32nd European Photovoltaic Solar Energy Conference and Exhibition

Transcription

1 FLIP-FLOP CELL INTERCONNECTION ENABLED BY AN EXTREMELY HIGH BIFACIAL FACTOR OF SCREEN-PRINTED ION IMPLANTED N-PERT SI SOLAR CELLS Henning Schulte-Huxel 1, Fabian Kiefer 1, Susanne Blankemeyer 1, Robert Witteck 1, Malte R. Vogt 1, Marc Köntges 1, Rolf Brendel 1,2, Jan Krügener 3, Robby Peibst 1,3 1 Institut für Solarenergieforschung Hameln (ISFH), Am Ohrberg 1, D Emmerthal, Germany 2 Institute of Solid-State Physics, Leibniz Universität Hannover, Appelstr. 2, Hanover, Germany 3 Institute for Electronic Materials and Devices, Leibniz Universität Hannover, Schneiderberg 32, D Hanover, Germany ABSTRACT: We present bifacial fully ion implanted and screen-printed n-pert cells, fabricated either by applying a single co-anneal process to cure the implant damage or by applying two separate anneals after boron/bf 2 and phosphorous implant, respectively. In the first case of boron implant and co-anneal our best cells achieve an independently measured front (rear) side efficiency of 21.0 % (20.43 %) and for the boron implant and separate anneal the efficiency is 21.5 % (21.31 %). To the best of our knowledge these values are the highest efficiencies reported so far for fully ion implanted and screen-printed bifacial n-pert cells. We furthermore show that light treatment of boron-implanted and co-annealed n-pert cells increases the cell efficiency by 0.6 % abs. This diminish the efficiency gap to separately annealed cells. We measure a bifacial factor of 99.4 % that is the highest value reported so far for any high-efficiency Si cell. The high bifaciality enables an adapted module interconnection scheme called here Flip-Flop, which is based on a front-to-front and rear-to-rear interconnection of cells with alternating orientation (n + or p + side facing up). Based on the measured IV-characteristic of a cell with a bifacial factor of 97 % ( conservative scenario ) we demonstrate that the Flip-Flop interconnection scheme has the potential for a module efficiency improvement of 0.5 % abs on aperture area (as compared to the conventional all cells emitter up configuration) despite the 3 % current mismatch. We experimentally demonstrate a monofacial 16-cell Flip-Flop module that achieves an aperture area efficiency of 20.5 %. Keywords: Bifacial solar cells, n-pert cells, ion implantation, module integration, Flip-flop module, planar interconnection, light soaking 1 INTRODUCTION There is growing interest in PV industry in bifacial solar cells [1], either for application in bifacial modules [2] or to raise the current generation in standard modules with a highly reflective back sheet [3,4]. These concepts are based on a conventional solar cell interconnection process with 2-5 cell interconnection ribbon (CIR) or wires, conducting the current from the front side of one cell to the rear side of the adjacent cell. In the standard cell interconnection process, a solder coated copper CIR is soldered to the front side of one cell and lead to the next cell, where it is soldered to its rear side (Fig. 1(a)). The requirement to lead the CIR from the front to the rear side has two draw-backs: (i) the mechanical stress at the wafer edge, which has the potential to induce cracks into the solar cell [6], and, in consequence, (ii) the requirement of a gap between the cells with a width of 3-4 mm. A large cell-to-cell distance increases the module size and thus decreases the module efficiency. Since numerous contributions to the system costs scale with the module size, smaller modules offer the possibility to decrease the levelized costs of electricity (LCOE). An alternative interconnection concept has been proposed by Kopecek et al. [7]: n-type and p-type cells (not necessarily bifacial) connected alternating in a string. This allows a planar interconnection, where the cell interconnectors (tabs or wires) conduct the current either only on the front side or only on the rear side from one cell to another (Fig. 1(b)). However, it might be challenging to process high-efficient n-type and p-type cells in the same production line. In parallel to this work Augusto et al. presented a similar approach using n-type cells with front and rear side emitter, however still employing two different cell types [8]. Alternatively, bifacial solar cells can be placed alternating with the p + and the n + side facing upwards. However, this demands cells with a similar energy conversion efficiency for both sides, which ideally equals a bifacial factor of unity. For example, the n-type passivated emitter, rear totally doped (PERT) solar cell, which is recently widely investigated [5, 9 13] exhibit a