External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell

Splitting Singlets Solar cell efficiency is limited because light at wavelengths shorter than the cell's absorption threshold does not channel any of its excess energy into the generated electricity. Congreve et al. (p. 334 ) have developed a method to harvest the excess energy in carbon-based absorbers through a process termed “singlet fission.” In this process, high-energy photons propel two current carriers, rather than just one, by populating a singlet state that spontaneously divides into a pair of triplet states. Although it works in a functioning organic solar cell, the efficiency needs improving.