structure. They result in reduced interface state density, decreased surface recombination losses and lower the emitter saturation currents. CZ: 19% MC: 16.8 17.5% CZ: 19 20% MC: 18 19% CZ: 20 21% MC: 18 19% CZ: 20 21% MC: 19 20% CZ n-type: 21 ~ 25% Figure 2: Process sequence of differing PV technologies Thin wafers Ag (screen print) ITO Ag (PVD) n-type c-si Figure 1: Schematic drawing of the HJT cell and the process steps required ITO (p) a-si:h (i) a-si:h (i) a-si:h (n) a-si:h a-si Front / Rear Side TCO / Metal Rear Contact Curing The relatively straightforward HJT production process takes place at low temperatures and requires fewer production steps compared to other high efficiency designs as shown in Figure 2, which is economically attractive as it results in significant energy cost savings. An important technological advantage of HJT cells is the excellent surface passivation of a-si:h which results in high open-circuit voltages and high cell efficiencies. The superior temperature coefficient of T C = -0.20%/K ensures higher energy yield during module operating conditions. As previously mentioned, low temperature processing (<250 C) saves energy during manufacturing, prevents bulk degradation and enables the use of thin wafers. Integrated development throughout the PV value chain (diamond wire wafering, heterojunction cell technology and SmartWire Connection Technology (SWCT) guarantees the maximum performance of HJT modules. Meyer Burger technology for high performance solutions ensures the perfect integration and harmonisation of all processes with the goal of lowest LCOE. Today, wafers with a thickness of ~180 µm are in standard use in cell manufacturing. A wafer thickness even below 100 µm is enough for absorbing light in silicon solar cells. Using thinner wafers leads to significantly lower material costs because a higher number of wafers can be cut from one brick. With decreasing wafer thickness, the influence of the bulk material quality also decreases resulting in less tight requirements for e.g. minority charge carrier lifetimes. Diamond wire sawing is the best approach for thin wafers. The use of diamond wire leads to fewer microcracks and the depth of microcracks is shallower. With decreasing wafer thickness and the corresponding higher surface to bulk ratio the surface recombination loss becomes dominant compared to recombination loss in the bulk. Consequently superior surface passivation techniques are mandatory. 2 3

HELiA PECVD : a-si:h coating for front and rear side HELiA PVD : TCO/ Metal coating for rear side contact In general there is an optimum wafer thickness for every cell technology. However as shown in Figure 3, the final cell efficiency for heterojunction cells is independent of the wafer thickness at least in the range of wafer thicknesses achievable with sawing processes. Figure 3: Impact of wafer thickness on efficiency of heterojunction cells including the potential with improved light management Process flow / Texturing An optimal texturing and cleaning process forms the basis for a successful production process for highly efficient HJT cells. Meyer Burger process knowhow sets the foundation for optimum passivation. The initial steps in the HJT process sequence are wet chemistry processes for saw damage removal (SDR), texturing, cleaning and hydrogen termination. The subsurface damages and microcracks have to be almost completely removed to achieve high efficiency HJT cells. Measuring the surface recombination velocity (SRV) indicates the necessary saw damage removal independent of the quality of the bulk. Figure 4 shows the impact of saw damage removal on the surface recombination velocity. The saw damage removal can be optimised with the texturing process. A final short dip in hydrofluoric acid terminates the silicon surface until final passivation in the subsequent PECVD process. The surface after this wet chemical treatment is crucial for the quality of c-si / a-si:h interfaces and therefore for the surface passivation. PECVD: a-si:h coating for front and rear side In order to minimise energy loss within the solar cell, the surface must be highly passivated. Low temperature passivation with a-si:h deposited in