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

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.