New research suggests roadmap for more efficient organic solar cells

The 2 key physical processes that allow solar cells to convert incoming light into electricity are: (i) the creation of negative and positive charge within the material when light shines on it (mostly affects device current), and (ii) the separation and capture of those charges (mostly affects voltage), which can then be used to drive an external circuit – much like a battery. PV technologies may differ in the materials used to achieve these goals, but all solar cells must successfully perform these two functions (charge creation and charge separation/capture) in order to produce electricity.

Almost all commercial solar cells today are made using semiconductors, a class of materials like silicon, whose unique electrical properties allow charge creation over a large range of the sun’s spectrum that shines on it (higher current) as well as effective charge separation and capture (higher voltages). The resulting high efficiencies (up to 25% in silicon PV) have made semiconductor-based solar cells the dominant commercial PV technology.

Organic solar cells, on the other hand, have only managed to achieve record efficiencies in the 10-11% range. This is because in organic solar cells, unlike in semiconductors, opposite charges are created on the same molecule when light strikes the cell. In order to produce high currents, these charges must be captured before they can recombine. Since charge creation and separation occurs in the same molecule, it has led to a widely held notion that the two processes always compete with each other. Most researchers have thus focused on minimizing this trade-off between high voltage and high current.

However, the new research sheds light on the exact mechanism of charge transfer/capture in organic molecules and, as Noel Giebink of Penn State explains, “dispels this perceived tradeoff.” According to the researchers, a quantum effect causes charges in an organic PV cell to exist not as single particles, but in wave-like states over separate regions (see top image) of the same molecule. When these wave-states collapse far enough away from each other, the charge effectively becomes separated, generating current in the process.

The researchers have also demonstrated that a new class of nano-particles, which, when attached to the molecule, can perform charge capture more easily – without compromising on charge creation. In light of this new discovery, the researchers have suggested a number of “design rules” for the technology that should lead to notable improvements in efficiency. Giebink explains, “This result should help people design new molecules and optimize” existing cell designs that will “help increase solar cell voltage without sacrificing current.”

The new research is a boost for organic PV, which has so far eluded commercialisation. However, the potential is huge since organic solar cells use inexpensive organic molecules that can be cheaply produced in huge volumes and easily scaled up in roll-to-roll processing environment. Silicon solar cells, in comparison, require high-cost raw materials and energy intensive processes that do not offer the same economies of scale. Organic solar cells are also suitable for certain niche applications where traditional solar cells have failed to make inroads, thanks to their transparency: for example, they can be used to make transparent, power-producing windows.

If the new findings lead to more efficient, cheaper organic PV devices as the researchers hope, we may see such technology finding its way into the market sooner than expected.

Top Image Credit: Noel Giebink (Penn State University) via NewsWise