CADBACK: The CdTe thin film solar cell - improved back contact

• Research unit: EU RFP
• Project number: 97.0397-1

Key words (English)

CdTe; solar cells; solar energy; thin films; photovoltaics; electrical contacts; stability

Alternative project number (English)

EU project number: JOR3-CT97-0218

Research programs (English)

EU-programme: 4. Frame Research Programme - 5.1 Nonnuclear energies

Short description (English)

See abstract

Partners and International Organizations (English)

ANTEC (D), Uni. Durham (UK), Univ. Parma (I), JRC (I/EC), Uni. Gent (B), ISFH (D)

Abstract (English)

Development of a stable and low-Ohmic resistance electrical contact (called back contact) on CdTe is essential for the high efficiency and long term stability of CdTe/CdS solar cells. CdTe semiconductor has a high electron affinity, therefore most metals form a non-Ohmic or even a Schottky barrier, which reduces the solar cell performance. Within the framework of the EU-program we have developed processes to make efficient and long term stable electrical 'back-contact' on CdTe layers grown by close space sublimation (ANTEC GmbH process) and high vacuum evaporation (IQE-ETHZ process). The industrial partner of this EU project, ANTEC GmbH is setting-up the first CdTe solar module production plant in Europe with a production capacity of 10 MW/yr. We have developed processes where vacuum evaporated Sb or Sb2Te3 buffer and Mo metallization layers are applied on CdTe to form efficient and stable back contacts, solar cells of more than 12% efficiency have been obtained. Accelerated stability tests have confirmed that these cells with efficiencies in the range of ~10% to 12% are 'stable' for periods corresponding to more than 50 years. It was observed that impurities in CdTe source material may also influence the performance of solar cells. These impurities affect the carrier concentration profile and tend to diffuse across the heterojunction, quite often they are detrimental for the efficiency. Efforts were made to develop solar cells where CdTe layers were not chemically etched, cell efficiencies were low because of a surface oxide layer on CdTe. However, evaporated Te-layers improve the efficiency of solar cells.The photoconductive gain in CdS is strongly affected by impurities, which diffuse into the CdS layer. Presence of oxygen seems to promote Cu migration along grain boundaries from the back contact into the CdS layer. Any indication of the Sb diffusion into CdS could not be found by means of AQE measurements.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 97.0397-1