PPV a Solar Cell Candidate

Organic polymers possess the ability to convert electrical energy into photon energy and vice versa. In combinations with the potentially high conducting properties they constitute an excellent platform for many applications such as organic light emitting diodes OLED and solar cells. While the OLEDs have already entered the market, organic solar cells still have a power conversion effciency that is too low compared to the inorganic devices. This is partly due to low intrinsic charge carrier mobilities which leads to current losses via recombinations. To increase the effciency of this type of device it is necessary to perform more profound studies of the processes that limit the power conversion. These processes include exciton generation and relaxation, charge separation, and charge transport. Here we study at all these processes, but most emphasis is put on the exciton relaxation and charge separation, where the latter is strongly dependent on the first. It turns out that highly excited electron-hole pairs are much easier to separate than the lower lying excitations. We have also studied the effect of adding lattice energy to the system, to simulate the effect an increased temperature can have on charge separation.

The Su-Schrieffer-Heeger SSH model is the most commonly used theoretical model for describing carbon based conjugated systems. Here we have used an extended version of the SSH model which includes a three-dimensional description of the system based on internal coordinates bond distances and bond angles as well as dihedral angles.

The SSH Hamiltonian with an additional term describing the external eletric field has the form

where the electronic part is defined as
In the simulations which include an external electric field E the following term has been added to the electronic part of the Hamiltonian,
The field is constant both sparially and in time after a smooth turn on.

The rest of the interaction in the system are described classically as:

The equation of motion for the atoms is
where M is the mass of a CH-group in PPV and a C atom in the case of the C60 fullerene group. The solution of the equation is

When optimizing the starting geometries of the molecules we minimize the total energy with respect to the atomic positions in all three dimensions, with the constraint to keep the molecular size constant. The atomic position and the charge density were numerically integrated in time by solving the coupled differential equations. This was done using a eigth order Runge-Kutta integration with step-size control which in practive means a time step of less than 10 as.

PPV and C60

The simulation was performed on both PPV only systems with several PPV chains and PPV together with C60 fullerene balls. The purpose was to study the interaction between an absorbing material (PPV) and a charge transfering material (c60). The study found that the an absorbed electron in PPV could be lead away by the C60 and thus give rise to a charge separation, and therefore a current. The figure below shows a charge separation where the net charges are shown for PPV and two C60 as a function of time. After 1 ps the charge separation is almost complete, consult paper [1] for more discussion.

[1] L. GisslÚn, A. Johansson, and S. Stafstr÷m, J. Chem. Phys. 121 , 1601 (2004).