full area BSF which provides lateral conductivity and thus enables similar finger-to-finger distances as on the front-side. Thus, a fine line screen print can be applied on both sides of the solar cell [5]. Together with the high stable minority carrier lifetimes of n-type silicon, n-pert cells with bifacial factors > 98 % [5] can be developed. This is the case for our ionimplanted and screen-printed n-pert solar cells with efficiencies up to 21.5 % and bifacial factors exceeding 99 %. Those solar cells give the opportunity to realize the interconnection scheme mentioned above, which we call Flip-Flop design. One should remark that any other cell concept with sufficiently high bifaciality factors, such as bifacial heterojunction cells, would also be compatible with the Flip-Flip interconnection scheme. Similar to the concept of Kopecek et al. [7], the Flip- Flop design proposed here offers several advantages: the distance between the cells is not limited by the CIR connecting the front and the rear side of the adjacent cells, allowing a smaller cell spacing and thus higher module efficiencies. The reduced current path in the CIR decreases the resistive losses and permits thinner CIR, reducing the shading losses on the cell. Also, the mechanical stress at the edge of the solar cell by the CIR is reduced. For small cell gaps the CIR requires a crimp, which significantly reduces the module lifetime [14]. Cell interconnection degradation is one of the most dominant failure mechanisms in the field [15]

2 Figure 1: Schematic drawing (a) of a conventional cell interconnection scheme with CIR from front to rear side, and (b) of the Flip-flop cell interconnection scheme with CIR only from front to front and rear to rear side. (c) Photograph of our first demonstrator Flip-Flop module. We present a 16-cell demonstrator module produced with this new interconnection process with an aperture area efficiency of 20.5 %. This promises a large potential for further improvement of the module efficiency. 2 EXPERIMENTAL DETAILS The solar cells are processed on mm² n- type pseudo-square Cz silicon wafers with a base resistivity of 3 Ωcm. First, saw damage removal is performed, the wafers are double-side textured and wet chemical cleaned. We use ion implantation and subsequent anneal to form the n + and p + regions. For the latter, we evaluate implant and anneal for elementary boron and BF 2 with a dose of cm² [16]. BF 2 is more attractive for industrialization due to the simplified implanter. technology (no mass separator [17]) and due to the reduced thermal budget required for annealing (950 C instead of 1050 C) [16]. After implantation and annealing, a stack of Al 2 O 3 /SiN x is deposited on the p + emitter side and a SiN x layer on the n + side for the surface passivation. We use a double print for the metallization on both sides of the cell with a finger opening of 30 μm in the stencil and 5 busbars. The emitter is contacted with an Ag/Al paste, and the n + side is contacted with an Ag paste, both pastes commercially available. The cells are fired in a conveyor belt furnace. We measure the IV characteristics of the solar cells from both sides using a LOANA system from pvtools on a brass chuck. The champion cells were independently measured at F-ISE CalLab. The module is made of cells strips with a size of mm². Therefore, the cells are laser-cut from the n + side into rectangular solar cells without the pseudosquare edges. One should remark that the rectangular geometry is desirable for high module efficiencies, but not absolutely necessary in general for the Flip-Flop concept. Also, full square wafers would have enabled a rectangular geometry without cutting the half cells from 78 mm width down to 62.5 mm. No degradation of the cell performance is observed due to cutting with ns-laser pulses. Instead, we determine a relative increase in efficiency (illumination from the emitter side) of 0.6 % rel in average comparing the initial cell performance and the rectangular half cells. This deviation is within the measurement uncertainty for the IV-tester of 1.0 % rel. After current matching, the wafers are connected into 8- half-cell strings using the Flip-Flop interconnection process with interconnector CIR with a cross-section of mm, which are covered with a highly reflective structured film. The cell spacing and the string-to-string distance is 1 mm. The cells are laminated with an ethylene-vinyl acetate (EVA) lamination foil with an improved UV-transparency, a 3.2-mm-thick front glass with anti-reflection coating (F solar) and a Coveme PYE 3000 white back sheet. The IV characteristic of the module is measured with an A + flasher from halm while using