Organic Photovoltaics
Organic Photovoltaic (OPV) devices convert solar energy to electrical energy. A typical OPV device consists of several photoactive materials sandwiched between two electrodes. Figure 1(a) depicts a typical bilayer organic photovoltaic device. An alternative structure is called bulk heterojunction (BHJ) where the two active layers are replaced by a blend of the same materials, Fig. 1(b).
Figure 1. Structure of (a) a bilayer organic photovoltaic device, (b) a blend organic photovoltaic cell.
In an OPV cell, sunlight is absorbed in the photoactive layers composed of donor and acceptor semiconducting organic materials to generate photocurrents. The donor material (D) donates electrons and mainly transports holes and the acceptor material (A) withdraws electrons and mainly transports electrons. As depicted in Figure 2, those photoactive materials harvest photons from sunlight to form excitons, in which electrons are excited from the valence band into the conduction band (Light Absorption). Due to the concentration gradient, the excitons diffuse to the donor/acceptor interface (Exciton Diffusion) and separate into free holes (positive charge carriers) and electrons (negative charge carriers) (Charge Separation). The exact process of charge separation has two stages. In the first a bounded polaron pair is formed at the interface. In the second step the polaron pair can either recombine or separate into individual polarons which are then diffuse to the electrode and collected there (Charge Extraction).
Figure 2. Functional mechanism of an organic photovoltaic cell without looking at the exact morphology/ (D = donor, A = acceptor) .