That’s been an option in the UK for a few years now, but it’s not sufficiently widely-advertised for people to take notice. The loan still accrues interest (at around 6%), but it’s government-backed and there are rules that ensure that you can’t end up paying out more than you’re taking in/saving as a result of the panels. Most folks pay off the loan in about 15 years, which still leaves a theoretical 10 years of value out of the panels before their warranty runs out.
The UK government has a lot of schemes under the “Green Deal” brand that work in similar ways: e.g. they’ll pay for you to get a more-efficient boiler, cavity wall insulation, loft insulation, etc. and you pay them back out of the savings you make. It’s not bad.
The other major way that people get solar panels here on existing properties is through a “rent a roof” scheme, whereby you let a company install their panels on your roof: they take the subsidy, and you get the energy savings (as payment for the use of your roof). These have proven popular because there’s no initial outlay, but some homeowners have been stung by contractual clauses that say that e.g. if you have to take the panels down for a week to repair the roof, then you have to pay the company to compensate them for their losses in the meantime.
I had a pile of money lying around, so I bought ours outright from day one!