Water is the best heat transfer fluid from the respect of efficiency. It is also the best heat storage medium from the respect of heat able to be stored per cubic foot.
If water did not freeze at 32 degrees F., then you would see water as a heat transfer fluid in every residential and most commercial solar water heating systems. So all solar water heating systems that operate all year round in the continental US must be freeze protected. There are 2 types of systems used worldwide for residential and most commercial solar heating/cooling systems, pressurized and drainback. You can read about how those systems work in my book (it is available for purchase and also should be available at your local library). Drainback systems use either pure water or a weak antifreeze solution as the heat transfer fluid and the fluid drains from the collectors every time the system turns off. In a pressurized system the collectors are filled with a non-tozic antifreeze.water mixture at all times.
solar water and space heating and cooling systems do not require temperatures above the boiling point of water, so there is no reason to go there. Because solar energy is low-grade heat, it is difficult to make steam on a reliable basis except in very specific places on earth and with very specialized collectors. These systems are called CSP (concentrating solar power) and are all large facilities that are in high desert regions of the world and are very sophisticated systems that typically operate around 600 degrees F. They in fact often use an oil as their heat transfer fluid. They do make steam and turn a turbine to make electricity. These plants are the cheapest way to make electricity on the planet, with the added benefit of no pollution.
I recommend my book as a resource for solar water heating systems. While there are plenty of good resources on the internet about this subject, I would caution you that there is a lot of mis-information out there too.