The fabrication of a Heterojunction with Intrinsic Thin-layer (HIT) device, including a thin (˜10 nm) amorphous silicon film on a single crystal substrate, has been shown to enable solar cells with high open circuit voltages, as a result of the larger band offset achieved between the wider band gap amorphous silicon and the single crystal silicon as well as the superior surface passivation characteristics of amorphous silicon. (M. Taguchi et al., Proceedings 5th Photovoltaic Science and Engineering Conference (1990) 689). Recent research at Sanyo has shown that reducing wafer thickness with HIT devices has the effect of increasing the open circuit voltage but with a reduction of the short circuit current due to loss of long wavelength infrared radiation as the wafer gets thinner. See FIG. 1, showing data from Taguchi et al. “High efficiency HIT solar cell on thin (<100 micron) silicon wafer” 24th EU PVSEC (2009). The plot in FIG. 1 shows values for short circuit current density, Isc, open circuit voltage, Voc, and efficiency, Eff, for two thicknesses of thin cells, where the values are given relative to those for a 165 micron thick cell. Sanyo's data is for wafer thicknesses down to about 75 microns. However, the latest data from Sanyo (37th IEEE PV specialists conference, 2011) has shown record open circuit values as high as 747 mV for a 58 micron thick wafer (where the wafer was mechanically thinned).
However, the fabrication by mechanical thinning and handling of these very thin wafers poses problems for manufacturing and is not economically viable—there is a need for improved methods of fabricating and handling thin silicon in the production of HIT solar cells.
Furthermore, as silicon wafer thickness is reduced to reduce silicon consumption and hence manufacturing costs of photovoltaic products, more of the long wavelength infrared radiation will pass through the thin wafer without being absorbed, due to the relatively poor absorption coefficient of silicon. Consequently cell efficiency is reduced for thin (less than roughly 50 micron) silicon cells. There is a need for solar cell structures which can compensate for the poor absorption of longer wavelength light.
Yet furthermore, even though the conversion efficiency of the conventional prior art HIT cells is roughly 23%, it is far from what may theoretically be achieved—there is still a need for improved cell efficiency.