Role of the Charge Transfer State in Organic Donor–Acceptor Solar Cells

Abstract

Charge transfer complexes are interfacial charge pairs residing at the donor-acceptor heterointerface in organic solar cell. Experimental evidence shows that it is crucial for the photovoltaic performance, as both photocurrent and open circuit voltage directly depend on it. For charge photogeneration, charge transfer complexes represent the intermediate but essential step between exciton dissotiation and charge extraction. Recombination of free charges to the ground state is via the bound charge transfer state before being lost to the ground state. In terms of the open circuit voltage, its maximum achievable value is determined by the energy of the charge transfer state. An important question is whether or not maximum photocurrent and maximum open circuit voltage can be achieved simultaneously. The impact of increasing the CT energy-in order to raise the open circuit voltage, but lowering the kinetic excess energy of the CT complexes at the same time-on the charge photogeneration will accordingly be discussed. Clearly, the fundamental understanding of the processes involving the charge transfer state is essential for an optimisation of the performance of organic solar cells.

Charge Transfer Dynamics in Donor–Acceptor Complexes between a Conjugated Polymer and Fluorene Acceptors

“…The charge-transfer state with the hole at the donor and the electron at the acceptor, both near the interface, has been recognized as a key intermediate state on the route from photon absorption to free charges in various donor-acceptor combinations. 27,31,42,73 Our results demonstrate that acceptors with a higher EA (i.e. those providing a stronger driving force for charge separation) result in an exponentially increased recombination rate from the CTC state which leads to reduction of the longlived separated charges.…”

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…Among others, charge generation and charge recombination might be two most fundamental yet most debated issues. [9][10][11][12][13][14][15][16][17] The difficulty in understanding charge generation in PSCs is rooted in the low dielectric constant of organic materials. Low dielectric constant results in weak dielectric screening of the Coulombic attraction.…”

On the Energetic Dependence of Charge Separation in Low-Band-Gap Polymer/Fullerene Blends

“…Often referred as charge transfer (CT) states, these e-h pairs have binding energies approximately one order of magnitude higher than kT. 7 Understanding what determines whether these interfacial states dissociate to form free charges is a key unresolved challenge for such organic optoelectronic devices.…”

“…[7][8] Building upon Onsager theory, Morteani et al and Peumans and Forrest proposed that excess energy is an important factor in overcoming the electrostatic eh attraction of the bound charges. 9,10 Two types of CT states were identified; relaxed CT states that predominantly recombine to ground state and "hot" CT states with enough excess energy to drive efficient charge dissociation.…”