Within this work, we provide a comprehensive overview about research activities and current status with respect to rotary printed metallization for silicon solar cells. We will present the major results of the research project "Rock-Star" and previous activities which focused on the metallization of Si solar cells using flexographic printing and rotary screen printing. We demonstrate that the rear side metallization of passivated emitter and rear cells (PERC) can be realized with rotary screen printing on the same quality level as state-of-the-art flatbed screen printing. Furthermore, it is shown that both rotary screen printing and flexographic printing are able to realize the fine line front side metallization. Fine line front side contacts down to approx. 40 mu m for rotary screen printing and around 30 mu m for flexographic printing are demonstrated. However, further optimization is required to reduce finger width, increase finger height and thus decrease the mean lateral finger resistance. A major result of project "Rock-Star" is the development of an innovative demonstrator platform to enable rotary printed solar cell metallization with high printing speed and low cycle time per wafer. Newly developed transport, alignment and printing concept enables a cycle time of down to 0.45 s/wafer. The concept and features of the demonstrator machine are presented within this paper.

Within this work, we will present the machine concept and the first results of passivated emitter and rear contact (PERC) solar cells fabricated on an innovative high-throughput rotary printing demonstrator machine which has been developed within the research project »Rock-Star«. The machine is designed to perform the metallization of Silicon solar cells with a printing speed of up to 600 mm/s and a cycle time down to 0.45 s/wafer using a newly developed shuttle transport system. Within a first experiment, multicrystalline Silicon (mc-Si) PERC solar cells metallized on the rear side with rotary screen printed obtained a mean conversion efficiency of η = 19.3 % which is on the same level as reference cells with standard flatbed screen printed rear side metallization (η = 19.3 %). Furthermore, a 9 cell demonstrator module with cells partly metallized on the demonstrator and SmartWire (SWCT) interconnection is presented.

One of the main challenges within Si solar cell production is the increase of throughput rates. With throughput rates in the range of 2400 Wafers / hour, metallization lines based on flatbed screen printing are typically the bottleneck of a modern solar cell production. Within this work we present innovative printing technologies allowing for higher throughput rates, e.g. rotational printing technologies as flexographic printing and rotational screen printing and as well multi-nozzle dispensing. In principle, flatbed screen printing processes can be completely replaced by using e.g. rotational screen printing for rear side printing and e.g. flexographic printing or dispensing for front side printing.. Hence, the realization of higher throughput rates and thus lower production costs is addressed. Moreover, new printing technologies allow for Ag paste savings up to 20% and efficiency increases of 1-2% relative by optimizing the finger geometry.

Rotational flexographic printing technology is a highly promising approach to increase the productivity of the cost-intensive solar cell metallization process. The ability to realize narrow contact fingers with very low silver consumption makes this technology particularly attractive for the front side metallization of busbarless solar cells in combination with multi-wire interconnection like Meyer Burger’s SmartWire Connection Technology (SWCT). Within this work, we investigate the feasibility of this approach on solar cells with 156mm edge length. Two types of silver inks are prepared and evaluated with focus on optical and electrical properties of the printed front side grid. Both inks achieve sufficient lateral finger resistances below 20Ω/cm. A low specific contact resistance of ρc,95%=3.0±0.6mΩcm2 is obtained with ink A. Using flexographic printing, Aluminum back surface field Czrochalski-grown Silicon busbarless solar cells with a maximum conversion efficiency of η=19.4% (η∅=19.0%) are fabricated and interconnected to a mini-module. The mini-module obtains an aperture conversion efficiency of η=15.8%. The origin of the cell-to-module (CTM) losses are examined in detail. It is shown that a certain part of the CTM-losses originates from the characteristics of the used GridTOUCH I–V-measurement device. Further sources of possible CTM losses are investigated using electroluminescence measurement (EL) and discussed in detail.

Flexographic printing is a high-throughput rotational relief printing method and represents a promising innovative approach for the front side metallization of silicon solar cells. The ability to realize narrow contact fingers with very low silver consumption makes this technology particularly interesting for busbarless solar cells with wire-interconnection like Meyer Burger's SmartWire Connection Technology (SWCT). This study intends to provide a comprehensive evaluation of the general feasibility for this approach. Fundamental printing tests are carried out to investigate important influence parameters like the flexible printing plate and the correlation between printing pressure and contact finger width. We found out that contact finger elements on elastomer printing plates down to an actual width of wf = 7 ± 2 μm can be realized using a high-resolution laser-engraving process. Furthermore, we show that printed contact finger width increases by approx. Δwf ≈ 10 μm for each Δploc = 0.05 MPa of pressure increase and thus has a strong impact on the contact grid. Busbarless Al BSF Cz-Si solar cells with flexo-printed front side metallization are fabricated and measured using Pasan GridTOUCH/SpotLIGHT-system. The best group of solar cells obtains a mean conversion efficiency of η = 19.0% and a maximum conversion efficiency of ηmax = 19.4%. A mini-module with the best two solar cells achieved a aperture conversion efficiency of η = 15.8%. Causes for cell-to-module losses are investigated and discussed in detail. In summary, we demonstrate the successful application of flexographic printing technology for the front side metallization of busbarless solar cells with multi-wire interconnection.

The current cost pressure within the photovoltaic industry requires innovative approaches to increase productivity and reduce production costs. The use of highly productive rotational printing methods offers the potential to increase the throughput of the cost-intensive metallization process by the factor 2 to 3. Within this work, we present two innovative rotational printing technologies – flexographic printing and rotary screen printing – for the front side metallization of Silicon solar cells. We show that both technologies can realize contact fingers with a lateral finger resistance RL below 10 /cm in one printing step which is sufficient for busbarless solar cells with multi-wire interconnection. For flexographic printing, we found a strong impact of the anilox roller properties on geometry and lateral resistance of the contact fingers. A finger resistance of RL = 8.5 /cm which is sufficient for wireinterconnected busbarless solar cells could be achieved using an anilox roller with a very large dip volume of VD = 16.5 cm3/cm2. For rotary screen printing, a considerable increase of the finger width due to spreading of the Ag-paste proved to be the major challenge for further development.

Rotational flexographic printing is a promising high-throughput technology for the front side metallization of silicon solar cells. Very low silver consumption and the possibility to realize narrow contact fingers make this technology particularly interesting for multi-busbar solar cells. Within this work, fundamental printing tests have been carried out on a flexographic roll-to-flat machine using an experimental anilox roll and elastomeric laser-engraved printing plates. A double printing process with intermediate drying step has been applied. Contact fingers down to 33μm in width and up to 8μm in height have been realized using this technology. Lateral resistances in the range 500 to 1500Ω/m have been determined by four point measurement method. These results underline the capability of flexographic printing for fine line metallization of multi-busbar solar cells.