The first solar cells ever processed with the metal aerosol jet system were printed with a nickel-containing ink and a silver ink designed for ink-jet applications (Fig. 10.20). The efficiencies for these untextured cells of 9% (silver- ink) and 12% (nickel ink) were quite promising (Table 10.3). The ink with nickel particles even etched through the antireflection coating. However, due to the possibility of using modified silver screen-printing paste which is already optimized for solar cell application, the emphasis for further solar cell processing was based on those inks, as presented in the following section.
Table 10.3: IV-parameters of first solar cells processed with the metal aerosol jet system. The low short-circuit current density is due to the non-textured surface and the conservative grid design shading about 15% of the solar cell surface. (ARC: antireflection coating)
ink ARC Voc [mV] jsc [mA/cm2] FF [%] [%]
η
no 600 21.5 71.5 9.2 ink-jet Ag yes 594 2.6 26.4 0.4 no 598 20.7 71.5 8.9 modified Niyes 612 27.8 69.6 11.8 Fig. 10.20: Picture of first pro-cessed solar cell.
10.5.2 Multicrystalline silicon solar cells
15.6 cm x 15.6 cm multicrystalline silicon wafers with a thickness of 220 µm were processed by an industrial partner up to the SiNX-layer with an emitter sheet
resistance of about Rsh = 55 Ω/sq and an iso-textured surface. At Fraunhofer ISE
the rear side of the wafers was conventionally screen-printed and dried. Then on the front side the contact layer was deposited using the metal aerosol jet system. For printing, a commercially available silver screen-printing paste, modified in its viscosity, was used. On each 15.6 cm x 15.6 cm wafer several grids with a size of 5 cm x 5 cm were printed. The busbar was created by printing several lines adjacent to each other with a slight overlap. Afterwards the cells were fired in an inline fast firing furnace at temperatures between 750°C and 900°C. The formation
of the second layer, the highly conductive layer, was performed by light-induced silver plating. Optionally, an annealing step at 400°C for 10 min in forming gas ambient followed. The solar cells were finally separated by rear side laser scribing and breaking. A picture of a solar cell is shown in Fig. 10.21 (right-hand).
Fig. 10.21 Left-hand: Microscope picture of contacts using different printing settings resulting in line widths of 70 µm and 160 µm after silver plating.: Right-hand: 5 cm x 5 cm sized multicrystalline silicon solar cell with metal aerosol jet printed contacts thickened by the light- induced plating process.
Using the same tip of an outlet of 200 µm diameter, metal lines with a width between 50 µm and 140 µm were deposited on different wafers. After the fast firing process step, the contacts were thickened for 10 minutes by the light-induced plating process resulting in a finger height between 10 µm and 15 µm. The total contact width increased by approximately 20 µm to 30 µm. Microscope pictures of plated contact fingers are presented in Fig. 10.21 left-hand. The IV-parameter of solar cells with 70 µm and 160 µm wide contact lines (after plating) before and after the annealing step are presented in Table 10.4.
The lower jsc for the solar cell with the wider finger can be explained with the
larger shaded area. The 25 fingers with a width of 160 µm shade about 8% of the 5 x 5 cm² sized solar cell surface compared to 3.5% for the solar cell with 70 µm wide fingers. This is the main reason for the about 4.5% lower jsc value. The low
fill factor of the cell with 70 µm broad contacts is probably caused by the low parallel resistance of rp = 700 Ω cm². Nevertheless, the series resistance for the
Table 10.4: IV-parameters of the best 5 cm x 5 cm sized multicrystalline silicon solar cells measured before and after the annealing step. The best screen-printed “reference” solar cell with a size of 15.6 x 15.6 cm² achieved a short-circuit current density of 33.5 mA/cm², a fill factor of 76.0% and an efficiency of 15.8%. Cell-ID Process step Voc [mV] jsc [mA/cm2] FF [%]
η
[%] rs [Ω cm²] rp [kΩ cm²] 160 µm finger width after plating 619 32.7 78.4 15.8 0.7 12.3 83.2 after annealing 617 32.7 79.4 16.0 0.5 12.2 70 µm finger width after plating 619 34.0 76.2 16.1 1.0 0.7 93.6 after annealing 618 34.2 77.4 16.4 0.8 0.7to be optimized to achieve a low contact resistance to the emitter surface, which is especially necessary for fine-line printed contacts.
