Bulk heterojunction organic-inorganic photovoltaic cells based on doped silicon nanowires

Heterojunction photovoltaic devices were fabricated using single crystal silicon nanowires and the organic semiconductor regioregular poly-(3-hexyl thiophene) (RR-P3HT). N-type nanowires were first grown on an n+ silicon substrate by the vapor-liquid-solid (VLS) method. Devices were then fabricated by filling the gap between the nanowires and a transparent indium tin oxide (ITO) glass electrode with a polymer. For initial devices the gap was filled with P3HT deposited from chlorobenzene solution. Device performance indicates that both silicon and P3HT act as absorbers for photovoltaic response, but that photocurrents were very low due to high series resistance in the cell. A second type of device was fabricated by depositing a thin layer of P3HT on the grown nanowires by dip coating from a dilute solution, and then filling the voids between nanowires and the transparent electrode with the conductive polymer poly-[3,4-(ethylenedioxy)-thiophene]: poly-(styrene sulfonate) (PEDOT:PSS). The relatively high mobility of this organic conductor results in much higher photocurrents in photovoltaic cells, but results in a dip in the spectral response of the cells in the blue-green region due to light absorption in the conducting polymer. These materials show promise for efficient low-cost photovoltaic devices, but the cell geometry and materials interfaces will need to be optimized to reach their potential.