“For the first time ever, we have direct experimental evidence that an external quantum efficiency above 100% is possible in a single photodiode without any external antireflection,” says Hele Savin, associate professor of Micro and Nanoelectonics at Aalto University in Finland. The results come just a few years after Savin and colleagues at Aalto University demonstrated almost unity efficiency over the wavelength range 250–950 nm in photodiodes made with black silicon, where the silicon surface is nanostructured and coated to suppress losses.

Noticing some curious effects in the UV region, Savin’s group extended their study of the devices to focus on this region of the electromagnetic spectrum. UV sensing has multiple applications, including spectroscopy and imaging, flame detection, water purification and biotechnology. While annual market demand for UV photodiodes is expected to increase to 30%, the efficiency of these devices has been limited to 80% at best. To Savin’s surprise, closer analysis of their device’s response to UV light revealed that the external quantum efficiency could exceed 130%. Independent measurements at Physikalisch Technische Bundesanstalt (PTB) verified the results.