Getting to a net-zero emissions economy requires faster development and diffusion of novel clean energy technologies. We exploit a rare natural experiment to study the impact of an open-access mandate on the diffusion of scientific research into patented technologies. From 2014 onwards, the U.S. Department of Energy (DOE) required its 17 National Laboratories (NLs) to publish all peer-reviewed scientific articles without a paywall. Using data from more than 300,000 scientific publications between 2012 and 2018, we show that scientific articles subject to the mandate were used on average 42% more in patents, despite embargo periods of up to 12 months. We also show that articles subject to the mandate were not cited more frequently by other academic articles. Our findings suggest that the mandate primarily contributed to technological development but has not led to additional academic research. Lastly, we show that small firms were the primary beneficiaries of the increased diffusion of scientific knowledge.