High open circuit voltages on < 50 micron silicon substrates by amorphous-silicon (a-Si) and quinhydrone-methanol (QHY-ME) passivation

Thin silicon solar cells offer the well-known advantages of cost reduction and higher efficiencies. A thinner solar cell may have a higher open circuit voltage than a thicker one assuming the surfaces are well passivated and the light trapping is included thus resulting in improved efficiencies. High open circuit voltage, Voc, of 720 mV [1] or above has been achieved from several technologies on conventional thickness wafers and has approached 740 mV on thinner wafers [2]. However, the theoretical limit from detailed balance calculations is between 830 mV and 850 mV (depending on spectrum) [3]. In order to achieve silicon solar cells which approach the detailed balance voltage limits, controlling the broad mechanisms that limit the open circuit voltage becomes very important and these are: (1) Auger recombination (e.g., by controlling dopant concentration); (2) the thickness of the material; and (3) the surface passivation. While high open circuit voltages have been demonstrated on thicker silicon solar cells, achieving the necessary surface passivation for "thin" solar cells (i.e., less than 50 micron thick wafers) to show increase in Voc has remained a challenge. The present research demonstrates via Implied-Voc measurements that amorphous-Si passivation as well as organic passivation based on quinhydrone-methanol has sufficient surface passivation such that open circuit voltage increases even at thickness of 35 microns. These results are also significant in demonstrating that non-ideal effects, such as high injection, play a significant role in determining Voc, but nevertheless experimentally still allow high open circuit voltages as the device is thinned.