GaAsP self-assisted core–shell p-i-n nanowires were grown on Si solar cells. The resulting tandem cell exhibited an enhanced Voc of 1.16 V, increasing from 0.54 V for the Si cell alone. Nevertheless, the efficiency of the tandem cell was only 3.51% as compared with 9.33% for the Si cell due to a current-limiting short-circuit current density from the nanowires. Further improvement in device performance can be realized by improving the nanowire open-circuit voltage and short-circuit current, related to doping of the nanowires.
A concept for a nanowire-based photovoltaic (PV) device is presented along with the requirements for achieving high photoconversion efficiency including nanowire morphology, crystalline structure, nanowire dimensions (diameter, period (spacing) and length), avoidance of misfit dislocations, low resistance contacts, controlled doping for p-n junctions, surface passivation, and current-matching. The state of the nanowire PV device field is presented.
The continuity and Poisson equations are solved numerically to obtain J-V characteristics and photoconversion
efficiency of a two-junction solar cell. The cell consists of a top junction comprised of nanowires with bandgap of 1.7
eV grown on a bottom junction comprised of a Si substrate. The lattice relaxation possible in nanowires permits latticemismatched
III-V material growth on Si, thereby achieving the optimum bandgaps in a two-junction cell. The model
indicates a limiting efficiency of 42.3% under a concentration of 500 Suns (AM1.5D spectrum). This limiting efficiency
is similar to that calculated for the planar lattice-matched triple-junction Ge/InGaAs/InGaP cell. Methods of fabricating
the nanowire/Si cell are discussed including requirements for nanowire sidewall surface passivation.
The templated growth of GaAs nanowires based on a catalytic seed particle is described. The optimum growth conditions for biosensor applications of the nanowires are presented including the catalyst formation process and nanowire growth temperature. We have also observed growth conditions that result in nanowires oriented parallel to the surface, which may result in a new paradigm for biosensor fabrication.