Hey Logan, that's a difficult question to quantify. Leading in something like technology can mean many things. The Japanese are probably leading the edge of advanced solar photovoltiacs, at least as far as efficiency, but the Americans came out with the first solar shingle. I would have to put my vote in the Germans however. There are basically 2 ways to generate electricity from the sun. One is to use photovoltiac panels, the ones you see on the space station, and on the roof of our garage. They work quite well, are very expensive, and only convert about 12 % of the suns energy to usable electricity. That sounds horrible, but consider that we run our entire home with an array no larger than the roof of our one car carport, and you see that space and efficiency are not really that important for most homes. The other way is to use the heat from the sun to make steam, which turns a turbine generator. Doing this boosts efficiency to about 60 or 70 %. A device like this will probably never be built for home use, but for utility scale projects, they work well. There are 2 such plants in the US, both experimental.
But enter the Germans. They are working on a plan to use large trough shaped solar collectors to heat oil to 600 degrees F. Then circulation pumps will pump the hot oil into a large insulated tank, something the size of a school building. Then they will meter water into the tank with a water to oil heat exchanger. This will allow the 600 degree oil to instantly boil the water into steam, again to run the turbine. This has 2 distinct advantages over just heating water: First, when you heat the water directly, there is lead time to heating into steam and turning the generator, so you have to fire it up long before you plan to need it. And if there is a reduction in electric use, then the excess steam has to be vented off to prevent the turbine from overspeeding, so there is some waste. With the water to oil exchanger, we can use an incoming metering valve like a gas peddle on a car. By allowing more water in, you can spool up the torque at the turbine almost instantly, and throttle it back down just as fast, so you have more control and less waste. Secondly, since that huge tank of oil is good and hot at sundown, it can continue to make steam into the night time hours until all the oil has cooled to about 250 degrees. Now we have access to solar power at night, something none of the previous technologies have pulled off.
There are probably other developements out there that are more exciting, but this one is here now and basically uses processes we have known about for years. The plan is to install a huge plant in the NW corner of the Sahara Desert that will run all of Europe. As an additional feature, since they will be using sea water for the steam production, one of the byproducts of producing the electricity will be fresh water, which will be used to irrigate certain high sun intensive agriculture, turning part of the desert into a garden with desalinated sea water. It all has lots of hope, personally I can't wait to see it in action, if it works as well as expected, I think you'll see additional plants in other parts of the world. Take care Logan, Rudydoo