Emitter formation and contact realization by diffusion for germanium photovoltaic devices

Standalone germanium solar cells are under development for application in high-efficiency mechanically stacked multijunction solar cells and thermophotovoltaic systems. To realize a suitable device, a more fundamental research has been done on germanium doping, surface passivation, and contact formation. In this paper, emitter formation and contact realization are discussed in detail. Emitter formation is done by phosphorous diffusion from a spin-on dopant (SOD) source. Critical parameters are the diffusion time, diffusion temperature, and phosphorous content in the SOD. Front contact formation is done by an innovative method, where the contacting metal is diffused through the amorphous silicon passivation layer. The specific properties of the diffusing metal, diffusion temperature, and diffusion time are important. Using the developed process, a stand-alone germanium solar cell has been realized with a world-class AM1.5G energy conversion efficiency of 7.8%.