Open circuit voltage and short circuit current are not enough to evaluate the efficiency, but the "fill factor" also needed. There are various types of loss of power. Quantum efficiency---efficiency of photons--curriers conversion, recombination---loss of the curriers and Ohmic loss. Each loss affect these factors in different ways. 

You say you have problems with I-V graph, do you mean you failed to measure the graph? Unfortunately, the efficiency can not be evaluated without the graph. So, it is better to find way to measure the graph. What problem do you have? We may find a solution for you.