Micromorph Solar Cell Optimization using a ZnO Layer as Intermediate Reflector

The insertion of a zinc oxide (ZnO) intermediate reflector (ZIR) between the top and bottom cell of a superstrate (p-i-n/p-i-n) micromorph tandem solar cell is analyzed, experimentally and by numerical simulation. Solar cells are deposited onto glass plates coated by surface-textured ZnO layers deposited by low-pressure chemical vapour deposition (LP-CVD). The gain in the top cell short-circuit current density (J_sc) obtained by ZIR insertion as well as the corresponding loss for the bottom cell are experimentally observed, for different values of ZIR thickness d. The gain and the loss depend nearly linearly on ZIR thickness for d < 100 nm, the maximum gain is almost 3 mA/cm². Experimental results are compared with an optical simulation. In the latter a three-layer effective media approximation is used for modeling of thin ZIR layers. Micromorph tandem solar cells were deposited on 2 different types of front LP-CVD ZnO layers: (a) a layer optimized for a-Si:H single-junction solar cells; (b) ZnO layers specially developed for muc-Si:H cells and having undergone a novel surface treatment. In case (a) J_sc=12.1 mA/cm² and initial conversion efficiency is 11.6 %; in case (b) J_sc=12.8 mA/cm² and initial conversion efficiency is 11.8 %. The open-circuit voltage (V_oc) value could be improved from 1.32 V to 1.41 V with an increased surface treatment time.