a temperature range between 150-250 C results in outstanding surface recombination velocities. This a-si:h is able to passivate all levels of n-type and p-type silicon. Doped a-si:h is used to form both the emitter and the back surface field (BSF). Additionally, doped a- cell efficiency [%] SRV Seff [cm/s] 16 14 12 10 25.0 24.0 23.0 22.0 21.0 20.0 8 6 4 2 80 100 120 140 160 180 200 220 240 260 mean cell thickness [µm] 0 4 6 8 10 12 14 SDR [µm/side] Figure 4: Impact of saw damage removal on SRV Si:H contributes to passivation due to its field effect properties. Roth & Rau, a member of the Meyer Burger Group, has developed the modular high throughput HELiA PECVD system specifically for wafer based silicon HJT cell concepts [2]. The heart of the HELiA PECVD system is the patented S-Cube, a sophisticated parallel plate plasma reactor with a box-in-box arrangement providing ultra-pure and uniform amorphous silicon layers. A 13.56 MHz RF source is used to minimise plasma damage during layer deposition. Thus the required quality of the amorphous silicon layers in terms of minority charge carrier lifetime and band gap are ensured. The key parameter to determine the quality of the passivation with the a-si:h layer is the minority charge carrier lifetime. The HELiA PECVD system has achieved values of more than 10 ms effective lifetime on FZ-wafers and more than 4 ms on CZ production wafers as shown in Figure 5. These sets of data both for the polished FZ-wafers and for the textured CZ-wafers demonstrate obviously the passivation capability of the HELiA PECVD system. For heterojunction cell concepts, intrinsic a-si:h layers are deposited on the front and rear sides of the wafer respectively, providing excellent surface passivation. Doped a-si:h layers are deposited on the intrinsic a-si:h layers in order to create an emitter on the front side of the cell (boron Measured lifetime [s] 10-2 Minority Carrier Density [cm-3] doped p a-si:h) and a back surface field (BSF) on the rear side (phosphorous doped n a-si:h). The emitter enables the separation of the charge carriers while the BSF drives the minority charge carriers away from the rear side to reduce rear side recombination losses. Fz1: 12378 µs Fz2: 12841 µs ST27-2205: 4472 µs ST27-2209: 5639 µs ST27-2222: 6769 µs ST27-2318: 4837 µs ST27-2324: 4752 µs ST27-2325: 4365 µs 10-3 10 14 10 15 10 16 10 17 Figure 5: Sinton lifetime data of an i-passivated polished FZ and CZ wafers (Numbers for internal reference of experiment) 4 n Automatic wafer transfer 3 i Automatic wafer transfer between s 2 p 1 HTO WF HTI Figure 6: Wafer flow within the HELiA PECVD system i In the single stripe: 1. Automatic carrier movement 2. Process sequence automatic Automatic wafer transfer 4 5

Lift WL Double LM BMi TCO BMo TM BMi Metal BMo UM WU Lift D - Unload Figure 7: HELiA PVD system configured for HJT cell processing To achieve a high quality surface passivation of a HJT cell, it is essential to avoid any cross doping. The HELiA PECVD system therefore performs these process steps in dedicated process strips to use an optimised process setting for each layer and prevent from any surface contamination. Figure 6 shows a schematic of a HELiA PECVD system including the wafer flow. The HELiA PECVD system in Figure 6 is designed for a throughput up to 2,400 wafers per hour. As the a-si:h layers are in the range of only a few nanometers, the deposition process is very short and the gas consumption is low. PVD: TCO / Metal coating for rear side contact In the modular HELiA PVD system, a sputter process is used in one of the chambers to apply a transparent conductive oxide (TCO) layer to the front and rear side of the wafer [2]. In addition to collecting the photo-generated current and forming an ohmic contact on the cell, the TCO layer on the front side acts as an anti-reflection layer. Especially for the front side of the HJT cell, the optical and electronic properties of a-si:h and TCO layers need to be adjusted with respect to each other. Indium tin oxide (ITO) is a favorable TCO material for HJT cells because it is very transparent and conductive Print front side Conventional crystalline silicon solar cell technologies use front side