Organic selective contacts for crystalline silicon solar cells

Abstract

The hole-conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) [PEDOT:PSS] is known to effectively passivate crystalline silicon surfaces and to simultaneously provide a low contact resistance for holes. PEDOT: PSS can therefore be applied as a hole-selective, passivating contact layer in silicon solar cells. In this thesis, PEDOT:PSS/c-Si solar cells with phosphorusdiffused front and PEDOT:PSS/c-Si heterojunction at the rear (‘BackPEDOT’ solar cells) are manufactured and the selectivity of PEDOT:PSS on crystalline silicon is examined. On random-pyramid-textured silicon surfaces covered with PEDOT:PSS we observe for the first time an additional thin organic layer, which has an increased sulfur content in comparison to the bulk PEDOT:PSS and which is responsible for the passivation of the silicon surface. The maximum shortcircuit current density increases with decreasing PEDOT:PSS layer thickness. In order to further reduce the parasitic absorption, the impact of additives to the PEDOT:PSS dispersion is investigated. We hereby demonstrate for the first time, that the admixture of sorbitol not only reduces the parasitic absorption in PEDOT: PSS, but also improves the passivation quality of PEDOT:PSS on silicon. A maximum energy conversion efficiency of 20.6% of a BackPEDOT solar cell was achieved for PEDOT:PSS with sorbitol admixture. This is the highest efficiency achieved so far for a c-Si solar cell with an organic selective contact. Furthermore, the band bending within the silicon bulk of the PEDOT:PSS/c-Si junction is examined by a newly introduced methodology based on the Depletion Region Modulation (DRM) effect. For the first time we demonstrate that admixture of sorbitol to the PEDOT:PSS dispersion improves the chemical interface passivation, but the band bending within the silicon bulk towards the PEDOT:PSS/c-Si interface remains unaffected.