A photodiode is made to detect light quickly a solar cell is made to collect energy from light. They are both typically silicon diodes, but modified to meet their dissimilar requirements. A photodiode has to be quick, which means low capacitance, which means small area of silicon. Thus, it is not very sensitive, and cannot produce much power from light. A solar cell has as large an area as you can afford to buy, getting watts per square inch. There are other differences in the way they are made, and how the PN junction is grown, but they all relate to this dissimilarity in purpose.