Improved surface passivation and reduced parasitic absorption in PEDOT:PSS/c-Si heterojunction solar cells through the admixture of sorbitol

We examine the impact of sorbitol admixture to the hole-conduction polymer PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)] on the characteristics of PEDOT:PSS/crystalline silicon heterojunction solar cells. We fabricate solar cells where the PEDOT:PSS layer is deposited as a hole-collecting contact at the cell rear, whereas the electron-collecting front is conventionally processed by means of phosphorus diffusion. Surprisingly, we observe that the admixture of the infrared-transparent sorbitol not only improves the short-circuit density of the solar cells due to the reduction of the infrared parasitic absorption, but also improves the passivation quality of PEDOT:PSS on silicon and hence the open-circuit voltage of the solar cells. The series resistance is not influenced by the admixture of sorbitol up to 4.0 wt.% sorbitol admixture in the PEDOT:PSS dispersion, but shows a pronounced increase for larger sorbitol contents. The optimal sorbitol content concerning efficiency is hence 4.0 wt.%, leading to an energy conversion efficiency of 20.4% at one sun, which is more than 1% absolute higher compared to the efficiency of the reference cells without sorbitol.