A Combined Theoretical and Experimental Study of Dissociation of Charge Transfer States at the Donor–Acceptor Interface of Organic Solar Cells

Abstract

The observation that in efficient organic solar cells almost all electron-hole pairs generated at the donor-acceptor interface escape from their mutual coulomb potential remains to be a conceptual challenge. It has been argued that it is the excess energy dissipated in the course of electron or hole transfer at the interface that assists this escape process. The current work demonstrates that this concept is unnecessary to explain the field dependence of electron-hole dissociation. It is based upon the formalism developed by Arkhipov and co-workers as well as Baranovskii and co-workers. The key idea is that the binding energy of the dissociating "cold" charge-transfer state is reduced by delocalization of the hole along the polymer chain, quantified in terms of an "effective mass", as well as the fractional strength of dipoles existent at the interface in the dark. By covering a broad parameter space, we determine the conditions for efficient electron-hole dissociation. Spectroscopy of the charge-transfer state on bilayer solar cells as well as measurements of the field dependence of the dissociation yield over a broad temperature range support the theoretical predictions.

Local Intermolecular Order Controls Photoinduced Charge Separation at Donor/Acceptor Interfaces in Organic Semiconductors

“…[ 19 ] One very common and quite intuitive argument is that local molecular order delocalizes the manifold of charge transfer states, leading to a strong reduction in the coulomb binding energy because the effective initial separation distance of electrons and holes is increased. [ 3,4,10,[20][21][22] This is consistent with several spectroscopic studies that indicate ultrafast generation of widely separated electron-hole pairs. [ 3,23,24 ] However, most of these theoretical pictures involve an interface between pure phases of donor and acceptor molecules, neglecting the widely observed "mixed phase" that generally appears in polymer/fullerene blends.…”

“…For example, it appears that fullerene aggregates enhance the production of free charges versus bound geminate pairs in donor/fullerene blends. [3][4][5][6][7][8][9] Similarly, donor polymers with longer conjugation lengths can enhance charge separation, [10][11][12] and How free charge is generated at organic donor-acceptor interfaces is an important question, as the binding energy of the lowest energy (localized) charge transfer states should be too high for the electron and hole to escape each other. Recently, it has been proposed that delocalization of the electronic states participating in charge transfer is crucial, and aggregated or otherwise locally ordered structures of the donor or the acceptor are the precondition for this electronic characteristic.…”

“Hot or cold”: how do charge transfer states at the donor–acceptor interface of an organic solar cell dissociate?

“…Fig. 59 We consider this agreement as an important support for the model. In the limit of complete localization, equivalent to m/m e = 10, E b converges to the value predicted from the point charge model.…”

“…26,27 CT states can also be generated by direct excitation below the absorption edges of donor and acceptor. 25,59 One cannot rule out that just after its creation an intermediate hot CT state has a higher chance for dissociation, but the overwhelming fraction of dissociation events proceed via cold CT states. by exciting either the donor, the acceptor or -directly-the CT state.…”

Promising Strategy To Improve Charge Separation in Organic Photovoltaics: Installing Permanent Dipoles in PCBM Analogues

“…[13][14] Recently, an experimental study performed by Vandewal et al 8 questions this proposal by revealing that the best materials systems show an internal quantum efficiency higher than 90% without the need for excess electronic or vibrational energy. [20][21][22] suggest that a dipolar layer can be formed at donor/acceptor interfaces.…”