Silicon nanowire development for solar cell devices

Silicon thin films are promising optical absorber layers for inexpensive thin film solar cells that are fabricated on foreign substrates, such as glass and stainless steel. While the reported efficiencies of nanocrystalline Si based thin film solar cells have been higher than the known stabilized efficiency of amorphous silicon based thin film solar cells, they don´t compare to the efficiencies reported for wafer based silicon solar cells. This is mainly due to the recombination of minority carriers at grain boundaries and intra-grain defects that are present in the nanocrystalline-Si absorber layer. Therefore, a device geometry that allows the benefits of nanostructure and also lowers grain boundary defects should greatly enhance efficiency, reduce the cost of the final solar cell and help increase market penetration. Vertically aligned silicon nanowire (SiNW) solar cells have already been shown theoretically to be much less sensitive to impurities, have low reflective losses and significantly higher optical absorption compared to planar Si Solar cells. A number of devices based on SiNW building blocks have been suggested; however, high performance and scale-up utilizing low cost processing approaches to produce effective devices remain a challenge. A high volume and large sample area catalytic chemical vapor deposition system has been designed and built to synthesize silicon nanowires using the vapor-liquid-solid growth process. The nanowires were fabricated on gold coated p-type Si substrates. The formation of nanowires was confirmed by secondary and transmission electron microscope analysis. Design of experiments was carried out to identify the SiNW process window for solar cell applications. The catalytic chemical vapor deposition at 550°C and 150 torr process pressure yield nanowire growth with higher populations. P-i-n type solar cell devices were fabricated by growing i-type SiNWs on a p-type wafer followed by forming a top emitter layer using n-ty- e spin-on-dopant. Further process optimization is on going for realizing high efficiency silicon nanowire solar cells.