a long (25 ms) flash in order to take into account the high diffusion capacitance of the cells. Module measurements are performed in-house so far. We determine the spectral miss match factor for this module according to the norm IEC and use this to correct the IV-characteristics according to the norm correction method RESULTS AND DISCUSSION 3.1. Bifacial n-pert cells Table I lists the IV parameters of our current champion n-pert cells with B or BF 2 implant for emitter formation, either annealed in a single co-annealing step or by separate annealing steps for B, BF 2 and P implant. To our knowledge, these are the highest values published so far for industrial ion implanted, fully screen printed n- PERT cells. In particular, the bifacial factor > 99 % is the highest value reported for any high efficiency cell concept [9]. This is the essential basis for the Flip-Flop module concept. For the demonstrator module, however, we use cells from a previous run (exclusively co-annealed cells) 2 408

3 with slightly lower efficiencies and bifacial factors, see Table I. For a sufficient number of comparable cells, we also mixed up split groups with B implanted emitter and designated area). Usually, the cells feature a cell spacing of 4 mm to avoid damage at the cell edges due to mechanical stress. The cell spacing has the potential to TABLE I. IV-parameters (full-area measurement on 239 cm 2 ) of our current n-pert champion cells with B or BF 2 implant for emitter formation, either annealed in a single co-annealing step or separate annealing steps for B/BF 2 and P implant. Additionally, the average values of the cells used for the module are given. *Independently measured by F-ISE CalLab. Emitter Anneal V oc [mv] J sc [ma/cm 2 ] FF η f b = η rear /η front B co-anneal 665* 39.8* 79.3* 21.0* 97.3 Best cells B separate anneal 671* 40.2* 79.7* 21.5* 99.4 BF 2 co-anneal 658* 39.9* 78.4* 20.6* 97.7 BF 2 separate anneal 664* 40.4* 77.6* 20.8* 98.6 Average values of B co-anneal cells for module BF 2 co-anneal with BF 2 implanted emitter. Since we wanted to use an UV-transparent EVA foil, we studied first the UV stability of our n-pert cells by illuminating the Al 2 O 3 passivated side of the cells with halogen lamps under approximately 1000 W/m² irradiation. Here, we made an unexpected observation, which is reported in the following. Figure 2 shows the preliminary results of our experiments. For the cells with the BF 2 implanted emitter no significant change in efficiency (Δη < 0.1 % abs ) after 24 hours of illumination is observed. In case of the cell featuring a boron implanted emitter we measure a significant increase in efficiency of 0.6 % abs. This increase in efficiency is due to an improvement of the open circuit voltage V oc, as well as due to an improvement of the pseudo fill factor, resulting from lower J 01 and J 02 values compared to the initial state. A similar behavior for the two cell types is also observed for groups of 8 cells each (B emitter average: Δη = 0.6 % abs ; BF 2 emitter average: Δη = % abs ). Since both cell groups feature the same passivation stack, we attribute the improvement in case of the B implanted cells to a curing of defects in the bulk and /or in the emitter. However, further research is required to understand the underlying mechanisms. Work in this direction is ongoing, and the results will be published elsewhere Flip-Flop module In order to demonstrate the potential of our Flip-Flop interconnection scheme for module efficiency improvement, we perform simulations based on IV curve measured on a representative half cell (Tab. 2, first rows). One should note that the bifacial factor of this cell of 97.4 % is slightly lower than our average value (98.3 %), thus representing a conservative scenario. According to the conventional interconnection scheme, the cell side with the highest efficiency faces the sun. In the hypothetical case of a loss-free interconnection, the full efficiency potential of the cell is transferred into the module, when referring to the cell area only (see Table. II third row interconnected conventionally (emitter up), Figure 2 Efficiency of n-pert solar cells with a B or BF 2 implanted emitter in dependence on illumination time. increase the current of the module, however, it also enlarges the module size and increases resistive losses in the CIR. The first effect can be taken into account as follows: We use the probability for the collection of light reflected from the back sheet between the cells as back sheet light recovery probability k as given in Ref. [18] for the best white back sheet tested in that work. However, one has to remark that these values are determined for monofacial