Trade-offs of the opto-electrical properties of a-Si:H solar cells based on MOCVD BZO films

Abstract

Boron-doped zinc oxide (BZO) films, deposited by metal–organic chemical vapor deposition (MOCVD), have been widely used as front electrodes in thin-film solar cells due to their native pyramidal surface structure, which results in efficient light trapping. This light trapping effect can enhance the short-circuit current density (J_sc) of solar cells. However, nanocracks or voids in the silicon active layer may form when the surface morphology of the BZO is too sharp; this usually leads to degraded electrical properties of the cells, such as open-circuit voltage (V_oc) and the fill factor (FF), which in turn decreases efficiency (E_ff) [Bailat et al., Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on. IEEE, 2006, vol. 2, pp. 1533–1536]. In this paper, an etching and coating method was proposed to modify the sharp “pyramids” on the surface of the BZO films. As a result, an evident enhancement was achieved for these modified, BZO-based cells' V_oc, FF, and E_ff, although the J_sc exhibited a small decrease. In order to increase the J_sc and maintain the improved electrical properties (V_oc, FF) of the cell, a thin BZO coating, deposited by MOCVD, was introduced to coat the sputtering-treated BZO film. Finally, we optimized the trade-off among the V_oc, FF, and J_sc, that is, we identified a regime with an increase of the J_sc as well as a further improvement of the other electrical properties.