Photovoltaics hold considerable promise as a sustainable and industrially viable means of energy generation. One low-cost approach is based on ‘tandem’ dye-sensitized solar cells (DSCs) containing both n-type and p-type DSCs. However, the realization of high-efficiency DSCs is hindered by the poor performance of p-DSCs. Now, Andrew Nattestad and colleagues in Australia and Germany1 report the realization of a p-DSC that can convert photons to electrons with an efficiency close to unity.

A crucial ingredient for the development of this efficient p-DSC was the specific tailoring of the constituent dye molecules. In p-DSCs, incident light is absorbed by a dye-sensitized p-type semiconductor film resulting in electron injection from the semiconductor into the dye. In the case of n-DSCs, dye-excitation results in the injection of electrons from the dye into the n-type semiconductor.

The photon-to-electron conversion efficiency of p-DSCs, however, has so far been limited by the recombination of electrons with holes. Nattestad and his colleagues resolved this obstacle through the use of a specific donor-acceptor dye containing an oligothiophene linker of variable length. This provided control over the distance between the surface of the p-type semiconductor and the negative charge remaining on the dye-molecule (the acceptor unit) after photoexcitation.

“This distance strongly influences the recombination rate,” says Udo Bach, one of the group leaders. “By increasing the spacer length from two to six thiophene units, we were able to dramatically reduce charge recombination so that every photon absorbed by our dye was converted to a charge carrier that could be extracted by the external circuit of the solar cell.”

Fig. 1: Energy level diagram illustrating the charge generation and flow in a tandem DSC containing n-DSC (left) and p-DSC (right) components. The incoming light is absorbed by the dye molecules (yellow arrows), generating electrons (blue) and holes (red) that propagate in opposite directions through the device, giving rise to a photocurrent.

The energy conversion efficiency of these devices is several times higher than the most efficient p-DSC known to date and opens up the possibility of realizing high-efficiency pn-DSCs (Fig. 1). “Multi-junction solar cells convert sunlight more efficiently because each junction is designed to convert photons of a specific band within the full solar spectrum,” says Bach.

The researchers have already demonstrated a working pn-DSC device, which has provided encouraging results. Although Bach notes that their prototype is only a proof-of-principle device, further work is already underway to optimize the complementarity of the spectral sensitivity of the two constituent DSCs to maximize its efficiency.