Molecular doping method with nm control applied to Si nanowires for solar cells.

Silicon nanowires (SiNWs) have been largely studied for application in microelectronics and photovoltaics. In both cases the control of their doping has demonstrated to be crucial as well as difficult. The usage of standard doping methods, such as ion implantation, can be critical in nanostructures for the difficulty to achieve abrupt junctions, for the stochastic spatial distribution of the implanted ion and for the severe crystal damage. Other approaches such as the formation of amorphous doped layers over the NWs present the drawback of creating heterointerfaces increasing trapping effects at the junction. An innovative solution to make controlled, uniform and conformal doping at nanoscale without bulk or surfacial defects has been recently proposed [Ho Nature 7 62 2008, Garozzo MSEB in press 2013]. This method consists in forming a monolayer of dopant-containing molecules from liquid solutions. We demonstrate the formation of junction depths as small as 10nm for both p- a nd n-type doping with peak concentrations of 1e19 cm-3 ca. The method is applied to an array of 1e10 cm-2 dense SiNWs with 1µm length and diameters up to 60nm ca. The optical functionality of the NW array is characterized and it is found that it shows a total reflectance, mainly consisting of the diffuse component, of 10% thus indicating that either the SiNWs strongly absorb the radiation or, alternatively, they have a relevant role in light trapping, making them particularly attractive for solar cells.