PEAK EFFICIENCIES WITH FALLING MANUFACTURING COSTS

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1 PEAK EFFICIENCIES WITH FALLING MANUFACTURING COSTS Simple and cost-effective introduction of PERC technology into the mass production of solar cells Kerstin Strauch, Florian Schwarz, Sebastian Gatz 1 Introduction Photovoltaics have developed dynamically and continually around the globe in recent years. Ever lower manufacturing costs are being demanded, with simultaneously increasing efficiencies. Passivated Emitter and Rear Cell (PERC) technology has been gaining more and more attention because, in addition to significantly higher degrees of efficiency of up to 21%, it also has no problems with wafer materials, which are becoming relentlessly thinner. Another advantage is its usability on both p-type and n-type silicon (Si). Converting an existing aluminum back-surface field (Al BSF) line takes only two additional process steps: the improved back-surface passivation, through the application of a dielectric layer to the back surface of the cell by means of CVD (chemical vapor deposition), and subsequent laser contact opening, which leads to a local BSF (compare Al BSF cell vs. PERC cell, see Fig. 1). Fig. 1 Schematic view of a typical industrial solar cell with whole-surface, screen printed Al BSF (left) and a PERC solar cell (right) with dielectric back-surface passivation and locally opened aluminum contacts. By applying a compact aluminum oxide (Al 2 O 3 ) layer, the charge carrier recombination on the cell back-surface is reduced and the internal reflection is increased in comparison to a conventional, whole-surface Al BSF cell. Because the deposition of Al 2 O 3 occurs at very moderate deposition rates and the layer is also not resistant to the aluminum back-surface contacting applied later, an aluminum- 1

2 resistant and temperature-stable cover layer made of amorphous silicon nitride (SiN x ) is added. This SiN x differs in its properties (layer thickness and refractive index) from the SiN x deposited on the textured front surface, which, as an antireflection layer, lowers the degree of reflection in order to increase the transmission of the incident light into the solar cell. In light of this new challenge for coating systems, Manz AG has designed an entirely new system principle for the deposition of highly developed passivation layers: the Vertical Coating System VCS1200, with a throughput of 1,200 wafers per hour. The development focused in particular on the high level of quality of the deposited layers as well as the productivity of the system. A very high molecular dissociation of process gases is guaranteed by the use of an inductively coupled, high-power plasma (HP-ICP: High Power Inductive Coupled Plasma, see article "Highly efficient RF impedance matching networks," R. Beckmann, VIP Vol. 26, 2014, pages 28-33). The example of mass spectrometry on a SiN x deposition in Fig. 2 shows how the gas portion of the precursor gases silane (SiH 4 ) and ammonia (NH 3 ) in the downstream flow drop below 5% with increasing plasma power. At a plasma power of 6.5 kw, the H 2 portion is at 93% and demonstrates the nearly complete decomposition and utilization of both process gases. This high decomposition results in the deposition being almost exclusively determined by radicals, and thus the Share of exhaust gas Fig. 2 The almost complete decomposition of precursor gases was demonstrated by mass spectrometry. desired dense passive layers are achieved. Furthermore, ion bombardment occurs at moderate energy between ev and can be regulated by plasma power and pressure. The HP-ICP technology thus combines the advantages of conventional direct and remote PECVD technology (see Fig. 3). In addition, there is another advantageous aspect: the hydrogen from the precursors dominates the plasma species and plays an important role in film formation. Separating weakly bonded surface groups generates adsorption sites and reinforces atomic bonding. The passive layers separated by HP-ICP technology, such as SiN x or Al 2 O 3, have high density, even at high deposition rates of up to 6 nm/s for SiN x and up to 2 nm/s for Al 2 O 3. The exceptional passivation properties are retained and can be achieved at moderate process temperatures below 400 C. 2

3 Fig. 3 Overview of the fundamental features of the most widely distributed plasma CVD processes. 2 Application example: VCS1200 a) FeinPass Grant Project For three years, Manz AG has worked on the introduction of a two-surfaced passivation for c-si solar cells, in cooperation with the Fraunhofer Institute for Solar Energy Systems (ISE) and SolarWorld Innovations GmbH in the "FeinPass" project, sponsored by the Federal Ministry for Education and Research (BMBF). Within this project, machines could be designed and processes optimized through the development of innovative production methods which lead to increased degrees of efficiency, with simultaneously lower manufacturing costs. Screen-printed PERC solar cells with peak efficiencies of 20.5% were achieved in an industry-oriented production process in a cell batch with the Institute for Solar Energy Research of Hameln (ISFH). The project was completed in May 2014 in connection with the Photovoltaics Innovation Alliance, through the presentation of VCS1200 as an industry-suited coating tool. 3

4 Fig. 4 HP-ICP system VCS1200 for dual-surface passivation of crystalline silicon solar cells (Image: Manz) b) VCS1200 Coating System The VCS1200, developed by Manz (see Fig. 4), is characterized by a very small footprint. The rotational processing unit takes up the majority of the total area of 5x4 m 2. In addition to the inlet chamber and the heating chamber, this includes the 500 x 500 mm processing chamber. There the plasma is ignited by applying a power of 12 kw to a rectangular induction coil with a high frequency of MHz; the wafers are not involved in plasma generation (i.e. not electrically contacted). They are located on two carrier plates, which are arranged symmetrically on both sides of the plasma in order to optimally utilize the plasma source (see plasma generation diagram in Fig. 3, top right). Homogeneous coating occurs on an area of 350 x 350 mm. The gas inlets and outlets are integrated in the chamber walls so that the best possible layer homogeneity is achieved. In addition to the processing unit, the tool also comprises a fully automated handling system, including SpeedPicker. The new system, which is tailored to the high demands of the solar industry, is based on full-surface electrostatic mounting of the substrate and vertical processing. This minimizes mechanical stress on the wafers and there is neither shadowing nor particle deposits which could lead to layer defects. The stationary process also ensures the high temperature stability required for uniform deposition. 4

