Organic Solar Cells

Organic photovoltaics have received extensive attention due to their excellent quality (the processability of the solution, adjustable electronic properties, low-temperature manufacturing and cheap and lightweight materials). Although several other photovoltaic technologies have higher efficiency, OPV still has advantages due to low toxicity, cost and environmental impact. So far, they have exceeded the certification efficiency of 13%, which is close to the efficiency value obtained by low-cost commercial silicon solar cells.

An OPV battery is a type of solar cell in which the absorber layer is based on organic semiconductors, usually polymers or small molecules. For organic materials to become conductive or semi-conductive, a high level of conjugation (alternating single and double bonds) is required. The conjugation of organic molecules causes the electrons associated with the double bond to become delocalized over the entire conjugation length, and these electrons have higher energy than other electrons in the molecule.

Fig 1. (a) Consecutive steps for photocurrent generation in a donor-acceptor heterojunction structure. (b) Schematic cross-section of planar donor–acceptor heterojunction. (c) Schematic cross-section of mixed or bulk heterojunction. (Cao W. R, et al. 2014)

However, in organic materials, these electrons do not occupy the valence band, but are part of the so-called "highest occupied molecular orbital" (HOMO). The first is called the lowest unoccupied molecular orbital (LUMO). There is an energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of OSC, which is usually called the bandgap of the material. As the conjugation increases, the bandgap will become small enough that visible light can excite electrons from HOMO to LUMO.

Compared with inorganic materials, the main advantage of OPV lies in its ability to be used in large-area and flexible solar cell modules as well as its lower cost and easier manufacturing than silicon-based materials. However, to catch up with the performance of silicon-based solar cells, both the donor and acceptor materials in OPV need to have a good extinction coefficient, high stability and good film morphology. Since the donor plays a vital role in absorbing solar photon flux, the donor material requires extensive light absorption to match the solar spectrum. Besides, another basic requirement of an ideal donor/acceptor is a large hole/electron mobility to maximize charge transport.

Fig 2. Schematic structure of organic photovoltaic devices with spontaneously-formed interdigitated structure. (Cao W. R, et al. 2014)