Electroluminescence from charge transfer states in Donor/Acceptor solar cells

Charge photocurrent generation is a key process in solar energy conversion systems. Effective dissociation of the photo-generated electron-hole pairs (excitons) has a strong influence on the efficiency of the organic solar cells. Charge dissociation takes place at the donor/acceptor interface via charge transfer (CT) excitons, which is Coulombically bound interfacial electron- hole pairs residing at the donor/acceptor heterojunctions. The CT state represents an intermediate state between the exciton dissociation and recombination back to the ground state. Since the recombination of photo-generated charges is a major limitation for the efficiency of the organic solar cells, a thorough understanding of this loss mechanism is crucial to improve the performance of the devices. Furthermore, examining this interfacial state is of great importance in order to maximize open-circuit voltage and photocurrent simultaneously as they both depend on it[1]. As a useful method, Electroluminescence (EL) measurements can be applied to examine CT-state emission. In this method[2], CT states are created by injection of electrons and holes into the device, and the luminescence originated from free carrier recombination at the donor/acceptor interface is detected. As a less studied system, we examine here the interfacial charge transfer state recombination in DBP:C70 thin-films. The weak EL from the small molecule solar cell biased in the forward direction gives valuable information about the CT state recombination, from which the maximum open-circuit voltage can be estimated, and further can be used in the modeling and optimization of the OPV devices.

[1] C. Deibe, T. Strobe, and V. Dyakonov, “Role of the charge transfer state in organic donor-acceptor solar cells,” Adv. Mater., vol. 22, pp. 4097–4111, 2010.
[2] K. Tvingstedt, K. Vandewal, A. Gadisa, F. Zhang, J. Manca, and O. Inganäs, “Electroluminescence from charge transfer states in polymer solar cells,” J. Am. Chem. Soc., vol. 131, no. 3, pp. 11819–11824, 2009.