All photovoltaic device efficiencies are limited by the 'threshold' process inherent in how photovoltaic devices work: a photon above a certain energy level is required to excite an electron that will later be extracted as electrical current. This sets a limit to the efficiency of solar power conversion to electricity of a 'single threshold' system to about 30%. Differentiating the threshold to two 'steps' increases the theoretical limit to 42%. One of the proposed ways to achieve this is by splitting the solar spectrum and guide each part to a different device, each with a different threshold energy, matching a different portion of the solar spectrum. If the devices are stacked, this is called a tandem configuration. To make such a approach worthwhile, a photovoltaic device that uses the high-energy portion of the solar spectrum efficiently is required. Current available options are extremely costly and are not feasible for large-scale application, or are insufficiently inefficient to make their use worthwhile.