Decrease of internal resistance of a solar cell while increasing the load resistor?
The solar cell can only produce an amount of current proportional to the incident light. If the load draws less current than the cell can produce then its output voltage doesn't drop much, indicating a low internal resistance. In this region resistance is dominated by series resistances of the bulk and sheet silicon, surface contacts and interconnects.   

As load resistance is reduced it draws more current until getting to the 'knee' where cell output is current limited. Then the voltage drops sharply,  which corresponds to a sharp increase in internal resistance. Here the primary internal resistance contributor is the limited current supply.  



Your formula will only give a sliding increase after the knee because you are using the total current and voltage (relative to open-circuit) instead of the incremental change. To get the true (dynamic) resistance you should use the differences between voltages and currents before and after each decrease in load resistance.