Interconnection of busbar-free back contacted solar ...

Interconnection of busbar-free back contacted solar cells by laser welding Henning Schulte-Huxel1, Susanne Blankemeyer1, Agnes Merkle1, Verena Steckenreiter1, Sarah Kajari-Schroeder1, Rolf Brendel1,2 1

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Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany

Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, D-30167 Hannover, Germany

Abstract We are presenting the module integration of busbar-free back-junction back-contact (BJBC) solar cells. Our proof-of-concept module has a fill factor of 80.5% and a conversion efficiency on the designated area of 22.1% prior to lamination. A pulsed laser welds the Al-metallization of the solar cells to an Al-foil carried by a transparent substrate. The weld spots electrically contact each individual finger to the Al-foil, which serves as interconnect between different cells. We produce a proof-of-concept module using busbar-free cell strips of 25×125 mm². These are obtained by laser-dicing of a 125×125 mm² BJBC solar cell. The fill factor of this module is increased by 3.5% absolute compared to the initial cell before laser-dicing. This is achieved mainly by omitting the busbars and reduction of the finger length. The improvement of the module fill factor results in an increase in the module performance of 0.9% absolute before lamination in comparison with the efficiency of the initial 125×125 mm² BJBC solar cell. Hence, this interconnection scheme enables the transfer of high cell efficiencies to the module. 1. Introduction Solar cells reaching a 24% efficiency on the basis of the back-junction back-contact (BJBC) concept have been developed by different groups [1–4]. In general, the metallization geometry of the solar cell features an interdigitated finger structure and busbars collecting the current from the individual fingers. Simulations and measurements have shown that the busbar regions of the BJBC solar cells reduce the fill factor and short-circuit current density [5–7]. Other extended structures like contact areas with several tens of square millimeters cause similar problems. Additionally, the transport of electric current in the metallization (fingers and busbars) leads to series resistance and, hence, to fill factor losses [7,8]. Due to the trade-off between a maximum emitter fraction and a minimum series resistance in the base finger metallization, the crosssection of the base and emitter regions is sometimes chosen to be asymmetrical [9]. However the small width of the base fingers leads to additional resistive losses. Therefore, an interconnection scheme which omits the busbars and decouples resistive losses in metallization from the width of the fingers has the potential for a significant increase in the cell performance of the module. A process for interconnection of busbar-free solar cells which decouples the resistive losses from the size of the base contacts was proposed by Verlinden et. al [8,10] for point-contact solar cells. There the transport of electric current of the two polarities is situated on two different levels. For example, the emitter current is carried in a metal layer covering the whole rear side of the solar 1

cell except the contacts of the second polarity, in general the base. The base current is carried by a second metal layer situated on top of the first. The two metal layers are insulated from each other by a stack of different dielectric layers. This process is called multilevel metallization. In this work we are presenting the module integration of busbar-free BJBC solar cells by means of a simplified multilevel metallization. We contact each individual base and emitter finger to an Al foil using the aluminum-based mechanical and electrical laser interconnection (AMELI) process [6,11]. A polymer layer insulates the base from the emitter metallization. In addition to the module integration process, we are presenting modifications to our previously-reported baseline BJBC cell process [6,12]. These modifications enable busbar-free interconnection. 2. Solar cell process and results Figure 1 shows a cross-section of the BJBC solar cell used for a busbar-free module integration. Since we aim at the laser welding of the thin base fingers to an Al foil, it is advantageous if these are elevated with respect to the emitter contacts, see Fig 1. SiNx ARC

