Development of stand-alone germanium solar cells for application in space using spin-on diffusants

The development of stand-alone germanium solar cells is essential for application of this type of cell in highly efficient mechanically stacked solar cells and in Thermo Photo-Voltaic (TPV) systems. Up to now the highest efficiency reported for this type of cell is 6.4 percent (AM0). Simulations show that in order to obtain high efficiencies it is essential to create a shallow emitter and sufficiently passivate the front and back surfaces. The research presented in this paper makes use of a cheap and easy cell process using a spin-on diffusant source to realise the emitter. Passivation of the front surface is obtained by applying amorphous silicon deposited by PECVD. An innovative process is used to create the contact with the emitter through the passivating layer. As a result a cell efficiency of 6.7 percent (AM1.5) is obtained.