Solar energy has considerable potential as a renewable energy technology, but most solar cell designs still have difficulties competing on the energy market because of a trade-off between energy-conversion efficiency and cost-effective mass-production. Dong Chan Lim, researchers from the Korea Institute of Materials Science, Young Dok Kim from Sungkyunkwan University and colleagues from several Korean institutions have developed an enhanced electron contact layer to construct organic solar cells with significantly improved energy-conversion efficiency.1

Fig. 1: Schematic diagram showing the structure of organic solar cells with a ZnO/carbon nanotube (CNT) buffer layer.

Depositing organic thin films by a wet chemical process is a cost-efficient way of fabricating solar cells. “However, a barrier for the application of films produced by a wet process is the low stability of the resultant organic layers owing to their rapid degradation by light,” says Lim. To circumvent this issue, inverted solar cell structures have recently been developed. In this architecture, the conventional order of construction is reversed — the active organic layer that converts photons into charge is deposited on the transparent conducting electrode rather than on top of the device, thus enhancing the stability of the organic solar cell to light-induced degradation. In this structure, however, to ensure that positive charges from the organic layer do not reach the negative contact where they would be lost, a buffer layer of a material such as zinc oxide (ZnO) is also inserted between the two layers.

Now, Lim, Kim and their team have improved the function of this buffer layer by incorporating carbon nanotubes into it during fabrication. The nanotubes enhance lateral electron transport through the ZnO layer and at the same time increase its surface area in contact with the organic layer, both of which improve electron collection from the active layer. The inherent toughness of the carbon nanotubes also means that the mechanical stability of the solar cells is enhanced considerably.

Overall, incorporating carbon nanotubes into the buffer layer of organic solar cells increased the devices’ long-term performance by almost a third. The researchers now plan to carry this scheme further toward a commercial fabrication process, says Kim. “We are working on the large-scale fabrication of these devices, possibly also using other nanomaterials such as graphene for further improved contacts.”