n-Si, 1.5 Ohmcm +

p -emitter +

n -BSF

SiNx

Al metallization

Al2 O3

Fig. 1 Cross-section of the BJBC solar cell. The n+-type BSF and p+-type emitter are situated on the top and in the grooves of the rear-side structure, respectively. The cell process starts with n-type wafers with a base resistivity of 1.5 Ω cm. These are damageetched and cleaned before a phosphorous diffusion from a POCl3 source. A full-area n+-type layer with a sheet resistance of 40 Ohm/square is formed on both sides of the wafers. The phosphorous glass is removed in an HF solution. In a wet oxidation at 900°C, a 200 nm thick layer of silicon dioxide (SiO2) grows on both sides of the wafers. This oxide layer acts as an etch and diffusion barrier during the next processes. In a single-side HF step we remove the SiO2 layer from the front side of the solar cells. We texture this surface with random pyramids in an alkaline solution and deposit a silicon nitride protective layer (SiNx) onto it. We use laser technologies to realize the interdigitated pattern on the rear side of the solar cells. A picosecond laser locally ablates the SiO2 layer on the rear side. This defines the emitter regions. We remove the laser-induced damage during the structure etching in a KOH solution. The remaining SiO2 layer protects the BSF regions and acts as an etch barrier. In this way, an interdigitated structure with a height difference about 12 µm is realized on the rear side of the solar cells. After a cleaning step a boron emitter with a sheet resistance of 60 Ohm/square is formed in a furnace diffusion process. After the removal of the protective layers in an HF solution and a subsequent cleaning step, the rear side of the cells is passivated with a full-area aluminum oxide and silicon nitride (Al2O3/SiNx) stack. These are deposited by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD). We use a picosecond laser to realize laser contact openings (LCO) by local ablation of the passivation stack on the emitter and the base regions. The front side of the cells is passivated with a double layer of SiNx. 2

Afterwards, the rear side of the cell is metalized with a 25-µm layer of aluminum covered by 400 nm of SiOx. Both are deposited in a mask-free single-vacuum evaporation process. The thin oxide layer protects the aluminum in the next wet-chemical etching step enabling a self-aligned contact separation on the steep flanks of the structure as proposed by Sinton et al. [13]. Using this process we produce 125×125 mm² solar cells with a pitch of 2300 µm and an emitter and base-finger width ratio of 18/5. Initially the cells have two busbars placed close to the opposing edges of the cell. The busbars are necessary for the measurement of the initial cells before interconnection. All the cell measurements we are presenting in this work are in-house measurements and have been realized under standard test conditions (25°C, 100mW/cm², AM 1.5G) with a LOANA system from pv-tools. As a reference cell we used one of our baseline BJBC solar cells calibrated at ISE Callab. Table 1 summarizes the measured one-sun parameters of a batch of five BJBC solar cells. The total area of the cells (t.a. 156.25 cm²) or a designated area (d.a. 132.67 cm²) is illuminated. For the designated area measurements a mask is used which excludes the two busbars and the two non-busbar edges. 3. Cell interconnection process Every module reported on in this work is build using 4 cell strips diced out of a large 125×125 mm² solar cell. We use four 125×125 mm² solar cells to produce four modules. We laser dice each solar cell into four strips 25 mm wide perpendicular to the fingers. These cells strips are 25×125 mm² in size and free of busbars. To reduce the recombination losses at the laser-diced edges, a SiNx layer is deposited at a low temperature (360°C), but no etching of the laser damage is performed. We apply a thin polymer layer on the total area of the rear side metallization serving as an insulating layer between the cell metallization and the Al foil. For this purpose we dissolve an encapsulant (Tectosil ®, Wacker) in isopropanol. The thickness of this layer after curing is 750 ± 250 nm. This layer protects the Al cell metallization from humidity [14] and avoids shunts between the Al foil and the emitter metallization.

Fig. 2 The AMELI interconnection scheme for busbar-free solar cells. The dashed red lines indicate the locations of the cross-section I and II, where the base and emitter regions are contacted, respectively. The cross-sections are not to scale. 3

