Enhanced energy conversion efficiency of Al-BSF c-Si solar cell by a novel hierarchical structure composed of inverted pyramids with different sizes

The fabrication of silicon hierarchical structures with both superior light trapping and efficient carrier collection is significant for improving the efficiency of c-Si solar cells. Here, we present a simple method to fabricate a novel hierarchical structure composed of inverted pyramid (IP) with different sizes by varying nanostructure rebuilding (NSR) ratio ([NaF]:[H2O2]) during NSR treatment of silicon nanostructures containing Ag nanoparticles (AgNPs), followed by a systematic investigation of the underlying mechanism behind the structure formation. The silicon nanostructure is obtained through a well-known Ag assisted chemical etching method. Independent of NSR ratio, large-size IP with AgNPs fixed at the vertex can always form by carefully modulating NSR duration. Interestingly, by changing NSR ratio to 1:2, 1:4 and 1:6, it is found that the dissolution rate of AgNPs at the vertex during NSR process rises accompanied with increasing the number of etching points by AgNPs redeposition on the IP side walls, forming hierarchical structures composed of IP with different sizes. Moreover, the weakening of masking effect of AgNPs is demonstrated with the increase of AgNPs dissolution, rendering the IP vertex from flat to sharp. By adopting the best hierarchical structure for light trapping, the corresponding Al back surface field (Al-BSF) c-Si cell can reach a high efficiency of 19.80% with a V-oc of 644 mV, J(sc) of 38.77 mA/cm(2), FF of 79.32%, which is 0.20% and 0.27% absolute higher than that of conventional random IP and micro pyramid (MP) based cell, respectively.