Using an as-deposited Al-doped ZnO (AZO) film synthesized by ultrasonic spray pyrolysis directly onto crystalline silicon (c-Si), we demonstrate a bifacial AZO/n-Si heterojunction solar cell (HJSC) with high efficiency of 8.3%/5.4% under front/rear illumination. To optimize fabrication process, the influence of substrate temperature T-D (in the range 310-460 degrees C), annealing, and film thickness d on the film and AZO/c-Si junction properties were studied systematically. SEM, ellipsometry, EDX spectroscopy, transmission, reflection, and external quantum efficiency spectra, resistivity rho, Hall, Suns-V-infinity, and light I-V measurements were used for the analysis. Annealed junctions, AZO/n-Si and AZO/p-Si, as well as AZO/p-Si junction with as-deposited films showed small open-circuit voltage V-oc (<300 mV). The highest V-oc (similar to 480 mV) showed AZO/n-Si junction with as-deposited film grown at 410 degrees C. We employed as-deposited AZO films grown at 410 degrees C in AZO/(nn(+))Cz-Si/In2O3:F bifacial heterojunction solar cells, which differed only in the AZO film thickness. Increasing d from 260 to 910 nm resulted in the following: (1) the photocurrent did not changed; (2) rho and R-sh of the film, Voc and the series resistance of the HJSCs decreased; (3) the fill factor FF and efficiency II increased (for front illumination, FF: from 29.1 to 57.3%, eta from 3.3 to 8.3%, respectively). At rear illumination, the best cell showed the efficiency of 5.4%. At 1-sun front illumination and 20-50-100% 1-sun rear illumination, such a cell will generate energy approaching that produced by a monofacial solar cell of 9.1-10.3-12.1% efficiency. (C) 2016 Elsevier Ltd. All rights reserved.