Not even waste a ray of sunshine by designing solar cells that surpass the 50% efficiency limit. This the wager that, launched a few years ago from the Milan-Bicocca University, is engaging Japanese researchers at Kobe, led by professors Kita Takashi and Asahi Shigeo. To win it, starting from cell design, the Japanese team is refining the innovative technology called up-conversion: A system capable of capturing two or more low energy photons and converting them into a higher energy one. Cell capacity is thus maximized even in poor light conditions. An important step forward if we consider that current PV technologies are unable to exploit the entire solar spectrum and at best only gather two-thirds of available light. The Sun in fact hits the Earth with an infinite spectrum of electromagnetic frequencies, a rainbow of colours with different energies that can be harvested to produce electricity, yet about 70% of this wealth is wasted by currently available solar cells. The work done by the Japanese team entails studying a new cell structure to provide them with a heterojunction interface that can hold together materials with different energy gaps in order to boost yield. "These results," the scientists explain, "suggest that the two-photon up-conversion system has a high potential for producing next-generation, high-efficiency solar cells." The result will be an increase in both photocurrent and voltage, with a new theoretical limit estimated at 63% compared to the current 30%.