Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge-Transfer State Electroluminescence

Charge‐transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free‐carrier bimolecular recombination at the donor‐acceptor interface, rather than a charge‐trap‐mediated process. The fingerprint of the presence of nonradiative trap‐assisted recombination, a voltage‐dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination‐intensity dependence of the open‐circuit voltage. Charge‐transfer state electroluminescence is investigated in polymer:fullerene solar cells. It is demonstrated that the charge‐transfer state emission originates from free‐carrier recombination, rather than a charge‐trap‐mediated process. The voltage dependence of the electroluminescence quantum efficiency can shed light on nonradiative recombination routes, revealing dominant free‐carrier bimolecular recombination in most organic solar cells. This is confirmed by measurements of the illumination‐intensity dependence of the open‐circuit voltage.