Four cell strips diced out of one solar cell are positioned on a 10-µm-thick structured Al foil attached to an encapsulant foil (Tectosil ®, Wacker). The Al-foil is structured with a laser. The spare Al-foil is then manually removed. Fig. 2 shows the resultant geometry of the Al foil. In order to ensure a good contact between the Al metallization of the cell strips and the Al foil during the interconnection process, the stack is placed on a vacuum chuck. This results in an atmosphere with a reduced pressure. Then we apply the AMELI process by focusing a laser beam onto the Al foil through the transparent encapsulant and both Al layers are welded [11]. We use single laser pulses with a pulse duration of 1.2 µs at a wavelength of 1064 nm. The welding process pierces the polymer layer as well as the SiNx and oxide layers deposited on the Almetallization of the cells. First we contact the narrow base fingers of the solar cells to the Al foil, see Fig. 2 cross-section I. The same piece of Al foil, which contacts the base of one cell, is after that welded to the emitter fingers of the next cell, see Fig. 2 top. In order to contact the emitter, the foil is structured in such a way that it is solely situated on top of the emitter fingers, see Fig. 2 cross-section II. After the interconnection of the cells we laminate the modules in a glass-glass lay-up using the same encapsulation foil as the substrate of the Al foil. 4. Proof-of-concept modules After interconnection we use an in-house flasher (h.a.l.m. Elektronik GmbH) to measure the I-Vcharacteristic of the modules. We measure the produce modules before and after lamination without a shadow mask. The measurements results are related to the area of the four interconnected cell strips (c. a. 4 cells × 25 mm × 125 mm = 125 cm²). Table 2 shows the one– sun parameters of the best cell and of the resultant module. For a convenient comparison the open-circuit voltage Voc and short-circuit current density Jsc values of the module are related to the corresponding values at cell level. Based on the assumption that the individual voltages of the cell strips connected in series are added up and they have identical voltages, we divide the module voltage by the number of cells strips. In the case of the current density, we assume a constant current over all the cell strips. The measured current density of the module is related to the module area. In order to relate this current to the area of an individual cell strip, we multiply the short-circuit current density of the module by the number of cell strips. The difference in the open-circuit voltage Voc of the initial solar cell measured on total area and the average Voc per cell after interconnection is less than 2 mV (see Tab. 2), which is within measurement uncertainty. By eliminating the busbars and therefore the electrical shading in this region, we expect the shortcircuit current density Jsc for each individual cell strip to be the same as the Jsc for the initial solar cell measured on the designated area. The measured Jsc for the module corresponds to the lowest current of the interconnected cell strips. We do not observe the expected gain in Jsc by eliminating the busbars. Compared to the cell measurement on the designated area (41.4 mA/cm²), the Jsc per cell before lamination (40.1 mA/cm²) there is in fact, a relative reduction of 3%. Fig. 3 shows an electroluminescence (EL) image of the module before lamination. In the case of the non-contacted individual fingers, dark horizontal lines should be present in the EL-image. It is obvious that all fingers are well contacted, so that a failed interconnection of the cell strips is not the reason for this current loss.

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Fig. 3 Electroluminescence image of the module before lamination at an applied voltage of 2.74 V on a linear scale. The three dark vertical lines correspond to the interconnection gaps (Alfoil) between the four cell strips of the module. The fill factor of the initial solar cell measured on the total area is increased from 77.0 % to 80.5% on module level. This gain of 3.5% absolute compensates for the losses in Jsc. The module has a conversion efficiency of 22.1%, which is as high as the efficiency of the 125×125 mm² initial solar cell measured on the designated area without busbars. 5. Analysis of gains and losses For a more general understanding we compare the relative losses of all the four produce modules with the four initial solar cells before laser-dicing. Figure 4 shows the relative changes in the I-V parameters of the non-laminated modules compared to both the solar cells before laser-dicing measured on the total and designated area, and the laminated modules. The cell efficiencies before laser-dicing and interconnection on the designated area are between 21.7% and 22.1%. Below we discuss these changes between modules and cells in detail.

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Fig. 4 Relative changes in the I-V-parameters between the non-laminated module taking into account the active cell area (c.a.) and the initial cell measured on the total area (t.a.), designated area (d.a.), and the module after lamination for the four modules we produced. Positive values correspond to an increase in the I-V-parameters of the non-laminated module compared to the initial cell. In the case of the comparison of the module before and after lamination, the negative values correspond to losses after the lamination process. Module 1 corresponds to the module presented in the previous section. a. Fill factor Comparing the initial solar cell measured on the total area and the module before lamination (c.a.) the most noticeable change occurs in the fill factor. There is a relative increase of between 3.7 and 5.2% (an absolute increase of 2.9% - 4.0%). We attribute this increase mainly to two possible sources: 1. The elimination of the emitter busbar: The emitter busbar causes significant resistive losses due to the long distance for the majority carriers generated in this region. As shown in Tab. 1., its omission results in a relative increase in the average fill factor (FF) of 1.5%, as determined by measuring the initial cells on the total area (t.a.) without a mask and on the designated area (d.a.) with a mask. 2. Reduction of the resistive losses in the metallization of the busbars and the long fingers: by omitting the busbars and shortening the fingers from 120 mm to 25 mm, a relative increase in the FF of up to 3.2% is observed, as shown by the comparison of the measurements of the initial cell on the designated area and of the non-laminated module (c.a.) (see Tab. 2). In order to determine the proportion of change in the fill factor between the initial cell measured on the designated area and the interconnected cell strips that cannot be attributed to 1., we approximate the impact of the resistive losses in the metallization on the fill factor (2.) by

