Influence of contaminations on the performance of thin-film silicon solar cells prepared after in situ reactor plasma cleaning

Abstract

This paper addresses the influence of the chemical memory effect (CME) of in situ plasma cleaning by using the fluorinated gases on the properties of subsequently deposited thin-film silicon solar cells and discusses methods to avoid or reduce this effect. Secondary ion mass spectrometry (SIMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) profiles analysis showed a high impurity concentration in the intrinsic (i)-layer of p–i–n solar cells prepared directly after in situ cleaning. With increasing number of cell depositions these contaminations decrease and the solar cell performance recovers to the standard value. Restoring solar cell performance is accompanied by a decrease of contaminants concentration in the i-layer. The intentional variation of the F-content in the i-layer obtained by adding SiF 4 to the process gas mixture during a-Si:H i-layer preparation reveals that for solar cells a fluorine content above 1.5 × 10 19 cm − 3 is critical. We applied NF 3 or SF 6 + O 2 as cleaning gases and optimized the cleaning procedure. In case of using NF 3 as the cleaning gas, the CME was less pronounced as compared to the SF 6 + O 2 case and by additional procedures, like increasing the total gas flow rate during deposition, hydrogen plasma treatment of reaction chamber, the high solar cell quality could be achieved directly after in situ reactor cleaning. Low concentration of impurities in such cells was observed. Also the long-term illumination test (light-soaking for 1000 h, AM1.5 radiation) shows the same stabilized efficiency as compared to reference cells.