The solar cells were characterized before and after the annealing step. For both cells the efficiency increased by 0.2% to 0.3% absolute after the annealing step, caused by a gain in the fill factor and a reduction of the series resistance. It is assumed that the contact between the deposited and the plated metal is improved or the plated silver has a direct contact to the silicon surface. The effect of the FF improvement will be further investigated in Chapter 11. The appropriate annealing temperature and time depends on the one hand on the emitter profile, on the other hand on the deposited ink. For a contact of good quality a high annealing temperature is preferable, but this increases the danger of damaging the space charge region at the same time. For the processed cells the optimum annealing temperature was between 300°C and 400°C for a time of about 5 minutes to 10 minutes.
10.5.3 Monocrystalline silicon solar cells
In a further experiment four large-area Cz-silicon solar cells have been processed with metal aerosol jet printed front contacts. These wafers had remained from another batch that was used for optimizing the hotmelt screen-printing process. For the hotmelt experiment 50 wafers were processed on 12.5 x 12.5 cm², 1 Ω cm, boron-doped Cz-silicon wafers with a thickness of 250 µm. The cells exhibit a textured surface covered by a SiNX antireflection coating and an emitter
sheet resistance of Rsh = 45 Ω/sq. The rear side was conventionally screen-printed
and dried. Most of the wafers were printed with hotmelt paste, a few were screen- printed with conventional paste and more than a month later four remaining wafers were printed with the metal aerosol jet system. For comparison, the hotmelt screen- printed and conventional screen-printed cell results will be briefly presented. These cells were fired in an inline fast firing belt furnace. While for the hotmelt cells a wide temperature variation was performed, the conventional screen-printed cells were fired at the standard firing temperature for that paste. The best of the 35 solar cells with hotmelt printed contacts achieved an efficiency of
η
= 17.9%, while the average efficiency of eight cells fired at optimum temperature wasη
avg= 17.5%(see Table 10.6). The relatively low efficiency of the best conventional screen- printed cell of
η
= 17.2% is probably due to a slightly too low firing temperature resulting in a FF of just 77% and an open-circuit voltage of 619 mV.For the front side grid of the metal aerosol jet printed cells, a final contact width of 70 µm and a height of 12 µm was assumed, so that in combination with other assumed cell data (e.g. jmpp, Vmpp) an optimum finger separation distance of
s = 1.9 mm was calculated by grid simulation (see Chapter 3), compared to the hotmelt printed cells having a separation distance of s = 2.3 mm. For each paste the contact layer of one wafer was single printed, the other one triple printed, with the purpose to reveal the effect on the contact height. The printing speed was set to vprint = 20 mm/s. This led to a total printing time of about 15 minutes and 45
minutes for a single and triple printed contact grid, respectively. Half the time was necessary to print the two 1.5 mm wide busbars. The finger height hf after the
firing process was between 1 µm and 2 µm for the single printed contact layer and between 3 µm and 6 µm for the triple printed one.
Two different pastes were used for printing. Paste “B” was the same paste as used for the multicrystalline cells (see above) with the addition of 1% by mass phosphorus powder. The idea was to form a higher doping concentration directly under the contact, in order to achieve good adhesion and low contact resistivity to the emitter surface [147,148]. Paste “A” was a different silver screen-printing paste (from another manufacturer), which was modified in its viscosity.
To keep full control of the optimum firing condition, the wafers were fired in a lab-type fast firing furnace, the front contacts were thickened by light-induced plating and the wafer’s edge isolated by laser scribing and breaking. After IV- measurement some solar cells were plated a second time to increase the finger
conductivity and annealed under room ambience for 10 minutes at 300°C. Fig. 10.22 illustrates a processed solar cell.
Fig. 10.22: Metal aerosol jet printed and plated solar cell of size 12.5 x 12.5 cm² achieving an efficiency of 18.3%.
IV-parameters of the best solar cell measured at different steps in the process sequence are presented in Table 10.5. This cell constitutes the highest efficiency (
η
= 18.3%) and highest fill factor (FF = 81.0%) achieved within this work for a large-area silicon solar cell. This cell featured a single printed contact using paste B. Whether the addition of phosphorus to the paste had an effect, could not be clarified within this work. After the first light-induced plating process the cell was limited by the high series resistance of rs = 2.1 Ω cm². To increase the fingerconductivity further, the fingers were thickened by the light-induced plating process a second time. The enlarged shaded area reduced jsc significantly, but due
to the boost in fill factor by 9%abs, the efficiency reached a value of
η
= 17.8%. InTable 10.5: IV-parameters of the best 12.5 cm x 12.5 cm sized monocrystalline silicon solar cell measured at different steps in the process sequence.
