Multijunction thin-film solar cells on flexible substrates for space power

Photovoltaic arrays have played a key role in power generation in space. The current technology will continue to evolve but is limited in the important mass specific power metric (MSP or power/weight ratio) because it is based on bulk crystal technology. Solar cells based on thin-film materials offer the promise of much higher MSP and much lower cost. However, for space applications, a 20% or greater AM0 efficiency (η) is required. The leading thin-film materials, amorphous Si, CuInSe₂ and CdTe have seen significant advances in efficiency over the last decade but will not achieve the required efficiency in the near future. Several new technologies are herein described to maximize both device η and MSP. One novel approach involves the use of very lightweight flexible substrates such as Kapton^™ and metal foil. Recently, prototype devices of thin-film materials have been fabricated on these substrates. We describe our efforts to enable this advance including materials processing and device fabrication and characterization on both polymer and metal foil substrates. Another approach involves stacking two cells on top of each other. These tandem devices more effectively utilize solar radiation by passing through nonabsorbed longer wavelength light to a narrow-bandgap bottom cell material. AMPS modeling of current devices in tandem format indicate that AM0 efficiencies near 19 % can be achieved. And with improvements in materials, AM0 efficiencies approaching 25% are achievable. Several important technical issues need to be resolved to realize these complex devices: development of a wide bandgap material with good electronic properties, development of transparent contacts, and targeting a 2-terminal device structure (with more complicated processing and tunnel junction) or 4-terminal device.