collector lines ( fingers ) and busbars. Reducing the consumption of the silver paste used in the fingers and busbars is key to reducing costs. Meyer Burger offers the busbarless SmartWire Connection Technology (SWCT) to minimise cell-to-module (CTM) loss and to optimise the module efficiency. Only the finger grid pattern on to the front surfaces of HJT cells will be formed by using a conventional screen printer and an epoxy-based low temperature paste. The busbarless design facilitates a fine-lined screen printed grid pattern. By replacing the busbars with a lined grid pattern with thinner and smaller fingers, silver consumption is significantly reduced. It also increases the cell conversion efficiency by decreasing shadow loss on the cells. Figure 8 shows the performance of HJT SWCT modules. The module power is increased while silver consumption is reduced. With the application of SWCT, the requirements for finger conductivity become less stringent compared to the requirements for conventional busbar designs. Finger resistance of up to 100 Ω/cm can while providing a good electrical contact with the doped a-si:h layers. In a second chamber of the same HELiA PVD system, a metal layer such as silver, or a stack of different metals such as a combination of silver and nickel vanadium is applied on the TCO layer as rear side metallisation. The composition of the metal stack is dependent on the downstream module process. The rear side metallisation provides an excellent electrical contact to the TCO layer as well as a good internal reflectance of long wavelength light. This is accomplished without having to break the vacuum or to turn the wafer between these coating processes. Figure 7 shows a HELiA PVD system configured for HJT cell processing. This configuration enables to achieve the edge isolation simultaneously avoiding extra laser or chemical steps. Rotating cylindrical sputter targets for TCO and metals on magnetrons enable a high target utilisation of over 85% to be achieved, ensuring a cost effective coating process. The HELiA PVD and HELiA PECVD systems are perfectly matched in terms of capacity and layer properties. Power [% rel ] 102.0 101.5 101.0 100.5 100.0 99.5 99.0 98.5 98.0 Screen A Finger width: 90-100 um 110 mg of Ag paste Screen B Finger width: 50-60 um 40 mg of Ag paste Figure 8: Power gain combined with reduced silver consumption of Heterojunction SmartWire Connection Technology modules be combined with SWCT without causing significant power losses. The shorter the distance between two finger contacts, the lower the impact of power losses in fingers. Curing A - Load After printing the front side, contact firing is not required with this HJT cell concept. The curing of printed HJT cells is a simple thermal process at temperatures <250 C in order to outgas the solvents of the low temperature paste. The temperature profile affects the conductivity of the screen printed lines and of the TCO lay- Testing HJT cells are high capacitance cells which require a measurement time of 400-600 ms. This is significantly longer compared to standard low capacitance cells. Pasan SA [3], a member of the Meyer Burger Group, in cooperation with the Institute of Micro Technology (IMT) at the University of Neuchâtel in Switzerland [4], has developed a new I-V curve cell tester series known as Spot LIGHT, which is available in two formats: Spot LIGHT 1 sec and Spot LIGHT HighCap. With its high quality A+A+A+ 5 ms length pulsed Xe light source, Spot LIGHT 1 sec is dedicated to the high-speed measurements that are required for in-line applications, such as end-of-line quality control in solar cell production lines or beginning-of-line quality control in solar module production lines. Spot LIGHT HighCap is dedicated to testing solar cells with high capacitances such as heterojunction cells. The Spot LIGHT HighCap combines light-emitting diodes (LEDs) to increase the pulse length from 5ms to 600ms (Figure 9). This hybrid light source measurement process had been validated by Fraunhofer ISE. The result is a system which provides dependable solar cell measurements while maintaining the total COO of the system at the same level achieved by the Spot LIGHT 1 sec. This