Organic light-emitting devices (OLEDs) are a promising technology for flat-panel displays and solid-state lighting. While OLED efficiencies have increased dramatically in recent years, further progress is complicated by the fact that the vast majority of organic materials are fluorescent and therefore emit only from molecular excited states ('excitons') with spin 0, or 'singlet' spin symmetry. Here, we demonstrate the ability to manipulate the fraction of excitons which form as singlets in fluorescent materials by altering the OLED structure. We insert a mixing layer that affects only charge-transfer (CT) states, which are the precursors to excitons. As a result, we triple the singlet fraction and the efficiency of the red fluorophore DCM2. We term fluorescence enhanced by CT spin mixing 'extra fluorescence', and show that its origin is in part an inversion of the usual energetic ordering of the singlet and triplet CT states.