(1) 6

since an increase in area-weighted series resistance Rs leads at the first approximation to a reduction in the voltage at the maximum power point. Therefore, the ΔVmpp is the voltage drop at the maximum power point due to the change in series resistance ΔRs and can be replaced by the product of the change in the area-weighted resistance and the current density at the maximum power point Jmpp. The area-weighted resistance in the finger can be determined in the case of linearly-increasing current by [15] (2) with ρ = 3.2 µΩ·cm being the resistivity of the evaporated aluminum [16], i = 2.3 mm the index width of the fingers, lF the length of the fingers and AF the cross-section area of the fingers perpendicular to the direction of transport of the current. The thickness of the Al layer is 25 µm. When considering the busbars, the approximation of linearly-increasing current is no longer valid as the maximum distance for the current in the busbar is only 9 times the index width. Therefore, in order to determine the series resistance, we determine the total loss in the busbar Ploss by adding up the power losses between the individual fingers Ploss,k (3) where I is the total current in the busbar and n the number of fingers. The losses in the individual finger k are (4) with j the current density, i the index, i.e., the distance between two fingers, lF the length of the fingers, and ABB the cross-section area of the busbar perpendicular to the direction of transport of the current. The calculated resistances are given in Tab. 3. In the case of the base fingers at module level, the Al foil, which contributes to the transport of the current, is taken into account. The difference in area-weighted resistances ΔRs between the initial cell and module level is 696±34 mΩ cm² and thus according to eq. (1) leads to an absolute reduction in the FF of 3.5±0.2%. The measured absolute difference in the fill factor between the initial cell (d.a.) and the module before lamination is between 1.2 and 2.5% (relative difference : 1.5 - 3.2 %, see Fig. 4 center). The difference between the measured and calculated values could be caused by the following effects: 1. We do not take into account the resistance of the ribbons for external connection, the Al foil between the 25×125 mm² cell strips and the contact resistance of the laser weld spots. 2. The laser-dicing of the cells induces shunt-like losses which reduce the fill factor. Nevertheless, the increase in the fill factor can be attributed to a large extent to the differences in the metallization schemes of the initial cell and the module produced. b. Open-circuit voltage Voc There are only minor losses (< 1% relative) in the open-circuit voltage Voc for all modules by comparing the cell measurements on the total or designated area with the module measurements. For comparison we take the voltages per cell in the case of the module. All measured variations are within the uncertainty of the measurements. Please note that the measurements are performed 7

with different devices. In our previous work we observed that laser-induced damage from the welding process is negligible [11]. In the case of non-homogeneous cells, series interconnection may result in a change in voltage. For the initial cells all parts of the cells are set to the same voltage. After interconnection of the cell strips in series, each cell strip is at its own voltage level. The individual voltages for the module are then added up. Cutting the base busbar, which is poorly passivated by the Al2O3 layer, should lead to an increase in voltage. This effect is not detectable and could be explained by the voltage losses caused by the poorly passivated edges of the cell strips after laser-dicing. c. Short-circuit current density Jsc The relative difference between the short-circuit current density of the initial 125×125 mm² solar cell measured on its total area compared to the current per cell strip in the module is less than 1%. However, as the cell strips are free of busbars, we expect the same short-circuit current density Jsc per cell strip in the module as for the initial cell measured on the designated area. The comparison of these data, see Fig. 4, however shows a relative reduction of 3.0 to 3.3%. One reason is that the current in the module is limited by the lowest current in the individual cell strips interconnected in series. An inhomogeneous current generation of the cell strips reduces the current in the module. Besides this matching effect, shunts or regions with an increased recombination can decrease the current. In order to reveal such losses, we perform infrared lightmodulated lock-in thermography measurements (ILIT) [17,18] on the interconnected cell strips at a photon flux corresponding to one sun.