Cell-ID Process Step Voc [mV] jsc [mA/cm2] FF [%] [%]
η
rs [Ω cm²] rp [kΩ cm²] LIP 625 36.5 71.3 16.3 2.1 10 36_3 2nd LIP 623 35.8 80.2 17.8 n.a. 10 Sintering 624 36.1 81.0 18.3 0.4 8a following sintering step the fill factor was further increased, resulting in the high- efficiency value of 18.3%.
IV-parameters measured after the plating process of all four wafers are presented in Table 10.6. The second wafer metallized with paste B, featuring a triple printed contact layer, also achieved an increased FF of 80.7%. Compared to the single printed contact jsc is lower, because the triple printed contact is broader.
The efficiency of
η
= 17.5% of the solar cell with a single printed contact layer using paste A was also on a high level. However, the parallel resistance rp andpseudo fill factor PFF were reduced, indicating increased recombination currents. Both, PFF and rp are further reduced for the triple printed contact layer. In
combination with the high series resistance, the measured efficiency for this cell was low. It is assumed that paste A has a more aggressive etching behavior and damages the space charge region, which might be avoided by reducing the firing temperature for this paste. Nevertheless, a single printed contact layer featuring a thickness of 1 µm to 2 µm was sufficient to achieve a contact of good electrical quality.
Table 10.6: IV-parameters of the processed 12.5 x 12.5 cm² sized monocrystalline silicon solar cells. Paste “B” is the same paste as used for the multicrystalline silicon solar cells with the addition of 1% red phosphorus by mass. Paste “A” is a conventional silver screen-printing paste modified in its viscosity from a different manufacturer. Hotmelt and conventional screen-printed (SP) cells were processed on wafers from the same batch.
Cell-ID Paste Prints Voc
[mV] jsc [mA/cm2] FF [%]
η
[%] PFF [%] rs [Ω cm²] rp [Ω cm²] 36_4 A single 623 35.3 79.5 17.5 81.9 0.6 4 000 36_1 A triple 623 34.5 68.1 14.7 80.9 2.6 700 36_3 B single 624 36.1 81.0 18.3 82.5 0.4 10 000 36_2 B triple 622 35.2 80.7 17.7 82.4 0.5 8 000Best hotmelt cell 624 36.2 79.1 17.9 Average of 8 hotmelt cells 624±1 36.0±0.1 78.1±1.2 17.5±0.3 Best conventional SP cell 619 36.1 76.7 17.2
10.6 Chapter summary
Metal aerosol jet printing in combination with a light-induced plating process has been developed for creating the front side contact layer of the proposed two layer contact structure within this work. Opposed to most other metallization technologies the metal aerosol jet system is a non-contact direct-write technique, making it suitable for printing on uneven surfaces and thin and fragile wafers. However, the biggest advantage of this technology lies in the working principle of the deposition head: The metal aerosol stream is surrounded by a second gas stream and then focused through the nozzle onto the substrate. This prevents clogging of the tip. The width of the deposited line is much smaller than the size of the nozzle outlet. Lines of 14 µm width were printed using metal organic inks and a nozzle with an outlet diameter of 100 µm.
Up to date a lot of improvements of the system have been applied by Optomec as well as by Fraunhofer ISE. Especially the uptime and the process stability of the machine could be significantly increased. For industrial mass production a multi nozzle system is required. In order to achieve a throughput rate comparable to screen-printing systems, a printing speed of 50 mm/s to 100 mm/s is required, assuming that each finger of the wafer is printed by one individual nozzle of the multi nozzle system.
Within this work 12.5 cm x 12.5 cm textured monocrystalline silicon solar cells with a standard Al-BSF have been successfully processed with a modified commercially available screen-printing paste. The best solar cell processed achieved an efficiency of
η
= 18.3% at a fill factor of FF = 81.0%.Nevertheless, the full potential of the metal aerosol jet system will be achieved when using optimized inks giving a low contact resistivity to a lowly doped silicon surface.
11 Microstructure analysis of contacts
The main focus of this chapter is the analysis of fine-line printed and plated contacts on a macroscopic as well as on a microscopic level. In the first part of this chapter, the current understanding of the contact formation process for screen-printed contacts is reviewed. In the second and third part, the dependencies of the contact resistance on the contact geometry and on the plating process will be discussed, which is based on electrical and optical investigations. The results are summarized and possible new current paths between semiconductor and contact demonstrated.