cells, which cause some uncertainty when transferring them to a module with bifacial cells. For a cell spacing of 4 mm, k is Thus, a cell spacing of 4 mm for the conventional interconnection scheme results in a reduction of the efficiency to 19.4 % when referring to the aperture area (cell and back sheet area). The Flip- Flop approach, where half of the cell face the sunny side with the surface having the highest efficiency and the other halve with the opposite surface, results in a decrease in module performance due to a slight current mismatch even when assuming loss-free interconnection. By interpolating the IV-characteristics as measured with illumination from the emitter and from the n + side and adding the voltage for equal currents, we generate an IV

4 Table. II Calculated 2-cell module efficiencies for the conventional emitter up and Flip-Flop interconnection scheme. The calculations are based on the measured IV data from a cell with a moderate bifacial factor of 97.4%, representing a conservative scenario for the Flip-Flop approach. Module design Single cell Emitter up Interconnected Flip-Flop Measurement Emitter up, full area (97.5cm 2 ) n + up, full area (97.5cm 2 ), η I sc [A] V oc [mv] FF P mpp [W] Cell spacing [mm] designated area aperture area designated area aperture area k characteristics of the series connected cells. When referring to the cell area (designated area), the resulting efficiency of the Flip-Flop module is 20.0 %. For smaller cell spacing, the recovery factor k increases [18]. The Flip-Flop approach enables a cell spacing of 1 mm, which is considered in the last calculation (Table II sixth row). For 1 mm, we determined a k = The resulting Flip- Flop module efficiency for 1 mm distance (aperture area) is 19.9 %, which is 0.5 % abs higher compared to the case of the conventional interconnection scheme using the same cells. This is a further benefit of our Flip-Flop interconnection scheme besides the reduction of mechanical stress in the cell. The efficiency for the emitter up configuration depends crucially on the cell spacing. Figure 3 shows the calculated aperture efficiencies for cell spacing d between 1 and 6 mm based on the cell data give in Table II. The k values are calculated for each d according Ref. [18]. The bifacial factor has no impact when the emitter of all cells is facing the sunny side. In case of the Flip-Flop configuration we keep the cell spacing constant at 1 mm and vary the bifacial factor by shifting the current I(f b ) = I 0 - I sc (1- f b ), (1) where I 0 is the measured front side current and I(f b ) is the calculated current for the bifacial factor f b. For a cell spacing of 3 mm in case emitter up the same efficiency can be achieved with a bifacial factor as low as 96 %. In the case of our average f b of 98.3 % the Flip-Flop configuration with 1 mm cell spacing is superior to the emitter up configuration with 2 mm cell spacing, which is a challenging cell spacing for the standard configuration (emitter up), since the CIR must be led from the front side a cell to the rear side of an adjacent cell. Figure 4 shows the experimental demonstrator Flip- Flop module with sixteen cells connected in series. The IV-parameters of the module and the expected characteristics based on the IV data of the individual cells are given in Table III for the case of no series resistance losses as well as including the resistive losses due the cell and string interconnect CIR. These calculations include no optical losses or gains due to changed absorption and reflection in the module. We expect a module efficiency on designated area (cell area only) of 20.2 % and a fill factor of 77.7 %. We determine an efficiency of 20.2 % on the aperture area (cell and back sheet). This is a very promising result for a first demonstrator module made of cells with only 20.3 % front-side and 19.8 % rear-side efficiency. Comparing the expected and the measured performance of the module, we observe no significant change in V oc due to the interconnection. The reduction of the fill factor of 0.5 % abs is moderate and can be explained by the resistive losses in the CIR (see Table III, lines 2 and 3). The short circuit current of the module is increased compared to the cells, which is caused by the light recovery form the interconnects, the fingers, and the back sheet as well as the reduced front side reflection at Figure 3 Aperture area (cell and back sheet area) efficiencies for emitter up configuration in dependence on the cell spacing d and for the Flip-Flop configuration with in dependence on the bifacial factor f b Figure 4 Photograph of our demonstrator Flip-Flop module with sixteen cells connected in series.

