Fig. 1

Different types of charge carrier-selective contacts. a Contact selectivity of traditional solar cells is determined by the doping density of the semiconductor directly underneath the metal contact. b Contact selectivity of heterojunction solar cells is determined by the induced accumulation/inversion region inside the semiconductor directly underneath the metal contact, due to the work function difference between heterojunction contact material and semiconductor. c Nanowires allow for a different type of charge carrier-selective contact; the carriers can be extracted parallel to the surface instead of perpendicular to it (as in doped- and heterojunction contacts) (red and blue arrows). This means that carriers are not extracted through the surface-gate layer. The cross-sectional image shows a possible band alignment and the blocking of radial hole transport at the surface-gate interface. In d−f, the band diagrams in the dark are drawn for the cases in a−c, respectively. We note that band bending in the dark is indicative for the selectivity of a contact but not a sufficient description. For an accurate assessment of charge carrier selectivity the quasi-Fermi level under illumination have to be considered, which have been omitted here for simplicity. The color gradient indicates the charge carrier selectivity with red being very hole selective and blue very selective for electron conduction