Fig. 5 ILIT cosine image of the four interconnected cell strips at a photon flux corresponding to one sun at open-circuit voltage. The image is on a linear scale. The white straight line indicates 8

the location of the line scan in arbitrary units. The three dark vertical lines correspond to the interconnection gaps (Al-foil) between the four cell strips of the module. The ILIT image in Fig. 5 reveals heat-dissipating defects that are located in scratches and residuals from handling of the cells. These features can also be observed in the EL image (Fig. 3). Additional locations of increased signal intensity can be observed at the short edges of the cell strips – outer non-busbar edges of the initial 125×125 mm² cell - and at the long laser-diced edges. The increased signal intensity at the laser-diced edges is clearly shown in the line scan in Fig. 5 and indicates higher recombination at these edges. Therefore, part of the losses in shortcircuit current density Jsc can be attributed to the edges, as the laser-diced 25×125 mm² cell strips have a 3 times larger ratio of edge length to cell area than the 125×125 mm² initial solar cells. Remarkable is the absence of local shunts after interconnection of the cell strips. This shows the very good insulation properties of the polymer layer. An influence of the same features described above can also be qualitatively observed in the photoluminescence (PL) image of module 1, as shown in Fig. 6. The intensity of the signal reduces towards the edges of the cell strips. Reduced signal intensity corresponds to less radiative recombination and is an indicator of other recombination paths, which lead to a reduction in the charge-carrier density. Especially the labeling by a diamond scriber on the front side of the cell strips, see bottom and upper right corner of Fig. 6, has a negative impact on passivation quality.

Fig. 6 PL image of module 1 after lamination on a linear scale at open-circuit voltage. The three dark vertical lines correspond to the interconnection gaps (Al-foil) between the four cell strips of the module. The impact of the edges on the short-circuit current can be verified by measuring the module with different shadow masks. Table 4 gives some details of the masks we used for measurements of 9

module 1. Taking the parameters of the laminated module 1 measured without a shadow mask as a reference, there is a relative change of -1.7% in Jsc when using mask 1 with an open area corresponding to the active cell area. Please note that a partial shading of the cell strips may occur due to non-perfect alignment of the mask. Using mask 2, which additionally shades one millimeter of the long edges of the cell strips, the relative change in Jsc is +1.2% compared to the figure without a mask. If 2.5 mm are shaded at all four edges by mask 3, relative current density Jsc increases by 2.5% compared to the non-shaded case and by 4.2% compared to mask 1. This indicates that the losses at the four edges of the individual cell strips lead to a reduction in module current density. Hence, the greatest proportion of the losses in Jsc can be assigned to the recombination of the charge carriers at the four edges of the individual cell strips used for production of the module. Please note that during the measurement of the initial cells on the designated area all four edges are also shaded. d. Efficiency The efficiencies of the initial cells measured on the total area are between 20.4% and 21.2%. Absolute module efficiencies increases by 0.9% to 1.0% (relative increase : 4.1 - 4.8%). This can mainly be attributed to the increased fill factors. The comparison of the module efficiencies to the cell efficiency measured on the designated area shows losses of between 0% and 1.9%. The efficiency drop is caused by the reduction in module current density mainly due to edge effects. e. Losses due to the lamination process Figure 4 also shows the comparison of the module parameters before and after lamination both in relation to the active cell area (c.a.). We observe a decrease in current densities Jsc and hence in module performances after lamination. A relative loss of about 4% can be assigned to reflective losses at the glass front side of the modules. Furthermore, for example in the case of modules 1 and 2, one single finger is disconnected after lamination resulting in an additional decrease in current density Jsc. The lamination needs to be further improved in order to preserve already existing good contacts with all individual fingers. 6. Comparison to existing busbar-free interconnection schemes We have presented a module integration process for busbar-free BJBC solar cells. The fingers of the interdigitated back junction structure are laser-welded to an Al foil using the AMELI process. This Al foil covers the largest fraction of the rear side of the solar cell, where it contacts the individual base fingers. To avoid shunts in this area between the Al foil and the emitter region a thin layer of encapsulant is inserted. For cell interconnection the same foil that contacts the base of one cell is led to another solar cell, where it is welded to the individual emitter fingers at one cell edge. Contacting individual fingers or isolated contact structures was realized also by other processes. Advent Solar presented a process [7] that interconnects busbar-free solar cells with line-shaped finger structure on the rear side. A copper ribbon is placed perpendicular to the direction of the fingers and conductively glued to them. In general conductive adhesives contain silver [19], which is a cost-intensive material. They also require an oxide-free contact area. Therefore, they cannot contact Al-layers [19] and, for example, silver contacts need to be applied. In order to avoid shunts between the two polarities, an insulation layer with openings for the electrical contact is necessary. [7]. This requires an aligned deposition of this structured insulation layer. In our interconnection process the insulation layer is pierced during welding and, hence, the 10

