Organic photovoltaic advantages.. or challenges?

Solar energy as one of the alternative and sustainable energy sources has a potential to replace fossil fuels. However, harvesting solar energy is still not economically competitive with the conventional energy sources. Many investigations are being made to reduce the manufacturing costs by developing new and innovative low cost materials, for example organic photovoltaic (OPV) materials. The efficiency of OPV devices is still considerably low, thus large efforts are also being made to improve it. The development of OPV devices is a highly active research area where very fast progress is being made. But still, the current generation of OPV devices are not competitive enough due to the limited fundamental knowledge about the optimal properties of the materials involved 123.

Currently the most common OPV materials are donor-acceptor blend. The most common mechanism of OPV devices consists of the following processes (see Figure 1), i.e. firstly light absorption of the donor or acceptor molecule leading to the formation of local exciton (D*A), then diffusion of the exciton towards the donor-acceptor interface. The electron is subsequently transferred from the donor to the acceptor molecule leading to the formation of a charge transfer state (D+•A-•). Finally, the free charges are transported to the electrodes and ultimately the photocurrent is generated.

Figure 1. The mechanism of an OPV device. a) Light absorption of the donor, leading to the formation of local exciton (D*A). b) Exciton diffuses to the donor-acceptor interface. c) Electron is transferred to the acceptor, the formation of charge transfer state (D+•A-•). d) Free charges are transported to the electrodes. D* = donor excited state, D+• = radical cation of donor, A-• = radical anion of acceptor, unfilled blue circle = hole, filled red circle = electron.
Figure 1. The mechanism of an OPV device. a) Light absorption of the donor, leading to the formation of local exciton (D*A). b) Exciton diffuses to the donor-acceptor interface. c) Electron is transferred to the acceptor, the formation of charge transfer state (D+•A-•). d) Free charges are transported to the electrodes. D* = donor excited state, D+• = radical cation of donor, A-• = radical anion of acceptor, unfilled blue circle = hole, filled red circle = electron.

One way to improve the efficiency of OPV devices is to generate multiple exciton. Singlet fission (SF) is a process in which singlet excited chromophore transfers part of its energy to the neighbouring chromophore to generate two triplet-excited states that are coupled into singlet. This process is illustrated in Figure 2. The most urgent issues in this field are finding the most potential chromophore and understanding its mechanism. Many studies have been done to find chromophores that satisfy the energetic criteria of the SF process,i.e. the energy of singlet excited state should be equal or larger than twice the energy of triplet excited state [E(S1) ? 2E(T1)]. In addition to this, the SF rates (electron transfer rates) should be considerably large 45.

Figure 2. The singlet fission process
Figure 2. The singlet fission process

Computational chemistry helps to predict the properties of various materials accurately and to screen the potential systems fast and cheap in comparison with wet chemistry (experimental study). Moreover, a careful analysis and interpretation of the computational results allow designing rules that can be used as a guidance for the development of new materials for more efficient OPV devices. A crucial process in the generation of charge carriers is the electron transfer between the materials in the donor-acceptor blend. A new method based on nonorthogonal configuration interaction (NOCI) will be employed to calculate the electron transfer rates. This method will be applied also to study the SF process thoroughly. This new method has main advantages, i.e. a high accuracy due to the inclusion of electron correlation and orbital relaxation effects and it allows a clear chemical interpretation due to the use of nonorthogonal orbitals in the construction of cluster base state 6.

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