Flexible micromorph tandem a-Si/μc-Si solar cells

The deposition of a stack of amorphous (a-Si:H) and microcrystalline (mu c-Si:H) tandem thin film silicon solar cells (micromorph) requires at least twice the time used for a single junction a-Si:H cell. However, micromorph devices have a higher potential efficiency, thanks to the broader absorption spectrum of mu c-Si:H material. High efficiencies can only be achieved by mitigating the nanocracks in the mu c-Si:H cell and the light-induced degradation of the a-Si:H cell. As a result, mu c-Si:H cell has to grow on a smooth substrate with large periodicity (>1 mu m) and the a-Si:H cell on sharp pyramids with smaller feature size (similar to 350 nm) to strongly scatter the light in the weak absorption spectra of a-Si:H material. The asymmetric intermediate reflector introduced in this work uncouples the growth and light scattering issues of the tandem micromorph solar cells. The stabilized efficiency of the tandem n-i-p/n-i-p micromorph is increased by a relative 15% compared to a cell without AIR and 32% in relative compared to an a-Si:H single junction solar cells. The overall process (T < 200 degrees C) is kept compatible with low cost plastic substrates. The best stabilized efficiency of a cell deposited on polyethylene-naphthalate plastic substrate is 9.8% after 1000 h of light soaking at V(oc), 1 sun, and 50 degrees C.