5 Fig. 5 Set-Up of VCS1200 The individual processes within the VCS1200 are highlighted in Fig. 5. First, the wafers are loaded onto the square carrier plates by the SpeedPicker (1). Then, the carriers are aligned vertically (2)+(3), transferred into the rotational processing chamber via a load lock (4) and heated (5). The actual deposition of single passivation layers or the layer stack occurs in the processing chamber (6). The carriers are then unloaded in the same manner. The processing chamber can be extracted to the rear by a pull-out (7) for cleaning and maintenance outside the tool. The vacuum pumps are housed under the carrier handling (8), in a spacesaving manner. A single tool can be combined into a cluster with additional tools in order to increase throughput. Both front-surface anti-reflection coatings as well as passive back-surface coatings can be carried out with this fully automated HP-ICP system. c) Anti-reflective coatings with SiN x The three properties of density, refractive index and hydrogen content are important for optimal effectiveness of the anti-reflective layer on the front surface of the solar cell. In general, a density of ~ 2.9 g/cm 3, a refractive index of between 2.05 and 2.15 and a hydrogen content of below an atomic percentage of 10 (at%) is desirable. Fig. 6 shows the behavior of these three parameters under the influence of the deposition temperature in the VCS1200. From a temperature of ~370 C, the deposited layer is so compact that the desired density of 2.9 g/cm 3 is achieved. The hydrogen content decreases with increasing temperature, but is still between 8-9% at 370 C, while the refractive index remains stable in a temperature range from 5

6 Density Hydrogen Content Refractive Index Temperature [ C] Temperature [ C] Temperature [ C] Fig. 6 Density, hydrogen content and refractive index of HP-ICP deposited SiNx films at various process temperatures C. With the VCS1200, an anti-reflective coating is therefore already possible at moderate temperatures around 370 C. The stationary process of the VCS1200 also enables the deposition of a SiN x /SiN y double layer in order to increase the module efficiency of the solar cells. By applying such a double layer with a highly refractive SiN x sub-layer and a lowrefractive SiN y capping layer, the internal quantum efficiency (IQE) of the solar cell can be optimized in the range from nm. Calculations have shown that the short circuit current density (J sc ) is barely influenced by the double layer, but an increased J sc can be achieved in the installed module through the additional glass pane. The open circuit voltage (V oc ) is likewise increased by the improved passivation quality (see following section). d) Passive Coatings with Al 2 O 3 /SiN x Stack In the case of aluminum oxide deposition from trimethylaluminum (TMA, for molecule see Fig. 7), nitrous oxide (N 2 O) and oxygen (O 2 ), the plasma chemistry differs considerably from that using SiH 4 and NH 3. The carbon portion supplied by TMA and integrated in the deposited Fig. 7 structural formula of TMA layer is also decisive in the quality of Al 2 O 3 films, in addition to density (typically about 3 g/cm 3 ) and refractive index (usually 1.65 or higher). Embedded carbon (C) leads to a reduction of the layer thickness and adversely influences the passive effect. The high rate of gas decomposition of the VCS, already demonstrated for the SiN x, enables the deposition of an Al 2 O 3 layer with minimal C content and thus improved passivation quality. The passivation quality is evaluated the charge carrier lifetime. This means the average time after which charge carriers generated by light (electrons and gaps) recombine and thus do not contribute to current. The measured total lifetime of a Si wafer is a function of the volume lifetime and the contribution of the surface. High quality Si material with a large volume lifetime is determined by as low contaminants from foreign atoms and displacements as possible. The surface lifetime can be influenced by applying passivation layers. 6

7 These reduce the surface state density and depending on charge state also drives away one type of charge carrier (electrons or gaps) from the surface through a socalled field effect passivation. In high quality Si material with a high volume lifetime (Floatzone Si wafers are particularly suitable here, having a very low degree of contamination due to their special manufacturing process), the total measured lifetime can be used as a measure of the quality of the passivation. Lifespan [s] Density of minority charge carriers Fig. 8 Lifespan and effective surface recombination speed S eff for Al 2 O 3 /SiN x passive stacks by means of HP-ICP The wafers must be passivated on both surfaces for this measurement. A subsequent heating process simulates the firing process of the later solar cell and leads to an improvement of the passivation effect. A p-type Floatzone silicon wafer (base resistance: 2 Ohmcm, wafer thickness: 200µm) with 20nm Al 2 O 3 and 75nm SiN x was coated on both surfaces for qualification of the Manz Al 2 O 3 layer. A constant lifetime of ~1.3ms at an injection level below cm -3 was achieved after a 10-minute heating process at 400 C [F.Schwarz et al. "Advanced Anti-Reflection and Passivation Layer Systems Produced by High-Power Plasma in the New Manz PECVD System," EUPVSEC, 2012]. Fig. 8 shows the recorded lifetime curve. The calculated surface recombination speed was less than 7 cm/s, which ultimately enables efficiencies >20%. 3 Conclusion In constructing a new coating system for dual-surface passivation of solar cells, Manz AG has developed a compact, fully automated and easy to maintaine platform. At the key features of this platform are the innovative HP-ICP technology and the modern electrostatic carrier system. The anti-reflective and passive layers it deposits for the front and back surfaces of the solar cell are characterized by high gas utilization, optimal layer quality and homogeneity and very high deposition rates, resulting in a very high throughput. 7