application of this layer requires no alignment or structuring. In addition, the laser welding also pierces the native oxide layer on the aluminum. Therefore, our process has the advantage of enabling the interconnection of Al-metalized solar cells without any additional solderable metallization such as silver or conductive adhesives. However, the laser welding process requires high precision in the location of the laser weld spots. Another contacting scheme for the busbar-free BJBC solar cell, Verlinden’s multilevel metallization, is based on aluminum [10]. It has been developed for contacting concentrator solar cells featuring point-shaped contacts. It requires complex processing: deposition of a first metal layer, contact separation, application of a stack of insulation layers (e.g. anodic oxidized Al and SiO), opening of the dielectric layers on the point contacts, deposition of a second metal layer and finally the module integration of the cells [10]. Compared to this process our interconnection scheme significantly reduces the number of process steps. After contact separation of the cell metallization, we apply a polymer-based insulation layer to the rear side of the solar cells and subsequently cure it. Both process steps occur in ambient conditions. Then we position the cells on the Al foil and perform the laser welding. After this step the module is ready for lamination. 7. Summary and Outlook The applicability of our process has been shown by production of proof-of-concept modules. We built four modules each by interconnecting four aluminum metalized busbar-free 25x125 mm² solar cell strips. These cell strips were obtained by laser-dicing 125x125 mm² BJBC solar cells with two busbars. The best module had a conversion efficiency of 22.1% prior to lamination. The initial 125×125 mm² solar cell, which provided the four cell strips used for this module, had an efficiency of 21.2% (22.1%) measured on the total area (designated area without busbars). Comparing the module with the initial cell before laser dicing and interconnection, there was an absolute increase in the FF of 3.5%. This can be mainly attributed to the reduction of the finger length and omission of the busbars. This resulted in an absolute increase in performance of 0.9 % before lamination. Hence, the interconnection scheme we developed enables the transfer of the high cell efficiencies of busbar-free solar cells to modules. Due to reduction of electrical shading one could also expect an increase of short-circuit current density compared to the cell measured on total area. However, due the damage induced at the cell edges during dicing of the cells, this increase was not observed. Nevertheless, this is related to the cell stripes used in this work and not to the interconnection process itself. After lamination the module efficiency relative to the active cell area was 21.0%. No shunts were observed, which demonstrates that our polymer layer insulates sufficiently well. Glasses without anti-reflection coating were used for encapsulation, resulting in significant losses in module current after lamination. The presented process is not limited to BJBC solar cells with interdigitated finger-like structures. It is applicable to various cell geometries including point contacts, which enable high cell conversion efficiencies [1,20,21]. Further, the AMELI welding process can also be used to contact screen-printed Al pastes [22]. Hence, module production with BJBC solar cells with screen-printed Al metallization using our interconnection scheme is also possible. This work presented the interconnection of 25 mm wide cell stripes, however, we also consider the up-scaling of the interconnection scheme to large-area busbar-free BJBC solar cells. For this, we propose the following modifications and requirements of the process: The Al foil would preferentially structuring first, e.g. by punching, and then be applied to the substrate. Since the used encapsulant foil is flexible, distortion of such a substrate may occur on large area. However, a slight distortion of the substrate is acceptable, since the solar cells are aligned to the individual 11

