Reduction of Parasitic Absorption in PEDOT:PSS at the Rear of c-Si Solar Cells

The hole-conducting polymer PEDOT:PSS is known to effectively passivate crystalline silicon (c-Si) surfaces and at the same time provide a low contact resistance for holes. PEDOT:PSS can hence be used as a hole-selective contact layer at the rear side of a silicon solar cell. We investigate the influence of the transparency and thickness of the PEDOT:PSS layer on the parasitic infrared absorption in the PEDOT:PSS layer on the rear of a silicon solar cell. We find that a decrease of PEDOT:PSS layer thickness reduces the parasitic absorption in the PEDOT:PSS layer. Reflectance measurements show an increase of escape reflection in the long-wavelengths range for decreasing PEDOT:PSS layer thicknesses. On solar cells with an active area of 4 cm(2), we observe an increase of the short-circuit current density J(sc) of (0.6 +/- 0.3) mA/cm(2), from (37.7 +/- 0.1) mA/cm(2) to (38.3 +/- 0.2) mA/cm(2), by decreasing the PEDOT:PSS layer thickness from (301 +/- 23) nm to (67 +/- 8) nm. In addition, we show that additives that increase the transparency of the PEDOT:PSS layer can be used for effectively reducing the parasitic infrared absorption in the PEDOT:PSS layer. One additive examined in this study is sorbitol. With admixture of sorbitol to the precursor solution we observe a decrease of parasitic absorbed current density by the PEDOT:PSS of 50 % for a sorbitol content of 2.1 %, and a decrease of 80 % for a sorbitol content of 8.4 %. On large-area (15.6 x 15.6 cm(2)) PEDOT:PSS/c-Si solar cells with phosphorus-diffused front emitter and screen-printed metal contacts on the front side and full-area PEDOT:PSS-covered rear, we observe an increase of the J(sc) value of 1.37 mA/cm(2) for 8.4 % admixture of sorbitol to the precursor dispersion.