You are correct, the sporophyte is the diploid stage. The easiest way to explain this to think about the terms diploid and haploid. Diploid or 2n means that there is a full set of chromosomes present - two copies of each - one from the female and one from the male. Haploid or n means that the chromosome number has been reduced by half - or we separate the pairs of chromosomes. The reason we need to have a haploid stage is to prepare for fertilization. An organism, whether a plant or animal, needs to maintain its chromosome number to be healthy. Because two sex cells fuse during mating, those sex cells have to have half of the chromosomes to make sure the resulting zygote gets the right number of chromosomes. So back to sporophytes - the sporophyte phase is when the sex cells (gametes) have fused to form the zygote and therefore have each donated their DNA (chromosomes) to give the offspring its full set. I have attached a diagram that might help you understand. I hope I made sense and that I was able to help. Feel free to message me if you need more information.