Al foil structures and not to the substrate. This alignment of the cells to the Al structure needs to be carried out only at one edge of the cells, where the emitter is contacted. This allows a tolerance of ± 250 µm depending on the cell´s metallization geometry. A higher precision is required for laser welding of the base and emitter regions, where the tolerance is in the order of ± 50 µm. But this is achievable with commercially-available image-recognition and scanner systems. For this reason, we consider an up-scaling of the interconnection scheme to large-area busbar-free BJBC solar cells to be feasible. Acknowledgments The authors wish to thank I. Feilhaber and T. Friedrich for solar cell processing, D. Münster and A. Pazidis for coatings, T. Neubert and D. Sylla for their support on laser processing, as well as F. Kiefer, M. Köntges, I. Kunze, R. Peibst, and C. Schinke for fruitful discussions. This work has been supported by the German Ministry for the Environment, Nature Conservation and Reactor Safety under Contract 0325192 (CrystalLine Project) and the State of Lower Saxony.

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Tab. 1 I-V-parameters of the best cell with measurement uncertainty and the average values with (maximum-minimum)/2 deviations for the cell parameters measured under standard test conditions for a batch of 5 BJBC solar cells. The total area of the cells (t.a. 156.25 cm²) or designated area (d.a. 132.67 cm²) is illuminated.

Cell

A [cm²]

η [%]

FF [%]

Voc [mV]

Jsc [mA/cm² ]

Best cell 21.2± 77.0± 686.3 40.2 156.25 t.a. 0.6 0.8 ±3.1 ±1.1 Best cell 22.1± 78.1± 683.8 41.4 132.67 d.a. 0.6 0.8 ±3.1 ±1.2 Average 20.7 76.7 683 156.25 t.a. ± 0.4 ± 0.7 ± 3.0

39.6 ± 0.4

Average 21.9 78.2 680 132.67 d.a. ± 0.2 ± 0.3 ± 3.0

41.1 ± 0.3

14

Tab. 2 I-V-parameters measured under standard test conditions for the best cell used for module interconnection. The module parameters before and after lamination are also provided. In addition, the open-circuit voltage and short-circuit current density of the module per cell are given for comparison. t.a. is the total area, d.a. the designated area of the cells and c.a. the active cell area of the module. A [cm²]

η [%]

FF [%]

Voc [mV]

Cell t.a.

156.25

21.2

77.0

686.3

40.2

Cell d.a.

132.67

22.1

78.1

683.8

41.4

Module c.a.

125

22.1

80.5

2740

684.9

10.0

40.1

Laminated module c.a.

125

21.0

80.8

2728

681.9

9.52

38.1

15

Voc/cell [mV]

Jsc Jsc/cell [mA/cm²] [mA/cm²]

Tab. 3 Area-weighted resistances at cell level calculated for the initial 125×125 mm² BJBC cells and at module level for the interconnected 25×125 mm² cell strips. The given uncertainties are related to the uncertainty in the geometry. Rs [mΩ cm²] Cell level Base busbar Emitter Busbar Base finger (lF = 120 mm) Emitter finger (lF = 120 mm) Total cell Module level Base (lF = 25 mm) Emitter (lF = 25 mm) Sum module

172±6 154±5 297±20 80±3 704±34 finger finger

4.4±0.1 3.5±0.1 7.9±0.1

16

Tab. 4. Masks used for measurements of the laminated module. The masks have 4 openings for the 4 cells with the corresponding widths and lengths. The area given is the sum of the 4 openings. In addition, the measured short-circuit current density Jsc and the relative change in Jsc compared to the module without a mask are given.

Mask

Width [mm]

Length [mm]

No

Area [cm²]

Jsc Rel. [mA/cm²] change

125

9.52

1

25

125

125

9.36

-1.7%

2

23

125

115

9.63

1.2%

3

20

120

96

9.76

2.5%

17

Jsc