Charge transfer state in DBP:C70 organic solar cells

Organic solar cells (OSC) are green solar energy technology, which can be fabricated from organic compounds with cheep techniques and on flexible or transparent substrates such as plastic or glass. OSCs are cost efficient, and lightweight devices that can exhibit high power conversion efficiency (PCE) under low light irradiation. These advantages have made OSCs of great interest, and they have been extensively studied over the course of past few years1. Despite these advantages, OSCs have low stability and lifetime, which puts a barrier between laboratory achievements and industrial scale requirements. Therefore, in order to closing this gap and to obtain further improvements in the device performance and long-term outdoor stabilities, a detailed understanding of the device degradation mechanism is required.
In an OSC, effective dissociation of the excitons takes place at the donor-acceptor interface via delocalized charge-transfer (CT) states, which represents an intermediate state between the exciton dissociation and recombination back to the ground state. In this work we perform the electroluminescence (EL) created by bimolecular free career recombination and sensitive external quantum efficiency measurements (sEQE) in DBP:C70 based SCs as a less studied system in order to study the energy and effect of CT states on degradation of the devices2. The results from these measurements reveal valuable information about the loss mechanism during the aging experiment as well as the energy of the CT states from which the maximum open circuit can be calculated and will set the base for modeling and optimizing the stability of the solar cells.
1. Cao, H. et al. Recent progress in degradation and stabilization of organic solar cells. J. Power Sources 264, 168–183 (2014).
2. Tvingstedt, K. et al. Electroluminescence from charge transfer states in polymer solar cells. J. Am. Chem. Soc. 131, 11819–11824 (2009).