That's largely due to timing and to the nature of the projects. Many venture capitalists invested in clean energy inventions in 2008. That year half of all cleantech deals included at least one new investor, the Journal reported. Since then, they've learned firsthand of the enormous risk and time commitment involved in developing cutting-edge technologies to commercialization, and have opted to wait for returns on what they have, rather than seeking new projects. Last year only 30 percent of deals had new backers.

Mix of Good and Bad News Comes in New Global Ranking

The first-ever survey, from the World Wildlife Fund and Cleantech Group, a San Francisco market research firm, found that the U.S. government spends the most worldwide on R&D for clean energy technologies, and it has the greatest number of cleantech startups and investors.

But the news is less hopeful when it comes to three other indicators: renewable energy use, strong and steady policies for encouraging clean energy generation and the amount of revenue its cleantech firms generate. Relatively low marks in these categories, among others, landed the United States a fifth-place ranking in the index.

Denmark, meanwhile, topped the list. Israel came in second place, Sweden in third and Finland in fourth.

The rankings are based on an analysis of each country's scores in 15 different categories. In addition to things like how much countries spend on R&D and how much renewable energy they generate, the report looked at whether their electric grids can support a large amount of green power, and if they have a streamlined application process for clean technology patents. It also examined the survival rate of fledgling startups, and the percentage of total workers that are employed in the cleantech sector.

The goal of the analysis is to show investors which countries are likely to thrive in the cleantech space over the next decade, while pointing out those that are lagging. It uses data between 2008 and 2011 from third parties and the Cleantech Group. The latest government spending figures were from 2009. In that year the Obama administration alloted more than $50 billion in federal stimulus for clean energy projects over four years.

But that could change. The country is planning to broaden its cleantech focus to include new technologies by pouring $18 billion in R&D funding over the next few years, according to a Lux Research report released on Tuesday, a move that could attract U.S. and European innovators to China.

The goal is to combine foreign expertise with Chinese spending. "This will create unforeseen opportunities for Western companies," Lux Research said via news release.