Basic efficiency limits, recent experimental results and novel light-trapping schemes in a-Si:H, μc-Si:H and ‘micromorph tandem' solar cells

<i>Theoretical</i>: Limits for <i>J</i>_sc, <i>V</i>_oc, FF and efficiency of single-junction solar cells and tandems are derived, as a function of <i>E</i>_g, showing that: (1) double-junction ‘micromorph' solar cells constitute an optimum combination of bandgap values; (2) main future gain for thin-film silicon solar cells will be increasing <i>J</i>_sc, by light trapping. Spectral ranges where light trapping has to act are presented, separately for a:Si:H and μc-Si:H. <i>Experimental</i>: For glass-pin type cells, light trapping is obtained at the front end, by use of rough TCO layers. For plastic-nip type solar cells, light trapping is obtained by texturing of the back reflector, which acts as basic layer for further growth. Both random texturing and periodic gratings have been used, but results presented are limited to single-junction a-Si:H and μc-Si:H solar cells. So far, the highest <i>J</i>_sc-enhancement obtained is approximately 20%.