ATAKAMA – Alternative Contacting Processes and Contact Materials for Highly Efficient Silicon Solar Cells with Passivated Contacts

Duration: November 2016 - October 2019
Contracting Authority/ Sponsors: Federal Ministry for Economic Affairs and Energy (BMWi)
Project Focus:
Niedertemperatur-Silberpaste nach thermischem Sintern bei 200°C. Der spezifische Fingerwiderstand und spezifische Kontaktwiderstand der gedruckten Struktur zum TCO beträgt 9.4 µcm bzw. 3.2 mcm².
© IEEE
Low-temperature silver paste after thermal sintering at 200°C. The specific finger resistivity and specific contact resistivity of the printed structure to the TCO is 9.4 µΩcm or 3.2 mΩcm².
Niedertemperatur-Silberpaste nach thermischem Sintern bei 350°C. Der spezifische Fingerwiderstand und spezifische Kontaktwiderstand der gedruckten Struktur zum TCO beträgt 3.5 µcm bzw. 6.8 mcm².
© IEEE
Low-temperature silver paste after thermal sintering at 350°C. The specific finger resistivity and specific contact resistivity of the printed structure to the TCO is 3.5 µΩcm or 6.8 mΩcm².
Spezifischer Fingerwiderstand als Funktion des thermischen bzw. photonischen Sinterprozesses.
© IEEE
Specific finger resistivity as a function of the thermal or photonic sintering process.

The metallization of solar cells with passivating contacts can lead to considerable electrical losses. The project ATAKAMA investigates innovative materials and processes that enable low-loss metallization. In particular, media based on nano-scale particles in pastes and inks are used, which are printed on TCO (transparent conductive oxide) by means of screen printing and inkjet processes and further processed with thermal and photonic sintering technology.

So far, conductive paths with a minimum structure width of less than 30µm have been realized using inkjet and screen printing technology. By using screen printing, specific resistances of 2.4 µWcm and specific contact resistivities (TCO-metal interface) of less than 5 mWcm² were achieved with a selected sintering process, which corresponds to the project objectives.

According to current market studies, solar cells with passivating contacts, such as TOPCon (tunnel oxide passivated contact) and SHJ (silicon heterojunction) solar cells, will quintuple their market share in the coming decade. In the project ATAKAMA, innovative metallization processes based on media with nano-scale particles (nano inks and nano pastes) and screen and inkjet printing processes as well as thermal and photonic sintering techniques are being developed in order to enable the large-scale production of corresponding next-generation solar cells and, thus, create the basis for potential technology transfers from the laboratory to the manufacturing industry.

The SHJ solar cells addressed in the project have temperature-sensitive, functional a-Si (amorphous silicon) and TCO (transparent conductive oxide) layers. Therefore, a metallization with typical metal pastes and firing processes is obsolete. Conductive paths based on nano pastes achieve adequate electrical properties even at low temperatures. Specific resistances and contact resistivities (TCO/metal) of 2.4 µWcm or less than 5 mWcm2 could already be measured in ATAKAMA. In order to reduce the cost-intensive consumption of Ag, the project is evaluating base metals such as copper Cu as an alternative to silver Ag.

Due to its high productivity, screen printing is well established in photovoltaics. However, the inkjet process implies special advantages due to contactless process control, minimal material consumption and digital templates which could be used in many ways for the resource-saving production of next-generation solar cells in digitized production facilities.

Photonic sintering is used in printed electronics, but due to its special features it can also be integrated into PV applications. Thermal sintering requires long process times. Throughputs that are suitable for PV can only be achieved with complex systems. Due to the special spectra and radiation intensities, photonic sintering enables the processing of substrates with a cycle time of a few milliseconds at low foot-print and is, therefore, a relevant process for the industrial transformation of TOPCon and SHJ solar cells. Within the project, the developed processes will be integrated into the procedures of the addressed cell concepts. Afterwards, they will be evaluated and the results will be published in renowned scientific journals.

Spezifischer Kontaktwiderstand als Funktion des thermischen bzw. photonischen Sinterprozesses.
© IEEE
Specific contact resistivity as a function of the thermal or photonic sintering process.