5 Table. III Measured IV-parameters (aperture area) of a proof-of-concept Flip-Flop module consisting of 16 bifacial solar cells before and after light treatment. For comparison, the expectations based on the measured IV curves of all interconnected cells are also shown (no current gain due to back sheet and no losses due to the encapsulation compared to the cell IV-measurements assumed). η I sc [A] V oc [V] FF P mpp [W] A [cm²] Calculated form cell IV-curves Calculated form cell IV with R s Measured after lamination Measured after light soaking the cell surface, which overcompensates the losses due to absorption and reflection of the front glass and EVA. The current could be further increased by using a back sheet with a high infrared reflectivity to benefit from the light transmitted through the cell. The used back sheet has a low reflection (~75 %) in the wavelength regime between 900 nm and 1200 nm [19]. As shown above on in section 3.1, an exposure to light has the potential to enhance the cell performance. The cells received no light treatment before interconnection, since the effect was discovered after finishing the module. Therefore, we irradiate the module for 25 hours. By this the open circuit voltage and the fill factor can be increased resulting in an aperture efficiency of 20.5 %. 4. SUMMARY We present bifacial n-pert cells with an extremely high bifacial factor of close to unity (99.4 %) and a module concept benefiting from this property. The cells are fully ion implanted and screen-printed. They are fabricated either by applying a single co-anneal process to cure the implant damage or by applying two separate anneals after boron or BF 2 and phosphorous implant, respectively. In the first case, our best cells achieve an independently confirmed front (rear) side efficiency of 21.0 % (20.43 %), in the latter case of 21.5 % (21.31 %). To our knowledge, these values corresponds to the highest efficiencies reported so far for this cell type. Moreover, the bifacial factor of 99.4 % is to our knowledge the highest value reported for any Si cell concept. Laser cutting of these cells shows no negative impact on the cell performance. We introduce a novel interconnection scheme for bifacial cells, which we call Flip-Flop. In the Flip-Flop module scheme every second cell is flipped upside-down, in order to interconnect them in a planer manner. Thereby the sunny sides of them are interconnected by a CIR enabling the series interconnection emitter contact of one cell and the base contact of the next cell. This offers several advantages: the distance between the cells is not primarily limited by the interconnection of the individual cells, allowing a smaller cell spacing and thus higher module efficiencies. The reduced resistive losses in the interconnectors allow the usage of thinner CIR, reducing the shading losses on the cell. We show that our cells with an average bifacial factor of 98.3 % lead to higher module efficiencies in the Flip- Flop configuration with a cell spacing of 1 mm compared to a conventional interconnection of the same cells with 4 mm cell spacing. Our first proof of concept module reaches an aperture efficiency of 20.2 %. The efficiency increases to 20.5 % after light treatment. The underlying effects enabling this efficiency gain need to be investigated further on cell level. The proposed Flip-Flip interconnection can be applied to any cell concept besides n-pert with sufficiently high bifaciality (compare Fig. 3), such as, for example, amorphous/crystalline Si heterojunctions cells. It is compatible with both interconnection via cell interconnection ribbons and via multiple wires. ACKNOWLEDGMENT The authors would like to thank Sabine Kirstein, Peter Giesel and Andreas Klatt for their help with sample processing, David Hinken for the spectral mismatch correction of the module. We gratefully acknowledge the support by the German Ministry for Economic Affairs and Energy under grants (CHIP) and (PERC-2-Module) as well as by the state of Lower Saxony. References [1] International Technology Roadmap for Photovoltaic, (2015) [2] U. A. Yusufoglu et al., Energy Procedia 55, pp (2014). [3] J. P. Singh et al., IEEE J. Photovoltaics 5(3), pp (2015). [4] T. Dullweber et al., Proc. 30 th EU PVSEC, Hamburg, Germany, pp (2015). [5] F. Kiefer et al., Sol. Energ. Mat. Sol. Cells 157, pp (2016). [6] M. W. P. E. Lamers et al., Prog. Photovolt: Res. Appl, 20(1), pp (2012). [7] R. Kopecek et al., Proc. 21 st EU PVSEC, Dresden, Germany, pp (2006). [8] A. Augusto et al., presented at 43 rd IEEE PVSC, Portland, OR (2016) [9] S. Gonsui et al., Proc. 28 th EU PVSEC, Frankfurt, Germany, pp (2012). [10] A. Frey et al., Proc. 29 th EU PVSEC, Amsterdam, the Netherlands, pp (2014). [11] P. Rothhardt et al., IEEE J. Photovoltaics 4(3), pp (2014). [12] A. Lanterne et al., Prog. Photovolt.: Res. Appl. 23, pp (2015). [13] V. D. Mihailetchi et al., Energy Procedia 77, pp (2015) [14] M. Pander et al., Proc. 28 th EU PVSEC, Frankfurt, Germany, pp (2012)

6 [15] E. Hasselbrink et al., 39 th IEEE PVSC, Tampa, FL, pp (2013). [16] J. Krügener et al., Sol. Energ. Mat. Sol. Cells 142, pp (2015). [17] V. Bhosle et al., Proc. of the 31 st EU PVSEC, Hamburg, Germany, pp (2015). [18] M. Köntges et al., to be presented at 32 nd EU PVSEC, Munich, Germany (2016). [19] M. Vogt, PhD thesis, Leibniz Universität Hannover, Hannover (2015)