is a unique characteristic among longpulse solar simulators. Measurement of high efficiency modules has different constraints: illuminating a large area during several hundreds of millisecond would not be cost effective, neither accurate. On the other hand, the capacity of a module is much lower than that of a cell. Therefore, Pasan has developed the DragonBack solution which is a dynamic sweep methodology that can be used with standard High LIGHT module tester with a 10 ms pulse length of A+A+A+ quality. Such industry-oriented and cost-effective solution enables to B - Process C - Process CALiPSO: flexible heat treatment furnace ers on the cells as well as the solderability of the cells. The Roth & Rau curing system is tailored to the HJT process in terms of process and productivity requirements [2]. measure the power of high capacitive modules, taking into account production tact-time, low TCO and high measurement accuracy. This innovative solution has been successfully validated by PI Berlin. High quality Xenon flash for calibration of LEDs LED flash up to 600ms Figure 9: Measuring methodology of the new Pasan spot LIGHT HighCap [3] 6 7

V OC : 44.38 V I SC : 9.01 A FF : 76.8% P MPP : 307.1 W HETEROJUNCTION TECHNOLOGY HJT A breakthrough in levelized cost of electricity Milestone: 307 watt module At 307 watts, Meyer Burger set a new photovoltaic record using a standard 60 cell solar module shown in Figure 10 [5]. A high level of process integration between the wafer, cell and module technologies made this developmental leap possible. The Meyer Burger HJT modules deliver an increased energy yield by combining HJT cells with a conversion efficiency of 21% and a very low temperature coefficient of just T C = -0.20%/K, together with the revolutionary Smart- Wire Connection Technology. Compared to standard cells which have a value of T C = -0.43%/K, a solar module using HJT cells from Meyer Burger can achieve >10% more energy yield on average. This results is a significant competitive advantage for cell and module manufacturers, as well as for end customers. SGS Fresenius Institute has already IEC certified HJT modules with SmartWire Connection Technology. Record Module! 60 cells, 156 mm x 156 mm 307 watt LCOE is one of the primary metrics used to measure the cost of electricity produced by both utility-scale and distributed power systems. A simulation for 100 kw PV power systems using silicon wafer and thin film PV modules was done for two different climatic conditions based on Germany which represents a cold climate and on India which represents a humid, subtropical climate. The calculation used PV SOL 6.0 Expert and the following parameters: Inverter Fronius Agilo 100.0-3, Modules Poly: 245 W, HJT 290 W, CdTe 90 W, CIGS 95 W and the following financial assumptions: 80% bank loan, 20% self invest, capital interest: 2%, loan interest 4%, 0,5% degradation/ year, 0,7% running cost/yr., base system cost 2015 with 20% margin. LCOE [$/kwh] 0.140 0.120 0.100 0.080 0.060 0.040 0.020 0.000 Site: Germany Standard Poly HJT (Meyer Burger) CdTe CIGS Figure 11 shows a comparison of both regions. For both regions, the LCOE achieved using the HJT PV system clearly outperforms the other PV systems. The HJT PV system benefits from its excellent temperature coefficient, higher conversion efficiency, higher energy yield and lower balance of system costs. As a result of this simulation LCOE of 9.1 $cent/kwh for Germany and 5.2 $cent/kwh for India are calculated. These attractive numbers underline the superiority of the Meyer Burger heterojunction technology compared to existing PV technologies currently available on the market. LCOE [$/kwh] 0.140 0.120 0.100 0.080 0.060 0.040 0.020 0.000 Site: India (Delhi) Standard Poly HJT (Meyer Burger) CdTe CIGS Figure 10: 307 watt record HJT module from Meyer Burger Figure 11: Levelized cost of electricity for a 100 kw PV power system 8 9

