Unfortunately not. Here are the applicable lines from the IRS instructions for form 5695.
"You will enter the amounts you paid for qualified solar electric property. See Qualified solar electric property costs, earlier."
"For purposes of both credits, costs are treated as being paid when the original installation of the item is completed," That would include the cost of materials, installation, and taxes found on the invoice.
For clarification on the second question; it is not whether or not a loan is secured or not that determines its deductibility, it has to do with what the loan is secured by. For example, home mortgage loan interest is deductible, but car loan interest is not. Here is link that discusses interest deductions.
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