I believe you simply connected your cell backwards, just swap the high and low connections. Solar cell J-V curves could be plotted in either the 2nd or 4th quadrant, but we have agreed to plot them all in the 4th quadrant by convention. It makes a bit more sense that way when you consider the path of an individual charge.
Dear Farzaneh it is impossible!!! James' answer is totally substantiated and i believe that the main problem is that you've connected the cells backwards. If your problem is still existed, then check the axis of your plots.