ABOUT US AUTHORS Meyer Burger Meyer Burger Technology Ltd. is the technology leader at every stage of the photovoltaic (PV) value chain from wafers to buildingintegrated PV systems. Processes and systems from the Meyer Burger group play a vital role in increasing overall performance and efficiency throughout the value chain. Benjamin Strahm graduated in Material Sciences from Swiss Federal Institute of Technology (EPFL) in 2004 and achieved his PhD in industrial plasma physics in 2007 in the field of silicon deposition in large area reactors. In 2008, he joined Roth & Rau Research (formerly Roth & Rau Switzerland) working in the field of plasma reactor design and crystalline silicon surface passivation by amorphous silicon for heterojunction solar cells. Since 2012, he is heading R&D activities in Roth & Rau Research. All core expertise under one roof Comprehensive, integrated technology portfolio Meyer Burger covers all important technology stages in the photovoltaic value chain. The interdisciplinary research and development work from wafers to the installed modules results in new standards in production procedures and processes and guarantees efficiency and performance throughout the entire system. We rapidly incorporate research results into commercial, integrated solutions. Sylvère Leu was born and educated in Switzerland, and graduated from ETH Polytechnic with a degree in Electronic Engineering and in Business administration at the University of St.Gallen (HSG). Sylvère Leu began working in Photovoltaics 25 years ago. At the end of 2005 he was charged with building up an integrated 250 MWp facility. In 2008 Sylvère Leu joined 3S Industries AG as COO. The company merged in 2010 into Meyer Burger Technology, where he is working as CIO/CTO of the group. Jens Krause studied semiconductor physics & technology at the University of Technology in Chemnitz, Germany. After spending three year at the Center for Microtechnologies of TU Chemnitz, he worked for more than ten years in the semiconductor industry. In 2009 he joined Roth & Rau, a member of the Meyer Burger Group, working now as head of strategic product management. USA Hillsboro (OR) Colorado Springs (CO) Columbia (NJ) Europe Moscow (RU) Dresden (DE) Hohenstein-Ernstthal (DE) Zülpich (DE) Eindhoven (NL) Reichelsheim (DE) Umkirch (DE) Thun (CH) Neuchâtel (CH) Asia Tokyo (JP) Seoul (KR) Zhubei City (TW) Shanghai (CN) Pune (IN) Singapore (SG) Georg Roters studied physics at the Westfälische Wilhelms Universität Münster, Germany. He received his PhD in physics from the Ruhr Universität Bochum, Germany and an Executive Master from the Boston Business School, Zürich. Georg Roters worked as Process Engineer, Project Manager and Product Marketing Manager in the semiconductor industry for 12 years. He joint Roth & Rau AG, a member of the Meyer Burger Group, in 2009 were he was building up and heading the Product Management department until 2012. Currently he is heading the Sales department within the Roth & Rau B.V. André Richter has a degree in electronic engineering (communication engineering, process measuring and control technology and environmental measurement) and he managed his own company specialising in electronic education systems for eleven years. Since 2001 his focus has been on photovoltaics. At Conergy AG he was involved in development and third level support of solar plants where he held the position of Technical Director in the Conergy solar plant located in Frankfurt (Oder). He then assumed the role of CEO for Conergy Electronics GmbH. In 2008 Andre Richter joined the Geneva based company, SES, as a consultant for the planning and construction of module lines in the United States. Since 2010 he has held the position of Senior Technical Developer at Meyer Burger Technology Ltd where he focusses on the establishment and realisation of strategic photovoltaic projects. References With around 1800 employees worldwide, Meyer Burger maintains close proximity to its customers. [1] Panasonic (http://panasonic.co.jp/corp/news) [2] Roth & Rau AG (http://www.roth-rau.com) [3] Pasan SA (http://www.pasan.ch) [4] IMT, Photovoltaics and Thin Film Electronics Laboratory (PV-Lab), EPFL, University of Neuchâtel, Switzerland (http://pvlab.epfl.ch) [5] Meyer Burger AG (http://www.meyerburger.com) 10 11

PASSIONATE ABOUT PV Meyer Burger Technology Ltd Schorenstrasse 39 / 3645 Gwatt (Thun) / Switzerland Phone +41 33 221 28 00 / Fax +41 33 221 28 08 mbtinfo@meyerburger.com / www.meyerburger.com HJT-EN-01