The following three problems on solar cells are studied in this paper： It is well known that the key element of a solar cell is a PN junction. However, the so called current-voltage characteristics curve (I-U curve) of a solar cell looks very differently with that of a normal PN junction, why? How does the I-U curve of a solar cell change exactly? How do the output current, output voltage, and output power of a solar cell change with the load resistance? The study on these problems has important significance in design and application of solar cells. The research results are as follows: The reason of first problem that the so called I-U curve of a solar cell is, in fact, the I-U curve of the load resistance in a solar cell circuit, not the I-U curve of the PN junction in the solar cell. The answerHYPERLINK"javascript:;" to the second problem is that the current monotonously decreases with increasing the voltage. The answerHYPERLINK"javascript:;" to the third problem is that for a solar cell circuit, with the increase of the load resistance, the output current monotonously decreases, and the output voltage monotonously increases, while the output power does not change monotonousl. There exists a maximum output power which correspods to an optimal load resistance.