Rear passivation of thin multicrystalline silicon solar cells

Thinner wafers, combined with improvements in multicrystalline substrate quality have led to minority carrier diffusion lengths that equal or even exceed the device thickness. In such devices, the passivation of the rear surface plays an increasingly important role in determining device performance. We have been using an aluminium paste to passivate the rear surface of the solar cells through the creation of a back surface field. This has led to devices with efficiencies that exceed 16% efficiency using an industrial process on large 10 x 10 cm(2) wafers of 300 mum thick. However the aluminium paste is expensive and a thick layer needs to be applied which leads to bending of thinner wafers below 250 mum. An alternative method uses a boron diffusion to form of a back surface field. The boron diffusions have been adapted from high efficiency cells (where high temperatures and long diffusion times are used) to multicrystalline cells that can only tolerate lower temperatures, and for industrial applications that require shorter diffusion times and simple industrially compatible application techniques. The application of boron diffusion sources via simple screen-printed pastes and spray-on diffusion sources is reported. The boron diffusions have been characterised through lifetime measurements on test samples and spectral response measurements on finished solar cells.