Nanowire device concepts for thin film photovoltaics

The realization of one-dimensional (1D) nanostructures, e.g. silicon nanowires (SiNWs) has opened up a new area for device applications in electronics, optoelectronics, thermoelectronics, photocatalysis, photovoltaics and sensing. For all device concepts based on SiNWs, the crystal structure, geometry (alignment of SiNW with respect to the substrate), interfacial properties between the SiNW and the substrate as well as the Si core and a potential oxide shell of the SiNWs (shell can either be native or thermally grown oxide), dopant concentrations and impurity levels are of key importance for functioning of the devices.
Large-area aligned Si nanowire (SiNW) arrays are fabricated on Si wafers and glass substrates via a metal-catalyzed wet chemical etching (WCE) process with or without the use of densely packed diameter-reduced polystyrene (PS) spheres as a mask. The diameter, length, packing density, and even the shape of SiNWs could precisely be controlled and tuned by adjusting either plasma etching duration or chemical etching conditions and the nominal diameter of the PS spheres. The antireflective properties of SiNWs and thus the extremely high absorption in thin SiNW layers are essential for NW based next generation solar cells. Several cell concepts with SiNWs are realized including most interesting ones:

(i) 	a hybrid organic/inorganic solar cell using SiNWs as absorber and PEDOT:PSS as a  conducting polymer. 

(ii)	a semiconductor-insulator-semiconductor (SIS) cell concept with SiNWs as absorbers and a tunneling barrier for charge carrier separation. The thin tunneling oxide is Al2O3 with a thickness of only a few Å and a transparent conductive oxide (TCO – here: Al:ZnO) are both grown conformally around the SiNWs using atomic layer deposition (ALD).

Results from the first solar cell prototypes and the influence of the thickness, chemical nature of the tunneling oxide and back and front contact’s structure in the SIS cell will be discussed in detail. Further optimization of the cells are realized (i) by using alternative TCOs as electrodes such as graphene ore silver nanowire webs. Preliminary electrical measurements will be shown. (ii) by modelling using numerical simulations and realization of enhanced absorption at reduced SiNW surface areas.   We see a real potential of the SIS SiNW based thin film cell for further improvement of cell parameters such as Voc to 600-700 mV and a power conversation efficiency of >15%.