Don't use the built-in shadow catch only feature.

Put this material on your ground. Do a test render, but do it with a background color so you can see if the ground is completely transparent. If it isn't, you need to raise the Diffuse_Value in the Diffuse node, until the lit part disappears.

Once you do that, change to Render Over Black, do your render, and save as PNG.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)